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62 DESIGN007 MAGAZINE I SEPTEMBER 2022 that 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% by weight 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 • ENEPIG (electroless nickel, electroless palladium, immersion gold) ENEPIG is a three-layer metallic coating of 1.97 µin (0.05 µm) minimum, 2-5 µin (0.05- 0.012 µm) typical, immersion-deposited Au over 2 to 12 µin (0.05-0.30 µm) electroless- deposited Pd over 118.1-236.2 µin (3-to 6 µm) electroless-deposited nickel. 1 is process plates a thin coat of nickel cov- ered by a thin layer of palladium, then gold. e gold provides a very good solderable sur- face. When components are soldered onto these pads, the palladium and gold diffuse into the solder joint. e gold layer is very thin so it won't reduce the solder joint strength. is process is generally not utilized for high reliability, long lifetime, or high vibration applications. is is used for wire-bonding applications. ENEPIG is typically used for flat surface/ fine pitch devices. e benefits of ENIPIG are a long shelf life and environmental resistance, as well as low contact resistance and excellent solder joint with SAC solders. • Gold Wire Bondable › Expensive › Cannot be reworked › Lossy RF (phosphorus nickel) › Palladium intermetallic bond to Pb in Sn/Pb is not ideal • IAg or ImmAg (immersion silver) ImmAg is a single layer metallic coating of 5 µin (0.12 µm) minimum, 5-20 (0.12 to 0.50 µm) typical, of immersion-deposited silver. 2 is process plates a thin layer of silver directly on top of the copper surface. As with the other immersion surface finishes, the finished product produces a very flat sur- face, which is ideal for fine pitch SMD arrays. is surface finish has the ability to maintain high solderability aer multiple heat cycles. is can also be used as an aluminum wire- bondable surface. It is compatible with no- clean assembly processes. is is becoming popular as a HASL replacement for lead-free

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