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38 PCB007 MAGAZINE I MARCH 2018 Goldman: Gene told me that you can image ex- tremely fine features. Can you tell me how that works? Favini: Yes, the tendency of the market is to move to the production of fine or ultra-fine- line PCBs. For this reason, we developed the FP400 dry film series and recently a new ver- sion named FP700. With this new dry film, we can reach a line/space resolution below 10 microns. The photoinitiators present in the resist, by UV light effects, trigger a radicalized action of monomers which join, causing their po- lymerization. During the exposure phase, the UV light impacts the dry film bottom-up, so there will be a difference of energy between the top of the dry film and its bottom, in touch with the substrate. That difference is propor- tional to the thickness of the resist, with an unavoidable difference of polymerization be- tween the top of the dry film and the bot- tom of the dry film. To minimize that gap the different photo initiators must be balanced. There are photoinitiators in the formulation of the dry film which help the polymerization of the top and photoinitiators that help the deep polymerization of the track, i.e., on the base of the resist. Their balance is required to obtain the vertical profile of the dry film. This type of characteristic in the dry film formula has grad- ually become more important over time in the market with the increased use of laser expo- sure units. In fact, in these exposure units it is particularly crucial to have an optimal balance in the formula of the three principal photoini- tiators to get very good verticality of the lines. This type of characteristic of the verticali- ty of the lines has become particularly critical with on-going growth of the dry film market for fine line and ultra-fine line applications. In the most recent project followed by Elga Eu- rope regarding the design and use on industrial base of dry film FP400 and FP700 which Elga Europe, through its customers' exclusive coop- eration, has been able to obtain very impor- tant technological results, producing by means of this dry film lines and spaces below 10 µm (Figure 2). Goldman: Most dry films have a cover sheet, and when you image them, do you still have the cover sheet on there, or do you take the cover sheet off and image right against the dry film? Favini: The dry film is made of three dif- ferent layers. The first one is PET, which is used as a carrier during the coating of the re- sist, which is the liq- uid dry film (before drying in the oven) with 50% of solvent used to properly mix all the raw material. On average, 25 dif- ferent raw materials are needed to pro- duce a dry film and Figure 2: Examples of FP400 photoresist capabilities.