Copper Wirebonding

            

Copper Wirebonding 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.  These advantages 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 elevated temperatures; and 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 wire, with an electrical resistivity of 0.017 micro-ohm-m at room temperature, is more electrically conductive by about 25%-30% than gold, which has a resistivity of 0.022 micro-ohm-m at room temperature.  This low electrical resistivity of copper results in better electrical performance.  In particular, copper wire is a preferred bonding wire material for high-current or high-power applications, since it can carry more current for a given wire diameter.

    

Copper also has about 25% higher thermal conductivity than gold (385-401 W m-1 K-1 for Cu and 314-318 W m-1 K-1 for Au).  Thus, copper wires dissipate heat within the package faster and more efficiently than gold wire, minimizing the thermal stress to which they are exposed.  Excessive heat on the wires can promote grain growth, which lowers the strength of the wires. The heat-affected zone (HAZ) formed on the wire during free air ball formation also tends to be shorter in copper wires because of their better thermal conductivity. The shorter HAZ in copper wires give them better wire looping capability than gold, an important aspect of die stacking.

    

Another advantage of copper over gold is its lower tendency to form intermetallic compounds with aluminum. The atoms of the gold wire have a high tendency to interdiffuse with those of the aluminum bond pad and form intermetallic compounds (IMC) with them. The high inter-diffusivity between gold and aluminum 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 wirebond reliability problems.  Aside from void formation, some of the intermetallic compounds formed by Au with Al are brittle and are therefore prone to fail by fatigue or stress cracking in the presence of thermo-mechanical loading.

   

Given the relatively high resistivities of the Au-Al IMC's, these intermetallics can induce additional heating when current is flowing through the wires.  The additional heat tend to accelerate the formation of more intermetallics, leading to a vicious cycle of IMC formation and heat generation.

   

On the other hand, intermetallic compound formation between the copper wire and the aluminum bond pad occurs at a higher temperature than Au-Al IMC formation. Studies by some experts have likewise shown that Cu-Al IMC growth is also 2.5 times slower than Au-Al IMC growth.  Because of copper's lower tendency to form intermetallic compounds than gold, copper bonds are deemed to offer a higher reliability at elevated temperatures.

   

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.

   

See Also:  WirebondingBonding Wires Bonding TheoryBonding Failures

         

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