Distribution in a Gold Ball Bond
grain size distribution
of a gold ball bond refers to the variation of the sizes of the grains
of the ball bond and the wire to which it is attached. A grain is a
volume of crystal of the same atomic arrangement orientation. In a
crystal, the interfaces that separate grains that have different
orientations in atomic arrangement are known as the grain boundaries.
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grain sizes exhibited by a wire affect the properties of the wire,
including its strength. Here are
some key points to remember about grain size distribution in a gold ball
gold ball wirebonding, the wire is melted to form the free-air ball used to
form the ball bond. This melting
of the gold ball results in coarse grains, since
travels up the gold wire and causes the grain size of the wire beyond
the ball bond to increase too.
increase in grain size diminishes as the distance from the ball bond
increases, because of the corresponding decrease in exposure to the high
temperature of the melt.
Thus, grain size is
maximum at the ball and decreases up the wire away from from the ball.
Eventually, a point in the wire will be reached wherein the grain size
is no longer affected by the heat, i.e., the grains still have the
original or unaffected grain size.
Excessive grain growth can result in what is known as a
characterized by grains that are larger than the wire diameter. Bamboo
structures are prone to grain sliding and wire
length of wire where the grain sizes have been modified from the
original size is known as the
recrystallization zone. The heat-affected zone is a 'weak' zone of
looping of the wire must not occur in the heat-affected zone.
Thus, the minimum loop height must be 80% of the heat-affected or
recystallization zone of the wire.
length of the recrystallization zone is determined by the type and
amount of dopant in the wire. Dopants increase the strength of the gold
wire by solid solution hardening. Common wire dopants include silicon,
beryllium, calcium, and lanthanides.
Excessive doping is also harmful to the wire, since this can result in
wire brittleness and voiding. Voids are due to the volatilization of the
dopants during wire melting.
Wire Bonding Theory
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