[sdiy] black flaky chip legs: what does this indicate?

[Steve Lenham]

I work extensively on Lexicon effects from the late '70s/early '80s 
(which are built form a LOT of 74-series logic) and see this all the time.

As others have observed, it is associated with TI parts from a 
particular era, though I have actually observed the same thing on some 
Z80 CPUs of similar vintage. It seems to happen purely from exposure to 
the atmosphere and does not require battery leakage or anything so 
dramatic (though Lord knows early Lexicons suffer from that too). ICs 
from other manufacturers of the same age and in the same board will be 
completely unaffected.

I'd be very interested to know exactly what the TI ICs are made out 
of/the chemistry of why this happens, not least because I dream of a 
quick chemical cleaning process for those that are not too far gone.

At a guess, the blackening looks rather like what you get on silver. It 
is easy to remove with a fibreglass pencil and this temporarily leaves a 
very shiny finish. However, on closer inspection the surface plating 
often seems to be pockmarked with corrosion and/or peeling off the pin. 
Unfortunately, once exposed, the underlying substrate seems to suffer 
from rust-like corrosion which weakens the pin. Eventually they snap off 
at their thinnest part.

Cheers,

Steve L.
Benden Sound Technology



On 02/11/2020 03:37, Doug Terrebonne via Synth-diy wrote:
> Yep see those blackened socketed TIs all the time especially in OB-X/Xas.
> 
> Doug
> synthparts.com
> 
> On Sunday, November 1, 2020, 3:03:36 PM PST, KA4HJH <ka4hjh at gmail.com> 
> wrote:
> 
> 
> 
>> On Nov 1, 2020, at 5:26 PM, Kenny Balys <kenny at beatkamp.com 
>> <mailto:kenny at beatkamp.com>> wrote:
>>
>> Yes!!!!!
>>
>> They were both TI and the device was manufactured in 1979.
>>
>> Both IC's were socketed. I had to pull disconnected legs out of the
>> sockets after removing the chips. They shed their legs.
> 
> I saw this happen once with a Bally pinball machine from the late '70s, 
> the first generation electronic ones. The logic board was damaged by a 
> leaking NiCd battery (the most common mode of failure) and when I tried 
> to pull the ROMs out of the sockets the legs just snapped off. I don't 
> recall the manufacturer.
> 
> I don't know if the battery had anything to do with it. It's mounted at 
> the bottom of the board and the firmware is up at the top. I've seen 
> these boards with much worse corrosion and the ROMs/EPROMs were OK.
> 
> 
> Terry Bowman, KA4HJH
> "The Mac Doctor"