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18 The PCB Magazine • February 2015 cuitry to the OSP solution? Certainly these free Cu+2 ions would then would be able to complex with the imidazole (OSP active ingredient) and form an organic film on the gold. The mixed metal compatible OSP additives complex these copper ions as they are formed in the OSP so- lution. Thus, this action prevents formation of the imidazole-copper ion complex that would deposit on the gold or other metal finish. How- ever, this action does not interfere with the ni- trogen functional group of the imidazole from forming an organic film on the copper. When OSP does coat the gold, there are adverse effects on insulation resistance (Figure 5). The Galvanic Effect The use of mixed metal finishes introduces another potential pitfall in fabrication. This is commonly known as galvanic corrosion or the galvanic effect (Figure 6). Galvanic corrosion/ degradation of dissimilar metallic surfaces in contact with each other is a well-recognized phenomenon. There exists a substantial body of work that has helped the electronics indus- try with understanding galvanic corrosion. Fon- tana and Greene [1] classified galvanic corrosion as one of eight forms of corrosion. The Depart- ment of Defense issued a military handbook, MIL-HDBK-729 "Corrosion and Corrosion Pre- vention Metals", in an effort to provide basic, fundamental information on corrosion and cor- rosion prevention [2] . The galvanic effect arises when two dissimilar metals are in contact with one another (such as nickel and copper with se- lective ENIG processing-basically copper-nickel gold interface). The issue is exacerbated by the type of micro-etch used. In addition, internal testing has shown that feature size particularly large area gold pads directly connected to cop- per traces leads to excessive etching of the cop- per at the interface. One may state that this is similar to a battery cell. When two metals (gold and copper) are in contact with each other and in an electrolyte (micro-etch), a strong potential exists between the two metals. With such a strong potential existing in this scenario, it is very likely that ag- gressive copper removal will be the result (where the copper meets the gold). An actual example of the galvanic etch is shown in Figure 7. It is quite evident that the gold plated trace makes intimate contact with the bare copper prior to OSP process. The aggressive etching that is taking place at the gold-copper junc- OSP AND SELECTIVE ELECTROLESS NICKEL continues feature Figure 5: Insulation resistance on gold when copper containing OSP is compared to mixed- metal compatible OSP. Figure 6: Schematic of galvanic etch mechanism.