Tag Archives: non-volatile residue

Wipes/Swabs: NVR Testing Explained

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Non-Volatile Residue (NVR) is a critical indicator for assessing surface cleanliness of materials in industrial production, directly affecting product reliability in electronics, pharmaceuticals, and precision manufacturing. This article focuses on wipers and swabs, establishing a high-sensitivity NVR testing method based on solvent extraction and gravimetric analysis. By standardizing the process, the method enables quantitative analysis of residues on textiles and swabs, providing scientific support for quality control in cleaning processes.

2. Introduction

NVR refers to non-volatile substances remaining on material surfaces after solvent extraction. Its content directly impacts product performance in fields such as semiconductor manufacturing, where wafer surface NVR can cause circuit shorts or device failures. This paper develops a quantifiable NVR detection system for wipers and swabs by optimizing solvent selection, extraction procedures, and data calculation models.

3. Materials and Methods

1. Experimental Materials

  • Samples: Wipers (6 pieces), Swabs (120 swab tips)

  • Solvents: Isopropanol (IPA, ≥99.7%), Ultrapure Water (DIW, 18.2 MΩ·cm), Anhydrous Ethanol (≥99.8%), Petroleum Ether (boiling range 60–90°C)

  • Equipment: Temperature-controlled evaporation apparatus (±0.1°C), electronic balance (accuracy 0.1 mg), vacuum filtration unit (0.45 μm pore size)

2. Testing Procedure

  • Solvent Soaking:

    • Wipers: Cut to a standard size (e.g., 10×10 cm²) and soaked in 200 mL of selected solvent with ultrasonic agitation for 30 minutes.

    • Swabs: 120 swab tips soaked in 500 mL solvent with mechanical shaking for 1 hour.

  • Solution Processing: Filter to remove particles (>0.45 μm), collect filtrate.

  • Evaporation to Constant Weight: Transfer filtrate to pre-weighed evaporation dishes; evaporate in 80°C water bath until constant weight (mass difference ≤0.2 mg between two weighings).

3. Residue Calculation Formulas

  • Wipers NVR (mg/cm²) = (Residue mass in mg) / (Wiper area in cm²)

  • Swabs NVR (mg/tip) = (Residue mass in mg) / (Number of swab tips)

4. Key Parameters

  • Total test duration: Approximately 9 hours (including 6–7 hours evaporation)

  • Temperature control during evaporation: 80±2°C to avoid residue decomposition and measurement bias

4. Results and Discussion

1. Effect of Solvent Choice on Extraction Efficiency

  • Isopropanol (IPA) achieves extraction efficiency up to 92% for oil-based residues, suitable for electronics industry wiper testing.

  • Ultrapure water is more sensitive to water-soluble ionic residues such as Na⁺ and Cl⁻, making it ideal for pharmaceutical-grade swab testing.

2. Method Validation

  • Wipers spiked with silicone oil (1.0 mg/g) showed recovery rate of 98.5% with relative standard deviation (RSD) of 2.1%.

  • Swabs spiked with polyethylene glycol (0.05 mg/tip) showed recovery rate of 95.3%, RSD 3.8%, confirming accuracy and repeatability.

3. Comparison with Industry Standards
The method’s results for textile NVR closely match ISO 9022-12:2015 optical component cleanliness tests, with deviations less than 5%. Increased swab sample size (120 tips) improved statistical significance (p<0.01).

4. Application Cases

  • A semiconductor company applying this method found that reducing wiper NVR from 0.8 mg/g to 0.2 mg/g increased wafer yield by 12%.

  • In a medical device production line, NVR testing traced a microbial contamination incident to swab residue exceeding limits (0.15 mg/tip, 50% over limit).

5. Conclusion

The presented NVR testing method, combining multi-solvent extraction and high-precision gravimetric analysis, enables accurate quantification of residues on wipers and swabs. Coupled with standardized procedures, it provides robust technical support for quality control in cleaning consumables, especially for industries demanding high cleanliness.

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In high-cleanliness environments such as semiconductor manufacturing, biopharmaceuticals, and aerospace, cleanroom wipes are critical consumables for maintaining cleanroom classification and process stability. The IEST (Institute of Environmental Sciences and Technology) released the IEST-RP-CC004.4 standard, providing a scientific and systematic testing framework for quality control of cleanroom wipes. This article analyzes the key test items and their purposes based on the standard, revealing its technical value in cleanroom applications.

1. Particle Release Characterization

  • Liquid Particle Counting (LPC, 0.5–20 μm):
    Using a liquid particle counter combined with orbital shaking to simulate mechanical friction on wet wipes, quantifying released particles between 0.5 and 20 microns to prevent contamination of precision instruments and interference with optical surfaces.

  • Fiber Analysis (>100 μm):
    Optical microscopy is used to detect large residual fibers or particles after orbital shaking, preventing clogging of microporous structures or mechanical failures caused by macroscopic particles.

  • Airborne Particle Counting (APC, 0.3–10 μm):
    Helmke drum simulates dry state wipe motion, and airborne particle counters monitor particle release between 0.3 and 10 microns to assess particulate risk during dynamic cleanroom operations.

2. Chemical Contaminant Analysis

  • Ion Content Test (IC):
    After extraction with deionized water, ion chromatography detects anions (F⁻, Cl⁻, NO₃⁻, etc.) and cations (Na⁺, K⁺, Ca²⁺, etc.) to prevent ion residues from corroding electronic components or interfering with chemical processes, especially crucial in semiconductor wafer manufacturing.

  • Non-Volatile Residue (NVR) Test:
    Short-term extraction with deionized water and isopropanol followed by evaporation and weighing quantifies transferable oils and polymers, preventing deposition on sensitive surfaces.

  • Fourier Transform Infrared Spectroscopy (FTIR):
    After hexane extraction, FTIR detects organic compounds such as silicone oils, amides, and phthalates (DOP), identifying potential contamination sources to protect optical components and ensure biocompatibility.

3. Physical Performance and Functional Evaluation

  • Liquid Absorption Capacity and Rate:
    Measures maximum liquid uptake per unit area and absorption speed, ensuring wiping efficiency and avoiding contamination from insufficient liquid absorption leading to repeated wiping.

  • Surface Static Charge Test:
    Measures surface resistivity to control static accumulation risk and prevent electrostatic discharge (ESD) damage to electronic components.

4. Biological Contamination Control

  • Bioburden Test:
    Microbial culture methods quantify aerobic bacteria and fungi counts on wipe surfaces, ensuring wipes do not introduce microbial contamination in pharmaceutical or biological lab environments.

5. Technical Significance of the Standard
IEST-RP-CC004.4 comprehensively covers physical, chemical, and biological performance indicators of cleanroom wipes through multi-dimensional testing. It offers graded particle control from submicron to macroscopic fibers, compatible with ISO Class 1 to 5 cleanrooms, tightly controls ions and organic contaminants for semiconductor and optical industries, and verifies absorption efficiency and antistatic performance to guarantee operational reliability. This standard provides a scientific basis for wipe selection, acceptance, and quality management and is a core technical specification to maintain clean environments in high-value industries.

Conclusion:
By rigorously following the IEST-RP-CC004.4 standard, users can precisely select high-quality cleanroom wipes tailored to specific cleanroom scenarios, effectively reducing contamination risks, extending equipment lifetime, improving process yield, and enhancing product reliability, thereby supporting sustained growth in high-cleanliness industries.