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
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