【作者】 李仕明；

【导师】 陆皓；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2009， 硕士

【摘要】 由于Pb元素具有毒性,各国相应的法规禁止使用含铅焊料,因此无铅焊料的研究迫在眉睫。目前已经有上百种无铅焊料。Sn-3.5Ag系无铅焊料具有高的力学性能和耐热疲劳性,常用于高温设备中,是目前最有可能替代Sn-Pb钎料的合金。而在热老化过程中,焊点界面IMCs层的厚度增加将引起接头的热疲劳寿命、抗剪强度和断裂韧性会减小。同时接头电迁移失效问题也引起了广泛的关注。本文根据设计了Cu/钎料(Sn-3.5Ag、Sn-3.5Ag-1.0Zn)接头用于热老化实验,Cu/钎料/Cu(Sn-3.5Ag、Sn-3.5Ag-1.0Zn)用于电迁移实验。所得的试样经过冷镶嵌、打磨抛光,采用SEM分析了热老化和电迁移阶段接头反应界面微观组织形貌及演变,界面相成分采用能谱仪EDX分析。主要结论如下:1.在热老化实验中,Sn-3.5Ag/Cu界面反应初生相为η-Cu6Sn5,在老化阶段,Cu6Sn5与Cu之间新生成一层较薄的ε-Cu3Sn层。化合物层的厚度随老化时间的延长而增加。而对于Sn-3.5Ag-1.0Zn/Cu界面初生相也为Cu6Sn5,热化过程中,ε-Cu3Sn并没有出现,取而代之的是Cu5Zn8,化合物层的生长有所被抑制。2.在电迁移过程中,互连接头两端电压变化可分为四个阶段,即迅速上升期、稳定期、电压波动期和继续增大失效期。迅速上升期时间较短,几分钟电压就能够达到稳定期。稳定期期间,对应着微型空洞的形成和转移,为电迁移的孕育期,经过短暂的波动期后,电压迅速升高,最终导致电迁移失效。3. Sn-3.5Ag和Sn-3.5Ag-1.0Zn焊料电迁移温度为常温,平均电流密度为1.0×103A/cm2和1.4×103A/cm2,远远低于普遍认为的电流门槛值1×104A/cm2,但是电迁移失效依然在负极的拐角处发生了。利用ANSYS模拟凸点中电流分布,发现了在阴阳极电子流入口附近电流密度达到了1.4×104A/cm2,而实验观察到的电迁移失效正好发生在该处,很好的说明了电流塞集引起了电迁移失效。4.由于电迁移的影响,IMCs生长出现明显的极性效应,Sn-3.5Ag和Sn-3.5Ag-1.0Zn焊料凸点互连接头正极界面的IMC都是随着电迁移进行单调增长的;而负极界面的IMCs都是随电迁移进行先增大后减小的,并且Sn-3.5Ag焊料接头是在5天时候IMCs到达最大值,而Sn-3.5Ag-1.0Zn焊料接头是在10天的时候到达最大值,同时同等条件下Sn-3.5Ag-1.0Zn焊料接头的IMCs都比Sn-3.5Ag焊料接头的小,说明Zn元素的加入有助于抑制凸点互连接头IMCs在电流载荷作用下的生长。5.热场和电场条件下研究了界面Kirkendall空洞的形成和演变过程,对于Sn-3.5 Ag/Cu界面,老化阶段,Cu/Cu3Sn界面的微空洞数量很少,而且增加不明显。然而,在1×103 A/cm2电流密度条件下,电流载荷会加速Cu/Cu3Sn界面Kirkendall空洞的形成,并且明显具有极性特征,即阳极Cu/Cu3Sn界面Kirkendall空洞的密度明显高于阴极。同时在Cu基板上镀一层Ni-P层可以起到抑制Kirkendall空洞的产生。而对于Sn-3.5 Ag-1.0Zn/Cu界面,在热老化和电迁移过程都没有产生Kirkendall空洞。更多还原

【Abstract】 Due to the toxicity of Pb, the elimination of toxic lead (Pb) from electronic products is a global tendency actively driven by legislation. So, the development of a good lead-free solder which is environmentally friendly materials is a great challenge. There are about 100 kinds of lead-free solders from various researches and public. The eutectic Sn-3.5Ag was one of the most promising and extensively studied lead-free solder, for its excellent wettability, thermal and mechanical properties. During aging, the intermetallic compounds(IMCs) are not stable and continue to grow in the stage of solidification. The electromigration(EM) has attracted lots of attention. In this paper, a Cu/Solder structure was designed for aging experiment and Cu/Solder/Cu sandwich was designed for EM test, where solder is Sn-3.5Ag and Sn-3.5Ag-1.0Zn. The samples were through cold inlay, polishing and eroded, the microstructures of the interface were analyzed by using scanning electron microscope (SEM), chemical components were analyzed by using Electron Probe Micro Analysis (EPMA). Main conclusions are as follows. 1. In the aging experiment, for Sn-3.5Ag/Cu interface, the initial phase at the interface was scallop like Cu6Sn5. Meanwhile, a new layer, which was determined as Cu3Sn, was formed at Cu6Sn5/Cu interface after isothermal aging. For Sn-3.5Ag-1.0Zn/Cu interface, the initial phase was also Cu6Sn5. During the thermal aging, the formation of Cu3Sn was depressed and replaced by Cu5Zn8. The growth of IMCs at Sn-3.5Ag-1.0Zn/Cu interface was depressed.2. In the EM experiment, the change of voltage of the solder bump joint can be divided into four periods, which are rapidly rising period, stable period, fluctuation period and failure period. The rapidly rising period is very short, about several minutes and then change into stable period, when the voids forms and transfers. And then, after a transient fluctuation period, the voltage rises rapidly and the failure happens.3. Even when the current density is less than the minimum current density inducing EM evolution, the EM failure occurred in the incurrent ingress of cathode. Using ANSYS simulating the distribution of current, we found there was current crowding in the solder bump. So the EM failure happened.4. Due to EM, the growth of IMCs had obvious polarity effects. For both Sn-3.5Ag solder and Sn-3.5Ag-1.0Zn solder, the IMCs in the anode interface, the thickness increased flatly. However, the IMCs in the cathode interface, the thickness increased first and the decreased. By comparing the thickness of the both solder, it was found that Zn could depress the growth of the IMCs under current.5. The formation and evolution of Kirkendall voids were investigated under thermal aging and current stressing. At the 150℃aging process, only a small amount of micro-voids were formed at Cu/Cu3Sn interface. While current stressing of 1×103A/cm2 was found to accelerate the formation and growth of Kirkendall voids. Moreover, the formation of Kirkendall voids had a polarity effect under the current stressing, i.e., much higher density of Kirkendall voids at the anode side than that at the cathode side. We also found that the Ni-P UBM on the Cu and the addition of Zn could depress the formation and evolution of Kirkendall voids.更多还原