Low Temperature Lead-Free Assembly via Transient Liquid Phase Sintering

摘要

Transient liquid phase sintering (TLPS) is a potentially attractive interconnect technology because it results in an electrically and thermally conductive bond that can be processed at temperatures lower than the final operating temperature of the device. In TLPS, a low melting temperature alloy melts and wets a high melting temperature metal. A chemical reaction occurs between the two phases, resulting in the generation of a network of solid intermetallic compounds and the elimination of the low melting temperature liquid. Our team developed a framework for evaluating TLPS systems, focusing primarily on the Sn-Bi-Cu system. We analyzed two commercial TLPS formulations supplied by Ormet Circuits Inc. with reported thermal conductivity of 25 W/mK. Differential scanning calorimetry, paired with the relevant phase diagrams, is used to determine which reactions occur and to what extent during processing. Energy dispersive X-ray spectroscopy, X-ray diffraction, and Rietveld refinement are used to quantify the elements present and the phase composition before and after processing. The evolution of the bond's structure during processing is examined at different heating rates and sintering temperatures. Our preliminary results indicate that the equilibrium phases should consist of only Cu, Cu_3Sn, and Bi. Consumption of the liquid phase occurs rapidly, with most liquid disappearing in as little as half an hour at 205°C, but does not necessarily result in the equilibrium phases. We will further investigate the kinetics of intermetallic growth and determine the effects of composition on the final microstructure. TLPS represents a new approach to electronic interconnects, and can work with or replace conventional solders in such areas as PCB fabrication, semiconductor packaging, and thermal interface management.