Interactions between liquid Sn-Bi based solders and contact metals for high temperature applications

Liquid solder interconnects are promising as an alternative approach to conventional high melting point solder interconnects for applications where the operating temperature is likely to exceed 125°C. In order to ensure that a liquid solder interconnect remains in contact with the terminations on the component and the substrate, and that electrical contact between them remains unbroken, there must be some growth of an intermetallic compound (IMC) at the interfaces between the solder and the contact metallizations. However, given that IMC growth is generally much faster when the solder is liquid, the growing IMC must act as a strong diffusion barrier to suppress further IMC growth. This paper presents preliminary studies of liquid-phase Sn-Bi based solders that result in stable interfaces between the solders and three common contact metallizations, consisting of electroless Ni(P)/Au, of Cu and of Ti-W. Small quantities (1 or 2 %) of an additional element, including Cr, Si, Zn, Ag, Au, Al and Cu, have been alloyed with the eutectic Sn-Bi composition to find an effective inhibitor additive that can achieve a strong IMC diffusion barrier. IMCs and their growth rates, as well as the consumption rates of the three contact metallizations in contact with the molten solders, were investigated. Storage temperatures of 200°C and 240°C were used, with storage times ranging between two hours and one month. Results to date show that suitable additives can significantly reduce IMC growth rates for both the Ni(P)-Au and Cu contact metallizations, while no appreciable IMC growth is observed for Ti-W in contact with both the original and the various alloyed Sn-Bi based solders. Based on the current results, criteria to further assist the design of feasible molten liquid solder – contact metallization systems have been deduced.