Al/SiC carriers for microwave integrated circuits by a new technique of pressureless infiltration
Materials with high thermal conductivity and thermal expansion coefficient matching with that of Si or GaAs are being used for packaging high density microcircuits due to their ability of faster heat dissipation. Al/SiC is gaining wide acceptance as electronic packaging material due to the fact that its thermal expansion coefficient can be tailored to match with that of Si or GaAs by varying the Al:SiC ratio while maintaining the thermal conductivity more or less the same. In the present work, Al/SiC microwave integrated circuit (MIC) carriers have been fabricated by pressureless infiltration of Al-alloy into porous SiC preforms in air. This new technique provides a cheaper alternative to pressure infiltration or pressureless infiltration in nitrogen in producing Al/SiC composites for electronic packaging applications. Al-alloy/65vol% SiC composite exhibited a coefficient of thermal expansion of 7/spl times/10/sup -6/ K/sup -1/ (25/spl deg/C-100/spl deg/C) and a thermal conductivity of 147 Wm/sup -1/K/sup -1/ at 30/spl deg/C. The hysteresis observed in thermal expansion coefficient of the composite in the temperature range 100/spl deg/C-400/spl deg/C has been attributed to the presence of thermal residual stresses in the composite. Thermal diffusivity of the composite measured over the temperature range from 30/spl deg/C to 400/spl deg/C showed a 55% decrease in thermal diffusivity with temperature. Such a large decrease in thermal diffusivity with temperature could be due to the presence of micropores, microcracks, and decohesion of the Al/SiC interfaces in the microstructure (all formed during cooling from the processing temperature). The carrier showed satisfactory performance after integrating it into a MIC.
Processing of Silvar for MIC Packaging Applications
Materials with high thermal conductivity and thermal expansion coefficient matching with that of GaAs are being used for packaging high-density microwave integrated circuits (MICs) due to their ability of faster heat dissipation. Powder metallurgy (P/M) has emerged as a promising technique for the processing of metal matrix composites (MMCs) in satellite applications such as thermal management materials. Individual components in these composites must retain their metallurgical identity. The present study investigates the processing of silvar (Fe-Ni-Co-Ag) alloys for thermal management materials. For these silvar alloys, composition was varied by varying the silver content from 15 to 35 wt.%. The compacts were pressed at 400 and 600 MPa and sintered/infiltrated at 1100degC, 1150degC,and 1225degC. Quantitative metallographic measurements were performed on all sample and the results were discussed. It was observed that all samples were sintered without any shape distortion. Microstructural evaluation reveals that higher compaction pressure resulted in highly contiguous structure. In spite of density differences between constituents, none of the alloys showed segregation. Coefficient of thermal expansion (CTE) values of the liquid phase sintered composites is near to the values obtained by rule of mixtures.
Fabrication of fibre composites using an aluminium superplastic alloy as matrix
An aluminium superplastic alloy has been used as the matrix for a variety of fibre reinforcements. It is shown that, by hot pressing in the superplastic regime of the alloy, a number of different reinforcements can be incorporated into the matrix. Tensile tests on composites with up to 25 vol % of reinforcement showed good agreement with the rule of mixtures.