Properties and reliability of SnZn-Based lead-free solder alloys

In green electronics, e.g. hybrids and MEMS (micro electromechanical systems) tin-lead-solder materials are substituted by lead-free solder materials. Several alloys could be used, for example SnCu, SnCuAg, or SnBi alloys, depending on the product and the working temperature. range between 205-216 /spl deg/C. Using tin-zinc solder materials the needed heat is smaller, electronic components will not be overheated and a high compatibility of all manufacturing processes could be assumed. On the other hand early reports in the literature indicate a sensitivity to intercrystalline corrosion. This work deals with the handling and the reliability of the eutectic Sn91Zn9 and the Sn89Zn8Bi3 alloy. The pad metallization of the substrate was Cu/Ni/Au or Cu and the metallization of the used electronic components was Sn, NiPd and Sn96.5Ag3.5. Shear tests before and after thermal cycling, creep tests at high homologous temperatures, metallographic investigations and migration tests were done after soldering to determine the reliability of these solder joints. A Sn60Pb40 solder alloy was used as a reference. The used solder profiles of a vapour-phase and a convective reflow soldering lead to good wettability of the liquid solder The solder joints show a higher shear force than the tin-lead alloy and the decrease of the shear force due to thermal cycles was lower than the tin-lead solder joints. The tin-zinc-alloy shows a higher creep resistance and a lower migration behaviour than the tin-lead alloy.