Electromigration in Flip Chip Pb-Free Solder Joints

Summary form only given. The demand of flip chip technology in high density packaging for advanced electronic consumer products is growing rapidly. Due to the decrease in device size and increase in functionality, electromigration has now become the most serious reliability problem in flip chip solder joints, especially the Pb-free. Electromigration in flip chip solder joints has several unique features that are very different from the electromigration in Al and Cu interconnects. Solder alloy has a very small critical product of electromigration. thus it can fail at 103 A/cm2. Owing to the line-to-bump geometry in flip chip, current crowding occurs at the contact between the line and the bump and the failure mode of electromigration is a pancake-type void formation at the cathode. Eutectic solder composition has a constant chemical potential below the eutectic temperature, so it enables up-hill diffusion and a nearly complete phase separation to occur in electromigration without resistance. Joule heating due to Al or Cu interconnects can cause a very large temperature gradient in the solder joint, so thermomigration accompanies electromigration. In this talk, the reliability issues of flip chip solder joints when electrical force is combined with thermal force, chemical force or mechanical force will be discussed.