【作者】 张伟；

【导师】 毛勇；

【作者基本信息】 云南大学 ， 材料物理与化学， 2014， 硕士

【摘要】 金锡共晶合金(Au-20wt%Sn)钎料作为一种无铅焊料,具有焊接温度适中、电导性好、钎焊强度和热疲劳强度高等优异性能,广泛应用于高可靠微电子器件封装。但金锡共晶合金本身很脆,加工成形困难,初始合金铸态组织对加工性能影响很大,深入研究金锡共晶合金初始凝固组织与加工性能的关系将具有重要的理论和实际意义。本论文采用不同的凝固工艺得到了不同初生相和片层结构的合金凝固组织,研究了不同初始凝固组织的热压缩变形行为,对比分析得到了具有最佳热加工性能的金锡共晶合金凝固组织,即细小全片层组织；并对这种细小全片层组织的金锡共晶合金采用均匀化热处理工艺,研究了其等轴化机制,针对等轴化的组织探索研究了室温轧制和室温压痕特性,以及热轧制过程中的变形行为,深入分析了裂纹特性、断口形貌及热轧制过程的组织演变,得到结论如下：(1)金锡合金的热压缩特性对合金初始组织十分敏感。初始组织为初生相与粗片层的合金热压塑屈服应力和塑性平台应力最高为38.3MPa和14.6MPa,而初始组织为细小全片层组织,其应力值分别为15.3MPa和6.9MPa。细小全片层合金热压缩前后流变软化程度最小为8.4MPa,其组织在热压缩过程中表现出了最佳的热压缩性能。(2)在热压缩过程中,粗大树枝晶初生相被分解为分开的枝晶臂,部分初生枝晶融入基体中；细小梅花状初生相发生压缩变形,部分融入基体中。初始组织中的初生相不利于合金的均匀变形,也不利于动态再结晶进程。(3)在热压缩过程中,粗片层组织热压缩后发生再结晶由片层转变为粗大等轴组织；而细片层组织再结晶等轴化为细小再结晶组织。热压缩变形后的片层组织演变规律为：片层团出现边界粗化,片层团边界贯穿形成更多细小的片层团,随着边界片层组织再结晶不断进行,边界组织不断向片层内部扩展,最终实现全部片层组织等轴化。(4)金锡共晶合金均匀化热处理组织演变过程为：首先层片共晶在高温下富集长大,接着长大至一定尺寸的新层片共晶开始扰乱和熔断,然后两组成相逐渐形成等轴组织直至完全等轴化,最后两组成相的平均尺寸随着退火时间的增加而增大。细小全层片金锡共晶合金凝固组织的最佳均匀化热处理条件为240℃,80min。(5)金锡合金冷加工性能表现出较小的塑性变性能力和易脆裂特性。室温轧制变形极易出现裂纹,裂纹沿AuSn相扩展,断口形貌呈河流花样为解理断裂,出现撕裂岭,表现出典型的穿晶断裂的特性,室温轧制塑性变形量小于3%。室温压痕在小载荷下合金出现塑性流动变形区,大载荷下容易出现滑移带和裂纹,且裂纹沿着滑移带产生和延展。(6)金锡合金具有良好的热轧制变形能力,采用控制轧制条件等温轧制与道次间退火,组织可以恢复为两相均匀分布的细小等轴晶,连续轧制可以得到0.1mmm以下的合金箔材,当中间道次退火不完全或合金变形量过大时,样品在边缘会出现裂纹,高温下的断裂属于准解理断裂。更多还原

【Abstract】 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.更多还原