A simulation study on periodical priority dispatching of WIP for product-mix fabrication

This paper proposes a new dynamic priority dispatching algorithm for the product-mix of work-in-progress (WIP) at a processing station with multiple machines. The algorithm is used to evaluate dispatching priority when each quantum starts. The priority is defined considering both the number of WIP and the decreasing rate of WIP in the incoming buffer. When the priority is high, a single sort of WIP is assigned to a machine for a given period, or quantum. This algorithm, named Pseudo Periodical Priority Dispatching (P3D), P3D was examined through Monte Carlo simulation. In a product-mix, a machine must be adjusted when the WIP is changed. The adjustment frequency and the degradation of the machine utilization due to the adjustment are discussed. The algorithm is able to achieve fair dispatching both at a bottlenecked processing step and a non-bottlenecked processing step.

1. Introduction

Currently, short time-to-market and cost reduction is strongly in demand for semiconductor manufacturing facilities. Both cycle time reduction and high machine utilization, which are conflicting requirements, must be achieved to meet those demands. In a product-mix, a machine is not used for only one kind of work-in-progress (WIP), and the machine must be adjusted when the WIP is changed. This adjustment, or setup, is an additional job for a manufacturing system, and the machine cannot be used for processing during the adjustment. In addition, the required cycle times of all types of WIP have to be maintained. Our most recent previously published work [1], [2] was on resource estimation and machine adjustment for product-mix. A new buffer management policy was proposed there. The purpose of this paper is to propose a new dynamic-priority WIP dispatching method based on that policy.