What Are the Best Practices for Selecting Cleanroom Wipers?

Cleanroom Wiper Selection is Critical to Maintaining Cleanliness

Maintaining a cleanroom’s cleanliness level is a vital but oftentimes overlooked job. However, it is critical to its continued function and use. Part of cleanroom maintenance is general facility cleaning, as well as critical workstation, product, and process area cleaning.

Wiper-based cleaning is one of the most effective methods to remove contaminants. To achieve effective cleaning and maintain compliance, selecting the appropriate wiper and wiping techniques are essential.

IEST Recommended Practice

The wiper contamination characteristics are summarized in the IEST recommended practice, IEST-RP-CC004: Evaluating Wiping Materials Used in Cleanrooms and Other Controlled Environments.

The most recent revision of IEST-RP-CC004 contains;

• Appendix A: Guidance for Wiper Selection by Environmental Cleanliness or Application. This guidance table is an attempt to summarize based on application (i.e. SEMI, MDM, Optics, Pharma) and overall cleanliness considerations, which wiper substrate material, edge treatment, and washing should be chosen. It provides a review of wiping options from absorbent fibers (that shed) to ultra-clean polyester wipers with sealed edges to everything in between.
• Appendix B: Method for Particle Enumeration or Analysis using Scanning Electron Microscopy provides guidance to perform scanning electron microscopy (SEM) to count particles <100µm at 200X and 3000X magnification.
• Appendix C: Alternative Test Method for Measuring the Rate of Sorption and Capacity of Wiping Materials provides standard test method guidance to wiper manufacturers that were established by industry-leading wiper manufacturing companies to achieve accurate and reproducible test results for absorbency and capacity of their wipers. The sorption capacity of the wiper is the amount of liquid the wiper can hold. The sorption rate describes how fast a wiper can sorb that liquid.
  
  

Performance Criteria

Wiper selection is based on four product performance factors: absorbency, particles, extractables, and ionic contamination. The wiper contamination characteristics are the particle and fiber, non-speciated extractable matter, and ionic contamination levels.

 

 

Other non-performance factors are the user’s process requirements, the impact of activities (e.g., cleaning spills and surfaces or holding manufactured parts or pieces) in the cleanroom, and the impact on product yield and manufacturing costs. These factors affect the type of wiper needed to achieve appropriate cleaning and contamination control.

 

Selection Process: Challenges and Solutions

Cleaning and wiping procedures are typically established in an organization’s internal standard operating procedures (SOPs) that are site-specific and based on:

• industry segment (e.g. advanced materials, life sciences)
• types of surfaces (e.g. rough, smooth)
• type of contamination
• cleaning agents
• safety

Part of the cleaning process may require the use of swabs for small, hard-to-reach areas. Large surfaces are best handled by using mops. Both pieces of equipment could be considered wipers of different sizes on a stick. Therefore, the selection of wipers, swabs, and mops should be based on the various internal SOPs for cleaning and these products should be evaluated based on the IEST test method results published by the manufacturer.

The most common challenge to wiper selection is that the most absorbent wiper substrate materials (e.g. non-woven and natural fibers) tend to shed more particles. Wipers with higher sorbent capacity will hold more cleaning chemistry, and clean larger surface areas. The downside is higher particle counts.

As with other cleanroom consumables, finding a balance between conflicting requirements is an ongoing operational challenge and is why understanding internal standard operating requirements is so important in deciding where product attribute compromises can and cannot be made in the selection process.

Testing Methods  

Testing results are largely influenced by your specific cleaning task.

Environmental/facility cleaning requires a more absorbent wiper to permit the cleaning of large surface areas. Wipers for critical product & process cleaning will require a wiper with lower particle counts, non-volatile residue, and detectable extractables. However, special applications (i.e. delicate optics, SEMI chamber cleaning, etc.) will require a wiper designed to succeed in that specific task.

The vendor(s) selected for wiper qualification can provide test results for the suitability of the product in relation to its application. These test results should include, at a minimum, the following technical performance attributes:

• Basis weight, according to TAPPI T-410
• Absorbency (including sorptive efficiency, capacity, and rate), according to IEST-RP-CC004.3, Sec 8.1, Sec 8.2
• Liquid Particle Counts (LPC), according to IEST-RP-CC004.3, Sec 6.1.3, Sec 6.2.1
• The non-volatile residue (with both DI water and IPA solutions), according to IEST-RP-CC004.3, Sec 7.1.2
• Extractable Counts (Ions), according to IEST-RP-CC004.3, Sec 7.2.2
  
  

Standards referenced: ISO 14644-1 and ISO 14644-2

IEST RPs referenced: IEST-RP-CC004 (other Tests mentioned: TAPPI T-410)

 

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