in the main page about wirebonding, bonding is achieved by interaction of these factors: ultrasonic
force, pressure, temperature, and time.
or cold welding is initiated between the wire and the bond pad by
placing these two metals under
contact with each other. In the case of gold ball bonding, for
instance, the free-air ball at the end of the wire is brought into
contact with the aluminum bond pad by a needle-like tool known as a
capillary, squashing the ball against the bond pad.
energy is then applied by the bonding tool to the bond which, in effect,
the bond against the bond pad. This scrubbing action cleans the
bond pad of debris and oxides, exposing a fresh surface of the bond pad
in the process.
Two examples of wirebonders
metallurgical bond or weld between the bond and the bond pad is further
enhanced with the continued application of ultrasonic energy, resulting
of the bond and bond pad against each other. Aside from the
physical contact and deformation of the metals unto each other,
interdiffusion of the bond
and bond pad metal atoms also occurs to further enhance the cold weld.
The process of bond and bond pad metal counterdiffusion
to form intermetallic bonds is known as
intermetallic formation (Fig.
The area of intermetallic formation between the bond and the bond pad is
intermetallic coverage (IMC). In general, bond
reliability increases with the IMC, as long as overbonding does not take
place. Many companies monitor the IMC as a means of detecting
bonding problems on the line. Good bonds generally exhibit IMC's
of the ball impression's area.
common reason for insufficient intermetallic formation aside from
is the presence of
on the surface of the bond pad. Frequently-encountered bond pad
contaminants include unetched glass, silicon saw dust, and process
7 shows a photo of a bond pad with unetched glass on the surface, which
resulted in less than 30% IMC and subsequent ball bond lifting.
Cross-section photo of a gold ball bond's intermetallics
Fig 7. Photo
of a bond pad with a lifted ball caused by insufficient IMC (<30%)
If the bond
and bond pad are composed of different metals, such as in the case of
gold ball/aluminum bond pad bonding,
required to 'soften' the harder metal (aluminum) to match their
hardness. Au-Al ball bonding generally takes place at 200-240 deg C. In
cases where the wire and bond pad have similar metallurgies, bonding may
occur at ambient temperature, such as in the case of aluminum
wedge/aluminum bond pad bonding.
is applied to the bonding tool to keep it in control as it scrubs the
bond against the bond pad. Without pressure, the bonding tool
would bounce around the bond pad as it receives and transmits the
The quality of a bond may be
assessed using bond strength
Effects of Grain Size Distribution on
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Bonding Failures; Bonding
Strength Tests; Bond
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