What does this equation mean?

The arithmetic behind that claim is a plain probability composition, and it is worth writing down because it is the one piece of real math in advanced packaging that most design reviews skip past. If each of n chiplets bonded into one package has an independent per-die yield $y_i$ , the assembly’s final yield is approximately $Y_{\text{package}} \approx \prod_{i=1}^{n} y_i$. which is a first-order model — it assumes bonding introduces no additional defects and that die yields are independent, both of which are simplifications addressed separately below. Even under this optimistic assumption, four chiplets each yielding 95% compose to roughly 81% assembly yield, and a twenty-die stack at the same per-die yield falls to roughly 35% — the industry’s own framing of why an unscreened stack becomes uneconomical fast as die count rises [ 4 ] . The entire purpose of KGD testing is to push each $y_i$ as close to 1 as pre-bond screening allows, since the model shows the product is unforgiving of even small per-die shortfalls once several dies are chained together.