Safe detection of hidden faults
number of ICs, • If there are higher components
the FOV is deci- in front of the tested pins,
sive for the time there might be overlaps in the
of inspection. angled field of view. One tries
• The to evade this situation with de-
depth of focus sign regulations; however, this
and the image is a question of practicability.
quality of the • A deflexion of the PCB during
angled view is the inspection process might
also decisive for cause a displacement of the
the whole FOV inspection positions in the
Figure 2. Examples of possible appearances of Lifted leads.
on the PCB as camera image of the angled
well as in regard view. To counteract this effect,
to the height adequate compensation algo-
Detecting critical faults—under all
extension of the components. rithm can help.
circumstances?
Consequently, it influences the • Components in an angled
Lifted leads belong to the most critical
inspection quality and speed. position other than 90° steps
faults in the production of PCBs. They re-
• The sole evaluation of the front cause extremely higher efforts
sult, for example, from lifted pins or from
menisci at pin solder joints regarding the programming
insufficient wetting (e.g., through oxida-
has only a limited informative time because the test samples
tion of a pin). In the electrical test (ICT,
value regarding the solder qual- that are defined in the library
boundary scan, functional test) they may
ity. An angled inspection of (test units, macros, etc.) cannot
perfectly show an electrical contact to the
the IC pins enables an optical be used, and a manual adapta-
pad on the PCB—however, in the actual
inspection that resembles an tion is necessary.
use there can be discontinuities and hence
IPC-conform test much more. • Besides the image capturing us-
a malfunction of the PCB.
• Especially for the inspection of ing angled view, an intelligent
The detection of lifted leads is complex
fine pitch ICs up to 0.3 mm, verify function is necessary in
due to their diversified appearance. The
a sufficient resolution is neces- order to reach a safe fault detec-
following parameters have a significant
sary for safe fault detection. tion with minimum false call
influence on the quality of the solder joint/
lifted lead:
• solder quantity
• width of the pad
• length of the pad
• solder flow at the pin foot
• flow behavior on the pad
• height of the pin
All these criteria are combined in reality so
that there are a variety of possible appear-
ances. In the daily use there may be lifted
leads that are detectable with orthogonal
inspection; however, this is significantly
dependent on the PCB layout and the ap-
plied solder quantity.
an angled view alone is not
enough...
As already mentioned, the angled-view
is considered as THE remedy for maxi-
mum fault detection. However, a critical
approach to the AOI systems is absolutely
necessary as a range of parameters have dif-
ferent impacts on the performance:
• The field of view (FOV) of the
angled view is responsible for
the inspection speed of the
complete system. As a compre-
hensive use of the angled view
is necessary for the inspection
Figure 3. Field of view of the angled-view module “Chameleon” by GÖPEL electronic GmbH with SO-IC at a
of complex PCBs with a high
45° angle.
www.globalsmt.net Global SMT & Packaging – October 2009 – 11
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