Dissolution behavior of Cu UBM in BGA structure Sn-3.0Ag-0.5Cu/Cu joints during liquid isothermal aging at and above the solder´s melting temperature

The dissolution of the under bump metallization (UBM) into the molten solder is inevitable during the soldering process. However, overly dissolved UBM can lead to the excessive growth of interfacial intermetallic compound (IMC) and change the solder´s composition as well as result in the formation of some bulk IMC in the solder matrix; consequently, mechanical properties of the solder interconnects can be deteriorated seriously. In this study, the dissolution behavior of Cu UBM in single-sided ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu joints, which were assembled by differential scanning calorimetry (DSC) incorporated into liquid isothermal reflow process, were studied systematically. Results show that, at a relatively low reflow temperature of 217°C, which is the melting onset temperature of Sn-3.0Ag-0.5Cu solder, the dissolution rate of Cu UBM is very slow and its consumption is mainly used for the growth of interfacial IMC. Clearly, a slight increase of temperature from 217°C to 218°C can significantly increase the degree of Cu UBM consumption during isothermal reflow process, the proportion of the Cu UBM dissolved into the solder matrix in the total consumption of Cu UBM also increases dramatically from 15.5% to 71.0%; as the isothermal temperature is increased to 230°C, the proportion just increases to 72.6% due to an almost complete saturation of Cu content in the solder matrix.