AUTHOR: EDWARD L. DREIZIN TITLE: DROPLET WELDING: A New Technique for Welding Electrical Contacts

The joining of wires to various substrates (wires, plates, foils and semiconductor packages) is a ubiquitous process in the manufacture and modification of electrical and electronic devices. While spot welding is used extensively to attach lamp and vacuum tube filaments to base wires and foils, soldering is almost universally used to make chip to package and wire to substrate interconnections in electronic devices (Refs. 1-3). Reliability problems with joints made by using these techniques stem from failures associated with mechanically and thermally induced stresses, surface contamination, the complex metallurgy of multiple layers and thermal expansion mismatch between the chip or substrate and the solder. This article reports the results of a feasibility study of a new joining technique that may overcome many of these problems. The new technique, called DROW (droplet welding), for joining (welding) metals on a scale appropriate for electronic and electrical connections is made possible by the recent development of the novel generator of monodisperse metal droplets (GEMMED) (Refs. 4-7). The GEMMED is a pulsed microarc device that produces spherical, 50-1000-μm-diameter, high-temperature metal droplets and uniquely provides uniform metal droplets with controlled, repeatable droplet size, temperature and trajectory. Droplet initial velocity varies in the 0.5-5 m/s range depending on the metal and the droplet diameter. Joining in DROW is achieved by deliberate deposition of high-temperature metal droplets (filler metal) on the site of contact of the joined components. Therefore, only the filler metal is heated by the microarc. When the droplet impinges on the surfaces, its heat content produces localized heating and melting of the components to be joined. The ability to control the size and initial droplet temperature allows one to tailor the extent of heating (melting) to the requirements of the joint. Because DROW applications can use essentially any metal as the weld filler, it is expected that DROW will allow filler material to be selected to reduce or eliminate mismatch problems and to simplify the metallurgy of the joints. The absence of fluxes in DROW should reduce surface contamination problems, and the mechanical strength of the welds should be higher than those of brazed or soldered connections. In contact welding (spot welding) applications, reliability is the major problem, especially when refractory metals are welded together as in lamp and vacuum tube applications. DROW is a completely new approach to the welding of small specimens of refractory materials; it makes a conventional weld joint instead of an electrically fused joint. Reliability and strength should also improve.