The presence of agitation at an air / solution interface and a foaming agent causes foaming. Most water soluble fluxes contain surficants. Surficants are foaming agents. First, it's likely this foaming is caused by poor control of the temperature of the rinse water. Even a drop of 5*C may instigate massive foaming. Contact your flux supplier for a recommended temperature. In the meantime, I suggest a 50*C control point. Next, as boards are cleaned, it forms water soluble soaps in the wash solution. These soaps appear as foam. This foaming may be suppressed, but only in moderation. If the soap concentration becomes too high, rinsing becomes difficult. In that case, excessive foaming is a useful warning to change the wash solution.
BR ... davef
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