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The thermal fatigue endurance of completely lead-free 95.5Sn4Ag0.7Cu/ plastic core solder ball (PCSB) composite joint structures in low-temperature Co-fired ceramic/printed wiring board (LTCC/PWB) assemblies was investigated using thermal cycling tests over the temperature ranges of )40°C-125°C and 0°C-100°C. Two separate creep/fatigue failures initiated and propagated in the joints during the tests: (1) a crack along the intermetallic compound (IMC)/solder interface on the LTCC side of the joint, which formed at the high-temperature extremes; and (2) a crack in the solder near the LTCC solder land, which formed at the low-temperature extremes. Moreover, localized recrystallization was detected at the outer edge of the joints that were tested in the harsh ()40°C-125°C) test conditions. The failure mechanism was creep/fatigue-induced mixed intergranular and transgranular cracking in the recrystallized zone, but it was dominated by transgranular thermal fatigue failure beyond the recrystallized zone. The change in the failure mechanism increased the rate of crack growth. When the lower temperature extreme was raised from )40°C to 0°C, no recrystallized zone was detected and the failure was due to intergranular cracks.

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Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in .20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/ 95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from À40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.

No abstract