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108 PCB007 MAGAZINE I OCTOBER 2018 is a catalyst that accelerates the tarnishing of the copper metal substrate. As the stripper is used, the dissolved copper content builds and the stripper becomes increasingly corrosive to the exposed copper metal substrate. This is a good reason to not use high pH resist stripper formulations with concerns such as the oxida- tion or tarnishing of the copper. Some fabrica- tors continue to employ low-cost, highly con- centrated caustic-based strippers, but they are mistaken because this approach is not actually low cost (more on this later). Thus, in reality, the tarnishing of panels in a used stripper is a failure of the anti-tarnish chemistry to deal with the increased corrosiv- ity of the copper-contaminated stripper. This unfortunate situation can be avoided by se- lecting a stripper with anti-tarnish chemistry robust enough to overcome the corrosivity of high levels of dissolved copper. This choice can result in saving a tidy sum of money on stripper chemistry. The stripper can then be used to its real capacity rather than being pre- maturely dumped because it is tarnishing the copper. This is the more obvious problem. A second issue relates to the dramatic de- crease in speed of stripping with usage of the stripping chemistry. This results in an increase in resist stripping cost. Costs can come in sever- al forms: lost productivity, potential for rejects due to resist residues remaining on the surface, adding fresh resist stripper to the working so- lution, and inadequate removal of resist skins from the sump. The addition of fresh strip- per is usually a losing proposition depending on the stripper formulation and other factors. Other factors include the amount of stripped resist film in the sump, the reactivity of the ex- posed resist with the stripper ingredients, and whether or not the resist film is breaking up into particles/skins or is partially dissolving. Figure 1 shows the relative behavior of a monoethanolamine (MEA)/choline-based re- sist stripper. These two chemicals are found in many of the commercially available resist strippers on the market today. Please note that in Figure 1, while representing a typical resist stripper operation, there are subtle differences from one resist to another. However, the gener- al shape of the curve is representative. Notice that the speed over the first 30% of the stripper bath is very high and almost com- pletely uniform. As the choline depletes, the stripper starts stripping with the MEA. Con- sequently, the speed drops suddenly and dra- matically and is uniform through its life. At this point, the stripper is acting as if it were a pure MEA-based product and choline was nev- er present. For an outer layer stripper formula- tion, it makes sense to not have an MEA/cho- Figure 1: Stripping behavior of a MEA/choline-based resist stripper over time.