[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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