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74 PCB007 MAGAZINE I NOVEMBER 2018 While attempting to enhance resist strip- ping for those more difficult to remove resists, here is a word of caution. Manipulating dwell times, concentrations, and operating tempera- tures will have adverse effects on metal etch resist attack, copper darkening, and potentially higher cost. This is where different resist strip- ping formulations should be evaluated. Tarnishing of the Copper Surface A second part of the cleanliness equation is the stripping process must leave the copper surface free of any staining, tarnishing, or any other cosmetic issues. High-pH resist stripping formulations tend to oxidize the bare copper. It is critical that most resist stripper formula- tions contain effective compounds to prevent oxidation. In addition, certain additives within the resist formulation itself can leave stains on the copper. These additives are dye precursors used to cause a color change during exposure. These dyes can form complexes with the cop- per surface. Thus, these stained complexes are difficult to remove. If this is an issue, the fabri- cator must ensure that the resist stripping for- mulation contains solvents to break that bond and leave the copper surface clean. Conclusion Yes, there is some art to the resist stripping operation. However, it is more about the sci- ence. In a future column, I will present process control methods for resist stripping as well as dive into additional troublesome activities re- lated to the process. PCB007 Michael Carano is VP of technology and business development for RBP Chemical Technology. To read past columns or contact Carano, click here. Figure 2: Same photoresist in each beaker, but two different resist stripper formulations (right shows more effective resist removal).