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FCBGA: Why does heating the inner solder of a chip 'revives' it?

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David Hess:
This problem crops up occasionally with hybrids and lead frame die attachments for ICs where solder reflowing may actually fix it but flip-chip packaging is more fragile for the reasons Ian.M gives.

mikerj:

--- Quote from: jotwerde on August 29, 2018, 04:08:37 pm ---
--- 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?

--- End quote ---

Ian is saying the die itself is likely not damaged unless the broken solder ball caused damaging currents to flow.  GPUs tend to have tight power sequencing requirements for their various supply rails, so if one of these became disconnected due to a broken solder bump between the die and carrier, then it's possible that damaging internal currents could flow through parasitic diodes etc.  If the broken solder bump was carrying a logic level signal then chances are the die is still fine.

Ian.M:
Yes.  Another permanent death scenario is loss of a control line causing bus contention resulting in I/O driver burnout.   *IF* one had the bare die, the resulting failure would often be quite obvious under an electron microscope with an irregular crater where there should be well defined quasi-rectangular metallisation and SiO structures.

Hmmm ...... I wonder if there is a market opportunity for an enterprising company with IC packaging and failure analysis experiance, to buy up unobtanium as N.O.S. high value flip-chip on carrier BGAs as cheap failed parts, separate the die from the carrier, auto-inspect for die damage, then re-bump and reflow and bond onto a new carrier, ball that, and test on a 'franken-card' that has had its key BGA replaced with a test socket, so the result can be resold with a 1 year parts only warranty?

jotwerde:

--- Quote from: mikerj on August 29, 2018, 08:22:35 pm ---Ian is saying the die itself is likely not damaged unless the broken solder ball caused damaging currents to flow.  GPUs tend to have tight power sequencing requirements for their various supply rails, so if one of these became disconnected due to a broken solder bump between the die and carrier, then it's possible that damaging internal currents could flow through parasitic diodes etc.  If the broken solder bump was carrying a logic level signal then chances are the die is still fine.

--- End quote ---

How much of a difference is there usually between logic level signal currents and power currents?

janoc:

--- Quote from: jotwerde on September 08, 2018, 07:37:15 am ---How much of a difference is there usually between logic level signal currents and power currents?

--- End quote ---

Like several orders of magnitude?

Logic inputs have high impedance, so you get at best short time current spikes while the input capacitances charge and discharge, but the average currents are low, less than 100mA for sure.

The power lines carry the power to the chip - if the card needs e.g. 100W of power, at 12V -> 100W/12V = 8.3A Of course, that is not going to be through a single line and not all that power is going to the GPU (some is also consumed by the RAM, voltage regulators, fans, etc.) but still. Gives you an idea of the magnitude difference.

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