Figure 34. The chamfer
diameter (B dimension consists of the hole and both sides of the IC. The function of the B
dimension is to capture and center the free air ball prior to making the ball bond. Most
thermosonic capillaries have IC=0.0004in./10µm or larger.
If the free-air
ball is formed to one side of the wire as a result of a poor tail bond, the resulting ball
bond can be golf club shaped. Reasons for a poor tail bond include:
chamfer too small
chamfer angle too steep
Poor lead metallization
golf club shaped ball bonds can also be caused by:
Inside chamfer is too small or
Wire is too hard
Wire tension/drag is not tight
Incorrect vertical distance
between the electrode and the end of the wire
Excessive tail length
If the B
Dimension is too large, the ball can be "swallowed-up" into the chamfer area. If
this happens, the ball may not have enough contact with the bonding pad resulting in weak
ball bond or "non-stick." It is also possible for the capillary's tip to touch
the pad. Touching the pad can damage the device and cause build-up of die pad
on the tip of the capillary.
the wire can be caused by:
the spool has scratches
This ball bond
was made by a worn capillary. The original shape of the inside chamfer is gone, leaving a
rough concave surface. This ball bond is strong and will pass bond shear tests, but could
fail optical tests. This capillary will eventually start producing bonds with flame-off
errors and tailing problems.
Figure 40. The capillaries
above have the same hole diameter but different dimensions. The smaller B dimension is
used when is attempting to form a smaller ball bond. A reduced free-air ball size is
recommended when the smaller B dimension is used.
The 90° I. C. angle is often combined with a smaller B dimension to
reduce the overall ball bond size. The B volume with a 90° I.C. angle is approximately
45% greater than with a 120° I.C. angle, allowing more of the free air ball to be drawn
up into the chamfer diameter. This forms a taller, smaller diameter ball bond than a 120°
I.C. capillary with the same B dimension.
the BASIC CAPILLARY BONDING and DESIGNdocument