process window index

**Process Window Index (PWI)** is a **quantitative metric that measures how centered the current operating point is within the process window** — expressed as a percentage where 0% is at the center (maximum margin) and 100% is at the edge (on specification limits). **How PWI Is Calculated** - **Per Response**: $PWI_i = |(y_i - target_i) / (USL_i - target_i)| imes 100\%$ (for upper half). - **Overall PWI**: $PWI = max_i(PWI_i)$ — the worst-case among all responses. - **Interpretation**: PWI < 50% = well centered. PWI < 100% = within spec. PWI > 100% = out of spec. - **Composite**: Composite PWI combines all responses into a single operating position metric. **Why It Matters** - **Process Centering**: PWI immediately shows if the process is centered or drifting toward spec limits. - **Monitoring**: Track PWI over time to detect drift before reaching specification limits. - **Comparison**: Compare PWI across tools or chambers to identify which needs attention first. **PWI** is **the speedometer for process centering** — a single number showing how far the operating point is from the safe center of the process window.