Mary Lou,
I have issues with most of the tape tests, ASTM or otherwise, as the 
results are far too often subjective and do not represent real world 
factors.  We used to use a version of the ASTM tape test here, where a 
process control coupon was coated, cured, and an "X" scribed into the 
coating.  Tape was applied over the top and pulled off.  Whether or not 
you passed that test was dependent on whether Bob the Gorilla or Millie 
the Mouse performed the test, was the razor blade sharp, did you snap pull 
the tape, was the tape new or old, was the correct tape used....?  We 
stopped using that test because it created too many false positives, 
showing problems where none existed. 

Secondly, until Boeing or Airbus start to install small elves that run 
around inside our avionics carving up the coating with razor blades, the 
tape test does not represent a valid failure mechanism for the coating. 
For us, as long as the conformal coating adheres through our climactic 
burn in testing, then it is acceptable.  Since coating adhesion generally 
increases with time, if it passes temperature-humidity cycling in the 
factor right after cure, it will last in the field. 

Now, if we do see loss of conformal coat adhesion after cyclical testing, 
then we have to track down why and determine root cause.  What was the 
pre-cursor to your Parylene failures?

Doug Pauls
Rockwell Collins



Mary Lou Sachenik <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
05/09/2009 06:47 AM
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Subject
[TN] Parylene Adhesion Tape Test






We recently had some issues with lack of adhesion of Parylene coating

What types of notes are being used on drawings to define parylene 
conformal coating adhesion test on pwa's?
Is the ASTM3359-08 (for assessing the adhesion of coating films to 
metallic substrates) the best spec to call out for this testing .
I presently don't have pwb coupons to perform destructive cross hatch 
test.

What type of tape is being used testing parylene adhesion on the PWA's? 
Some specs call out 3M-250 tape, the ASTM D 3359 calls out the Permacel 
99.
Actually I prefer the Wescorp 79210 clear Anti-static Tape, does this seem 

acceptable?

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