Adhesion of precision welded lead-free electrical interconnects formed by molten droplet deposition

An existing process, droplet welding, has been proposed for the production of precision, high-temperature, lead-free electrical joints. A modified metal inert gas (MIG) welding plasma is used to produce molten metal droplets, which then fall on a part to make an electrical joint. The subject of the present paper is an investigation of the factors affecting successful welded joint formation for a given droplet material and target, with the goal of providing the basis of a computer model to enable rapid process set-up on a production line. It is found that a parameter space can be identified for good adhesion of a droplet to a target, characterized by droplet temperature and target thickness, for each droplet material/target material combination. Essentially adhesion can be viewed as determined by competition between the delivery of thermal energy from the droplet to the target immediately underneath the droplet, and the removal of the energy from the interface region to the rest of the target, with no role played by the droplet kinetics after impact. It is therefore concluded that a relatively simple thermal model could be used by production-line engineers to identify the parameter space for rapid process set-up with new material combinations and products. The conclusion is supported by evidence from high-speed video images of droplet impact. Such a simple thermal model is proposed and is found to be capable of predicting adhesion between droplet and target. The results are discussed in the context of the extensive literature on molten droplet impact and solidification.