【作者】 杨莹；

【导师】 毛勇；

【作者基本信息】 云南大学 ， 材料加工工程， 2015， 硕士

【摘要】 Au-20Sn(质量分数,%)共晶合金作为一种钎焊材料,具有导电性及钎焊流散性好、钎焊强度高和抗蠕变性能优良等优异特性,广泛用于高可靠性微电子和光电器件封装。合金的加工使用性能与其凝固组织密切相关。已有的研究表明,熔体混合工艺可以有效地改善Au-20Sn合金的初始凝固组织和性能,且将发生规则层片共晶(regular lamellar eutectic)向非规则异常共晶(anomalous eutectic)的组织转变。所以,深入研究Au-20Sn合金在熔体混合过程中的组织演变规律及非规则共晶的形成机制对该合金的加工制备及应用具有重要的理论和实际意义。本文采用高低温熔体混合法,深入研究了高温熔体温度(记为TH)对Au-20Sn共晶合金凝固组织的影响及其组织演变规律；采用电子背散射衍射技术(EBSD)深入研究了Au-20Sn共晶合金熔体混合凝固组织中初生相、规则层片共晶、网格状过渡组织及非规则共晶的晶体学取向特征,并对比分析了细小全层片共晶均匀化热处理后的球化组织及其晶体学取向特征,从而探讨了Au-20Sn合金熔体混合非规则共晶的形成机制,得到了以下结论：(1)TH对Au-20Sn合金熔体混合凝固组织产生了显著影响。①当TH较低时(370℃～390℃),凝固组织中形成了近球状ξ’-Au5Sn初生相。随着TH的升高,共晶组织发生了从规则层片共晶到网格状过渡组织,再到非规则共晶组织的转变。当TH为377℃时,得到细小的网格状过渡组织；当TH为382℃和386℃时,得到非规则共晶和网格状共晶的共存组织,但共晶组织中ξ’-Au5Sn和δ-AuSn相的相互桥连逐渐减弱,两相明显粗化和长大。②当TH较高时(392℃～410℃),凝固组织中形成了具有小平面生长特性的δ-AuSn初生相。共晶组织主要为非规则共晶和网格状过渡组织的共存组织。但随着TH的升高,共晶两相粗化和熟化的程度加强,得到较典型的块球状非规则共晶组织。(2)实验探索出了优化的Au-20Sn合金EBSD制样方法,即在低转速下进行(100-200r/min)机械抛光,并用W0.02的SiO2悬浮液进行长时间最终抛光,此种方法制得的试样能够满足EBSD测试的要求,标定率为85%以上。(3)对Au-20Sn合金熔体混合凝固组织的EBSD取向分析表明：①当TH较低时(370℃～390℃),凝固组织中规则层片共晶、网格状共晶、非规则共晶均具有相似的取向特征,共晶ζ’-Au5Sn和δ-AuSn相的晶体学取向都非常集中。这表明这种非规则共晶的形成机制为层片共晶过热——重熔——粗化及δ-AuSn碎片的旋转。②当TH较高时(392℃～410℃),规则层片共晶组织中ζ’-Au5Sn和δ-AuSn相的取向高度集中,而网格状共晶组织中ξ’-Au5Sn相却表现出分散的取向,非规则共晶组织中ξ’-Au5Sn相甚至呈现出随机取向的趋势。这种非规则共晶是过冷合金液相中共晶两相独立形核、长大的产物。(4)对Au-20Sn细小全片层共晶均匀化热处理后的球化组织的EBSD取向分析表明：在浓度梯度的驱动下,ξ’-Au5Sn和δ-AuSn层片间元素互相扩散,层片组织粗化、断开,逐渐转变为球化组织。这种球化组织的晶体取向和显微形貌都与TH较低时(370℃～390℃)得到的网格状共晶组织十分相似。这也进一步证明了当TH较低时,非规则共晶源于层片共晶组织的过热、熔断、粗化。更多还原

【Abstract】 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.更多还原