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78 SMT Magazine • October 2017 Summary ENEPIG PCB surface finish reliability char- acteristics for RoHS solder joint assemblies have been the subject of numerous papers showing favorable results. However, reliability data for tin-lead solder is scarce—sometimes showing negative effects on reliability. This paper ad- dressed the ENEPIG with tin-lead for high reli- ability electronics hardware to see if there were any apparent issues. For this reason, many ac- celerated thermal cycles and shocks along with isothermal aging were performed using the LGA1156 assemblies to determine the integrity of the solder/ENEPIG interface after each envi- ronmental exposure. A summary of findings is listed below: • After 200 thermal cycles (TC, -55°C to +125°C), no failures were detected by daisy-chain monitoring and no microstructural anomalies occurred at the ENEPIG/solder interfaces • After 200 TC plus 324 hours of aging at 125°C, the ENEPIG/solder microstructural changes at the interfaces were within normal expectations • After 200TC plus 100 thermal shock cycles (TS, -65°C to +150°C), the LGA1156 assemblies failed in the daisy-chains showing resistance opens, but no significant degradations were detected at the ENEPIG/solder interfaces by cross- sectioning and SEM elemental evaluation The preliminary test results showed the ac- ceptance of the ENEPIG PCB finish with tin- lead solder for short-term duration in high-re- liability applications. It also revealed the short- term thermal cycle reliability acceptability of LGA1156 assemblies under standard harsh TC (-55°C to +125°C), but not under a more severe thermal shock cycle (TS, -65°C to +150°C). Acknowledgments The research described in this publication is being conducted at the Jet Propulsion Labora- tory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Copyright 2016 Califor- nia Institute of Technology. U.S. Government sponsorship acknowledged. The author would like to acknowledge the support of the team in this work, especially D. Hunter, A. Mehta, and E. Bradford. The author also extends his appreciation to the program managers of the National Aeronautics and Space Administration Electronics Parts and Packaging (NEPP) Program. SMT Figure 12: Representative SEM photomicrographs image with EDS elemental analysis of LGA1156 solder conditions after 200 thermal cycles (-55°C/+125°C) and an additional 100 thermal shock cycles (-65°C/+150°C). RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING