Interfacial reaction and wetting behavior in eutectic SnPb solder on Ni/Ti thin films and Ni foils

Ni-based under bump metallization (UBM) is of interest in low cost flip chip technology primarily due to a slower chemical reactions with high-Sn solders such as eutectic SnPb as compared to Cu-based UBM. We studied wetting behaviors and interfacial reactions of the eutectic 63Sn-37Pb on Ni foils and Ni/Ti thin films using transmission electron microscopy (TEM), scanning electron microscopy, and energy dispersion x-ray analysis. Wetting angle, morphology of solder surface, and the rate of consumption of Ni have been studies as a function of reflow time at the temperatures of 200, 220, and 240 °C. From the TEM analysis, we found that Ni forms a single layer of scallop-type Ni₃Sn₄ compound with the eutectic SnPb. During the isothermal annealing, we observed the spalling of Ni₃Sn₄ compound from the Ni/Ti thin films. The spalling phenomenon is similar to that of Cu₆Sn₅ from the Cu/Cr thin films, yet the rate is slower. The spalling of Ni-Sn compound eventually caused dewetting of the molten solder from the Ti surface. © 1999 American Institute of Physics.