About Wire Bonding

Local Time at Hybond: Wednesday, 22 February 2012, 1:18 PM

Wire bonding development:

Wire bonding was discovered in Germany in the 1950's through a fortuitous experimental observation and has subsequently been developed into a highly controlled process. Today it is used extensively for electrically interconnecting semiconductor chips to package leads, disk drive heads to pre-amplifiers, and many other applications which allow everyday items to become smaller, "smarter", and more efficient.

What is wire bonding?

Wire bonding is the method by which a length of small diameter soft metal wire is attached to a compatible metallic surface without the use of solder, flux, and in some cases with the use of heat above 150 degrees Celsius. Soft metals include Gold (Au), Copper (Cu), Silver (Ag), Aluminum (Al) and alloys such as Palladium-Silver (PdAg) and others.

There are three types of wire bonding:

1. Thermocompression Bonding:

A process which involves the use of force, time, and heat to join the two materials by inter-diffusion. The wire (heated in some cases) is pressed against the hot surface (at 150 deg. C or more) at high force for a limited period of time to achieve the bond. No friction is used. This process uses gold wire and a gold bond surface and it was originally associated with Ball Bonding. To this day there are still some people who will use the phrase "Thermocompression Bonding" as a synonym for Ball Bonding even if it now uses ultrasonic friction.

2. Thermosonic Bonding:

A process which involves the use of force, time, ultrasonics and heat to join two materials. The wire (heated in some cases) is pressed against the hot surface (at 150 deg. C or less) at low force and vibrated for a limited period of time to achieve the bond. This process uses gold wire and a gold bond surface and it was also originally associated with Ball Bonding because the first time ultrasonics were used as a bonding parameter it was done with Ball Bonding.. To this day there are still some people who will use the phrase "Thermosonic Bonding" as a synonym only for Ball Bonding, even if it now is used for wedge bonding too.

a.) Gold Ball Bonding:

So named because it is the bonding of a gold wire which, in its initial stage, has a ball or sphere formed on the end. A "flame-off" is used to make the ball. Originally the flame-off was done with an open hydrogen flame which would rotate towards the end of the wire and melt it, creating a sphere at the end of the wire. Currently the ball is made with an EFO (Electronic Flame Off) which creates a spark to melt the end of the wire. This bonding process uses force, time, ultrasonics and heat to make bonds. As of recent years copper wire has begun to be used for this process but equipment must have modifications to prevent oxidation of the wire and especially the ball during its formation at flame-off.

b.) Bump Bonding or Stud Bumping:

This is a variation of Gold Ball bonding. In this form of wire bonding, only the "bump" or ball is bonded to a surface. No wire extends from the first bond. This way of bonding is used for making gold bumps on die that will later be "flip-chip" bonded. Bump bonding is also used to connect surfaces whose planes are 90 degrees from each other but whose contacts are close together.

c.) Wedge Bonding:

This process was originally exclusive to aluminum wire, and no heat was used to create bonds. With time, heat was added to the bonding surface and gold wire was used for thermosonic wedge bonding of the gold wire. This is now a common form of bonding gold wire or ribbon. It should be noted, however, that some people still use the phrase "wedge bonding" as a term for ultrasonic (explained below) and not thermosonic wire bonding. Regardless of the traditional meaning of the phrase "wedge bonding", the thermosonic wedge bonding process uses force, time, ultrasonics and heat to make bonds.

3. Ultrasonic Bonding:

A process which involves the use of force, time and ultrasonics to join two materials. The wire is pressed against the surface (both at ambient temperature) at low force and vibrated for a limited period of time to achieve the bond. This process can be done with gold, aluminum, copper, palladium, silver or platinum wire or ribbons and to bond surfaces of the same materials. Originally, this form of wire bonding was done only with aluminum wire so to this day there are still some people who will use the phrase "Ultrasonic Bonding" as a synonym only for aluminum wire Wedge Bonding, even if it now is used for gold wire wedge bonding and other materials as well.

a.) Wedge Bonding:

This process uses force, time and ultrasonics with the bonding surface at room/ambient temperature to make bonds. Although originally exclusive to aluminum bonding applications, today there are many other materials and alloys that can be bonded by the ultrasonic method, including some thought at one time to only be effective if heat was applied.

b.) Peg Bonding:

This process uses force, time and ultrasonics with the bonding surface at room/ambient temperature to make bonds. Although Peg Bonding is exactly the same as Wedge Bonding, and can be done thermosonically as well as ultrasonically, the main difference with the techniques described earlier is that in a Peg bonder, wire is not fed from a spool of wire by the wire bonder. In a Peg Bonding application, the wire or ribbon (or any conductor in general) is either pre-aligned over the bond pad or it is manually introduced under the bonding tool (peg) to be bonded. A more common name to this process is TAB (Tape Automated Bonding) or Single Point TAB. Hybond has given the name Peg to this type of bonding because naming it TAB would lead to the assumption that a Tape feeding mechanism would be included as part of the equipment and in reality it is not. The name Peg is also given by Hybond because the tool used for bonding usually looks like a Peg just as the tool used in Wedge bonding looks like a wedge.

Today, ultrasonic bonding is a different process from what it was originally thought to be. The concept of interfacial rubbing is not valid. Ultrasonic energy, when applied to metal to be bonded, renders it temporarily soft and plastic. This causes the metal to flow under pressure. The acoustic energy frees molecules and dislocates them from their pinned positions which allows the metal to flow under the low compressive forces of the bond. Thus heat at the bond site becomes a byproduct of the bonding process, and the external heat becomes unnecessary to form the bond. This is also called a "cold weld".

The friction of the wire breaks up and sweeps aside some contaminants in the weld area exposing clean metallic surfaces which promote the metallurgical bonds. It is important, however, to begin with a clean surface to avoid difficulties or failures in bonding. In some cases the ultrasonic scrubbing may not be able to remove contaminants as in the case of lubricants.

It was also found that the bonding tool moves cyclically across the top of the wire. A regular flat tool may not grip the wire well enough, and this causes the wire to slip back and forth across the bond surface of the tool. For this reason, there are tools which are made with more porous materials such as ceramic-metal alloys or that have special features like cross grooves or grids which assist in the gripping of the wire during ultrasonic and thermosonic bonding.