Wire bonding is a method used to connect a fine wire between an on-chip pad and a substrate pad. This substrate may simply be the ceramic base of a package or another chip. Common wire materials include gold, aluminium and copper.
The main advantage of wire bonding technology is that it is low-cost.
Copper wire bonding refers to the wire bonding process that employs copper wires for interconnection, instead of the gold and aluminum wires traditionally used in semiconductor packaging.
Copper is rapidly gaining a foothold as an interconnection material in semiconductor packaging because of its obvious advantages over gold, which include:
1) Cost reduction of up to 90%
2) Superior electrical and thermal conductivity
3) Less intermetallic growths
4) Greater reliability of the bond at higher operating temperatures
5) Higher mechanical stability.
Copper is inherently 3 to 10 times cheaper than gold, so substituting gold wires with copper wires can realize tremendous annual cost savings for a semiconductor packaging company.
Copper also has about 25% higher thermal conductivity than gold and copper wires dissipate heat within the package faster and more efficiently than gold wire, minimizing the thermal stress to which they are exposed.
The disadvantages of copper wires versus gold wires include:
1) Copper tends to undergo oxidation at relatively lower temperatures
2) The hardness of copper wire require a bonding parameter (bond force and ultrasonic energy in particular) optimization to achieve effective bonding without causing cratering
3) Copper wire introduces a few failure analysis difficulties
4) Being relatively new, copper wire bonding technology is not yet as well-understood as gold ball bonding technology.
The oxidation of copper wire may be addressed by conducting the free air ball formation in an inert atmosphere. However, such a bonding process modification introduces new complexities into the assembly operation, such as parameter optimization for the nitrogen or forming gas used.
Pure gold wire doped with controlled amounts of beryllium and other elements is normally used for ball bonding. This process brings together the two materials that are to be bonded using heat, pressure and ultrasonic energy referred to as thermosonic bonding. The most common approach in thermosonic bonding is to ball-bond to the chip, then stitch-bond to the substrate. Very tight controls during processing enhance looping characteristics and eliminate sagging.
Alloyed aluminum wires are generally preferred to pure aluminum wire except in high-current devices because of greater drawing ease to fine sizes and higher pull-test strengths in finished devices. Pure aluminum and 0.5% magnesium-aluminum are most commonly used in sizes larger than 0.004 inch.
All aluminum systems in semiconductor fabrication eliminate the "purple plague" (brittle gold-aluminum intermetallic compound) sometimes associated with pure gold bonding wire. Aluminum is particularly suitable for ultrasonic bonding.
In order to assure that high quality bonds can be obtained at high production speeds, special controls are used in the manufacture of 1% silicon-aluminum wire. One of the most important characteristics of high grade bonding wire of this type is homogeneity of the alloy system. Homogeneity is given special attention during the manufacturing process. Microscopic checks of the alloy structure of finished lots of 1% silicon-aluminum wire are performed routinely. Processing also is carried out under conditions which yield the ultimate in surface cleanliness and smooth finish and permits entirely snag-free de-reeling.
There are two main classes of wire bonding:
Ball bonding
Wedge bonding
Ball bonding usually is restricted to gold and copper wire and usually requires heat. Wedge bonding can use either gold or aluminum wire, with only the gold wire requiring heat.
In either type of wire bonding, the wire is attached at both ends using some combination of heat, pressure, and ultrasonic energy to make a weld.
Ball bond:
Wedge bond:
For the application of wirebonding method, terminals of chips have to be arranged at the periphery of the chips, otherwise short circuit is easily caused. Therefore, wirebonding technique is difficult for high I/O (>500) interconnections.