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12 SMT Magazine • January 2017 by Ben Gumpert, William Fox, and C. Don Dupriest LOCKHEED MARTIN Abstract The surface finish of a printed circuit board provides a number of functions, with impacts starting at the point of design and continuing through the life of the assembled product. Elec- troless nickel electroless palladium immersion gold (ENEPIG) is a surface finish that has been demonstrated to have a variety of benefits, and to be suitable for both SnPb and Pb-free circuit card assembly. Extensive testing of ENEPIG has demonstrated the reliability of this surface fin- ish and resulted in the creation of an industry standard for its application: IPC-4556 Specifi- cation for Electroless Nickel/Electroless Palladi- um/Immersion Gold (ENEPIG) Plating for Print- ed Circuit Boards. When soldering to ENEPIG, all of the palladium is dissolved into the solder joint, and creates a palladium-rich region at the base (Pd source) of the solder joint. This palla- dium-rich microstructure can spall off, exhib- iting a columnar shape. With ever decreasing size of parts used in electronics assembly, the size of the solder joints correspondingly contin- ues to shrink, which causes the relative size of this palladium-rich microstructure to grow rela- tive to the overall joint thickness. In this study, the impact of industry standard Pd thicknesses on thin solder joints is evaluated through shear testing. Introduction ENEPIG is a multi-layer surface finish for cir- cuit boards. For soldering, the gold and Pd are applied to protect the solderability of the un- derlying Ni, and they are ultimately dissolved into the solder joint. Excessive amounts of these metals in the solder joint can potential- ly cause weakness of the solder joint, impact- ing reliability. There is an industry specification (IPC- 4556) that describes the requirements for EN- EPIG, and the finish has some use in the indus- try, but is limited due to availability and the in- creased cost of the printed board (PB) relative to other surface finishes. The plated layers of EN- EPIG are very flat, as all plated surface finishes Evaluation of the Use of ENEPIG in Small Solder Joints FEATU RE

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