How to control contamination from flexible packaging materials

How to control contamination from flexible packaging materials

By Ed Weggeland, Richmond Technology Inc.

The cleanroom of the `90s and beyond is being utilized for the manufacture of every conceivable type of device from medical applications to PC boards to wafer fabrication and disk drives. The diversity of devices being manufactured and classes of cleanrooms means that all the films being discussed in this article can, and are, being used in cleanrooms. Contamination issues are not limited to particles, but include processing additives and chemical compatibility. There is no single answer to questions often posed such as “what film should be used?” and “how clean should it be?”

As sloughing is critical to a discussion of the various packaging materials available, we need to define the term. “Sloughing” is the process of material shedding particles when flexed. This occurs on all flexible, unsupported packaging films to a greater or lesser degree depending on the softness of the film (i.e., polyethylene sloughs particles up to 100 microns in size while nylon won`t slough particles greater than 50 microns).

Materials available

Polyethylene is the lowest cost packaging film available. The problems encountered with polyethylene are sloughing of particulate and transfer contamination of processing additives.

Polyethylene resins are normally one of several types — linear low density (LLDPE), low density (LDPE), medium density (MDPE) and high density (HDPE). MDPE and HDPE are the top choice of the polyethylenes as they can be extruded without additives such as slip and antiblock (which enable the user to open the bag). These additives are a primary source of transfer contamination to the product being packaged. MDPE and HDPE are also stronger films which limit sloughing of particulate from the material itself when handled or when product is inserted or removed from the bag. Polyethylene is a better gas barrier than moisture barrier. When used in a vacuum and double packaging process, the outer layer of polyethylene should be at least 6 mil in thickness in order to hold a vacuum for any length of time.

In normal handling, particles from five microns to 25 microns in size will be sloughed from MDPE and HDPE. Particles to 100 microns in size are sloughed from LLDPE and LDPE.

Nylon film is either blown extrusion or a cast extrusion. The normal thickness available is 2 mil. One of the many positives associated with nylon is the strength of the film, which limits sloughing of particulate to 50 microns in size. It is extremely puncture resistant, vacuum sealable and a good gas barrier. By its nature, nylon is hygroscopic, absorbing moisture from the air, which makes it a poor moisture barrier. This also can lead to heat sealing problems when the film is dry and brittle (this can be solved by preconditioning in a moist atmosphere). Nylon is commonly used as an intimate contact material (or inner bag) when stringent cleanliness is required. Polyethylene can then be used as a less expensive outer bag.

Moisture barrier films are available for the cleanroom in several configurations — Tyvek/foil/poly, nylon/foil/poly, nylon/ PET/poly. Tyvek is utilized for maximum puncture resistance, foil is for moisture/EMI/RFI/ESD protection and nylon as a clean, strong layer. Structures are available with a static dissipative inner layer to prevent triboelectric charging of the contents during handling and shipment. However, these structures can also be supplied without a static dissipative inner layer for critically clean applications.

One problem typically seen in the high-end cleanroom is the generation of static charges through tribocharging, both on the packaging film and the product being packaged. Tribocharging occurs during the process of contact and separation of any material(s). When used in conjunction with ESD-sensitive components, antistatic materials and/or ionization may be required for product protection. Static charges present on the material can lead to electrostatic attraction of particulate to the material and the product.

When static dissipative additives are utilized on the inner structure, contamination issues can arise from transfer or outgassing of the additive onto the product. These can be controlled for critical applications by the selection of the agent and the amount utilized that is available to the surface of the film.

Cleaning techniques

Non-critical applications sometimes utilize a post-cleaning process at the converter level. This means that a converter will purchase a previously manufactured bag, typically polyethylene, then bring it into a cleanroom for wiping and packaging. Also typical is normally extruded material being brought into a cleanroom for conversion into a bag. This generally leads to a great inconsistency in materials received by the end user, both in particulate levels and additive contamination. A post-cleaning process can remove gross contamination on a bag; however, particles still remain at the seal edges and the bottom of the bag. These are next to impossible to remove in this type of procedure.

