Pure silver (4N Ag)
Highest visible reflectivity and electrical conductivity of any metal. Soft and easy to bond, but silver tarnishes in sulphur-bearing air and can migrate under bias in humidity, so the package must protect it well.
Semiconductor interconnect guide
Bonding Wire for LED Packaging
LED bonding wire is the fine metal thread, usually 15 to 50 µm across, that carries current from the package lead frame to the LED chip. Because the wire also sits inside the optical cavity, its reflectivity affects light output, so most lighting and display LEDs now use silver alloy wire, with gold, gold-coated silver, copper and aluminium kept for specific reliability or cost needs. This guide explains how each wire behaves and how to choose one. Metline supplies the aluminium bonding wire; silver, gold and copper wire are compared to support selection.
- Typical diameter
- 15 – 50 µm (0.6 – 2.0 mil)
- Main LED choice
- Silver alloy wire
- Bonding method
- Thermosonic ball / stitch
- Key specs
- Diameter, break load, elongation
What bonding wire does inside an LED package
In a typical SMD, COB or lamp-type LED, the chip is glued or eutectically attached to a silver-plated lead frame or substrate. One or two fine wires then join the chip's bond pads to the package terminals. The wire has three jobs: carry the drive current, survive the thermal and mechanical stress of the encapsulant for the life of the device, and avoid absorbing the light the chip produces.
That third job is what makes LED wire selection different from ordinary IC packaging. In a logic chip the wire is hidden under black moulding compound and its colour is irrelevant. In an LED the wire loop arches through clear or phosphor-filled silicone directly above the emitting surface. Every photon that strikes the wire is either reflected back into the useful beam or absorbed as heat. Across millions of devices, a few percent of extra luminous flux is a real commercial advantage, which is why wire reflectivity is quoted on LED data sheets.
The wire is also the element most likely to fail mechanically. Silicone encapsulants expand far more than metals when they heat up, so each power cycle pulls and pushes on the loop. A wire that is too brittle cracks at the neck above the ball; one that is too soft sags and can touch the chip edge. Choosing the right alloy, diameter and loop profile therefore matters as much as choosing the chip.
The main LED bonding wire families
Six families cover nearly all LED production. They differ in reflectivity, chemical stability, hardness of the free-air ball, cost and the equipment changes needed to run them.
High-silver alloy
Silver alloyed with small amounts of gold, palladium or other elements. Keeps most of the reflectivity of pure silver while improving resistance to sulphidation, electromigration and ball corrosion. The mainstream choice for lighting and display LEDs.
Gold-coated silver
A silver alloy core with a thin gold surface layer. The gold shields the silver from the environment and helps form a stable free-air ball, giving better reliability in hot, humid or sulphur-rich conditions.
Gold and gold alloy
Pure 4N gold is the reference for chemical stability and bonding window. Gold-silver alloys add reflectivity and cut cost. Gold remains the default for automotive and other high-reliability LEDs.
Copper and coated copper
The cheapest conductor with good conductivity, but the free-air ball is harder, which raises the risk of cratering soft LED pads, and it oxidises, so bonding needs forming gas. Palladium-coated copper reduces oxidation.
Aluminium
Used mainly for wedge bonding in LED modules, driver electronics and power stages rather than inside small optical packages. Low cost and no intermetallic issues on aluminium pads.
LED bonding wire comparison table
The table compares the families on the properties that drive LED selection. Ratings are relative and reflect general industry experience; supplier data sheets give exact values for each grade.
| Wire type | Visible reflectivity | Environmental stability | Ball hardness / pad risk | Relative cost | Typical LED use |
|---|---|---|---|---|---|
| Pure silver (≥99.99 % Ag) | Excellent across the spectrum | Moderate: needs low-sulphur, low-chlorine package | Soft, low risk | Low–medium | High-efficacy lighting packages |
| High-silver alloy | Excellent | Moderate to good | Soft, low risk | Low–medium | General lighting, SMD display, backlight |
| Lower-silver alloy (higher alloy content) | Good | Good | Slightly harder | Low | High-volume indoor devices |
| Gold-coated silver | Excellent | Good to excellent | Soft, low risk | Medium | Outdoor displays, humid or sulphur-bearing service |
| Gold alloy (Au-Ag) | Good | Excellent | Soft | Medium–high | High-power and long-span packages |
| Pure gold (4N Au) | High for red, poor for blue | Excellent | Soft, widest window | High | Automotive and high-reliability LEDs |
| Copper / Pd-coated copper | Moderate | Moderate (oxidation) | Hard, higher risk | Lowest | Cost-driven lines with proven copper processes |
| Aluminium (wedge) | Not used in the optical path | Good | Wedge bond, no ball | Low | Module, driver and power interconnects |
Reflectivity matters most for blue-pumped white LEDs. Published measurements show gold reflects around 95 % of red light near 650 nm but only about 40 % of blue light near 450 nm, so a gold loop in a blue chip package absorbs a noticeable share of the emitted light. Silver and high-silver alloys stay highly reflective across the whole visible range, which is the main reason they displaced gold in mainstream white LEDs.
