【作者】 韦小凤；

【导师】 王日初；

【作者基本信息】 中南大学 ， 材料科学与工程， 2014， 博士

【摘要】 AuSn20焊料具有良好的热导率、抗疲劳和抗蠕变性能,被广泛应用于高端电子产品的封装。但是常规熔铸法制备的AuSn20共晶焊料极脆,很难加工成为电子封装所需的箔带材。本文作者采用叠层冷轧+合金化退火法制备AuSn20箔带材焊料,利用SEM(EDS)、XRD和DSC等实验手段研究叠轧和退火过程中焊料的组织演变和性能,并制定最佳轧制和退火工艺。根据AuSn20焊料的实际应用情况,采用实验模拟的方法研究AuSn20焊料的焊接性能、焊点的可靠性、以及焊接界面失效的影响因素；结合理论计算和实验验证,探讨AuSn20/Ni(Cu)焊点界面金属间化合物(IMC)层的生长动力学,并在此基础上评估焊点的力学可靠性。本文的主要研究工作及结果如下：(1)采用叠层冷轧复合技术制备Au/Sn复合带,研究叠合层数和轧制工艺对复合带组织、成分和性能的影响。结果表明,当叠合层数为7层时,采用多道次、小道次压下量的轧制工艺可以制备出组织相对均匀、成分和熔点接近Au-Sn共晶合金的复合带。在叠层冷轧过程中,当道次压下量较大(最大道次压下率为47.6%)时,Au层和Sn层发生不均匀变形,Au/Sn复合带中Au含量偏高,熔点上升。当采用小道次压下量(最大道次压下率23.8%)时,Au/Sn复合带变形相对均匀。(2)探讨不同退火工艺下Au/Sn界面扩散机制,并在此基础上优化出实现Au/Sn复合带完全合金化的最佳退火工艺。在退火过程中,Au/Sn界面的AuSn、AuSn2和AuSn4复合IMC层随退火时间延长和退火温度升高而逐渐长大,Au、Sn单质逐渐减小,直至完全反应。理论计算和实验验证表明,AuSn20焊料完全合金化退火的最佳工艺为220℃退火12h。(3)采用回流焊技术制备AuSn20/Cu (Ni)焊点,研究其组织和剪切强度的随钎焊和退火工艺的演变规律,证实焊点的力学可靠性及失效断裂模式与IMC层的厚度和形貌有关。在310℃下钎焊时,AuSn20/Ni焊点界面处形成(Ni,Au)3Sn2IMC层。焊点的室温剪切强度随钎焊时间延长逐渐降低。在120、160和200℃下老化退火时,界面形成(Au,Ni)Sn和(Ni,Au)3Sn2或(Au,Ni)Sn、(Ni,Au)3Sn2和(Ni,Au)3Sn复合IMC层。随退火时间延长和退火温度升高IMC层的厚度逐渐长大,焊点的剪切强度逐渐减小。(4)采用扩散偶方法研究AuSn20/Ni焊接界面的IMC层的生长动力学。实验结果证实：Ni/AuSn20/Ni焊接界面的IMC层生长均符合扩散机制,其厚度l变化遵循公式：l=k(t/t0)n。在120、160和200℃下退火时,AuSn20/Ni界面复合IMC层的生长比例系数k分别为5.71×10-10m、3.24×10-9m和1.34×10-8m,时间指数n分别为0.514、0.471和0.459。在不同退火温度中焊接界面IMC层的生长均以体积扩散为主。(5) Cu/AuSn20/Ni焊点在钎焊和退火过程中的耦合界面反应使焊点的组织和性能发生变化。在310℃下钎焊时,在Cu/AuSn20界面形成胞状ζ-(Au,Cu)5Sn层,在AuSn20/Ni界面形成(Ni,Au,Cu)3Sn2四元IMC层。Cu/AuSn20界面的Cu原子在钎焊过程中穿过焊料到达AuSn20/Ni界面参与耦合反应。IMC层生长动力学数据表明Cu的耦合对Ni-Sn化合物的生长起抑制作用。焊点的剪切强度随钎焊时间的延长呈先增大后减小的趋势。在退火过程中Cu/AuSn20界面形成AuCu和Au(Cu,Sn)复合IMC层,以体积扩散机制随钎焊时间逐渐生长,而AuSn20/Ni界面的(Ni,Au,Cu)3Sn2层以反应扩散机制生长。剪切强度随老化退火时间的延长逐渐下降,剪切断裂随焊料/Cu界面的IMC层厚度的增大逐渐往Cu界面迁移,在Cu侧IMC层内发生脆性断裂。更多还原

