Authored By:
Ben Gumpert, William Fox, C. Don Dupriest
Lockheed Martin
Ocala, FL, USA
Grand Prairie, TX, USA
Summary
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. Electroless 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 finish and resulted in the creation of an industry standard for its application: IPC-4556 Specification for Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Plating for Printed 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 palladium-rich microstructure can spall off, exhibiting a columnar shape. With ever decreasing size of parts used in electronics assembly, the size of the solder joints correspondingly continue to shrink, which causes the relative size of this palladium-rich microstructure to grow relative to the overall joint thickness. In this study, the impact of industry standard Pd thicknesses on thin solder joints is evaluated through shear testing.
Conclusions
The thick solder mask present on the PBs in this test not only had an impact on the solder joint geometry, it also had an apparent impact on solder volume applied during the card build, which affected the resulting metal content of the solder joints. This resulted in a lower level of Pd than originally intended for this study. As expected, the Pd formed a lamellar intermetallic with the Sn in the solder, but did not create a continuous layer due to the limited amount of Pd.
Incomplete curing of the PB appears to have contributed significantly to lifted pads in the unaged test samples, and brittle fracture of the solder joints on ENEPIG was often associated with those lifted pads. Isothermal aging of the test samples decreased the occurrence of both lifted pads (on both HASL and ENEPIG samples) and brittle fracture failure (on ENEPIG samples). It should be noted, however, that bulk solder failures on the ENEPIG samples occurred at the narrow point of the solder joint (the middle of the hourglass shape), and this solder joint geometry may have an impact on the failure point.
The results confirm that the shear strength of SnPb solder joints on ENEPIG is similar to that of solder joints on HASL (when the results of this study are adjusted for solder joint geometry.) The specific cause of the brittle fracture on ENEPIG, however, was not determined in this study, and so the influence of the Pd content is not known.
The use of a Ag bearing solder had a small impact of shear strength, but there was not a significant difference of this impact when HASL and ENEPIG are compared.
Initially Published in the SMTA Proceedings
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