Heat resistance of Sn-9Zn solder/Cu interface with or without coating

The potential of the newly developed Sn-9Zn solder paste as a lead-free sol der, especially focusing on the stability at high temperature, was examined . The initial interface strength between Sn-9Zn and Cu, about 50 MPa by the tensile test, is higher than other interfaces such as Sn-37Pb/Cu. While th e Sn-9Zn/Cu interface maintains the high strength level after heat exposure at 125 degrees C, the heat exposure at 150 degrees C degrades strength ser iously. The degradation at 150 degrees C is caused by dissipation and by di sruption of the Cu-Zn reaction layer at the interface. Where the Cu-Zn laye r is eroded to form a whole, Sn directly reacts with a Cu substrate to form a thick Sn-Cu reaction region, Such an interfacial morphology change cause s the serious degradation, With the Ni/Pd/Au coating on a Cu substrate, the interface becomes much stronger than the direct interface. Even after heat exposure at 150 degrees C, strength degradation is not so significant. Zn segregates into the coating layer. During high-temperature exposure, Ni and Pd diffuse each other. Zn also diffuses into the coating layer to form com pounds, and as a result, a depleted zone of Zn is formed in the solder clos e to the interface.