Tuesday, March 31, 2009

Determining Particle Count in Clean Packaging Film

Given the lack of up-to-date standards and the importance of optimizing communication between vendors and users, the contamination control industry needs a universal procedure for the testing of clean film packaging. This article provides concise, definitive directions for doing that.

The procedure begins with ensuring that all parts, components, assemblies and subassemblies, systems, and related equipment required for the cleanliness testing of clean film packaging have been cleaned to the highest levels of cleanliness and inspected in accordance with testing procedures and testing parameters for the utmost cleanliness required. A minimum Class 100 facility for quality testing is essential; if a Class 100 facility is not available, a Class 100 laminar flow hood located within a Class 1000, 10,000 or 100,000 facility can be utilized. Before testing begins, a trained technician should ensure that the lab facility is clean and clear of any obstructions or non-essential equipment and articles. All apparatus should be pre-cleaned and assembled for ease of operations.

The technician should wear a static-shielding garment with the head fully protected by a full hood (eyes-only) or bouffant hat. If a full hood is not worn, a mustache or face covering of the highest quality should be worn. The technician’s hands should be covered with long or ultra-long vinyl or latex gloves overlapping the sleeve areas. The operator should wipe down the face, arms, and sleeves of the garment, as well as the gloves, prior to initiating the test.


Isopropyl alcohol (IPA) should be a prime solvent for test fluids. This should meet TT-I-735 Grade A. Other applicable solvents would be DI, UPW, or ethyl alcohol. Preparation of solvents should be the highest grade of solution, and should be primarily filtered into an ultra-clean solvent dispenser.

The initial filtration unit at the exit of the solvent dispenser should be a Pall type with an absolute filter of 0.2 micron efficiency, which is compatible with the solvent being used. The next filtration position would be at the dispenser end, with a clean, pressurized dispensing gun recommended. At the nozzle of this gun should be a 0.45 micron membrane filter. The combination of these two filtration units on the test solvent container, in conjunction with filtration of the solvent going into the dispenser, will ensure the utmost quality and purity of the test solvent.

Additional equipment would include the following:

> A stainless steel membrane funnel assembly, which will allow for capturing of any solvent during the test process

> A Pyrex-style flask suited to the funnel assembly and adapted for a vacuum system attachment

> 0.45 micron porosity, gray or black background gridded, membrane filters for observation

> Stainless steel scissors

> Tweezers

> 2 x 2-in. glass microscope slides

> 45 mm membrane petri dishes

> An oil-less vacuum pump

All apparatus and equipment should be pre-cleaned and laid out in an orderly manner on a pre-cleaned table in the Class 100 room or laminar flow hood. Using an appropriate tack cloth, the operator wipes down all garments, sleeves, and gloves prior to initiation of the test. All equipment is prewashed prior to the initiation of the testing, using the pre-filtered and pre-cleaned test solvent.

The first step in all testing is to run a solvent blank on the solution being used. A 0.45 micron, 45 mm membrane filter is removed utilizing pre-cleaned tweezers, placed on a pre-cleaned 2 x 2-in. glass slide, and rinsed off in an angled solvent wash to remove any contamination of the membrane filter. This is then placed on the pre-cleaned membrane funnel assembly, and the funnel assembly is put together. Approximately 100 ml of the test solvent is poured into the pre-cleaned funnel to be filtered through the 0.45 micron membrane filter. Vacuum is applied to the flask bottle, drawing a vacuum through the membrane filter from the funnel assembly to speed the flow of fluid or solvent through the membrane. Once the membrane appears to be dried, the vacuum pump is turned off. The membrane is removed and placed in a pre-cleaned petri dish. The petri dish is then covered with a pre-cleaned lid and placed on a microscope for inspection.

The inspection of the membrane is performed at 40 power through a binocular or trinocular microscope assembly, equipped with a certified and calibrated scale, or through a microscope video monitor setup that allows the viewing of the membrane not only through the trinocular microscope but also through a calibrated monitor and sizing assembly. Any particulate viewed is calibrated or sized at 100 power, either by changing the reticle setup of the binocular microscope or by resetting the monitor system to view particles at 100 power.

Once the entire surface of the membrane is inspected and any particulates are identified and sized, the solvent is then proven to be clean. The next step in the setup is to take a sample of clean film product for testing, isolating approximately 0.1 m2 (1 ft2) of surface area, which is equal to a 6 x 12 in sealed area, or an assemblage of bags to equal 0.1 m2 (1 ft2 or 144 in2) of surface area. (The surface area of a bag includes both sides of the bag.) When sheeting is utilized, both sides are to be rinsed off to equal 0.1 m2 (1 ft2) of surface area. In lay-flat tubing, an equal amount of tubing is to be utilized to equal 0.1m2 (1 ft2) of surface area.

Once the surface area has been isolated or sealed on all four sides, the outside of the test sample is pre-cleaned by first wiping down, and then rinsed off with the pre-cleaned and pre-validated test solution. Utilizing the pre-cleaned stainless steel scissors, a corner of the bag or test sample is carefully slit (not cut), to make an opening. Slitting is performed by gently closing the scissors into the film, and then pushing through in a slicing action. This will prevent any contamination due to the scissoring or cutting action of the scissor against the film being tested.

