Review of metal matrix composites with high thermal conductivity for thermal management applications

1. Introduction
The increasing requirement imposed on thermal management materials in microelectronics and semiconductors drives the development of advanced metal matrix composites (MMC) with high thermal conductivity (TC) to effectively dissipate heat and tailorable coefficient of thermal expansion (CTE) to minimize thermal stresses. This is of vital importance to enhance the performance, life cycle and reliability of electronic devices.
Metal matrix composites with high volume fraction of reinforcement are attractive in view of the possibility to further enhance TC by the use of high TC components and the flexibility to adjust the CTE by controlling the content of reinforcement. Al and Cu were usually used as metal matrix due to their high TCs, and the reinforcements involved SiC, carbon and diamond. On the consideration that the composites produced from the same reinforcement have similar difficulties during fabrication, the composites with high TC were divided into three main categories: SiC/metal, C/metal and diamond/metal composites. Owing to the fact that specific thermal conductivity (thermal conductivity divided by density) of Al-based composites was higher than that of Cu-based composites, Al-based composites are more desirable in avionic applications where light weight is demanded.
Great breakthrough was achieved in SiCp/Al composite, but its TC is still relatively low for many thermal applications. With the intention to further increase thermal properties, the reinforcements (carbon and diamond) with higher TC were introduced. C/metal composite is attractive because of its ease of machinability, while diamond/metal becomes a hotspot for its extremely high TC . However, non-wetting characteristic and undesirable interfacial reaction make great difficulty on the fabrication process and greatly limit the improvement of thermal properties of C/metal and diamond/metal composites. Thus, improving wettability and optimizing interfacial structure become challenging issues. Additionally, advanced metal matrix composites are hard to be machined into complex shape. In response to this problem, near-net shape technology was developed in our group by the combination of pressureless infiltration and powder injection molding. Brazing technique is another important aspect, but little work is done especially about Cu-matrix composites.
2. Thermo-physical properties
Thermo-physical properties were affected by matrix, reinforcement and interfacial structure. Interfacial characteristic can be controlled by modification of metal matrix, surface treatment of reinforcement and selection of suitable processing parameters.
2.1. SiC/metal composites
SiCp/Al composites are the key packaging materials at this time. The major problems encountered in the processing are bad wettability in SiC-Al system and the undesirable interfacial reaction:
2.2. Carbon/metal composites
The ease of machinability is the most attractive property of carbon/metal composites, while the bottleneck is their low value of TC.
2.3. Diamond/metal composites
Diamond exhibits exceptionally high TC (600–2300 W/(m·K)), but the TC of diamond/metal composites has not been taken into full play.
3. Processing of metal matrix composite
3.1. Liquid infiltration
Liquid infiltration involves two main steps: preform preparation and infiltration of molten metal into porous ceramic preforms.
3.2. Powder metallurgy (PM) route
For solid-state PM route, the reinforcement and metal powder are mixed and consolidated by hot pressing, high-temperature and high-pressure (HTHP) or spark plasma sintering (SPS). HPHT produces the highest thermal properties in the case of C/metal and diamond/metal composites . SPS is favorable for the suppression of interfacial reaction due to the lower sintering temperature and fast heating rate . Metal coating on the filler is necessary to assure homogeneous distribution of reinforcement and suppress the interfacial reaction. However, PM is limited to simple-shaped components with low content of reinforcement.
3.3.Rapid Solidification
4. Electronic packaging processing
Packaging process is another important factor that impacts the heat dissipation of packaging material. Conventional metal packaging process is mainly comprised of four steps. Firstly, Kovar substrates and Kovar enclosures are brazed with Ag-Cu eutectic alloy at about 830 °C to form a cavity. Secondly, the Au-Si eutectic alloy is utilized in order to attach the die to the cavity. During this process, a gold preform is placed at the top of cavity while heating the package. As the die is mounted over the gold preform, Si from the die backside diffuses into the gold perform, resulting in the formation of Au-Si alloy. A further diffusion of Si into gold preform enhances the Si-to-Au ratio of the alloy until the eutectic ratio is achieved. The Au-Si eutectic alloy contains 2.85% of Si and melts at about 363 °C. Hence, in order to obtain the eutectic melting point, normally 380430 °C, the temperature of attached die must be reasonably high. The third step is wire bonding that supplements the electrical connection between silicon chip and external leads of the semiconductor device by using very fine bonding wires. Finally, Sn-based solder is used to seal a lid to the package at 200–330 °C. For the brazing of advanced composite, the biggest challenges are surface finishing and brazing processes.
4.1. Surface finishing of composites
4.2. Brazing of Al-matrix composites
5. Future prospects and recommendations
Advanced metal matrix composites are still far from wide use due to the limit of thermo-physical property, manufacturing process, brazing technique and cost. The advantage of carbon/metal and diamond/metal composites has not been taken into full play because of the high interfacial resistance. Fundamental research on the improvement of wettability, controlling of interfacial structure and thermal conductance mechanism is of vital importance. Novel composites with co-continuous structure of hybrid reinforcement need to be emphasized. Near-net forming technique is also a key consideration. Continue improvements in packaging design and process, as well as new packaging solutions, are required.