Jan,

Here is my two cents worth.

<< I'm just wondering about a no-clean flux made by InterFlux called
 interflux2005M.

We have worked with a couple of manufacturers, commercial and military, who
have qualified the 2005M for their assembly process.  The material appears to
be good from an ionics standpoint (halide free) and had good SIR performance,
when used properly.


>> Sometimes we need to put some extra flux on certain kinds of components,
 and if there's not enough heat it won't burn away and leave a white
 powder left on the PCB. This is to my knowledge Adipic acid (spelling?).
 
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.  If you do have to apply additional flux in hand
soldering, use a flux pen, not a squirt bottle.

>> 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.

>> Of course we can use a waterwash or any similar kind of washer, but we
 don't wan't that extra step in the production. Many people here at
 Ericsson have diffrent theorys about if the flux do harm or not.
 Anybody have any experience in this matter?

Unless you modify your method of flux application (i.e. going to the flux
pen), you may need to do the cleaning.  Of course, no one wants an additional
process step - costs time, costs money.  If you want to eliminate that step,
eliminate the source of the problem.

We deal with companies who are trying to determine the root causes of field
failures or process failures.  People have had problems with the situation
you describe, not so much from metal migration or corrosion from the weak
organic acids, but due to the other effects (not reaching activation temps,
residual isopropanol, flux going where it is not supposed to go, WOAs forming
insulating barriers on switches, etc.).

Hope this was helpful.

Doug Pauls
Contamination Studies Laboratories
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