Preparation and Agitation of Flux Slurry
In the simplest operation, the lid is removed and flux is manually scooped out of the drum (with a large plastic scoop) and added to the flux slurry reservoir tank. The flux should always be added to water and never scooped into an empty tank. Aerosolization should be controlled by a local exhaust ventilation system (LEV). The operator will likely need to wear a dust respirator and PVC gloves, goggles and an adequate protective coverall.
For large volumes of flux slurry preparation, it is also common to dump the entire drum contents into the reservoir with a forklift truck. Again, care should be taken to avoid dusting and aerosolization.
All slurries must be agitated to hold the flux particulate in suspension. Allowing the flux particles to settle out in the mixing tanks or containers will result in inconsistent flux loadings. During a shutdown period (maintenance, holidays etc.), the agitators may be turned off. Upon start up, it must be ensured that all settled flux is brought back into suspension prior to starting the fluxing operation. Ideally, the flux slurry should be slowly agitated during shutdown for ease of start-up.
Since the flux is insoluble in water and the goal is to keep the flux in suspension, the natural tendency is to use high agitation speeds which creates high shear forces. The high shear forces will break up particles of flux and over time (even a few hours), shift the particle size distribution to smaller particles, even to the sub-micron range. These very small particles tend to be “sticky” and when collected in one place, will acquire a gel like appearance. Furthermore, once the flux has acquired this sticky property, it is very difficult to bring this flux back in suspension after a shut-down period.
These effects may be seen even if the speed of the agitator has not changed, but the slurry consumption has decreased (e.g. one less work shift per day). In other words, the same flux is being agitated for a longer time than before and therefore may be shifting to a smaller particle size as a result of the increased residence time in the tote.
The key to agitation for flux slurries is low speed – low shear agitation to just keep the flux in suspension. Faster is definitely not better when it comes to keeping the flux slurry suspended.
Flux which has acquired a gel like consistency caused by high shear stresses may lead to strainer clogging. Even if the individual particles are small enough to pass through the mesh, once one particle sticks to the screen, others will stick to it and eventually accumulate to such an extent as to clog the strainer. Gelled flux is very difficult to bring back into suspension because it does not break up easily – the flux sticks to itself. This gelled flux will clog a small mesh size strainer in no time at all. The stickiness of sub-micron particle size flux has been associated with many blockages and is often seen to clog nozzles.
Large agglomerates are most often formed by the flaking off of flux that has dried on the walls of the spray cabinet or other nearby structures such as exhaust hoods. The best practice to avoid the formation of these agglomerates is to have a regular clean-out procedure. When this practice is not carried out, flux solids will settle out within individual droplets and form clumps or agglomerates. These agglomerates can be very hard and are also often associated with blockages.
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