[volt-nuts] volt-nuts Digest, Vol 7, Issue 9

[Bruce Griffiths]

Depends on the curing chemistry.
Some epoxies release amines during the curing process.
Amines attack copper.

Bruce

Dick Moore wrote:
> Alan, if it's possible to just leave the hole in the board, that may be the best solution overall. BTW, Nichicon has always had a very good reputation for quality electrolytics, as have the Japanese suppliers in general. The 2001 is certainly worth the effort to repair. I had a similar problem with a PCB in a Fluke 732A DC Reference. An epoxy coated Ta cap burned up and took a lot of the board with it and then a resistor burned as a result and took more board. I scraped and dug away the carbon and left the board as is -- of course this was not a multi-layer board. Most instruments don't need the sealer protection once in a lab in relatively dry, warm air where they'll be used for a long time. I wouldn't be sure that using the automotive epoxy would be a good idea...
>
> Dick Moore
>
>
> On Mar 17, 2010, at 5:00 AM, volt-nuts-request at febo.com wrote:
>
>    
>> Message: 6
>> Date: Wed, 17 Mar 2010 11:20:37 -0000
>> From: "Alan Scrimgeour"<scrimgap at blueyonder.co.uk>
>> Subject: Re: [volt-nuts] Keithley 2001 Multimeter Fault - Update
>> To:<jfor at quik.com>,	"Discussion of precise voltage measurement"
>> 	<volt-nuts at febo.com>
>> Message-ID:<AE27F40615CE4CA08494C3AD153F4414 at AlanPC>
>> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>> 	reply-type=original
>>
>> 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
>>      
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