Ball vs Wedge Bonding: Key Differences Explained
Both are wire bonding methods that connect the electrode pads of a chip to a lead frame or substrate with a thin metal wire. Ball bonding melts the tip of the wire with an electrical discharge to form a ball, then bonds it with heat, force and ultrasound. The ball has no direction, so the wire can be led out in any direction and bonding is fast, which makes it the most widely used method for IC packages. Wedge bonding presses the wire flat from the side and bonds it mainly with ultrasound. It can use aluminum wire and ribbon directly, so it is common in power semiconductors that carry large currents.
| Aspect | Ball bonding | Wedge bonding |
|---|---|---|
| Bond shape | The first bond is a ball; the second bond is a stitch where the wire is pressed flat | Both the first and second bonds are wedge-shaped, with the wire pressed flat |
| Tool | Capillary (a thin ceramic tube with a hole in the center) | Wedge tool (feeds the wire at an angle from behind and presses it with the tip) |
| Bonding energy | Thermosonic, combining heat, force and ultrasound. The substrate is heated to about 150 to 200 °C | Mainly ultrasound and force. Aluminum wire can be bonded at room temperature |
| Wire material | Gold, copper (including palladium-coated copper) | Aluminum (thin and thick wire), aluminum ribbon, gold. Thick copper wire is also used for power |
| Wire diameter | About 15 to 50 µm | Tens of µm for thin wire, about 100 to 500 µm for thick power wire. Flat ribbon is also used |
| Directionality | The ball has no direction, so the wire can be led out in any direction | Bonds are made along the wire feed direction, so the direction is fixed. The head or workpiece is rotated for each bond to align the direction |
| Speed | Fast. Bonds can be made continuously without regard to direction | Slower. Requires motion to align the direction |
| Bond size and pitch | The ball is about twice the wire diameter. Supports narrow pad pitches of a few tens of µm | The bond is narrower than a ball, so thin wire also suits narrow pitches |
| Loop shape | The wire rises from the ball, so some height is needed. There are techniques to lower it | Low loops are easy to make |
| Reliability concerns | With gold wire on aluminum pads, growth of intermetallic compounds and voids. With copper wire, cracking under the pad (cratering) | With thick aluminum wire, bond lift-off and heel cracks caused by repeated temperature cycling |
| Typical uses | IC packages in general, such as QFP, BGA and stacked memory chips | Power semiconductors and power modules, automotive electronics, RF and optical devices, connections with aluminum wire |
Figures are typical orders of magnitude and vary by process generation and product.