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.
Microstructure and properties of SiCp/Al electronic packaging shell produced by liquid–solid separation
Abstract The electronic packaging shell of high silicon carbide (54%SiC, volume fraction) aluminum-based composites was produced by liquid–solid separation technique. The characteristics of distribution and morphology of SiC as well as the shell's fracture surface were examined by optical microscopy and scanning electron microscopy, and the thermo-physical and mechanical properties of the shell were also tested. The results show that Al matrix has a net-like structure while SiC is uniformly distributed in the Al matrix. The SiC p /Al composites have a low density of 2.93 g/cm 3 , and its relative density is 98.7%. Thermal conductivity of the composites is 175 W/(m·K), coefficient of thermal expansion (CTE) is 10.3×10 −6 K −1 (25–400 °C), compressive strength is 496 MPa, bending strength is 404.5 MPa, and the main fracture mode is brittle fracture of SiC particles accompanied by ductile fracture of Al matrix. Its thermal conductivity is higher than that of Si/Al alloy, and its CTE matches with that of the chip material.
Achievements and contradictions of the Peruvian agrarian reform
Different paths of transition to capitalist agriculture, as well as different types of agrarian reform, can be distinguished. The Peruvian agrarian reform is analysed within this context and evaluated in terms of its contribution to rural development. The Peruvian case underlines the importance of understanding the rationality of the pre‐existent agrarian system for a successful restructuring of the agrarian economy and society. The contradictions which arose between the State's model and the peasants’ model of agrarian reform, as well as those between different types of peasants, are discussed. Peru's experience is also a reminder that a land reform is not a sufficient condition for rural development unless accompanied by proper price, marketing, credit and investment policies.