Physical properties of bonding wire metals
Bulk values for the pure metals at about 20 °C are shown below. Alloyed and coated wires differ slightly: adding alloying elements to silver, for example, raises its resistivity, which is why high-silver alloy wire conducts less well than pure silver while still matching or beating gold.
| Property (pure metal) | Gold (Au) | Silver (Ag) | Copper (Cu) | Aluminium (Al) |
|---|---|---|---|---|
| Electrical resistivity (µΩ·cm) | 2.2 – 2.4 | 1.59 | 1.68 – 1.72 | 2.65 – 2.8 |
| Thermal conductivity (W/m·K) | 317 | 429 | 401 | 237 |
| Density (g/cm³) | 19.3 | 10.5 | 8.96 | 2.70 |
| Melting point (°C) | 1064 | 962 | 1085 | 660 |
| Oxidises in air during bonding | No | No (but sulphides) | Yes, needs forming gas | Yes, oxide broken by ultrasonic wedge |
Values are standard handbook figures for high-purity bulk metals; fine wire properties also depend on drawing and annealing.
Density also has a cost implication that buyers sometimes miss. Bonding wire is sold by length, but its metal value tracks weight. A metre of 25 µm silver wire contains just over half the metal mass of the same gold wire, and silver costs a small fraction of gold per gram, so the material saving from switching is large even before reflectivity gains are counted.
How to choose the right wire for your LED
- Define the package and environmentIndoor lighting, backlight, outdoor display, automotive, horticulture or UV. Note the expected humidity, sulphur exposure (rubber, adhesives, industrial air) and operating temperature.
- Set the optical priorityFor blue-pumped white LEDs where luminous efficacy matters, start with pure silver or a high-silver alloy. For red or infrared devices, gold's low blue reflectivity is irrelevant.
- Check reliability requirementsAutomotive and outdoor products usually need more margin against corrosion and migration: consider gold-coated silver or gold. Plan reliability tests such as temperature cycling and temperature-humidity-bias before release.
- Choose diameter from current and spanLow-power SMD chips commonly use 15 – 20 µm wire; mid- and high-power chips 23 – 30 µm or more, sometimes with two wires. Longer spans and higher current call for larger diameter or a stiffer alloy.
- Match break load and elongation to the bonderHigher elongation gives a softer wire that loops easily; lower elongation gives a stiffer loop that resists sweep during encapsulation. Agree target BL and EL ranges with the supplier.
- Qualify on your lineRun bond pull and ball shear tests, check intermetallic coverage on the chip pad and ageing performance, then lock the specification for repeat orders.
Diameters, spools and packaging
Fine bonding wire is specified in micrometres and, in North American practice, in mils (thousandths of an inch). The table lists common LED sizes and where they are used.
| Diameter (µm) | Diameter (mil) | Typical LED use |
|---|---|---|
| 15 | 0.6 | Small indicator and mini SMD chips |
| 17.5 – 18 | 0.7 | Low-power SMD and display pixels |
| 20 | 0.8 | General SMD lighting chips |
| 23 | 0.9 | Mid-power lighting packages |
| 25 | 1.0 | Mid- to high-power, standard reference size |
| 30 – 33 | 1.2 – 1.3 | High-power chips and COB arrays |
| 38 – 50 | 1.5 – 2.0 | High-current or long-span connections |
Typical industry format
- Diameter tolerance
- Commonly ±1 µm for fine wire, or as agreed
- Spool
- Standard 2-inch bonding spools compatible with automatic wire bonders
- Length per spool
- Commonly 500 m to several thousand metres
- Packaging
- Individually sealed, often under vacuum or inert gas, with desiccant
- Documentation
- Certificate of analysis with diameter, break load, elongation and lot number
Silver and copper wires have limited floor life once opened; follow the supplier's storage and use-by guidance.
Failure modes and how to avoid them
Silver sulphidation
Sulphur compounds from rubber seals, gaskets, adhesives or polluted air diffuse through silicone and turn silver black. This lowers reflectivity and can weaken the bond. Use alloyed or gold-coated wire and keep sulphur sources away from the luminaire.
Silver migration
Under voltage and moisture, silver ions can move and form dendrites between conductors. Alloying silver with palladium or gold slows this, and good package sealing prevents the moisture that drives it.
Neck and heel cracking
Thermal cycling of the encapsulant fatigues the wire just above the ball (neck) or at the stitch (heel). Control loop height and shape, and use a wire whose heat-affected zone length suits your loop.
Wire sweep
Dispensing or moulding the encapsulant can push long, soft loops sideways until they touch. Stiffer alloys, shorter spans or larger diameters reduce sweep.
Pad cratering
A hard free-air ball, as with copper, can crack the semiconductor beneath the pad. Optimise ultrasonic power and force, or stay with softer gold or silver wire on delicate LED chips.
