Thought I would add my two cents on the subject.
We are on a steep learning curve with respect to plastic parts. One of
several problems we encountered was dewetting at conformal coat. After
working the issue for a while, we have concluded that plastic parts are
inherently harder to wet than other parts and that some conformal coating
products are inherently better at wetting than others. A formula to
predict wetting would be nice but we so far we have progressed by trial
and error.
The first coating that we tried was our
'good-old-stand-by-coat-anything', single component, solvent based,
MIL-I-47058 UR. Wetting was spotty and unpredictable.
We were also working with the manufacturer of our solvent based coating
to evaluate their line of UV cure, single component, 100% solids,
MIL-I-47058 coatings. We tried their 100% solids coatings on the plastic
parts and had even less success. The dewetting of the plastic parts was a
text book example of 'crawling' or 'retraction'. Wetting was good
everywhere except on the plastic parts. Cleaning the plastic parts did
not improve wetting.
We then tried a Dymax product, a single component, 100% solids, UV cure,
acrylic copolymer (meets AR, ER, and UR.) We had much better luck. So
far this product coats plastic parts without a problem. It seems very
robust with respect to plastic parts.
Wetting is a function of the surface tension of the liquid and the free
energy of the surface. If I understand the physics, wetting will not
occur unless the surface tension of the liquid is less than the critical
free energy of the solid. I assume that these parameters vary over a
wide range for the materials in question - but there is very little data
available. Even without the mold release variable, plastic is a low
energy surface relative to metal and ceramic and, is therefore, more
difficult to wet. I assume that solvent in a conformal coating improves
wetting and, also, that the wetting characteristics of 100% solids
products are greatly affected by the selection of monomers.
Hope this information is of some help
Mary Davis
Sr. Material & Process Engineer
Alliant Techsystems
206-356-3311
[log in to unmask]
----------
From: CINDY KEMP ORLANDO ISC *8-306-6[SMTP:[log in to unmask]]
Sent: Wednesday, October 23, 1996 5:44 AM
To: Technet
Cc: GSPLASV
Subject: FWD: pwb-cca : conformal coating
Technet,
I'm forwarding this from a colleague.
Cindy Kemp
Lockheed Martin
Orlando, FL
**************************************************************************
*****
*
From: [log in to unmask]
Date: Tue, 22 Oct 96 16:35:22 EDT
Message-Id: <[log in to unmask]>
To: [log in to unmask]
Subject: pwb-cca : conformal coating
Sender: [log in to unmask]
Precedence: bulk
hi group;
We are finding that in designs using conformal coating (UR) over plastic
parts
that the cc is dewetting on the parts.
Question - is this seen as a problem ?
I am proposing that a note should be added to our designs as follows:
CONFORMAL COAT DEWETTING ON PLASTIC ENCAPSULATED MICROCIRCUIT
COMPONENT BODY IS ACCEPTABLE.
If there is a better way please let me here from you.
What is the real would doing ?
thanx; Skip Greb
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