A technique for the design of 3-D RF welding electrodes by first modeling the surface of the welded part perimeter as a spline in a 3-D CAD program, and then dividing the spline up into multiple, small straight segments. The straight segments are offset simultaneously in opposite directions from the spline, to form two sets of straight segments, each set being equidistant from the spline, with the gap between the two sets equal to the target thickness of the welded package. New die faces are constructed from each of the sets using smoothing techniques, and 3-D models of the die faces are manufactured to check accuracy of the design, making sure that all the corresponding points of the pair of electrodes are equidistant, thereby creating a strong weld with good cosmetic results all along the parts being joined.