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Fix a bottleneck by pacing wafer releases, identifying the tool that is actually limiting flow, protecting it with a small buffer, and moving engineering attention to it. Starting more lots while downstream capacity is full usually increases work-in-progress (WIP) and queue age rather than good-die shipments. The same overload can lower yield because lots spend longer waiting and face more process and scheduling risk.
This guide applies those principles to Fab Floor, the browser simulation in which you release wafer lots and follow them through lithography, etch and metrology. It shows how to distinguish a starved tool from a blocked one, when to use engineering focus or preventive maintenance, and how to verify that a change improved output instead of merely moving the constraint.
What a bottleneck is in Fab Floor
A bottleneck is the step whose effective capacity limits the entire line. The line can contain several fast tools, but practical output cannot exceed what the tightest step can process after interruptions, maintenance and other losses. Kuvia Corp summarizes the rule as “The bottleneck sets output.” Its companion warning is equally important: “Pushing more work in doesn’t ship more.”
In Fab Floor, the objective is not simply to keep every tool busy. You are balancing good-die output, yield, controlled WIP, healthy tools, cash and reputation. A busy floor with growing queues can therefore be performing worse than a deliberately paced floor with less WIP.
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Capacity is effective, not just the number on a tool card
Nameplate speed is only a starting point. A tool that is nominally fast can lose practical capacity when it alarms, waits for work, lacks material, or cannot hand lots to the next operation. Judge the behavior you can observe in the simulation: whether lots are arriving, whether they are waiting, how long queues remain, and whether the tool is available to process them.
Why the largest pile is not always the root cause
A large queue shows where flow is accumulating, not necessarily why. The pile may be in front of a true capacity constraint, or it may be downstream of a tool that is stopped. Conversely, a tool can be the constraint while its own queue looks modest if upstream release is inconsistent. Use the tool’s state and its neighboring queues together.
Find the active constraint before changing anything
Pause the instinct to release another lot or upgrade the most visible machine. First take a short operating snapshot. Record the current output, total WIP, queue age at each major step, tool utilization, tool health and yield. You need a before state so that a later change can be judged rather than guessed.
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Signals to inspect
| Signal | What it tells you | How to use it |
|---|---|---|
| WIP location | Where lots are accumulating | Inspect the tool immediately after the pile and the step immediately before it. |
| Queue age | How long work has been waiting | Prioritize an old queue even when its visible size is not the largest. |
| Tool utilization | How continuously a tool is processing | High utilization with a persistent upstream queue suggests a capacity constraint. |
| Tool state | Whether the machine is starved or blocked | Use the direction of the diagnosis: upstream for starved, downstream for blocked. |
| Yield and good-die output | Whether additional flow is creating valuable product | Do not treat higher starts as success if good output or yield declines. |
| Alarms and health | Whether interruptions are removing capacity | Recover the affected tool before adding more work to its queue. |
Starved versus blocked: the key diagnostic split
| State | What you see | Where to investigate | Typical response |
|---|---|---|---|
| Starved | The tool is ready but has no lot to process. | Upstream release, supply, dispatching or the preceding operation. | Restore a deliberate feed or correct the upstream shortage; an upgrade to this idle tool will not create work. |
| Blocked | The tool has completed work but cannot send it onward. | Downstream capacity, an outlet problem or a full receiving queue. | Relieve the downstream constraint and avoid feeding still more lots into the blocked path. |
ReliaSim’s concise rule is useful here: “A starved machine points upstream and a blocked one points downstream.” Apply it to the tool’s actual state, not to the size of the nearest queue.
Check the constraint under interruptions
A nominally fastest operation can become the active constraint if it loses time to alarms or maintenance. Compare effective behavior over the period you are managing. A slower but healthy tool may be delivering more usable capacity than a faster tool that repeatedly stops.
Control wafer starts and WIP
Fab Floor explicitly warns that “More starts are not useful if the downstream line cannot clear them.” Release lots only at a rate the constrained path can absorb. If downstream tools cannot clear the work, each additional start lengthens queues without raising shipments.
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Use a deliberate release loop
- Observe: check the active constraint, its queue and the next downstream outlet.
- Release a limited amount of work: add lots only when the bottleneck is ready to process them or its small protective buffer is being consumed.
- Watch WIP and output together: a rising WIP count without a corresponding rise in good-die output is evidence that starts are outrunning capacity.
- Stop feeding the line when the outlet is blocked: let the downstream path recover before creating more upstream inventory.
