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62 DESIGN007 MAGAZINE I DECEMBER 2022 should be part of any decision to choose a final finish: • Lead-tolerant or lead-free (LF) process • Shelf life • Flatness • Lead or ball pitch • Wire bondability • Lead insertion • Solder joint integrity • Corrosion resistance • Potential problems • Cost Lead-free Finishes Lead-free finishes are considered "RoHS" compliant (< 0.1% BW of finish, for Pb, Hg, or Cd) with the single exception of tin/lead HASL. e RoHS compliant finishes include the following: • ENIG • ENIPIG • ENEPIG • ImmAg • ImmSn • Electrolytic nickel/gold • LF HASL • OSP Lead-free PCBs require that the standard HASL surface finish cannot be used. ere is still a significant amount of discussion on what the long-term surface finishes will be. Cur- rently, the immersion silver and OSP surface finishes are the most prevalently specified sur- face finishes for solderable PCBs. Immersion tin is the prevalent surface finish for press-fit backplanes. Please contact your PCB fabrica- tor for current information on where industry specifications are heading. Final Finishes Electrolytic Nickel/Gold (Electrolytic nickel and hard or soft gold plating) Electrolytic nickel/gold is a two-layer metal- lic plating of a varied thickness of either hard or so electroplated gold over a varied thickness of "low stress" electroplated sulfamate nickel.* Electrolytic nickel/gold has the best shelf life and environmental resistance, as well as a low contact resistance. Selective finishes are possi- ble with electrolytic nickel/gold (< 15 µin sol- dering, > 30 µin contacts and wire-bondable so gold). Gold Wire Bondable • Most expensive (thicker gold = $$$) • Selective finishes more expensive (filled vias required in some cases) • Cannot be reworked • Moderately lossy RF (nickel) *Per IPC-4556, nominal pad size of 0.060" x 0.060" (1.5mm x 1.5mm). Understanding the cost drivers in PCB fab- rication and early engagement between the designer and the fabricator are crucial ele- ments that lead to cost-effective design suc- cess. Following your fabricator's DFM guide- lines is the first place to start. DESIGN007 Anaya Vardya is president and CEO of American Standard Circuits; co-author of The Printed Circuit Designer's Guide to… Fundamentals of RF/Microwave PCBs and Flex and Rigid-Flex Fundamentals; and author of Thermal Management: A Fabricator's Perspective. Visit I-007eBooks. com to download these and other educational titles. He also co-authored "Fundamentals of Printed Circuit Board Technologies" and provides a discussion of flex and rigid-flex PCBs at RealTime with… American Standard Circuits.