Would you like paper or plastic bags for your groceries? This question from the checkout clerk begs a simple answer. Either type of bag will provide adequate protection for your cereal, soup, frozen foods, and bread. Possibly the capability to recycle one or the other enters into the equation, but your choice is no earth-shattering decision.
The question that was simple at the grocery store becomes quite complex when it relates to handling and storage bags in your electronic assembly area. You can opt for metallized, static-dissipative, or antistatic containers for your components and subassemblies. If you select the metallized bags, your choices are metal-in, metal-out, or moisture vapor barrier.
Long-term protection from both ESD events and moisture is a consideration. Then there’s the matter of identifying the contents visually or with a bar-code reader. Also, the bag must be durable, not for just a few minutes but for days, weeks, or even months—especially if it must be sealed, opened, and reused. If you will use the bag in a clean room, this raises the question of contaminants.
And like the grocery bag, you must consider how your static-protection container will be discarded when it reaches the end of its useful life. Many users are driven by government regulating agencies to find 100% recyclable bags. When you have a simple, one-material product, this is easy. However, an effective antistatic bag requires a polyethylene layer and a metallized layer, and the combination of materials makes it very difficult to recycle the finished product.
Finally, unlike the grocery bag, you must consider cost. These bags aren’t free, you know.
This article addresses the basic types of ESD control bags on the market today. The accompanying comparison chart looks at the thicknesses, sizes, construction, and special features of individual bags.
Even though there are some variations, most bags fit into one of three generally accepted categories:
- Metal-in (MI), the most commonly used of all bag types, has an outer layer of dissipative polyester and an inner layer of dissipative polyethylene. Between them is a layer of aluminum. It is superior in durability and less expensive than metal-out (MO) construction.
The ESD protection, not quite as good as in the MO bag, is satisfactory for most applications. The aluminum minimizes the generation of triboelectric charges when items move around inside. Typically, it has about 40% light transmission, which makes it possible to identify the contents visually although it may be difficult to read the bar code.
- The MO bag has an abrasion-resistant coating, then a layer of aluminum or nickel. The third layer is polyester followed by an inside surface of dissipative polyethylene. Transmission of light generally is greater than 40%. When placed on a dissipative mat or other grounded surface, an ESD charge reduces more rapidly on an MO bag than on an MI bag, a feature that makes the MO bag quite popular.
- Moisture vapor barrier (MVB) bags were developed to protect today’s surface-mounted components. Before the film for MVB bags was available, moisture had to be baked out of surface-mount components before they were exposed to high soldering temperatures. With MVB bags, components are stored virtually free of moisture, contributing to higher yields and better reliability. The price is higher than with other bags; but for many users, protection outweighs price.
There are two types of four-layer MVB construction. One uses a dissipative outer layer such as nylon over layers of aluminum, polyester, and dissipative polyethylene. The other has similar third and fourth layers, but the outer layer is dissipative Tyvek™ and the second layer is more resistant to punctures. The contents of the typical MVB bag are not visible, although a translucent MVB product can be obtained.
Clear dissipative bags provide tribocharge and contamination protection. Films used in clear dissipative bags are permanently dissipative and made of polyethylene and ethylene vinyl acetate or 100% recyclable polypropylene. Typically used for storage of less sensitive components, the bags can be ultra cleaned for use in a clean room. Most of these products are clear enough for the contents to be easily identifiable.
Black conductive, the first type of static-shielding bag developed and the least expensive, is constructed of carbon-loaded polyethylene with a conductive surface. There are some disadvantages: you can’t see through it, and the carbon coating tends to be too contaminating for many clean-room applications.
“The sensitivity of your products to static damage will determine the optimum type or types of bags for your application,” said Dave Bermani, corporate marketing director at Desco Charleswater. “Puncture resistance, seam strengths, electrical properties, and sizing are very significant for storage of subassemblies with sharp corners. Details on rugged bags are available from the manufacturers.”
The properties that are most important for one application may not be relevant for another. Each must be evaluated carefully. “Some trade-offs may be necessary. For example, you are not going to find the best moisture barrier and greatest transparency in the same type bag because opaque materials provide better moisture barrier protection than clear materials,” explained Todd Somers, manager of BayStat.
An Unusual Application
The typical bag is used for relatively small components or subassemblies, and the contents remain in the bag for just a few weeks at the most. An interesting exception is the FR2192 family of heavy-duty bags. According to Jim Hiller, general manager of Specialty Bags, these bags conform to MIL-B-81705B, Type I and MIL-B-81705C, Types I, II, and III. They have metallized Tyvek on the outside, and aluminum foil forms a conductive inner layer with a resistivity of 104 W or lower. Boeing uses the large bags to store bombs, and the design life is 20 years with no ESD-related problems.
Driving Forces in the Industry
While there haven’t been any major breakthroughs in protective film in recent history, there are some relatively new constructions such as air-cell bags and pouch bags. Also, clean-room bag manufacturers are working to reduce particle contamination, ionic contamination, organics, and outgassing in their offerings.
“The disk-drive industry is the biggest driving force in static-control packaging today,” said David Swenson, technical service specialist at the Electronic Handling and Protection Division of 3M. “Disk-drive manufacturers demand low contamination with extremely good static control. This stretches the limits of material availability, especially since costs must be kept at a reasonable level.”
Published by EE-Evaluation Engineering
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