Effect of aging on fatigue crack growth at Sn-Pb/Cu interfaces

The effect of isothermal aging on fatigue crack growth behavior at the Sn-Pb solder/Cu interface was examined, with emphasis on the role of interfacial microstructure. Flexural peel interface-crack specimens were made from the joints of eutectic Sn-Pb solder and Cu and were further aged at 443K for 7 and 30 days. Kinetics of fatigue crack growth along the solder/Cu interfaces were measured from flexural peel specimens as a function of strain energy release rate. Aging was found to induce not only microstructural changes in the solder and at the interface, but also degradation in fatigue crack growth resistance of the interface from the fatigue threshold to the fast fracture. The fatigue threshold decreased from 25 to 20 J/m exp 2 after aging for 7 days and to 10 J/m exp 2 following aging for 30 days. The degradation in the fatigue crack growth resistance is related to the formation of a Pb-rich layer at the interface.