Grain Size Distribution in a Gold Ball Bond


The 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.


The 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 bond:


- During 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 heating causes larger grain size


-  Heat travels up the gold wire and causes the grain size of the wire beyond the ball bond to increase too.


-  The 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 'bamboo' structure, characterized by grains that are larger than the wire diameter. Bamboo structures are prone to grain sliding and wire fracture.


-  The length of wire where the grain sizes have been modified from the original size is known as the heat-affected zone or the recrystallization zone.  The heat-affected zone is a 'weak' zone of the bondwire.


-  The 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.


-  The 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.


See also:   Wire Bonding; Wire Bonding Theory




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