Solder ball failure mechanisms in plastic ball grid array packages

摘要

Plastic ball grid array packages were aged at 125 and 150℃ for up to 2000 hours. Various solder ball pad metallurgies were studied. These included a pure electrolytic Ni barrier layer with an Au protective layer from 0.48 to 1.27 μm thick, an electroless plated Ni-P barrier layer with 0.48 μm Au protective layer and a Ni-Co baffler layer with an Au layer from 0.52 to 1.46 μm thick. Solder ball shear tests were conducted at a range of ageing times. The solder ball shear strength was found to decrease after an initial hardening stage. The deterioration of solder ball shear strength was found to be mainly caused by the formation of intermetallic compound (IMC) layers, together with microstructural coarsening and diffusion related porosity at the interface. Sn was found to form different intermetallic compound layers for the different types of Ni barrier layer. For the ball pad metallurgy, two distinct intermetallic compound layer structures were observed to have formed after ageing. For electrolytic Ni there was found to be a critical Au thickness below which separate (Au,Ni)Sn{sub}4 IMC particles formed on the surface of the Ni{sub}3Sni. However, if the Au layer was thicker than this critical value, then a continuous (Au,Ni)Sn{sub}4 layer formed on top of the Ni{sub}3Sn{sub}4. A thick Au layer and high ageing temperature resulted in the rapid formation of a thick (Au,Ni)Sn{sub}4 intermetallic compound layer. For electrolytic Ni/Co plating, the critical Au thickness is thinner than pure electrolytic Ni plating in terms of continuous (Au,Ni,Co)Sn{sub}4 intermetallic compound layer formation. Once two continuous intermetallic compound layers formed fracture tended to occur at their interface. It was found that the bonding strength between (Au,Ni,Co)Sn{sub}4 and (Ni,Co){sub}3Sn{sub}4 is higher than that between (Au,Ni)Sn{sub}4 and Ni{sub}3Sn{sub}4. For the ball pad metallurgies which do not form two continuous intermetallic compound layers, the shear strength still decreased, due to the coarsening of the microstructure, intermetallic particle formation and diffusion related porosity at the surface of the Ni{sub}3Sn{sub}4.