Wetting, solder viscosity

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). 

Temperature is of vital importance when soldering. Several key characteristics strongly depend on it, including surface tension, viscosity, and wetting force. What has been observed for leadbearing solders also holds true for lead-free alloys Wetting performance is better at higher temperatures. 

Besides viscosity, the surface-wetting ability of underfills is critical to capillary flow. For capillary flow to occur, the underfill material must wet the surfaces so that the advancing contact angle is less than 90°. " Also, for capillary flow, the intramolecular forces of attraction among adhesive molecules must be weaker than the intermolecular attraction of the adhesive for the die, the substrate, and the solder surfaces.t This occurs when the surface tension of the underfill is lower than the surface energy of... 

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