Solid-State Spalling of Ag3Sn in an Eutectic SnPb Solder Joint with an Ag Thin Film/Ge Cell

Spalling of intermetallic compounds is a common and detrimental phenomenon in the wetting reaction between Sn-based solders and under-bump metallizations; however, it is still not clear whether spalling will appear in solid-state aging. Here, the solid-state spalling of Ag-Sn intermetallic compounds at the interface between an eutectic Sn-Pb solder and an Ag/Ge substrate are examined during aging. Scanning electron microscopy (SEM) observations showed that Ag-Sn intermetallic compounds departed from the Ge substrate after the silver thin film was completely depleted during solid stateaging. This solid-state detachment of Ag-Sn intermetallic compounds was substantiated using deep etching. Despite this significant change in interfacial architecture caused by spalling, there is no considerable decline in the shear strength, implying that the interfacial joining was still effective. Thermodynamic analysis suggests that the spalling may be attributed to the open rapid diffusion paths among scalloped Ag3Sn in solid-state aging, limited thickness of the Ag layer and poor adhesion between Ag3Sn and the Ge substrate.