By E. N. Borson, Swales Aerospace

Introduction and background
IEST-STD-CC1246D was published in 20021 and is the latest revision of the standard that was first published in 1962 as MIL-STD-1246.2 The standard was written in response to a need to define quantitative cleanliness levels for products that included components and fluids. Levels were defined for both particulate and nonvolatile residue (NVR) molecular contaminants.

Based on the technology and the needs at that time, it was determined that particles of sizes less than 1 μm and NVR levels of less than 1 mg were not significant.

Experiments showed that particle distributions on or in clean products could be represented by a lognormal distribution with a peak at 1 μm. Cooper3 compared three different ways to describe particle distributions. For the standard, the lognormal, cumulative distribution was selected and approximated by a log-log2 equation as follows:

N =

number of particles per 0.1 ft2 of surface area, 1 ft3 of gases, or 0.1 liter of liquids

0.926

is the absolute value of the slope of the line (also expressed as the tangent of the angle between the line and the horizontal axis)

x =

particle size in micrometers

L =

the cleanliness level

This lognormal approximation provides a good representation of particles on or in precision-cleaned or filtered products. Larger particles tend to be more easily removed than smaller particles, resulting in larger numbers of small particles remaining in or on the product.

IEST-STD-CC1246D uses the same equation, but the units were changed to be consistent with the International System of Units/metric (SI) usage. The ft2 was replaced with 0.1 m2 and all fluid volumes are now per 0.1 liter. The difference between ft2 and 0.1 m2 was deemed to be insignificant considering the uncertainties in counting particles, especially using manual counting methods.

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Figures 1 and 2 show the particulate cleanliness levels as defined in IEST-STD-CC1246D.

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Originally, the range of particulate cleanliness levels was from 10 to 1000 and was extended to 1 in revision B.4

NVR levels were originally specified as mg/ft2 for surfaces and mg/0.1 liter for liquids. Gases were not included.

Revision C

The U.S. Army Missile Command approached the IEST to revise the standard. IEST Working Group 901 was formed to work on the revision, and MIL-STD-1246C was published in 1994.5

The rapid growth in contamination-control technology and the demand for more stringent cleanliness requirements resulted in significant changes to the standard.

Alternative methods to the Cleanliness Level equation for specifying particles were included. This allowed for the inclusion of particles of less than 1 μm in size, and considered newer techniques for measuring particles and the effects of particles on product performance.

Particle deposition could be specified as percent area coverage (PAC) in place of or in addition to specifying particle sizes and Cleanliness Levels. This was needed for specifying the cleanliness of optical devices such as solar cell panels, spacecraft thermal radiator surfaces, and instruments. The term PAC was selected over the previously used term obscuration because it was defined as the sum of the projected areas of the particles, whereas obscuration can be optical attenuation that is not directly related to area coverage.

When particle deposition within cleanrooms was plotted on the log-log2 graph, the resulting curve no longer followed the Cleanliness Level equation. Hamburg and Shon6 had shown that particle fallout in cleanrooms could have slopes that were up to approximately -0.3 when the equation was adjusted to fit actual and theoretical fallout data, and depended on the class of cleanroom air and operations in the cleanroom. “Dirty” products have particles that are made up of two or more distributions of particles. Any one particle size distribution depends on the source of the particles.

The smallest maximum allowable NVR level that could be specified was changed to 0.01 mg/0.1m2 and 0.1 mg/L, responding to the need for cleaner products.

A section on statistics was added, and “visibly clean” as a cleanliness specification was defined. Visibly clean inspection methods have been shown to be quantifiable in many applications when using trained personnel.7

Revision D

In 1997, the Army Missile Command commissioned IEST to revise and adopt MIL-STD-1246 as an industry standard as its usefulness had expanded far beyond military applications, and U.S. policy was requiring agencies to convert government standards to nongovernmental standards where practical.

IEST-STD-CC1246D reduced the smallest maximum NVR level that can be specified to 10 ng per 0.1 m2 or 0.1 L.

Table I of the standard (see Figure 2) was revised in the harmonization activity, including rounding off particle numbers to three significant digits or one decimal place, whichever was appropriate. The use of a fraction of a particle raised some questions, but when data are normalized or averages calculated for statistical purposes, fractional numbers might result. Detailed requirements on the use of Figures 1, 2 (Table I), 3 (Table III), and other tables are found in the standard. The appendix on cleaning and packaging was eliminated as other documents were available that covered the subject adequately.

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The informative list of test methods was updated.

During the drafting of Revision D, ISO 15952-2 was being developed8 within ISO TC20/Subcommittee 14, Space Systems and Operations. Cooperative work with ISO TC20/SC14 harmonized the two standards so that the cleanliness levels are consistent. The harmonization of international and national standards is continuing for new standards and current standards as they require review. These efforts involve IEST, ASTM, ISO and other organizations.

Next revision

IEST policy requires that standards start the review process three years after initial publication. IEST-STD-CC1246D is now at this stage. New test methods have been published, and some clarification of requirements and wording are necessary. Any comments on IEST-STD-CC1246D should be submitted to IEST. Participation in the review and any revision effort is encouraged. It is important to get comments and suggestions from the people in industry and government who are using the document.

References

1. IEST-STD-CC1246D, Product Cleanliness Levels and Contamination Control Program, Jan. 2002.
2. MIL-STD-1246, Product Cleanliness Levels and Contamination Control Program, Dec. 1962.
3. Cooper, D.W., “Comparing Three Environmental Particle Size Distributions: Power Law (FED-STD-209D), Lognormal, Approximate Lognormal (MIL-STD-1246B),” Journal of the IES, Vol. 34, No. 1, Jan/Feb 1991, pp. 21-24.
4. MIL-STD-1246B, Product Cleanliness Levels and Contamination Control Program, Sept. 1987.
5. MIL-STD-1246C, Product Cleanliness Levels and Contamination Control Program, April 1994.
6. Hamburg and E. M. Shon, Particle Size Distribution on Surfaces in Clean Rooms, Proc. IES, May 1984, pp. 14-19.
7. J.T.Sanders, Precise Visual Inspection, 1998 Proceedings of the IEST, Phoenix, Ariz., May 1998.
8. ISO 14952-1, Space systems-Surface cleanliness of fluid systems-Part 2: Cleanliness levels, Available from American Institute of Aeronautics & Astronautics, http://www.aiaa.org.

Gene Borson holds a BS in Mechanical Engineering from the University of Calif. at Berkeley (1950) and did post-graduate studies at the University of Calif. at Los Angeles. He has more than thirty-five years experience in spacecraft systems, including contamination control for ground and flight operations. He joined The Aerospace Corporation in 1962 and has been a consultant on contamination control and materials applications for space systems since retiring in 1991. He is currently a part-time employee of Swales Aerospace, working on NASA space projects. He is a member of IEST WG-901 that wrote MIL-STD-1246C and IEST-STD-CC1246D and is active on other IEST working groups. He is also active in national and international technical matters and is a founding member of ASTM Committee E-21 on “Space Simulation and Applications of Space Technology.” He also represents ASTM E-21 on the U.S. Technical Advisory Groups (TAGs) for ISO Technical Committee 209, “Cleanrooms and Associated Controlled Environments” and ISO TC 20, “Aircraft and Space Vehicles,” Subcommittee 14, “Space Systems and Operations.”