Abstract: A tool for use in ultrasonically bonding a wire to a metal surface has a tip having a compound curved channel having a central concave surface and a peripheral non-convex surface along each edge of the central surface. The central surface has a cross-sectional shape similar to that of a portion of the wire's cross section. The total cross-sectional area of the channel should be at least 50% and less than 100% of the wire's cross-sectional area. In a preferred embodiment, the cross section of the central surface is an arc of a circle approximately centered on a line drawn between the outer edges of the peripheral surfaces of the channel. The preferred embodiment also has a beveled surface intersecting an end of the channel to provide a gradual transition for the wire shape from the bond to the undistorted wire.The tool when used to bond an appropriate wire by an ultrasonic bonding machine produces a wire whose outer shape in the bond area conforms to the shape of this tool's channel.