Gold wire bonding attaches a fine gold wire from a die pad to a package lead using a thermosonic ball bond — heat plus ultrasonic energy, with nothing melting. Gold gets used because it does not oxidise, which is exactly what lets it form a free-air ball and a repeatable ball-stitch bond. But gold is not always the right wire, and knowing when aluminum wins is half of choosing correctly.
Why gold, and how the ball bond works
A gold ball bond starts by melting the wire tip into a free-air ball with a spark, then pressing that ball onto the pad under heat and ultrasonic scrub. The metal does not oxidise, so the ball forms cleanly and stays bondable — try the same free-air ball on a metal that grows an oxide and the ball is contaminated before it lands. The stage sits around 150 °C, which lowers the gold's flow stress and speeds diffusion so a clean weld forms in milliseconds. That heat-plus-ultrasonic combination is thermosonic bonding, the standard behind gold ball-stitch work, and it is covered in thermosonic wire bonding.
Which diameter
Wire diameter is set by two things: how much current the connection carries, and how much room the bond pad and pitch allow. Thicker wire carries more current and is mechanically tougher; finer wire fits tighter pads and finer pitch. We stock fine gold and aluminum wire from 15 µm to 50 µm, and the right diameter is the smallest that carries the current with margin and lands cleanly on the pad.
Fine-pitch capability is equipment-dependent — set by the wire bonder, not by a number we can quote in the abstract — so the honest answer to "how fine can you go" is to send a pad layout and pitch, and we confirm it against the tool for that specific die.
The intermetallic caution
Gold wire on aluminum pads is the standard metallurgy, but the gold-aluminum system forms a series of intermetallic phases over time, accelerated by heat — the effect the industry nicknames "purple plague." A well-controlled bond and process window keep it benign; an over-bonded or dirty interface lets it concentrate stress and voiding at the bond. It is a reason bond quality is verified rather than assumed, and a reason a hot, long-life part gets a closer look at the wire choice.
When aluminum wins
Aluminum wire bonds by a different mechanism — ultrasonic wedge bonding at room temperature — and it wins in three places:
- Power — heavy aluminum wire carries high current in thick loops, and there is no gold-priced way to do that.
- Temperature-sensitive assemblies — the wedge bond needs no heated stage, so a die or an already-attached component that cannot take 150 °C stays cold.
- All-aluminum metallurgy — aluminum wire on aluminum pads avoids the gold-aluminum intermetallic system entirely.
The full ball-versus-wedge, gold-versus-aluminum decision is laid out in ball vs wedge, gold vs aluminum, and the three bond families we run — gold ball, aluminum wedge and ribbon — in types of wire bonding.
How the bond is verified
Gold and aluminum wire bonds are verified by destructive wire pull to MIL-STD-883 Method 2011 and by ball or wedge 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 — the standard defines the floor and a healthy process sits well above it. Heisler bonds gold and aluminum wire only, no copper.
More on wire types, diameter, loop control and testing on our wire bonding services page.
Answered.
Why is gold used for ball bonding?
Because gold does not oxidise. Ball bonding starts by melting the wire tip into a free-air ball, and a clean, bondable ball only forms on a metal that will not grow an oxide skin before it lands on the pad. Gold's freedom from oxide is what makes thermosonic ball-stitch bonding repeatable at around 150 °C.
What wire diameter is used for gold wire bonding?
It depends on the current the connection carries and the room the bond pad and pitch allow. Thicker wire carries more current and is tougher; finer wire fits tighter pads. We stock fine gold and aluminum wire from 15 µm to 50 µm and pick the smallest diameter that carries the current with margin and lands cleanly.
What is purple plague in gold wire bonding?
A nickname for the gold-aluminum intermetallic phases that form at a gold-wire-on-aluminum-pad bond over time, accelerated by heat. A controlled bond and process window keep it benign; an over-bonded or contaminated interface lets it concentrate stress and voiding. It is one reason bond quality is verified by pull and shear rather than assumed.
When should I use aluminum wire instead of gold?
For power loops that need heavy wire to carry high current, for temperature-sensitive assemblies where the room-temperature aluminum wedge process avoids a heated stage, and for all-aluminum metallurgy that sidesteps the gold-aluminum intermetallic system. Gold ball bonding stays the default for high-I/O, fine-pitch die.
Do you offer gold wire bonding in the US?
Yes — thermosonic gold ball bonding and ultrasonic aluminum wedge bonding, with fine wire from 15 µm to 50 µm and ribbon on request, in Halethorpe, Maryland, on US soil. We bond gold and aluminum only, not copper. Bonds are verified by pull to MIL-STD-883 Method 2011 and by shear.