Role of minor Zn addition in the interfacial reaction between lead-free solders and Cu

Sn-Ag-Cu alloys are considered to be the most promising lead-free alloy systems in packaging industry. However, it required the addition of minor alloy elements to improve the properties of the SnAgCu solders to pass various reliability tests and Zn is one of the candidates for the elements. In this study, the effects of Zn content and solder volume on the interfacial reaction will be explored. Experimentally, bulk solder ingots (6 g) and solder joints (10 mg) with different Zn concentration were used to study the Zn concentration effect as well as the solder volume effect. Interestingly, the intermetallics formed at the interface were strongly dependent on the Zn concentration. When Zn concentration was 0.5 wt.%, the reaction product was Cu₆Sn₅-based compound. As the Zn concentration increased to 2 wt.%, the reaction product switched to Cu₅Zn₈-based compound. In addition, the reaction product was observed also depended on solder volume. In solder joints, the volume effect should be considered as the volume of solder determines the total available Zn. As the size of the joints shrink, the supply of Zn becomes even more limited, and the variation in Zn concentration becomes more critical. The available Zn in solder was depleted as the Zn-containing IMC(s) growth, and the equilibrium phase at interface would shift. In this study an extra Sn-Zn-Cu solder would also used to investigate the effect of Cu addition in Sn-Zn solders. In Sn-Zn-Cu/Cu reaction, Cu atoms reacted with Zn to become CuZn particles inside the solder thus reduced the content of Zn in solder. Both Cu content and Zn content have to be taken into account as Zn-doped SnCu and SnAgCu solders are used in actual products.