Transient Liquid Phase Sintering Pastes as Solder Alternatives in High Temperature Applications

Abstract

High operating temperature electronics are a growing market in the electronics industry. Most of the components and substrates necessary to support the production of these harsh environment devices are now available, but the interconnect materials for component, module and board level assembly are lagging in development. Currently the high operating temperature markets are being served by lead-bearing solders and expensive alloys such as gold-tin or gold-germanium. Lead has been banned from the majority of electronics applications in many areas of the world, but so far has been exempted in the high temperature solder applications due to the lack of an adequate replacement. Even without the impending regulatory restrictions, the melting temperature of the lead containing solders is marginal for the next generation of high operating temperature electronics. Transient liquid phase (TLPS) sintering paste compositions are a new class of solder replacement materials that can be processed at typical reflow temperatures, but which do not remelt when subjected to subsequent thermal excursions. TLPS pastes combine solder alloy particles and reactive metal particles in proportions such that the solder ‘thermosets’ during a typical solder reflow cycle. This ‘thermosetting’ behavior results in a joint that does not remelt at the original reflow temperature, and thus enables the highly reliable electrical interconnect essential for high operating temperature applications. TLPS pastes are similar to solders in many respects. The electrical, thermal and mechanical performance is similar to conventionally used tin-based solders. The TLPS pastes are stored and applied like solder pastes. The reflow cycles used to form the TLPS paste joints are also similar to those for forming solder joints. However, unlike solder, TLPS materials do not change shape during reflow or wet substrates beyond the deposition footprint. Also unlike solder, the metals within the bulk of the TLPS interconnect react with one another resulting in a stable interconnect with a thermally robust bonding structure to joining surfaces, even after subsequent thermal cycles. Though not a practical replacement for solder in common assembly operations, TLPS technology is an attractive solder alternative for specialty interconnect applications. In this paper, the nature and characteristics of TLPS pastes will be will be explored in comparison to solder paste materials commonly used in high operating temperature applications.