Issue link: https://iconnect007.uberflip.com/i/950499
40 PCB007 MAGAZINE I MARCH 2018 so it's very important that the mixing process is perfect. After coating, the resist, the second layer, goes into an oven 62 meters long and after the evaporation of the solvent we have what we call "dry film." To keep the different layers of the master roll from stick- ing together a third lay- er of polyethylene is ap- plied before the winding of the dry film and which is what you call the cover sheet (Figure 3). Goldman: Ah, that's what I was calling the cov- er sheet. Favini: The polyethylene is removed just before the lamination of the dry film to the substrate. Then the dry film is exposed, and then also the PET is removed to proceed with the dry film developing process. Goldman: The PET sheet stays on during expo- sure. How thick is that? Favini: It depends, but about 18 microns. Goldman: Does that somehow affect the image resolution? Favini: No, the image resolution is affected by the transparency of the PET. A too-thin PET could break up during the removal process af- ter dry film exposure. This is a characteristic of low quality dry film with the intention of the dry film producer to reduce the production costs but it creates problems for the PCB pro- ducers as mentioned. What is also very impor- tant is the thickness distribution of the dry film on the master roll during the coating process because this affects the resolution of the lines during PCB production. Elga Europe guaran- tees to the customer a maximum variation of ±1 micron on the master roll width, while some other dry film producers guarantee a maximum of ±4 microns. This worse distribu- tion also creates wrinkles on the master roll during its aging and so an even higher thickness differ- ence further worsening possible problems to the customers. It also creates an edge fusion problem on the dry film rolls. Goldman: Because you've got a little bit of standoff from the resist, does that affect the shape of the line and the width? Favini: Yes, it's very important to use a PET with high transparency to guarantee a very low refraction effect so that the light will not be di- verted during the dry film exposure process. This can also be adjusted by regulating the ex- posure energy. Of course, with the increase of the PET quality, the purchasing cost will be higher, but these features are very important for production of high quality PCBs and man- datory for fine line and ultra-fine line work to guarantee a very good line definition. For this reason, to produce our ultra-fine line dry films we use a special and dedicated PET with even higher quality than the ones used for the other dry films. For example, Elga supplies a custom- er in the U.S. with the FP720 dry film of 20 mi- crons thickness; with it they can produce five- micron lines and spaces. So it's clear how the quality of all the raw materials involved in the dry film production are important. Goldman: That's very good. Was this done with direct imaging, perhaps laser? Favini: In this case, it is a conventional one with a UV light. But this kind of dry film can also work with direct image with DDI, LDI, or laser diode sensor. Goldman: Okay, thank you so much. This has been a very interesting discussion. Favini: Thank you, Patty. PCB007 Figure 3: Schematic of dry film layers.