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Testing of
Wire Bonds
The DELVOTEC automatic
bonders measure the cross-sectional deformation of the bond wire during the
bonding process and monitor the applied ultrasonic power and the bonding time.
This gives a certain
indication of the quality of the bonds, is however not sufficient to assess
yield, reliability and longevity of the bonds.
In addition to an accurate
optical inspection with an appropriate microscope (magnification x200)
by an experienced specialist and metallurgical analyses of the bond pads, bond
pull tests provide quantitative information on the bond strength and
variability.
A
pull
test is performed by means of a micro-hook
which pulls the bond wire vertically upwards. The control of the geometrical
variables are essential for a quantitative test result.
In the two
bonds are on the same level (H=0) and the loop is pulled in the center (e=0),
the force in the bond wire is:
For a
given bond strength the result varies obviously with the ratio d/h or with the
angle q.
The ratio of
the force applied to the bonds f1, f2 and the hook force F is expressed by the
correction factor a, tabled below:
|
q (°)
|
factor a
|
| 10 |
2.88 |
| 20 |
1.46 |
| 30 |
1 |
| 40 |
0.79 |
| 50 |
0.65 |
| 60 |
0.58 |
| 70 |
0.53 |
| 80 |
0.51 |
| 90 |
0.5 |
One
distinguishes destructive and non-destructive bond pull tests.
Destructive pull testing uses manual,
semi-automatic or automatic wire pull dynamometer to measure breaking strength
of wire loop. Typical bond breaking strength for 25 microns diameter wire is
~8g.
Non destructive pull test is similar but a fixed maximum pull force is applied which should be at the
minimum required strength.
Non-destructive pull tests are intended to reveal
weak bonds while avoiding damage to good bonds. This means that the force has to
be chosen such that good bonds are not stressed beyond their elastic limit.
In both test modes, statistical analysis of the
results (in terms of average value, is of interest to make predictions for large
volume productions.
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