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74 The PCB Magazine • October 2014 DRY FILM PHOTORESIST ADHESION TESTS continues Figure 2 introduces the concept of a peel test that measures dry film adhesion on copper. In real- ity, such a 90-degree pull test (e.g., with an Instron instrument) does not work with dry film, because the film will show cohesive failure before adhesive failure. This test arrangement is, however, used to test copper adhesion to the underlying dielectric. Overview of Adhesion Tests The effect of copper surface differences on dry film adhesion can be evaluated by the fol- lowing criteria: • Instant adhesion • Resolution after development, as judged by – Minimum isolated line remaining (surviving development) – Minimum line/space resolution • Survival of small resist features (e.g., dots) through development or development and etching • Degree of resist lifting in pattern plating • Yield (print & etch; AOI data) Instant Adhesion The instant adhesion test is a pull (peel off) test performed at different time intervals after resist lamination (0, 15, 30, 60 minutes, and 24 hours). An adhesive tape is pressed against the laminated resist, the tape is pulled off, and the area-percent of the resist remaining on the copper surface is re- ported. There is no pre-scoring or cross-hatching of the resist before applying the adhesive test, as it is practiced in other standard adhesion tests (e. g., in the paint industry). Adhesion immediately after lamination tends to be lower than adhesion measured later, whereby in most cases the mea- sured adhesion levels off after about 15 minutes of hold time after lamination. Resolution after Development Resolution after development can be judged by the smallest lines and spaces resolved, and the smallest, isolated line "held." A multiple pitch pattern with 20–200 micron lines and spaces, isolated and nested (paired) lines, in a print & etch version and the corresponding pat- tern plate version, can serve as a test vehicle. Survival of Small Resist Features Through Development or Development and Etching A related test, introduced by Asahi, measures resist adhesion under actual wet processing con- ditions, through development or through de- velopment and etching. The resist is laminated, exposed, and developed to form a dot pattern of different diameter dot sizes. The test board is figure 2: Concept of adhesion test by peel test.