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46 PCB007 MAGAZINE I DECEMBER 2019 Electroless nickel immersion gold (ENIG) has been around the printed circuit industry for more than 25 years. The first version of the IPC-4552 ENIG specification was issued in 2002. Initially, the specification only addressed tin/lead solder; now, lead-free solder, like SAC 305 and its variants, dominate soldering in electronics. Although the occurrence of corro- sion was recognized, a better understanding of the defect has led to a series of improvements over time. Today, it is well es- tablished that Ni cor- rosion occurs in the immersion gold step, and the most impor- tant method for elim- inating the defect is through process con- trol. ENIG is a com- plex chemical process with multiple process steps, and each step must be completed successfully before proceeding forward. ENIG remains a very popular surface fin- ish and offers a series of benefits at assembly: it is easy to inspect, has an extended shelf life, and is suitable for a wide range of assembly applications. The IPC-4552 Rev A, issued in 2017, speci- fies the deposit thickness: nickel from 3–6 µm (120–240 µnis) and gold from 0.04–0.1 µm (1.6–4.0 µins). The upper limit for gold at 0.1 µm (4.0 µins) would require an extended dwell time in the immersion gold bath. The ex- tended dwell time makes the deposit suscep- tible to nickel corrosion. The recommended immersion gold deposit thickness is 0.04–0.07 µm (1.6–2.8 µins). If a higher gold thickness is a design requirement, an alternative to im- mersion gold should be used for deposition. Two available alternatives are reduction-assist- ed immersion (RAI) gold and electroless gold. ENIG Pre-plate: Tin Stripping and Solder Mask Preparation Controlling the outcome of ENIG plating starts with the parts coming to the line; parts must be free of tin and or- ganic residues. Tin is used as an etch resist during the cir- cuitization of the board. Tin must be completely stripped to allow for uniform cat- alyzation of the copper surface. Residual tin interferes with the deposition of the palladium catalyst or activator. The copper surface coming to the ENIG line, in most cases, follows the application of sol- New Developments in ENIG The Plating Forum by George Milad, UYEMURA

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