Reducing hot spots and junction temperatures of integrated circuits using carbon composite in a printed circuit board and substrate

Abstract

Carbon composite laminate is used in the integral structure of printed circuit boards (PCB) today to spread the heat from the heat source mounted on the surface. The thermal conductivity of carbon fiber used in the composite is lateral and ranging from 10W/m.K to 600W/m.K. This lateral property results in high thermal conductivity in-plane, opposed to the through-plane of composite. This anisotropic thermal property has the unique advantage of spreading heat throughout the entire surface area of the printed circuit board. The composite layer can be used as an internal thermal plane layer and heat from the heat source can be conducted to the carbon composite layer through thermal vias. The composite layer can also be used as a ground or power layer to improve heat conduction from the heat source to the carbon composite layer through all ground or power via connections. Carbon composite also acts as an internal heat spreader thus, concentrated heat from the high power ICs can be spread out to entire plane area. This reduces or eliminates localized and reduces junction temperature of the IC components. Further heat can be removed from the PCB surface using a forced or natural convection cooling mechanism. This type of internal heat spreading method can be extremely advantageous in an application such as a memory module which uses stacked devices. In a stacked memory module, the inside DRAM is trapped between outside DRAM and the PCB. The outside DRAM receives fanned air but the inside DRAM has no way to dissipate heat to the environment. A series of tests have been performed and have shown 8-12degC temperature differences from standard materials using carbon composite in the PCB. Beside the thermal benefits, carbon composite has several other benefits. It allows the designer to tailor the coefficient of thermal expansion (CTE) of the PCB to match with the CTE of components, it increases rigidity/stiffness by magnitudes, and it does not add any weight over FR4, polyimide and standard materials. A designer can reduce hot spots and the junction temperature of the hot components by selecting the proper carbon composite type and selecting the proper number of composite layers in the structure of the PCB. Several other materials can also be used as a thermal plane such as copper, copper-moly-copper (CMC), copper-Invar-copper (CIC), aluminum nitride and aluminum silicon carbide (AlSiC)