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46 PCB007 MAGAZINE I OCTOBER 2022 in manufacturing facilities. ese were hot air solder leveling (HASL) and reflowed tin/lead for surface contacting (insertion) and electro- lytic tab plating of nickel/gold as needed. As solder mask over bare copper (SMOBC) made its debut, reflowed tin/lead fell out of favor. In the next generation (lighter, smaller, and faster), two major manufacturing develop- ments dominated PCB designs, namely sur- face mount technology (SMT) and ball grid array (BGA) to meet the requirements of newer designs. SMT and BGA created challenges at assem- bly. Coplanarity was an absolute must. HASL formed a meniscus that interfered with the application of solder paste on surface mount pads. is requirement made organic solderabil- ity preservative (OSP) and electroless nickel/ immersion gold (ENIG) come to the forefront for these applications. e next major evolution in PWB manufac- turing was the elimination of lead from solder. A new generation of lead-free (LF) solder— tin/silver/copper (SAC) alloys—were the pre- vailing replacement for tin/lead. e SAC fam- ily of alloys have a melting point of 217–219 o C, with a peak liquidus temperature of 240 o C for complete wetting and for forming a consistent intermetallic compound (IMC), as compared to tin/lead which averaged 187 o C for a melt- ing point and a peak soldering temperature of 215 o C. To accommodate the elimination of lead from solder, equipment makers made design changes to accommodate the higher reflow temperature of LF alloys. Although HASL only provides a soldering surface, it remains a viable surface finish today for products that have adequate spacing between pads and do not require contacting or bonding. In the same way, OSP suppliers were able to produce the next generation, namely OSP-HT (high tem- perature). OSP and OSP-HT are widely used worldwide. For designs that require high temperature soldering and coplanarity, immersion silver and immersion tin filled that need. However, neither could meet all the demands of solder joint reliability and long shelf life. Silver is sus- ceptible to tarnishing and creep corrosion, and tin requires a thick immersion coating to retain its solderability. Over time, copper will diffuse into the immersion tin, forming a non- solderable Cu/Sn IMC. Both immersion silver and immersion tin remain viable surface fin- ishes with allowances made to overcome their shortcomings. ENIG, although it is a more complex and more costly process compared to immersion silver and immersion tin, was successful in filling the need for a surface finish that is sol- derable with LF solder, aluminum wire bond- able, and a good contacting surface with an extended shelf life. ENIG had a challenging start when it was first introduced; there were incidents of nickel corrosion under the immer- sion gold. e corrosion, if excessive, would interfere with IMC formation and the affected part would fail to form a reliable solder joint. IPC ENIG Specification 4552 Rev B, issued in 2021, spelled out a method to evaluate and measure the extent of ENIG nickel corrosion. Now that there is a method to measure and quantify nickel corrosion, the defect is on its way to being eliminated. "You can't fix what you can't measure." ENIG remained a popular finish for parts that required its attributes. e next chal- lenge was the need for an additional attribute and that was gold (Au) wire bonding. ENIG is not Au wire bondable because of the possible The next major evolution in PWB manufacturing was the elimination of lead from solder.