The Journal of
The  Institute of Circuit Technology
Vol 17 No 1
January 2024

Links to Contents 

Lessons Learned While Investigating Microvia Reliability Failures
Atotech, Germany.

Tobias Bernhard
et al. 

10

Richard Wood-Roe
Journal Editor

In this issue of The Journal our first paper is fom Atotech and outlines the lessons learned while investigating microvia failures. There is a thorough review of the examination techniques and the recommended assessment procedures used.

Our second paper is from Resonac Group of Japan and describes the development of a low loss adhesive film for bonding multilayer PTFE substrates.

Finally we have a paper outlining the latest developments of laser drilling using high frequency burst mode from The Applied Laser Photonics Group and Schmoll Maschinen

Pete Starkey has provided an excellent review of our Christmas symposium at Harrogate. I have found a generous amount of items to add to the industry and membership news sections.

Our future events include the spring seminar in March and the popular Foundation Course which will will be in April this year at Chester University.

I hope you find the contents of the Journal interesting. I welcome your input for the next edition. This email address is being protected from spambots. You need JavaScript enabled to view it.

Bill Wilkie
Technical Director and Membership Secretary

           2024

            Details 

ICT Spring Seminar and AGM on 5th March 2024

At the Manor Hotel, Meriden

The AGM this year involves the election of a Chairman and Vice Chair. Nominations are invited for these posts and also for the election of new Council Members.

Annual Foundation Course on 15th - 18th April 2024

The Foundation Course will be held at Chester University, with the first day hosted by the Merlin Group at their Deeside facility.

Paper 1

Lessons Learned While Investigating Microvia Reliability Failures.

Tobias Bernhard, Roger Massey, Senguel Karasahin, Frank Brüning
Atotech GmbH, Berlin, Germany

Abstract.
Micro vias, be they mechanically, or more typically laser drilled have revolutionized the technical capabilities of today PCBs. Their high hit rate, and low cost during manufacture, in combination with their small real estate requirements enable the High Density Interconnects that have allowed PCB and product designers to push their applications to capabilities beyond anything thought possible with more traditional drilling techniques. That being said, with their number easily running into the many 1000s in a single PCB assembly, there is increasing concern about the long term reliability of micro vias, especially when used in a stacked configuration.

Over the last few years, there has been a slow, but increasing number of reports concluding that stacked micro vias are failing preferentially when compared to an alternative staggered via design. However, while a staggered interconnect arrangement could be seen as a solution to satisfy the reliability demands, they are usually undesirable as they consume larger amounts of real estate which can’t be tolerated in many applications.

With strong evidence available, the preferential failure of stacked micro vias can’t be denied, and as such, there is now a growing number of investigations ongoing to examine microvia reliability and determine if and why staggered micro vias fail preferentially, and also trying to identify what is the ideal microvia structure for best reliability performance.
As a result of the ongoing need to understand the nature of any failure, there is an ever increasing array of analysis tools being drawn on in order to inspect microvia structures. Optical microscopy has in many cases been superseded by the Scanning Electron Microscope, and the SEM is now being supplemented with other tools such as the Focused Ion Beam Microscope and Tunneling Electron Microscope to name a few. However, with each new analysis tool there comes a wealth of new information, and this needs to be understood and interpreted before that information can be valuable.

As part of investigations into microvia failures, we review the published data and find that while these new analysis tools are being readily used, there is what we consider to be some misinterpretation of the data, leading to inaccurate route cause diagnosis, and conclusions that are questionable at best, or wrong and misleading at the worst.

This paper summarizes these initial investigations and discusses the misinterpretation of data as well as offering some insight into other microstructural characteristics which will likely impact the physical properties and reliability of plated blind micro vias.

Background.
Mechanical and laser formed blind micro vias (BMV) have become a standard technology for high density PCB applications as their minimal footprint enables numerous real estate savings compared to traditional through hole techniques. While the use of staggered patterns (Figure 1a) is still widely used, this approach requires a larger footprint compared to an equivalent stacked approach (Figure 1b) and so the use of stacked BMVs has become the state of the art solution where the highest PCB densities are needed.

As with all production processes, there is a learning curve, in-which, products are initially prone to increased production variances, which lead to performance and quality issues, and as is normal, these diminish over time until a high volume, high reliability process is established. Laser drilled BMVs have undergone such a cycle, with BMVs generally now being considered as a robust interconnect method. As mentioned previously, stacked BMVs are often preferred by product designers as they utilize not only a smaller footprint, enable higher track densities with smaller pad pitch, but also allow for enhanced thermal paths which are also a necessity in many modern electronics units where, heat flow can be as critical as electrical flow.

One ongoing concern with stacked BMVs is the impact arising from heat, namely the expansion which occurs during PCB assembly as well as once those populated PCBs reach their final service application. With a series of stacked BMVs being essentially a “solid” column of Copper, the thermo-mechanical properties of the BMV stack is dramatically different to that of the surrounding dielectric matrix, and so there is understanding that significant stresses can arise at the interfaces between each individual via as well as between the complete stack and its encapsulating glass/epoxy resin. While this concern has always existed, and it has been proven to exist, where there have been concerns for final product reliability, it has been found that a staggered BMV pattern can accommodate the applied thermal strains and offer an improved product lifetime, and so have been adopted. Unfortunately, with modern electronic applications demanding the highest possible densification in the smallest possible area, the use of staggered BMV designs is becoming unacceptable and the need for a high reliability stacked BMV design is re-emerging. It must be pointed out however, that stacked BMVs are very widely used within many of today’s PCB designs without excessive field or assembly failures, however, there are increased reliability applications such aeronautics, space and military, where there is a rapidly growing concern over the long term reliability of stacked BMVs.

It is fair to say that while this concern has been steadily increasing, it has been predominantly within those users located in the USA, however, with many PCB users and producers now having a presence and operations in many countries, this concern has begun to spread and has become more of a global issue than it has been in the past. In mid-2018 IPC released a white paper^[1] which as its name implies raised a “call to arms” among the PCB industry as a whole, to review the performance of BMVs, and since that time, there have been an increasing number of papers at technical conferences reiterating and highlighting the need to investigate BMVs further.^[2]

This paper, summarizes a number of investigation techniques used when examining failures occurring within stacked BMVs following thermal screening, or during field service, and highlights the need for the correct analytical tools to be used otherwise there is a risk of root cause being incorrectly identified.

Micro Via Formation and Failure.
There are multiple specific methods, for the formation of a BMVs during PCB production, however such methods can generally be summarized into a generic process flow as shown in Figure 2, it should be noted that there are no significant differences depending on whether the via itself is formed through mechanical drilling or laser ablation.

In terms of failures exhibited by BMVs, the most commonly reported is one referred to as capture, or target pad separation, an example of which can be seen in Figure 3a. For such failures, it is typically found that the failure occurs at the base of the BMV where the plated deposit connects with the previously formed target pad.

From Figure 2 it is clear that this target pad to plated layer interface is a complex structure comprising of the target pad itself, underlying an electroless Copper (ELESS Cu) or other metallization layer, which is then covered with an electrolytic plated Copper (ELYTC Cu) layer, (Figure 3b), as such, it can be appreciated that there are multiple potential locations for failure. (Table 1)

While failure at locations 1 and 5 in Table 1 are possible, they are rarely reported, with the most common failures being attributed to poor “adhesion” of the ELESS and/or ELYTC Cu, or failure within the ELESS Cu itself, although this is rarely so simple to identify.

Typical Investigation Techniques.
As its name suggest “failure analysis” (FA) occurs after a component has failed, and as such offers a vital insight into why that component did fail. Historically, FA has made use of cross sectioning to gain access to the point of failure, followed by optical microscopy, and over time this has been supported or even replaced by Scanning Electron Microscopy (SEM) which offers not only much higher magnifications but elemental detection and analysis.

Electro Polishing.
Mechanical polishing followed by chemical etching is a widely utilized technique, and will likely remain one of the initial methods of sample investigation, however, this can be considered a “macro” process as there can be substantial amounts of material removed. A more “refined” method, which can still be used in combination with SEM etc, is that of electro polishing. This is still considered an etching process, but the material removal is significantly lower than that with wet etching, and can lead to significant inspection benefits, which may be missed through other techniques. As with most etch type operations, the degree of etch penetration or treatment, varies depending upon the properties of the material being treated, so electro polishing, can often highlight interfaces between complex layered structures like those at the base of a BMV.

