Graham Sutherland / Polynomial
@greg for the same reason, capacitor manufacturers tend to do failure analysis by physically cutting the part out of the board, rather than desoldering it. the heat would alter the lattice and mess up any measurements or crystallographic analysis they want to do.
it's also why you shouldn't run calibration on test equipment immediately after doing hot air rework. best to wait a few days for the initial rapid capacitance decline to settle down.
Graham Sutherland / Polynomial
@greg this is also why it only ends up working temporarily in some consoles. if other components (particularly electrolytic caps) are showing their age, they start to have higher impedance, which causes more noise on the power rails and more undershoot during load transients, which can cause the CPU/GPU to glitch out. when you do one of the hot air tricks you de-age the MLCCs, which is just about enough to compensate for the other parts aging, but then that early rapid aging stage kicks back in.
Graham Sutherland / Polynomial
@greg so by heating up the MLCCs, you end up bringing the lattice up to the Curie temperature, which reforms the tetragonal lattice, recreating all the spontaneously polarised electric domains.
and that's why sticking a towel around your xbox works. it's also why the "reflow the BGA" trick also seems to work. nothing to do with BGA balls, it's just the hot air gun resetting the MLCC aging.
(note: temperature is weird at the atomic scale; you don't actually need the ambient to get up to 120°C)
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