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90 PCB007 MAGAZINE I NOVEMBER 2022 Everyone has their favorites. e simplest and cheapest is OSP, followed by lead-free HASL, but they have their pros and cons. Immersion silver is quite popular in North America and immersion tin is popular in Europe. ENIG has been a long-time standard, but the nickel now comes into play because it does not dissolve in the solder as the others do. Enter palladium to replace the nickel as a barrier metal. e new finishes are Pd/Ag, Ni/Pd, Ni/Pd/Au, Pd/ Au, and Au/Pd/Au—all developed to replace the ENIG for solderability, shelf-life, and wire bonding. During development of ENEPIG, it was rec- ognized that the addition of a palladium (Pd) layer between the nickel and gold enabled both gold and aluminum wire bonding oper- ations, in addition to the normal solder- ing application. In addition, the Pd layer was found to limit the corrosion of the nickel by an overly aggressive immersion gold process. Introduction e late Karl Dietz never wrote on the topic of palladium as a final finish, but he did write about gold plating as a final finish and had an excellent discussion on copper plating 1–5 . But palladium now has a renaissance as a final fin- ish. It was very popular in the 1970s, as the only other final finishes were tin-lead reflow, nickel-gold, OSP, or immersion tin. Palladium was very popular with the automotive industry then and a major supplier of boards was Photo- circuits of Glenn Cove, New York. Final Finishes As semiconductors (IC transistors) become smaller and have faster rise times, the signal is more sensitive to any ferromagnetic metal in its path. So many alternatives have been devel- oped to remove nickel as a barrier metal on copper for final finishes, especially in ENIG to improve insertion loss. Palladium as a Final Finish Happy's Tech Talk #14 by Happy Holden, I-CONNECT007

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