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82 The PCB Magazine • September 2015 tion thereof. Figures 2a-c illustrate how sur- face particles of different sizes on polyester films image in a non-collimated printer: The imaged imperfections in the resist, pockmarks on the surface and a fluted sidewall shape, be- come less pronounced as the polyester surface becomes smoother. Laser direct imaging (LDI) could be considered an exposure with a highly collimated light source. Therefore, the imag- ing of matte particles on the polyester is an issue in LDI. Figures 3a and 3b show the effect of colli- mation for a given polyester surface structure: Higher collimation gives a sharper picture of the particles. Not only are the matte particles in the polyester creating unwanted images, dirt will also image better, and matte features on the phototool surface can image. Even scratches on the backside of the phototool, further away from the resist, can image. Therefore, the in- troduction of highly collimated printers has to go hand in hand with improved handling and clean room standards. The selection of the polyester film thickness is a compromise between processability, cost, and resolution requirement. The thinner the polyester film, the better the contact between the phototool and the resist layer, and the bet- ter the resolution. Thinner polyester is also less expensive, but very thin polyester film can lead to film wrinkling in the hot roll laminator. Typi- cal polyester film thicknesses are in the range of 20–25 microns. The composition of the photosensitive layer was covered in detail in References 2 and 3 and will be summarized briefly as follows: Photoinitiator System The simplest "system" is a single component photoinitiator, which is activated by the UV radiation, generates a radical, and initiates the chain polymerization. More often, a system of sensitizers and activators or co-initiators form the active radicals. Occasionally, chain transfer agents or hydrogen donors are part of the pho - toinitiator system. They form secondary radi- cals, which are better suited to initiate photopo- lymerization. Typical structures found as part of the initiator system are aromatic carbonyl com- pounds and hexaarylbisimidazoles (HABIs). figure 2a: coversheet with coarse matte particles. figure 2b: coversheet with small matte particles. figure 2c: clear coversheet. kArl'S tECh tAlk DRy FILM PHOTORESIST STRuCTuRE AND COMPOSITION continues

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