Research On Microstructure Evolution Of Micro-solder Joint Induced By Temperature Gradient

With the miniaturization trend and multi- functional demand in electronic packaging field, the feature size of the interconnecting solder bump in electronic package is reduced increasingly, and the electric current density is increased significantly, which cause that the thermomigration(TM) phenomenon accompanied with electromigration(EM) phenomenon in solder joint become a reliability issue. In order to study TM phenomenon separately, here we designed a TM experimental apparatus to study the microstructure evolution and mechanical property of different micro-solder joint induced by temperature gradient.The growth behaviors of the interfacial intermetallic compounds(IMCs) in Cu/Sn/C u and Cu/Sn0.7Cu/Cu solder joints were systematically investigated under a temperature gradient of 1046 °C/ cm for 250, 500, and 750 h, respectively. Thermomigration(TM) caused the IMCs(mainly C u6Sn5) accumulation at the cold end and disintegration at the hot end, and the evolution rate in C u/Sn0.7C u/C u solder joint was faster than that in Cu/Sn/C u solder joint. In addition, it was observed that C u3 Sn IMC both at the cold end hot end grew with TM time in C u/Sn0.7C u/Cu solder joint, and the Cu3 Sn at the hot end grew faster than at the cold end. However, no obvious change was observed in C u/Sn/Cu solder joint.For comparison, the K irkendall voids were found in interfacial C u3 Sn after aging, while in interfacial C u6Sn5 after TM. The microhardness detection indicates that TM can cause microhardness reduction in the two solders, and the microhardness gradually increased from the hot end to the cold end, which was attributed to grain coarsening and higher vacancy concentration at the hot end. Besides, the shear fracture path in Cu/Sn0.7Cu/Cu solder joint changed from the middle of the solder matrix to the position near the interface of hot end with the increasing of TM time.In addition, the effect of TM and aging on the interfacial reaction of N i/Sn/C u solder joint under same conditions was also investigated. The interfacial IMC at N i/Sn and Sn/C u interface was(C u,N i)6Sn5 after reflowing, aging and TM. The interfacial IMC thickened with TM time, no matter the N i/Sn interface is the cold end or hot end; but the interfacial IMC after aging grow fastest, and the IMC as hot end thicker than as cold end. The Cu atoms increased with TM time at N i/Sn interface as cold end; while decreased as hot end. Besides, the interfacial IMC at C u/Sn interface as cold end grow fastest, and the IMC after aging faster than as hot end. Moreover, the N i atoms increased as cold end; while decreased as hot end. The results show that the TM effect can be slow down when the N i/Sn interface as hot end in Ni/Sn/Cu solder joint.For comparison, the TM and aging effect of C u/Sn Pb/C u solder joint was investigated under same condition. The results showed the dendrite structure was decomposed after TM and aging. However, the TM caused Pb and C u toward cold end, which resulted in the white α-Pb phase coarsening and the interfacial IMC thickening at the cold end; while the α-Pb phase were distributed relatively equally among β-Sn phase after aging.