Thermal tests and analysis of thin graphite heat spreader for hot spot reduction in handheld devices

Very thin heat spreaders made of materials with high in-plane thermal conductivity are desirable for thermal solutions to eliminate hot spots on packages of highly converged handheld electronic devices with shrinking form factors. Graphite is used commercially in these applications because of its unique anisotropic properties of high in-plane thermal conductivity and low thru-thickness thermal conductivity. This paper presents application-level thermal performance tests on graphite based spreaders attached to a plastic substrate. Two graphite materials, provided by GrafTech International Holdings Inc., with different thicknesses and in-plane thermal conductivities, were tested. Two types of spreader constructions were also compared. The thermal performance tests emulate the typical heat dissipation scenario for handheld device packages; a constant and uniform heat flux on a very limited area on one side of the spreader and a natural convection environment on the other side. The thermal resistance of the spreader assembly is examined as a function of the different materials and constructions. Additionally, samples were subjected to three types of accelerated life tests (ALT). The thermal performance of these samples before and after ALT were compared to determine the effects of the ALT sequences. Statistical procedures including Analysis of variance and the paired student t test were used to analyze the data.