Substrate thermal interface employing helium gap

Abstract

A method and exemplary apparatus are disclosed for increasing the heat transfer capabilities between a nominally planar, but actually slightly concave, first surface (such as the substrate of a micropackage) and a planar second surface such as a heat sink. This result is obtained by filling any non-contacting space intermediate the heat transfer surfaces of the thermal interface with helium. In a test environment for a micropackage, the heat transferring surfaces are, respectively, a substrate outer face of the micropackage and a planar surface of a test fixture heatsink. Helium is introduced, under pressure, through one or more apertures provided in the heatsink planar surface to displace air and occupy any non-contacting space between the heat transferring surface which result from the presence of non-planar regions of the substrate outer face, particularly those resulting from quasi-spherical surface distortion. Such quasi-spherical surface distortion is often observed in micropackages and is typically in the form of a generally slightly concave surface configuration caused by stresses applied to the inner face of the substrate by the overlaying circuit laminations. In one embodiment of the invention, the helium is supplied at sufficient pressure to ensure a constant leak of helium to the ambient at the periphery of the thermal interface ot further promote temperature uniformity by convection as well as by the superior heat transfer characteristics of helium over air.