Analysis and Experimental Verification of the Volume Effect in the Reaction Between Zn-Doped Solders and Cu

Abstract

The volume effect in the reaction between Zn-doped solders and Cu is reported and explained in this article. Zn-doped solders with two different volumes, bulk solder baths (6 g) and small solder balls (10 mg), were reacted with Cu substrates. It was found that the reaction at the interface depended not only on the Zn concentration but also on the volume of the solder. For the bulk solder reaction the Zn concentration stayed nearly constant, whereas in the small solder joint case the Zn concentration changed substantially with the reaction time. This was because the Zn concentration during reaction was different for different solder volumes when the volume effect became important. For bulk solder baths, when the Zn content was 0.5 wt.%, the reaction product was a Cu₆Sn₅-based compound. As the Zn content increased to 2 wt.%, the reaction product switched to a Cu₅Zn₈-based compound. When the Zn content was 1.0 wt.%, Cu₆Sn₅ and CuZn formed simultaneously. When the solder volume became smaller, the Zn concentration was no longer constant, and the phase equilibrium at the interface changed with time. It was shown that the shifting of the equilibrium at the interface caused the massive spalling of CuZn.