Figure 7. The zero degree (0°) face
angle provides the greatest downward force on the stitch bond; however, it may result in
heel cracking problems.
To reduce this problem, 0° face angle capillaries are designed with a larger outside
radius of 0.0025"/64u or larger. 0° face angle capillaries have shown to work well
for lead frames with lead planarity problems.
8. The eight degree (8° ) face
angle is probably the most successful dimensional characteristic of modern capillary
design. The 8° face angle provides adequate force down on the stitch bond and limits heel
crack problems. This is due to the thicker cross-sectional area of the wire under the face
up to where the outside radius begins. The 8° face angle capillary works well over the
broadest range of moralizations; i.e., thick film, thin film, soft and hard moralizations.
Figure 9. The four degree (4°) face
angle provides a compromise between the 0° face angle and the 8° face angle capillary.
The 4° face angle provides more force down on the stitch bond than the 8° face angle.
This allows for a stronger stitch bond but can lead to the possibility of heel cracks or
chopped off stitches. This is due to the thinner cross-sectional area of the wire under
The 4° face angle is recommended when power and time (instead of
force) are used to correct stitch bond nonsticking problems.
Figure 10. Causes
for a week transition from the stitch bond to the Wire include:
Face angle of capillary too
Too small or sharp outside
radius size due to design or wear
Excessive force setting,
causing tip to penetrate bonding pad
Excessive power setting
especially when force setting is too high
Improper lead clamping
allowing shifting of the bond
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