Studies were conducted on Pb-free solder interconnects after aging for up to 1,000 h at temperatures of up to 150°C and submitting to drop testing. Cracks were shown to propagate along interphase boundaries (bulk solder/Cu 6 Sn 5 , Cu 6 Sn 5 /Cu 3 Sn, Cu 3 Sn/Cu, and bulk solder/Ag 3 Sn) and along the bulk solder. Intergranular propagation in the solder was rarely observed, although cross sectioning showed evidence of that particular path. There is a transition in preferred crack propagation with aging time and temperature, from the intermetallic layer to the bulk solder. Sulfur was found in the fracture surfaces and may be a weakening agent in these solders.
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