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92 PCB007 MAGAZINE I NOVEMBER 2022 An electrolytic nickel/gold fin- ish was typically the process of record (POR) for such wire bond- ing needs. ENEP investigations into the ENEP process, which is essentially the ENEPIG process without the immersion gold step, actually followed the introduc- tion of ENEPIG to the market. In comparison to ENEPIG, ENEP appears to offer a surface finish with benefits like stronger solder joint reliability under Pb-free con- ditions, lower cost (by elimination of gold) and, as recently investi- gated, Cu wire bonding capability. Palladium Finish Used in Automotive Early in the 2010s, palladium reemerged as a final finish. Electronic chemicals company OMG introduced its new palladium finish, Pal- laGuard. (OMG is now part of MacDermid.) Some of its features were: • Eliminates black pad • A single coating on copper • Excellent silver replacement • Short process time of 15 minutes • Excellent shelf life • Can be used to replace carbon switches • Excellent replacement for harsh environments • High gold bond shear test results Properties e properties of palladium, which was dis- covered in 1803 by the English chemist Wil- liam Hyde Wollaston, are many: • Pure palladium deposit, atomic number 46 • Melting point is 2,830.82°F (1,554.90°C) • Palladium deposit with 2-3% phosphorus (Pd-P) • Pure palladium 50% soer than Pd-P (Figure 1) • Specific gravity is 11.9 • Resistivity, microohm-cm at 0°C: 10.7 • Electrical conductivity, percent IACS: 16 • ermal conductivity, CGS units: 0.17 • Deposit thickness of 2–5 microinches for soldering and 5–15 microinches for wire bonding • Pd has a hardness in the range 300 to 400 V.P.N. and is relatively free from internal stress, so that the plate does not suffer from curling and peeling troubles • When solder joint strength is critical, or gold wire bonding is required • Pure Pd has a better wetting angle and reflow spread than Pd-P (Figure 2) New Applications of Palladium For electronic assembly reliability espe- cially for harsh environments, palladium has many advantages over nickel, except cost. For improved wire bonding reliability, a minimum of 5 microinches of electroless palladium over nickel or copper will eliminate any 0.7 mil gold wire surface failures and provide over 6 grams of pull strength. Figure 1: Comparison of hardness of palladium-phosphorus and pure palladium autocatalytic deposits 6 .