Effects Of Alloy Composition And Process Conditions On Microstructure Of Au-Sn Alloys

Gold-tin eutectic alloy solders are widely applied in microelectronic device packing because of its advantages such as high brazing strength and thermal conductivity,great creep resistance and fatigue resistance.It was found recently that gold-tin eutectic alloys composed of two compounds exhibit good thermal deformation ability after they are undercooled and annealed.However,microstructure evolution of gold-tin eutectic alloys through undercooling solidification and annealing is little studied.In the present thesis,the undercooling methods of gold-tin eutectic melts were explored first.Microstructure evolution of near-eutectic gold-tin alloys was then investigated under different conditions of undercooling and annealing.The effects of additions of third elements,Ag and In,on microstructure evolution during undercooling and annealing were investigated.In the experiments,the undercooling behavior of gold-tin eutectic melts was investigated by using different fluxing reagents first.The near-eutectic gold-tin alloys were also undercooled by quenching their molten melts onto a chill substrate.Phase constituents and microstructure of solidified alloy samples were examined using X-ray diffraction and scanning electron microscopy.Microstructure of the annealed alloy samples was examined.Additionally,effects of alloying additions,Ag and In on microstructure evolution of gold-tin alloys during undercooled solidification and annealing were studied.The main conclusions are as follows:(1)The exact eutectic composition of gold-tin alloy is determined to be Au-20.4Sn by differential scanning thermal analysis and microscopic studies.The undercooling experiment showed that maximum undercooling of 15 K,27 K and 28 K were achieved in gold-tin eutectic alloys using silicone oil,ternary molten salt,ternary molten salt and resin as the fluxing reagent,respectively.The best method is using 7.5NaCl-23.9KCl-68.6ZnCl₂ ternary molten salt and a small amount of rosin as the fluxing reagent.In this method,the obtained maximum undercooling in near-eutectic gold-tin alloys is 20?28 K(about 5%of the melting point).(2)The solidification structure of Au-20.4Sn alloy is composed of lamellar eutectics at low undercoolings.An entangled two-phase structure is formed together with lamellar eutectics as undercooling rises.The solidification structure consists mainly of anomalous eutectic at high undercoolings.The primary phase of Au-18Sn and Au-19Sn hypoeutectic alloys is dendritic Au₅Sn phase.There is an entangled two-phase structure in addition to the primary dendrites and lamellar eutectics.As undercooling rises,the size of dendrites decreases.At high undercoolings,Au-18Sn hypoeutectic alloys are solidified into a divorced eutectic structure,and Au-19Sn hypoeutectic alloys are solidified into a fully anomalous eutectic structure.The primary phase of Au21Sn and Au-22Sn hypereutectic alloys is dendritic AuSn,and an entangled two-phase structure is formed in interdendritic regions.The anomalous eutectic structure is also formed at high undercoolings.The anomalous eutectic with larger particle sizes is due to ripening of the entangled structure,which is formed by the uncoupled growth of Au₅Sn and AuSn.The anomalous eutectic with smaller particle sizes is due to remelting and ripening of the lamellar eutectics.(3)The lamellar eutectics are evolved into the entangled structure and anomalous eutectic during annealing.It is easy to obtain anomalous eutectic,of which the matrixes are connected together at high annealing temperatures because spheroidization of regular lamellar eutectics is promoted significantly.The high annealing temperature also makes the transformation of chemically ordered ?'-Au₅Sn into disordered ?-Au₅Sn.Both of the two changes in the microstructure are supposed to be beneficial to improvement of the deformation ability of the matrix.(4)The addition of third elements,Ag or In,changed the solidification behavior of the Au-20.4Sn eutectic alloys.At high undercoolings,the addition of the third element refines the solidification microstructure significantly.During the annealing,the third elements reduce the growth rates of grains.Their additions also suppress the solid-solid transformation of the gold-tin eutectic alloys.Thus,they help retain the chemically disordered ?-Au₅Sn phase,which is supposed to be beneficial to the improvement of the deformation ability of the gold-tin alloys.