Preparation And Related Fundamental Research On AuSn20Eutectic Solder For Electronic Packaging

AuSn20solder has been widely used in high-end electronic products packaging because of its advanced properies in good thermal conductivity, excellent resistance to fatigue and creep. However, the AuSn20alloy is brittle under the common prepared process, resulting in the difficulty to fabricating into Foil-strip products to meet the requirements for electronic packaging. The AuSn20solders were prepared by the laminated-rolling and alloyed-aging process in this study. The Micro structural evolution and the properties of the Au/Sn laminate layer during the rolling and aging process were investigated with the SEM (EDS), XRD, and DSC experimental technologies, and the best process parameters were researched. We investigated the soldering property, reliability of the AuSn20solder joints by the experiments designed according to the real usility enviroment of the AuSn20solder, and analysed the joint failure factors. The growth behavior of the intermetallic compound (IMC) layer in the interface and the mechanical property of the joints were discussed by combining the theoretical calculation with the experimental verification. The main works are summarized as follows:(1) The Au/Sn composite belts were prepared by laminate-rolling process, and the effects of the laminate numbers and rolling technology on the microstructure, composition and properties of the Au/Sn composite belts were investigated. The results show that the Au/Sn strip with7layers and uniform microstructure, eutectic compositeon, and melting point can be prepared by the rolling technology with multi-pass and trail time reduction. In the laminate-rolling process, the inhomogeneous deformation of Au and Sn layers occurs under a large reduction (the maximum is47.6%), the Au content in the composite belt is higher than that of the eutectic composition, and the melting point rises. Under trail time reduction (small than23.8%) condition, the Au/Sn composite deformation is relatively uniform.(2) The aging process of Au/Sn composite belts completely alloying by diffusion was optimized based on the diffusion mechanism of the Au/Sn interface. After aging at solid-state temperature, the thickness of AuSn, AuSn2, and AuSru conpound IMC layer in Au/Sn interface grow gradually, and the Au and Sn layer were consumed accordingly. Combining the theoretical calculation with the experimental results, the optimum technology for AuSn20solder completely alloying is aging at220?for12h.(3) The AuSn20/Cu(Ni) solder joints were prepared during the reflow process.The mechanical reliability and the failure mode of the solder joints are related to the thickness and micro structure of the interfacial IMC layer according to the investigation of the microstructure and shear strength evolution of the joints after reflowing and aging. After soldering at310?, the (Ni,Au)3Sn2layer is fabricated at AuSn20/Ni interface. With the extension of reflowing time and the decrease of cooling speed, the shear strength of solder joint at room temperature declines gradually. During the process of aging at120?,160?and200?,(Au,Ni)Sn,(Ni,Au)3Sn2or (Au,Ni)Sn,(Ni,Au)3Sn2and (Ni,Au)3Sn compound IMC layers were formed at the interface.With the extension of aging time and increasing of aging temperature, the thickness of IMC layer gradually grows up, whereas the shear strength of solder joint decreases.(4) The growth behavior of IMC layers in AuSn20/Ni interface was investigated with diffusion couples. It shows that the thickness/grows following the formula:l=k(t/to)n. During aging at120?,160?, and200?, the proportionality coefficient k for the composite IMC layer was5.71×10-10m,3.24×10-9m, and1.34×10-8m, repectively, and the exponent n was0.514?0.471and0.459, repectively. This phenomenon indicates that the volume diffusion was contributed to the growth of the IMC layer at all aging temperature.(5) Coupling interface reaction occurred at the Cu/AuSn20/Ni joints during the reflow and aging process, which changes the structure and property of the soldering joints. After reflow at310?, the ?-(Au,Cu)5Sn cell structure formed at the Cu/AuSn20interface and the (Ni,Au,Cu)3Sn2quaternary IMC layer formed at the AuSn20/Ni interface. Copper atomics at the Cu/AuSn20interface passed through the AuSn20solder and took part in coupling reaction at the AuSn20/Ni interface. The kinetics of the growth for (Ni,Au,Cu)3Sn2quaternary IMC layer indicated that the coupling reaction of copper restrained the growth of Ni-Sn compounds layer. The shear strength of the joints has a trend to increase and then decreased with prolonging of reflow time. During the aging process, the AuCu and Au(Cu,Sn) layers forme at the Cu/AuSn20interface, which is controlled by the volume diffusion. The (Ni,Au,Cu)3Sn2grows by reaction diffusion at the AuSn20/Ni interface. The shear strength gradually declines with prolonging of aging time. The brittle shear fracture position moves to the copper interface with the growth of the IMC layer at the solder/Cu interface.