Minor Ag induced shear performance alternation in BGA structure Cu/SnBi/Cu solder joints under electric current stressing

In this work, the effect of minor Ag addition on shear performance of ball grid array (BGA) structure Cu/SnBi/Cu solder joints under electric current stressing was investigated. The results show that the shear strength of solder joints decreased with increasing current density, and the effect of Ag addition on shear strength changed from an elevating state to a deteriorating state. The elevating effect of Ag addition was due to the dominance of Sn/Bi phase refinement and Ag3Sn dispersion, which was weakened gradually with the increase in current density. The deteriorating effect of Ag addition was attributed to the more inhomogeneous distribution of current density and higher thermal gradient induced by the finer phase of the solder matrix as well as the consequent severer strain mismatch at the Sn/Bi phase interface. The finer Sn/Bi phase was also conductive to easier lattice atom migration under current stressing, resulting in a rapid decrease in shear strength. Moreover, the dominant factor on the shear strength reduction tended to change from Joule heating to athermal effect of current stressing. The fracture of solder joints indicated an insignificant influence by the Ag addition, which all occurred in the solder matrix in a ductile mode.