To all folks out there who think of a coating as a barrier (i.e.
something that prevents) especially against water:
Conformal Coatings are not water proof.
In fact I know of nothing that is truly water-proof, even glass.
Water is a really nasty substance, given enough time it will go
through everything, it can even kill you.
Conformal coatings are a "thin, plastic protective film that conforms
to the varied profile of an electronic circuit assembly". They help
reduce the effect of moisture and condensation (and other surface
contaminants) and will, for a relatively short time, help stop
running water from affecting electronic circuit perfomance. Of course
they should have minimal effects on the electronic circuit operating
beneath them.
BTW, the running water needs to be pretty pure, please don't take a
coated circuit and drop it into a bucket of tap water and expect it
to work, it probably won't. Use de-ionised water and it should be OK
for a time.
Like you and me they breathe. In so doing they permit air, that may
contain moisture, to migrate in and out of the film. If there are
nasties like ionics present beneath them, then they (the coatings)
can cause more problems than they solve. The ionics, usually salts,
behave like magnets and swell as they take up moisture and in worst
case, then bridge between adjacent conductors and your circuit ain't
like it used to be! So clean them off first or make sure that
whatever residues are present, will behave themselves in company.
I must be among very few in this industry where my customer only used
my product under extreme duress. It must be so nice to feel wanted....
Kindest regards,
Graham
[log in to unmask]
On 6 Dec 2006, at 12:56, Bill Kasprzak wrote:
> Good morning,
>
> I am surprised that there are such opposing views on whether conformal
> coating is effective as a moisture barrier.
>
> My impression has always been that the primary purpose for coating
> boards
> was to act as a moisture and corrosion barrier. It has dielectric
> strength
> as a secondary property.
>
> So, I am I hearing from Brian that conformal coating is useless as a
> moisture barrier on PC Boards?
>
> Bill Kasprzak
> Moog Inc
> Process Engineer, Electronic Assembly
>
>
>
>
> Brian Ellis <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 12/06/2006 01:31 AM
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Brian Ellis <[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> Re: [TN] conformal coatings as water proofing
>
>
>
>
>
>
> I disagree with your disagreement. CC CANNOT protect against
> atmospheric
> moisture as they are all as porous as a sieve. The best they can do is
> to delay the onset of undesirable effects. CCs SHOULD cover everything
> except contacts on 2- or 3-D assemblies and, as such, protect against
> the effects of condensation. They do not allow the passage of water in
> liquid phase, only in vapour phase.
>
> Brian
>
> Edward Mines wrote:
>> 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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