A body side wafer of a stoma system is bonded to a further component thereof using an ultrasonic welding apparatus including an anvil and a horn. The horn includes a first welding surface and the anvil includes a second welding surface. At least one of the two welding surfaces includes a pattern of projections, preferably a plurality of adjacently placed conical projections such as pyramids, truncated pyramids, prismoids, cones, truncated cones or cylinders. The steps include clamping the parts to be welded between the first and second welding surface, and ultrasonically vibrating the horn with respect to the anvil. The resulting welded structure includes a first and a second welding area, respectively, which are bonded to each other by ultrasonic welding. At least one of the two welding areas includes a pattern of depressions, preferably a plurality of adjacently placed conical depressions, preferably in a goffered, honeycomb or diamond pattern.