PCB007 Magazine

PCB-Oct2014

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72 The PCB Magazine • October 2014 Laminate construction, chemical composi- tion of the copper foil surface and its topogra- phy, 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 surfac- es and process conditions with regard to dry film adhesion. The constant in such studies is a given dry film resist that is tested on differ- ent copper surfaces and under different process conditions. Conversely, one can keep the lami- nate construction and copper foil preparation as well as lamination conditions and hold times the same, while testing the adhesion character- istics of different films. There are several failure modes, or sources of yield losses, if the copper surface is not prop- erly prepared. Failure may be due to insufficient or excessive adhesion (Figure 1): 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 plat- ing process will cause tin/lead underplating, ul- timately leading to shorting defects ("shorts"). 3. Failure to achieve good release of unex- posed resist during development can cause etch retardation in a print-and-etch process, ulti- mately leading to shorts. 4. Failure to achieve good release of unex- posed resist during development in a plating pro- cess can cause poor adhesion of the plated cop- per 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 mul- tilayer 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 retar- dation. by Karl Dietz Karl dietz Consulting llC Dry Film Photoresist Adhesion Tests C o l u m n karl's tech talk figure 1: failure modes due to insufficient or excessive adhesion.

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