Bonjour Sylvain
Interesting case. This needs more than a visual inspection to be sure
what happened, but my guess is that the area may have been bathed in a
water-soluble flux for hand-working/retouching and washing was
forgotten. What is interesting is that you may have tracking between the
2 ceramic capacitors to the south-west of the main area which could have
sublimed conductive nasties elsewhere, causing a kind of chain reaction.
Is there a relatively high voltage between these 2 components? (Or it
nay have been vice versa.)
Also of interest are the white spots scattered more or less everywhere,
especially on the left, well away from the damaged area. This should not
be, but I cannot be sure what the cause is just from a photo.
You say function generator. Does it produce very large voltage changes
in the ps or ns range? The dV/dt gradient can sometimes play havoc with
contaminants and even with FR-4 laminates.
FYI copper oxide is never green. Copper salts, especially chloride, may
be. If, indeed, it is copper chloride, then something is really wrong.
OTOH, your solder mask is green, so we cannot be categorical about it.
If he is still with CERN, Mr A. Gandi should be the best person to help
you identify the residues or, at least, guide you in the right
direction. If you see him, please pass on my best regards.
Brian
Sylvain Kaufmann wrote:
> Hello technetters,
> Sorry for the subject but that's what came to my mind when I faced this case :) I never saw this previously, even when I burnt 2kW/chan amps in rock shows the result was not so catastrophic!
>
> I would be very glad to have your thoughts about some pictures I sent to Steve.
> http://stevezeva.homestead.com/files/G_n__fonct_pignard_03_crop.JPG
>
> http://stevezeva.homestead.com/files/G_n__fonct_pignard_closeup_16.jpg
>
> Additional pictures has been sent to Steve this morning (relatively to my location).
>
> Here is the case:
> One customer from us came up with one function generator with its case open; he opened it because it stopped working and had a bursty odor (smell?) and he wanted to see what was this odor and if he could change a fuse and let it work again. When he saw what was on the board, he came up to me to take pictures and have little explaination of what went wrong on this board. The customer bought this generator one year and a half ago, so it's still under guarantee, but he wanted to protect himself against a manufacturer's complain that could have said: "you did a short on the output, this repair doesn't fall into guarantee scope, so you pay for it" (now that he opened the case, maybe the guarantee is allready gone, but whatever). This generator has not been used a lot, understand it's not a generator that is installed in an automatic test setup that runs 24h a day.
>
> The facts:
> - Presence of flux residues on both sides
> - Presence of several different copper oxydes (I don't know their english correct names, but the green ones, the green gelatin like ones, the red ones)
> - PCB's temperature increased up to at least 300°C or so
> - Generator was not working in a harsh environment; it was lying on a table in engineer lab, so, except if there was some fog from the open window near to the table, this generator never saw external humidity :)
>
> My guess:
> - The manufacturer/assembler did a rework/touchup on a QFP on the top side of the PCBA; to perform it, the operator used a very active flux that he spread around the component he wanted to rework; some of flux went on the other side through the PCB vias.
> - Then the generator worked properly for about one year and a half, but during this time, some aggressive compound eat gently the copper barrel in several vias, leading to a increase of the barrel electric resistance, up to the time the resistance was so high that the dissipated power elevated the temperature to about 300°C.
> - Or this could be the result of electromigration and a hard short-circuit that happened on the board.
> - This part of the design has a control loop and power was first provided by control electronics up to the time this power could not be dissipated anymore and the PCB broke and so the generator stopped working
>
> My concern:
> - As I see this happening without other factors than ones related to assembly (no humidity, no high voltage, no thermal shocks, no vibrations), I wonder what are the circumstances that lead to such destruction; is it only the flux chemistry? If I see some green gelatine like copper oxydes in some vias on one of our board, should I consider that this extreme end could happen to my product? Is it one isolated case (PCB destruction with risk of fire) or is it how it will allways end? Etc etc
>
> Thank you for your inputs and have a nice Friday
> sly
>
> S. Kaufmann
>
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> Sylvain Kaufmann
> CERN / European organisation for nuclear research
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