Expedite Versus WIP: Dispatch Decision on Two SMT Lines
An electronics plant runs two SMT lines and a final assembly cell. MES shows 18 active work orders for the next 48 hours, all with material kitted. Two orders share a stencil that requires a 90-minute changeover, and one is an expedite for a customer with a contractual ship date in 26 hours. Line 2 is scheduled for a 6-hour preventive maintenance window starting in 30 hours. Current WIP is 1,240 boards, 35% above the target, and it consumes 80% of the buffer space before conformal coat. Operators report frequent short stops at AOI, and the MES OEE dashboard shows availability at 72% against a 90% target. The planner wants to release three more orders to keep both lines fed. The production manager asks you to decide the next 24-hour dispatch sequence and whether to release the new orders.
1. Recommend the dispatch sequence for the next 24 hours and justify the scheduling rule you use.
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Use finite-capacity, due-date-aware sequencing rather than one global rule. First check the MES routing and setup matrix to see whether the expedite order uses the stencil already loaded on Line 1. If it does, run it next there: this avoids the 90-minute changeover and protects the 26-hour ship date. If it does not, compare the expedite's remaining slack with the changeover cost and with Line 2's PM window. Running it on Line 2 may be better only if the 6-hour PM can still start within its 30-hour window. Group the two stencil-sharing orders consecutively so the changeover is paid once. Place short, low-changeover orders ahead of the PM window so Line 2 does not enter maintenance with a long run in progress. Do not sequence only by shortest processing time; that raises throughput but can miss the expedite. Recalculate after each completion because availability at 72% and AOI short stops add variance. The governing rule is: protect the contractual due date, minimize total changeover, and keep the PM feasible.
2. Should the planner release three more orders? Explain your decision using the WIP and buffer data.
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No. WIP is already 1,240 boards, 35% above target, and the pre-conformal-coat buffer is 80% full. Releasing three more orders increases queue time, worsens the buffer constraint, and hides the AOI short-stop problem rather than fixing starvation. Use a CONWIP-style release: set the cap at target WIP and release one order only when a completion drops WIP below the cap. If the lines risk starving, check whether the cause is AOI downtime rather than lack of released orders. A practical release gate is: release the next order only if buffer occupancy is below 70% and the line has less than two hours of work queued. This is a judgement call because customer service matters, but adding WIP does not shorten lead time when the constraint is already congested; it lengthens it.
3. What metrics would you review after 24 hours to judge whether the decision worked, and what would make you change course?
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Review WIP level versus the 1,240-board baseline, buffer occupancy before conformal coat, expedite completion against the 26-hour promise, changeover minutes, AOI short-stop frequency and duration, and Line 2 PM start time. Also track schedule adherence and the number of orders released versus completed. If WIP falls below target and expedite is on time, the release cap is working. If buffer occupancy stays above 80% or AOI stops rise, tighten release further and escalate the AOI issue to maintenance and engineering. If the expedite misses its date because of the stencil changeover, revise the setup matrix or pre-stage the stencil earlier. Change course if the PM window is threatened or if the expedite's slack falls below four hours; at that point, resequence or add overtime rather than release more orders.