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70 The PCB Magazine • July 2015 by Karl Dietz Karl DieTz coNsulTiNg llc karl's tech talk Dry Film Photoresist Thickness Selection Criteria Column Dry film photoresist suppliers typically indi- cate for which process (e.g., print and etch, pat- tern plate, tent and etch, gold plating) a certain resist type is suitable; however the fabricator still has to select the resist thickness that match- es his needs best. The most important thickness selection criteria that come to mind are price, resolution, and yield. Dry film photoresist with a thinner photo- sensitive layer tend to have a lower price because of the lower material cost of the resist layer. So it makes sense to use the thinnest resist that still satisfies yield and resolution expectations. For print and etch, one would choose the thinnest film that still gives acceptable yields. The thinnest resist will also give the best resolu- tion in print and etch because of the more fa- vorable etch channel aspect ratio that will give the best etch factor. The etch channel aspect ra- tio is defined as the ratio of the total thickness of the resist and the copper to be etched, over the width of the space to be etched. The etch factor is the ratio of vertical etching over lateral etching whereby a high number is more favor- able for resolution. High etch channel aspect ra- tios will result in poorer etch factors. Therefore thinner resist (and thinner copper) give better resolution. Potential problems that one can get into with films that are too thin for the application are poor resist conformation to a relatively rough copper surface, because there is not enough re- sist mass to flow and fill the recessed areas of the copper topography, which lead to circuit open defects or so called dish-downs. Another poten- tial problem could be the formation of wrinkles during hot roll lamination. Print and etch resists are typically 25–35 microns thick. The thinnest available dry films are about 20 microns. For tent and etch, one will have to pick a film that is thick enough to form 'tents' over the metalized through-hole openings to hold up against mechanical and/or chemical attack. Historically, these films had to be 50 microns to hold up. In recent years, resists have been improved to give good performance at only 40 micron thickness; in some cases it may be only 35 micron thickness. The tenting strength can be measured with an instrument like the one shown in Figures 1 and 2. Figure 1: instron ® stress/strain tester. Figure 2: instron ® probe testing tent strength.

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