You bring up an interesting point. The activators in the "no-clean"
fluxes are (usually) dibasic carboxylic acids, although mono- and
tri-basic ones are not unknown. These react with metal oxides to form
metal salts. These are all weakly ionic and can contribute to problems.
However, many of the acids themselves, also weakly ionic, form the bulk
of the residues and they are designed to sublime off, at room
temperature, over a period of weeks to months after soldering. The salts
do not sublime. The fact that the acids disappear contribute to the
subsequent reliability of the assembly. If you coat over them, they
cannot sublime; they are there for ever and able to cause more damage
than they would without a coating.
However, consider the case where the metals being soldered are heavily
oxidised. That means the salt:acid ratio in the resides will be much
higher, after soldering. Now it is one thing having a pair of -COOH
groups attached to a molecule with a high bond strength, but it is a
totally different chemical kettle of fish having -COOCuOOC- (for
example) joining two molecules. This kind of structure would have
electrical and chemical characteristics totally different from the
original acids, including a reduced solubility, hence one of the causes
of "white residues". This reduced solubility is one of the factors that
helps render the residues reasonably innocuous but I can't help feeling
that the -COOH groups at the outer ends of metallic organic (NOT
organometallic) salt molecules, as well as unreacted ones in the acids,
will attract humidity through a coating, over time, under humid
conditions. If this happens, then expect trouble.
I, for one, always feel that using chemicals in a way for which they
were never designed is playing the sorcerer's apprentice. If you get
away with it after due and extensive qualification tests, well and good,
you may wish to take the risk and hope your process never goes off the
rails. If you don't do the tests, then don't be surprised if the
floodgates of trouble open.
Brian
Somtawin Pornpottanamas wrote:
> Has anyone ever seen white contaminate ( like a crystal) on assembly
> boards , between IC lead? I found white residue on some board after
> reflow . I try to clean by used IPA, flux removal solvent and also DI
> water. But could not remove it. I also found that after we used the
> solvent to clean, the white residue increase more.
>
> Our process used no clean . What is this residue? What type of solvent
> that should be use to clean flux residue when we have to rework or
> Dedug?
>
>
>
> Thanks & Regards,
>
>
>
> Somtawin Pornpottanamas
>
> Material Engineer
>
> Kimball Electronics (Thailand)
>
> Tel. 038-401566 ext. 211
>
> Fax. 038-495029
>
>
>
>
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