From Figure 4a and 4b it is clear that there is a line of voids at the base of the BMV, what is not known however, is the exact location of those voids, namely do they occur at the target pad – ELESS Cu interface, within the ELESS Cu layer itself, at the ELESS Cu – ELYTC Cu interface, or within the bulk plated Cu? This knowledge is essential in understanding the root cause of the defects, and identifying the process area in need of troubleshooting activities. Following the electro polishing operation, it becomes possible to distinguish the boundaries where the target pad, ELESS and ELYTC Cu layers occur. In view of this, in Figure 4c, it is clear that the line of voids occur not at the target pad – ELESS Cu interface as is often assumed, but actually at the ELESS Cu – ELYTC Cu interface.

Scanning Electron Microscopy with Focus Ion Beam.
It is reasonable to state that SEM has become a standard technique used during surface failure analysis, yet, more recently there has been a wider adoption of SEM utilizing a Focus Ion Beam (FIB) as this allows selective removal of material and offers additional inspection “within” the sample that is not possible with surface techniques such as standalone SEM (Figure 4). SEM-FIB has been used within the IC fab environment for many years, and is now gaining in popularity within, but is still not common, to the PCB industry.

In order to open up such an inspection window in the sample, an incidental ion beam is used to “mill” or ablate material, and through automated scanning of the ion beam, a high level of control can be achieved, enabling detailed penetration into the sample.

One of the first points typically noted during FA of BMVs is the general crystal structure within the plated Copper, and this generally falls into 2 distinct groups. The first showing a single uniform crystal structure where the interface between the target pad and the plated Cu cannot be distinguished (Figure 6a), or a structure where there is a break in the crystal continuity and one or more clear interfaces can be seen (Figure 6b) While, it could be argued which of the two structures would offer the “best” reliability performance, it is offered that a single crystal pattern would be preferential, as any marked changes in crystal structure would generally enable some form of stress concentration, and could exacerbate failure. It should also be considered, that such changes in structure typically occur around the target pad – ELESS Cu – ELYTC Cu interfaces, and as this is commonly the point of BMV failure, it would be sensible to propose that a continuous metallography would be desirable.

Such structures arise due to the crystallization of plated deposits that occurs following plating. Immediately after deposition the plated layer has no defined crystal structure, but will begin to develop one over a period of time, ideally this recrystallization occurs in a “bottom up” fashion, with each new layer following the crystal orientation of the underlying layer, i.e. the ELESS Cu continues the orientation of the target pad Cu, and then the ELYTC Cu follows the orientation of the ELESS Cu layer. Known as epitaxial growth, this process occurs naturally, although it should be acknowledged that there are influences within the process and the chemical constitution of the plating chemistries which can impact the recrystallization characteristics of plated layers.

While optical microscopy and SEM, in combination with etching processes will enable inspection of a BMV crystal structure, SEM-FIB is becoming a popular means to achieve this as, once the ion milling has opened the inspection surface, there is no need for secondary etching and there is minimal risk of over or under etch leading to ambiguous results.

Possible Mis-Categorization with SEM-FIB.
As discussed previously, SEM-FIB enables removal of the sample material in a direction normal to the sample surface. The major benefit of utilizing such a “FIB cut” is that it can reveal features within a sample that may otherwise be damaged or influenced through traditional methods such as cross sectioning.

One point to consider, and is often overlooked, is the removal of the sample material itself during the “cut” or milling operation, and where that material goes once it has lifted from the sample. As with all SEM units, there is a series of vacuum and detector systems that remove and make use of these “sample” streams, yet the volume of material removed during a FIB- cut is substantially higher than that created when performing simple elemental analysis, yet it is assumed that the operating vacuum and exhaust system in the SEM chamber is sufficient to accommodate this high material removal rate, yet this is not necessarily the case.

Figure 6 shows a prepared SEM-FIB sample, where the initial “cut” has been made manually in order to approach a Copper filled Through Silicon Via (TSV) Once the TSV has been reached, the automatic milling and serial imaging process was begun. During this automatic operation there are two major points of note.

1. As expected, the Cu filled TSVs become revealed as the cut proceeds through the sample
2. There is build-up of material occurring within the milling box as the operation occurs.
a. At 0 automatic slices, there is no material visible in the milling box
b. After 80 slices, redeposition can be seen on the side walls of the milling box
c. After 160 slices, there is moderate redeposition in the milling box
d. After 240 slices, there is significant redeposition in the milling box

It is point 2 that is of most interest as this material build up contains the plated Cu from within the TSV itself. As such, it is a redeposition of the material removed during the cut or milling operation. For the images shown, the redeposition was not considered an issue as it did not impact the investigation being made, as there were no structural defects within the TSV itself, but when such a technique is used to investigate a cracked BMV, there is a serious cause for concern.

The primary requirement during FA is that the techniques used do not influence, or change the sample being inspected, otherwise there is a strong likelihood that the collected data can be misleading. If material redeposits on the sample during a FIB-SEM investigation then this primary requirement is broken, and the conclusions drawn from the analysis must come under question. In the case of a cracked BMV, there is a very high likely hood that the redeposition will occur not only within the milling box but within the crack under investigation, in view of this, the crack may appear to be coated with Cu which can be mis-interpreted as a defined layer or coating.

Figure 8 shows a series of SEM-FIB images taken while investigating a failed BMV joint, clearly the point of failure is located at the base of the BMV and it would be reasonable to anticipate that it would be located close to, or cognizant with one of the target pad – ELESS Cu – ELYTC Cu junctions. Through the use of an automated milling procedure, an inspection plane normal to the crack has been created. In Figures 8c and 8d there appears to be a distinct layer of material on either side of the failure crack, and when measured, this is found to be approximately, 1um in thickness, which is comparable to that of the deposited ELESS Cu layer. As such it would be reasonable to conclude that the crack arose due to cohesive failure within the ELESS Cu layer as this can be clearly seen on either side of the crack surface.

In order to support this conclusion it would be beneficial if some additional analysis could be performed in order to locate and confirm the ELESS Cu layer, and typically this would be supplied through Energy-Dispersive X-ray spectroscopy (EDX) targeting elements unique to the ELESS Cu process. In such cases, Palladium or Nickel would be suitable; Pd is commonly used as a catalytic seed layer, so its presence can be used as a reference from which the location of the ELESS Cu layer can be determined, and in the case of Ni, it is often co-deposited with the Cu in order to influence the physical properties of the final ELESS Cu layer, and so, Ni presence can directly indicate the location of the ELESS Cu layer itself. Unfortunately the energy of the incidental electron beam used in SEM-EDX investigations is of a sufficiently high order that the beam penetration can be in excess of 1um in depth, resulting with a high “background” reading. In the case of BMVs, this means that there is a high Cu measurement, and a very low, or no concentration of Pd or Ni detected, thus making ELESS Cu location extremely challenging. When this occurs, without the supporting elemental evidence, the original conclusion would remain, and the failure is attributed to a “brittle” ELESS Cu layer, which would subsequently generate investigations and actions focused around the applicable process.

Transmission Electron Microscopy.
With the limitations noted above in relation to SEM and SEM-FIB, the use of Transmission Electron Microscopy (TEM) now becomes attractive as the technique offers a lower detection limit for elemental analysis, potentially useful in identifying Pd and Ni in ELESS Cu layers, as well as a higher resolution which may yield more clear details on metallographic features in and around the BMV failure.

As the name, suggests, TEM is a transmission method, meaning that the detectors are typically positioned beyond the sample, and they collect particles following their transport through the sample, unlike SEM where the detectors are commonly in front of the sample and collect particles that have been scattered, reflected or emitted off the sample surface. TEM does require specialist sample preparation, typically involving SEM-FIB to cut a thin lamellae which can then be examined transmissibly. (Figure 8)

As a TEM sample is substantially thinner than a SEM sample, in the region of 50-100nm, there is a much lower effective beam penetration depth compared to SEM, thus enabling a lower “background” analysis during EDX and an increased elemental sensitivity detection, meaning that the lower Pd and Ni contents in an ELESS Cu deposit are more likely to be detected.

Figure 10 shows representative results through TEM investigations, and as can be seen, the primary image in combination with the EDX analysis allows relative locations of the ELESS and ELYTC Cu layers to be determined.

