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Ball bonding is a type of wire bonding, and is the most common way to make the electrical interconnections between a bare silicon die and the lead frame of the package it is placed in during semiconductor device fabrication.
Gold or copper wire can be used, though gold is more common because its oxide is not as problematic in making a weld. If copper wire is used, nitrogen must be used as a cover gas to prevent the copper oxides from forming during the wire bonding process. Copper is also harder than gold, which makes damage to the surface of the chip more likely. However copper is cheaper than gold and has superior electrical properties, and so remains a compelling choice.
Almost all modern ball bonding processes use a combination of heat, pressure, and ultrasonic energy to make a weld at each end of the wire. The wire used can be as small as 15 µm in diameter—such that several welds could fit across the width of a human hair.
A person upon first seeing a ball bonder will usually compare its operation to that of a sewing machine. In fact there is a needle-like disposable tool called the capillary, through which the wire is fed. A high-voltage electric charge is applied to the wire. This melts the wire at the tip of the capillary. The tip of the wire forms into a ball because of the surface tension of the molten metal.