Ask what type of wire bond a part needs and you get two questions, not one: how the weld is made, and what the wire is made of. Ball or wedge sets the geometry and the tooling. Gold or aluminum sets the metallurgy, the temperature budget, and how much current the loop can carry. Most bonding decisions come down to matching those two axes to the pad, the pitch, and the reliability target — not to whichever setup the shop already has warmed up.
This is the general engineering picture. It is worth understanding before a design review, because the wire-bond choice ripples backward into pad metallization and forward into how the part gets screened.
Ball bonding and wedge bonding
Ball bonding starts by melting the wire tip into a free-air ball with a spark, pressing that ball onto the first pad, then paying out wire in a loop to the second pad and forming a stitch. The capillary tool is omnidirectional — it can loop in any direction without rotating the part — so ball bonding is fast and comfortable at fine pitch. It is almost always run in gold.
Wedge bonding forms no ball. The wire is fed under a wedge tool and ultrasonically scrubbed flat onto the pad, then carried to the second pad and scrubbed again. The tool is directional, so the part or the bond head has to rotate to line up each bond. That makes wedge bonding slower, but it leaves a smaller footprint, and it is the natural route for aluminum and for heavy wire. Aluminum wedge bonding runs at room temperature.
Thermosonic, thermocompression, ultrasonic: the energy behind the weld
Underneath ball and wedge sit three ways to actually make the solid-state weld, defined by which mix of heat, force, and ultrasonic energy does the work.
- Thermocompression. Heat plus force, no ultrasonic. The historical gold ball method, but it needs a hot substrate — often 300 C and up — to form the weld. That thermal budget is hard on real die, which is why pure thermocompression is now rare.
- Ultrasonic. Force plus ultrasonic scrubbing at room temperature, with no substrate heat. This is the standard route for aluminum wedge bonding and heavy-wire power loops, and the friendly option for die that cannot see a hot stage.
- Thermosonic. Heat, force, and ultrasonic together. Moderate stage heat — well below thermocompression — plus ultrasonic energy makes a reliable gold ball bond at a temperature the die survives. It is the workhorse for fine-pitch gold today.
Gold wire and aluminum wire
The metal is the other axis, and it changes almost everything about the process.
Gold is a noble metal with no surface oxide, so it ball-bonds cleanly by thermosonic and forms strong, round, repeatable loops that hold fine pitch. The catch is the gold-aluminum couple: on aluminum pads, gold grows brittle intermetallic phases — the notorious purple-plague AuAl2 among them — and Kirkendall voiding can hollow the interface. That growth is driven by diffusion, so temperature and time in bonding, cure, and service all feed it. Bounding the thermal budget and screening with thermal cycling is how the gold-on-aluminum interface is kept honest.
Aluminum wedge-bonds ultrasonically at room temperature, needs no stage heat, and matches aluminum bond pads metal-to-metal, sidestepping the gold-aluminum couple entirely. It also scales up: heavy aluminum wire carries current that a fine signal wire cannot, which is why power loops are almost always aluminum. The tradeoff is that wedge geometry is directional and the finest pitches are harder to automate than they are in gold.
Heavy wire and ribbon
Two variants sit outside the fine-wire signal world. Heavy aluminum wire handles high-current power interconnect, where a signal-diameter wire would simply not carry the load. Ribbon — a flat conductor instead of a round wire — lowers inductance and spreads current, which makes it useful for RF paths and power alike. Both come off the same bonding discipline; they are choices within the toolbox, not separate crafts.
How the choice actually gets made
Strip away the taxonomy and the decision is a short set of questions:
- Pad metallurgy. Aluminum pads point toward aluminum wedge, or gold ball with a bounded thermal budget.
- Pitch. The finest pitches favor gold ball bonding and its omnidirectional loop.
- Current. Power loops want heavy aluminum wire, not fine signal wire.
- Temperature budget. Temperature-sensitive die favor room-temperature ultrasonic aluminum, with no hot stage.
- Reliability precedent. A long MIL-STD history behind a given metallurgy can settle a program on its own.
The practical answer is to run both metallurgies and both geometries, then let the device pick. That is how we approach wire bonding — matching gold ball, aluminum wedge, or ribbon to the pad, pitch, current, and reliability target, and developing the bond program and the pull-and-shear data together. It also runs downstream of the attach step, since the die is placed and cured before it is ever bonded; that is a separate reliability decision covered under die bonding.