If we now refer back to the example in Figure 8 where the BMV failure is attributed to cohesive failure within the ELESS Cu layer. Through TEM analysis, we can now determine the location of the ELESS Cu layer itself, which, as shown in figure 11b, is found to be not only wholly intact but located to the left side of the crack, implying that the material found on the inner faces of the fracture surface are in fact redeposited artifacts arising from the FIB cut. In view of this, it can now be concluded that the BMV failure itself could be associated with adhesive failure between the ELESS Cu and ELYTC Cu layers.

Similar to most microscopy methods, TEM has a number of applicable inspection techniques, including “Bright” and “Dark Field”, BF and DF respectively. When utilizing BF, the detectors receive a high intensity of directly transmitted electrons, enabling good clarity of crystal orientation, whilst DF has a lower intensity of scattered electrons, making it more suitable for detection of small voids etc within the sample. (Figure 12)

We have reported elsewhere[3,4,5] on the presence of nano voids found during TEM-DF investigations, and this has led to a unique and increased understanding on potential failure mechanisms for BMV structures, but has importantly also enabled us to offer a “best practice” process for similar investigations.

Suggested “Best Practice” for BMV Failure Inspection.
As would be expected, a recommended analytic approach is a sequential operation, with each step in the sequence building on the knowledge gained from the preceding step.

It should be noted that investigations of separated joints with visible cracks is not recommended, due to preparation artefacts from cross sectioning and the high potential for redeposition layers forming during ion milling operations with SEM-FIB. However, if this is not possible, care should be taken to ensure that such artefacts do not lead to misleading conclusions.
Step 1 – Optical Microscopy.
Determine general presence and location of point of interest.
Chemical etching can yield additional data regarding basic crystal structures.

Step 2 – SEM (+EDX).
Surface investigation at the point of interest.
Chemical etching can yield additional data regarding basic crystal structures.
Electro polishing may yield outline of layered structures but is not always conclusive. “Macro” elemental analysis.

Step 3 – SEM-FIB (+EDX).
Inspection normal to the point of interest.
Determine crystal structures and any breaks in epitaxy across the target pad – plated Cu interface. Approximate locations of layers may be possible.
“Macro” elemental analysis.
Caution should be taken when inspecting open cracks as redeposition can occur during milling operations.

Step 4 – TEM (+EDX).
Inspection of nano scale features
“Micro” elemental analysis of the point of interest Confirm location of ELESS Cu
Identification of trace impurities

Summary.
There is an increasing awareness and concern regarding the reliability performance of stacked microvia in advanced PCBs leading to extensive works investigating BMV failure. Through a number of associated investigations, it has become apparent that the use of SEM-FIB has become popular as it offers an enhanced understanding of the complex crystallography present within the microstructures of a plated and filled BMV. Care should be taken however when utilizing SEM-FIB as there is a risk of preparation artifacts which can impact the conclusions drawn, as such, and based on our own experiences, a recommended best practice for the investigations of failed BMV is offered.

References
[1] – Magera J. et al. IPC-WP-023 “IPC Technology Solutions White Paper on Performance-Based Printed Board OEM Acceptance. Via Chain Continuity Reflow Test: The Hidden Reliability Threat — Weak Microvia Interface. (May 2018)
[2] – Magera J. Strickland J.R “Copper Filled Microvias – The New Hidden Threat“. Proceedings of IPC Apex EXPO (2019)
[3] - Bernhard T., Gregoriades L., Branagan S, Stamp L., Steinhäuser E., Schulz R., Brüning F. “Nanovoid Formation at Cu/Cu/Cu Interconnections of Blind Microvias: A Field Study” IMAPS International Symposium on Microelectronics Proceedings (2019)
[4] - Bernhard T. Branagan S., “The formation of nano voids in electroless Cu layers”, MRS Spring conference (2019), submitted to MRS Advances (2019)
[5] – Zarwell. S, Bernhard. T. Steinhäuser E., Kempa. S. Brüning F. “The Effect of Cu Target Pad Roughness on the Growth Mode and Void Formation in Electroless Cu Films” IWLPC Proceedings (2019)

Previously published in IPC Expo proceedings

Paper 2

Development of Low Loss Adhesive Film for Multilayer PTFE Substrates 
Yusuke Watase, Tetsuro Iwakura and Masaki Yamaguchi.
Resonac Group, Japan

Yusuke Watasa

Resonac Group

Abstract
Fluororesins such as PTFE, which have excellent low-dielectric properties, are used as substrates for high-capacity, high-speed transmission, and their application is increasingly promising in beyond 5G and 6G society. However, the fabrication of multilayer PTFE substrates requires high temperature pressing (about 350 °C), and its via fabrication is challenging. Against this background, a novel bonding film and thermoset resin has been developed for PTFE multilayer substrate using unique resin technology. This bonding film has low-dielectric properties (Dk = 3.0 / Df = 0.0023 at 10 GHz) and high adhesive strength with PTFE substrate. Thus, this bonding film enables a multilayer PTFE substrate which maintains the low loss properties of PTFE to be fabricated under low temperature press (200 °C). In addition, due to its good desmear processability, blind vias with high connection reliability can located in multilayer PTFE substrate.

Introduction
In recent years, the spread of safe driving support systems for automobiles has been advancing by using millimeter wave. Research aimed at realizing autonomous driving is also flourishing, and the demand for in-vehicle millimeter wave radar is expected to increase rapidly in the future.^[1] The next-generation millimeter wave radar is being examined for communication at frequencies around 77-81 GHz. Therefore, the use of resin substrates such as fluororesins (PTFE) and liquid crystal polymers (LCPs) featuring low dielectric properties is being studied around the antenna portion.^[1] In addition, the application of the millimeter wave band has progressed with the realization of 5G (fifth-generation mobile communications), which is a next-generation communication technology. Low-dielectric substrate materials are also required for antenna components of smartphones and base stations. Figure 1 (left) represents the normal antenna construction. However, by utilising the multilayer construction as shown in Figure 1 (right), the antenna can be used for  large-capacity, high-speed transmission as required in the 5G and 6G networks. However, a fabrication of multilayer PTFE substrate requires high-temperature pressing (about 350 °C), and its via fabrication presents challenges. Therefore, based on a new thermosetting resin obtained by a unique resin technology, a novel low-dielectric film has been developed that combines dielectric properties and excellent workability comparable to PTFE and LCP substrate materials. Furthermore, the application of this film as an adhesive layer used to realize multilayer PTFE substrates was examined. This paper outlines the developed low-dielectric film and reports on the status of application studies to bonding films.

Development Concept
To obtain good antenna characteristics, the antenna substrate material must have a small transmission loss.^[1] The transmission loss consists of dielectric loss based on the dielectric properties of the insulating material and conductor loss due to wiring resistance.

Formulas 1 and 2 show that the higher the frequency, the greater the loss, so an adaptation to high frequency is essential in millimeter wave band applications. Therefore, it is required to use a low dielectric material as the insulating substrate material.

Similarly, regarding conductor resistance, a wiring material (copper foil in general) having a small surface roughness is required to avoid the skin effect on the surface layer of the transmission line ^[1] accompanying high frequency. On the other hand, when copper foil with low surface roughness is used, there is a concern that the adhesion strength will decrease because the copper anchor effect will be small.^[1]
To address these concerns, a novel thermosetting resin has been developed, which has low dielectric properties and high adhesive strength with low roughness copper foil, as well as an insulating film based on the resin. The relationship between Dk and Df of thermosetting and thermoplastic resins is shown in Figure 2. Since PTFE and LCP exhibit low dielectric properties, they are used as low-dielectric substrates, but their substrates have a problem of weak adhesion to copper foil. On the other hand, the novel new resin has low dielectric properties (Dk : 2.4, Df : 0.0016) and high adhesion to low roughness copper foil. Based on this new resin, a new material can be used as an insulating substrate material. The novel resin is filled with inorganic filler to achieve heat resistance, processability, and reliability. Filling with inorganic filler lowers the coefficient of thermal expansion (CTE) and improves processability. By applying organic/inorganic composite technology, the interface interaction between the new resin and inorganic filler can be adjusted, and the novel resin achieves properties such as low thermal expansion, low water absorption, and high heat and heat resistance.

