Here we go again. All polymers, including plastics used to encapsulate
electronics, as well as all conformal coatings, have well documented moisture
vapor transmission rates. So, it makes no practical difference if the body of
the device is coated, or not. Although your customer amy demand complete coating
of all surfaces on the board, if all exposed, conductive elements (leads and
traces) are properly coated you will see no performance differences between
boards with fully coated packages, and those with poorly coated packages.
If, on the other hand, you rely on the coating to "stake" the device to the
board, to reduce vibration induced stress, then the device should be clean and
coated.
By the way, if you are not dipping your boards in the conformal coating,
I can assure you that you have uncoated surfaces somewhere on your board.
The opinions stated herein are not necessarily those of my employer.
Lyn R. Lynch
602.276.7361
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: RE: pwb-cca : conformal coating
Author: [log in to unmask] at internet
Date: 10/30/96 12:23 PM
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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