Semiconductor bonding wire connects chip pads to the leadframe or substrate, and choosing it is a process decision as much as a materials one. Ball bonding runs on gold, copper or silver-based wire; wedge bonding runs on aluminium. Diameter follows pad pitch and current, and the mechanical window (breaking load and elongation) decides how the wire behaves on your bonder. Metline supplies aluminium bonding wire; gold, copper and silver are described here so you can see where aluminium is the right answer.
Two bonding processes account for almost all wire-bonded packages, and each works with a different family of wires. In ball bonding, an electric spark melts the wire tip into a free air ball, which is pressed onto the pad with heat and ultrasonic energy through a ceramic capillary; the second bond is a crescent-shaped stitch on the lead. In wedge bonding, the wire is pressed sideways under a grooved wedge tool and welded by ultrasonic energy, usually at room temperature, on both ends.
Ball bonding is fast and direction-free: after the first bond the capillary can move to the second bond in any direction. That suits high pin-count ICs bonded at thousands of wires per hour. It needs a wire that forms a clean, round, repeatable ball, which gold does naturally, and copper and silver alloys do under protective gas or with engineered dopants and coatings.
Wedge bonding is directional, because the wire must be aligned with the tool, but it needs no heat, makes very small fine-wire bonds and scales up to heavy wire and ribbon for power devices. Aluminium is the natural wedge-bonding wire: it welds well ultrasonically, it matches the aluminium pads on most chips and it is inexpensive. Gold wire can also be wedge bonded, typically in RF and microwave assemblies.
Usually 4N (99.99 %) gold with ppm-level dopants that set strength and loop behaviour, plus lower-purity gold alloys. Easy to ball bond, no oxidation, long track record. Its drawback is cost, which pushed most volume packages to copper.
Bare high-purity copper or, more commonly, palladium-coated copper (PCC), sometimes with a thin gold flash. Better conductivity than gold and much lower cost, but harder, so pad cratering and second-bond control need care, and bonding is done under forming gas.
Silver alloyed with gold, palladium or other elements, sometimes coated. Widely used in LEDs for reflectivity and cost. Pure silver is prone to electromigration and sulphur tarnish, hence the alloying.
Al-1 % Si fine wire for wedge bonding, and high-purity or lightly alloyed aluminium heavy wire for power devices. Bonds at room temperature, forms monometallic joints on aluminium pads and costs least of all four.
| Property | Gold | Copper | Silver | Aluminium |
|---|---|---|---|---|
| Resistivity of pure metal, µΩ·cm | 2.44 | 1.68 | 1.59 | 2.65 |
| Density, g/cm³ | 19.3 | 8.96 | 10.5 | 2.70 |
| Annealed hardness | Soft | Harder than gold | Soft | Softest (pure Al) |
| Main process | Ball (and wedge for RF) | Ball, under N₂/H₂ forming gas | Ball | Wedge, ultrasonic |
| Typical fine-wire diameters | 15 – 50 µm | 15 – 50 µm | 15 – 30 µm | 18 – 75 µm (Al-1 % Si) |
| Heavy wire | Rare | 5 – 20 mil class power wire | Rare | 100 – 500 µm, standard for power |
Resistivity and density are standard values for the pure metals. Diameter ranges are indicative of common commercial practice; the exact range depends on alloy and supplier.
Diameter is set by three limits: the bond pad and pitch (the bond must fit on the pad without touching its neighbour), the current the connection carries (the wire must not overheat or fuse), and loop length (long, thin wires sag and sweep during moulding). Bonding wire is often specified in mils in the industry; 1 mil equals 25.4 µm.
| Diameter | In µm | Typical use |
|---|---|---|
| 0.6 – 0.8 mil | 15 – 20 µm | Very fine-pitch ICs and memory, ball bonded |
| 0.8 – 1.0 mil | 20 – 25 µm | Mainstream IC and LED packages |
| 1.0 – 1.25 mil | 25 – 32 µm | Standard Al-1 % Si wedge bonding in hybrids, sensors, COB |
| 1.25 – 2.0 mil | 32 – 50 µm | Higher-current IC leads, discrete devices |
| 3 – 5 mil | 75 – 125 µm | Small power devices, light heavy-wire bonding |
| 5 – 20 mil | 125 – 500 µm | IGBT, MOSFET and SiC power modules, battery interconnects (heavy aluminium wire) |
When current is the limit, several wires in parallel or a ribbon are often better than one very thick wire, because they spread current over the die metallisation and reduce the risk of a single bond lifting. For heavy aluminium wire, published fusing-current curves and the module maker's own thermal tests, rather than a simple rule, should set the number of wires.
