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74 SMT Magazine • October 2017 daisy-chain resistance measurement that also showed no increase in resistance after 200 ther- mal cycles. Generally, more than 20% resistance increases, within a short time increasing to mega ohm resistance, are associated with crack- ing and opens in the solder joints. Characterization of the solder/ENEPIG in- terface is critical to be established after cycling, but before a subsequent isothermal aging for comparison. Figure 7 illustrates representative SEM images with SEM EDX/EDS (energy disper- sive x-ray) elemental analysis (e.g., showing ex- istence of Ni, Cu, Sn, and Pb) at the solder inter- face after 200 thermal cycles (-55°C to +125°C). 200 TC (-55°C/+125°C) + 324 hr Aging (125°C) for LGA1156 To determine microstructural changes due to isothermal aging, thermally cycled LGA1156, as well as the half of the unused samples from the microsectioned parts were subjected to iso- thermal aging at 125°C for 324 hours. A section from the cut sample was purposely used to elim- inate the contribution of manufacturing vari- ables on the ENEPIG/solder interface interme- tallic growth. This enabled only the contribu- tion of isothermal aging for direct comparison of the microstructural changes at the interface before and after aging. Note that this approach cannot be used for thermal cycling since half of the package induced a different CTE mismatch than a full size assembled package. The test re- sults could not have captured the effect of the CTE mismatches. Figure 8 compares represen- tative SEM images of samples before and after isothermal aging. No significant changes appear at this magnification due to the additional 324 hours isothermal aging. RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING Figure 6: Representative optical and SEM images of LGA1156 solder joints at 200 thermal cycles (-55°C to 125°C). Acceptable LGA solder joint quality with non-solder mask defined (NSMD) or pad defined configurations are also apparent.