Physical properties of bonding film
Table 1 shows the physical properties of the novel resin. This film exhibited good dielectric properties of Dk = 3.0 and Df = 0.0023 at 10 GHz. In addition, it was low CTE (36 ppm / °C) and exhibited high adhesion with low roughness copper foil. In addition, T-300 (according to IPC TM-650), which was a heat-resistant characteristic, was 60 min or more, and the pyrolysis temperature (5 % weight reduction) was 460 °C, so it had excellent heat resistance.

Characteristics of novel resin
When the novel resin is applied as a bonding film for multilayer PTFE substrates, characteristics such as (1) good embedded ability, (2) high adhesion with PTFE substrates, and (3) high reliability are required as well as press moldability at low temperatures (PTFE substrate molding temperature is above 350°C).

Embedded Ability
We have demonstrated the embedding ability of this novel resin. A 18μm thick copper was embedded with a 25μ thick novel resin pressed at 200 °C. Figure 3 shows a cross section image after embedding a copper pattern. It was found the embedded ability of novel resin was good without voids.

Figure 3. Cross section image after embedding copper pattern

Adhesion to PTFE substrates
Adhesion between PTFE substrate and the novel resin was evaluated. Table 2 shows the adhesion and heat resistance with the three types of PTFE substrates pressed at 200 °C. It was found the novel resin had high adhesion to any PTFE substrates, and the solder heat resistance at 288 °C float was good.

Via fabrication of PTFE substrate with novel resin

We evaluated via fabrication of PTFE substrate with the novel resin. The via fabrication process showed in Figure 4. Via formation was performed by CO2 laser (conformal method), and dry and wet processes were used to remove smear, resin residue, from the bottom of the vias.

Figure 4. Via fabrication process of PTFE substrate with novel resin.

Figure 5 shows the cross section image of fabricated via, fabricated through the process shown in Figure 4.

Figure 5. Via fabrication of PTFE substrate with the novel resin.

Figure 5 shows that the shape of the formed via is good, and there is no smear residue at the bottom of the via, so that the novel resin has good desmear ability.

Reliability testing of multilayer PTFE substrate
Finally, multilayer PTFE substrates were fabricated and the reliability of the substrates was evaluated. The structure of the fabricated substrate (4-2-4) is shown in Figure 6. Its substrates contain an interstitial via hole (IVH) / 0.125 / 0.15 / 0.175 mm) and a through-hole (TH) / 0.2 / 0.3 / 0.4 mm). The fabricated substrate had a well-shaped IVH and TH.

Figure 6. Cross-section of multilayer PTFE substrate.

Reliability evaluation results using this substrate were shown in Table 3. We evaluated reflow heat resistance (260 °C, 10 pass), connection reliability (-65 °C ~ 125 °C, 1000 cycles), and insulation reliability (85 °C, 85% RH, 16 V, 1000 h).

Conclusions
A novel resin, a bonding film for PTFE multilayer substrate, has been developed based on a unique resin technology containing a novel thermosetting resin. The novel resin has low-dielectric properties (Dk = 3.0 / Df = 0.0023 at 10 GHz) and high adhesive strength with PTFE substrate. Thus, the novel resin enables a multilayer PTFE substrate which maintains the low loss properties of PTFE to be fabricated under low temperature press (200 °C). In addition, due to its good desmear processability, blind vias with high connection reliability are able to be located in multilayer PTFE substrate. Bonding film is a useful material as a bonding film for PTFE substrates.

References
[1] Japan Marketing Survey (2017). Market Outlook for Millimeter Wave Radar/Next-Generation (5G) Communications and High-Speed and High-Frequency Substrate Materials.
[2] Masaru Nakamura, Takafumi Hasegawa (1994). Fluorine resin. Plastics vol.45 No.9, Pages 42 - 46.
[3] Takao Tanigawa (2015). New low-transmission loss material for millimeter wave lasers "AS-400HS". Hitachi Chemical Technical Report vol.58, Pages 18 - 19.
[4] Fumiaki Baba (2005). Development and application of polymer materials for high frequencies. CMC Publishing.
[5] Katsuhiko Nakamae (1998). Chemistry and application of adhesives. Dainippon-tosho, The Chemical Society of Japan, Pages 7 - 10.

Previously published in IPC Expo proceedings.

Paper 3

Picosecond Laser Microvia Drilling of ABF Material Using MHz Burst Mode

Daniel Franz¹ , Tom Häfner², Kay Bischoff¹, Jonas Helm¹, Tim Kunz², Stefan Rung², Cemal Esen³ and Ralf Hellmann^1
1 Applied Laser and Photonics Group, University of Applied Sciences Aschaffenburg, Würzburger Straße 45, 63743 Aschaffenburg, Germany
² Schmoll Maschinen GmbH, Odenwaldstraße 67, 63322 Rödermark, Germany
³ Applied Laser Technologies, Ruhr University Bochum, Universitätsstraße 150, 44801 Bochum, Germany

Abstract

We report on a comprehensive study of laser percussion microvia drilling of Ajinomoto build-up film (ABF) material using an ultrashort pulsed laser in MHz burst mode. After laser processing, microvia drilling quality is was evaluated by observing the fabricated diameter and taper using laser scanning microscopy and metallography. The influences of the incubation effect, heat accumulation and shielding effects as a result of pulse to pulse interactions are assessed on the ablation threshold, penetration depth and laser microvia drilling quality. We found that an increasing heat accumulation in MHz burst mode processing is responsible for the void formation and delamination of the insulating ABF layer.

Therefore, the parameter clearance was introduced to evaluate these effects on the microvia sidewalls. For a comparable clearance, applying 2 intra-burst pulses achieves an average reduced taper of down to 19.5% compared to single pulse mode. At the same time, a reduced laser drilling time of 16.7% per microvia highlights the enormous potential of the MHz burst mode for laser drilling of ABF material in printed circuit board fabrication.

1. Introduction

The trend towards miniaturized high-performance electronic components driven by increasing demands on their functionality requires a high-density electronic packaging and interconnection technology. Hence, laser drilling has been intensively studied for various materials used in interconnect devices, such as, e.g., ceramics or different printed circuit board (PCB) materials^[1–6]. In particular, ultrashort pulsed (USP) laser percussion drilling has been established for the fabrication of blind vias, so-called microvias, to inner conductive layers in multi-composite PCB material^[5-8] with small diameters of <10 μm^[9-10]. To ensure reliable electrical interconnections with a high thermal durability, the capture pad should be spatially exposed as much as possible, typically in a range of 70%–90% with respect to the upper microvia diameter^[11]. In addition to the most common printed circuit board substrate FR-4, an epoxy resin filled with spherical glass particles called Ajinomoto build-up film (ABF) material is gaining interest as an insulating layer^[7-11,14]. 

A USP laser enables material ablation with small focal diameter and negligible thermal stress due to its short laser pulse duration in the picosecond and femtosecond range^[8-15]. Characteristic nonlinear absorption mechanisms are the result of very high pulse peak intensities of above 1013–1014 W/cm2^[16-17], allowing an absorption of even transparent materials such as polymers and glasses^[18-19]. The tremendous interest and potential of USP lasers has led to the ongoing development of high average powers. 1kW and more are nowadays available for laser micromachining.^[20-23] Consequently, the arising laser pulse energies of up to some 100 μJ lead to a laser fluence that is significantly above the well-known optimum for maximizing the ablation efficiency and rate^[24]. For scaling the laser pulse energies closer to the optimum ablation efficiency, so-called laser burst mode can be used to split a laser pulse into at least two and typically up to a few hundred intra-burst pulses with a temporal intra-burst interval in the nanosecond range.^[24-31]

However, within the timescale of a few nanoseconds between successive intra-burst laser pulses, several pulse to pulse interactions occur and their significant influences on processing efficiency and quality in laser micromachining have to be considered^[6,32-38]. Even with ultrashort laser pulses, a fraction of the laser pulse energy remains in the substrate as heat during the ablation process^[26,36]. Heat accumulates over time if the time interval between successive laser pulses is insufficient to dissipate the induced heat into the surrounding material, which in turn is limited by the material-specific thermal conductivity^[37,39]. Furthermore,
interaction of laser pulses with laser-induced plasma and ejected particles has to be taken into account, since individual intra-burst pulses can be absorbed, scattered or reflected^[32,40]. These shielding effects are perceptible in laser micromachining of metals using MHz burst mode, observing a remarkable decrease of the ablation efficiency for an even number of intra-burst laser pulses^[26,34,41]. In addition, a change of the absorbance of ultrashort pulsed laser processed surfaces is reported for metals and dielectrics^[33,42-43] for laser bursts in the MHz range. For example, the absorbance of copper is 49% after processing in single pulse
mode and increases to 60% when using MHz burst mode with a number of intra-burst pulses of NBP = 3 and ata wavelength of λ = 532 nm^[42].

