Dear Brian/Grant,
Amen. Supporting customers using conformal coating products, as I do, this
is one of the most commonly asked questions, and I agree with Brian's
answer.
In fact this is why we use SIR as a research tool in development and many
people who chose to coat over no-clean validate their mix of process
chemicals and process settings with SIR.
Other residues can also come from handling, especially if staff have been
used to a process where the boards were cleaned in the past.
Regards,
Alan Brewin.
Concoat Ltd.
-----Original Message-----
From: Brian Ellis <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 25 March 2000 13:56
Subject: Re: [TN] Coating of assemblies with no clean fluxes
>Grant
>
>As a general rule, coating over flux residues is dangerous IMHO. There
>may be a few exceptions, but these are generally unavailable to the
>common mortal. Let us look at what is left on a board:
>- unmodified flux residues (it is BS to say that they are "burnt off"),
>possibly a dibasic carboxylic acid, maybe with organic halides, maybe
>with rosin or something similar
>- ionic salts (tin, lead, copper etc. carboxylates, halides, etc.)
>- polymerised flux residues
>- contaminants from boards and components, which may include
>inadequately washed water-soluble fluxes etc.
>- residues from rheological modifiers, maybe glycols etc. (hygroscopic)
>
>If you leave all these on the board and then coat over them, you are in
>DEEP trouble.
>
>One possible exception is where the flux is a fairly concentrated adipic
>acid type and where the coating is an epoxy which requires adipic acid
>for cross-linking, but this is hairy a) because it will be impossible to
>guarantee stoichiometry, b) it will not be homogeneous and c) you still
>have all the other gunge under the coating. The automobile industry use
>this technique (ever wonder why there are so many modern cars with
>failures of the electronics?).
>
>The short answer is your vendor is a liar.
>
>Brian
>
>
>Grant Emandien wrote:
>>
>> Hi all,
>>
>> The debate on cleaning of fluxes as well as the use no-clean fluxes seems
to
>> hang around forever. We have been informed by our solder an flux supplier
>> that assemblies with no clean fluxes can be conformally coated without
the
>> need to clean the boards.
>>
>> The argument is that the high temperatures of the reflow oven 'burns off'
>> the active ingredients, leaving only inactive residues. These flux
residues
>> do not have to be cleaned and can supposedly be coated. The rep. believes
we
>> are taking a backward step by possibly opting to use water-based flux. Is
>> this pure sales talk?
>>
>> Is anyone able to offer any advice or insight, or been able to assess the
>> environmental and electrical performance of such assemblies.
>>
>> Regards
>> Grant
>>
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>--
>Brian Ellis
>Protonique SA
>PO Box 78
>CH-1032 Romanel-sur-Lausanne, Switzerland
>Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
>E-mail: [log in to unmask]
>URL: Technical and consultancy divisions:
> http://www.protonique.com
> Web services division:
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>
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