Enhancement of TBGA substrate in packing drop test

The drawback of Sn/Ag/Cu solder alloy on selective Ni/Au plating (SMOBC) TBGA substrate is weakened solder joint, which promote the embrittlement and affects long-terms reliability. The ability of electronic packages and assemblies to resist ball drop failure is becoming a growing concern recently. In this paper, test vehicles of Sn-Ag-Cu (SAC) solder alloys with different Ni thickness of 3, 5, 8 and 10 mum 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 thickness metallization. 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 is identified due to the solder mask (SM) tail in thin Ni, where Cu diffusion could get the simple channel through SM and also provides 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 and 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, resulting to the acceleration of 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 the dropping test. In contrast, a good diffusion barrier is demonstrated by thick Ni solder system, exhibiting a slower IMC growth and lower consumption rate of Ni layer, which evidences a higher shear loads and good performance in the dropping test. In conclusions, a minimum 5 mum thick Ni outperforms 3 um thin Ni in shear te- st 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.