Institute of Circuit Technology Evening Seminar - Imaging
The Norfolk Arms, Arundel
18th November 2008

Arundel CastleThe ICT revisited the historic West Sussex town of Arundel for its November evening seminar on 'imaging'. With almost forty registrants, the meeting was particularly well attended, with some ICT members having travelled from as far away as Scotland.

The seminar was opened by the ICT's Chairman, Steve Payne, who welcomed everyone to the splendid Norfolk Arms Hotel. Steve also thanked Global-Ventek Laminates UK Ltd, who had kindly sponsored the seminar.

The first presentation of the evening was given by Grant Bradley of Electra Polymers and his chosen theme was on ‘The Chemistry of Imaging’. Grant began by asking what was actually meant by the term imaging and how images could be created. The wide range of possible techniques included screen printing, laser ablation, ink jet deposition and imprinting, but the process most important in the PCB industry was photoimaging. (Laser Direct Imaging (LDI) was described as being a subset of photoimaging). The positive and negative working resist approaches were described and it was stated that negative working resists could be divided into two types, namely those based on cationic processes and those employing free radicals. The cationic type of process used direct epoxy polymerisation, was relatively slow and typically included an extensive dark reaction. Conversely, free radical polymerisation processes were faster and exhibited little dark reaction. The key to successful free radical polymerisation was to use the appropriate photoinitiator and some key examples were illustrated. A resin binder and a monomer were also needed and chemical structures such as the novolac binders were shown. In addition, a range of specialist and often proprietary additives was also needed for a specific successful resist formulation. The reactions taking place during the exposure process were then detailed. Grant described the significance of exposure in terms of time and light intensity. Sidewall definition was also critical, since the lower light dosage at the bottom of a layer of resist could lead to undercutting. This could be improved by keeping the polymerisation time down. Adhesion was another critical issue and it was stated that acrylate polymerisation could often exhibit a shrinkage of up to 30%, with faster curing compounds actually shrinking the most. One way of improving the adhesion was by reducing the level of acrylate present in the formulation. Another way was to use monomers that were more flexible. The acrylate polymerisation process involved the formation of peroxides which could cause copper corrosion and also contribute to plating failures. The resolution achievable was complex and influenced by a number of factors in addition to the imaging chemistry. It could be enhanced by careful development control and by optimising the glass transition temperature of the formulation. The resist chemistry also had a large impact on solder and plating resistance, as well as on legend adhesion and conformal coating compatibility. The ultimate resist formulation was typically based on a number of compromises in order to offer good processability over a wide range of conditions and equipment types. However, formulations could also be customised as required for specific equipment requirements.

Between the two presentations, Steve Payne announced that the ICT’s 2009 Annual Symposium would be a special event as it marked the Institute’s 35th anniversary.

The second presentation was given by Russ Crocket of DuPont Circuit Materials and was entitled ‘Imaging in a Tsunami’. Russ gave an overview of DuPont’s electronics technologies. DuPont was a science based company operating in five specific sectors. One of these was ‘electronics technologies’. The Chinese were currently describing what was happening across the world in the electronics industry as a tsunami; plants were being mothballed and there was a dramatic reduction in orders. In Europe, there was also a significant slowdown in orders and the economic view was that the recession would be worse than the one that occurred during the 1981 to 1982 period. On the positive side, inventories were relatively lean compared to previous downturns and oil prices were still falling, which would offer some help.

Russ then moved on to discuss dry film resists and he stated that Riston had now been available for 40 years. He said that, in the current circumstances, it was very important for suppliers to secure their own supply chains. Dry film resists had now gone below 10 micron resolution and could also give high productivity and lower processing costs at these fine resolutions. For high volume and high productivity, dedicated dry films were required for plating or etching, as good adhesion on all copper and direct metallisation surfaces was required. There should also be no leaching during plating and the resist must have good stripping characteristics, even if over plated. In tent and etch applications specialist dry films were also needed, although applications were still somewhat limited in Europe (but not in the Far East and the USA). Clearly, good adhesion on fine lines and annular rings was critical. The drive to laser imaging was continuing because of the promise of artwork elimination and reduced process costs and times. LDI was very sensitive to ‘dust imaging’ and thus good panel cleaning was essential. However, it could offer significantly increased resolutions and yields with higher photospeeds. The newer resists also had better adhesion and flexibility along with enhanced plating resistances and faster, cleaner stripping. A common problem with high resolution resists could be sludge formation. Russ then emphasised the importance of process control, especially in environments where there were reduced staffing levels, such as might occur during the current downturn. Factors that were also important were cleaning, lamination temperatures, development conditions and the use of proper drying. Examples of fine line features produced using Riston dry film were shown.

There were then questions from the audience about the conditions needed for producing 10 micron lines. These would need a collimated light source and, although the resolution could be achieved, the biggest challenge was to get resolution with the required yields. For this type of resolution, the resist would typically be in the region of only 15 microns thick. For very fine lines in high volume production, most manufacturers were using wet lamination, although this was not taking place much in Europe. Russ also stated that exposure was typically performed with the Mylar coversheet in place. The LDI resists didn’t normally contain any fillers and this helped with light transmission to the base.

This was another very successful, well attended and useful seminar that illustrated the value the ICT brings to its membership. Steve Payne concluded the seminar by thanking the speakers and the sponsors.

Martin Goosey 18th November 2008

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