Bonding Wires

The integrated circuit is connected to the leads of its package by bonding wires, which are very fine wires composed of  high-purity gold, aluminum, or copper.  Wirebond reliability depends greatly on the proper choice of the bonding wire, especially in advanced packaging technology which requires special loop profiles and extremely small bond spacing (fine-pitch bonding).

The first consideration in choosing a bond wire is the type of package it will be used for. Gold wire can't be used in hermetic packages because it won't be able to withstand the high temperature of hermetic sealing. Aluminum wire is the standard choice for hermetic assembly. For plastic packages, however, gold wire is the more logical choice because it is faster, easier to use, and therefore more cost-effective.
   

The diameter of the wire is the next important consideration. Thinner wires will be required by circuits with smaller bond pad openings, while circuits that draw large currents or require thermomechanical robustness would do better with thicker wires.

Another consideration when choosing a bonding wire is its tensile strength. The wire will be subjected to a lot of tensile stresses throughout its lifetime, e.g., during bonding itself, during encapsulation, during board mounting, during usage. Needless to say, the higher the tensile strength the better.
  
The elongation property of the wire is also an important consideration in the selection of the wire. Wires with higher elongation are more difficult to control during loop formation at wirebonding. Thus, it is better to choose a wire that doesn't elongate much during the bonding process.
  
The last major consideration is the length of the heat-affected zone of your wire. When the end of the wire is melted by flame-off to form the free air ball prior to bonding, the high temperature enlarges the grain structures of the zone closest to the ball. These larger grain structures are more vulnerable to shearing stresses that cut across the wires. Wires with a longer heat-affected zone can not be used in low-loop wirebonding because the heat affected zone may be subjected to the shearing stresses of loop formation. Normally the wire manufacturer will indicate whether the wire is for low-loop or for high-loop applications.

Copper wire is becoming one of the preferred materials for wirebonding. Copper wire of smaller diameter can achieve the same performance as gold wire of bigger diameter.  Copper wire, needless to say, is also more economical than gold wire.

With proper set-up, copper wire can be successfully wedge-bonded and be used as alternative to aluminum wire, especially in applications where higher current-carrying capacity is needed or complex geometry problems are encountered.

Copper wire is harder than gold and aluminum, so it has a higher tendency to contribute to die damage if the bonding parameters are not put under tight control. It is also inherent for copper to oxidize, which if left unchecked can lead to storage and shelf life issues.