Thermal fatigue damage of quad flat pack leads and Sn-3.5Ag-X (X=Bi and Cu) solder joints

Quad flat pack (QFP) leads/Sn-3.5Ag-X (X=Bi and Cu) joint was thermally cycled between 243 K and 403 K or 273 K and 373 K, and relationship between thermal fatigue of QFP solder joint and isothermal fatigue properties of bulk solder has been discussed. Both metallographic examination and mechanical pull test were preformed 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 joints. The behavior observed here reflects the isothermal fatigue properties of bulk solder because thermal fatigue crack initiates at the surface of the solder fillet and propagates within the fillet in an early stage of fatigue damage.