Mary Lou
If you are not cleaning the assemblies before coating with Paralene,
this might well be the cause of your problem. It is my understanding
that Paralene exhibits poor adhesion to "no-clean" residues.
...and in corroboration of Dougs comments, if the coating was a
silicone, then the tape won't stick to it and you will have a
potentially false positive.
Regards
Graham Naisbitt
Email: [log in to unmask]
Phone: +44 (0)12 5252 1500
Web: www.gen3systems.com
On 11 May 2009, at 12:33, Douglas Pauls wrote:
> 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
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Mary Lou Sachenik <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> 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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