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High Performance Pyrolytic Graphite Heat Spreaders – Near Isotropic Structures with Metallization
Keywords: pyrolytic graphite, metallization, CTE
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 ºK in the x-y plane of the material (7 W/m ºK 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. A case study is presented involving a laser diode application where the spreader/die thermal flux is nearly two-dimensional. PYROID® HT pyrolytic graphite acting as a heat spreader is 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) is oriented where the minimal conduction is required along the die. An additional case study is presented where a three-dimensional spreader is required and a laminated composite structure of PYROID® HT is used where the equivalent isotropic conductivity is over two times greater than the conductivity of copper. The graphite layers are metallized to allow solder bonding of the material as well as direct die attachment. The adhesion of the metal layers to the graphite was tested with several industry accepted techniques. This presentation also discusses the analysis of CTE matching, the thermal performance modeling and graphite metallization results that enable bonding of dies and devices to the graphite.
Richard J. Lemak, General Manager
MINTEQ International, Inc.
Easton, PA

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