Thermal conductivity of composite materials

A composite material can be defined as a material in which two or more different materials are bonded together. It is sensible to make a distinction between macrocomposites and microcomposites. The first group usually has a layered structure, and PCBs are the main application. The second group looks uniform to the naked eye; materials from this group are often used for packages and heat spreaders.

It appears that a combination of two or more materials can have much better overall properties than the individual materials. The following quote is taken from ref. 1:

Various new advanced composite materials are now available, which provide great advantages over conventional materials for electronic packaging thermal control, including:

• Extremely high thermal conductivities
• Low, tailorable coefficients of thermal expansion
• Extremely high strength and stiffness
• Low densities
• Low cost, net shape fabrication processes

Apart from the topic of the last bullet, the other claims are related to physical properties. This technical brief focuses on thermal conductivity only, but I would like to stress that solving thermal mismatch problems is one of the great challenges for the future. Promising materials are for example Al/SiC, with a thermal conductivity about equal to Al, but with a lower density and a much more attractive CTE.

Back to thermal conductivity. As becomes evident from the table, many composite materials tend to be anisotropic.

Ref. 1: C. Zweben, High Performance Thermal Management Materials, ECM vol.5: 3, pp. 36-42