Solidification behavior of lead free and tin lead solder bumps

An optical observation method was used to characterize the solidification behavior of WLCSP and Flip Chip solder bumps on actual product devices. The WLCSP solder ball diameter was 350um, and Flip Chip diameters were 110um and 80um. Sn3.5Ag, Sn2.3Ag, SAC305, Sn0.7Cu, and 63Sn37Pb alloys were investigated. Cooling rates were varied between 0.10C/sec and 3.9C/sec. In addition, Vickers Hardness testing was employed to look for any correlation between undercooling and mechanical response of the alloy. Solidification was found to occur 4C to 60C below the solidus temperature (undercooling = 4C to 60C). The range of undercooling for the bumps on a given sample was between 8C and 40C. The sequence of solidification was quite random. Cooling rate had a minor effect on undercooling behavior for rates between 0.5C/sec and 3.9C/sec. For the very slowest rate of 0.1C/sec, the amount of undercooling decreased. 63Sn37Pb and 96.5Sn3.5Ag had the least amount of undercooling and the tightest range of undercooling. 96.5Sn3.0Ag0.5Cu and 99.3Sn0.7Cu had the most undercooling and the widest range of undercooling. Flip chip bumps had 5C to 10C more undercooling than WLCSP balls. There was no correlation between undercooling and Vickers hardness, within the range of conditions studied here. There was a slight increase in hardness for SAC305 samples with a faster cooling rate during solidification. Finally, understanding of the solidification behavior in Flip Chip joints was used to interpret a specific reliability test result.