Study On Hot Compression Deformation Behavior And Processing Performance Of Au-20Sn Eutectic Alloy

Au-20Sn eutectic alloy solders are widely used in the high-reliability microelectronic device packaging due to their excellent electrical conductivity and thermal conductivity, high soldering strength and good thermal fatigue behaviors. But its inherent brittlement makes it difficult to be processing deformed, and the initial microstructures of alloy have great impact on processing performance. To do a further research on the relationship between initial solification microstructure and processing profermance will have important theoretical and practical significance for the brittle Au-20Sn eutectic alloys.In the study, Au-20Sn eutectic alloys with different initial solidification microstructure were preparaed and the thermal compression behavior of the different initial microstructure alloys were researched. The phase evolution and effect of initial microstructure on compression behavior were also carefully examed to explore the most excellent hot workability of solidification microstructure, namely fully fine lamellar microstructure. The further research was done on the deformation behavior and microstructure evolution in the Au-20Sn alloy during homogenization heat treatment, cold deformation and hot rolling. Main conclusions are drawn as follows:(1) The compressive behaviors at220℃of Au-20Sn alloy are very sensitive to the initial solidification microstructure. The yield strength and steady-state flow stress is38.3MPa and14.6MPa for the initial coarse lamellar (0.4μm) eutectic with the large dendrites, and only15.3MPa and6.9MPa for the fine fully lamellar (0.08μm), respectively. It indicates that the refinement of eutectic lamellar and the elimination of the primary phase are very helpful for the improvement of the deformation performance of the Au-20Sn eutectic alloy.(2) During compression, the large primary phase tended to be formed separate dendrites or split into small dendrites and some roselike primary phase was dissolved in the eutectic. The large primary dendrites in matrix had great impact on the process property. And it brought about the uniformed deforming, showing high yield strength and steady-state flow stress during the compression. When the primary phase changed to rose-like, its influence on deforming become limited.(3) During compression, the coarse lamellae and fine lamellae transformed to recrystallized structure which exhibited different grain size. The fine lamellae exhibit fastest recrystallizing and lowest yield strength and steady-state flow stress in compressive test. And the coarse lamellae showed less voluntary to be deformed and tend to recrystallized structure which exhibited larger grain size comparing the fine lamellae. The coarse lamellae and fine lamellae both transformed to recrystallization structure through three steps:lamellar boundary coarsen, boundary structure calculated and lamellar colony refined, and then fully equalaxed structure formed.(4) The microstructure evolution of lamellae structure under spheroidizing heat treatment is:lamellae coarsen, lamellae spheroidized through fusing and spheroidized structure grouth. The optimum heat treatment condition of Au-20Sn eutectic alloy is annealling at240℃for80mins.(5) In cold deforming, the Au-20Sn alloy showed small plastic deformation and brittle inherent. During cold rolling, the crack went along the AuSn phase and the fracture morphology was the pattern of cleavage fracture, showing typical transgranular fracture properties. In room temperature, the alloy showed less than3%rolling deformation. Indentation experiment at room temperature showed that the plastic flow deformation zone occurred with small load, while with large load, cracks and slip bands occurred, and cracks expand and extend along the slip bands.(6) In hot deforming, the Au-20Sn alloy showed excellent performance. With a proper intermediate annealing, the alloy could be continuously rolled to0.1mm foil. The fracture mechanism of Au-Sn eutectic alloy in the hot working belongs to quasi-cleavage fracture, because the ζ’-Au5Sn phase had ductility, in contrast, the δ-AuSn phase are brittle structure. The cracks initiated around ζ’-Au5Sn phase.