Several studies and simulations of laser drilling of metals^[28,41,44], dielectrics^[45] and glasses^[46-47] have reported improved ablation efficiency, ablation rate, ablation volume, penetration depth and drilling quality as a consequence of using laser bursts in the MHz regime. The results are mainly attributed to heat accumulation and are discussed in terms of laser-induced shielding effects by plasma or ejected particles and incubation effects as a result of pulse to pulse interactions of successive intra-burst laser pulses. In contrast to these studies on laser drilling of single component materials with USP lasers in MHz burst mode, the effects of pulse to pulse interactions in laser drilling of multi-composite materials such as ABF material for PCB fabrication have not yet been investigated. Hence, the aim of this study is to fundamentally understand the impact of pulse to pulse interactions in laser percussion drilling of ABF substrate with MHz bursts on the ablation threshold, penetration depth as well as drilling quality of microvias. Therefore, bore craters were generated into ABF material under various MHz burst configurations and evaluated by laser scanning microscopy and metallography.

2. Experimental setup and sample preparation

Experimental investigations of laser microvia percussion drilling in ABF material were performed using a high-power USP laser (Lumentum, Picoblade 3) with a laser pulse duration of τ = 10 ps, a wavelength of λ = 532 nm and a beam quality of M2 < 1.3 in MHz burst mode. In all experiments, GX-T61 ABF PCB material with a layer thickness of 30 μm and a 16 μm thick inner copper layer was examined, see figure 1 (a).

An overview of the thermal and dielectric properties of the studied ABF substrate is provided by Hichri et al^[14] and Nair et al^[48]. The simplified optical setup for sample processing is depicted in figure 1(b), wherein a combination of biconvex and plano-concave lenses is used to adapt the raw beam diameter to d₀ = 3mm (1/e2). For deflection and subsequent focusing of the laser radiation, a combination of a 2D galvanometer scanner (Newson, RTA-AR-800-3G) and a telecentric f-theta lens (LINOS F-Theta-Ronar 515–540 nm, fused silica) with a focal length of f = 100 mm is applied. The laser focus diameter of d_f = 29 μm (1/e2) is calculated by substituting the experimental used data in equation (1)^[6].

Equation 1

The study evaluates the ablation behavior of ABF material and copper as well as the laser microvia drilling quality using a number of intra-burst pulses of NBP = 1–6 and an intra-burst pulse repetition rate of fBP = 82 MHz (consequently a time interval between successive pulses of ΔtBP = 12 ns), see figure 1(c). For each bore crater, both the total energy input and the intra-burst pulse energy distribution was maintained constant. Therefore, the pulse energy in MHz burst mode has been set accordingly by adjusting the gain factor of each intra-burst pulse. For example, to compare the results with a single pulse mode (1 intra-burst pulse) at a laser pulse energy of EBP = 30 μJ, the laser pulse energy is reduced to EBP = 10 μJ using 3 intra-burst pulses. First, the damage threshold method based on Liu^[49] is used to determine the ablation thresholds of GX-T61 ABF material and pure copper material with a sample thickness of 500 μm at a laser burst repetition rate of fB = 10 kHz and 1 MHz with laser fluences of up to 7.6 J cm−2 and a number of laser bursts of NB = 15. To evaluate the ablation threshold of the PCB materials, first the laser fluence FBP is calculated according to equation 2.

Equation 2

Where EBP is the laser pulse energy and w0 the radius of the beam waist (1/e2) ^[6]. The logarithmic representation of the squared drilling radii generated as a function of the laser fluence is then used to determine the material-specific ablation threshold Fth.
After studying the ablation thresholds, the microvia formation is evaluated in terms of ablation volume, diameter and depth in increments of 2 laser bursts using a 3D laser scanning microscope (Keyence VK-X200) at a laser burst repetition rate of 200 kHz.
Furthermore, the percussion drilling quality of microvias in ABF substrate is evaluated using different MHz burst configurations in terms of fabricated microvia diameter and taper. The taper is defined by the ratio of the upper (Dtop) and the lower (Dbot) microvia diameters [5, 6] and is determined by optical microscopy (Leica DM6000 M), see figure 1(a). For metallographic preparation, drilling grids were generated into target material applying laser pulse energies of EBP = 2.5–30 μJ, a number of intra-burst pulses of NBP = 1–6 and a number of laser bursts between NB = 30–36 at a laser burst repetition rate of fB = 1 MHz. After laser processing, top view images were first taken using optical microscopy to determine the diameter D of the microvias, cf figure 1(d). Afterwards, drilling grids were embedded by a 2-component synthetic material based on modified polyester resin (Demotec 15 plus). Finally, the hardened material was removed with a grinding machine (Latzke LS3V) to examine the microvia drilling quality in cross sections. As voids and delamination occur within the ABF layer after laser drilling, the parameter clearance is introduced for its evaluation. The clearance is in turn defined by the maximum deviation with respect to a line between the lower and upper microvia diameter for each sidewalls, see figure 1(a). To obtain high-resolution images of the microvias, the polished samples were first treated with acetone and then sputtered with a thin conductive gold layer. Images were then captured using a scanning electron microscope (SEM) (Tescan MAIA3) with an acceleration voltage of 5 kV. A detailed overview of the laser parameters used in the experimental investigations of laser microvia
percussion drilling in MHz burst mode for analyzing of the ablation threshold, penetration depth, microvia diameter and taper is provided in table 1.

3. Results and discussion

3.1. Ablation behavior in MHz burst mode

3.1.1. Ablation threshold. In order to evaluate the potential of picosecond laser pulses in MHz burst mode for percussion drilling of microvias into PCB material, firstly, a fundamental characterization of the ablation behavior of processes involved and materials is mandatory. Hence, the ablation thresholds of GX-T61 ABF material (dielectric layer) and pure copper (conductive inner layer) are determined.

An overview of the achieved ablation thresholds Fths for copper and ABF depending on the number of intra-burst pulses is shown in figure 2. It is evident that the ablation thresholds are consistently decreasing with an increasing number of intra-burst pulses for both ABF material and copper, whereby the drop of the graphs are significantly greater for copper. This results in a lower ablation threshold for copper compared to ABF material using 6 intra-burst laser pulses for both laser burst repetition rates, although the ablation threshold for copper is up to 2.3 times higher in single pulse mode. At laser burst repetition rates of fB = 10 kHz and 1 MHz, the ablation threshold of ABF in single pulse mode of Fth = 0.24 J cm−2 is more than halved by using MHz burst mode. For copper, a substantially drop of up to 90.4% is obtained for applying 6 intra-burst pulses compared to the single pulse threshold of Fth = 0.51 J cm−2.
The decrease of the ablation thresholds for both materials with an increasing number of intra-burst pulses is attributed to the incubation effect ^{[29, 50, 51]}. This phenomena is ascribed to the accumulation of laser-induced chemical and structural changes as well as plastic deformation of the substrate as a result of thermal stress fields ^{[52, 53]}. In addition, an enhanced absorbance of the laser processed surface using MHz burst mode as a result of its higher roughness can support the ablation process. This was demonstrated by Jaeggi et al ^[33] and Neuenschwander et al ^[42] for copper and silicon with an increasing number of intra-burst pulses.
In contrast to the dielectric, copper exhibits a dependency of the ablation threshold of the laser burst repetition rate. The ablation threshold is consistently about 0.08 J cm−2 lower for fB = 1 MHz compared to fB = 10 kHz. This is associated to the different thermal properties and especially heat dissipation of both materials. Whereas copper provides a high thermal conductivity between 377–385 W/mK [54, 55] and is thus capable to dissipate the occurring heat at higher laser burst repetition rates into the surrounding material, the multi-component ABF substrates are assigned to have low values between 0.5–2.5W/mK [56]. Therefore, we assume that strong heat accumulation already occurs at a laser pulse repetition rate of fB = 10 kHz for processing ABF material. For this reason, no significant difference in ablation threshold is observed compared to a higher laser burst repetition rate of fB = 1 MHz.

