A Study of Intermetallic Compounds in Tin Bumps during Multi-Reflows

An electroplating pure tin bumping technology has been developed in house with commercial chemicals. Sputtered TiW and Cu were used as under bump metallurgy (UBM). The tin bumps were peripheral distributed on the die. The electroplated bumps had mushroom shape, and the height is about 70mum. The as-reflow bumps had sleek shape, and the diameter is about 100mum. For as-reflow bumps, scanning electron microscope was used for observing the microstructure of interface, and energy dispersive X-ray microanalysis was used for investigating the composition of the intermetallic compounds. Multireflow tests were used as accelerating tests for tin bumps. The samples reflowed 1 times to 10 times respectively. Ball shear tests were used for evaluate the strength of different samples of multi-reflow tests. There are two layers of intermetallic compounds between copper and tin, which are Cu6Sn5 and Cu3Sn. For as-reflow bumps, the thickness of Cu3Sn layer is too thin to detect, and its growth need additional contact time. As the reflows time increased, Cu6Sn5 and Cu3Sn layers both grow, and the thickness of Cu6Sn5 is thicker than that of Cu3Sn. The total thickness of intermetallic compound has an exponential relationship with the times of reflow. The mechanism of intermetallic compounds growth is the diffusion of liquid tin to solid copper. The simplified diffusion model in the intermetallic interface is similar to one dimension nonequilibrium diffusion in the plate. The shear strength results of different samples which had multireflow tests were found to have little discrepancy with each other, that means the performance of tin bumps were stable