RELATIONSHIP BETWEEN THE IMPACT STRENGTH AND MICROSTRUCTURE OF LEAD-FREE SOLDERS
Authors: Keith Sweatman, et al. Company: Nihon Superior Co., Ltd Date Published: 5/16/2008
ICSR (Soldering and Reliability)
Abstract: Concern about the failure of lead-free BGA and CSP packages when portable devices such as cell phones are accidentally dropped and a general concern about the resistance of these packages to shock loading has prompted an interest in the impact strength of the soldered BGA connection. Results have already been reported on the measurement of the impact strength of lead-free 1.6mm diameter BGA spheres on 0.42mm solder mask defined pads on copper/OSP and ENIG substrates using recently developed equipment that can load individual BGA spheres at high strain rates in shear and tension. Two lead-free solders, Sn-3.0Ag-0.5Cu (SnAgCu) and a silver-free Sn-0.7Cu-0.05Ni-0.006Ge (SnCuNiGe) were studied with Sn-37Pb included as a benchmark. To study the effect of intermetallic growth on impact strength BGA were tested after up to two reflow cycles and 200 hours ageing at 150°C. BGA were tested in shear at speeds of 10, 100, 1000, 2000 and 4000mm/s and in tension at speeds of 1, 10, 100, 200, and 400mm/s. While at load rates lower than about 100mm/s in shear and 10mm/s in tension the energy required to detach the SnAgCu was higher than that required to detach the SnPb or SnCuNiGe sphere, at higher speeds the SnAgCu failed in a brittle manner with low impact energy while the SnCuNiGe alloy required more energy even than the SnPb and exhibited a high proportion of ductile fracture. The analysis of the results of this test program has continued and in this paper the authors will report on the investigation of the relationship between the energy absorbed during impact fracture and the microstructure of the solder, in particular the intermetallic at the solder/substrate interface. These results indicate that because of the stability of their microstructure some leadfree solders are more suitable than others for applications where they might be subjected to shock loading.