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80 The PCB Magazine • September 2015 The three-layer structure of dry film photo- resist (Figure 1) is quite familiar to those in the PCB industry, but the functions of the layers and the composition of the photosensitive layer are less understood. A solution of the photoresist composi- tion is coated onto the polyester support film, then dried, and covered with the polyethylene film before the dry film resist wide-stock roll is formed. The primary role of the polyethylene is to assure that adjacent laps of dry film don't stick to each other. Thickness uniformity of the polyethylene is important so that the photosen- sitive layer remains of uniform thickness. Gel spots in the polyethylene need to be avoided to assure that the photoresist has no localized thin spots that can lead to dish downs in print and etch processes or underplating in pattern plat- ing. Catalysts and processing aids in the poly- ethylene may diffuse into the photoresist layer, potentially causing unintended interactions with resist components, especially the photo- initiator system. The adhesion force between the polyeth- ylene and the photosensitive layer needs, of course, to be lower than the cohesion forces within the resist and the adhesion force be- tween the resist and the polyester base so that the polyethylene peels off cleanly before dry film lamination. The polyester film serves several purposes. It is the base onto which the photosensitive layer is coated. It needs to be transparent in the expo- sure wavelength range of approximately 340– 400 nm, which it is. The polyester has to have sufficient tack to the resist to stay in place dur- ing board handling in lamination and exposure to provide some protection against mechanical damage, but it needs to peel off cleanly before development. The polyester also provides a barrier against oxygen diffusion into the resist layer. Oxygen is an inhibitor to radical polymerization. It scavenges the radicals formed by the photons before the radicals can initiate photopolymer- ization. Therefore, the oxygen dissolved in the resist needs to be consumed before the acrylate monomers can polymerize, and replenishment of the oxygen from the air needs to be blocked, which is one function of the polyester film. The dry film polyester film and the phototool are often supplied with a matte surface[1]. This matte surface is typically created by small par- ticles, partially embedded in the surface, to fa- cilitate air escape during film roll formation and during vacuum draw-down in contact printing. Depending on the size at these particles, their proximity to the resist, and depending on the degree of collimation of the light source, these features will actually image. The imaging may be due to the opaqueness of the particle, or the difference between the refractive index of the particle and its surrounding media, or a small air gap between the two media, or a combina- by Karl Dietz Karl Dietz consulting llc karl's tech talk Dry Film Photoresist Structure and Composition Column figure 1: three-layer structure of dry film photoresist.

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