Work-In-Progress (WIP) management controls the number of wafer lots actively moving through the fab at any time. The goal is to balance throughput against cycle time—too much WIP creates long queues and extended cycle times, while too little WIP starves tools and wastes capacity.

Finding the Sweet Spot

The ideal WIP level keeps bottleneck tools busy without creating excessive queuing everywhere else. This balance is critical because inventory sitting in queues costs money and extends delivery times.

Key Metrics

• WIP turns: Throughput divided by average WIP (higher is better) • X-factor: Actual cycle time divided by raw process time (ideal = 1.0, typical fab = 2-4) • WIP-by-step report: Shows where lots accumulate at each process step

Management Approaches

CONWIP (Constant WIP) releases new lots only when completed lots exit the fab. Push systems release on schedule regardless of downstream status. Pull systems release based on actual downstream capacity. Many fabs also set WIP caps per process area to prevent congestion, and operations teams conduct daily WIP reviews to adjust priorities based on current conditions.