Missing solder ball failure mechanisms in plastic ball grid array packages

Plastic ball grid array packages were aged at 125 and 150 degrees C for different time intervals from 4 to 2000 hours. Various solder ball pad metallurgy including pure Ni barrier layer (electrolytic plating) with Au protective layer from 0.48 to 1.27 mu m, Ni-P barrier layer (electroless plating) with 0.48 mu m Au protective layer and Ni-Co barrier layer with Au layer from 0.52 to 1.46 mu m were studied. Solder ball shear test was conducted at each time interval of aging. Solder ball shear strength decreased after initial hardening stage. The deterioration of solder ball shear strength was found mainly caused by the formation of intermetallic compound (IMC) layers, together with microstructure coarsening and diffusion related porosity at the interface. Sn forms different intermetallic compound layers with different Ni barrier layer. For ball pad metallurgy in this studied, two intermetallic compound layers formed after aging. A critical Au thickness value was found between 0.48 mu m to 0.7 mu m for electrolytic Ni. If Au protective layer is thinner than the critical value, separate (Au,Ni)Sn-4 IMC particles form on surface of Ni3Sn4. If Au layer is thicker than the critical value, a continuous (Au,Ni)Sn-4 layer forms on top of Ni3Sn4. Thick Au layer and high aging temperature result in formation of thicker (Au,Ni)Sn-4 intermetallic compound layer in short time. For electrolytic Ni/Co plating, the critical Au thickness is thinner than pure electrolytic Ni plating in terms of continuous (Au,Ni,Co)(x)Sn-y intermetallic compound layer formation. In shear test, fracture occurs at either interfaces or in the layer with the lowest shear strength. Once two continuous intermetallic compound layers formed, the fracture tends to occur at their interface. It was found that the bonding strength between (Au,Ni,Co)(x)Sn-y and (Ni,Co)(3)Sn-4 is higher than that between (Au,Ni)Sn-4 and Ni3Sn4. For ball pad metallurgy do not form two continuous intermetallic compound layers, the shear strength decrease due to the coarsening of microstructure, intermetallic particle formation and diffusion related porosity on surface of Ni3Sn4. A Phosphorus rich layer forms at the interface between Ni3Sn4 and Ni-P barrier layer after aging, fracture at this interface is not the dominate failure mode for electroless Ni/Au metallurgy.