Thermosonic wire bonding is a solid-state welding process that attaches a fine wire — almost always gold — to a bond pad using ultrasonic energy and heat together, with the part held at roughly 150 °C, so the joint forms without either metal ever melting. It is how the overwhelming majority of gold ball bonds are made, and if your die needs high-I/O, fine-pitch interconnect, it is very likely the process you are actually buying.
Three bonding regimes, one difference
Ultrasonic, thermosonic and thermocompression bonding are all solid-state welds. The tool presses the wire or ball against the pad hard enough for plastic flow and intimate contact, and energy disperses the surface contaminants so the two clean metals interdiffuse. The three processes differ only in what supplies that energy:
| Process | Energy source | Typical stage temperature | Bond time | Typical wire and use |
|---|---|---|---|---|
| Ultrasonic (US) | Ultrasonic scrub only | Room temperature | ~10–20 ms | Aluminum wedge–wedge; heavy Al wire and ribbon for power and RF |
| Thermosonic (TS) | Ultrasonic scrub + heat | ~150 °C (often 120–170 °C) | 10–15 ms | Gold ball–stitch; high-speed autobonders, high-I/O fine-pitch die |
| Thermocompression (TC) | Force + heat only | 300 °C and above | Long dwell | Gold; largely obsolete for wire |
Why gold gets heat and aluminum does not
The ultrasonic mechanism needs relative motion at the interface — in the classic characterisation work, when no sliding occurred, there was no bonding. That microscopic scrubbing is what wears away the contaminant film so clean metal can meet clean metal.
Aluminum wire carries a hard, brittle native oxide that shatters and sweeps into debris zones under ultrasonic scrub alone, which is why aluminum wedge bonding works at room temperature. Gold does not oxidise, but a gold ball is harder to get flowing and the pad's own contaminants still have to be dispersed — so heat is added. Warming the stage to around 150 °C lowers the gold's flow stress, enhances diffusion and helps disperse the contaminant film. Ultrasonic energy plus that heat is, by definition, thermosonic bonding.
Why thermocompression faded
Thermocompression is the oldest of the three: force and heat with no ultrasonic assist. It still exists as a concept — and has returned in other forms elsewhere in packaging — but as a wire process it is largely obsolete, for three compounding reasons. A stage at 300 °C or more damages plastic packaging. The process is far more sensitive to surface contamination, because nothing is scrubbing the interface. And its long dwell cannot feed a modern autobonder whose thermosonic bond time is 10–15 milliseconds per weld. Thermosonic bonding is what replaced it: the ultrasonic scrub buys back the cleaning and the speed, and the temperature drops by half.
Who does thermosonic bonding as a service in the US
Search for thermosonic bonding and most of the names that come back are equipment makers — companies that build and sell the bonders. That answers the wrong question. If you have bare die and substrates and need them interconnected, you need an assembly service, not a machine.
Heisler Semiconductor provides thermosonic wire bonding as a service in the US, in Halethorpe, Maryland, at prototype and low volume. Gold ball–stitch thermosonic bonding at ~150 °C is our high-I/O, fine-pitch default, run on automated ball bonders with 25 µm and 33 µm gold wire stocked, alongside ultrasonic aluminum wedge bonding for power, RF and temperature-sensitive assemblies. Die attach, wire bond and inspection happen under one roof, so the bond recipe is developed against the real die, pad metallisation and substrate rather than a generic coupon.
How the bonds are qualified
A thermosonic bond is verified destructively by wire pull to MIL-STD-883 Method 2011 and ball shear, with non-destructive pull to Method 2023 available where a programme screens every wire. Acceptance is set as a multiple of the Method 2011 minimum for the wire size — the standard defines the floor, and a healthy process sits comfortably above it. Per-lot pull and shear monitoring is how a bonding window proven at qualification stays proven in production.
Where to start
Three facts scope a thermosonic bonding job:
- The pad metallurgy and pad pitch — gold ball on aluminum pads is the standard metallurgy, and the pitch decides the capillary and wire.
- What temperature the assembly can tolerate — the ~150 °C stage is gentle by packaging standards, but an already-attached component or a temperature-limited die can push the job to room-temperature ultrasonic wedge instead.
- The qualification target — a commercial pull-and-shear sample plan and a MIL-STD-883 screen are different amounts of work, and it is cheaper to know which before the recipe is developed.
More on wire types, loop control and testing on our wire bonding services page, and the neighbouring decision — which bond family fits which part — is covered in ball, wedge and ribbon bonding compared.
Answered.
What is thermosonic wire bonding?
A solid-state welding process that attaches a fine wire, almost always gold, to a bond pad using ultrasonic energy and heat together, typically with the part held at around 150 °C. Neither metal melts: the ultrasonic scrub disperses surface contaminants while the heat lowers the gold's flow stress, and the two clean metals interdiffuse to form the weld. It is the standard process behind gold ball–stitch bonding.
What is the difference between ultrasonic, thermosonic and thermocompression bonding?
All three are solid-state welds that differ only in what supplies the energy. Ultrasonic bonding uses ultrasonic scrub alone at room temperature — the aluminum wedge process. Thermosonic adds heat to the ultrasonic scrub, typically a stage at 120–170 °C — the gold ball process. Thermocompression uses force and heat only, at 300 °C and above, and is largely obsolete for wire because the temperature harms plastic packaging and its long dwell cannot match a 10–15 ms thermosonic bond.
Why does gold ball bonding need heat when aluminum wedge bonding does not?
Aluminum carries a hard, brittle native oxide that the ultrasonic scrub shatters and sweeps away at room temperature, so aluminum wedge bonds cold. Gold has no oxide, but a gold ball is harder to get flowing and the pad's contaminant film still has to be dispersed — heating the stage to around 150 °C lowers the flow stress and enhances diffusion, which is exactly what makes the process thermosonic rather than ultrasonic.
Is thermocompression bonding still used?
Rarely, for wire. The 300 °C-plus stage temperature damages plastic packaging, the process is highly sensitive to surface contamination because nothing scrubs the interface, and the long dwell is incompatible with modern autobonder speeds. Thermosonic bonding replaced it for gold wire by adding ultrasonic energy and cutting the temperature roughly in half.
How are thermosonic wire bonds tested?
Destructive wire pull to MIL-STD-883 Method 2011 and ball shear, with non-destructive pull to Method 2023 where a programme screens every wire. Acceptance is set as a multiple of the Method 2011 minimum for the wire diameter, and per-lot pull and shear monitoring keeps the qualified bonding window honest in production.
Who provides thermosonic wire bonding as a service in the US?
Heisler Semiconductor provides thermosonic wire bonding as a service in Halethorpe, Maryland, at prototype and low volume — gold ball–stitch bonding at ~150 °C as the high-I/O, fine-pitch default, with 25 µm and 33 µm gold wire stocked and ultrasonic aluminum wedge bonding alongside. Most names a search returns for thermosonic bonding are equipment manufacturers; if you have bare die that need interconnecting, you need an assembly service, not a machine.