Pronounced effects of Zn additions on Cu-Sn microjoints for chip-stacking applications

The strong effects of Zn addition on the reaction between Sn-based solders and Cu substrate are widely known. Nevertheless, all of the previous studies are limited to the situation in which the solder volume was relatively large, and the effects of Zn on the much smaller microjoints remain unclear. In this work, the effect of Zn addition on the reaction in microjoints is investigated. Sandwich structures of Cu/Sn/Cu and Cu/SnxZn/Cu (x = 0.4, 0.7, and 1 wt%) are prepared by thermal compression bonding. The thickness of Sn or SnxZn layer is controlled at 10 μm. It is found that Zn addition significantly retards the growth of Cu3Sn, and the growth of Cu6Sn5 is also slowed down. The results of this study suggest that it is possible to avoid the total conversion of Sn into brittle intermetallics during the lifetime of microjoints by the addition of a proper amount of Zn.