Missing solder ball failure mechanisms in plastic ball grid arraypackages

摘要

Plastic ball grid array packages were aged at 125 and 150° Cfor different time intervals from 4 to 2000 hours. Various solder ballpad 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-Cobarrier layer with Au layer from 0.52 to 1.46 μm were studied. Solderball shear test was conducted at each time interval of aging. Solderball shear strength decreased after initial hardening stage. Thedeterioration of solder ball shear strength was found mainly caused bythe formation of intermetallic compound (IMC) layers, together withmicrostructure coarsening and diffusion related porosity at theinterface. Sn forms different intermetallic compound layers withdifferent Ni barrier layer. For ball pad metallurgy in this studied, twointermetallic compound layers formed after aging. A critical Authickness value was found between 0.48 μm to 0.7 μm forelectrolytic Ni. If Au protective layer is thinner than the criticalvalue, separate (Au, Ni)Sn₄ IMC particles form on surface ofNi, Sn₄. If Au layer is thicker than the critical value, acontinuous (Au, Ni)Sn₄ layer forms on top of Ni₃Sn₄. Thick Au layer and high aging temperature result information of thicker (Au, Ni)Sn₄ intermetallic compound layerin short time. For electrolytic Ni/Co plating, the critical Au thicknessis thinner than pure electrolytic Ni plating in terms of continuous (Au,Ni, Co), Sn_y intermetallic compound layer formation. In sheartest, fracture occurs at either interfaces or in the layer with thelowest shear strength. Once two continuous intermetallic compound layersformed, the fracture tends to occur at their interface. It was foundthat 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 padmetallurgy do not form two continuous intermetallic compound layers, theshear strength decrease due to the coarsening of microstructure,intermetallic particle formation and diffusion related porosity onsurface of Ni₃Sn₄. A Phosphorus rich layer formsat the interface between Ni₃Sn₄ and Ni-P barrierlayer after aging, fracture at this interface is not the dominatefailure mode for electroless Ni/Au metallurgy