Hi David - One problem you are encountering in your attempt to characterize
what elements are associated with the white residue is the resolution of
your analytical methods. Scanning Electron Microscopy (SEM) is a great tool
and its Energy Dispersive xray spectroscopy (EDS) capabilities can find
lots of things but there are limitations. Typical detection limits are 0.5
atomic % using very good calibration standards. The sampling field is also
pretty large (in simple terms a teardrop shape zone is sampled where the
electron beam hits)  and the overall sample geometry (e.g. a flat surface
versus a 3D surface)  will impact your analysis resolution. Other
techniques such as Auger spectroscopy and Secondary ion mass spectroscopy
(SIMS) can be used to supplement the SEM-EDS to get better resolution if
needed. Also - to look get an idea of what the flux may be contributing to
the white residue the use of Fourier Transform Infrared analysis (FTIR) and
Ion Chromatography (as suggested by Terry Munson) are very useful. Good
Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]




David yeh <[log in to unmask]> on 05/28/99 07:02:03 AM

Please respond to [log in to unmask]

To:   [log in to unmask]
cc:
Subject:  Re: [TN] White Residue on solder joints




Tegehall,

Thanks for sharing. I have not ruled out the flux to cause these residues.
But what puzzled me is why elements of flux are not captured in EDX? I
expect to see some carbon - that is what I thought I should see in flux.

I also thought the water soluble flux would be soluble in DI water- that is
what my supplier claimed.

The only concern I have over this residue is that it is corrosive in
nature, otherwise it should be harmless like you said.

The residue only showed up after few days and only on some substrate. I am
in the process of simulating this effect by subjecting in different
temperature profile, storing in different environment, having the substrate
washed and unwashed. Please share if you have additional input you like me
to look at.

Thanks once again.



----------
> From: Tegehall Per-Erik <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re: [TN] White Residue on solder joints
> Date: Wednesday, May 26, 1999 10:07 PM
>
> I have seen the type of flux residues you describe a number of times. In
> most cases, it has turned out to be a lead salt, for example lead
adipinate
> formed when adipic acid in the flux (an activator) react with lead oxide
on
> the surface to be soldered. My experience is that these salts are very
> difficult to dissolve whatever solvent you use. Since the residues are so
> difficult to dissolve, they are probably rather harmless for most
> applications (but I have no proof for that statement). However, if you
will
> conformally coat the assemblies, they may cause poor adhesion of the
> coating. What puzzle me is that you say it was manifested a few days
after
> aqueous wash. It should have been there immediately after cleaning.
>
> Per-Erik Tegehall
> IVF
> Sweden
>
> > ----------
> >
> > Has anyone ever come across white residues that looks like snow cap
> > covering the solder joints? The phenomenon only manifested  a few days
> > after Aqueous wash and EDX captured high lead and oxygen. The white
> > residues are so stubborn that they could not be dissolved in DI water
and
> > IPA. We use a water soluble paste and clean our assembly twice
> >
> > All inputs are welcome and appreciated.
> >

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