Manufacturers of modern high performance military and commercial electronics are steadily increasing the power density of the components of their devices. The increase of small component size and/or higher power densities results in large component heat flux levels. In applications from laser diodes, radar transmit/receive (T/R) modules and LED lighting, solutions to thermal issues through advancements in heat spreaders and heat sink design are needed to continue the performance improvement of these devices and structures.
Advanced materials for heat spreaders should have high thermal conductivity, a coefficient of thermal expansion (CTE) that matches the electronics die, and a reasonable cost. Diamond, for example, has the highest thermal conductivity of any known solid material at room temperature, combined with a high modulus of elasticity, but has a CTE mismatch with silicon (die material) and a high relative cost.
As an alternative material, PYROID® HT pyrolytic graphite has a thermal conductivity of 1700 W/m °K in the x-y plane of the material (7 W/m °K in the z plane), costs more than 250 times less than diamond produced by chemical vapor deposition (CVD), and has a modulus of elasticity much lower than diamond. The lower modulus of elasticity of this material results in an order of magnitude lower thermal stress level between the spreader and the die than diamond.
In one case study involving a laser diode cooling application, the spreader/die is nearly two-dimensional. Thus, pyrolytic graphite can be oriented with high conductivity in the direction into the spreader and away from the die (x-y plane), and the low conductivity direction (z direction) along the die where minimal conduction is needed. For the cases where a three-dimensional spreader is required, laminated composite structures of pyrolytic graphite have been developed where the equivalent isotropic conductivity is over two times greater than the conductivity of copper. The graphite layers are metallized so that they can be bonded together by soldering and/or dies can also be attached by soldering. The adhesion of the metal layers to the graphite has been tested with several standard techniques. Graphite metallization techniques, thermal performance modeling and analysis of stress levels due to CTE mismatching will be discussed in this paper.