Abstract: Method of bonding a chip to a Cu heat sink comprises depositing a layer of Ni upon the sink, depositing a layer of Pt over the NI, and thereafter annealing the sink in a partial vacuum containing a reducing gas. After the annealing step, a metallic wetting layer of Au and a layer of In are sequentially deposited upon the Pt, and the chip is soldered to the heat sink by means of a flux free soldering process. The pure Pt is very dense and stable, does not have microholes therein which would otherwise allow the Cu ions to penetrate the Pt. For a BeO heat sink, a Ti layer is deposited over the sink rather than Ni, and a layer of Pt is thereafter deposited over the Ti layer. The Ti, Pt and BeO interact to form a good bond of high thermal conductivity, which also blocks ion migration upwardly toward the solder and the electronic chip.

Abstract: An element is soldered to a heat sink by a flux-free technique in which flux-free solder is first deposited onto the heat sink surface, then the element is placed into position on the surface, then the heat sink is placed into and held at a constant temperature in a pressure controlled chamber to a level below the melting point of the solder while undergoing a plurality of cycles of introduction and purging of an oxygen stripping gas.