Dispatching is the decision logic that determines which waiting lot a tool should process next. Good dispatching rules maximize throughput and on-time delivery while respecting critical queue-time limits.

Common Dispatching Rules

FIFO (First In, First Out) processes lots in arrival order—simple and fair, but it ignores priorities. Priority-based dispatching processes hot lots and engineering lots first regardless of arrival time. Critical Ratio calculates priority as (time remaining until due date) / (remaining process time)—when the ratio drops below 1.0, the lot is behind schedule. Shortest Queue Next sends lots to the tool group with the shortest queue.

Advanced Dispatching

Modern systems use Q-time aware dispatching that automatically escalates lots approaching queue-time limits to prevent scrap. Setup minimization groups lots requiring the same recipe to reduce tool changeover time. Bottleneck starvation avoidance prioritizes lots heading to bottleneck tools so the constraint never sits idle.

Implementation

The MES (Manufacturing Execution System) enforces dispatching rules automatically. Dispatch lists update in real-time as lots move and tool status changes. Operators follow the dispatch list unless overridden by engineering for special circumstances.