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January 2017 • SMT Magazine 21 add only a small amount of gold to the solder joint and which, in combination with the Au present on the PB, would not be a major factor to integrity of the joint. Actual measurements of the part, however, indicated that it had a thick- er gold finish (14.3–15.5 µin), which resulted in the higher level of gold in the final solder joint. The high level of gold measured at the fracture surface indicates that could have played a role in the brittle nature of the failure. Conclusion 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 intermetal- lic 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 as - sociated with those lifted pads. Isothermal ag- ing of the test samples decreased the occur- rence 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 sam - ples 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 geom- etry). The specific cause of the brittle fracture on ENEPIG, however, was not determined in this study, and so the influence of the Pd con- tent 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. SMT References 1. Chen, Y.J.; Huang, K.Y.; Chen, H.T.; Kao, C.R., "Au and Pd embrittlement in space-con- fined soldering reactions for 3D IC applications," Advanced Packaging Materials (APM), 2013 IEEE International Symposium, pp. 102–112. 2. P. T. Vianco, "Lead-Free Surface Finishes: Compatibility with Assembly Processes and In- terconnection Reliability," (Edina, MN: Surface Mount Technology Association, SMTA Webinar, January, 2007), pp. 35–36 3. Pun, K.; Islam, M.N.; Tin Wing Ng, "ENEG and ENEPIG surface finish for long term sol- derability," Electronic Packaging Technology (ICEPT), 2014 15th International Conference, 2014, pp. 1–5. 4. Rowland, R. and Prasad, R., "Comparing PCB Surface Finishes and their Assembly Pro- cess Compatibility," Proceedings of SMTA Inter- national, Sep. 27–Oct. 1, 2015, Rosemont, Illi- nois. 5. Wolverton, M., "Quality, Reliability and Metallurgy of ENEPIG Board Finish and Tin- Lead Solder Joints," Proceedings of SMTA Inter- national, 2011, pp. 960–965. Editor's Note: This paper was originally pub- lished in the proceedings of SMTA International, 2016. C. Don Dupriest is LM Fellow at Lockheed Martin. Ben Gumpert is on the manufacturing engineer staff at Lockheed Martin. William Fox is on the material & process engineering staff at Lockheed Martin. EVALUATION OF THE USE OF ENEPIG IN SMALL SOLDER JOINTS

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