Module of a wave soldering system to spray flux on the underside of a passing board. The bristles of a horizontally mounted brush absorb, at their lowest point in turning, flux from a reservoir through which they pass. The wet bristles continue to rotate upwards, passing by an equally horizontal mounted sharp blade. Thereby the bristles get initially tensioned, and. after having passed the blade, suddenly relieved. The flux that had been carried by the bristles is now sprayed onto the underside of the pcb.