Using pre-cleaned, gloved hands, the corner of the film that is slit is now pinched open carefully. Again, 100 ml of test solution is introduced into the packaging material. The opening is folded over to prevent any leakage of solution, and the product is then agitated with the solution inside for approximately 10-12 seconds. It is critical to agitate the solution long enough in all surface areas and crevices of the film, but not long enough to allow degradation of the film area or film material by the test solution. (See NASA’s KSC-C-123H Method I, Surface Cleanliness of Fluid Systems.1)

The funnel assembly is reassembled using a second pre-cleaned 0.45 micron, 45 ml dark gridded membrane and pre-cleaned funnel assemblies as before. The solvent is then poured into this assembly. The vacuum is drawn through the funnel until the most of the solution has been pulled through the membrane. An additional amount of pre-cleaned and blanked test solution is then utilized to final rinse the funnel assembly.

Once again, the membrane is then dried slightly by the vacuum, removed carefully from the funnel assembly, placed into a pre-cleaned petri dish and covered. This pre-cleaned and covered petri assembly, with the membrane from the test procedure, is again placed under a binocular or trinocular microscope. The viewing of the membrane is done again at 40 power. The typical scanning process is to view the individual squared-off or gridded areas of the membrane, excluding the initial or intimate contacted squares on the petri dish. This means that the squares or grids around the lidded areas of the petri dish are excluded because of the possibility of contamination from the closure of the petri dish.

Starting from the upper left-hand corner, the gridded membrane is viewed in a lateral process from left to right or right to left, moving down row by row and viewing all grids. When particulate is identified, the operator will change the 100 power reticle viewing for sizing on a calibrated reticle in a binocular scope or for viewing across the monitor at 100 power via a calibrated sizing apparatus equipped with a trinocular video monitor setup.

Once the entire gridded surface of the membrane has been inspected and all particulate sized, based on groupings of 5-15, 15-25, 25-50, 50-100, 100-150 and 150+ microns, the total count should then be accumulated.

Particulates smaller than 5 microns are normally not sized in referencing Mil Standards, Mil Specs, NASA specifications, or customer specifications such as Lockheed Martin’s, since these particulates have not been determined to be a critical matter. It should be noted that in the smaller than 5 micron size range, if the membrane is obscured or covered with particulates, the test should be determined to be a failure. As technology has advanced, however, some companies are testing for particulate levels as low as 0.2 microns. Because this size range is extremely difficult to test by microscopic methods, liquid particle counters equipped with sensors are now in use.

Throughout the entire operation, care must be taken so that the operator’s or technician’s hands or garments do not enter the downstream area of the filtered air flow or cross over the testing apparatus, test samples, membranes, or petri dishes during any test. This is extremely critical because any such motion or cross-contamination may cause contamination in the test membrane.

Care must also be taken in sizing of particulate so that the longest dimension of a particle should be examined as the maximum length. In terms of fibers, anything more than 50 microns in length should be classified and identified as either a particle or fiber, with both cross and length dimensions noted. Once the entire membrane surface has been examined, particulates sized and counted, these numbers are cross-referenced to the appropriate specification. Determination of cleanliness relative to pass-fail criteria is then made.

During this process, if a membrane is identified as having too many particles to be counted, it should be classified as a possible reject. However, per ASTM F312-69, Standard Test Methods for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters,2 a second method can be utilized. Again, by scanning the entire gridded surface, starting in one corner of the membrane, the number of gridded units and particulate by unit measure are recorded. When the numbers become too extreme for continued counting, an extrapolation of particle counts, or grid counts, can then be utilized, per the ASTM specification, based on percentage of grid units counted per entire gridded surface and the number of particles counted in the counted gridded units.

This extrapolation will therefore result in an estimated count on the total surface area. For utmost accuracy, however, the entire surface should be counted when possible and feasible. The quality control department should then review the test results against the required specification. When an extreme abnormal count is seen, a second microscopic inspection can be utilized to confirm the original numbers.

In conjunction with this, a similar method for testing can be utilized using many of the procedures and processes described above except for the replacement of the microscope and membrane units with the quantitative and qualified liquid particle counter to be utilized; however, liquid particle counters have been proven by many organizations to be inaccurate and not accepted to be a non-viable source of particulate counting, due to the variations in the test solution and the sensing of the particle counter, whether it be light sensing or laser sensing units.

The approved and recommended method for determination of particulate counts on clean film has always been unquestionably the microscopic inspection method.


1 NASA KSC-C-123H Method I, Surface Cleanliness of Fluid Systems.

2 ASTM F312-69, Standard Test Methods for Microscopical Sizing and Counting Particles from the Aerospace Fluids on Membrane Filters.

Other documents used in the development of this procedure, but not referenced in the text, include:

SAE ARP-743, Procedure for Determination of Particulate Contamination of Air in Dust Controlled Spaces by the Particulate Count Method.

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