锡基无铅焊料互连体在电流作用下的损伤行为

IMR OpenIR

	锡基无铅焊料互连体在电流作用下的损伤行为
	刘海燕
学位类型	博士
导师	尚建库 ; 王中光
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	无铅焊料电迁移空位浓度应力松弛位错攀移 Lead-free Solder Electromigration Vacancy Concentration Stress Relaxation Dislocation Climb
摘要	" 在当前的主要封装形式如Flip Chip、BGA、CSP等先进封装形式中，焊料凸点为电子元件提供了机械支撑和电气连接。焊料凸点的尺寸和焊点之间的间距日益降低，使无铅焊料连接的可靠性问题变得尤其重要。电迁移过程中焊点内部产生大量的空位、孔洞等微观缺陷，必然会对焊料的力学性能产生影响；电流产生大量的焦耳热，导致焊点在服役中受到热场和电场共同作用。本论文着重研究了Sn-3.8Ag-0.7Cu和纯锡焊点在电流作用下的力学性能弱化行为，并通过添加第四组元Zn有效抑制了Sn-Ag-Cu焊点的电迁移。此外，对电迁移与热迁移共同作用下的焊点失效也做了相应研究。在高密度电流作用下，Sn-Ag-Cu焊料中的锡原子向阳极发生定向扩散。大量成分为Sn的凸起在焊点表面产生，经统计发现所有凸起的体积总和随通电时间呈半对数直线增长。与此同时，焊点的阴极附近空位不断积累，空位浓度随通电时间的增长规律与表面凸起的增长规律一致。电迁移促进了Sn-Ag-Cu焊料的应力松弛，随电迁移时间的增长焊点的应力松弛速率提高。分析认为其原因是过饱和空位对位错产生了攀移力作用，导致位错攀移速率提高，进而提高了焊点的应力松弛速率。在纯锡焊点中，电迁移后样品表面出现了很多晶界沟槽和晶界裂纹，其深度和宽度随电迁移时间的增长而增加。晶体结构和物理性质的各向异性导致电迁移后Sn晶界不同部位的空位浓度存在差异，该浓度差对Sn晶粒产生扭矩作用并导致Sn晶粒发生扭转。电迁移后焊点的应力松弛速率加快，应力松弛机制由通电前的位错攀移转变为晶界扩散机制。随着电迁移时间的增长晶界附近空位不断聚集，而空位浓度的提高有利于晶界扩散和晶界滑移，从而提高了焊点的应力松弛速率。 Sn-1Ag-0.5Cu-1Zn焊点的界面金属间化合物分为两层，其中靠近焊料的化合物层成分为Cu5Zn8，电迁移后焊点界面化合物层厚度没有发生变化，极性效应被有效抑制。这是由于Cu5Zn8的化学性质非常稳定，从而抑制了Cu原子的溶解和扩散。电迁移过程中Sn原子的定向迁移引发了与电子风力方向相反的回流力，该回流力驱使Zn原子发生反向迁移。Zn向阴极的聚集抑制了阴极附近空位浓度的增加，使电流对焊料力学性能的弱化作用得到有效抑制。在电流作用下，焊点内部存在较大的温度梯度。焊点在服役过程中，焊点中的金属原子受到电迁移和热迁移的共同作用，当热迁移与电迁移作用方向一致时焊料内原子的扩散加剧，从而加速了焊点的失效。降低互连体环境温度、降低焊点内部温度梯度有利于抑制焊点金属原子的扩散，并有效抑制了微焊球互连体的失效。"
其他摘要	" In microelectronic packaging, components are often mounted on a substrate through the solder interconnection that provides both mechanical support and electrical connection. As the trend of miniaturization and high performance in electronic products goes forward, the package density increased continuously and the size of the solder connection becomes smaller and smaller. The miniaturization of the solder connection will inevitably place challenges on the reliability of the solder connection. Electromigration damage was found in solder joints by creating voids, hillocks and interfacial failures. The microstructure defects including vacancies and voids may affect mechanical properties of solder joints. Meanwhile, the electric current in the solder interconnects generated great joule heat and the temperature of components is raised. As a result, the solder joints are subjected to both electric field and thermal field. In the study, we focused on the mechanical degradation of Sn-3.8Ag-0.7Cu and pure tin solder joints under electric current. The strength reduction under electric current was suppressed by the Zn addition in the Sn-1Ag-0.5Cu-1Zn solder interconnects. The failure of solder interconnects under combined effect of electromigration and thermomigration was also investigated. After applying a direct electric current, a large number of Sn hillocks were found on the surface of Sn-3.8Ag-0.7Cu solder joints. The volume of all hillocks in a fixed area was measured, and a nearly linear relationship was found between the total volume and the electromigration time. At the same time, a corresponding vacancy flux caused by electromigration moved from the anode to the cathode. Analysis of the vacancy flux indicated that the increase of the vacancy concentration at the cathode side with electromigration time agreed with the growth kinetics of the hillocks observed in the experiments. By modeling the stress relaxation as a climb-assisted dislocation glide process, it is shown that the vacancy accumulation induced by electromigration enhanced the dislocation climb rate, resulting in a large increase of the stress relaxation rate. For pure tin solder, grain boundary grooves appeared on the surface of the solder joints and deepened as the current stressing time was prolonged. A divergence of vacancy concentration at the grain boundaries existed after electromigration and generated a torque, and consequently the Sn grain was rotated. The stress relaxation rate was increased after electromigration and the dominant mechanism was changed from dislocation climb to grain boundary diffusion. The atomic diffusion rate along grain boundaries was enhanced by the increase in the vacancy concentration at the grain boundary, leading to higher stress relaxation rates. In the as-reflowed Sn-1Ag-0.5Cu-1Zn solder joints, two flat layers of IMCs existed between solder and Cu substrate. The IMC layer adjacent to the Cu substrate was Cu6Sn5, and layer adjacent to the solder was Cu5Zn8. After electromigration, the IMCs remained almost unchanged, indicating that the polarity effect on interfacial IMC growth was suppressed. Due to the strong binding of Zn with Cu in Cu5Zn8 IMC, the dissolution of the IMC at the cathode of the Sn-1Ag-0.5Cu-1Zn solder was effectively suppressed. In tensile tests, the strength of the Sn-1Ag-0.5Cu-1Zn solder interconnects remained about the same after electromigration. Since the abnormal polarity effect of the Zn elements counteracted the increasing vacancy concentration resulting from the Sn migration, the vacancy induced strength reduction was successfully inhibited. After direct electric was applied to the Sn-Ag-Cu solder joints in the flip-chip configuration, the temperature of chip side was notably higher than PCB side and a huge thermal gradient existed in solder joints. Under electromigration, the Sn and Cu atoms in the solder diffused from the cathode to the anode, however, under thermomigration, Cu atoms diffused to the PCB side while Sn atoms diffused to the chip side. The atomic diffusion was enhanced if the direction of both driving forces were the same. The diffusion was inhibited by decreasing the temperature and thermal gradient in the solder joints, such that the failure of the solder interconnects was suppressed."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64459
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	刘海燕. 锡基无铅焊料互连体在电流作用下的损伤行为[D]. 北京. 中国科学院金属研究所,2012.