Wetting, solder dynamics

Many wave designs are available from the various wave-solder machine manufacturers.There are wave machines that provide multiple smaller, turbulent wave(s), which are best for leadless components such as surface-mount passive devices (resistors, capacitors, etc.). Smoother-flowing waves are recommended for components with leads and through holes as well as coarse-pitch surface-mount devices. Wave dynamics are dictated by process values as well as the materials in contact with the wave. As the solder wets to the circuit board materials, solder wetting contact angle and solder viscosity imposes wave peel-off characteristics (see Fig. 47.23). 

In general, a complete understanding of the wetting balance force-time curve for solders awaits a rigorous theoretical analysis capable of resolving aU the dynamic aspects of the solder flow noted above. 

The logical implication of Eq. (46) for dynamic contact angles (i.e., for a moving contact line) is that solders will wet a partially oxidized surface at a slower rate than the pure metal. The rate of advance of the contact line over the patch depends on pinning dynamics. The shape of the contact line is distorted at each point the liquid encounters an oxide patch. Contact line motion on heterogeneous surfaces typically exhibits an erratic behavior often referred to as stick-slip motion. It is possible that a gaseous inclusion will be formed when the solder negotiates a patch if the surfaces of the deformed liquid on either side of the patch coalesce before the entire patch is wetted. 

Tsung-Yu Pan, J.M. Nicholson, H.D. Blair, R.H., et al. Dynamic wetting characteristics of some lead-free solders. Proceedings of the 7th International SAMPE Electronics Conference, Parsippany, NJ June 20-23, 1994 343-354. 

---