Investigation of the undercooling of SnCu solder spheres

This study focuses on the influences on undercooling of SnCu solder alloys. Therefore Differential Scanning Calorimetry (DSC) measurements were carried out on commercially available solders with varying Cu content (from 0 wt.% to 3.0 wt.%). Furthermore the diameter of the investigated solder spheres was varied from 131 μm to 1120 μm in order to represent typical interconnect sizes of electronic packages. The results showed only little influence by composition and volume, which was unexpected. Further experiments showed that the undercooling of the investigated solder alloys is probably suppressed by an impurity which triggered heterogeneous nucleation at higher solidification temperatures. By now we were not able to detect the responsible impurity in our samples, but it was possible to reproducibly and permanently decrease the solidification temperature of our samples (e.g. SnCu0.7 from 210 °C to 150°C) by thermal preconditioning above 370 °C. These findings open the opportunity to directly modify undercooling in SnAgCu solders.