Optimization of a Flux-Less Metal Thermal Interface Material by Reflow in a Vacuum Furnace

Abstract

This paper describes the work performed with a pure metal thermal interface material (TIM) for the sole purpose to improve the transfer of heat from the die to the metal cover case. A flux-less reflow process is employed in order to reflow the indium TIM material. This operation is performed in a vacuum furnace utilizing heat, vacuum, and pressure in a specific sequence in order to wet the metal lid and the backside of the flip chip die. The initial objective was to demonstrate minimal voiding of the TIM and subsequently limited flow out of molten solder from and along the sides of the die. A series of experiments were employed where acceptance criteria is evaluated by a) X-Ray, b) scanning acoustical microscopy (SAM), and c) cross-section. Acceptance criteria consists of 1) indium wetting of both lid to indium interface and indium to silicon interface die, 2) indium bond line (BLT) thickness, 3) lid tilt, and 4) lid shear strength. Acceptance is determined after a subsequent 4X ball grid array (BGA) reflow in a conventional belt reflow furnace with minimal voiding, no popcorn or blistering of the laminate substrate, and TIM thickness and solder flow out at sides of the die within the acceptable limits of the above mentioned criteria.