Direct Solar-Thermal Formation of Graphitic Heat Spreaders on Organic Substrates

Limited heat spreading in electronic devices is a significant issue that affects their overall performance and thermal management, especially in power amplifiers and integrated circuits. A heat-spreading layer can be synthesized by various methods, such as chemical vapor deposition and substrate bonding. In this study, a green manufacturing approach based on concentrated solar irradiation directly converts organic substrates into graphitic form with short-duration (tens of seconds) and high-flux irradiation (1 - 2 kW) from a solar simulator. In this custom-designed high-flux instrument, a 10 kW
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xenon light source produces a peak heated region about 3 cm in diameter and a maximum heat flux of 4.5 MW/m2 (4500 suns). Process optimization is pursued to produce graphitic layers thicker than 100 µm, large converted areas of the substrate, good uniformity, and substrate adhesion by varying parameters, including the number and duration of flashes, the time between flashes, off-focal plane location, and irradiation power. The heat-affected region is confirmed as graphitic by analyzing peaks from Raman spectra. Surface reflectance is measured to investigate how much light is absorbed by the sample. Graphite thickness is measured by cross-section field emission scanning electron microscopy. The performance of graphitic layers as a heat spreader is confirmed from heat spreading temperature data using an infrared camera while operating a GaN chip, where chip is mounted on the organic substrate by thermal compression bonding. This solar-thermal graphitization work represents a green manufacturing milestone for fabricating a heat spreader using solar energy.