Corrosion on DIP pins

[Alex Eisenhut]

So I retrieved my PET 4032 from my parent's house. The 4032 is a 40 year old computer. It was in the basement for 30ish years. There was no flooding.
How do some chips have rusty pins but the next one over doesn't?

Those are RAM chips, but various 74LS also have the same rusty pins while the next one doesn't.
And the rusty/non-rusty RAMs are within one bank of 8 chips, so the difference can't be that one was wave soldered and the other by hand.

That rust means the end of that chip as the corrosion will crack open the two halves of the body and let humidity in the guts, right?

[inse]

Not sure whether it is rust or flux residue.
Why not brush the entire PCB with a stiff paintbrush and PCB cleaner?
If you are afraid of corrosion, you could apply conformal coating after completely overhauling the PCB.

[Haenk]

I would rather guess these have been reworked/replaced (maybe even in factory) with a different type of solder and flux has not been cleaned, most types of flux will corrode things over time.
You might want to check the caps as well, they don't seem to have leaked, but 50 years are 50 years - therefor they might have dried out.

[tooki]

I wouldn't be surprised if the caps had leaked, and the electrolyte is the reason for the rust.

[strawberry]

wont split open but rust will get to bondwires if it continues
even solder is tarnished 

I wouldn't be surprised if the caps had leaked, and the electrolyte is the reason for the rust.

capacitors are not as corroded

[edavid]

The insertion process can scrape the tin off the edges of the DIP pins, leaving the Kovar exposed and vulnerable to rust.

However, plastic DIP packages aren't hermetic, so there was already "humidity in the guts".  The ICs should keep working until the pins completely rust through.

[tooki]

wont split open but rust will get to bondwires if it continues
even solder is tarnished 

I wouldn't be surprised if the caps had leaked, and the electrolyte is the reason for the rust.

capacitors are not as corroded

Not surprising, since the capacitor’s leads have to be made of a material compatible with its own electrolyte!

[strawberry]

Not surprising, since the capacitor’s leads have to be made of a material compatible with its own electrolyte!

capacitor leads are tin plated iron (rarely copper)
tin solder tarnish at super high humidity level

[rsjsouza]

Assuming there was no ingress of any kind, as mentioned before the manufacturing of the computer might have either scratches some parts or left residue that slowly corroded over the decades. I have seen many boards like that.

One additional point is that there were crap and good leadframes as well especially from that era (1970s-1980s), even with parts from the same manufacturer but different lots or country of origin. Standards for handing and storage of ICs were still being developed.

Just an additional annoyance: MOSTEK is known nowadays to have a relatively high incidence of memory rot, which renders the IC unusable due to internal degradation even on ICs that are physically in good shape.

Good luck in your repair/restoration!

[edavid]

Just an additional annoyance: MOSTEK is known nowadays to have a relatively high incidence of memory rot, which renders the IC unusable due to internal degradation even on ICs that are physically in good shape.

That's a problem with mask ROMs like the MK36000.  Aren't the ICs in the photo DRAMs?

[Alex Eisenhut]

Turns out my Dad was over-watering plants in the general area and it dripped through the floor... yup... It's rust.

I took the machine apart and there is a dried puddle around where the CRT sits on the enclosure. And it makes sense, the corrosion is pretty much where you'd expect drops to fall through the case.

Lesson: don't leave semi-precious old computers in someone else's basement for 30 years, I guess? 

[rsjsouza]

Just an additional annoyance: MOSTEK is known nowadays to have a relatively high incidence of memory rot, which renders the IC unusable due to internal degradation even on ICs that are physically in good shape.

That's a problem with mask ROMs like the MK36000.  Aren't the ICs in the photo DRAMs?

I have seen DRAMs fail as well - you can also check a few of the vintage folks (Adrian's basement, 8-bit guy, etc.) comment this as well.

[Alex Eisenhut]

I have seen DRAMs fail as well - you can also check a few of the vintage folks (Adrian's basement, 8-bit guy, etc.) comment this as well.

Micron Tech 4164s in the 64 are almost certain to fail. I don't know much about MOSTEK DRAMs.

[tooki]

Turns out my Dad was over-watering plants in the general area and it dripped through the floor... yup... It's rust.

Hahahah my sister did the same thing to her Time Capsule (the little router/NAS Apple used to sell): it was on a shelf below some plants. The Time Capsule only has ventilation slots on the bottom, so water very slowly crept up, until eventually shorting the power supply.

Luckily replacement power supplies are readily available online (all appear to be salvaged) at low cost, so I just swapped it out.