The wire bond is the most common of all packaging methods for connecting a device or “chip” to a leadframe or substrate.  Automatic gold wire ball-bonding machines are the most common in use today producing 10-15 ball bonds per second. The less frequently used wedge bonders are popular in the packaging of medium to high power Radio Frequency (RF) devices  power semiconductors, and High power, insulated gate bipolar transistor (IGBT) based modules.

The main difference between ball bonding and wedge bonding machines is basically a trade-off of speed for flexibility. For example, ball bonding machines can run at twice the speed of wedge bonders, but can only use gold or copper as the feed stock material.  Wedge bonders can stitch with gold, copper, aluminum, platinum, and alloys.

The wedge bond (Figure 4-1) is formed through a combination of ultrasonic action and slight pressure of the bond head to the metal surface. Low frequency (20-60 KHz) is applied at room temperature for large diameter wires (10-20 mil diameters) and 120 KHz for fine wire applications (2-5 mil).  In contrast, the gold ball bond (Figure 4-2) is created by thermo compression bonding at 90 to 150 degrees centigrade with or without ultrasonic action added.  It should be noted that while the wedge bond can be made at room temperature, the most reliable wire bonds will come from the optimal combination  of all the variables - time, temperature, pressure, and ultrasound. Figure 4-3 shows a stacked die package with a unique style of low profile attachment method known as Reverse Standoff Stitch Bonding (RSSB).

There are applications where wedge bonding has a distinct advantage over ball bonding.  For example, flex circuits are not good candidates for gold ball bonding due to the potential for the adhesive layers on the flex to heat up and soften in the bond tool. The softened flex absorbs more of the ultrasonic energy than the metal surface creating a low strength connection with low reliability. Advantages and disadvantages of each bonding method should be explored for a given application to ensure the desired outcome.

A popular board finish today is Electroless Nickel Immersion Gold (ENIG) over copper. Since the gold in this finish is thin (2 to 8 micro-inches), heating of the printed circuit board (PCB) during assembly processing can cause the nickel to diffuse through the gold. Once this happens, the nickel will readily oxidize, inhibiting the formation of an intermetallic bond between the gold wire and the PCB or package pad metal surface. The lack of sufficient bonding may prevent gold ball bonds from sticking and greatly reduce the reliability of the finished product. In contrast, an aluminum wedge bond can be made to nickel oxide, copper, gold, aluminum, and many other metal finishes with good electrical bonding and adhesion strength.  This makes the wedge bond an appropriate option in adverse process conditions on mixed hybrid assemblies.

Wedge bonding has been found to be very effective in the manufacturing of high-end RF products. Designs for radios operating in the microwave and millimeter wave range must be bonded with 18 to 25 micron diameter gold wire or 50 x 10 micron gold ribbon bonds. With severe restrictions on wire diameter and wedge shape, many tool providers now offer special tools to make precision shaped, gold wedge bonds for high frequency applications.

Recently, the EMPF has started a program with Orthodyne EquipmentCorporation to develop a wedge bump capability.  The EMPF is actively pursing wedge bumping using gold and aluminum, and will investigate novel alloys and machine configurations to deliver fine pitch or high standoff wedge bumping.