Bonjour Brian,
Thank you for your input that is, as usual, very valuable and relevant; here are the answers I can provide now:
- I cannot say anything about the faulty electronic block and its functions, so I cannot answer about the presence of high voltage on capacitors (interresting point, the sublimation of metal, never heard of it before; is it related to electromigration?)
- The generator is not a top speed one - 300MHz; as disscussed with the engineer, this generator doesn't have big dV/dt or I would say it's not the fastest you could find
- Unfortunately, we could not do any test (well, we didn't want to scrape a surface or even touch it) because of the warranty, we just wanted to see and build an idea about the failure, so we will never know the exact nature of these copper compound (if they are)
- Mr Gandi is not working anymore at CERN, but I discussed with his successor about some of these compounds on a previous case (he is the responsible of our internal PCB workshop); we are not chemists and we certainly call every sulfides, salts or oxides as oxides and that's absolutely wrong, sorry for all of you specialists in this field. When I showed him the "green gelatin like oxide" he told me it was one of the numerous copper resulting compound called (in french, sorry for others) "oxyde pulvérulent" which is the proof that there was water+copper+another chemical agent and the result is something that could be interpreted as a soldermask rework compound (same colour and this polymer-like shape); I have sent some pictures to Steve showing this compound coming out of vias.
- Did you allready face cases where the green colour of such chemical was related to soldermask and not any copper related? I find this statement very logical now that you talk about it, but I have to admit that the idea that soldermask could have been dissolved by the agressive chemical did not pass through my head because I had the idea the soldermask is not removable from a PCB; wrong, it seems...
Anyway, we will not be able to have a final answer to this case because we don't have a hand on how it works and how it was manufactured, we only have these pictures to state on the case. But still, thank you for your help
sylvain
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: vendredi, 9. novembre 2007 11:56
To: [log in to unmask]
Subject: Re: [TN] Flux residues - great balls of fire
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
>
> ************************************************************
> Sylvain Kaufmann
> CERN / European organisation for nuclear research Site de Meyrin
> 1211 Geneva 23
> Switzerland
> Dpt: TS-DEM-WS (int mail J06600)
> Phone: +41 22 767 37 02
> Fax: +41 22 766 87 77
> E-mail: [log in to unmask]
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> http://cern.ch/dem
> ************************************************************
>
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