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84 PCB007 MAGAZINE I AUGUST 2021 Surface finish R&D departments on the sup- plier side have been very busy coming up with new finishes to meet the everchanging de- mands of the electronics industry. Today, de- signers have wide variety of finishes to choose from: HASL (hot air solder leveling), electro- lytic nickel gold, OSP (organic solderabilty preservative), ENIG (electroless nickel/im- mersion gold), I-Ag (immersion silver), I-Sn (immersion tin), EN- EPIG (electroless nick- el/electroless palladium/ immersion gold), EPIG (electroless palladium/ immersion gold), EPAG (electroless palladium/ auto-catalytic gold), DIG (direct immersion gold), and ISIG (immersion sil- ver/immersion gold). In addition, most finishes of- fer variations within their process; specifying thick- ness of the layers and the type of electrolyte is used to meet specific performance criteria. Historically, HASL was among the first fin- ishes that saw wide use as a solderable finish. HASL was accompanied by tin-lead reflow where it was suitable, as long as tin-lead sol- dering remained the primary assembly meth- od. Both HASL and tin-lead reflow required a thermal excursion. e technology (mostly double-sided) at that time could tolerate it. OSP was introduced as a simpler, faster, and safer procedure. OSP gained market share as long as the primary assembly method required only a single excursion to tin-lead reflow tem- perature. With the introduction of SMOBC (solder mask over bare copper), the dominant finish- es were HASL and OSP for pads and electrolyt- ic nickel/electrolytic hard gold for contacting fingers. As MLB (multilay- er boards) and surface mount were introduced, HASL topography (lack of planarity) became a problem at assembly as the pad size contin- ued to shrink. ENIG and OSP became the better choices. ENIG had the advantage over OSP in that it withstands mul- tiple thermal excursions required at assembly. ENIG was also aluminum wire bondable and a good metallic contacting surface. All these advantages justified the higher cost and complexity of processing associate with ENIG. For a few years, ENIG, HASL, OSP, and electrolytic nickel gold were the staple for the industry, with HASL and OSP for simpler An Overview of Surface Finishes The Plating Forum by George Milad, UYEMURA

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