Bonding wire is an indispensable material in the world of semiconductor.
Nonetheless, we are not able to see it in our daily life.
What’s the role of it? How are they bonded?
We will answer your questions.
Bonding wire is used as an interconnect material for a semiconductor package. It is a thin metal wire to carry electric signals from a semiconductor to an external part. Semiconductors are used in various kinds of stuff that support our daily life from PCs, home appliances, and to automobiles. In general, a semiconductor package is sealed with a mold resin after wire bonding process, thus bonding wire is not seen from outside.
Gold, silver, copper, and aluminum are used as a material for bonding wire. Gold had been almost exclusively used for ball bonding process, but the invention of palladium coated copper (PCC) wire (EX wire) has changed the dynamics of bonding wire market in the early 2010s. Now the PCC wire is the most widely used material in the market. In recent years, the use of silver wire is also increasing in various types of package application as an alternative to gold wire. Aluminum wire is mainly used for wedge bonding process mainly for power semiconductors.
The size of bonding wire varies significantly depending on the application with the finest down to 15μm and the thickest up to around 500μm. As of writing this article (2023), a high-end package uses more than 2,000 wire bonds per package with a bond pitch of around 30-50μm. As the device becomes more functional and more miniaturized, the wire being used also gets thinner and thinner, which in turn increases the technical challenge for wire manufacturer. We are coping with this technical challenge with our high R&D capabilities.
In ball bonding, the tip of wire is melted by an electric discharge and a ball (Free Air Ball) is formed. The ball is bonded onto a semiconductor element or a pad (1st bond) by mechanical force, ultrasonic power, and heat. A connection to an external terminal is made by a stitch bond (2nd bond).
In wedge bonding, a ball is not formed, and the bond is formed only by mechanical force and ultrasonic power. It is mainly used for power semiconductor.