A comparison of production planning formulations with exogenous cycle time estimates using a large-scale wafer fab model

A key parameter of the linear programming (LP) models widely used for production planning in industry and academia are the lead times, the estimated delay between material becoming available to a resource and the completion of its processing at that resource. Lead times are commonly treated as exogenous, workload-independent parameters and assumed to be integer multiples of the planning period. Although formulations with non-integer lead times have been proposed in the past, we are not aware of any studies that systematically evaluate the benefits of using non-integer lead times in LP models. In this paper we implement LP models to plan the releases of wafers into a large-scale wafer fabrication facility and compare the performance of LP models with and without non-integer lead times by simulating the execution of the resulting release plans. We find that the models with non-integer lead times yield substantially improved performance, and are quite straightforward to implement.