Retardation of thermomigration-induced Cu substrate consumption in Pb-free solder joints by Zn addition

Abstract

Thermomigration (TM) caused by temperature gradient in micro solder joints is recently recognized as a crucial reliability concern for 3D packaging technologies. In the present study, the interfacial reactions in Cu/Pb-free solder/Cu solder joints have been in situ investigated under a simulated temperature gradient of 508 °C/cm by synchrotron radiation real-time imaging technology. Serious TM-induced Cu substrate consumption at hot end and abnormally fast TM-enhanced intermetallic compound (IMC) growth at cold end are clearly observed in pure Sn and SnAgCu solder joints. However, significant retardation of the TM-induced Cu substrate consumption and the TM-enhanced IMC growth is realized by introducing Zn into the solders. The dissolution rate of the hot end Cu substrate and the growth rate of the cold end IMCs both decrease about three times as the Zn content increases from 0 wt% to 9.0 wt%. The improvement in TM resistance by Zn addition is attributed to the formation of Cu5Zn8 or Cu6(Sn,Zn)5 IMC at the hot end, the weakened diffusion of Cu atoms and the lowered driving force for IMC precipitation at the cold end. The molar heat of transport Q* of Zn is calculated to be 2.31 kJ/mol.