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High reliability of advanced thermal interface materials with carbon nanotubes
Keywords: Carbon nanotubes, TIMs, Reliabitity
Carbon nanotubes (CNTs) are promising materials for future electronic devices due to their excellent physical properties such as high thermal conductivity and mechanical property. Therefore, various applications with CNTs have been suggested, including transistors, interconnects, and thermal bumps. Thermal interface materials (TIMs) is one such application and is used to thermally connect a heat spreader and processor tip. Although an individual CNT exhibits extremely high thermal conductivity, around 3000 W/mK [1, 2], there are issues to be addressed to fabricate CNT-TIM. One issue is poor CNT density. After vertically aligned CNTs are grown on a silicon substrate, they are applied to the CNT-TIM directly. Therefore, the density of as-grown CNTs determines the number of heat passes from a heat source (processor tip) to a heat sink (heat spreader). The packing ratio of as-grown CNTs is known to be relatively very low (around several percent) due to difficulties in CNT growth with high density. Furthermore, the reliability of CNT-TIM, including adhesion, is unknown because CNT-TIM is a new material that differs in terms of adhesion from indium film and/or thermal grease, which are generally used as TIMs. We propose a method for densifying CNT by using silicone rubber and compare the thermal resistance of CNT-TIM fabricated from a densified CNT sheet with indium film. After densification, the density of the CNTs was three times higher than that before densification without causing damage. The thermal resistance of the CNT-TIM was found to be as low as that of indium film by using a temperature gradient method. We also conducted a temperature cycling test (from -40ºC to 125ºC) on the CNT-TIM to evaluate its reliability. The CNT-TIM was found to be stable regarding its thermal property and adhesion over 1500 cycles. These results suggest that CNTs may be a candidate as advanced TIMs having high thermal conductivity and reliability.
Daiyu Kondo,
Fujitsu Laboratories Ltd.
Atsugi, Kanagawa

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