Issue link: https://iconnect007.uberflip.com/i/1533339
58 PCB007 MAGAZINE I MARCH 2025 Black Pad-Nickel Corrosion e use of an electroless palladium (EP) layer between the electroless nickel and the immersion gold will eliminate any nickel cor- rosion (Black Pad), and the thicker the Pd, the less immersion gold is deposited, thus reduc- ing costs. e explanation is that the gold ions, at the lower EP thickness, had access to the underlying nickel and deposited at an accel- erated rate, producing nickel corrosion. Lack- ing access to the underlying nickel, the immer- sion gold (IG) reaction with phos-palladium becomes self-limiting to 2 µin (0.05 µm). Solderability Pure Pd has a better wetting angle and reflow spread than Pd-P (Table 1) and is similar to ENIG but is best aer one or more Pb-free sol- dering reflow cycles. A Case Study of HP's Extensive Use of Palladium Hewlett-Packard successfully developed the first low-profile keyboard in 1970 for its new HP-35 calculator. Unfortunately, a thick gold deposit was required on the annealed beryl- lium/copper keystrip in order to maintain their long-useful life (Figure 2). is keyboard was selected to be used in HP's line of desktop computers (the 9800 series) but an even longer life for the key- board was desired. HP then developed a mod- ified version of the unique plated metal— tri-metal or sometimes called "white bronze" as it was an alloy con- taining copper, tin, and zinc (HP called it 1. "Circuitry Solutions," MacDermid Alpha Electronics Solutions. 2. "Electroless Palladium Plating—Correlating Plating Solution and Deposit Properties," by Britta Schafstellar, IPC APEX EXPO 2022. Happy Holden has worked in printed circuit technology since 1970 with Hewlett-Pack- ard, NanYa Westwood, Mer- ix, Foxconn, and Gentex. He is currently a contributing techni- cal editor and columnist with I- Connect007, and the author of Automation and Advanced Procedures in PCB Fabrication, and 24 Essential Skills for Engineers. To read past columns, click here. Table 2: Palladium characteristics for soldering, contact switching and wire bonding. (Source: Atotech) AB alloy). This bright white, very ductile fin- ish was used as an undercoat for a selectively plated spot of palladium on the beryllium/ copper key strip, thus saving a lot of gold costs and extending the life of the keyboard. Summary The resurgence of the use of palladium as a final finish or a barrier over copper or nickel has solved several problems. This is especially useful when the PCB may be used in harsh environments or in the presence of clorine (Cl). Palladium is very dense and can be used at lower thicknesses than gold. PCB007 References Figure 2: HP's gold plated Be/Cu key strip welded to the gold PC board for the HP-35 calculator.