Preparation And Study On The Diamond (Boron Carbide)/Metal Composites Used In Electronic Packaging

Metal matrixes composites are very promising electronic packaging materials, for composite possess high thermal conductivity and appropriate coefficient of thermal expansion. As a result, Ceramic particle reinforced metal copper-based composites have been introduced to prepare heat sink materials. In this study, we have developed boron carbide-copper composites and diamond-copper composites for different applications. Electroless copper plating process is introduced to improve the bonding of ceramic particles and metal matrix as copper does not adhere well to diamond, and a new pretreatment process is performed to improve the bonding strength between the diamond reinforcement and the copper matrix. A series of analysis technique including XRD, SEM, EDS and TEM were employed to investigate the pretreatment process parameters to improve thermal conductivity of metal matrix composites. The main results and new findings in this work are summarized as follows:(1) B4C/Cu composite with enhanced thermal-physical properties was fabricated after the process of electroless deposition of copper on the boron carbide particles, in which the salt-based colloid palladium activation process was introduced into the electroless deposition and uniform copper coating was formed on the surface of B4C particles. The process of electroless depositon includes the concentrated nitric acid through roughening pretreatment process, ultrasonic cleaning in alcohol process, the sensitized and activated process in salt-based colloid palladium and electroless depositon of copper on B4C particles. Electroless plating copper on B4C particles is beneficial in improving the uniformity of the distribution of copper in the composites and bonding of interface between copper and B4C. The microstructure of the sintered sample are investigated by SEM, which shows that high quality copper deposits are obtained on the B4C particles surface accompanied by the good interfacial contact between the copper and the pretreated B4C powder. Composites with relatively low coefficient of thermal expansion and an efficient thermal conductivity will be obtained, when the volume fraction of B4C is40%～70%. (2) Boron nanostructure was fabricated on diamond particle surface by a new heat treatment process, in which the boron powder was mixed with copper powder and diamond powder at high temperature. This work demonstrates that the morphology of the surface of the diamond particles is dependent on the treatment temperature during the heat treatment process. Boron nanowires was fabricated on the diamond particle surface when the diamond powder was mixed with copper powder and boron powder at1060℃, while nanopollars and nanosheets were grown on the surface of diamond powder when the heat treatment process performed at1120℃. The boron nanostructure was confirmed to be crystalline rather than amorphous material, and its structure was identified to be hexagonal structure. The role of the liquid phase copper during the process of boron nanostructures grown on the diamond surface is that the fluidity of the liquid-phase copper at high temperature transmits boron atoms to the surface of diamond, which improves the uniformity of BNWs on the diamond particles surface. Diamond/Cu composites were prepared after the pretreatment of diamond powder, and the thermal conductivity of composites pretreated with B powder reaches660W/(m·K) when the diamond particle were coved by a three dimension network of nanowires which effectively increase the contact area between the diamond and copper.(3) Cu/diamond composites were fabricated by spark plasma sintering (SPS) after the pretreatment of the diamond powders. The pretreatment coating method is better than the other coating technology in the diamond particle surface treatment, which including the vacuum micro-deposition technology and the sputtering technology. The new pretreatment coating method not only solves the interface problem with a lower price by a simple vacuum furnace but ensures the thermal conductivity of the copper matrix without any formation of copper alloy in copper matrix. The carbide former boron appears to be the best candidates for diamond powder pretreated process, and the carbide former tungsten enhances more effectively the thermal conductivity of Cu/diamond composites than the diamond pretreated with the other carbide former metal powder. Apparently, the formation of large number of nanopillars and nanosheets on the diamond particle surface makes the boron element the better candidate for the pretreatment process, because the TC of B-coated diamond/Cu composites are better than the W-coated diamond/Cu composites, when200μm diamond particle with the diamond volume fraction of44%are used in the fabrication process of Cu-diamond composites. The intrinsic conductance of the interfacial layer play a important role in the thermal conductivity of the composites which is another reason we choose W as the carbide former, because the WC and W-Cu pseudo alloy layer have a better thermal conductance than the other carbide layer formed in the Cu-X/diamond(X=Cr, B and Ti) system.(4) The pretreatment temperature and the diamond particle size play a crucial role in the thermal conductivity of the composite at a fixed particle volume fraction of46%. The thermal conductivity is up to631W/(m·K) and can achieve to the maximum value of672W/(m·K), when the200μm and300μm diamond particle was pretreated at1293K and1313K, respectively.(5) The effective thermal conductivity of each type of material is analyzed using four models for composites (Maxwell model, PG Klemens model, Hassel man and John-son model and Generalized Self-Consistent Scheme model). These analytical models were developed using effective medium theory and differ in their assumptions concerning inter-particle interaction and particle distribution, shape and size, and are valid for relatively small filler amounts. The effective conductivity depends primarily on the diamond contents of filler particle and thermal conductivity of the matrix.