What does this equation mean?

This is the direct rationale, stated by imec’s own R&D leadership, for preferring die-to-wafer hybrid bonding over wafer-to-wafer bonding for heterogeneous stacks: die-to-wafer bonding permits stacking known-good dies of unequal sizes, achieving a higher compound yield than committing an entire wafer’s worth of die positions to a single bonding pass [ 2 ] . The underlying arithmetic is straightforward and worth making explicit, because it is the actual reason “known-good die” testing exists as a discipline: if a stack of n dies each independently has yield y , and defects are uncorrelated across dies, the compound yield of the finished stack before any pre-screening is approximately $y^n$ — falling multiplicatively with every additional die stacked. Testing and discarding bad dies before they are committed to an irreversible bond recovers the yield loss that would otherwise compound across the stack, at the cost of the test infrastructure itself: burn-in racks, edge probing at package-relevant pitch, and — increasingly, as hybrid-bonding pitch falls below what a needle probe can reliably contact — specialized fine-pitch test structures built into the die specifically to make pre-bond testing possible at all.