Effect of Ag on ripening growth of Cu6Sn5 grains formed between molten Sn-xAg-0.5Cu solders and Cu

In microelectronic packaging, the reliability of Pb-free solder joint is critically influenced by the interfacial reactions with under bump metallurgies (UBMs), and therefore controlling the intermetallic compound (IMC) formation and growth is recognized as a very important factor for improving the reliability of the solder joints. A silver (Ag) alloying element is broadly used in Pb-free solder alloys since it is beneficially effective on the decrease of melting point, and the improvement of wettability and mechanical properties, such as tensile strength and fatigue. However, the Ag element is often neglected in controlling the IMC growth, because it does not form any IMCs with two common UBMs, such as Cu and Ni(P) UBMs. Actually, in the interfacial reactions between Sn-rich solders and Cu UBMs, Cu6Sn5 IMCs are mainly formed during reflow, and Cu3Sn IMCs are additionally formed between Cu6Sn5 IMCs and Cu UBMs during thermal aging. In addition, most of the previous studies have reported that the morphologies and growth of Cu6Sn5 IMCs are controlled by the reflow temperature or time, the cooling rate, the addition of minor alloying elements (e.g. Ni, Co and Zn), and the crystal orientation of Cu UBMs. In this study, the lateral growth (or ripening growth) of Cu6Sn5 grains formed between molten Sn-rich solders and Cu UBMs was investigated with Sn-0.5Cu, Sn-1.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu solders. After reactions with Cu UBMs, the size and morphologies of Cu6Sn5 grains were compared each other through the observation of top views. The Cu6Sn5 IMCs formed at the interface of Sn-3.0Ag-0.5Cu solders exhibited much smaller grains, but their thickness was not much different from those that formed in Sn-0.5Cu and Sn-1.0Ag-0.5Cu solders. In other words, the ripening growth of Cu6Sn5 grains during reflow was reduced - - in Sn-3.0Ag-0.5Cu solders. To understand the effects of Ag on ripening growth of Cu6Sn5 grains, the angles of two neighboring Cu6Sn5 grains formed at the interface of each solder were compared, and the interfacial energies between Cu6Sn5 grains and molten solders were discussed. In addition, the volume effect of Ag on the ripening growth of Cu6Sn5 grains and the effect of smaller Cu6Sn5 grains on reducing the formation of Cu3Sn IMCs during thermal aging were also reported.