3.1.2. Penetration depth, diameter and ablation volume. The ablation volume, penetration depth and diameter were determined using laser scanning microscopy in increments of two laser bursts. The averaged results are shown in figure 3 for a sample size of 3, a number of intra-burst pulses of NBP = 1–6 and a laser burst repetition rate of fB = 200 kHz.

An overview of top view images of percussion drilled microvias into ABF material in increments of 2 laser bursts in dependence of the number of intra-burst pulses is provided in figure 3(a). It is clearly visible that for applying the same total energy, the inner copper layer is reached with fewer laser bursts for MHz burst mode in comparison to single pulse mode. The evaluation of the penetration depth as a function of the number of applied laser bursts and intra-burst pulses is depicted in figure 3(b) and reveals that the 30 μm thick ABF insulating layer is drilled through by using 18 laser bursts for single pulse mode. In contrast, applying a number of intra-burst pulses of NBP = 2, 3 and 6 intra-burst pulses, only 16, 14 and 12 laser bursts, respectively, are required to reach the inner copper layer. From this point of view, throughput in PCB fabrication can be improved by reducing the laser microvia drilling time of 16.7%–33.3%. It is also worth noting that after reaching the conductive layer in MHz burst mode, the penetration depth in copper is not affected and is comparable to that in single pulse mode.
The increasing penetration depth in ABF during laser percussion drilling using MHz burst mode is assigned to accumulated residual heat within a microvia, which in turn alters the absorption properties of the ABF material, allowing a deeper penetration. This assumption is also reported by Metzner et al ^[57], obtaining up to 20 times higher penetration depth for laser structuring of silicon and cemented tungsten carbide applying an intra-burst pulse repetition frequency of 80MHz and a number of intra-burst pulses of up to eight. In contrast to previous mentioned studies on laser burst processing of metals ^{[26, 34, 41]}, shielding effects by laser-induced plasma and ejected particles are not identified for an even number of intra-burst pulses. Since these effects are in turn both material and fluence dependent ^[26], we assume based on our observations that shielding effects have a negligible impact on laser microvia percussion drilling of ABF material at higher intra-burst pulse repetition rates and that incubation effect and heat accumulation play a dominant role. Therefore, shielding effects are not considered in the following evaluations.
Furthermore, consistently smaller microvia diameters are determined for a number of intra-burst laser pulses with NBP = 3 and 6, figure 3(a) and (c). After laser processing using 30 laser bursts, a reduction in hole size of approximately 10% is observed compared to a diameter in single pulse mode of D = 37 μm, which is a result of the significantly lower fluence of each intra-laser burst pulse. In contrast to this, 2 intra-burst laser pulses obtain a similar diameter range, although the laser energy density is halved. In turn, this contradictory result confirms the assumption that occurring shielding effects are negligible and for this case, the reported
beneficial ablation behavior using MHz bursts counteracts the lower laser fluence. In addition, this is evident by consideration of the ablation volume in figure 3(d), whereby the highest averaged volume of V = 24,332 μm3 is ablated for using 2 intra-burst pulses. However, to completely determine the influence of the MHz burst on the
microvia drilling geometry, a metallographic preparation of the microvias is essential, which is discussed in the following section.

3.2. Evaluation of the microvia drilling quality

3.2.1. Taper, diameter and damage of inner copper layer
The taper and diameter of drilled microvias are illustrated in figure 4 for a number of intra-burst pulses of NBP = 1–6, laser bursts of NB = 30 and NB = 36 and laser pulse energies between EBP = 2.5–30 μJ at a laser burst repetition rate of fB = 1 MHz.

As an overall result, significantly smaller taper ratios are observed for a higher number of laser bursts with a negligible increase in microvia diameter, figures 4(a) and (b), respectively. Thus, in combination with the analysis of microvias in increments of 2 laser bursts for MHz burst mode from figure 3, it can be concluded that after reaching the inner copper layer, additional laser bursts mostly contribute to the reduction of the taper. In addition, the resulting taper ratios show a trend towards steeper microvia walls with increasing number of intra- burst pulses. Therefore, the smallest averaged taper of 117.4 ± 6.3% is achieved with 6 intra-burst pulses with a pulse energy of 5 μJ and 36 laser bursts. For the same total energy input, the taper is thus 13.7% smaller than in the single-pulse mode. The slope of the graphs decreases more rapidly with the laser pulse energy in MHz burst mode than in single pulse mode and again the decrease is more pronounced for a higher number of intra-burst pulses. A similar trend is identified when evaluating the microvia diameters. The smaller diameters for the MHz burst mode result from of a decreasing laser fluence with an increasing number of intra-burst pulses. The smallest hole sizes are 26.1 ± 0.2 μm for 6 intra-burst pulses and 30 laser bursts.
We mainly ascribe the widening of the microvia at the bottom area to an increasing heat accumulation during laser drilling using MHz burst mode. If an intra-burst pulse is absorbed by the insulating ABF substrate, a certain amount of the absorbed laser fluence below the ablation threshold is converted to heat in the material ^[58]. In turn, due to the low thermal conductivity of ABF substrate of <2.5 W/mK ^[56], the induced heat can not
dissipate into the surrounding material within the time scale of successive intra-burst pulses of ΔtBP = 12 ns.
Therefore, in laser microvia percussion drilling, heat accumulates as the number of laser bursts and intra-burst laser pulses increases, producing a smaller taper value. As reported by Metzner et al ^{[57, 58]}, the temperature may even exceed the evaporation temperature of the substrate using high laser burst repetition rates of several MHz and thus leading to an additional removal mechanism which contributes to the total ablated material.
In addition, it is worth mentioning that for all MHz burst configurations no measurable damage in the inner copper layer is attained, except for applying a number of intra-burst pulses of NBP = 6 and 36 laser bursts. Here, an average damage of the inner copper layer of 4 ± 1.3 μm is observed, as illustrated by selected optical microscope images and SEM images of microvia cross sections in figures 5 and 7. This, in turn, confirms the favorable ablation behavior for both copper and ABF from the previously reported studies for a higher number of intra-burst pulses.
Furthermore, molten and re-solidified copper is evident at the inner copper layer towards the sidewalls of the microvias in the cross sections. On the one hand, this is again attributed to increasing heat accumulation at the bottom of the hole during percussion drilling process using MHz burst mode. On the other hand, occurring melt formation may lead to a modification of the ablation characteristics during laser bursts with a temporal separation of a few nanoseconds, which is comparable to that of a nanosecond laser pulse, as reported by Domke et al ^[25].

3.2.2. Clearance of microvia walls. Although the laser drilling quality is improved by accumulated heat using MHz burst mode, negative side effects occur. Voids are created and even delamination of the insulating ABF layer is observed for 6 intra-burst laser pulses. For illustration, selected cross sections of laser drilled microvias with 30 and 36 laser bursts and a number of intra-burst pulses of 6 are shown in figure 5 using optical microscopy and SEM imaging.

Since the formation of voids can lead to a significant reduction of the electrical connection lifetime after electroplating by more than 90% ^[59], a characterization of the voids is indispensable. Hence, the previously introduced parameter clearance is used to evaluate the size of these modifications, figure 1(a). This parameter is also retained to quantitatively describe the effect of accumulated heat when using MHz bursts.
The results of the clearance are shown in figure 6(c) for single pulse mode with a laser pulse energy of EBP = 30 μJ and its comparative MHz burst modes with the same total energy input. For an overall comparison
of the microvia drilling quality, the taper and microvia diameters for these laser burst parameters are extracted

from figure 4 and additionally depicted in figure 6(a) and (b). Obviously, the clearance is independent of the selected number of laser bursts of NBP = 30–36, but increases continuously with the number of intra-burst pulses, reaching highest averaged values of 2.44 ± 0.76 μm for 6 intra-burst pulses. Compared to the single pulse mode with small clearance values of 0.93 ± 0.2 μm, the values are doubled with 1.96 ± 0.43 μm for 3 intra-burst pulses and in a comparable range with 1.16 ± 0.3 μm for 2 intra-burst pulses. From this point of view, the evaluation of the clearance represents well the evolution of the voids with an increasing number of intra-burst pulses. This is also in good agreement with selected optical microscope images and SEM images of microvia cross sections shown in figure 7.

