Title Conformal coating process control and inspection
Bob Willis
Conformal coating process
control and inspection
There is definitely an increasing interest product, material, volume, application and
in conformal coating for electronics. This design density. Checking the wet coating
is based on recent workshop and seminars thickness on the surface of the board after
run by the SMART Group in the UK and application, or alternatively a blank panel,
other organisations worldwide. Over the will make measurement simpler. Using a
last 12 months your columnist has been wet depth gauge, check the coating in two
rekindling his interest and knowledge on or three areas on a sample board. Ideally
the subject for a workshop session at SMT check more than one sample in a batch.
Nuremberg. Things have moved on a long Record the reading, location, material and
way since my time as an apprentice using process parameters for future reference.
brush and manual spray coating tech- Coating a blank test board prior to
niques within GEC. Producing operator
Coating contamination
running production or using a proprietary
instructions and inspection criteria for the matrix test card like that used by Nordson/
number of brush hairs allowed in a coated Asymtech is a good way of checking spray
seems like a lifetime ago. process parameters. The test board or card
Some of the most common process can be inspected with a long wave UV light
defects include: for coating definition, overspray or coating
run on. Process parameters like solids
content, temperature, nozzle type, speed,
height and pressure can be recorded with
the sample and retained for reference
Dry coating measurement can be
performed on a test label applied to the
surface of a board prior to coating. The
thickness of the dry label can be recorded
Mask de-wetting
prior to coating; subtracting the thickness
of the label provides the coating thickness
in that location. The label can be re-
tained for future reference and is ideal for
Non or de-wetting
engineers to review during process audits.
Digital thickness measuring systems are
available for logging data for SPC.
With care, coated boards can be
examined directly after spray coating, using
a UV light source on an out-feed conveyor.
Care needs to be taken to avoid con-
tamination and disturbance of the coating.
Surface contamination with hairs, fibres or
Coating bubbles
other particulates can be a problem with a
delay before curing. The recent introduc-
tion of automatic optical inspection (AOI)
Simple process control checks can be
specifically designed for coated boards can
invaluable during conformal coating.
Delamination
be used as off-line or in-line inspection
These may vary depending on the type of
with software which has been specifically
4 – Global SMT & Packaging – March 2009
www.globalsmt.net
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