Studying Of The Solidification Behaviors Of Undercooled Au-Sn Alloy

Au-Sn alloy solder with the melting point between 280 ~ 360? has important application in the field of microelectronic device packaging becase to its advantages of good weldability and thermal fatigue resistance.Under the condition of conventional coagulation,The microstructure of Au-Sn alloy is bulky,which seriously affects the processing properties of the alloy.But the microstructure of undercooled Au-Sn alloy will be refined,which effectively improve thermal performance of the alloy.Therefore,this paper studies the microstructure and corresponding evolution mechanism of undercooled Au-Sn eutectic alloy(AuSn20.5),Au-Sn hypoeutectic eutectic alloy(AuSn20)and Au-Sn hypereutectic alloy(AuSn24?AuSn26).The study is important to understand the microstructure and evolution mechanism of undercooled Au-Sn alloy.Applying cyclical superheating,the microstructure and corresponding evolution mechanism of the undercooled AuSn alloy were studied: the solidification curve of undercooled Au-Sn alloy was monitored by the infraed pyrometer,the microstructure of undercooled Au-Sn alloy was analyzed by SEM.And using the LZ model,BCTmodel,Karma model and dendrite remelting model,the microstructure's evolution mechanism of undercooled Au-Sn alloy was comprehensively analyzed.Eventually,the results are obtained as follows:? ? Studying of the solidification behaviors of undercooled eutectic alloy(AuSn20.5):(1)Under the condition of cold coagulation(?T<32 K),lamellar eutectic will precipitate from in undercooled eutectic alloy.With increasing of the undercooling,the spacing of the layers decreases to 2 ?m when undercooling is 32 K.(2)The theory calculated results adopted by the LZ model and BCTmodel show that when the undercooling is below 35 K,a fully eutectic structure will form.??Studying of the solidification behaviors of undercooled AuSn20 alloy :(1)Under the condition of cold coagulation(?T<30 K),the microstructure of AuSn20 alloy is composed of ?'-Au5 Sn primary phase and eutectic structure.(2)Through the calculation of the LZ eutectic growth model and the BCT dendrite growth model,it is found that when the undercooling is below 44 K,the ?'-Au5 Sn phase become the primary phase.The refining mechanism of the ?'-Au5 Sn primary phase is mainly dendrite remelting and dendrite breaking.??Studying of the solidification behaviors of undercooled AuSn24 and AuSn26 alloy:(1)Under the condition of cold coagulation(?T<30 K),the microstructure of AuSn24 and AuSn26 alloys are composed of ?-AuSn primary phase and eutectic structure.And some eutectic layers in the AuSn24 alloy have evolved into a club-like tissue.Non-regular eutectic structures were found in AuSn26 alloy.(2)Through the calculation of the BCT dendrite growth model,it is found that when the undercooling is small,?-AuSn phase precipitates from supercooled AuSn24 alloy melt and AuSn26 alloy melt as primary phase.The refining mechanism of the ?-AuSnprimary phase is mainly dendrite remelting.