What does this equation mean?

The second ceiling is statistical and softer, but it bites earlier. Defects arrive on a wafer at some density, and the probability that a given die contains none of them falls as its area grows. Feng and Ma, in the cost model they published for multi-chiplet architectures, use the negative binomial form $Y(A) = \left(1 + \frac{D_0 A}{c}\right)^{-c}$. with defect density $D_0$ , die area A , and a clustering parameter c that captures the fact that defects are not independently scattered [ 2 ] . The exact parameters matter less than the shape. Yield falls monotonically with area, so cost per good die rises faster than area, and past some size the marginal square millimetre costs more than the function printed on it is worth. Feng and Ma note that the reason this became urgent is that two trends met: process improvement slowed while chip area approached the reticle limit, so the historical route to more transistors per part closed from both ends.