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Wedge bonding is a solid state process. The primary bonding parameters in
wedge bonding are ultrasonic power, force and time. When bonding gold wire,
the fourth parameter of heat is added. Wedge bonding joins two pieces of
metal using a process of diffusion. The bonding wire is mechanically bonded
to the die pad or package metallization by rubbing the surfaces
of the wire and moralizations together.
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This rubbing occurs by vibrating a clamped wedge at ultrasonic frequency
while the wire is held under the bond foot of the wedge. Commonly called
scrubbing, this rubbing disturbs any surface oxide film, exposes clean
metallic surfaces and forms a metallurgical weld. This metallurgical weld is
not degraded significantly by heat.
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Microwave
diodes, transistors and GaAs devices generally have gold as their surface
layer and have small bonding pads. Some of these bonding pads measure less
than 0.002in./51µ inches across and less than 0.004in./102µ on center.
The advantage
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of wedge bonding over ball bonding is the ability to form bonds on such
narrow closely spaced bond pads. Typical wire diameters for microwave
applications range from 0.001in./25µ to 0.0005in./13µ. Most microwave
devices are wedge bonded with gold wire.
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