A bond made by pressure and vibration
The wire is not soldered in place. In a common process called ultrasonic wire bonding, a tool presses the wire against a small metal pad on the chip while vibrating it at high frequency. The motion scrubs away surface films and helps the metals deform into intimate contact, forming a bond without melting them. The tool then makes a carefully shaped loop of wire and bonds its other end to a contact on the package.
Gold and aluminum are useful because they can be drawn into very fine wire and bonded reliably to compatible metal surfaces. Different wire-and-pad combinations call for different bonding methods: some use a tiny ball formed at the wire’s end, while others use a wedge-shaped tool.
The loop is part of the design
A chip’s pads and a package’s contacts rarely line up directly. The wire loop bridges that gap, and its shape must leave enough slack to tolerate small movements as materials expand and contract with heat. At the same time, the wires must stay separated and clear of neighboring structures. Too little clearance can cause a short; a weak or poorly formed bond can interrupt the signal.
That makes wire bonding a miniature assembly operation, not merely a way to attach metal. The finished chip depends on many individual connections surviving handling, temperature changes, and years of use. A wire almost too small to see can therefore determine whether the packaged device works at all.
