Paste Processing and Structural Properties of Copper Metal-Matrix Composites Containing Short Carbon Fibers

Abstract:
This paper reports on the processing behavior of copper sintering paste admixed with short carbon fiber and structural properties of the resultant metal-matrix composite after sintering. Carbon fibers have zero or negative axial thermal expansion coefficient and high tensile strength, promising effective mechanical and thermal properties suitable for doublesided die attach of wide bandgap power semiconductors operating at temperatures above 200deg C. Pastes with filler fractions of 2 to 50% by wet volume were prepared and stencil printed to produce 50 to 200 µm thick paste layers, which were susequently dried and sintered in 100% H2 (pressureless, for 30 min at 290deg C) and 100% N2 (for 15min at 260deg C). The fibers had a nominal diameter of 7 µm, a 110 nm Ni coating to improve bonding strength with the Cu matrix and were cut to lengths of 100 to 250 µm. The stencil printing process was found to cause fiber orientation with small outof- plane tilt and preferential alignment with the printing direction. The composite sintering pastes were then used to sinter silicon dummy chips to copper-clad substrates. Die-shear strengths were determined for a selection of filler concentrations, and correlated with transverse and horizontal cross-sections using optical imaging and scanning acoustic microscopy. Unlike metal-matrix composites formed by hot pressing or melt-stirring, composite sinter pastes combine integration into pressureless and pressure-assisted sintering steps for die attachment with taylored material properties to address thermal fatigue in the immediate vicinity of power semiconductors and in their active structures.