Hi David.

This is what is called "no clean" flux I believe.

I've seen lots of pcbs assembled in China when nobody
bothered to wash them up at the end and all over covered
with residues of white conductive stuff.


WBR Dmitry.



David Van Horn wrote:
> 
> I have a problem with a system that measures current using an AVR's ADC.
> It's doing a differential high side measurement of current into a
> battery.
> 
> The initial symptom was that a high percentage (20%-ish?) had very low
> charge current. This was verified, and after looking over the system
> very carefully, I came to the realization that the ADC was reporting
> roughly twice (not exactly twice) the current that was actually flowing.
> The only anomaly I could see in the circuit, was that the in-circuit
> values of two of the input resistors were about 20% lower than those on
> a known good board. Out-of-circuit measurements show the resistors are
> good, well within their 1% tolerance.
> 
> After looking at it a while, it appears that the problem is ionic
> contamination of the board.
> 
> When operating, heating the input resistors even slightly, causes the
> current measurement to go completely insane, and the board may drop to 0
> output current, or up to 3X normal, depending on how you heat.
> 
> Hitting the board with degreaser while operating, produces similar
> results.
> 
> Measuring the affected resistors, and the reference, show no significant
> change over a fairly wide temperature range.
> 
> A thoroughly cleaned board performs normally.
> 
> Many of these have been built, over about 2-3 years, without ever seeing
> this problem, or anything remotely like it.
> 
> Visually, under 10X mag, I can seem some faint whitish "stuff", and some
> particulate gunk, and the solder joints are grey, not silver.
> 
> Further, I know that since the last run, there has been a major shakeup
> at the factory, and the fellow who was in charge of process control is
> gone. I don't have a ton of confidence in his replacement, but up to
> this point, I hadn't seen anything directly wrong with him either.
> 
> So: How can I <PROVE> that these are, or are not, contaminated?
> This will be a big issue, the factory is not going to want to hear this.
> 
> 
> 
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