And I respectfully disagree with your disagreement. I think some education
is in order.
First, MIL-I-46058 and IPC-CC-830 are not magic documents. A listing on
QPL-46058 is no guarantee that any coating will work on your intended
application. Both 46058 and 830 are procurement documents. They set a
minimum bar for coatings to be used in military or commercial products. I
know of products not on the QPL that work just fine for their intended
purpose. I know of some on the QPL that I would not go near. The minimum
or maximum thickness in those documents refers to the qualification testing
range. Yes, you can go thicker than what is listed in these documents, but
then you are on your own.
Anytime you go outside the manufacturer's recommended thickness and
process, you are on your own. Doesn't mean its not going to work.
In the same manner, J-STD-001 gives you a target range of 1-5 mils of
coating thickness, but the language is also clear that if you go outside
that range, you have to do the additional testing to make sure it works.
Again, doesn't mean it won't work. You don't see condensation references
in 46058 or 830, because those are user-specific applications. Not
everyone needs to have that level. The testing specified in those
documents sets the minimum level of testing, not all possible tests or
scenarios. That is why we do extensive testing in condensing environments
to make sure the coating will work in our end use environment.
Are there ignorant engineers who blindly assume that conformal coating is a
bulletproof barrier against everything? Absolutely, I deal with them every
day. They just need education in materials science. Patient teaching
usually does the trick.
But Ed, you are wrong about the condensation. Our products, as is true for
Honeywell, Harris, Smiths, Boeing, ........, all go into condensing
environments. It is not unusual, and is frequently the case, where
avionics go through dew point condensation and the conformal coating has to
protect against that. Not once, not twice, but for 20-30 years or more.
It is also a common occurrence for it to be raining in avionics bays as the
plane descends. So the coatings have to withstand rain and flowing water.
We have been making conformal coating do this consistently for over 40
years, as have others. You have to make sure that your application and
cure system gives you consistent coverage and that you have selected the
right material and the right process.
If you want to say that anything thicker than 5 mils is an encapsulant,
that to me is semantics.
If you want to say that it can't be done consistently and cover 3D
components, we disagree. We can and we do.
As I have said in this forum in the past many times, going forward with a
material without understanding it thoroughly and the associated processes
and what the susceptibilities are, is just asking for trouble and generate
many of the nightmare scenarios we have all seen. But don't say that
because some people can't use it properly, nobody can.
My 4 cents worth.
Doug Pauls
Edward Mines
<[log in to unmask]
OM> To
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Subject
12/05/2006 11:18 [TN] conformal coatings as water
AM proofing
Please respond to
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; Please respond
to
Edward Mines
<[log in to unmask]
OM>
Doug Pauls wrote than conformal coatings protect against condensation. I
disagree.
Conformal coatings protect against atmospheric moisture. It is giant leap
of faith that many engineers make that they protect against condensation.
When I left the industry 2 years ago there was no reference to condensation
in IPC-CC-830 or MIL-I-46058 or UL746.
Conformal coatings do not always completely cover all conducting items on
3 dimensional boards.
Doug's employer is not the only one that uses a thicker layer of
conformal coating or similar material to protect against moisture.
IPC-CC-830 (and predecessor MIL-I-46058) specify a maximum coating
thickness for conformal coating. I am sure there are reasons for this. By
putting on more coating (a thicker layer) the assembler is out of
compliance with IPC & MIL
conformal coating specs.
If industry wants to use a material to protect PC boards from liquid
water that material should be called a different name (I called the one we
sold at HumiSeal a selective encapsulant). Tests should be developed to
insure that that material does the job for which it was intended; a QPL
should be established.
IPC, MIL & UL tests for conformal coating all include accelerated aging
and thermal cycling tests for conformal coatings within a specified
thickness range. Would qualified coatings pass those tests in thicknesses
outside those ranges? Can the user be certain that conformal coatings cure
properly in thicknesses outside those ranges? Might overly thick coatings
crack if subjected to those tests? Or in the field? If a board fails
because the conformal coating was used incorrectly is the board
manufacturer negligent? I am aware of one instance where this last issue
was seriously addressed in the automotive industry.
Are the tests in MIL-I-46058 (and grandfathered into IPC-CC-830)
realistic? Are they overkill?
Is there a reason for the thickness limitation?
At HumiSeal I heard of several instances each year where customers tried
to put more conformal coating on a board than specified by MIL-I-46058 and
recommended by HumiSeal (it seems like more would be better). I know for a
fact that some conformal coatings do not cure properly above a certain
thickness.
Edward Mines
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