Static-Shielding Bags Offer Safe Passage Through ESD Battlefield

Makers of electronic products must wage a constant battle against the lightning-fast static charge and discharge that can ruin their hard work in one costly instant. The challenge is to get their product safely from the point of origin to the customer. One of the many options available is the cost-effective static-shielding bag.

Static-protecting bags typically are categorized in one of three classifications: static- dissipative, static-shielding, and static-shielding with a moisture barrier, said John Jaran, president of Web Technologies. The static-dissipative version prevents static generation but does not offer protection from it. The static-shielding bag augments the dissipative film with a static-protective metal barrier. The third type is similar to the static-shielding bag with the added protection of a moisture vapor seal.

Static-shielding bags are manufactured in metal-out and metal-in formats. The metal-out type has a metal layer applied near the exterior surface. A Faraday cage effect is created by a thin, transparent, conductive layer of aluminum, said Mr. Jaran. The layer is placed near the surface of the material for conduction of charge. These bags have a protective, abrasion-resistant dielectric coating over the metal.

The metal-in type, sometimes called a buried metal bag, has an electrically conductive metal layer bonded between sheets of polyester and polyethylene. The polyethylene layer is on the inside of the bag, and it provides an antistatic capability to minimize the generation of triboelectric charges caused when the contents move inside the bag.

The antistatic agent can be a coating on the polyethylene for single-use applications, or it can be compounded into the polyethylene for more exacting static requirements. The metal is buried more deeply than a metal-out version and used when enhanced durability is required, said Mr. Jaran.

But it is incorrect to generalize that a metal-out bag always protects a product to a specific charge level, such as 25 V, and that a buried metal bag will work well from 100 V and up, said Mr. Jaran. This may not be the case.

The bag material must be from a reputable supplier with film that achieves the required charge-protection levels. The real test of bag functionality is directly attributable to how well the material is processed through a bag- making machine.

The best static-shielding film can be rendered ineffective if made into a bag incorrectly. To assure customers of the highest quality, the bags manufacturer must

have test equipment to check the performance of the material after construction.

Bar-Code Reading

Although most static-shielding bags have the classic gray, transparent appearance, there is a tremendous variation in the performance of these bags, added Mr. Jaran. Reputable bag manufacturers offer a product with reproducible performance. They track all raw materials and follow documented manufacturing procedures, and they check the bags for quality with the appropriate test instruments.

Generally, a metalized layer attenuates a charge placed on the outside of the bag, said Todd Somers, international sales manager at BayStat. The thicker the metal layer, the greater the charge the bag material attenuates.

Unfortunately, the metal layer can impede bar-code reading, making it difficult for companies to verify the contents of a shielding bag. The light transmission rate of a material is a significant factor in allowing a bar code to be read through a flexible package, said Mr. Somers. The thicker the metal, the lower the light transmission rate. Light transmission rates of >40% allow many scanning systems to effectively read a bar code on a board inside a bag.

The interface of the metal and polyester layers scatters the bar-code scanning beam, making bar-code reading through a shielded bag tenuous at best, said Brent Beamer, director of technology at Static Control Components. Reducing the amount of metallization can improve bar-code readability at the cost of a more fragile metal layer. Claims for bar-code-readable shielded bags should be challenged by testing the bag with the same scanning equipment and bar-coded items that will be used in production.

Bar codes are used for inventory control on printed circuit boards and for tracking products. If a shielded bag is too opaque or glossy for a bar-code reader to decipher, it may provoke you to open the bag, making it easy for a static event to occur.

Bar-code reading is becoming an important issue, said Mr. Jaran of Web Technologies. Readability is attributed to a number of physical properties, including the surface gloss of the product. It has a profound effect on the readability, second only to the transparency of the bag.

Bar-code reading through packaging material also is dependent on the reading equipment as well as the optical characteristics of the packaging material, said Dave Swenson, manager of applications development and technical service at the 3M Electrical Specialties Division. Providing shielding that meets the necessary specifications may not allow a great deal of optical transparency.

The user and the specifier of the materials must evaluate the desired package for protection, based on test methods and the ability to read bar codes through the packaging material. Presently, it is doubtful if any supplier of static-shielding materials could state with 100% accuracy that a given bag will work all the time with all bar-code readers.

Volume-Loaded or Coated

The dissipative characteristic of a film allows a charge to spread out over a surface. It is accomplished by volume loading or surface coating. For volume loading an additive is mixed with the film resin. The additive increases the conductivity of the surface of the film. For the coating process, the film is made, and then the surface is treated with an additive that increases the conductivity of the surface and allows local charges to be dissipated.

Which should you choose? It depends on how long you need the protection. A coated material typically provides in-use service for a relatively short time because the coating tends to wear, said 3M’s Mr. Swenson. Volume-loaded materials may provide longer-lasting performance because the surface can renew its static-dissipative and low-charging qualities from the built-in reservoir of chemicals in the polymer layer.

Typically, the price of surface-coated materials is lower than the volume-loaded types, but the volume-loaded materials ultimately may be less costly per cycle if they are reused several times, said Mr. Swenson. Specifiers of bag material often choose surface-coated materials for one-time tasks and volume-loaded materials for multiple occasions.

Both volume and surface-coated methods provide appropriate surface resistivity and reduce tribocharging, said Static Control Components’ Mr. Beamer. Coatings perform slightly better in low relative humidity and are less costly and easier to apply.

Many users are looking more closely at the interior of the bags, said Web’s Mr. Jaran. They realize that it is important to specify as well as to know how the static- dissipative properties were obtained.

Today, volume-loaded and surface-treated polyethylene films are offered. No matter which version you select, investigate the issue of contamination and how the antistatic-generating technique will affect your product. Ask how the wash coat of amines or amides is applied to the polyethylene.

Amines and amides are chemical additives that improve the dissipative qualities of the polyethylene resin. The compatibility of the additive and the polyethylene determines the amount of diffusion of the additive.

Too much affinity between the additive and the resin prevents the additive from reaching the surface and aiding static dissipation. Too little compatibility and the film dissipates static but the surface is excessively greasy, and the additive may contaminate the contents of the bag.

Also ask for ion extraction data. It is another source of contamination.

Some shielding-bag specifications indicate a polyethylene layer that is static dissipative, and others indicate antistatic. Static-dissipative material may not be antistatic or low charging, but antistatic material almost always is static dissipative, said 3M’s Mr. Swenson.

Most static-discharge shielding bags have interior polyethylene that meets the ANSI ESD S11.11 surface-resistance requirement of <1 × 10¹¹ W for static-dissipative material, continued Mr. Swenson. However, claiming antistatic performance is more complicated. Understanding the triboelectric nature of the bag, requires a description of the contacting surfaces and the items in the bag. You must identify the contacting surfaces and the measurement of charge generated when they separate.¹

All bags are not created alike, but you can narrow the information gap by asking the hard questions. Ask for the performance data of the product. It is important because shielding protection can vary among suppliers and even from lot to lot. Reputable suppliers will have lot data.

Reference

1. Swenson, D., “Triboelectric Charge Generation – What’s Important? What Does It Mean?”, EOS/ESD Symposium Proceedings, 1992, pp. 209-217.

Copyright 1997 Nelson Publishing Inc.

October 1997