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Date: | Thu, 14 Jun 2007 11:15:44 -0500 |
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Thank you Ed. And thanks to the many who offered advice and suggestions
both on line and off line. Much appreciated. Helluva resource this group
is.
Doug Pauls
Edward Mines <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
06/14/2007 10:59 AM
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[TN] black light intensity measurement
Doug-
You ask an interesting question. I don't believe there are any standards
regarding the intensity of UV light for inspection of conformal coatings.
Before working for HumiSeal I worked for Polychrome, then one of the
bigger manufacturers of presensitized printing plates. The photosensitive
coatings on these plates are exposed with high intensity lamps in a vacuum
frame. Sometimes it was necessary to calibrate the exposure units and we
had a device we called a "puck". At least some exposure units use UV
lamps. Some lamps loose illuminating strength over a period of time.
There used to be a company in Chicago (UV Process Supply?) which sold
everything related to UV curing including these "pucks". Spectronics and
other manufacturers of UV lamps may know where to get UV intensity
measurement instruments too. At HumiSeal we had a similar instrument to
meassure the strength of our UV cure unit.
The intensity of UV is proportional to the distance between the light
source and the object being illuminated SQUARED (or maybe even to a
higher power?). You couldn't look at the light sources we were using at
both employers.
Now is a good time to mention that the glowing power of the fluorescent
dye in conformal coatings sometimes declines with age. We suspected that
this had to do with exposure to light. HumiSeals 1B31 & 1A33 contain a lot
of fluorescent dye; it's easy to dissolve in the solvents in those
coatings. It's not so easy to dissolve in some of the other coatings. So
the glow strength will vary from coating to coating. I think it's
dependent on coating thickness too.
Don't discount that different operators will count different numbers of
defects from the same screen. When I worked at Kodak (I got around) we saw
this many times with microscopists.
Don't get hung up on minute voids in conformal coatings. There are
breaks in the coating that you can't see - the backs of flat leads, bends
in flat wire (we called them "knees"). A different material should be used
for absolute coverage.
Edward Mines BEChE, MEChE
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