Research On Preparation And Performance Of Short Graphite Fiber/Al Electronic Packaging Material

High performance mesophase pitch-based graphite fiber (Grf) has become a promising reinforcement for electronic packaging composites owing to its high axial thermal conductivity (TC) to900W·m-1·K-1, negative coefficient of thermal expansion (CTE) to-1.45×10-6K-1. Therefore, this type of graphite fiber reinforced aluminum matrix (Grf/Al) composite, if processed properly, can offer a well combination of high TC, tailorable CTE, low density and superior machinability, which makes it very interesting material for thermal management applications.In the present study, surface modification of Grfs and optimization of fabrication processing parameters were used to ameliorate the interfacial bonding and improve the properties of Grf/Al composites. Surface modification of Grfs was processed by the electroless plating and molten salts method. The Grf/Al composites were prepared by vacuum hot pressing (VHP) and vacuum pressure infiltration (VPI). The processing parameters of Grf surface modification and composite fabrication route were optimized. The microstructures, interfacial structures, thermophysical and mechanical properties of Grf/Al composites were systematically investigated. The theoretical models were also used to reasonably predict the thermal properties of Grf/Al composites. The results indicated that:Five different coatings with homogenous, integrated morphologies and mean thickness of0.5μm were formed on Grfs by electroless plating and molten salts method with appropriate processes. For the electroless nickel coating process, NaH2PO2·2H2O was used as reducing agent. The nickel coating layer consisted of mainly Ni and slightly Ni3P phase. Only Cu phase was identified after electroless copper plating on Grf surface. In the chromium carbide molten salts coating process, Cr3C2was nucleated first. With the prolonged plating time, Cr atoms continuously reacted with Cr3C2to form Cr7C3. As a result, the major component of chromium carbide coating was Cr7C3. TiC and slight Ti both appeared in Ti-coated Grfs. With the plating time increasing, Ti phase was gradually transformed into TiC until the coating was totally composed of TiC. Only Mo2C was detected in the molybdenum carbide coating layer. In the molten salts process of Grfs and MoO3, with the increase of plating temperature, the formation of Mo2C occurred in two steps:the reduction of MoO3to MoO2and reduction of MoO2to Mo2C. The surface modification of GrfS had a positive effect on improving the interfacial bonding between Al matrix and Grf, and in turn facilitated the densification of overall coated composites. Sintered at650℃under a pressure of60MPa for40min by VHP, aluminum matrix composite reinforced with20-40vol.%coated Grfs obtained relative density up to98.8%. Due to the unidirectional press process, the fibers mostly lay in perpendicular to press direction (i.e. X-Y direction) and thus the Grf/Al composites showed strong anisotropic microstructure and properties. Proper Grf preform was fabricated by paraffin wax-based and starch-based binders. Infiltrated at800℃under infiltration pressure of1MPa for10min and solidification pressure of30MPa for30min when cooled to600℃by a modified two-step VPI, aluminum matrix composite reinforced with40-60vol.%coated Grfs achieved relative density up to99.1%. The composites with higher fiber volume fraction could be densified by two-step VPI. Besides, for coated Grf/Al composite with the same fiber volume fraction of40%, the TC of composite by two-step VPI was higher than that of composite by VHP. VPI was more applicable for the preparation of high performance Grf/Al electronic packaging materials.The combined Maxwell-Garnett effective medium approach and acoustic mismatch model were used to predict and analyze the TCs of Grf/Al composites. Theoretically, the uncoated composite was the most favorable since a diffusion-bonding layer contributed to the perfect interfacial bonding between pure Grf and Al. However, as the experimental results, the TC of uncoated composite was the lowest owing to the weak interface including pores, cracks and undesirable interfacial reaction which impaired the thermal transfer at the interface. The coating layer greatly strengthened the interfacial bonding between Grf and Al resulting in higher TC of coated composites. Aluminum matrix composites reinforced with carbide coated Grfs by molten salts method achieved the highest TC since carbide coating formed continuous combination of Grf and Al. Nevertheless, the diffusion between coating element and aluminum varied in which the diffusion of Ti was farther than that of Cr, Mo. The Ni, Cu coating by electroless plating improved the wetting between Grf and Al by the diffusion of nickel, copper into the matrix and formation of Al3Ni, Al2Cu. However, it was difficult to form a continuous intermetallic compound transition layer at the interface leading to lower TC than that of carbide-coated composites. Therefore, it was necessary to choose a coating with high TC itself and little diffusion into aluminum matrix in order to obtain Grf/Al composite with high TC. The descending orders for the TC of coated composites were the Mo2C, Cr7C3, TiC, Cu and Ni-coated Grf/Al composite. The CTE of composite decreased with fiber volume fraction increasing. The higher interfacial bonding strength, the lower CTE of coating layer material, the lower CTE of the composite was. Thus, the ascending orders for the CTE of coated composites were the Mo2C, TiC, Cr7C3, Ni and Cu-coated Grf/Al composite.The Mo2C coating was found to be the most effective in improving the thermophysical properties. Besides, Mo2C-coated composite exhibited the best mechanical properties owing to its continuous, smooth and strongly bonded interface, combined with the moderate diffusion in aluminum matrix. The bending strength of composites slightly decreased (from217MPa to169MPa) with increasing fiber volume fraction from40%to60%due to the incremental defects in composites. As the results, the Mo2C-coated Grf/Al composite achieved TC of221-228W·m-1·K-1, which agreed reasonably well with the theoretical predictions, as well as CTE of (6.0-9.4)×10-6K-1,which made it well satisfy the comprehensive performance requirements of electronic packaging material in modern thermal management.