The fabrication of functional gradient hypereutectic Al-Si composites by liquid-solid separation technology

A liquid-solid separation technology (LSS) is applied to fabricate near-net shape packaging shells made of hypereutectic Al-Si composites with gradient distributions of the microstructure and properties. This new technology solves the problem of the formation of coarse flake primary Si, which will seriously deteriorate the mechanical properties of the materials, and unavoidable with conventional casting method. The microstructure of the hypereutectic Al-Si composite were characterized using scanning electron microscope, energy dispersive spectroscopy and transmission electron microscope. In order to eliminate the influence of crystal defects and stress, the effects of annealing treatment on the thermal expansion coefficient and thermal conductivity of the composites were also investigated. The results show that the volume fraction of Si decreases along a gradient from the bottom centre of the packaging shell, which houses the chips, to the top of the packaging wall, which is welded with the cover materials. The bottom of the shell has a CTE of 11.7 × 10−6 K−1, TC of 124.3 W m-1 K-1, and primary Si content of 53.7%. The top of the shell has a CTE of 18.4 × 10−6 K−1, TC of 151.2 W m-1 K-1, and primary Si content of 23.3%. Such a distribution enables a good matching of the thermal expansion behaviour of the packaging shell with the chips, which have typically low CTE, while avoiding the issue of poor soldering performance of Al/Si alloys. In addition, it reduces the internal stress of the shell due to integrated forming technology.