Jud
Strock, Technical Editor, reported about ESD control bag types in the October
2000 issue of Evaluation Engineering.
Choosing
an ESD bag is not as simple as choosing paper or plastic at the grocery story.
An ESD bag must provide protection from moisture and ESD events and must protect
from contaminants if used in a clean room environment. It must be durable, lasting
for days, weeks, or even months especially if it is to be sealed, opened, and
reused. There is also the aspect of responsible recycling as required by government
agencies. An effective antistatic bag requires a polyethylene layer and a metalized
layer. This combination of materials makes recycling more difficult. Also, unlike
grocery store bags, ESD bags are not free; and cost plays an essential role when
choosing the right ESD bag for your electronic assembly needs.
There
are three generally accepted categories of ESD bags:
- METALIZED:
Metal In (MI) is the most commonly used bag type.
It consists of an outer layer of dissipative polyester and an inner layer of dissipative
polyethylene with a layer of aluminum between. This bag is superior in durability
and more cost effective than the metal-out (MO) construction. The ESD protection
is satisfactory for most applications but not as good as the MO bag. The aluminum
minimizes the generation of triboelectric charges when items move around inside.
Light transmission is at about 40%, which permits visual identification of the
contents but not easy readability of the bar code.
MO bag has an abrasion-resistant coating, then a
layer of aluminum or nickel with polyester as the third layer followed by an inside
surface of dissipative polyethylene. Transmission of light is generally greater
than 40%; and when placed on a dissipative mat or other grounded surface, an ESD
charge reduces more rapidly using the MO bag versus the MI bag.
MOISTURE VAPOR BARRIER BAGS were developed to protect
surface-mounted components. Previously, before surface-mount components could
be exposed to high soldering temperatures, the moisture had to be baked out. However,
with the development of the MVB, components are stored virtually moisture free.
These bags are more expensive than other bags but protection outweighs price for
many users. There are two types of 4-layer MVB construction. One type consists
of a dissipative outer layer such as nylon over layers of aluminum, polyester,
and dissipative polyethylene. The other type has similar third and fourth layers
but the outer layer is dissipative Tyvek ™, and the second layer is more resistant
to punctures. A translucent MVB product is available; but typically the contents
of an MVB bag are not visible.
- CLEAR DISSIPATIVE: The clear, dissipative
bags can be ultra cleaned and used in a clean room. Contents can be easily identified.
They provide tribocharge and contamination protection. Permanently dissipative
clear bags are used for storage of less sensitive components and are composed
of polyethylene and ethylene vinyl acetate or 100% recyclable polypropylene.
- BLACK
CONDUCTIVE: Constructed of carbon-loaded polyethylene with a conductive
surface, black conductive was the first type of static-shielding bag developed
and is also the least expensive. Disadvantages are: It is opaque and the carbon
coating is not suitable for 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."
Determining
the bag most relevant for your needs may require careful evaluation. "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.
Boeing
uses the FR2192 heavy-duty bags to store bombs and the design life is 20 years
with no ESD-related problems. These bags conform to MIL-B81705B, Type I and MIL-B-81705C,
Types I, II, and III. They have metalized Tyvek on the outside. Aluminum foil
forms a conductive inner layer with a resistivity or 10 4 !& or
lower.
FOR MORE INFORMATION
concerning ESD bags, visit EE website evaluationengineering.com and access the
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