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 corrosion was recognized, a better understanding of the defect has led to a series of improvements over time.
Today, it is well established that Ni corrosion occurs in the immersion gold step, and the most important method for eliminating the defect is through process control. ENIG is a complex chemical process with multiple process steps, and each step must be completed successfully before proceeding forward. ENIG remains a very popular surface finish 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, specifies 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 extended dwell time makes the deposit susceptible 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 immersion gold should be used for deposition. Two available alternatives are reduction-assisted immersion (RAI) gold and electroless gold.
To read this entire column, which appeared in the December 2019 issue of PCB007 Magazine, click here.