Parallel print & inspection processes to achieve 100% post-print inspection
however, is the use of area cameras for
post-print inspection. Although these
cameras are ideally suited for aligning the
substrate with the stencil, they have a
key disadvantage for inspection; the small
field of view (FOV). The mechanical
restrictions of the screen printer and
method for aligning the substrate and
stencil do not permit further expansion
with respect to inspection. The limits of
this traditional technology have
been reached.
In working on this problem, assembly
equipment manufacturer ERSA developed
Line Scan Technology (LIST), which
uses a line-sensor-based camera for both
alignment and inspection. Similar to a
Figure 2. Distribution of potential errors across the sub-processes.
standard scanner, the patent-pending LIST
camera has top and bottom line sensors,
One reason for this may be budget • Camera ‘flashing’ when moving and
but at a resolution never before seen in
restrictions. If you are only able to install capturing the images
a scanner. Standard cameras take many
one inspection system, you will naturally • Reducing the core cycle time permits
pictures of a very small area. The LIST
want to inspect the final PCB quality more time for inspection
camera scans at a speed of 35 mm per
after soldering. While the errors that are • Parallel process to clean templates and
second. Prior to the development of LIST,
detected here can prevent a defective inspection
the fastest inspection speed in a printer
assembly from leaving the factory, they
has been 1,200 mm
2
. The LIST camera
often require very expensive repair While such activities clearly improve
achieves 9,100 mm
2
.
procedures. Production management existing concepts, they do not represent
While the use of a line sensor camera
must be aware of the error potential in real innovations. All too often, equipment
substantially enlarges the area that can
the printing process. If the budget for a manufacturers are so deeply rooted in their
be inspected and will achieve a cycle
separate post-print inspection system is core technology that it is almost impossible
time that is quicker than the line cycle
not available, then the printing machine for them to change them.
for 100% inspection of the substrate for a
itself should have an inspection capability By taking a step back and viewing the
wide range of users, there are users whose
integrated. This is a cost-effective total process from a distance, new ideas
line cycle time is under 20 seconds and
alternative, and nearly 90% of the in-line can result.
who want or are obliged to carry out
screen printers sold today have this option State-of-the-art in a printing machine
100% post-print inspection. For these
available. up until today has been a system with
users, LIST alone is not enough; a parallel
The performance of integrated a camera placed between substrate and
process is necessary With current screen
inspection systems, however, varies stencil. This technology has proven itself
printers, the substrate is printed first and
significantly with respect to inspection and provides the greatest possible precision
then inspected. ERSA’s VERSAPRINT
task and inspection speed. Most systems combined with low cycle times. One
P1 stencil printer combines these two
available in the market can detect the limitation with today’s screen printers,
most time-consuming steps by changing
paste on the pad, solder bridging and print
the alignment of the camera axis and
alignment. All of these systems, however,
using a three-part transport, the Triple
share critical disadvantages. None of the
Rail Transport (TRT), with the resting
integrated inspection systems currently
position of the line sensor camera at the
available are able to provide 100%
right exit transport section, making it
inspection without delaying the cycle time
possible to completely inspect (100%) one
of the complete production line. Printed
circuit board while printing the next. So
substrates, then, can only be partially
long as the inspection time is less than or
checked.
equal to printing time, parallel inspection
It goes without saying that all
with LIST and TRT effectively divorces
manufacturers are working intensively
inspection time from cycle time. It also
to improve their integrated inspection
eliminates the need for an AOI machine
systems. The desire and necessity for 100%
stationed after the stencil printer.
inspection has not been a secret but rather
Because this inspection process is
an all-important goal. Even if the objective
essential for each PCB, it can rightly be
is clear to all, however, the methods of
called a real parallel process.
achieving it vary.
New technologies such as LIST and
These include:
parallel printing/inspection capability
• Expanding the field of view
can help manufacturers meet the ever-
• Quicker movement of the camera axis
Figure 3. The Line Scan Technology (LIST)
increasing demands on production
by using linear drives
camera scans at a speed of 35 mm/second.
processes with economical efficiency.
44 – Global SMT & Packaging - August 2008 www.globalsmt.net
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