Transient Liquid Phase Sintering (TLPS) Conductive Adhesives for High Temperature Automotive Applications 2014-01-0797

Power electronics products such as inverters and converters involve the use of Thermal Interface Materials (TIMs) between high power packages and a heat exchanger for thermal management. Conventional TIMs such as thermal greases, gels, solders and phase change materials (PCMs) face challenges to meet the need of these products to operate reliably at much higher temperatures. This has driven the development of new TIMs such as Transient Liquid Phase Sintering (TLPS) Conductive Adhesives. TLPS adhesives have been developed for many potential applications due to various advantages like lead free, flux-less and particularly their low temperature processability, which enables the use of heat sensitive components in the design. With all these motivations, a project was launched and completed to assess TLPS adhesives as a unique TIM for high temperature automotive applications due to its high bulk thermal conductivity and metallic joint formation at interfaces.

This paper reports the evaluation of three different TLPS conductive adhesives with different formulations. All the TLPS adhesives contain lead-free eutectic SnBi fillers, together with other fillers with much higher melting point (Tm) so as to process them using low temperature profiles. Different processing profiles were applied to process different TLPS adhesives. After processing, the assemblies could meet high temperature application requirements due to the shift of Tm of SnBi alloy from 139°C to a much higher value >200°C. Three different heat exchanger materials including bare Cu, Al and AlSiC with solder wettable surface finishes were assessed. X-ray was used to check void formation and Scanning Electron Microscopy (SEM) was conducted on the cross sections to study the quality of the whole metallurgical network. Die shear force was used to verify the joint strength. Thermal resistance measurement was carried out to verify thermal performance of TLPS assemblies. Thermal cycling test (−40 °C to +135 °C) had been performed to assess the reliability performance of the assemblies. TLPS samples built on Al and Cu substrates exhibited higher thermal resistance due to higher mis-match of coefficient of thermal expansion (CTE) with that of AlN, while TLPS samples assembled on AlSiC substrates were much more reliable and showed low thermal resistance throughout the thermal cycling test.