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FCBGA: Why does heating the inner solder of a chip 'revives' it?
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jotwerde:
So, I've watched this video from Louis Rossmann:




If I got it right, then what he says is that reballing a dead flip-chip GPU doesn't repair it and that you have to replace the chip to repair it properly, as the problem is that the GPU has died and not the solder balls cracked.
He also mentions that, while reballing a GPU the inside of the chip also gets heated, reflowing the solder INSIDE the chip and that this is why the chip will work for a while afterwards, not the reballing.

I guess the solder inside the chip he talks about are the solder bumps in the following pics:

--- End quote ---


--- End quote ---


But why does reflowing the solder-bumps inside a dead GPU (essentially 'inner'-reflowing of the chip, if I get this right) revive it? I mean, the gpu is dead, why would reflowing the bumps fix it?

Thanks.
Ian.M:
Its probably not 'dead' dead unless the ball that failed was supplying a critical power rail with tight sequencing requirements.

Thermal expansion and contraction of the die relative to the carrier causes the fault by fatiguing the inner balls till they crack, then expansion and contraction of the underfill 'mushes' the metal at the crack surfaces until they are no longer in contact.  If you heat the chip to inner ball reflow temperature, surface tension evens out the surfaces either side of the crack, probably doming them towards the center of the pad.  However with no opportunity to add fresh flux, the solder either side of the crack doesn't reunite due to surface oxidisation and possible contamination from the underfill outgassing.  As the undefill cools it forces the solder surfaces together and the contact pressure resulting from the difference between the reflow temperature and the max die temperature keeps it working until the solder mushes too much at the point of contact by underfill expansion/contraction due to repeated thermal cycling, especially switching off.
jotwerde:

--- Quote from: Ian.M on August 29, 2018, 03:05:29 pm ---the ball that failed was supplying a critical power rail with tight sequencing requirements.

--- End quote ---

I don't really understand what you mean here, could you explain this sentence a little more? Thanks.
If I got the rest of your answer right, then what I misunderstood was that with 'dead GPU' he doesn't mean that the GPU has completely failed, but that there's no way to fix the inner solder cracks, right?
ArthurDent:
What Ian.M says makes sense to me but his saying: “Its probably not 'dead' dead….” reminds me of this Monty Python skit.



I admit that I know little or nothing about this problem but it seems to me if there are also solder bonds inside the chip, the alloy must be such that the melting point is much higher than what would be used to solder the chip to the board, otherwise any time you soldered a chip on a board, you’d stand a chance, however small, of messing up the connections on the innards of the chip. If the problem is between chip and board could it be exacerbated by ROHS?

If the chip/board bond is broken then I would think a chip removal, cleaning, new flux, and new solder balls would be needed otherwise any apparent repair from just reheating might not last long. If the problem is internal then a new chip is probably called for.
Gyro:
Just to save some time looking for previous references / discussions ...

https://www.eevblog.com/forum/repair/reball-repair-(the-rossman-video-and-such)/msg1621612/#msg1621612

https://www.eevblog.com/forum/repair/reballing-is-not-repair/msg852540/#msg852540

https://www.eevblog.com/forum/repair/my-experiences-reflowing-videocards-(so-far-100-success-rate)/msg872296/#msg872296

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