Intermetallic growth
Over time at temperature, the wire and pad metals form intermetallic layers. Some growth is healthy; excessive or voided growth weakens the bond. Qualify by high-temperature storage and shear testing.
Testing methods and standards for bonding wire
Wire quality is checked at two levels: the wire itself, and the bond it makes in your package. Both should be part of incoming inspection and process control.
| Test | What it shows | Reference |
|---|---|---|
| Break load and elongation | Tensile strength and ductility of the wire lot | Supplier certificate; ASTM F219 test methods |
| Diameter measurement | Size and consistency along the spool | Weight-per-length or optical methods |
| Wire pull test | Strength of the complete bonded loop and its failure location | MIL-STD-883 Method 2011; ASTM F459 |
| Ball shear test | Strength of the ball bond to the chip pad | JEDEC JESD22-B116; ASTM F1269 |
| Temperature cycling | Resistance to fatigue from encapsulant expansion | JEDEC JESD22-A104 |
| Temperature-humidity-bias | Corrosion and migration resistance | JEDEC JESD22-A101 |
| Lumen maintenance | Long-term light output of the finished LED | IES LM-80 with TM-21 projection |
Gold wire has long been specified to ASTM F72 and aluminium-1 % silicon fine wire to ASTM F487, although many LED makers now work to supplier data sheets and their own qualification plans, particularly for silver alloys that these older standards do not cover.
How LED wire is bonded: ball and stitch
Gold, silver and copper LED wires are joined by thermosonic ball bonding. An electric spark melts the wire tip into a free-air ball, which the capillary presses onto the heated chip pad while ultrasonic energy scrubs through surface films. The capillary then draws a loop to the lead frame and forms a crescent-shaped stitch bond before cutting the wire. Silver and copper need a protective gas around the spark to keep the ball round and clean; gold does not.
Aluminium wire uses ultrasonic wedge bonding at room temperature, with a wedge tool flattening the wire against each pad. Because there is no ball, the bond direction is fixed by the first wedge, which suits linear module and driver layouts better than the tight, three-dimensional loops inside small LED packages.
Common mistakes when specifying LED wire
- Copying an IC wire specification. IC packages ignore reflectivity; an LED package should not.
- Switching from gold to silver without re-qualifying. Silver needs different bonding parameters and sometimes a different capillary, and the package must control sulphur and moisture.
- Ignoring the luminaire. A well-made LED can still blacken if the fixture uses sulphur-cured rubber gaskets or high-sulphur adhesives.
- Choosing diameter only on current. Span, loop height and encapsulant stress also decide whether a wire survives.
- Leaving opened spools on the bonder for days. Silver and copper surfaces change once the seal is broken, which shifts ball formation and bond strength.
- Specifying only diameter. Without break load and elongation ranges, two wires of the same size can behave very differently on the line.
References and standards
- ASTM F72 – Standard Specification for Gold Wire for Semiconductor Lead Bonding
- ASTM F487 – Standard Specification for Fine Aluminum-1 % Silicon Wire for Semiconductor Lead-Bonding
- MIL-STD-883, Method 2011 – Bond Strength (Destructive Bond Pull Test)
- JEDEC JESD22-B116 – Wire Bond Shear Test Method
- IES LM-80 – Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules
LED bonding wire: frequently asked questions
Why do most LEDs use silver wire instead of gold?
Silver reflects blue and white light far better than gold and costs much less. In blue-pumped white LEDs a gold loop absorbs part of the emitted light, so silver alloy wire raises luminous flux while cutting material cost. Gold remains in use where maximum chemical stability is required.
What diameter of bonding wire is used for LEDs?
Most LED packages use 15 to 50 µm wire. Small SMD and display chips run 15 – 20 µm, mid-power lighting chips 20 – 25 µm, and high-power or COB devices 25 µm and above, sometimes with two wires per pad.
Does silver bonding wire turn black?
Pure silver can tarnish when sulphur or chlorine compounds reach it through the encapsulant, which reduces reflectivity. Alloyed silver and gold-coated silver resist this much better, and keeping sulphur-bearing materials out of the luminaire prevents most problems.
Can copper wire be used in LED packages?
Yes, in cost-driven production with a mature copper process. Copper needs forming gas to stop oxidation during ball formation, and its harder ball raises the risk of damaging delicate LED pads, so bonding parameters must be carefully optimised.
Where is aluminum bonding wire used in LED products?
Aluminium wire is wedge bonded and is mainly used outside the optical package: in LED driver electronics, power stages and module-level connections, where its low cost and compatibility with aluminium pads are useful.
What do BL and EL mean on a bonding wire certificate?
BL is the break load, the force in grams-force needed to break the wire in a tensile test; EL is the elongation in percent at break. Together they describe how strong and how ductile the wire is, which controls looping behaviour on the bonder.
Specifying aluminium wire for LED drivers or modules?
Send the pad metallisation, wire diameter and BL/EL targets to sales@metline.in, and our team will review the requirement and quote aluminium bonding wire.
