Missing solder ball failure mechanisms in plastic ball grid array packages

Plastic ball grid array packages were aged at 125 and 150°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 μm, Ni-P barrier layer (electroless plating) with 0.48 μm Au protective layer and Ni-Co barrier layer with Au layer from 0.52 to 1.46 μm 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 μm to 0.7 μm for electrolytic Ni. If Au protective layer is thinner than the critical value, separate (Au, Ni)Sn₄ IMC particles form on surface of Ni, Sn₄. If Au layer is thicker than the critical value, a continuous (Au, Ni)Sn₄ layer forms on top of Ni₃Sn ₄. Thick Au layer and high aging temperature result in formation of thicker (Au, Ni)Sn₄ 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), 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),Sn_y and (Ni, Co)₃Sn₄ is higher than that between (Au, Ni)Sn₄ 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