Abstract: Workpieces, such as printed circuit boards, are processed by passing the boards in contact with a standing wave of molten solder formed by circulating molten solder upwardly through a nozzle to overflow at least one edge thereof to form a standing wave which is accelerated by gravity so as normally to have an upper surface ballistically curved in the direction of flow. In accordance with the invention, the undersurface of the standing wave is supported to have a ballistic curvature, in the direction of flow, such that a major portion of the upper surface of the wave is substantially planar with the cross-sectional area of the wave varying, in the direction of flow, in a manner similar to the variation of the cross-sectional area of an unsupported wave following a ballistic curve in is trajectory.