Effect of Multiple Reflow Cycles on Ball Impact Responses of Sn-4Ag-0.5Cu Solder Joints with Immersion Tin Substrate Pad Finish

We report in this work the effect of multiple reflow cycles on ball impact test (BIT) responses and fractographies obtained at an impact velocity of 500 mm/s on Sn-4Ag-0.5 Cu solder joints. Solder balls were mounted on Cu substrate pads with immersion tin surface finish, supplied by two vendors. For these particular test vehicles and test conditions, fracture near the interface between the interfacial Cu₆Sn₅ intermetallic compound (IMC) and Cu pad was identified as the only failure mode induced by BIT. Measurement results indicate that BIT characteristics in general degrade as the number of reflow cycles increases. Furthermore, scanning electron microscopy observations show that the thickness and grain size of interfacial Cu₆Sn₅ increase with increasing number of reflow cycles. This correlation confirms the familiar notion that a thicker Cu₆Sn₅ degrades the interfacial strength.