Failure mechanisms and crack propagation paths in thermally aged Pb-free solder interconnects

Studies were conducted on Pb-free solder interconnects after aging for up to 1,000 h at temperatures of up to 150 degrees C and submitting to drop testing. Cracks were shown to propagate along interphase boundaries (bulk solder/Cu6Sn5, Cu6Sn5/Cu3Sn, Cu3Sn/Cu, and bulk solder/Ag3Sn) 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.