refers to the wire bonding process that employs
copper wires for interconnection, instead of the gold and
aluminum wires traditionally used in semiconductor packaging.
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
with an electrical resistivity of 0.017
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.
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.
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.
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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
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.
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.
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
Devices that are bonded with
copper wires are more difficult to subject to failure analysis.
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.
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
Bonding Theory; Bonding Failures
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