Every bonding wire specification gives a breaking load (in grams-force or centinewtons) and an elongation range for each diameter. They come from the final anneal and are the most important numbers on the certificate after diameter.
Higher breaking load gives stiffer loops that resist sweep during moulding, but needs more bonding energy and can stress the pad. Lower breaking load is gentler on fragile dies.
Higher elongation means a softer wire that forms long, low loops and survives thermal cycling with less heel cracking, but sags more. Low elongation suits short, stiff loops.
A narrow window from lot to lot matters more than the absolute value, because bonder parameters are tuned to it. Ask for the range, not just a nominal figure.
Wire hardness after ball formation (for Au, Cu, Ag) or as-drawn (for Al) affects pad damage. Copper's higher hardness is the reason for extra care on thin or low-k pad stacks.
For aluminium heavy wire, purity and dopants are the main levers: 5N aluminium is very soft and protects the die, while doped wires trade a little softness for better strength, corrosion resistance or power-cycling life. For Al-1 % Si fine wire, the silicon content and the final temper set the window.
| Package or device | Process | Common wire | Key selection factors |
|---|---|---|---|
| ICs and discretes on leadframes or substrates | Ball | Pd-coated Cu, Au, Ag alloy | Pad pitch, low-k pad stack, loop height, humidity and HTS reliability |
| LED packages | Ball | Au, Ag alloy, Au-coated Ag | Reflectivity, sulphur resistance, thermal cycling |
| MEMS and sensors | Wedge or ball | Al-1 % Si, Au | Low process temperature, clearance, vibration |
| Hybrid microcircuits and COB | Wedge | Al-1 % Si, Au | No board heating, monometallic Al-Al joints |
| Power modules (IGBT, MOSFET, SiC) | Heavy-wire wedge | High-purity or doped Al; heavy Cu | Current, power cycling, die top metallisation |
| Battery packs | Heavy-wire wedge | Aluminium wire | Current per cell, fusing behaviour, vibration |
Bond quality is then verified in the package. The test methods most often quoted are MIL-STD-883 Method 2011 (destructive bond pull) and JEDEC JESD22-B116 (wire bond shear). For automotive parts, AEC-Q100 and AEC-Q101 qualification applies, with AEC-Q006 adding requirements for copper wire. ASTM F487 once covered fine aluminium-1 % silicon wire but was withdrawn in 2024, so aluminium wire is now specified by the user's own drawing and the supplier datasheet.
Start from the process. Ball bonding uses gold, copper or silver alloy wire; wedge bonding uses aluminium. Then consider pad metallisation, pitch, current, cost and the reliability tests required. Aluminium is the usual choice for wedge-bonded hybrids, sensors and all heavy-wire power applications.
Not directly. Copper is harder and oxidises, so it needs forming gas at the bonder, adjusted parameters and a pad stack that resists cratering. Most packages that moved from gold to copper went through a full requalification.
It depends on pad size and pitch, current and loop length. Fine-pitch ICs use about 15 – 25 µm, typical Al-1 % Si wedge bonding uses about 25 – 32 µm, and power modules use heavy aluminium wire of roughly 125 – 500 µm, often several in parallel.
Pure aluminium is too soft to draw reliably to very fine diameters. About 1 % silicon strengthens the wire so it can be drawn fine and handled on the bonder, while keeping it soft enough for ultrasonic wedge bonding.
Fine-wire bonding (below about 75 µm) makes signal connections in ICs, LEDs and sensors. Heavy-wire bonding (about 100 – 500 µm) carries high current in power semiconductors and battery packs and is done almost entirely with aluminium wire.
Metline supplies aluminium bonding wire, including Al-1 % Si fine wire and high-purity or doped heavy aluminium wire, plus aluminium ribbon. We do not supply gold, copper or silver bonding wire.
Send the wire type, diameter, mechanical window, bonder information and sample quantity. Our team will propose a matching aluminium wire and arrange samples.
