Influences Of Rare Earth Additions On The Microstructure And Interfacial Reaction Of SnBi Solder Alloys

The interfacial compound in the solder alloy acts as thermal, electronic and mechanical connections in microelectronic packaging process, and their morphology and distribution influence the solder connection directly. Solder-substrate reaction determines the reliability of the solder joint and then affects the lifetime of the whole electronic equipment. Rare earth (RE) elements, as active elements added into solder alloys, get many attentions by many researchers. In this paper, different amounts of RE elements were added into Sn30Bi0.5Cu, Sn35Bi1Ag and Sn57Bi1Ag lead-free solders respectively. Microstructure evolutions and the microhardness of the solder joints and the influence of RE additions on the growth of IMC in solder joints were investigated. The results are given as follows:The results indicated that with the additions of RE, the melting property of the SnBiCu–xRE and SnBiAg–xRE (x= 0, 0.25 and 0.5) solder alloys changed slightly. The microstructures of theβ-Sn phase, Cu6Sn5 and Ag3Sn intermetallic compounds (IMCs) in the solder matrices were refined due to the surface adsorption effect of RE. In addition, the microhardness of the solder alloys increased remarkably with the additions of RE because of the refined microstructures. Moreover, the formed Cu6Sn5 IMC layers in SnBiCu–xRE/Cu and SnBiAg–xRE/Cu solder joints were much thinner than those in the undoped-RE SnBiCu/Cu and SnBiAg/Cu solder joints after reflowing and aging tests. This is because the RE decreased the interfacial energy of Cu6Sn5 IMC layer and then suppressed the Cu atoms to react with Sn atoms to form Cu6Sn5 IMC layer. The results indicated that with the increase of RE additions, the solidus temperature decreased and the mushy temperature zone increased slightly in SnBiAg–xRE (x= 0, 0.25, 0.5, 0.75 and 1.0) solder alloys. The microstructures of the Bi–rich dendrites in SnBiAg–xRE solder alloys were refined by the additions of minor RE elements.However, too many RE elements added into solder matrices led to the formation of large RE(Bi,Sn)3 intermetallic compound (IMC) which weakened the adsorption effect of RE elements on the Bi–rich dendrites. In addition, the thickness of the Cu6Sn5 IMC layers of SnBiAg–xRE/Cu solder joints were reduced remarkably due to the adsorption effect of RE elements at the interfaces of the Sn element and the Cu6Sn5 IMC layer. Evolution of IMCs in SnBiAg-xRE/Cu solder joints during aging were investigated by scanning electron microscopy. The results indicated that the growth and thickness of Cu6Sn5 IMC layers were influenced by the absorption of RE on the grain boundaries of Cu6Sn5 phase. The refinement of Bi-rich phase increased the growth of IMC layers, while the reduction of surface energy of the Cu6Sn5 phases suppressed the growth of IMC layers after the RE additions. In addition, the thickness of Cu₆Sn₅ IMC layers in SnBiAg-0.5RE/Cu solder joints were the lowest during aging.The microstructure evolution of Sn58Bi/x (x=Cu, Ni/Cu, Au/Ni/Cu, Ag/Cu and Zn/Cu) solder joints were investigated. In the Sn58Bi/Au/Ni/Cu and Sn58Bi/Ag/Cu solder joints, the elongated AuSn4 and Ag₃Sn phases were formed in the solder matrices and the AuSn4, Ag3Sn and Ni₃Sn₄ phases were formed in the solder joints. The microstructure of the Bi-rich phase was refined and the size was decreased in the Sn58Bi/Au/Ni/Cu and Sn58Bi/Ni/Cu solder joints. In addition, the thicknesses of the IMC layers in the electroplated solder joints were thicker than the unelectroplated solder joint.