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54 PCB007 MAGAZINE I SEPTEMBER 2018 wear gloves? It would be a major mistake to think strong cleaners will remove human fin- gerprints, so tread cautiously. Surface cleanliness is critical to ensure pho- toresist adhesion. In addition, topography pro- vides a mechanism to enhance adhesion of the resist to the copper surface. However, the crit- ical aspect is to have an overall roughness of the surface area that is uniform and not over- ly roughened with chemical etchants or me- chanical means. A surface that has major de- viations from peak to valley along the surface profile will adversely affect resist adhesion. Thus, there is the potential for lock-in on one extreme and resist lifting on the other. Best practice dictates diligence with sur- face preparation, resist lamination, hold times between process steps (in-process is best due to minimal holds), and all processes as- sociated with the resist exposure and develop- ment. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. • Dry lamination issues: This holds especially true if the post-lam- ination hold time is too long or tempera- ture and humidity are too high during hold time. Such conditions favor copper oxida- tion. In terms of oxi- dation, there will be a strong tendency for the laminated resist to strongly adhere • Exposure of the cop- per surface to hydro- chloric acid fumes: This may cause resist lock-in even if post- lamination hold times and conditions are normal. As in often the case, the resist-laminated panels for inner- layer processing may contact acid fumes from the cupric chloride etching process • Excessive pressure on the hot roll lamina- tors or too high a temperature • Excessive surface roughness (topography) due to overly aggressive chemical clean- ing or scrubbing of the copper surface: If you suspect lock-in due to a non-uniform or over-roughened surface, a profilometry study to measure surface roughness might be in order • Developer/unexposed resist residues re- maining on the copper surface in a random fashion: This happens due to insufficient rinsing or inadequate control of the devel- oping solution. In the latter case, the main culprit is too low a pH in the developer so- lution. Another cause is partially polymer- ized resist that was not completely devel- oped away Of course, extraneous copper can sometimes be linked to obvious causes, but that does not make it any less frustrating (Figure 2). While improper handling is somewhat of a rare oc- currence compared to other issues, this must be considered. For example, did the operators Figure 2: Copper not removed due to a fingerprint from poor handling.

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