Graham, Doug,
I am "listening". This is perhaps not a direct answer to your question Doug,
but the experience we gained from our study may be of interest. IVF
performed an study of conformal coatings in the early nineties with the
objectives to learn the pros and cons of conformal coatings and how to use
them in order to get the optimum reliability of printed board assemblies.
That was during the phase out of CFC´s so compatibility with various type of
flux residues was a large part of the study.
The idea, at start, was to verify the reliability of coated assemblies
according to standards, but we couldn´t find any standard specifying how to
verify the reliability of coated assemblies. MIL-STD-2000 and the IPC
standard (I have forgot the number) specified coating thickness requirements
and that the coating should not have any defects that could affect the
sealing properties. It was specified how the thickness should be measured
(on a flat, unencumbered, cured surface) but not how to verify that the
sealing properties are acceptable.
The coating thickness of a flat surface is usually of little interest, it is
non-flat surfaces such as solder joints and component leads that are the
critical areas to protect, but the standards have no requirement about
coating thicknesses on these, except that the sealing properties should not
be affected. Thus, you need to test the sealing properties. How do you do
that? That is up to you to find out. In the procurement documents,
MIL-I-46058 and IPC-CC-830, this is done by SIR measurements of comb
patterns on, "guess what", flat unencumbered surfaces. That is OK for
qualifying a conformal coating material but not for verifying the sealing
properties on a coated PBA. Therefore, we decided to use a test board
equipped with open circuit components that enabled us to measure the current
leakage between the solder joints to the components.
The "SIR test" was performed per IPC-TM-650, Method 2.6.3.1 (Condensing
Service Environment). To our surprise, the best results were achieved for a
non-coated reference board (when cleaned boards were tested). We found out
that no condensation occurred during the test (and it shouldn´t do if you
have a good test chamber and test PBAs with low thermal mass). Thus the test
conditions were much more benign than many field environments that coated
BPAs are expected to be used in. When we designed a test where we simulated
condensation of a 20 micrometer thick water film we got completely different
results, especially if performed some thermal cycling before the
condensation test. In short, the results can be summarised with that only
assemblies coated with Parylene passed the condensation test. The other
industry processes that were used for coating assemblies, both spray and
dipping, did not give a coating with good sealing properties on solder
joints and component leads when applying a coating with a thickness of 25 to
75 micrometer on a flat surface, which is the thickness range that coatings
usually have.
Thus, our conclusion was that, although it is important to verify that you
have a "quality" coating, it is far more important to verify that you have a
"quality" process that gives you a coating with good sealing properties. How
should you do that? Well, i doubt that you can find such a standard. It is
up to you to determine how you should do it. IPC is currently working with
writing a conformal coating guideline (IPC-HDBK-830, Guideline for Design,
Selection, & Application of Conformal Coatings). It is not released yet but
you can probably get a copy of the present proposal. It is probably the best
information available today. You can get much guideline in it, but still, it
does not tell you how you should verify the reliability, only that you ought
to do it.
We focused mainly on the sealing properties and resistance against corrosive
environments, but we also found that Parylene and some silicone coatings
improved the fatigue life of solder joints to LCCC components.
Regards
Per-Erik Tegehall
IVF
SWEDEN
http://www.ivf.se
-----Ursprungligt meddelande-----
Från: Graham Naisbitt [mailto:[log in to unmask]]
Skickat: den 4 september 2001 19:18
Till: [log in to unmask]
Ämne: Re: [TN] Conformal Coating
Doug - good evening from jolly olde England.
The most comprehensive study completed on Conformal Coatings - that I know
of - is the work done by the IVF in Sweden by Per-Erik Tegehall and his
team.
We have extracts of all this work however, it would be more polite to refer
any request to IVF for copies.
If Per-Erik is "listening" maybe you can confirm the situation. If not, let
those who are interested call me and I will do the necessary to get you
copies.
Regards, Graham Naisbitt
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of
[log in to unmask]
Sent: Tuesday, September 04, 2001 01:54
To: [log in to unmask]
Subject: [TN] Conformal Coating
Good morning all,
I hope you all had a nice weekend.
Last week (and a few weeks preceding) I have been part of a group that is
examining our conformal coating process looking towards the future. Right
now, I am in the middle of a classic engineering vs. manufacturing head
butting exercise. For some of our engineers, manufacturability seems to be
a completely foreign term.
I have been tasked to gather information. I figure this group is a great
place to start. I have two basic questions:
1) What specifications do you use to determine if you have a "quality"
coating in production? Internal workmanship standards? IPC-A-610?
MIL-STDS? I'm not talking about MIL-I-46058 or IPC-CC-830, since those are
primarily procurement documents.
2) Can you give me references for papers on the engineering aspects of
conformal coatings? Our engineers claim benefits like added vibration
resistance, thermal emissivity, etc., in addition to the usual moisture and
crud barriers.
Thanks in advance.
Doug Pauls
Rockwell Collins.
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