Elemental Redistribution and Diffusion Path Configuration Near the Edge of Sn-95Pb Solder Bump/Ni-Cu UBM Joints after Aging in the Flip-Chip Technology

The formation and growth of intermetallic compounds (IMC) play a critical role in the solder joint reliability. In this study, joint assembly of high lead Sn-95Pb/Ni-Cu was employed to study the interfacial reaction and related IMC formation after aging at 150degC for 168, 500, 1000, 2000, and 3000 hrs, respectively. Special effort was focused on the investigation near the edge of solder joints. Field-emission SEM and electron probe microanalyzer (EPMA) were used to evaluate the interfacial morphology and the compositions of IMC, respectively. For joints aged for 168 hrs, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were observed. However, another IMC (Cu,Ni)₃Sn was revealed after more than 500 hrs aging. The elemental redistribution near the edge of the solder joint was further probed by the X-ray color mapping technique in EPMA. In combination of the quantitative analysis for IMC in various region of the joint, the measured compositions could be mapped onto the ternary isotherm of Sn-Ni-Cu to elucidate the phase transformation among various types of IMCs. On the basis of the elemental redistribution of Sn, Ni, and Cu under various periods of aging, the kinetics process of related IMC formation was formulated. Series of diffusion paths were then constructed, which, in turn, provided an effective approach in the interpretation of the phase transformation in related IMCs