Doug Pauls wrote:
>  
> This brings me to a general caution.  Many assemblers have used the no-clean
> flux, intended for spray application in wave solder, as an additional flux to
> aid in touch up or repair soldering.  This practice should be viewed with
> extreme caution.  Most low solids fluxes (no cleans) are 95% isopropanol (not
> counting the water bourne ones).  The low surface tension of the isopropanol
> and capillary action cause the flux to go places it is not intended.
>  Additionally, much of this flux does not see the activation temperature
> necessary, since the hand solder is a very localized application of heat.
>  The white powder you are seeing is most likely adipic acid, but white
> residues are common for no-clean fluxes.  In your case, the white residue is
> the no-clean flux activator, usually a weak organic acid (such as adipic or
> succinic acid) reacting with water, either from a cleaning operation or from
> the water vapor in the air.

Adipic acid does not react with water.  It is sparingly soluble in 
water (2 g / 100 ml) and will dissociate a little.  Adipic acid will
not pick up much water vapor on its own.  The presence of other hydrophilic
material in the flux, such as a surfactant or polyglycol, may allow the 
flux residue to pick up water anyway.  Sometimes
a few slow wet/dry cycles will make an organic acid residue more 
visible.  Sometimes an organic acid residue will react with solder during
water washing, making tin carboxylate salts, that are are white residues.

Rosin does react with water during cleaning, leaving a visible opaque
white residue.  Some no-clean fluxes contain 1-5% rosin.  

> If you do have to apply additional flux in hand
> soldering, use a flux pen, not a squirt bottle.
> 
This is good advice.

> >> How does this affect the components and PCB if it's in one kind of a
>  place where you can't brush it off?
>  
> It depends.  In general, as other TechNetters have commented, adipic acid,
> and weak organic acids in general, are relatively benign materials.  The only
> place I foresee a problem is the crystalline nature of the residue.  If you
> are conformally coating the board, moisture could swell the crystal to
> several times is size in service, causing a blister (mealing), allowing
> moisture to collect, possibly shorting.
>
While many crystalline materials (CaCl2 for example) pick up
water, causing the crystal to swell (but not several times), 
this is not the case for adipic acid. Other material would
have to be present for moisture to accumulate.  This may indeed
be true, since soldering fluxes contain several materials other 
than the flux activator.  The presence of moisture may allow galvanic
or electrolytic corrosion to occur, and corrosion products are usually
much bulkier than the original metal.  

-- 
Karen Tellefsen
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