Room temperature lead-free soldering of microelectronic components using a local heat source

This paper describes a new joining process that enables fluxless, lead-free soldering of similar and dissimilar materials at room temperature with no thermal damage to surrounding components. The joining process is based on the use of a reactive multilayer foil as a local heat source. The foils are a new class of nano-engineered materials, which consist of thousands of alternating nanoscale layers comprised of elements with large negative heats of mixing. With a small thermal or electrical stimulus, a controlled, self-propagating reaction can be initiated in these foils at room temperature. By inserting a multilayer foil between two solder layers and two components, heat generated by the reaction melts the solder and consequently bonds the components. Since the heat generated is localized to the bonding interface, components are not exposed to high temperature and hence thermal damage is avoided. Materials with dissimilar coefficients of thermal expansion can also be joined, due to the localized heating of the components. This paper focuses on an application where surface mount connectors are joined to printed circuit boards using a eutectic Au-Sn solder alloy. Details on thermal exposure of the components during joining, performance verification testing, and the process advantages are presented.