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Those were binned cards, though.

Binned for compute failures, not memory failures.

That's why this works. The yield on those HBM stacks is excellent. The binned cards had failures in the logic die, and hyperscalers want uniformity in their clusters. That's why the mining market took the downbinned cards.

Do you not test the logic die before mounting the memory on top?

back then, no

Some testing can be done with a wafer probe, but other tests require stressing the die -- high current, heat output -- so it has to be in-package to do those tests.

Mounting the HBM on the interposer has to happen before putting the interposer in the package.

So it's the "must be in-package" yield tests that drive this.

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