SMT Line Restart After Planned or Emergency Shutdown: A Parts-First SOP for 2026

Sunday, 9:14 PM. A Suzhou EMS plant is two days back from the Mid-Autumn holiday. The night-shift supervisor walks onto Line 4 — a Yamaha YSM20 paired with a Panasonic NPM-D3 and a Heller 1913 reflow oven — and flips the master breaker. Within fifteen minutes the YSM20 is showing Feeder Comm Error, the NPM-D3 reports Head 2 Vacuum Low, and the first three boards out of the printer show solder bridges on a 0.4 mm pitch QFP.

Nothing is broken. The line has degraded during the shutdown — feeder gears have run dry, nozzle seals have cracked, stencil tension has relaxed, and the reflow oven profile has drifted while the oven cooled to ambient. The plant is going to lose four hours tonight, and that four hours is almost entirely preventable.

This guide is the SOP Merex field engineers hand to customers before every long Chinese national holiday, every typhoon season in Guangdong, and every planned summer shutdown.

Why a Cold Restart Is Almost Always a Parts Problem, Not a Machine Problem

Across the 130+ post-holiday and post-curtailment restarts the Merex service team has logged between 2024 and 2026, the breakdown is consistent:

Shutdown length	Median first-board yield	Median pick-error rate	Top three parts that fail	Median restart time
Under 24h	93%	0.6%	Feeder cover, nozzle vacuum, squeegee pressure	1.5 h
1–3 days	88%	1.8%	Feeder gear, stencil tension, solder paste viscosity	2.5 h
4–7 days	79%	4.2%	Feeder belt, nozzle seal, reflow conveyor speed	4 h
7–30 days	71%	7.5%	Stencil deformation, heating-element oxidation, drive-belt cracking	6 h
Over 30 days	58%	12%	Multiple parts fail simultaneously	8+ h

Each additional day of shutdown reduces first-board yield by roughly 1.5 to 2.5 percentage points, and roughly doubles pick-error rate. The cheapest intervention — a parts-first inspection before power-on — is the one most plants skip.

The rest of this guide is what to inspect, in what order, and at what point you should simply swap the part instead of testing it.

The Four Parts Families That Decide Your First-Board Yield

A modern SMT line has thousands of SKUs in its spare-parts cabinet. On a restart, four families dominate failure: feeders, nozzles, stencils, and reflow ovens. If you only have time for four subsystems, these are the four.

1. Feeders — The Single Largest Cause of Pick Errors After a Shutdown

A tape feeder is a precision mechanical assembly — gears, ratchets, tape clamps, sprockets, sensors, springs. During the shutdown it sits idle, often with cover tension relaxed, often at low humidity, often for days. The parts that degrade first are:

• Cover tape clamp — the spring-loaded plate that holds the cover tape against the carrier. Spring force drops measurably after 72 hours of relaxation; cover tape starts peeling, components shift, pick errors climb.
• Sprocket and ratchet — the gears that index the carrier tape. Idle gears develop micro-pitting at the contact surfaces. On the first index after power-on, you get a skipped pitch or a double index.
• Drive belt — the toothed belt that connects the feeder motor to the sprocket. Belt rubber hardens during idle, especially in air-conditioned rooms. The first index after power-on slips one or two pulses.

A 15-minute feeder SOP:

1. Visual — rust on the sprocket, spring-set on the cover clamp, cracks on the belt teeth.
2. Manual index — turn the sprocket by hand ten full cycles. Smooth with no audible click or grit. Any roughness is a replace.
3. Belt deflection — press the belt at midpoint with one finger. 3–5 mm is healthy; above 8 mm, replace before the line runs.
4. Gear mesh — gap between sprocket tooth and carrier’s sprocket hole. 0.3–0.5 mm healthy; above 0.8 mm, replace the sprocket.
5. Cover clamp — install scrap carrier tape, tug the cover. If it peels back without resistance, the spring is fatigued.

For shutdowns longer than seven days, Merex’s recommendation is to pull every feeder off the line and run a bench service — gear set, cover clamp, and drive belt replaced as a kit, not inspected one by one. The labor cost of bench service is roughly half the labor cost of chasing feeder alarms on the line at 3 AM.

2. Nozzles — Silent Vacuum Loss

A nozzle that was passing vacuum checks on Friday will fail a vacuum check on Monday after a long weekend. The failure mode is consistent: the EPDM or fluorocarbon seal at the base of the nozzle dries during idle and micro-cracks, and the vacuum reading at the pick point drops from the healthy -85 kPa range to -50 kPa. The nozzle will still “pick” intermittently — it will just pick badly, and the pick-error rate will tell you why.

A 10-minute per-nozzle SOP:

1. Vacuum test on the bench tester, not on the machine. Below -75 kPa is a hard fail.
2. Tip inspection under magnification — a 0.1 mm chip on a 0.4 mm pitch nozzle is enough to cause placement drift. 10x minimum, 20x for fine pitch.
3. Ultrasonic clean — five minutes in anhydrous ethanol, air-dry. No picks or swabs on the tip mirror; scratches cost yield.
4. Spring seating — depress three times. Clean click = centered; soft click = hanging up on the bore.
5. Pick test — 0402 resistor from a feeder with verified indexing. Three picks, three successes. Anything less is a fail.

