| abstract |
Micro-addition of a metal to a Sn-based lead-free C4 ball is employed to enhance reliability. Specifically, a metal having a low solubility in Sn is added in a small quantity corresponding to less than 1% in atomic concentration. Due to the low solubility of the added metal, fine precipitates are formed during solidification of the C4 ball, which act as nucleation sites for formation multiple grains in the solidified C4 ball. The fine precipitates also inhibit rapid grain growth by plugging grain boundaries and act as agents for pinning dislocations in the C4 ball. The grain boundaries enable grain boundary sliding for mitigation of stress during thermal cycling of the semiconductor chip and the package on the C4 ball. Further, the fine precipitates prevent electromigration along the grain boundaries due to their pinned nature. |