There is a tape test in the IPC TM 650 (method 2.4.28); this is available off the IPC web site.  This test is sited in IPC Conformal coat material spec (IPC-CC-830) in paragraph 3.5.2 and it notes that the adhesion of any coating differs significantly when tested on metal vs. laminate; the summary requirements also differ by Class (although most coatings meet Class 3, there aren't really coatings sold only for class 1 or 2).

                                Table 2 Adhesion Requirements
                                Over Metal/Plated Surfaces      Over Bare Laminate
                Class l ............................20% Class l          ..............................2%
                Class 2 ...........................10% Class 2            .............................5%
                Class 3 .............................5% Class 3           .............................0%

We use polyurethane, acrylic and silicone coatings and some silicones are the easiest to 'peel up' but we don't typically have a problem meeting the requirments above.  I would say that if we did, the first place we'ld look would be PWB/PWA cleanliness.  The fact the vendor "does clean the PWAs" doesn't necessarily mean they do a good job at it.  Typically, we find lack of adhesion related not to flux residues as much as cleaning solutions not being fully rinsed off (e.g. soaps or solvents) and leaving a "film".

If you have any other questions, call or contact myself or our coating "guru", Willy Chang at 253-657-8904.

Jim Maguire

> ----------
> From:         Jean-Francois Bissonnette[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Jean-Francois Bissonnette
> Sent:         Friday, December 03, 1999 7:26 AM
> To:   [log in to unmask]
> Subject:      [TN] Conformal coating pelling off
>
> Hi!
>
> We use silicon conformal coating on some boards and some of these
> show a lack of adhesion of the coating.  It peels off very easily
> with gently scratched.  I also heard of some peeling test that could
> be done, can it be done with any kink of tape?  The same coating on
> the other boards wont peel if I try to scratch it off.
>
> Can anyone shed some light on this?  I tought that it would be a
> cleaning problem but the supplier says that they do clean it.  Can
> the board material be the source of that problem?
>
> Thank You!
>
> JF
>
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