Interfacial Reactions Of Sn-8Zn-3Bi-X And Sn-3.5Ag-xZn Lead-free Solder And Cu Substrate

The composition, the morphology and the thickness of the Intermetallic Compounds (IMCs) have a fundamental effect on the reliability of solder joints in electronic packaging. Cu is most commonly used for the under bump metallization in the flip chip technique in electronics assemblies. Therefore, it is necessary to investigate the interfacial reactions between solders and Cu substrates. In this research, we investigated the effect of the additions of Ag, Ni and Cu to the Sn-8Zn-3Bi solder on the interfacial reactions with Cu substrates during soldering at 250 oC and aging at 150 oC for different durations. And The effects of Zn (1, 3 and 7 wt.%) additions to Sn-3.5Ag solder and various reaction times on the interfacial reactions between Sn-3.5Ag-xZn solders and Cu substrates during liquid-state aging were also investigated in this study.During soldering process, the Cu₅Zn₈ and CuZn₅ phases formed at the Sn-8Zn-3Bi/Cu interface. And the Cu₅Zn₈ and AgZn3 phases formed at the interfaces of the solder with addition of Ag. With the increasing content of Ag, the thickness of the Cu₅Zn₈ layer decreased gradually, which indicated that Ag addition to the Sn-8Zn-3Bi solder could suppress the growth of the interfacial IMC layer. When the solder was added with Ni, only the Cu₅Zn₈ phase was found at the interface. Moreover, the thickness of the IMC layer at the Sn-8Zn-3Bi/Cu interface was thicker than that at the Sn-8Zn-3Bi-xNi(x=0.5, 1)/Cu interface. The reduction effect of Ni addition to the solder on the interfacial reaction might be attributed to the formation of the Ni5Zn21 IMC in the solder bulk, which effectively suppressed the diffusion of Zn atoms to the interface to react with Cu. When the solders were added with Cu, Cu₅Zn₈ particles formed in the solder bulk, which retarded the diffusion of Zn to the interface and suppressed the reaction between Cu and Zn. In all samples, the interfacial IMC layer grew thicker with the reaction time following a parabolic law, which suggested the interfacial reactions were diffusion controlled. During the isothermal aging at 150 oC, the Cu₅Zn₈ IMC layer at Sn-8Zn-3Bi-X/Cu(X=1Ag, 1Ni, 1Cu) interfaces grew thicker with increase aging time. Meanwhile, Sn atoms diffused through the Cu₅Zn₈ layer to Cu substrate and reacted with Cu to form the Cu6Sn5 IMC. Comparing with Sn-8Zn-3Bi, the thickness of Cu₅Zn₈ IMC layer at Sn-8Zn-3Bi-X/Cu(X=1Ag, 1Ni, 1Cu) interfaces was thinner. It suggested that Ag, Ni and Cu addition to the Sn-8Zn-3Bi solder can effectively suppresse the growth of the Cu₅Zn₈ layer at the interfaces during aging.The composition and morphological evolution of interfacial IMCs changed significantly with the Zn concentration and reaction time. For the Sn-3.5Ag-1Zn/Cu couple, the CuZn and Cu6Sn5 phases formed at the interface. With increasing aging time, the Cu6Sn5 IMC layer grew thicker, while the CuZn IMC layer drifted into the solder and decomposed gradually. The Cu₅Zn₈ and Ag5Zn₈ phases formed at the interfaces of Sn-3.5Ag-3Zn/Cu and Sn-3.5Ag-7Zn/Cu couples. With increasing reaction time, the Cu₅Zn₈ layer grew and Cu atoms diffused from substrate to the solder, which transformed the Ag5Zn₈ to (Cu,Ag)5Zn₈. The Cu6Sn5 layer formed between the Cu₅Zn₈ layer and Cu is much thinner at the Sn-3.5Ag-7Zn/Cu interface than at the Sn-3.5Ag-3Zn/Cu interface. We measured the thickness of interfacial IMC layers and found that 3Zn addition to the solder was the most effective at suppressing the IMC growth at the interfaces.