How to Reduce Secondary Contamination in Laundry Centers

In electronics manufacturing, semiconductors, and pharmaceuticals, cleanroom garments, cleanroom wipes, cleanroom shoe covers, and wiping cloths are not absolutely clean before use — they have been repeatedly contaminated on the production floor by oil, solder residue, adhesive stains, particles, and human oils. The real risk is this: if these consumables are recontaminated during laundering, every prior cleanroom control measure is invalidated.

A Class 10 laundry center (ISO Class 10 / ISO 14644-1 Class 100) is an independent clean production environment that provides washing services for cleanroom areas. Its mission is simple: return contaminated consumables to a cleanliness state more reliable than newly purchased ones, without letting the washing process itself become a new contamination source.

Secondary contamination typically occurs at five points in the process. Each has a specific countermeasure.

1. Control What Comes In: Contamination Classification and Pre-Removal

The most overlooked — and most failure-prone — stage is before washing. If soiled items are dropped straight into a cleanroom washer, contaminants are carried directly into cleanroom core equipment and recirculating water circuits.

Core practice: pre-removal and segregation.

• Color/zone-based collection: used cleanroom wipes and unused wipes must be physically separated and never mixed.
• Particle pre-removal: before entering the laundry center, perform vibration de-dusting, blow-off, or vacuuming to remove over 80% of loose particles.
• Chemical pre-treatment: for solder paste residue, silicone stains, and oil, apply specialized degreasing agents first, rather than trying to "scrub hard" with heavy detergent inside the cleanroom.
• Heavily soiled items never enter the clean zone: items with heavy oil, metal swarf, or physical deformation are routed through a separate low-cleanliness passage and dedicated equipment.

The principle: dirty items never enter the clean room. This is the first iron rule of a Class 10 laundry center.

2. Washing Parameters: Low Temperature and Low Residue Are What Matter

Washing is not "the cleaner the better" — it is achieving cleanliness while minimizing residue and fiber damage.

• Water temperature: generally controlled within a 30–60 °C range. Higher temperatures damage fibers and accelerate structural aging and shedding of cleanroom wipes.
• Wash time: overly long cycles also cause fiber fatigue, shortening service life and paradoxically increasing subsequent contamination risk.
• Detergent selection: use only specialized low-residue, low-ion, low-volatility detergents. Avoid complex formulations such as fabric softeners and optical brighteners — these residues migrate to the fiber surface after drying and become a source of ionic contamination.
• Rinsing must be thorough: use the conductivity of the final rinse water as one of the release criteria, ensuring no detergent remains.
• Filters are the core components of the laundry center: filters on the washer inlet, recirculation water circuits, and spin-dryer exhaust must be replaced on a defined differential-pressure schedule. This is exactly where "consumable replacement intervals" land in the laundering scenario — pressure exceeding the limit means the filter element has failed and water quality and cleanliness are already compromised.

3. Drying: Never Let Moisture Give Cleanliness Back

If consumables are not dried thoroughly, the damp fabric will absorb airborne particles and encourage microbial growth.

• Drying temperature and time: excessive temperature damages fibers and degrades antistatic performance.
• Dedicated cleanroom drying: drying equipment should be located inside the cleanroom or equipped with HEPA exhaust. Never air-dry outdoors — outdoor drying re-exposes clean consumables to contaminated air, the most common and most fatal secondary contamination path.
• Maintain positive pressure in clean drying cabinets: keep slight positive pressure inside the cabinet to prevent backflow of external air.

4. Packaging, Storage, and Clean Delivery

After washing, packaging and storage must be completed within a clean environment.

• Packaging method: use clean-room poly bags with vacuum sealing or sterile packaging, ensuring the packaging process introduces no particles.
• Storage conditions: store in partitioned clean cabinets, managed by batch and class, following first-in, first-out, to prevent aging or damaged outer packaging during long storage.
• Delivery route: the path from the laundry center to the production line must be fully enclosed, using dedicated clean carts and clean containers; direct hand contact is prohibited, and the route must never pass through contaminated areas.
• Personnel segregation: staff in the washing area and the clean packaging area must be separated, preventing personnel from crossing zones and carrying contamination in.

5. Management System: Data-Driven with Regular Verification

Risk reduction cannot rely on "being careful" — it requires verifiable mechanisms.

1. Establish standard operating procedures (SOPs)​: define washing parameters, drying parameters, packaging methods, storage limits, and ownership for each consumable category.
2. Set storage limits: clean-packaged cleanroom wipes and garments must have a defined shelf life (e.g., use within 30 days of clean packaging); expired items must be re-washed.
3. Periodic surface resistance and ionic residue testing: sample each batch to ensure washing has not degraded performance.
4. Regular cleanliness verification: perform particle counting and settle-plate microbial monitoring on the laundry center itself, confirming it consistently maintains Class 10 status.
5. Training and qualification: laundry center personnel must complete cleanroom behavior training to prevent operational errors from causing contamination.
6. Exception traceability: if the production line detects an abnormality in a batch of consumables, immediately quarantine the same batch and trace back the washing records.

Conclusion

Secondary contamination control in a laundry center is fundamentally about zonal isolation + low-residue process + an end-to-end clean chain. Keep contamination outside the door, drive residue to the lowest achievable level, and preserve cleanliness from the moment washing ends all the way to the production line — that is the true value of a Class 10 laundry center: ensuring every cleanroom consumable enters the cleanroom in a verifiable state of cleanliness.