Furthermore, the clearance parameter suggests that residual heat accumulates rapidly with the number of applied intra-burst pulses. We assume that the generated heat within a microvia exceed the boiling point of the ABF substrate and thus lead in some areas to melting and evaporation of the insulating layer. As already mentioned, such a second removal process as a result of increasing heat accumulation is also suggested by Metzner et al ^{[57, 58]} for MHz bursts. For applying 2 intra-burst pulses, these effects lead to an improvement of the laser drilling quality by widening the microvias at the bottom area. However, using a greater number of intra-burst pulses, accumulated heat within a microvia is too extensive and thus the generation of voids and even delamination of the insulation ABF layer occur.
Comparing the evaluation of the clearance with the achieved taper and microvia diameters of figure 6(a) and (b), respectively, we find an improvement in drilling quality in terms of reduced taper of down to 19.5% using MHz burst mode with 2 intra-burst pulses and 30 laser bursts compared to single pulse mode. In addition, this burst configuration allows an optimization of throughput in PCB fabrication compared to single pulse mode, since a comparable microvia drilling quality is accomplished with 6 fewer laser bursts. Consequently, a reduction of laser drilling time for an individual microvia of 16.7% is demonstrated.

4. Conclusion
We have reported on the influence of the MHz burst mode on laser microvia percussion drilling of ABF substrate for PCB fabrication using an ultrashort pulsed laser with a wavelength of 532 nm. In the study, the impact of pulse to pulse interactions are being discussed on the ablation threshold, ablation volume and the penetration depth as well as the laser drilling quality, which in turn is defined by the taper and the microvia diameter. It has been shown that shielding effects of laser-generated plasma and ejected particles have a negligible impact on both ablation behavior and microvia drilling quality. In contrast, increasing heat accumulation significantly influences the microvia geometry during percussion drilling process with MHz bursts. On the one hand, it has been proven that the fabricated taper is reduced to a value of 117.4% due to accumulated heat in MHz burst mode. On the other hand, for a higher number of intra-burst pulses, the heat accumulation has a negative impact on the drilling quality, as voids and even delamination are observed in the insulating ABF layer. As a particular result, the use of 2 intra-burst laser pulses is beneficial to the microvia drilling quality, as the taper is reduced by down to 19.5% without the generation of voids. In addition, the drilling time per microvia with comparable characteristics can be reduced by up to 16.7% with 2 intra-burst pulses, which in turn allows an enhancing of the throughput in PCB fabrication.

Acknowledgments
This research was funded by the German Federal Ministry of Education and Research (project MOSES, grant number 13N16330) and by the Bavarian Ministry for Science and Arts (project LEZ@THAB, grant number H.2- F1116.AS/34).

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Editors Note

This paper is licensed by Open Access 4.0. It is published in full with no modifications.

https://creativecommons.org/licenses/by/4.0/
Originally published 15 September 2023 • © 2023 The Author(s). Published by IOP Publishing Ltd
Materials Research Express, Volume 10, Number 9
Citation Daniel Franz et al 2023 Mater. Res. Express 10 096301
DOI 10.1088/2053-1591/acf7b0

The Institute of Circuit Technology Christmas Seminar 2023

December 5th 2023, Harrogate

“The next industrial revolution isn’t on Earth” is the tagline for Space Forge, a British aerospace manufacturing company developing fully reusable satellites designed to take maximum advantage of the unique environmental benefits of space in manufacturing next-generation super-materials. Darren Cadman introduced the company and its strategy.

He described Space Forge as Europe's fastest-growing space-tech start-up, with funding from the UK Space Agency and the European Space Agency, together with several international investors. The company’s objective is to establish a microgravity research center to drive advanced material research and production, primarily focused on growing defect-free inorganic crystal structures in microgravity conditions for use in electronics. The huge demand for such a service is currently not being met. Not only is there a heavy reliance on the International Space Station but there is a lack of both a dedicated platform and a cost-effective soft-return mechanism. The key attribute of Space Forge is its world-first reusable, returnable orbital manufacturing platform, with its patented system to enable re-entry without burning up and a subsequent soft landing to protect fragile payloads, while reducing stress on the vehicle and minimizing the cost of refurbishment in preparation for re-launch.

Cadman explained that in-space manufacturing offers unique environmental benefits that can facilitate the creation of large, near-perfect crystals by chemical vapor deposition and enable the production of semiconductor products with performance superior to anything that can be achieved on Earth, where defects can result from gravity, causing buoyancy effects and preventing perfect mixing of gases, solutions or alloys of different densities. Furthermore, atmospheric contamination issues are avoided in space, where ambient pressure is 10 trillion times lower, and temperatures close to absolute zero can be achieved by radiators facing cold space with no need for cryogenics.

The Space Forge manufacturing platform is designed to be launched to a low-Earth orbit at a height of 550 km, with a three-month operational phase followed by a precision de-orbit and recovery from the sea, then taken back to base for inspection of vehicle and payload, refurbishment, and return to service. The system has the potential to prevent the generation at least 15 tons of carbon dioxide for every kilogram of semiconductor material produced for next-generation electronics, telecommunications and automotive applications.

Next to speak was Andy Cobley, professor of electrochemical deposition and leader of the functional materials research group at Coventry University, discussing the development of non-noble metal activators for electroless plating processes, in a project carried out in collaboration with Université de Mons in Belgium.

Reviewing the history of electroless copper plating in the electronics industry, Professor Cobley commented that its capability to metallise non-conductive materials has established it as an enabling technology in printed circuit manufacture and latterly in the rapidly evolving field of electronic textiles. Electroless deposition requires the non-conductive surface to first be treated with a “catalyst,” an activator that initiates the redox reaction whereby copper is reduced from the ionic state in solution and deposited on the surface as metal. Formaldehyde is typically used as the reducing agent, and subsequent deposition of copper proceeds autocatalytically.

The catalyst of choice has traditionally been colloidal palladium. But palladium is scarce, much in demand, and currently priced at more than £25,000 per kilogram, justifying the evaluation of lower-cost alternatives.

Using palladium as a baseline, screening tests were carried out on three candidate materials: titanium dioxide, zinc powder, and cobalt powder. A polyester textile was used as the substrate. Professor Cobley explained the experimental procedure and summarized the results in terms of the effects of concentration, immersion time, temperature, dispersion method, and particle size. He used a series of SEM micrographs to illustrate the physical differences in deposit characteristics.

It was observed that both zinc and cobalt could initiate the electroless copper process, and the dispersion method and particle size has significant effects on the final properties of the copper deposit. However, the initiation mechanism was found to be different from that of palladium, since neither zinc nor cobalt showed any catalytic activity. In contrast, palladium acted as the catalyst in oxidizing formaldehyde resulting in copper from solution being reduced to form the initial metal deposit upon which subsequent copper was deposited by autocatalytic reduction. In the case of zinc and cobalt, the initial copper deposit was formed by a simple displacement reaction by a metal lower down in the electrochemical series.

The final presentation came from Steve Payne, project manager at iNEMI, the international electronics manufacturing initiative with a mission to be the premier collaboration forum to forecast and accelerate improvements in the electronics manufacturing industry.

He explained the structure of iNEMI, its high-profile global membership, and how its roadmap identifies, forecasts, and prioritizes the future technology requirements, evolution, and infrastructure of the global electronics manufacturing industry in terms of applications and market drivers, technical needs, maturity of technical solutions and expected gaps and challenges—all with a 10-year outlook. He described how individual roadmap teams focus on the full lifecycle and ecosystem of a product, from design through manufacturing and operation to end-of-life, leveraging resources across the supply chain and collaborating to address common industry knowledge gaps.

Payne reviewed the portfolio of current iNEMI projects, highlighting those with particular relevance to printed circuit fabrication issues, and described in some detail the work on reliability and loss properties of copper foils for 5G applications, with particular reference to the surface treatment processes used to improve adhesion to resin systems and their effect on signal loss and integrity at high frequencies. Specific objectives were to:

Determine and characterize the best non-contact method for measuring the surface topography of very-low-profile copper foil finishes
Characterize signal loss for various surface profiles
Characterize methods for measuring copper-to-resin adhesion in terms of durability and reliability for low-profile copper foil and low-loss resin systems
The outcomes would be to:

1. Establish consistency in specifying surface topology of copper foil and bonding treatments
2. Provide better assurance for meeting PCB electrical performance characteristics
3. Improve the predictability of multilayer reliability
4. Reduce product qualification costs and timescales

Electrodeposited and rolled-annealed foils were included in the program.

