What does production control do in a chip fab, and how are due dates set?

Production control (sometimes called production planning and control) is the team in a chip fab that turns the quantities and dates customers ask for into how many wafers to start each week and each day, and which lots go first. When a tool stops and lots fall behind, the person who has to answer "so when can that lot ship?" is usually Production control.

What is the job like?

Picture ordering a birthday cake at a bakery. The clerk doesn't say yes right away. First they flip through the order book: how many cakes are already booked for that day, how many the oven can bake in a day, how many days this kind of cake takes. Only then do they tell you, "You can pick it up Saturday afternoon."

Production control is the person flipping through that order book in a fab. The difference: a fab's "cake" takes months to make, the "oven" is hundreds of tools that each do one kind of work, and orders from many customers are often in the queue at once (on fabs that make chips for other companies, see The news says "foundry". What is it?).

What kinds of plans does production control make?

Broadly, a fab's production plans can be sorted by how far ahead they look:

1. Years ahead: are there enough tools, and should more be added?
2. Months ahead, week by week: how many wafer starts each week, how many wafers finished, which lots ship in which week. This is the master production schedule.
3. Today: how many wafers each part of the line should finish today, and which lots have the highest priority.

Production control's work sits mostly in the last two. Books on production planning in wafer fabs split the decisions into the same kind of layers: how much to make in each period, when to start how much, and which lot goes on which tool first.

How is a due date worked out?

From wafer start to finished wafer takes months (why the count starts that day: "What is a wafer start, and why does a fab count from that day?"). So the schedule for a due date is worked out backwards from the day the customer wants the wafers:

• Take the day the customer wants them, subtract how long the wafers take, then leave a little extra time before that. The result is the latest day to start the wafers.
• That extra time is the buffer. If some step runs a few hours late, the buffer absorbs it and the ship date doesn't move.

A made-up example: if a lot takes 12 weeks on average and you leave 1 week of buffer, it has to start 13 weeks before the due date.

Because it takes so long, production control often can't wait for orders to arrive before thinking about them. One approach is to reserve capacity from the customer's forecast first, then take each real order out of that reserved space when it comes in. Before saying yes to a customer, the planner also checks whether the busiest tools have room.

Which numbers does production control watch every day?

• WIP: work in process, the lots on the line that aren't finished yet. Which step each lot is at, and how long it has been sitting there (Move and WIP).
• Cycle time: how long a lot takes in total, from wafer start to finished.
• Capacity: how much the tools can finish in a day. The slowest, most loaded step on the line is the bottleneck; whatever it can do in a day is the most the whole line can put out in a day.

These numbers are tied together. There is a simple relationship between the lots on the line, how much gets finished each day and how long each lot stays, explained in "What does the manufacturing department do in a chip fab?".

There's also something that runs against intuition: the fuller the tools are booked, the longer lots wait, and the wait doesn't grow in proportion. When the tools are close to full, squeezing in a little more makes the queue suddenly much longer. It's like a highway: a few extra cars mean nothing when traffic is light, but when the road is nearly full, a few extra cars bring everything to a stop. So production control has to balance two things when it plans: tools that aren't sitting idle, and lots that aren't waiting too long.

For the daily plan, a common approach in the research is to split the month's quantity into how many wafers each part of the line should finish each day, then check whether the lots are spread evenly. Where too many pile up, they wait in a queue; where the next part has nothing, its tools sit idle.

What happens when a customer wants lots sooner?

This is the question production control gets most, and the hardest to answer. There are roughly three cards to play:

1. Make it a hot lot: a hot lot goes to the front at every step. But it moves fast because other lots make way for it; with many hot lots, everything else slows down noticeably.
2. Use a lot that's further along: sometimes another lot of the same product is already near the end of the line. It can go to the rushed order first, and the order it was meant for is filled by a later lot.
3. Start more wafers: this card does nothing for a date that's close. New wafers have to go through every step from the first one, which is months again.

Whichever card is played, some other lot ends up later. So before production control says "yes", it works out: once this lot cuts in, who will be late, by how much, and can those orders still make their dates? Research papers on rush orders do this step first too, before deciding whether to accept one.

That's why a good answer often comes with conditions, like "It can go out Thursday, as long as the measurements at that step come back normal tomorrow." The lots that get pushed back are worth telling the sales rep about early, too.

How do production control and Manufacturing split the work?

Both look after the lots, but over different stretches of time. Broadly it works like this, and the real split differs from fab to fab:

Who	How far ahead	Questions they ask
Production control	days to months	How many to start? Which lots first? Will we make the due date?
Manufacturing	this shift, the next few hours	Which tool does the next lot go on? Which tool is down and needs a way around?

Production control makes the plan: how many to start, which lots come first, when each one should be out of which step. Manufacturing follows that plan on the line, dispatching lots from tool to tool. When a tool stops, Manufacturing first sorts out how the lots on the line get around it; production control works out whether the ship dates move and tells the sales rep, who tells the customer.

In fab24hr

In fab24hr, tap "Clock in", choose "A day in the fab", then on the "Your role today" card tap "Production fab" and swipe to Production control. You don't touch the tools, and you don't judge whether wafers are good; all four shifts answer the same question: when can the lots ship? On the late-night shift it's 05:30 and you're at home, reading the overnight WIP report on your phone: the etch tool ETCH-03 stopped in the night, with the hot lot H-2417 stuck inside. The sales rep calls to ask whether the ship date will move. The first steps are half a day behind, and you have to decide what to tell the sales rep: can the buffer after them absorb it? On the morning shift, at the 09:00 production meeting, ETCH-03 is down to one chamber and nine lots are waiting, and the first to look after are the one close to its Q-time and the hot lot that ships tomorrow. A part being installed doesn't count: the tool is only back once the qual run passes. On the afternoon shift, a manual run next door on ETCH-05 goes wrong and the whole of P-4K26 goes on hold. You re-plan the week's shipments and decide about the lot queued behind it: let it slip a day and tell the sales rep today, or ask the night shift to try to rush it. On the evening shift, this month's output is a little short and the department manager wants everyone to work the weekend; first you need to see whether the days that dipped came from tools stopping and lots on hold, or from too few people. Play Manufacturing instead, and on the afternoon shift the person who phones in a super hot lot is Production control. The company, people, tool numbers, lot numbers and figures in the game are fictional; what production control does and how due dates are set follow public, general sources, and the real division of work differs from company to company. For other jargon, see Chip fab jargon in plain English.