<title>Evaluation of high-thermal-conductivity graphite fibers for thermal management in electronics applications</title>

Abstract

ABSTRACT High thermal conductivity carbon fibers are orthotropic, having significantly different properties in the longitudinal(650-1 100 W/mK) and radial directions (15-50 W/mK). These fibers can function like thermal pipes to directionallytransport heat from one location to another. The potential of such high thermal conductivity materials to facilitate enhancedthermal management in miniaturized electronics systems used in space applications is considered. Visualization of heat flowusing time-resolved infrared imaging and analysis of the capabilities of different fabricated structures for directing heat flowwill be described.Keywords: thermal management, thermal properties measurement, carbon fibers; graphite, ultra-high thermal conductivitygraphite fibers, electronic packaging 1. INTRODUCTION The management of excess heat in electronics applications is becoming an even more significant problem with new developments in the microelectronics industry. The drive towards pushing microelectronics components down to smallerand smaller sizes, packed into closer and closer proximity on circuit boards greatly increases the demands on thermalmanagement systems. Decreasing the temperature at the integrated circuits has a marked role in increasing the reliability andlifetime ofthese devices1. There are a wide variety ofapproaches currently in use as well as another whole suite of conceptsunder development. These include both active and passive means of cooling. One example of an active approach is the useof spray cooling. Here a fluid is sprayed over the components requiring cooling to remove heat by boiling of the fluid. Thefluid is then collected, circulated past a heat exchanger and then used again. Convective cooling using forced air blown overthe part is another example of active cooling which is very widely practiced.Passive approaches include the use ofheat pipes2 which are closed tubes containing a material such as ammonia. There is aphase change in this material which allows heat to be extracted from the part of concern. Another passive method is toimprove the conduction from the high power dissipating component to the backplane of the circuit board or to a dedicatedheat sink. Different materials systems have been used in these heat spreading efforts ranging from the aluminum and copperplates which have had a long history of application to the more recent developments in diamond films3. Another recentdevelopment has been the use of composite plates incorporating graphite fibers with very high thermal conductivity, ic, in thelay-up46. This approach is very attractive for space applications where minimizing weight is a key design goal.The specific thermal conductivity ofthese high ic graphite fibers is particularly impressive as shown in Table I .