Heat transfer apparatus which accommodates elevational disparity across

Abstract

A heat transfer apparatus is presented accommodating elevational disparity of an upper surface of a semiconductor device with respect to the component side of a PCB without adversely affecting system reliability. The heat transfer apparatus includes a thermally conductive cap structure positioned between the semiconductor device and an ambient and thermally coupled to the semiconductor device. One embodiment includes one or more spacers which maintain a space between the cap structure and the semiconductor device. A chip mounted to the substrate of a ceramic BGA package is mechanically isolated from the cap structure by the spacers, preventing chip damage due to shock and vibration. A backing plate on a side of the PCB opposite the semiconductor device provides PCB structural support and additional heat transfer. Several fasteners attach the backing plate to the cap structure. A portion of the fasteners exert a force which urges the cap structure toward the semiconductor device, while the remainder of the fasteners fix the relative positions of the backing plate and the cap structure. Fixing the relative positions of the backing plate and the cap structure reduces creep problems and promotes stress relief in viscoplastic elements. A first thermal interface layer between the cap structure and the semiconductor device achieves an acceptably low thermal resistance value under a relatively small amount of pressure. The spacers are omitted in an alternate embodiment wherein the first thermal interface layer has a uniform thickness and a consistency which permits the thickness to be maintained during installation.