The inherent problems with utilizing a bag that has been “post cleaned” are lack of consistent resin (i.e., additives) as well as molecular forces at play which will not allow removal of smaller particulate which have been attracted and bonded to the material during extrusion and handling.

As particle sizes decrease, forces increase to bond the particle to the substrate. Intermolecular forces increase two orders of magnitude for each order of magnitude decrease in particle size. For example, a particle 700 microns in size has a theoretical attraction force of 1 G, while a particle 70 microns in size has an attraction force of 10,000 Gs. Electrostatic forces can increase these figures by several magnitudes.[1]

Liquid cleaners, such as DI water or solvent cleaners, can be used in the cleaning process; however, you have the same concerns with purity and re-deposit of contamination upon removal from the “bath” as with any other cleaning process in your cleanroom. Material compatibility with the selected cleaning agent must also be determined.

Common contamination issues

Mil-Std-1246C defines contamination as “unwanted material.” This includes particulate, ionic contamination, outgassing and transfer contamination.

Particulate testing. Liquid particle counters can be purchased to automate this process and will count and measure particle size depending on the requirement needed. However, the age-old method is still employed when sophisticated equipment is not available. This method utilizes a beaker, microscope and membrane filter. Typically, a 12-by-12-inch bag sample is fabricated; approximately 100 ml of solvent (DI water, IPA, Freon TF) is introduced into the bag; the bag is then agitated for a prescribed time period; the fluid is poured into a beaker and vacuumed through a filter. The particulate trapped on the filter is then sized and counted. [2]

Ionic contamination. Ionic contamination describes matter that is not electrically neutral but is either charged positively or negatively. Typical ionic contaminants are the anions — fluorine, chlorine, bromides, nitrates, nitrites, phosphates and sulfates as well as the cations — lithium, sodium, ammonium, potassium, magnesium and calcium. The main problem with ionics is that they cause corrosion.

NVR testing. The filtered solvent from particle testing is then heated and analyzed for total non-volatile residue and is measured in milligrams per square foot.

Outgassing. Conduct a head space analysis of volatile chemicals using gas chromatography/mass spectrometer. The old method utilized by the aerospace industry is ASTM 595-90 “Standard test method for total mass loss and collected volatile condensable from outgassing in a vacuum environment.” A small amount of material, typically less than 10 grams, is heated in a vacuum chamber to 125 degrees Celsius for 24 hours. A collector plate is maintained at 25 degrees C to gather and condense any outgassing products. In the test environment, material is subjected to a brute force that will cause outgassing of the test material. This outgassing is caused by a combination of the low vacuum and high temperatures.

“Total mass loss” (TML) is the material that is outgassed from the test specimen. It is calculated by taking the difference of the specimen mass before and after the test. It is expressed as a percentage of the initial specimen mass.

Outgassing data is critical to the disk drive and semiconductor industries as well as in optical applications.

“Collected volatile condensable material” is the amount of material that is outgassed and collected on the collector plate. It is expressed as a percentage of the initial specimen mass.

“Water vapor regained” (WVR) is the mass of water vapor regained by the specimen after reconditioning.

In cases such as nylon, a relatively large TML figure will be seen; however, when the WVR figure is noted, it can be seen that the greatest part of the TML was moisture.

How clean should material be?

Cleanliness is not an absolute. It is a relative condition denoting the degree to which a product can be isolated from, or cleaned of, contaminants. The level of required cleanliness must first be established for a product before determining what techniques and packaging material are needed to achieve the desired result. [2]

The following documents were referenced in preparing this report: ASTM 595-90, ASTM 312, Mil-Std-1246C.


1. Brandreth, D.A. and Johnson, R.E., “Particulate Removal in Microelectronics Manufacturing,” DuPont Co., Petro Chemicals Department, Wilmington, Delaware, 1978.

2. Weggeland, E.M., “Cleanroom Packaging Concepts,” Richmond Technology, Inc., Redlands, CA, 1994.


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