Inside every wire-bonded LED package there is a thin filament connecting the die pad to the lead frame or substrate. It is 18 to 50 micrometres across, it carries the full forward current, and it also acts as a heat path out of the die. Whether it is gold or copper changes cost, process difficulty and the way the part eventually fails.
What the Wire Has to Do
The wire carries current from the external pad to the die, and it carries some heat in the same direction. During thermal cycling the die, the wire and the substrate expand at different rates. Because the wire is thin and compliant, it absorbs much of that mismatch, which is why it survives thousands of cycles in a part whose other joints are far more rigid.
The bond itself is a metallurgical joint. The wire is welded to the die pad and to the lead by thermosonic ball bonding or wedge bonding. The quality of those two joints sets the parts reliability more than the wire material alone.
Gold Wire
Gold is the traditional choice. It is soft, it resists oxidation at room temperature, and it forms a reliable ball bond with a wide process window. Thickness control is easy, and 25 micrometre gold wire is still the default for many packages.
Its weaknesses are cost and the intermetallic it forms with aluminium pads. At high temperature, gold and aluminium grow intermetallic compounds and leave Kirkendall voids along the interface. A high junction temperature accelerates that growth, and the joint eventually lifts.
Copper Wire
Copper conducts better on both counts being asked of it. Resistivity is 1.68 micro-ohm-centimetre against 2.44 for gold, and thermal conductivity is around 400 W/m.K against roughly 317. It is also cheaper by a wide margin and mechanically stronger, so it resists wire sweep during moulding.
The trade-off is process control. Copper is harder and stiffer, so the ball bond presses harder into the pad, which raises the risk of cratering in the silicon beneath. Copper also oxidizes, so bonding needs a protective atmosphere or a palladium coating on the wire surface.
| Parameter | Gold wire | Copper wire |
|---|---|---|
| Resistivity | 2.44 micro-ohm-cm | 1.68 micro-ohm-cm |
| Thermal conductivity | about 317 W/m.K | about 400 W/m.K |
| Material cost | High | Low |
| Bonding window | Wide, thermosonic | Narrower, tighter control |
| Stress on die pad | Low | Higher |
| Oxidation | Resistant | Needs coating or inert atmosphere |
Failure Modes Worth Knowing
Lifted ball bond from intermetallic growth at the gold-aluminium interface. Wire break at the neck, usually after repeated thermal cycling. Cratering under the pad when the bonding force is too high for the pad stack. Kirkendall voids visible in cross section after high-temperature storage.
Copper-aluminium joints form a different intermetallic system and generally grow more slowly than gold-aluminium, which is one reason copper wire parts often post better high-temperature storage results. The pad metallization still has to be designed for the harder bond.
Test Data to Request
A supplier should be able to supply ball shear values in grams and wire pull values in grams, with the sample size and the rejection criteria. Ask for high-temperature storage at 150C and thermal cycling between minus 40C and 125C, both with a stated number of cycles and the failure criteria. Cross-section images after high-temperature storage show whether voids are forming at either interface.
Bond wire diameter belongs on the drawing. A 1A part bonded with 25 micrometre wire and the same part bonded with 38 micrometre wire behave differently under surge and under thermal cycling, even when the electrical ratings on the front page are identical.
Key Takeaways
Gold and copper are not simply better and worse. Gold buys process margin and pad friendliness; copper buys lower resistance, a better heat path and lower cost, and pays for it in tighter bonding control. The reliability question is decided by the joint, not by the metal name, so the shear and pull data and the storage test results matter more than the material listed on the datasheet.
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