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'[OT] or maybe [EE]? Ionic contamination on PCBs'
2005\08\16@144105 by David Van Horn

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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.




2005\08\16@145728 by Roberts II, Charles K.

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There is a test that you can have done by an analytical lab to give you
a number for ionic contamination. They wash the board with DI water and
do some chemistry magic to give you a percent of contamination. There is
an IPC spec for what is and is not acceptable.

There is a lab in Alabama that I know of that does this sort of stuff.

http://www.solderingtech.com/

http://www.solderingtech.com/engineering/lab.shtml

Here is there lab page, look under general testing capabilities.

Good luck.





Charles K Roberts II



{Original Message removed}

2005\08\16@150703 by Wouter van Ooijen

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> and the solder joints are grey, not silver.  

Maybe not important, but did the factory switch to lead-free?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu



2005\08\16@162659 by Spehro Pefhany

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At 01:42 PM 8/16/2005 -0500, you wrote:

>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?

Measure leakage between two unconnected points on a board from the factory.
Compare with a similar unpopulated cleaned board that they have not
touched.

>This will be a big issue, the factory is not going to want to hear this.

Might be a change to a water-soluble flux. That's burnt a lot of people.
Some no-clean is bad too.

The white stuff sounds a bit like undissolved activated flux particles,
though.

good luck with it.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\08\17@152017 by Peter

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On Tue, 16 Aug 2005, David Van Horn wrote:

If it is soluble contamination, wash de board in distilled water, then
put it overnight in a beaker of distilled water. In the morning you can
measure conductivity and maybe even detect what the contaminant is
(f.ex. indicator reaction). Use as little water as possible and wash the
beaker with the water first.

Peter

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