Wedge bonding is performed using a wedge-shaped bonding tool. The wire in wedge bonding is addressed at an angle (30° to 60°) through the rear of the wedge. When special clearance is necessary the wire will be fed at 90° through a hole in the shank for maximum clearance. Low angle wire feed style gives best placement control and tail consistency under the bond foot. High angle wire feed is only used when absolutely necessary due to high package walls where the bonding to the edge of the die is necessary. Tail control and bond placement accuracy is less consistent due to the steep feed angle which causes the wire tail to contact the pad prematurely causing the wire to shift away from underneath the bonding tip or be pushed back into the feed hole. Unlike at lower feed angles (30° to 45°) the wire is in line with the tip. Foot profile of the wedge can be either flat or concave. Most of the automatic aluminum wire applications use the concave foot to reduce wire positioning errors. The flat foot is used mainly with gold wire or with aluminum wire to obtain extremely short bonds. A groove foot has been designed for gold wire wedge bonding to improve the wedge-to-wire gripping. The material used for the wedge is dependent on the bonding wire material. For aluminum wire, the wedge is made of tungsten carbide. For gold wire, the material used is titanium carbide or cermet tip. The cermet tip wedge is most commonly used in applications where low temperature Au to Au bonding is required.
The parameters of the wedge can greatly affect the wire-bond characteristics. For the first bond, pull strength is affected by back radius (BR), bond location is influenced by hole size (H) and tail length is controlled by feed angle, hole shape and surface quality. The main wedge parameters that affect looping are hole size and shape, as well as feed angle. For the second bond, pull strength is defined mainly by front radius (FR), bond length and tail consistency is affected by back radius (BR).
|Wedge for Al wire
|Wedge for Au
Another major feature of ultrasonic wedges is the Bond Flat (BF). The Bond Flat is defined as the length of the bonding tool foot that appears flat when measuring with a microscope at 300X will give a very close approximation to the actual bond length achieved during the bond process.