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Copper Wire Bonding Process

Wire bonding process is commonly used to interconnect chips to the outside world using gold wire since its inception in the mid 1950 using thermo-compression, an application of heat and force. However, it was not enough to form a more reliable oxide free ball and stitch bonds interface until the introduction of thermosonic bonding in 1960 incorporates ultrasonic energy. For decades, continuous progression to improve the device-package reliability has been the primary goal while cost of ownership has become one of the driving forces to make all the electronic gadgets available nowadays, affordable to the masses.

In general, the copper wire bonding process is very similar with gold wire bonding as it basically uses the same wire bonder equipment with minor hardware and software retrofits. Instead of gold wire, it is replaced by copper wire, though not limited; the range is typically from 15µm to 50µm in wire diameters depending upon the package-device application. Copper wire bonding offers significant advantages over gold – superior product performance in terms of electrical and thermal conductivity; better product reliability due to slower intermetallic growth that causes voids; and higher break load during wire pull testing.

One of the early day drawbacks of using copper wire in the wire bonding process is oxidation problem which can impact the reliability and integrity of the encapsulated device inside the electronic package. As we all know, oxidation retards the welding of deformed ball into the bond pad, and stitch into the lead frame or substrate. Today, this has been overcome due to the vast improvement in the wire bonding technology and processing of different materials
(e.g. copper wire, lead frame or substrate, device metallization, etc…) to complement the use of copper wire.
 
The utilization of forming gas (a mixture of 95% Nitrogen and 5% Hydrogen)- for an oxidation free process during the formation of copper free-air-ball (FAB).
All automatic wire bonders used for copper wire bonding process are all equipped with copper kit, comprising of EFO (electronic flame-off kit) with provision to ensure optimum flow of forming gas.
Palladium coated copper (Pd coated Cu) wire is an alternative to choice retard oxidation.
Software enhancements integrated in the new generation of copper wire bonders to improve ball bondability with minimal aluminum splash-out and programmable segmented stitch features.
Special type of capillary surface finishing with granulated protrusion for better gripping and to reduce short tail related stitch bondability problems.


 
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