Dynamics of soldering reactions: microscopic observations

This paper provides a summary of some in situ, high-resolution studies of solder spreading reactions on microelectronic circuit metallizations. Experiments are described that focus on the use of the environmental scanning electron microscope, or ESEM. Those experiments have been complemented by studies using optical hot-stage microscopy and have been supplemented by additional analytical tools such as energy dispersive X-ray microanalysis, Auger and ESCA to evaluate chemical processes. Two general results from dynamic scanning electron microscope observations are that 1) molten solder alloys undergo a segregation process during spreading in which a "precursor" film spreads in advance of the bulk solder and 2) the spreading front, which may be enriched in Sn from Pb-Sn or Bi-Sn solders, or In from Pb-In solders, spreads along high-reactivity features of the metallization surface as a reacting "precursor" film. A third observation from these tests is that, in unconfined geometries, the reactive metallization, if not sufficiently thick, can be dissolved by the solder before wetting is complete, leading to dewetting of the solder. Both the kinetics and extent of spreading of these films and the relationship of these phenomena to the commonly measured contact angle and wetting forces are currently being examined by a range of complementary techniques. Information gathered in these studies shows that process temperature as well as composition, reactivity, and relative amounts of the solder and metallization species should all be factors of interest to the those responsible for control of soldering processes.