Effect Of Thermomigration On Microstructure And Mechanical Properties Of Lead-free Micro Solder Joint

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 and simulate the flip-chip(Flip-Chip) and 3D-IC package interconnect pads, 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) and mechanical properties at different height of Cu/Sn0.7Cu/Cu solder joints are first systematically investigated in our study. The samples which have the height of the solder joints are 0.8mm and 0.4mm are taken to a temperature gradient of 1250°C/cm for 250, 500, and 750 h, respectively. The results show that the IMCs accumulation at the cold end and disintegration at the hot end. The phenomenon is happened at both heights of solder joint. Moreover, Shear strength at both sizes are getting smaller and smaller and solder joint fracture mode transforms from the ductile fracture to brittle fracture with the increasing of TM time. The shear fracture path in solder joint changed from the middle of the solder matrix to the position near the interface of hot end at both sizes as well.In addition, the effect of TM on the interfacial reaction of Ni/Sn/Cu solder joint which changed its material of hot and cold end under same conditions was also investigated. The samples which exchanged material of hot and cold end are taken to a temperature gradient of 1250°C/cm for 250, 500, 750, 1000 h. The interfacial IMCs at both hot and cold end are becoming thicker and thicker with TM time, no matter the Ni/Sn interface is the cold end or hot end. The thickness of IMC is bigger when Ni as hot end compared to Ni as cold end. Furthermore, with the increasing of TM time, Shear strength decreased at condition of whether the both material of hot and cold end changed. The solder joint fracture mode and shear fracture path are both the same as Cu/Sn0.7Cu/Cu solder joints under the TM effect.Finally, the effects of the thermomigration at a temperature gradient of 2000°C/cm on the Cu/Sn0.7Cu/Cu are studied to know more about the growth behaviors of the interfacial intermetallic compounds(IMCs) and mechanical properties. The results show that the both sides of IMCs are getting thicker during the TM. The thickness of the IMC at cold end is bigger than hot end during the same loading time. Shear strength are getting smaller and smaller and fracture mode transforms from the ductile fracture to brittle fracture under TM. The shear fracture path in solder joint changed to the position near the interface of hot end.