Copper Wire Bonding

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.

Another advantage of copper over gold is its reduced tendency to form intermetallic compounds with aluminum bond pads. The atoms of the gold wire have a greater affinity to interdiffuse with those of the aluminum bond pad and form intermetallic compounds (IMC) with them. These IMC’s can create voids at the bond interfaces. The presence of such voids weaken the bond and can lead to bond lifting as well as other wire bond reliability problems. Other issues with IMC’s include stress, cracking and increased heat formation at the joint area problems. Copper wire can achieve greater mechanical stability than gold wire.

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.

In summary, copper wire bonding offer many advantages over gold and aluminum wire bonding. However, it also comes with certain technological challenges that need to be overcome. The achievement of reliable fine-pitched copper wire bonds requires the formation of consistently round and reproducible free-air balls. This necessitates the prevention of oxidation in free-air balls, which can be attained by creating an inert atmosphere around it during electronic flame-off. Optimized bond parameters and well-designed bonding capillaries are also needed for reliable copper bonds.