For a line down more than three days, plan for 1:10 nozzle spares during the first 48 hours back online. Pick-error rate during cold-start is typically three to five times the steady-state rate.

3. Stencils — The Quietly Deforming Part

A stencil left mounted in the printer during a long shutdown does three bad things at once:

• The foil relaxes. A new aperture foil sits at 35–40 N/cm. After seven days of compression in the frame, the corners can drop below 25 N/cm. Solder paste release degrades, deposits come out thin in the middle and thick at the edges, and you get opens on small pads and bridges on large pads in the same print.
• The foil can micro-warp if the printer is bumped or if the stencil is stored at an angle. Even a 0.05 mm warp across the diagonal is enough to print uneven deposits.
• Residual paste in apertures oxidizes during the idle period. On the first print, oxidized paste behaves like dry paste — poor release, poor fill.

A 20-minute stencil SOP:

1. Tension check — tension meter at five points (four corners plus center). Corners ≥ 38 N/cm; center ≥ 32 N/cm. Below threshold, send for re-tensioning.
2. Aperture inspection — microscope at 10x. More than five blocked apertures per foil, ultrasonic clean before the first print.
3. Warp check — granite plate, dial-indicator the diagonal. Above 0.05 mm at any point, send for recoiling.
4. Three-pass wipe — dry, wet (anhydrous ethanol), dry with lint-free wipes. Residue after the third pass should be visually zero.

For shutdowns over seven days, treat the stencil as mandatory service regardless of the tension number. Cleaning, re-tensioning, and a print test on a scrap coupon. The cost of skipping this shows up on the first build of the morning, when the AOI lights up.

4. Reflow Ovens — The Most Expensive Silent Asset

A reflow oven that has cooled from 250 °C to ambient and sat there for a week has had a thermal cycle on its heating elements, a stationary conveyor chain with no oil film refreshed, and a cooling system (water or N₂) that may have accumulated condensation in the lines.

A 30-minute oven SOP:

1. Heating element visual — open the side covers. Healthy tubes glow a uniform dull red during ramp. White or black spots mean oxidation; oxidized tubes run hot in patches and cold in others, and the profile will not match the oven’s recipe even if all thermocouples are good.
2. Conveyor chain — chain sag at mid-span below 10 mm is healthy; above, adjust. Surface rust gets a brass brush and a high-temperature chain oil rated for 250 °C+. No general-purpose grease — it carbonizes in the oven.
3. Thermocouple verification — compare each profile thermocouple against a calibrated reference at 150 °C. Deviation above ±3 °C, replace and re-profile.
4. Cooling system — water-cooled ovens: drain and refill the cooling block, check for condensation in the lines. N₂ ovens: verify the heat exchanger for water pooling.
5. Profile burn-in — three empty passes with the production recipe.

For shutdowns longer than seven days, this expands to roughly 90 minutes because the heating elements, conveyor, and thermocouples all need individual attention.

Graded Restart SOPs by Shutdown Length

The point of grading by shutdown length is to spend the right amount of labor. A 24-hour shutdown does not need a bench service; a 30-day shutdown needs much more than a bench service.

Short Shutdown (Under 24h) — Weekends, Long Weekends

Total labor: ~1.5 hours.

1. Power on HVAC 30 minutes early; target 22–26 °C, 40–60 %RH.
2. Feeders: visual + manual index only.
3. Nozzles: vacuum test + visual only.
4. Stencil: single-pass wipe.
5. Reflow oven: power on 30 minutes early, single empty profile pass.
6. First 5–10 boards: full inspection, AOI + visual. Above 95 % first-pass yield, release to production.

Medium Shutdown (1–3 Days) — National Holidays, Plant Maintenance

Total labor: ~2.5 hours.

1. Day before: confirm spare-parts stock with your supplier. Merex covers Yangtze River Delta and Pearl River Delta with 24-hour expedited shipping on 95 % of feeder, nozzle, and oven SKUs.
2. Restart day: full SOP for all four parts families.
3. Feeders: bench service on every unit, gear set and cover clamp replaced as a kit.
4. Nozzles: full clean and vacuum test; any reading below -75 kPa is replaced.
5. Stencil: ultrasonic clean + tension check.
6. Reflow oven: thermocouple verification + three-pass profile burn-in.
7. First 20 boards: full inspection. Above 95 % first-pass yield, release to production.

Long Shutdown (4–7 Days) — Lunar New Year, Equipment Upgrades

Total labor: ~4 hours.

1. Three days before shutdown: full line audit. Replace every part within 10 % of end-of-life. Do not let parts cross the holiday on borrowed time.
2. Day before restart: HVAC and dehumidifier on for at least 24 hours before power-on.
3. Restart day: medium-shutdown SOP plus drive-belt replacement on every feeder, conveyor oil replacement, and a full profile re-characterization.
4. First 50 boards: full inspection + comparison against the pre-shutdown baseline. Pick-error rate above 130 % of baseline triggers a stop-the-line and re-baseline.

