Wire bonding

Wire bonding makes the electrical connections between a semiconductor die and the surface it sits on: a fine wire, typically gold or aluminum 18–25 µm in diameter, is welded first to a metal pad on the chip and then to a pad on the submount, package lead or circuit board. It is the most common chip interconnect in electronic and optoelectronic packaging. A 1 mm gold wire of 25 µm diameter has a resistance of about 0.05 Ω and an inductance of roughly 0.9 nH; at high frequency the inductance matters far more than the resistance.

Ball and wedge bonds

Ball bonding uses gold or copper wire fed through a ceramic capillary. An electric spark melts the wire tip into a ball, which is pressed onto the first pad with ultrasonic vibration while the substrate is heated, typically to 100–200 °C (thermosonic bonding). The capillary then moves to the second pad, forms a loop and makes a crescent-shaped stitch bond. The wire can leave the ball in any direction, which suits fast automated assembly.

Wedge bonding presses the wire against the pad under a wedge-shaped tool, usually with ultrasonic energy alone at room temperature. Aluminum wire is normally wedge bonded, and gold wedge bonds are used where a low, short loop is wanted, as in microwave and optoelectronic modules. The bond direction is fixed by the tool, so wedge bonding is slower to automate.

Ribbon bonds use a flat ribbon in place of the round wire; it has lower inductance per unit length and is used for high-current connections and microwave transitions.

Inductance and bandwidth

A thin wire above a ground plane behaves as an inductor in series with the signal. A common rule of thumb is about 1 nH per millimeter of length; the free-space formula for a straight round wire gives 0.87 nH for 1 mm of 25 µm wire and 2.0 nH for 2 mm, the inductance per unit length rising slowly with length. In series with a 50 Ω load driven from an ideal voltage source, an inductance $L$ adds a pole at

$$f_L = \frac{R}{2\pi L} .$$

A 0.5 nH bond into 50 Ω has an $L/R$ time constant of 10 ps and a pole at 15.9 GHz; a 1 nH bond lowers it to 8.0 GHz. At 25 GHz the reactance of 0.5 nH is 78.5 Ω, larger than the load itself. In a real module the wire also forms a resonant circuit with the pad and junction capacitance: 0.5 nH with 100 fF resonates at 22.5 GHz, producing peaking in the response near that frequency followed by a steep roll-off, a feature the simple RC time constant model omits.

For directly modulated lasers, electro-absorption modulators and high-speed photodiodes, the bond wire is therefore often the factor that sets the package's modulation bandwidth once the chip itself is fast enough. Several measures reduce it:

• Short wires and low loops. Placing the chip pad close to the submount transmission line, with the chip surface level with the line, keeps wires to a few hundred micrometers.
• Parallel wires. Two wires in parallel lower the inductance, but less than by half because of their mutual inductance: two 1 mm wires 100 µm apart give about 0.64 nH, against 0.87 nH for one.
• Compensation. The wire inductance can be absorbed into a matching network with the pad capacitance, or a series resistor can be placed on the submount to damp the resonance.
• Flip-chip bonding. Solder bumps replace the wires entirely and bring the parasitic inductance down to typically tens of picohenries, the approach taken for flip-chip bonded lasers and many co-packaged optics assemblies.

For bias, heater and thermistor lines the inductance is harmless, and wire bonding remains the cheapest method.

Reliability and inspection

Bonds are tested destructively by pulling the wire with a hook until it breaks, where the failure location shows whether the bond or the wire is the weak point, or by shearing the ball off the pad. Common failure mechanisms are intermetallic growth at gold–aluminum interfaces at elevated temperature, cracking of the wire heel from thermal cycling, and pad lift on soft or contaminated metallization. A failing bond shows up as a rise in series resistance or as an intermittent open circuit during burn-in. On laser chips the bonding force and ultrasonic energy must be applied away from the ridge and facets, since the stress can damage the active region.

Photonic wire bonding

Photonic wire bonding is an unrelated optical technology that borrows the name: polymer waveguides written in place by two-photon lithography to connect fibers and photonic chips.

Common questions

What is the inductance of a wire bond?

About 1 nH per millimeter for a 25 µm wire is the usual working figure. Height above the ground plane, loop shape and nearby wires change the value by tens of percent.

Gold or aluminum wire?

Gold is ball bonded thermosonically, does not oxidize and is the standard for optoelectronic packages and gold-metallized chips. Aluminum is wedge bonded at room temperature, avoids gold–aluminum intermetallics on aluminum pads and costs less. Copper is used in high-volume electronics as a lower-cost alternative to gold.

References: G. G. Harman, Wire Bonding in Microelectronics, 3rd ed. (McGraw-Hill, 2010); L. A. Coldren, S. W. Corzine and M. L. Mašanović, Diode Lasers and Photonic Integrated Circuits, 2nd ed. (Wiley, 2012); F. W. Grover, Inductance Calculations (Van Nostrand, 1946).