A number of high performance materials now available for electronic housings and packages that can provide advantages over traditional choices. Most of these materials are advanced composites that are lighter, more weather resistant, with better thermal properties.
Some advanced materials provide thermal conductivities more than four times those of copper. The benefits from this include improved thermal design, and the ability to eliminate heat pipes, fans, and liquid loop cooling systems, including their space requirements and noise. Weight savings can reach 90% and size reductions up to 65%. Using advanced composites can also reduce power consumption, thermal stress and warping. CTEs for such materials can be more precisely matched, allowing direct bonding attach. Systems using these composites have increased reliability and improved performance. For OEMs, these materials increase manufacturing yields and may lead to part and system cost reductions. 
Advanced Material Classes
Advanced composite materials with thermal properties suitable for electronic packaging include monolithic carbonaceous materials and carbon matrix composites, metal matrix composites (MMCs), ceramic matrix composites (CMCs), and polymer matrix composites (PMCs).
All of these composites consist of two or more physically and/or chemically distinct components that together provide characteristics superior to the individual materials. Typically, a reinforcing component is distributed in a continuous or matrix component. When the matrix is a metal, the composite is termed a metal matrix composite, or MMC. In MMCs, the reinforcement usually takes the form of particles, whiskers, short fibers or continuous fibers. 
Composites are nothing new in electronic packaging. For example, E-glass fiber-reinforced polymer PCBs are PMCs; and copper/tungsten and copper/molybdenum are MMCs, rather than alloys. There are also many ceramic-particle and metal-particle-reinforced polymers used for thermal interface materials (TIMs), underfills, encapsulants, and electrically conductive adhesives. All are PMCs. 
Monolithic Carbonaceous Materials and Carbon Matrix Composites
Advanced carbon materials used for electronic packaging and electronics thermal management are typically monolithic carbonaceous materials. These include graphite and diamond, along with carbon matrix composites. They may be combined with metals or other materials to yield complex materials that are easier to process into manufactured components. Graphitic materials are lower density than metallic materials such as copper and aluminum, but provide equal or higher thermal conductivities.
Figure 1. (a) Phase Diagram of Carbon, and Carbon Allotropes: (b) Diamond, (c) Graphite, (d) Fullerene, and (e) Carbon Nanotube. 
Carbonaceous materials can be processed to form low-density thermal insulators to protect electronics from heat sources.
Metal Matrix Composites
Metal matrix composites, or MMCs, are made by dispersing a reinforcing material into a metal matrix such as aluminum, magnesium or titanium. Reinforcing materials include SiC (silicon carbide) and carbon and diamond. Among other characteristics of the reinforcement generally distinguish MMCs: short fiber- or whisker-reinforced MMCs, and continuous fiber- or layered MMCs. Continuous fibers typically provide the highest degree of load transfer from matrix to reinforcement, owing to their high aspect ratio. Particle and short fiber reinforced metals have a lower aspect ratio, and thus exhibit lower strengths than their continuous fiber counterparts. MMCs can be tailored to have optimal thermal and physical properties for packaging systems including substrates, carriers and housings. Continuous boron fiber-reinforced aluminum composites have been used as heat sinks in chip carrier multilayer boards. 
Figure 2. Silicon Carbide Fiber-Reinforced Copper Metal Matrix Composite. 
Several MMCs with high thermal conductivity and adjustable coefficients of thermal expansion are used in electronic packaging for thermal management. An issue with MMCs is their fabrication into usable parts. For example, while C/metal composites are easily machine-fabricated, diamond/metal composite has non-wetting characteristics and presents undesirable interfacial reactions in the fabrication process.
Qu et al  looked at key factors hampering the manufacture of MMCs, and ways to improve their thermo-physical performance. They developed methods to overcome MMC difficulties in terms of thermo-physical properties, processing methods and electronic packaging processing. The authors found that a combination of pressure-less infiltration and powder injection molding could produce near optimal shape composites. Improving wettability and optimizing interfacial structure were prerequisites for successful fabrication and further enhancement of thermal properties. 
Ceramic Matrix Composites
Ceramic matrix composites provide high thermal conductivity and low CTE. Other desirable characteristics of ceramic matrix composites (CMCs) include high-temperature stability, high thermal shock resistance, high hardness, high corrosion resistance, light weight, nonmagnetic and nonconductive properties, and versatility in providing unique engineering solutions. CMCs consist of ceramic fibers embedded in a ceramic matrix, forming a ceramic fiber reinforced ceramic (CFRC) material. Common fibers used in CMCs include SiC and alumina. More recent CMC materials include reaction-bonded SiC composites, aluminum-toughened SiC composites, and ceramic-based nanocomposites. 
Figure 3. A Highly Heat Resistant CMC Combustion Chamber Element from Airbus Defense and Space. 
Their intrinsic ability to be tailored as composites make CMCs highly attractive in a vast array of high heat applications, most notably internal engine components, exhaust systems and other “hot-zone” structures, where CMCs are envisioned as lightweight replacements for metallic super alloys. 
Polymer Matrix Composites
Both thermosetting and thermoplastic polymer matrices present with reasonable thermal conductivity. Different kinds of fillers play important roles in maximizing polymer performance and production efficiency. Cost reduction, density control, optical effects, thermal conductivity, magnetic properties, flame retardancy, and improved hardness and tear resistance have increased the demand for high performance fillers. Several types of reinforcements, especially nanoparticulate fillers, have been used in polymer matrix composites: vapor grown carbon fiber (VGCF), carbon foam, carbon nanotube (CNT), and other thermally conductive particles, such as ceramic, carbon, metal or metal-coated particles, as well as metal or carbon foams. Nanoparticles of carbides and nitrides can be used to reinforce polymer matrix nanocomposites to improve thermal conductivity, mechanical strength, hardness, corrosion resistance, and wear resistance. 
Figure 4. Carbon Fiber-Reinforced Polymer Matrix Composites with Pitch-based Fibers Provide Thermal Conductivities Up to 1100 W/m·K. 
In a study of polymer composites by Lee, et al, different inorganic fillers including aluminum nitride, wollastonite, silicon carbide whiskers and boron nitride with different shapes and sizes were used alone or in combination to prepare thermally conductive polymer composites. The use of hybrid fillers was found to be effective in increasing thermal conductivity. For a given filler load, the use of larger particles and surface treated fillers resulted in composite materials with enhanced thermal conductivity. The surface treatment of filler also allowed producing the composites with lower CTE. 
Materials used for electric and electronic enclosures – steel, aluminum and polymers – date as far back as the nineteenth century. Many applications now require enclosures that are lighter and/or more thermally conductive than can be obtained with these materials. This applies to commercial, military and aerospace applications. We now have several advanced materials that can provide significant thermal and mechanical improvements, and as the result are being used in an increasing number of applications where cost is not challenged, yet performance and application require these.
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