Electromigration induced microstructure evolution and damage in asymmetric Cu/Sn-58Bi/Cu solder interconnect under current stressing

The electromigration induced microstructure evolution and damage in asymmetric Cu/Sn-58Bi/Cu solder interconnects were investigated by in-situ SEM observation, focused ion beam (FIB) microanalysis and finite element (FE) simulation. The SEM results show that the electromigration-induced local degradation of microstructures, i.e., segregation of Bi-rich phase and formation of microcracks, in the asymmetric solder interconnects is much severer than that in the symmetrical ones. FIB-SEM microanalysis reveals that the microregional heterogeneity in electrical resistance along different electron flowing paths is the key factor leading to non-uniform current distribution and the resultant electromigration damage. Theoretical analysis and FE simulation results manifest that the current crowding easily occurs at the local part with smaller resistance in an asymmetric solder interconnect. All results indicate that the asymmetric shape of the solder interconnect brings about the difference of the electrical resistance between the different microregions and further results in the severe electromigration damage.     

高密度LED组装Sn-3Ag-0.5Cu焊点显微组织和力学性能的分析

分别用Sn37Pb钎料和Sn3Ag0.5Cu钎料组装高密度LED灯板,试验观察了焊后和老化后焊点的显微组织,测试了焊点的抗剪强度.结果表明,Sn3Ag0.5Cu焊点的金属间化合物层明显大于Sn37Pb焊点,但都在合格范围内,不会造成脆断,且其β-Sn初晶晶粒被网状共晶物包覆的特殊结构使其抗剪强度明显大于Sn37Pb焊点;老化后,两组焊点的抗剪强度均有所减小.虽然无铅焊膏代替Sn37Pb焊膏是电子工业界的发展的必然趋势,但是利用无铅焊膏组装小尺寸、高密度LED组件需要更精确的回流工艺.     

Approach to disrupting thick intermetallic compound interfacial layer in friction stir brazing (FSB) of Al/Cu plates

Friction stir brazing (FSB) was developed for dissimilar joining to overcome the problems associated with friction stir lap welding (e.g. pin wear, narrow bonded area, hook defect) by metallurgical reaction instead of plastic flow. To enhance the thermomechanical effect of the rotating shoulder, a pin free tool with a large shoulder of 30 mm diameter was used in FSB of 5 mm thick Al/Cu plates in stepped lap configuration with Zn braze. Compared to the case using common tool of 20 mm diameter, it was found that although the intermetallic compound (IMC) layer grew thickly, it was disrupted into particles, and end crack within thick IMC layer was eliminated. As a result, the joint fractured not along interface and the fracture load increased by as much as 2271 N. This work proposed an approach to disrupting continuous IMC into particles by enhancing the mechanical effect of rotating shoulder.