Research On Melt-mixing Solidification Structure Of Au-20Sn Eutectic Alloy Based On EBSD Analysis

Au-20Sn (mass fraction,%) eutectic alloy is a kind of solder. It has excellent properties, such as good electrical conductivity, outstanding brazing flowability, high brazing intensity and good creep resistance, so extensively used in high reliability microelectronics and optoelectronics packaging. Processing and properties of alloy are closely related to its solidification structure. Existing researches have shown that, the microstructure and properties of Au-20Sn alloy can be improved availably by melt mixing method, meanwhile some Au-20Sn regular lamellar eutectic transformed into anomalous eutectic. So it has important theoretical and practical significance to further study microstructure evolution and the formation mechanism of anomalous eutectic during melt mixing.Using high-low temperature melt-mixing method, the effect of temperature of high-temperature melt (denoted by TH) on solidification structure and solidification structure evolution of Au-20Sn eutectic alloy are deeply studied in this paper. Meanwhile the crystallographic orientations of primary phases, regular lamellar eutectic, grid transition structure, anomalous eutectic and spheroidized structure after homogenization heat treatment of Au-20Sn fine fully lamellar eutectic were analyzed by electron back-scattering diffraction (EBSD). So then, anomalous eutectic formation mechanisms in the melt mixing Au-20Sn alloys were further studied. The main conclusions are as follows.(1) TH effects on the melt-mixing solidification structure of Au-20Sn eutectic alloy obviously.①When TH is relatively low (370℃～390℃), spherical ξ’-Au5Sn primary phases are formed in solidification structure. With the increasing of TH, regular lamellar eutectic transforms into grid transition structure, and then grid transition structure transforms into anomalous eutectic. When TH is377℃, thin grid eutectic is formed. When TH is382℃or386℃, a mixture of anomalous eutectic plus grid eutectic is formed, in which the bridging between ξ’-Au5Sn and δ-AuSn phases decline gradually and the eutectic phases coarsen and grow up obviously.②When TH is relatively high (392℃～410℃),5-AuSn primary phases of faceted growth and a mixture of anomalous eutectic plus grid eutectic are formed in the solidification structure. However, with the increasing of TH, two eutectic phases further coarsen and ripen, and a kind of typical globular anomalous eutectic is formed finally.(2) The optimized method of the sample preparation for Au-20Sn eutectic alloy on EBSD is that mechanical polishing at low speed (100～200r/min), and polishing with SiO2suspension of WO.02for a long time finally. In this way, samples can meet the requirements of EBSD test, and the indexed rates can be more than85%.(3) EBSD analysis of melt-mixing solidification structure of Au-20Sn alloy shows that:①When TH is relatively low (370℃～390℃), the regular lamellar eutectic, grid eutectic and anomalous eutectic in solidification structure keep similar characteristics of crystallographic orientation. Both δ-AuSn phases and ξ’-Au5Sn phases are well orientated. These indicate that the anomalous eutectic results from the overheating, remelting, coarsening of lamellar eutectic and the rotation of δ-AuSn fragment.②When TH is relatively high (392℃～410℃), ξ’-Au5Sn phases and δ-AuSn phases in lamellar eutectics are all well orientated, but ξ’-Au5Sn phases in grid eutectic exhibit scattered orientations. Even orientations of ξ’-Au5Sn phases in anomalous eutectic are nearly random. This kind of anomalous eutectic is a product of independent nucleation and growth of eutectic phases in supercooled liquid.(4) EBSD analysis of spheroidized structure which transformed from fully fine lamellar eutectic of Au-20Sn alloy after homogenization heat treatment shows that:Driven by the concentration gradient, elements between ξ’-Au5Sn and δ-AuSn layers diffuse mutually.Lamellar structure become disconnected and coarse, gradually transforms into spheroidized structure. The similar orientation characteristics and micro-morphology of this spheroidized structure and grid eutectic when TH is relatively low (370℃～390℃), also further illustrated that this kind of anumalous eutectic results from the remelting and ripening of lamellar eutectic.