【Abstract】 AuSn20solder has been widely used in high-end electronic products packaging because of its advanced properies in good thermal conductivity, excellent resistance to fatigue and creep. However, the AuSn20alloy is brittle under the common prepared process, resulting in the difficulty to fabricating into Foil-strip products to meet the requirements for electronic packaging. The AuSn20solders were prepared by the laminated-rolling and alloyed-aging process in this study. The Micro structural evolution and the properties of the Au/Sn laminate layer during the rolling and aging process were investigated with the SEM (EDS), XRD, and DSC experimental technologies, and the best process parameters were researched. We investigated the soldering property, reliability of the AuSn20solder joints by the experiments designed according to the real usility enviroment of the AuSn20solder, and analysed the joint failure factors. The growth behavior of the intermetallic compound (IMC) layer in the interface and the mechanical property of the joints were discussed by combining the theoretical calculation with the experimental verification. The main works are summarized as follows:(1) The Au/Sn composite belts were prepared by laminate-rolling process, and the effects of the laminate numbers and rolling technology on the microstructure, composition and properties of the Au/Sn composite belts were investigated. The results show that the Au/Sn strip with7layers and uniform microstructure, eutectic compositeon, and melting point can be prepared by the rolling technology with multi-pass and trail time reduction. In the laminate-rolling process, the inhomogeneous deformation of Au and Sn layers occurs under a large reduction (the maximum is47.6%), the Au content in the composite belt is higher than that of the eutectic composition, and the melting point rises. Under trail time reduction (small than23.8%) condition, the Au/Sn composite deformation is relatively uniform.(2) The aging process of Au/Sn composite belts completely alloying by diffusion was optimized based on the diffusion mechanism of the Au/Sn interface. After aging at solid-state temperature, the thickness of AuSn, AuSn2, and AuSru conpound IMC layer in Au/Sn interface grow gradually, and the Au and Sn layer were consumed accordingly. Combining the theoretical calculation with the experimental results, the optimum technology for AuSn20solder completely alloying is aging at220℃for12h.(3) The AuSn20/Cu(Ni) solder joints were prepared during the reflow process.The mechanical reliability and the failure mode of the solder joints are related to the thickness and micro structure of the interfacial IMC layer according to the investigation of the microstructure and shear strength evolution of the joints after reflowing and aging. After soldering at310℃, the (Ni,Au)3Sn2layer is fabricated at AuSn20/Ni interface. With the extension of reflowing time and the decrease of cooling speed, the shear strength of solder joint at room temperature declines gradually. During the process of aging at120℃,160℃and200℃,(Au,Ni)Sn,(Ni,Au)3Sn2or (Au,Ni)Sn,(Ni,Au)3Sn2and (Ni,Au)3Sn compound IMC layers were formed at the interface.With the extension of aging time and increasing of aging temperature, the thickness of IMC layer gradually grows up, whereas the shear strength of solder joint decreases.(4) The growth behavior of IMC layers in AuSn20/Ni interface was investigated with diffusion couples. It shows that the thickness/grows following the formula:l=k(t/to)n. During aging at120℃,160℃, and200℃, the proportionality coefficient k for the composite IMC layer was5.71×10-10m,3.24×10-9m, and1.34×10-8m, repectively, and the exponent n was0.514、0.471and0.459, repectively. This phenomenon indicates that the volume diffusion was contributed to the growth of the IMC layer at all aging temperature.(5) Coupling interface reaction occurred at the Cu/AuSn20/Ni joints during the reflow and aging process, which changes the structure and property of the soldering joints. After reflow at310℃, the ζ-(Au,Cu)5Sn cell structure formed at the Cu/AuSn20interface and the (Ni,Au,Cu)3Sn2quaternary IMC layer formed at the AuSn20/Ni interface. Copper atomics at the Cu/AuSn20interface passed through the AuSn20solder and took part in coupling reaction at the AuSn20/Ni interface. The kinetics of the growth for (Ni,Au,Cu)3Sn2quaternary IMC layer indicated that the coupling reaction of copper restrained the growth of Ni-Sn compounds layer. The shear strength of the joints has a trend to increase and then decreased with prolonging of reflow time. During the aging process, the AuCu and Au(Cu,Sn) layers forme at the Cu/AuSn20interface, which is controlled by the volume diffusion. The (Ni,Au,Cu)3Sn2grows by reaction diffusion at the AuSn20/Ni interface. The shear strength gradually declines with prolonging of aging time. The brittle shear fracture position moves to the copper interface with the growth of the IMC layer at the solder/Cu interface.更多还原