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Karl's Tech Talk
By Karl Dietz
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Dry Film Photoresist Thickness Selection Criteria
Dry film photoresist suppliers typically indicate for which process (e.g., print and etch, pattern plate, tent and etch, gold plating) a certain resist type is suitable; however the fabricator still has to select the resist thickness that matches 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 resolution in print and etch because of the more favorable etch channel aspect ratio that will give the best etch factor. The etch channel aspect ratio 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 favorable for resolution. High etch channel aspect ratios will result in poorer etch factors. Therefore thinner resist (and thinner copper) give better resolution.
Editor's Note: This article originally appeared in the July 2015 issue of The PCB Magazine.
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