- Reassess after flow stabilizes: compare queue age, yield and good output rather than reacting to a single moment of utilization.
This is not a command to keep every queue empty. A completely empty queue can starve a bottleneck. The aim is controlled WIP: enough work to keep the constraint supplied, but not enough to bury the floor in waiting lots.
Why increasing starts can reduce yield
When starts rise faster than the line can process them, queue age and cycle time increase. In the game’s model, longer waits expose lots to more process and scheduling risk. That means a full-looking line can produce a lower-yield result than a paced line, even though more wafers entered the fab. Evaluate starts by good dies shipped, not by starts alone.
Protect the bottleneck with a small buffer
The supporting bottleneck simulation explains that output follows the constraint. A small buffer immediately before the constraint protects it from brief upstream variation. Releasing work at the bottleneck’s pace keeps that buffer available without creating an uncontrolled WIP surge.
Set the buffer by observation
- Identify the tool that is limiting effective output.
- Watch whether it repeatedly becomes idle because the preceding step delivers unevenly.
- If it starves, release enough work to restore continuity, then stop adding lots once the buffer is replenished.
- If the tool is blocked, do not enlarge its input buffer; investigate the downstream outlet instead.
- After any change, verify that the buffer remains small and stable rather than growing each cycle.
The buffer is protection, not a target inventory level. Its purpose is to prevent starvation of the constraint, not to make WIP appear high.
Move engineering focus to the active constraint
Fab Floor treats engineering focus as an active control. The game describes focus as improving local processing capacity and slightly reducing wear. Leaving focus on one tool group for an entire shift can waste that benefit when the constraint has moved.
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- Move focus when a different tool group becomes the sustained constraint.
- Move it after a maintenance recovery changes which step limits flow.
- Move it when a queue has cleared and another queue begins aging rapidly.
- Do not move it solely because a tool is busy for a moment; confirm the pattern with WIP, queue age and output.
Focus cannot compensate for a blocked outlet or an absent upstream feed. Apply it after you know whether the tool is starved or blocked, and then watch whether effective capacity and queue behavior improve.
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Respond to alarms and preventive maintenance
An alarm removes capacity immediately and can stop every lot behind the affected tool. When a tool alarms, take the maintenance kit to that tool and start preventive maintenance. Prompt recovery protects both throughput and the queue age of waiting lots.
Maintenance response procedure
- Identify the exact tool reporting the alarm.
- Take the maintenance kit to the affected tool rather than servicing an unaffected machine.
- Start preventive maintenance and confirm that the tool returns to an available state.
- Check the queue behind it for accumulated age and the downstream outlet for a new blockage.
- Resume deliberate releases only after the recovered tool and its receiving step can accept work.
Do not judge the success of maintenance by utilization alone. A repaired tool may immediately process a backlog, making utilization high while the next step becomes the new constraint. Follow the queue and output after recovery.
Choose the right intervention
When several fixes are possible, compare them against the same five questions: does the action increase capacity at the current constraint, what happens to WIP and queue age, does it improve tool health or alarm recovery, does it protect good-die yield, and does it simply move the bottleneck downstream?
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|---|---|---|---|
| Reduce or pace starts | WIP and queue age are rising while downstream capacity is fixed. | Over-correction can starve the constraint. | Stable small buffer, lower queue age and unchanged or improved good output. |
| Restore upstream feed | The active tool is starved. | Adding feed to a blocked path creates more WIP. | The tool processes continuously without a downstream pile growing. |
| Relieve downstream capacity | The active tool is blocked. | Extra upstream starts hide the outlet problem. | Completed lots leave the tool and the downstream queue stops aging. |
| Move engineering focus | The constrained tool group needs local capacity or wear reduction. | Focus on a non-constraint has little line-wide effect. | Effective processing improves at the target and the constraint either clears or moves predictably. |
| Run preventive maintenance | An alarm or unhealthy tool is removing capacity. | Servicing the wrong tool spends time without restoring flow. | The alarm clears, availability returns and waiting lots begin moving. |
| Upgrade or add capacity | The same operation remains the constraint after flow and health are controlled. | The bottleneck may shift to the next step, so WIP can relocate. | Good output rises without a new queue-age or yield problem downstream. |
The table does not imply that every option is available at every moment. Select the smallest intervention that addresses the diagnosed state, then measure before choosing another.
A complete bottleneck-recovery playbook
Use this sequence whenever output falls, WIP surges or yield drops.
- Freeze blind starts. Stop adding lots merely to keep the floor busy.
- Capture a baseline. Note good-die output, yield, total WIP, queue age, utilization, tool health and alarms.
