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52 PCB007 MAGAZINE I SEPTEMBER 2018 It happens when you least expect it. You think you have all the process controls in place to prevent issues after etching, but either cop- per remains where there should be none or photoresist remains on the copper. Regardless, it is a problem and must be fixed; what are some of the causes? Resist lock-in comes in many shapes and siz- es. Figure 1 shows an example of extraneous copper. The root cause of the remaining cop- per is often resist lock-in. In addition, several causes for the extrane- ous copper defect can be attributed to: • Excessive resist lamination temperatures: This can cause thermal cure of some photo- resists and difficulties in developing cleanly • Oxidation of the panels prior to resist lam- ination • Wet lamination: A thin layer of water is applied to the copper surface prior to re- sist lamination and requires resists that are compatible with wet lamination in order to avoid resist lock-in, which will lead to ex- cess copper. According to Dr. Karl Dietz, one must strictly adhere to hold time and its conditions Understanding Resist Lock-in and Extraneous Copper Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Extraneous copper.