STRENGTH OF LEAD-FREE BGA SPHERES IN HIGH SPEED LOADING
Authors: Keith Sweatman, Shoichi Suenaga, and Tetsuro Nishimura Company: Nihon Superior Co., Ltd Date Published: 1/24/2008
Pan Pacific Symposium
Abstract: Concern about the failure of lead-free BGA packages when portable devices such as cell phones are accidentally dropped and a general concern about the resistance of these packages under shock loading has prompted an interest in the impact strength of the soldered BGA connection. This paper reports the results of the measurement of the impact strength of lead-free 0.5±0.01mm 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. Impact strength is measured as the energy required to detach the sphere from the substrate to which it has been soldered. Two lead-free solders, Sn-3.0Ag-0.5Cu and a silver-free Sn-0.7Cu-0.05Ni-0.006Ge 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/sec and in tension at speeds of 1, 10, 100, 200, and 400mm/sec. Fracture surfaces were studied using scanning electron microscopy and the relative amounts of ductile and brittle fracture noted. Spheres were cross-sectioned to observe the effect of ageing on the growth of interfacial intermetallic. While at load rates lower that about 100mm/sec in shear and 10mm/sec in tension the energy required to detach the SnAgCu was higher than that required to detach the Sn-Pb or SnCuNiGe sphere, at higher speeds the SnAgCu failed in a brittle manner at low impact energy while the SnCuNiGe alloy required more energy even than the Sn-Pb and exhibited a high proportion of ductile fracture. This difference between the SnAgCu and the SnCuNiGe alloy increased after ageing and this could be correlated with the greater increase in the thickness of the intermetallic layer in the SnAgCu.