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

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.

back then, no