PAPER, PLASTIC, OR ESD-SAFE
By: Constance Harness
Jud Strock, Technical Editor, reported
about ESD control bag types in the October 2000 issue of Evaluation
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
There are three generally accepted
categories of ESD bags:
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.
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
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