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Laminate construction, chemical composition of the copper foil surface and its topography, resist composition, lamination conditions, and hold times all affect dry film photoresist adhesion, conformation, and, ultimately PWB yields. This area has been studied extensively over the years.
A number of resist adhesion test methods have been employed to test different surfaces and process conditions with regard to dry film adhesion. The constant in such studies is a given dry film resist that is tested on different copper surfaces and under different process conditions. Conversely, one can keep the laminate construction and copper foil preparation as well as lamination conditions and hold times the same, while testing the adhesion characteristics of different films.
There are several failure modes, or sources of yield losses, if the copper surface is not properly prepared. Failure may be due to insufficient or excessive adhesion:
1. Failure to achieve good adhesion in a print-and-etch process will cause etchant attack under the resist and ultimately an “open” defect.
2. Failure to achieve good adhesion in a plating process will cause tin/lead underplating, ultimately leading to shorting defects (“shorts”).
3. Failure to achieve good release of unexposed resist during development can cause etch retardation in a print-and-etch process, ultimately leading to shorts.
4. Failure to achieve good release of unexposed resist during development in a plating process can cause poor adhesion of the plated copper to the copper base (copper-copper peelers).
5. Failure to achieve good release of exposed resist in a print-and-etch process on innerlayers can inhibit the formation of copper oxide multilayer bonder (or alternative bonders) on such a copper surface.
6. Failure to achieve good release of exposed resist in a plating process can cause etch retardation.
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Editor's Note: This column originally appeared in the October 2014 issue of The PCB Magazine.
