Process of monitoring for the reflectivity change in indium phase transition soldering

摘要

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. Then the heat sink is raised to a temperature sufficient to cause a solid to liquid phase change to the solder layer in the presence of the oxygen stripping gas which is marked by optically detecting the change by monitoring the reflectivity change characteristic of the phase transition then holding the solder in a liquid state for a second interval of time sufficient to allow the majority of the solder oxidation at the interface between the solder and the element to be removed by the action of the oxygen stripping gas, and then rapidly cooling the heat sink and the element to room temperature.