Effect of Thermomigration in Eutectic SnPb Solder Layer

Due to the miniaturization trend and functional demand in high-density microelectronic packaging, the thermomigration in flip chip solder joints owing to the joule heating becomes a serious reliability issue. In this study, a novel apparatus which can provide a sufficient temperature gradient cross the solder joint in the specimen was used to carry on the thermomigration experiment separated from electromigration behavior. A constant temperature gradient above 2000 degrees C/cm was applied on the eutectic SnPb solder layer of specimen and the load duration were 20h, 40h, and 80h respectively. The result reveals that in eutectic SnPb solder layer of specimen, an obvious mass diffusion from hot side to cold side occurs and Pb is estimated as the primary diffusion element under a high enough temperature gradient. EDX result also proves that the percentage of Pb element in the cold side is higher than that in the hot side. With the temperature gradient load time goes up from 20h to 80h, the phenomenon of Pb migration to cold side turns to be much severer. Due to the redistribution of both Pb and Sn element, the morphology and thickness of IMC in both hot side and cold side changes obviously comparing that in as-reflowed solder layer. Meanwhile, the tensile test also shows that the mechanical strength of eutectic SnPb solder layer becomes weaker with the thermomigration load duration increases gradually, and this may be owing to the defects existing in the solder caused by mass diffusion.