Effect of Nickel Pad Metallization Thickness on Fatigue Failure of BGA Lead-Free Solder Joints

The purpose of this paper is to present our work on the effect of nickel (Ni) pad metallization thickness on fatigue failure of lead-free solder joints after thermal aging. Cyclic three-point bending tests were carried out to investigate the relationship between intermetallic compound (IMC) layer development and the fatigue life of solder joints. After the fatigue tests, cross-sections of solder joint were observed by scanning electron microscopy (SEM) and crack propagation paths were investigated. The results show that when the thickness of the Ni pad metallization layer was too thin, voids in the solder were observed and the fatigue life of the solder joints was shortened. On the other hand, when the thickness of the Ni pad metallization layer was too thick, thick IMC layers between the solder layer and the Ni layer developed after thermal aging. The optimum thickness of Ni pad metallization layer in this study was either 0.08 or 0.12 mum. For these thicknesses, the Ni layer diffused into the solder and was consumed completely. For these conditions, the development of an IMC layer did not occur during the thermal aging, and good fatigue strengths were obtained.