Payne briefly discussed non-contact roughness measurement and explained the different parameters:

Sa: Representing average surface roughness
Sdr: Representing the increment of the interfacial surface area relative to a flat plane baseline
The next phase in the program was to measure surface conductivity using dielectric resonators and he referred to work being done in Poland using the Fabry-Perot resonator for precise measurement of the electromagnetic properties of low-loss dielectric materials. It was capable of operating at higher frequencies than sapphire-ruby resonators. If this non-contact measurement technique could be correlated with roughness and loss, it offered an effective in-line process-control tool for foil manufacturers and laminators for checking copper surfaces.

The current phase of this program is the fabrication of microstrip circuits for the correlation of signal loss with surface conductivity. The next step will be reliability testing, looking for better information than could be gained from traditional peel testing.

Other current iNEMI projects mentioned by Payne were: PCB characterization for CAF and ECM failure mitigation, hybrid PCBs for next generation applications, and the PCB connector footprint tolerance project.

PCB initiatives in development include immersion cooling, embedded components, ultra-HDI challenges, halogen-free laminate flame retardancy challenges, PCB-level optical interconnects, and pad cratering mitigation.

Payne’s presentation certainly captured the attention of the audience and initiated an energetic Q&A session, capably moderated by Hudson, which continued through the buffet supper and into the hotel bar, where it metamorphosed into a welcome networking opportunity for PCB people.

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Section 8

Industry News

TIME has named Ken Logan, Group Sustainability and Regulatory Director of A-Gas, to the inaugural TIME100 Climate List, recognising the 100 most innovative leaders driving business climate action.

To assemble the list, TIME's editors and reporters fielded nominations and recommendations from industry leaders and partner organisations, like Global Optimism and The B Team, as well as TIMECO2’s (the climate arm of TIME) Advisory Council, then worked to assess the candidates on a variety of factors, including recency of action, measurable results, and influence. Read more......

It is with enormous pride that Adeon Technologies can announce it has received multiple purchase orders from Teltonika from Vilnius, Lithuania for supplying numerous types of equipment to their new Printed Circuit Board manufacturing Plant.

The initial conversations started early in the year 2022 and revolved around offering solutions and providing comprehensive plans for installation training and long term support. The Teltonika crew at the forefront consisting of Mr. Rico Schlüter, Mr. Tomas Auruškevičius & Mr. Arturas Kliobavicius took up the challenge to design and create the first new volume PCB factory to be build in Europe since more than 20 years.

The factory will see an extreme high level of automation being introduced. This team has admirably worked their way from concept to plan to execution of this challenging task.

Many thanks to Hollabbi Nataraja and the team at Process Automation International Limited (PAL) plus Darren Watson from GR8 Electrical Engineering ( www.gr8elec.co.uk ) on helping install and starting of our brand-new plating line which is now in full production mode!

The machine is equipped with the latest generation pulse plating process with an integrated copper Via Filling system, the design is a result of an intensive R&D work done by combined team from Exception PCB Ltd, Process Automation Limited Hong Kong ( www.palhk.com ) and Atotech an MKS Brand.

With other consortium partners GSPK has now completed seven quarters or a two-year project to help develop an Enhanced Liquid Immersion Power Supply (ELIPS). The project is funded by Driving the Electric Revolution, an Industrial Strategy Challenge Fund delivered by UK Research and Innovation.

GSPK and its partners are delivering this design-to-manufacture project to build products through a UK supply chain to produce the ELIPS and GaN based technology across Power Electronics Machines and Devices sectors.

This autumn, HMGCC marked the 80th anniversary of mathematician and computer scientist Alan Turing’s arrival at Hanslope Park.

A range of activities, talks and interactive events were hosted across Hanslope Park recently to mark this important time in the site’s history, involving colleagues from HMGCC, Foreign, Commonwealth and Development Office and FCDO Services. Read more....

Jiva Partners with OnAsset Intelligence to Develop the World’s First Biodegradable Tracking Device.

In a groundbreaking partnership, OnAsset Intelligence, a leading provider of asset tracking solutions, and Jiva Materials, a pioneer in sustainable materials, have joined together to develop the world’s first biodegradable tracking device.

This exclusive collaboration aims to combat the increasing problem of electronic waste (e-waste) and offer a sustainable solution to the environmental challenges posed by the disposal of single-use data logging and tracking devices. Read more...

Addressing PCB Environmental Supply Chain Issues with Direct Metallisation Technologies.

Supply chain management plays a crucial role in meeting the demand for printed circuit boards (PCBs) in today’s global electronics marketplace. Many complex logistical issues must be considered when developing a robust supply chain strategy. From labor costs and lead times to geopolitical tensions and environmental impacts, the challenges are numerous. Read more...

New investments in staff and machinery have led to a 6700sq ft expansion programme for both the PCB fabrication factory and office facilities.

We have had the first planning meeting with the contractors and the
building works are due to commence in February 2024. Read more...

Speedstack 2024 release candidate 24_01 available.

Customer requests for increased security mean we have made a major migration from legacy dotnet framework up to dotnet 4.8. Functionality of Speedstack is unchanged from 23_09 in this release however major code changes are involved. Significant range of manufacturers latest pre-pregs and cores added. Read more...

The automotive sector’s demand for revolutionary roofing glass aesthetics is one of the most adventurous challenges. And it is our engineering team’s innovation that is delivering sleek, capable, and robust electronic connector solutions to our Automotive Glazing clients for their PDLC Switchable Smart Glass Roofing designs.

Our own connector design shown here, encompasses typical features we can include in our flexible printed circuit solutions. 

Ventec Giga Solutions, the equipment division of Ventec International Group announces it has been appointment by Surge Robotic as sales agent and distributor of Optical PCB Layup Systems. Surge Robotic was founded in 2020 and launched the industry’s first four-camera optical PCB laminating system. The latest version employs four corner etch targets to align and detect PCB inner layer deformation to calculate and modify placement. PCB manufacturers can use inner layer measurement data to monitor and adjust their processes accordingly, thus enabling Smart Manufacturing. Read more...

Membership News

New Members

 ICT Register of Members
With the assistance of the ICT Treasurer, we continue to deep clean the ICT Register, archiving 56 Members, but recruiting 70 new members as of December 2023. This leaves us with a total of 422 members, an increase of 14, pending news of some company closures/takeovers. The large increase in recruitment was due in no small part to the success of the Supplier Webinar Sessions which we held in September and we hope to repeat this idea in 2024 with advance training sessions.

ICT Membership Grading 2010 - 2023

Section 11

ICT Council Members

Council Members

Emma Hudson (Chair), Andy Cobley (Past Chairman), Steve Payne (Hon Deputy Chairman), Chris Wall (Treasurer), William Wilkie (Technical Director, Hon Sec, Membership & Events), Richard Wood-Roe (Journal Editor & Web Site),  Jim Francey, Martin Goosey, Lynn Houghton, Pete Starkey, Francesca Stern and Bob Willis, 

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Section 12

Editors Notes

The ICT Journal

Richard Wood-Roe
Journal Editor

Instructions / Hints for Contributors

1. As it is a digital format the length is not an issue. Short is better than none at all!

2. Article can be a paper or a text version of a seminar or company presentation. Please include data tables, graphs, or powerpoint slides. We can shrink them down to about quarter of a page. Obviously not just bullet points to speak from.

3. Photo's are welcome.

4. We would not need  source cross references

5. Title of presentation - Of course! Date, Job title of Author and Company represented.

6. An introductory summary of about 150 words would give the reader a flavour of what it's all about.

7. Style - we don't want out and out advertising but we do recognise that the author has a specialism in the product or process that will include some trade promotion. Sometimes it will be a unique process or equipment so trade specific must be allowed.

8. Date and any info relating to where or if this article may have been published before.

9. We can accept virtually any format. Word, Powerpoint, publisher, PDF or Open Office equivalents. 

10. Also, to make it easy, the author can provide a word file to go along with his original powerpoint presentation and I/we can merge it together and select the required images. 

11. A photo of author or collaborators.

I really do look forward to receiving articles for publication.

Richard Wood-Roe

This email address is being protected from spambots. You need JavaScript enabled to view it.

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