Copper (Cu) Wire Bonding or Copper Wirebonding

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Copper Wirebonding (Page 2 of 2)

Studies have shown that copper wire can achieve greater mechanical stability than gold wire. Standard bond strength tests such as the wire pull test and the ball shear test have demonstrated that copper ball bonds exhibit 25%-30% higher readings than comparable gold ball bonds. In fact, copper wire bonds can be so strong that the wire itself does not break during wire pull testing, resulting instead in bond pad metal lifting. It is for this reason that non-destructive wire pull testing, wherein only a specified pull load is applied, is recommended for copper wires.

The disadvantages of copper wires versus gold wires include the following: 1) copper tends to undergo oxidation at relatively lower temperatures; 2) the hardness of copper wire require 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; and 4) being relatively new, copper wirebonding technology is not yet as well-understood as gold ball bonding technology.

The high tendency of copper wires to oxidize can result in excessive formation of oxide layers on its surface. Excessive oxide layers on the wire surface will prevent the formation of round free-air-balls - a prerequisite of a good ball bonding process. Highly oxidized copper wires are likewise harder and inherently more difficult to bond. Copper oxidation are also known to cause corrosion cracks.

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.

Since copper wire is harder than gold wire, it is more difficult to bond. Effective bonding can be achieved by increasing the bond force and ultrasonic energy used. However, there is a limit to which these parameters can be increased, since excessive force and power can damage the silicon substrate under the bond pad, a phenomenon known as cratering.

Devices that are bonded with copper wires are more difficult to subject to failure analysis. For one, the copper wires exhibit no contrast with the copper leadframes during x-ray inspection. Secondly, copper wires react with nitric acid, preventing conventional jet etching from being utilized for package decapsulation.

In summary, copper wirebonding offer many advantages over gold and aluminum wirebonding. However, it also comes with certain technological challenges that need to be overcome. The achievement of reliable fine-pitched copper wire bonds require 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.

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