Is it safe to piggy back chips?

Smart Bomb

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If i have an assumed bad chip on a pcb, which is not socketed, is it safe to take a new identical chip and "piggy back" it over the soldered chip?...Just slip it on top to where the legs of new chip are touching the exisitng chips legs.

Is this safe to do? Will it ruin the new chip or anything on the pcb?

I have two soldered chips on a pcb and im.thinking i may be able to see if my assumtions are correct if i attempt this method of a quick test.
 
it depends. if the chip you are testing is dragging down a pin then the bad chip will also drag down the good chip. ram and rom chips and ttl logic chips have a better probabilty of showing differences. I personaly would never do it to any thing related to power regulation or amps
 
Piggybacking is hit/miss for RAM.
I'd only do this with RAM.
TTL failure is too random to get a valid result.

Ideally, OP should have an HP 10529a comparator (or similar) to do this type of testing on TTL ICs etc.

https://forums.arcade-museum.com/showthread.php?t=274541&highlight=comparator

QhCzx.jpg
 
i guess if you're thinking you have a stuck output on a chip you could bend the output leg in question away from the chip and then piggyback it to see if replacement behaves as expected by monitoring that output

if that makes any sense
 
I'd like to go on record and give a direct impactful answer to this question.

✔ 1 accepted answer: ALWAYS a resounding NO!

Don't stack/piggyback your IC's period. Ever. Don't be lazy. Don't be that guy. do the work!
 
Last edited:
✔ 1 accepted answer: ALWAYS a resounding NO!
Like most "accepted" answers online, this one is wrong too.

Piggybacking chips when troubleshooting boards been been done for MANY years, and is a perfectly fine way to troubleshoot, as long as you understand that it's not guaranteed to pinpoint your problem, and in some cases could damage the donor chip that you're piggybacking (if the original chip is bad with a stuck output, and the new chip can't handle the current from the conflicting output).

Think about the cases... in the case that you suspect a bad chip, but it's not actually bad, the chip that you piggyback and the original one will output the exact same data, and nothing will happen. In the case that you piggyback a good chip over a bad one with a weak/open output, it'll fix the problem. In the case that you piggyback a good chip over one with a shorted output, they will conflict, and may meet in the middle, or one might overdrive the other. In some cases this will look like nothing happened, or it might drive it enough to make it marginal or even look fixed. Even subtle clues like that are very welcome when troubleshooting boards.

A logic comparator is nice as well, since it does actually tell you that the logic differs... though they're not perfect either. Plus, piggybacking is possible on chips where the logic comparator doesn't work (analog chips, RAM with multiple voltages, etc).

So, like most things... it's one tool in your toolbox. As long as you understand what it can/can't do, and you know the risks... feel free to use it when appropriate.

Obviously when you do determine what the problem chip is, properly remove and replace the bad chip... don't just solder the new one over the top of the bad one.

DogP
 
I agree with DogP. My experiences have been that it works in about 1 out of 4 bad chips.
If you piggyback it upside down, you will fry the new chip, and likely burn your fingers removing it.
 
I'd like to go on record and give a direct impactful answer to this question.

✔ 1 accepted answer: ALWAYS a resounding NO!

Don't stack/piggyback your IC's period. Ever. Don't be lazy. Don't be that guy. do the work!

I was able to track down my RAM issue in Warlords by piggybacking the RAM. Once I determined the bad RAM I, of course, replaced it though.
 
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