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NOVEMBER 2022 I PCB007 MAGAZINE 93 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). Summary e resurgence of palladium as a final finish or a barrier over copper or nickel has solved a number of problems. is is especially use- ful when the PCB may operate in a hazardous, corrosive, or extreme environment, such as with vehicles. When compared to its predecessors—ENIG and electrolytic Ni/Au—ENEPIG has been shown to offer stronger solder interconnects and wire bonds. Pull and shear forces needed for failures on ENEPIG were a minimum of 5% higher than ENIG and Ni/Au when SAC3xx solders were used. Furthermore, the fail- ure modes in the ENEPIG/SAC3xx solder interconnects were primar- ily ductile in the bulk, while failure modes in ENIG/SAC3xx and Ni/ Au/SAC3xx were primarily brittle in the IMC, even aer precondition- ing steps were taken. Ductile fail- ure in the bulk implies that ENEPIG has better solderability with SAC3xx solders than ENIG and Ni/Au 7 . PCB007 References 1. Tech Talk #54: Organic Additives in Copper Plating Baths (Part 1), by Karl Dietz, CircuiTree Magazine, February 2000. 2. Tech Talk #55, Organic Additives in Copper Plating Baths (Part 2), by Karl Dietz, CircuiT- ree Magazine, March 2000. 3. Tech Talk #145, Staying out of Trouble in Gold Electro-plating, by Karl Dietz, CircuiTree Magazine, Oct. 2007. 4. Tech Talk #154, Advances in Copper plating (Part A), by Karl Dietz, CircuiTree Magazine, July 2008. 5. Tech Talk #155, Advances in Copper plating (Part B), by Karl Dietz, CircuiTree Magazine, August 2008. 6. "Benefits of pure palladium for ENEP and ENEPIG surface finishes," by Mustafa Ozkok, ESTC Conference, October 2010. 7. "Review of Capabilities of the ENEPIG Surface Finish," by M. Ratzker, M. Pecht, G. Milad, Journal of Electronic Materials, Vol. 43, No. 11, 2014. Happy Holden has worked in printed circuit technology since 1970 with Hewlett-Pack- ard, NanYa Westwood, Merix, Foxconn, and Gentex. He is currently a contributing tech- nical editor with I-Connect007, and the author of Automation and Advanced Procedures in PCB Fabrication, and 24 Essential Skills for Engineers. To read past columns or contact Holden, click here. Figure 2: Comparison of wetting angle test results for Pd-P (left) and pure Pd (right) for various aging conditions. 6

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