spray impact:
flux removal

Removing solder flux from densely populated surface-mount printed circuit boards is, in contrast, primarily an erosive process.

above: count mapping of droplets meeting micro-jet threshold

Flux removal is further challenged by the exceedingly small clearance between the surface-mount components and the board substrate. Studies performed at spray research utilizing our high-speed video system have demonstrated the importance of  high-velocity lateral micro-jets to the effective removal of flux from under the surface-mount components. These micro-jets are produced by the reflected impact shock wave off the trailing air-liquid interface. The height of the micro-jets is dependent on a number of factors, the predominant ones being the droplet diameter and velocity. As the micro-jet height approaches that of the clearance below the components, flux removal rates fall precipitously. To optimize  flux removal it is therefore necessary to produce a spray that contains few droplets with diameters and velocities that would create micro-jets thicker than this limit.

Although this de-scaling nozzle would not normally be employed in flux removal applications, the above plot illustrates the impact frequency [in impacts per square centimeter-sec] of droplets whose velocity and diameter are such to produce lateral micro-jets of appropriate thickness. The contour scale is logarithmic [i.e. - red = 100,000 counts, blue = 10,0000, cyan = 1,000 etc.].