4 Crews, 3 Shifts: The Arithmetic of a 24/7 Chip Fab
Korean memory fabs run on four crews and three 8-hour shifts. Heavy industry in Korea has been moving to four crews and two 12-hour shifts. The two sound very different, but the arithmetic says otherwise: per person, both come to 2,190 hours and 730 legal night hours a year. What changes is how often people commute and how their days off are bunched.
KEY TAKEAWAYS
1. One 24/7 post needs 8,760 hours a year. Split across four crews, that is 2,190 hours per person, about 42 hours a week, under either shift system.
2. Korea's legal night window (22:00–06:00) is 8 hours a day, so each person carries 730 night hours a year either way, paid at a premium of at least 50%.
3. Shift length is the real difference. POSCO's days off went from 103 to 191 a year after moving from 8-hour to 12-hour shifts.
Where Korean chipmakers stand
Maeil Business reported in July 2023 that memory lines run four crews on three 8-hour shifts, and that, unlike petrochemicals and steel, chipmakers were not discussing a switch. Hankyung Business listed the movers: POSCO in 2011, S-Oil in 2021, SK Innovation's Ulsan complex in 2023. There has been some experimentation in chips. The Korea Economic Daily reported in December 2022 that Samsung was piloting a weekend-dedicated schedule for equipment staff, with 12-hour shifts on Fridays through Sundays and public holidays.
How a roster rotates
Under three shifts, the day splits into day, evening and night; three crews work and one is off. A common style of rotation, two days of each shift and then two off, works 6 of every 8 days. Under two 12-hour shifts, two crews work and two rest, so people work 4 days in 8. Read any column of the grid and every shift is covered by exactly one crew. The post never goes empty; only the person changes, with three handovers a day under one system and two under the other.
Same hours, different split
A post staffed around the clock needs 24 × 365 = 8,760 hours. Four crews make that 2,190 hours each. At 8 hours a shift, that is about 274 shifts a year; at 12 hours, about 183. In days off, roughly 91 versus 182. POSCO's reported 103 and 191 run a bit higher, presumably because of training and cover arrangements.
Night work follows the same logic. Korea's Labor Standards Act (Article 56) defines night as 22:00 to 06:00. Eight hours a day times 365 is 2,920 hours, or 730 per person across four crews. Even the number of night shifts matches, about 91 a year, a third of 274 or half of 183. Only the length of each night shift differs, 8 hours versus 12.
Why chips stick with 8-hour shifts
Twelve-hour shifts nearly double days off, but the end of a long shift is where fatigue builds. Hankyung Business noted that Korail moved to four crews on two shifts, saw accidents rise and was ordered back to a three-crew, two-shift system. In a fab handling wafers that have been through hundreds of process steps, that risk is part of why the industry says a switch is not on the table.
Twelve-hour shifts are not foreign to chipmaking, though. TSMC's Arizona technician postings specify compressed 12-hour day or night shifts on a rotating 3/4–4/3 schedule. It is a trade between shift length and longer blocks of rest, and labor norms in each country tip it one way or the other.
One step further: why four crews, and what 730 night hours cost
Change the crew count and the logic of four becomes clear. Three crews would mean 2,920 hours each, 56 a week, above Korea's 52-hour weekly cap (40 regular plus 12 overtime). Five crews would cut that to about 1,752 hours, 34 a week, but need 25% more people per post. Four is the smallest crew count that stays inside the cap.
The 50% night premium on 730 hours adds the equivalent of 365 regular hours of pay per person, about one-sixth of the 2,190 hours worked. So staffing one 24/7 post costs four people times 2,190 hours plus 365 hours' worth of premium. That is the number automation is measured against: removing one round-the-clock post removes four people and their night premium, not one. Health is part of the ledger too; the International Agency for Research on Cancer classified night shift work as probably carcinogenic to humans (Group 2A) in 2019.
What I actually watch
| Signal | Why it matters |
|---|---|
| Shift-system talks at Korean chipmakers | A switch would reshape staffing and labor cost |
| New fab ramp schedules vs. hiring | Each 24/7 post needs four hires |
| US fab staffing progress | 12-hour norms and local hiring set ramp timing |
Value chain read-through
| Segment | Link to shift work | Indicator |
|---|---|---|
| Fab automation | Replaces four people per post | Automation capex |
| Labor cost | 50% night premium | Labor share of COGS |
| New fabs | Posts × 4 hiring | Job postings |
| Overseas fabs | 12-hour compressed shifts | Local staffing |
Risks to this view
• The 2,190 and 730 hour figures are my simplified calculation and exclude leave, training and cover shifts.
• The four-crew, three-shift description of memory lines comes from press reports; arrangements vary by company, role and site.
• POSCO's 103 and 191 days off come from steel, not semiconductors.
That closes a four-part look at what working in a fab involves: contamination control, automation, hiring and shift work. Back to memory cycle data next.
Sources: Maeil Business (2023-07-13); Hankyung Business (2023-03-08); Korea Economic Daily (2022-12-11); Korea Labor Standards Act, Article 56; TSMC careers, "2025-2026 TSMC AZ Technician"; IARC Monographs Vol. 124 Q&A (2019).
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