Effect of Additional Element (Bi and Cu) on the Thermal Fatigue Strength of QFP/Sn-3.5Ag Solder Joint

摘要

Quad Flat Package (QFP) Leads/Sn-3.5Ag-X (X= Bi and Cu) joint was thermally cycled between 243K and 403k or 273K and 373K, and both metallographic examination and mechanical pull test were performed to evaluate thermal fatigue damage of the joint. The addition of bismuth drastically degrades the thermal fatigue resistance of Sn-3.5Ag solder. On the other hand, the pull strength of Sn-3.5Ag-Cu solder joints slightly decreased with increasing number of thermal cycles, though it still remains higher in comparison to that for conventional Sn-37Pb or bismuth containing solder joint. The behavior observed here reflects the isothermal fatigue properties of bulk solder, because thermal fatigue crack initiates at the surface of solder fillet and propagates within the fillet in an early stage of fatigue damade. Furthermore, the lead phases lying at the interface between lead-frame and bismth containing solder joint may promote the crack propagation at the interface, resulting in the extemely low thermal fatigue resistance of the joint.