[volt-nuts] Keithley 2001 Multimeter Fault - Update
Alan Scrimgeour
scrimgap at blueyonder.co.uk
Wed Mar 17 11:20:37 UTC 2010
Perhaps only the copper was 'etched', but I can imagine even epoxy being
attacked by hot electrolysis taking place right next to it. Electrolysis
could produce some very active compounds, depending on the electrolyte (and,
I'm not sure, but in such close proximity to the electrodes there may also
be some extremely reactive short lived species, or is that just Sci-Fi?).
But by some mechanism the upper layer of glass fiber in the board was
visible before I started digging it away.
The excavation is progressing with care. In the centre the damage has gone
right through the board to virtually the other side. Presumably air cooling
stopped or slowed the progress of the damage actually through the very last
layers of the board. It appears that once the hot electrolysis had begun to
damage the pcb it carbonised and in that conductive state, drew current and
generated yet more heat leading to a chain reaction in the form of a growing
carbonised region.
I've just had to cut a wide buried copper track in order to be able to
remove the carbonised pcb beneath it, which is disconcerting, but it will
just need soldering, or replacing with a piece of wire. I'm more worried
about what to use as a 'filling' in this cavity. I said I'd use epoxy resin,
but the usual stuff is damaged by soldering temperatures. I have some 'Auto
Weld' which says it's resistant to a constant 300C and should do. Once I
fill that hole back I'll never get it out again, so I'd better fix it
properly!
I'd like to add that I'm feeling pretty angry about those electrolytic
capacitors. They are sheer vandalism! Time for some companies heads to bow
down and appologise! Those unstable low dropout regulators are another
annoying self destruct mechanism too!!!
Alan
----- Original Message -----
From: "J. Forster" <jfor at quik.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Wednesday, March 17, 2010 3:12 AM
Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault - Update
>> I've had a closer look at the 'corrosion' damage from leaked capacitor
>> electrolyte using a stereo microscope. Some very fine tracks have been
>> completely eaten away in one area, but the path of the track is easy to
>> follow in the green solder mask layer making it easy to e bypass with a
>> wire.
>>
>> Looking in more detail at the 'Heat Damaged Area' I can see that the
>> chemical attack from the capacitor leakage is far more advanced. In
>> addition
>> to copper track, it's eaten right through the solder mask and deeper down
>> into the PCB resin leaving bare woven glass fibers visible.
>
> I find it hard to believe anything has eaten the epoxy. It's pretty inert
> to almost any chemicals.
>
>> There are vias in this area and I'm concerned there may be internal layer
>> connections that are damaged.
>>
> -John
>
> =============
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