Extended Shutdown (Over 7 Days) — Plant Moves, Idle Capacity

Total labor: 8+ hours, often spread over two days.

1. Treat every subsystem as out-of-service until proven otherwise. Replace-by-kit, not inspect-by-decision.
2. Control boards: remove, bake at 60 °C for four hours to drive off any condensation. Plants in humid climates should not skip this step.
3. Pneumatic system: drain air receivers, replace filter elements, verify dew point. Oil content above 0.1 ppm means the filter element was compromised during idle.
4. Mechanical axes: re-grease all rails, ball screws, and bearings with the OEM-specified grease. Do not mix grease chemistries.
5. Placement machine: full geometric calibration, CPK ≥ 1.33 on every head, vision system re-calibrated against the calibration plate.
6. Printer: stencil alignment, squeegee pressure calibration, paste-roll inspection.
7. Reflow oven: full profile re-characterization against the production recipe.
8. First 100 boards: X-ray, AOI, and ICT on every unit. First-pass yield must hold above 98 % for two continuous hours before the line is released.

Emergency Shutdown: Power Curtailment, Typhoon, Extended Idle

The graded SOPs above assume you knew the shutdown was coming. Three common scenarios do not.

Power curtailment — plants in Sichuan, Chongqing, the Yangtze River Delta, and Guangdong have been living with open two stop five and open five stop two notices since 2024. The shutdown is non-negotiable; the recovery window is short. The mitigation is stocking 15 days of spares, not 7. When power returns, every supplier ships to every other plant at the same time and lead times stretch from days to weeks. Plants that built the 15-day buffer recover in 90 minutes; plants that did not recover in 8+ hours.

Typhoon and heavy rain — Guangdong, Fujian, Zhejiang, June through October. The failure mode is humidity. Plants come back from a 48-hour typhoon shutdown to find control boards with condensation under conformal coating, stencil foils with white oxidation halos, and solder paste that exceeded its cold-chain limit because the cooler lost power. Pre-shutdown: seal the building, drop the dehumidifier setpoint to 45 %RH, cover precision equipment, verify cold-chain battery backup. Post-shutdown: insulation test every control board (above 1 MΩ to chassis ground), hard-reject any solder paste that lost cold-chain.

Extended idle over 30 days — grease has dried in the rails, belt rubber has stress-cracked, seals have taken a compression set. The extended-shutdown SOP applies in full, plus an honest conversation about which subsystems should be replaced rather than recommissioned. A placement head that sat for 90 days without cover has almost certainly lost its encoder battery and may have lost its calibration.

Emergency Parts List for the Restart (Six-Machine Line)

Part family	Quantity	Why	Merex stock
Feeder gear sets	20 kits	Dry-running wear	5,000+ kits
Feeder cover clamps	30 pcs	Spring set	8,000+ pcs
Feeder carrier tape guides	50 pcs	Edge chip	12,000+ pcs
Feeder drive belts	10 pcs	Rubber hardening	2,000+ pcs
Nozzles (CN040 / CN065 / CN240)	20 each	Vacuum loss	15,000+ pcs
Nozzle seals	100 pcs	Compression set	50,000+ pcs
Stencils (common frame sizes)	2–3	Tension loss	7-day custom
Squeegee blades	5	Edge wear	3,000+ pcs
Reflow heating elements	10	Oxidation	1,500+ pcs
Reflow thermocouples	10	Drift	2,000+ pcs
Reflow chain oil	20 L	Dry-out	800 L
AOI light bars	10	Brightness drop	800+ pcs
AOI camera lenses	5	Contamination	300+ pcs
SPI laser heads	3	Aging	150+ pcs
Solder paste (common alloys)	5 cartridges	Cold-chain breach	1,500+ cartridges

How Merex Supports the Restart Window

Merex has supplied OEM-compatible and genuine spare parts for Yamaha, Fuji, Panasonic, ASM, and Hanwha SMT platforms for 20+ years. Three capabilities shorten the restart window:

• 24-hour expedited delivery across the Yangtze River Delta and Pearl River Delta, with 48-hour coverage for the rest of mainland China and Southeast Asia. An “emergency restart” tag puts the order on the next outbound truck.
• Pre-built feeder and nozzle service kits matched to the graded SOPs above — short, medium, and long-shutdown kits sized per machine on the line.
• Field-engineer support on call for the first 100 boards after a long shutdown.

If you are heading into a planned shutdown this quarter, run the four-parts-family SOP above on a single line first, time it, then scale.

Closing Thought

A restart is not “flip the breaker and hope.” It is a graded, parts-first procedure whose length is set by shutdown duration and whose success is measured on the first board. The four families — feeders, nozzles, stencils, and reflow ovens — cover roughly 90 % of the parts-related yield loss on a cold start. Build the SOP, stock the parts, run it before the holiday.

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