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76 SMT Magazine • October 2017 Higher magnification was required to de- termine microstructural changes at the solder/ ENEPIG interface. Figure 9 compares the higher magnification SEM images after 200 TC and after additional 324 hours of aging. It also in- cludes the EDX/EDS elemental mapping analy- ses performed at the solder/ENEPIG interface showing the key elements to be tin and lead distribution. Grain growth is apparent, which is normal for aging. There is no abrupt change due to isothermal aging. 200 TC (- 55°C/+125°C) + 100 TS (-65°C/+150°C) for LGA1156 To determine the effect of higher tempera- ture cycling ranges on interface microstruc- tural changes, the thermally cycled LGA1156 assemblies (200 TC, -55°C/+125°C) were sub- jected to 100 more severe thermal shock cy- cles (TS, -65°C/+150°C). Contrary to the ther- mal cycle condition, which was performed in one chamber, the thermal shock cycle used two chambers and test vehicles were shuttled between the hot and cold chambers. A rep - resentative thermal cycle profile is shown in Figure 10. Only three TVs were subjected to this thermal shock cycle regime. All TVs failed between 20 and 60 cycles, but continued to 100 TS to possibly cause further opening of the solder joints. The one with an open after 60 TS was selected for cross-sectioning and the microstructural evaluation of solder and EN- EPIG interface. Figure 11 shows representative SEM images of the LGA assembly after 200 TC +100 TS. The cross-sections cover the areas with microvia, which was one layer down to provide daisy- chain pairs for continuity test. Figure 12 shows the SEM microstructure with EDS elemental analysis detailing the microstructural changes at the solder/ENEPIG interface. Finally, three SEM images, with their EDS elemental analyses, comparing the effect of additional aging and thermal shock cycles are shown in Figure 13. They cover SEM images af- ter 200 TC, after 200 TC plus 324 hours of aging at 125°C, and after 200TC plus 100TS. No ap- parent anomalies were observed. RELIABILITY OF ENEPIG BY SEQUENTIAL THERMAL CYCLING AND AGING Figure 9: Representative SEM photomicrographs with EDS elemental analysis of LGA1156 solder conditions after 200 thermal cycles (-55°C/+125°C), left, and after subsequent 324 hours of isothermal aging at 125°C (right). Samples were the same—half used after TC—and another half used for subsequent aging, polishing, and SEM imaging, and elemental analysis.