- Map the flow. Follow lots through lithography, etch and metrology and mark where waiting begins.
- Inspect the suspected constraint. Determine whether it is processing, starved or blocked.
- Trace in the correct direction. For starvation, inspect release, supply, dispatch and the preceding step. For blockage, inspect the receiving step and downstream capacity.
- Recover stopped capacity. If an alarm is present, take the maintenance kit to that tool and start preventive maintenance.
- Set a small protective buffer. Feed the constraint at its pace, stopping before WIP expands through the rest of the line.
- Move engineering focus. Place focus on the active bottleneck, not permanently on a favored tool group.
- Make one change. Do not alter starts, focus and maintenance simultaneously unless an emergency stop requires it; otherwise you cannot tell which action worked.
- Observe a complete response. Wait long enough to see queue age, output and yield move rather than judging the first completed lot.
- Record the result. Keep the change if good output improves without an unacceptable WIP or yield trade-off; reverse it if the constraint worsens.
- Repeat only after the line settles. A successful fix often reveals the next bottleneck. That is normal; treat the new constraint as the next diagnostic target.
Why a busy fab can have low output
Lots are waiting, not processing
Utilization somewhere on the floor can look impressive while lots spend most of their cycle time in queues. Waiting contributes to cycle time but not to completed good dies. Inspect queue age and the tool state instead of using overall busyness as a success metric.
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The constraint is hidden downstream
Upstream tools may continue completing work, creating a visible pile before a slower or interrupted downstream operation. Those completions do not become shipments until the constrained path clears. Release control and downstream recovery matter more than keeping upstream machines occupied.
An alarm erased nominal capacity
A stopped tool can make every upstream operation appear productive while the line’s practical capacity collapses. The maintenance-kit response restores the specific lost capacity; starting more lots only increases the backlog.
Engineering effort is pointed at the wrong group
Because focus improves local capacity and slightly reduces wear, leaving it on a previously important group after the constraint moves gives no line-wide benefit. Re-evaluate after each major flow or maintenance change.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Diagnose common symptoms
“The queue is huge. Should I upgrade that tool?”
First determine whether the tool is blocked or processing continuously. A huge queue before a blocked tool may be an outlet problem. A huge queue before a healthy, continuously processing tool is stronger evidence of insufficient effective capacity. Check the next step before spending an upgrade.
“The machine is idle, so why is output low?”
An idle machine may be starved. Trace upstream to release, supply or dispatch rather than increasing its capacity. A faster idle tool still produces nothing.
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“I released more wafers and yield fell.”
Compare queue age and cycle time before and after the release increase. If WIP expanded and waits lengthened, the extra starts exceeded downstream clearing capacity and increased the game model’s process and scheduling risk. Return to paced releases and protect the constraint with a small buffer.
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“Maintenance finished, but another queue appeared.”
Capacity recovery can expose the next limiting step. Re-map the flow, identify whether the newly visible tool is starved or blocked, and move focus only if that tool is now the sustained constraint.
“Everything is busy, but shipments do not rise.”
Busy tools may be feeding a single constraint or processing lots that are waiting downstream. Compare good-die output with WIP and queue age. If WIP rises faster than output, stop increasing starts and work on the active constraint.
Measure whether the fix worked
After each single intervention, compare the same measures used for the baseline:
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- Good-die output: the primary result; higher starts do not count if shipments do not rise.
- Yield: confirms that additional flow is still producing acceptable product.
- WIP: reveals whether the line is accumulating work faster than it clears it.
- Queue age and cycle time: show whether lots are waiting longer even when tools remain active.
- Tool health and alarms: confirm that lost capacity has actually returned.
- Constraint location: tells you whether the intervention relieved the bottleneck or merely shifted it.
A good result is not necessarily a permanently empty queue or maximum utilization. It is a stable flow in which the constraint has a small supply buffer, WIP is controlled, queue age is not accelerating, tool health is maintained and good-die output improves or remains protected.
Operating routine for sustained performance
At the start of a shift, inspect the baseline measures and identify the current constraint. During operation, release lots deliberately and check for starvation or blockage whenever the flow pattern changes. Respond to alarms immediately with preventive maintenance. Reassign engineering focus when the sustained constraint moves. At the end of a change, record what happened to output, WIP, queue age, yield and health before making another adjustment.
This routine turns bottleneck management into a feedback loop rather than a one-time upgrade decision. Because every capacity improvement can expose another limiting step, the correct question is always “What constrains good-die output now?”
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