Ball bonding is a type of wire bonding

Wire bonding

Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...

, and is the most common way to make the electrical interconnections between a chip and the outside world as part of semiconductor device fabrication.

Gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 or copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 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

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 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

Sewing machine

A sewing machine is a textile machine used to stitch fabric, cards and other material together with thread. Sewing machines were invented during the first Industrial Revolution to decrease the amount of manual sewing work performed in clothing companies...

. 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

Surface tension

Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 of the molten metal.

The ball quickly solidifies, and the capillary is lowered to the surface of the chip, which is typically heated to at least 125°C. The machine then pushes down on the capillary and applies ultrasonic energy with an attached transducer

Transducer

A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...

. The combined heat, pressure, and ultrasonic energy create a weld between the copper or gold ball and the surface of the chip - which is usually copper or aluminum. All aluminum systems in semiconductor fabrication eliminate the "purple plague" (brittle gold-aluminum intermetallic compound) sometimes associated with pure gold bonding wire. This property makes Aluminium ideal for ultrasonic bonding. This is the so-called ball bond that gives the process its name.

Next the wire is passed out through the capillary and the machine moves over a few millimeters to the location that the chip needs to be wired up to (usually called the leadframe). The machine again descends to the surface, this time without making a ball so that the wire is crushed between the leadframe and the tip of the capillary. This time the surface is usually gold, palladium

Palladium

Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...

, or silver - but the weld is made in the same way. The resulting weld is quite different in appearance from the ball bond, and is referred to as the wedge bond, tail bond, or simply as the second bond.

In the final step the machine pays out a small length of wire and tears the wire from the surface using a set of clamps. This leaves a small tail of wire hanging from the end of the capillary. The cycle then starts again with the high-voltage electric charge being applied to this tail.

The process where wire is cut right after ball is formed is also called stud bumping. Stud bumping is used when stacking chips in system in package

System in package

A system-in-a-package or system in package , also known as a Chip Stack MCM, is a number of integrated circuits enclosed in a single package or module. The SiP performs all or most of the functions of an electronic system, and are typically used inside a mobile phone, digital music player, etc...

(SIP) modules.

The current state-of-the-art machines can repeat this cycle about 20 times per second. A modern ball bonder is fully automatic and is essentially a self-sufficient industrial robot, complete with a vision system, sensors, and complex servo systems.