Application of ESEM to fluxless soldering

Abstract

The ESEM is ideally suited to study soldering processes. We have used it to observe solder reflow and joining in ambient gases. It reproduces effects of atmospheric pressure reflow in a hot stage light microscope, but with much better clarity and depth of field. Compared to a regular SEM, the ESEM offers advantages of atmosphere control and ability to observe the solder samples without carbon or gold coating. These coatings could interfere with the oxidation/reduction reactions which occur at the solder/ambient gas interface. Very thin surface films, especially oxide layers, dramatically influence the flow of liquid solder and the ability of solder to wet or join to another surface. Fluxless processes in particular are ideally suited for study in the ESEM. We have used the ESEM to observe dynamic fluxless soldering and have recorded events on videotape for later stop-action still pictures and slow motion photography. Examples of these processes are shown to illustrate the ESEM capability. Included are solder deformation structure, balling reflow of eutectic solder in hydrogen, balling reflow of eutectic solder in nitrogen, joining of two solder disks in nitrogen, and dynamic melting and freezing of an off-eutectic dendritic alloy. All of these are observed in the absence of flux.