I'm not an expert in this but have had some experiences of this in
recent months, as we've had problems with applying conformal coating
over boards assembled with a "no-clean" process.
Our customer had decided to use conformal coating as extra insulation in
HV areas of the board ,so the coating was selectively sprayed over
roughly half of the PCBA area. Basically the coating didn't adhere to
the board well enough, and it was noticed that we were getting arcing
between adjecant points when high voltages were applied to the circuit.
We were using Electrolube coatings, and after much discussion with them,
they advised us that the coating wouldn't adhere properly if the boards
weren't cleaned. We asked our board assembler to change processes, the
boards are now throughly cleaned before the coating is applied, and the
problems are much reduced.
Another supplier of coatings did take a different view, suggesting that
clean boards weren't essential to the process, but our experience seemed
to indicate that, in this application at least, cleaning the board prior
to coating was essential.
Suggest you discuss this with the coating supplier to get their advice.
Lorraine
-----Original Message-----
From: Peter Lee [mailto:[log in to unmask]]
Sent: 28 February 2002 05:26
To: [log in to unmask]
Subject: Re: [TN] No No-clean?
I am also facing the decision of whether to choose no-clean/no wash
process for boards to be conformally coated at customer's site. I am
conducting full qualifying run to confirm the compatibility between
paste/flux and coating material. Unfortunately you rarely can get
confirmation from flux manufacturer about reliability of conformal
coated over noclean boards.
Can anyone share their experience on the risk, precaution steps, and
process control for conformal coat over noclean?
Rgds,
Peter
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
Sent: February 27, 2002 6:18 PM
To: [log in to unmask]
Subject: Re: [TN] No No-clean?
Brian,
Can you elaborate on the dangers of conformally coating a no-clean
product?
I would have thought, in my ignorance, that processing in that manner
would
be most preferable. What kind of problems could or would one expect?
Thanks,
Leland Woodall
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Wednesday, February 27, 2002 2:03 AM
To: [log in to unmask]
Subject: Re: [TN] No No-clean?
Jim
Quite an interesting one this, with implications beyond the obvious.
Firstly, despite what Mike says, I think most of us would agree that a
properly cleaned board, even soldered with a reasonably aggressive flux,
is less likely to be unreliable than an uncleaned one, under some given
conditions. Experience has proved this.
Then there are some "no-clean" fluxes/pastes which are REALLY dangerous,
so it is easy to imagine that someone could have been bitten.
However, I strongly suspect, reading between the lines, that this notion
may apply to conformally-coated boards. Here, there is a distinct
problem with "no-cleans" unless the homework is VERY carefully done and
the process is constantly requalified, including checking the
contamination on every batch of incoming components. Personally, I would
never advise a client to use "no-clean" products if the products were to
be coated. It could be a time bomb waiting to go off under your feet
even years later (especially, no pun, with armament electronics which
may be stored for 15 years and then used for the first and only time
during a period of a few minutes).
Finally, a PCB design which is optimised for use with a mild "no-clean"
wave-soldering flux or paste would be better redrafted if it were to be
changed to a less mild flux and cleaned. Many such boards would be
totally unsuitable for cleaning and the result may be worse than ever,
in terms of reliability, if there were no re-design.
I would therefore want the rumour to be substantiated before any action
was to be taken.
Brian
Jim Jenkins wrote:
>
> Hi everybody,
>
> I was just approached by one of our engineers whose customer told her
that
> the Air Force has issued a directive disallowing the use of no-clean
fluxes
> on any of their hardware. The customer is flowing down that
requirement
to
> us. Do any of you know of this directive or what they could be
referring
to?
>
> Thanks,
>
> Jim
>
>
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