Effect of Ni Layer Thickness on Intermetallic Formation and Mechanical Strength of Sn-Ag-Cu Solder Joint

Au/Ni/Cu under bump metallization (UBM) in tape ball grid array (TBGA) package with Sn-Ag-Cu solder is commonly used for electronic IC packaging due to environmental and business concerns. The drawback of Sn-Ag-Cu solder on selective Ni/Au plating (SMOBC) TBGA is a weakened solder joint, which provides the embrittlement and affects long-terms reliability. The ability of electronic packages and assemblies to resist ball drop failure is also becoming a growing concern. Test vehicles of Sn-Ag-Cu (SAC) solder alloys with different Ni thickness of 3, 5, 8 and 10 ?m are prepared. Firstly, the effect of Ni thickness on the mechanical strength of solder joint was investigated. Secondly, packing drop test was performed to correlate the ball drop level with different Ni metallizations. In experimental, the ball shear strength on the SMOBC TBGA with thin Ni was found to be lower as-reflow and in the early stage of high temperature storage, compared to that with thick Ni. This could be due to the solder mask tail in thin Ni, where Cu diffusion could gets the simple channel through solder mask and also provide a fracture location for the weakened strength of solders joints and mixed brittle fracture failure in the shear test. With the extend high temperature storage or multiple reflows, it is found that the brittle TIMC (Ternary-Intermetallic-Compound) growth is governed by the amount of copper diffusion and the dissolution rate of Ni layer. With thin Ni, more Ni was consumed and the resistance of copper diffusion was dropped, leading to accelerate the interfacial TIMC growth and to reduce the shear strength in shear test with the brittle fracture failure. The weakened resistance of copper diffusion by thin Ni could be directly attributed to the missing ball issue on the assembled package in packing drop test. In contrast, a good diffusion barrier is demonstrated by thick Ni solder joint, exhibiting a slower IMC growth and lower consumption rate of Ni layer, which evidences a hi- gher shear loads and good performance in dropping test. In conclusions, a minimum 5 ?m thick Ni outperforms 3 ?m thin Ni in shear test and passes all of packing drop test cycles. Thus, this gives the evidence that the increase in Ni thickness for Ni/Au finish plays a critical role for improving the robustness of Sn-Ag-Cu solder joint in the SMOBC TBGA.