First Published in EOS/ESD
Technology June/July 1988
A Beginner's Guide
to ESD-Protective Containers
Properly selected handling and shipping containers
can limit ESD damage,
both to electronic devices and to your budget. Here are some
basic guidelines for container selection.
by Raymond P. Becker
President, Conductive Containers Inc.,
Static electricity has the potential
to cause billions of dollars in damage every year if it is not properly
controlled in areas where microelectronic parts are manufactured,
assembled, stored or shipped.
Both human beings and objects can generate
static charges, ad all microelectronic parts are sensitive in some
degree to the electrostatic discharge that can result.
While we cannot eliminate static electricity
(it is a part of nature), we can control it in at least three ways.
1. Reduce the potential for generating
static discharges by eliminating static generating material.
2. Shield static-sensitive parts and assemblies in uncontrolled
3. Ensure a safe path to ground to prevent accumulation of charge.
Because parts and assemblies must be
stored, moved, and shipped in a manner compatible with ESD control,
an important part of any ESD-control program is selecting containers
(e.g., bins, tote boxes, PCB racks, etc.) for ESD-sensitive parts.
This article offers a groundword on which you can build a set of
specifications for ESD-protective containers suitable to your application.
For any container to be effective it
must offer certain electrical, chemical, mechanical and value-related
|Fig. 1. Static-protective tote boxes come in
a variety of forms, ranging from low-cost, limited-lifetime
cardboard totes (a) to more durable cast- or molded- plastic
A suitable container must offer the
right combination of grounding, attenuation ad static-generation
Grounding is a connection to earth
to establish zero potential relative to ground. All materials can
be charged to some degree, but some dissipate this charge more easily
than others. Decay rate is the time required for charge to
dissipate almost completely.
Charge accumulated on highly conductive
objects will rapidly dissipate when the object is grounded. Insulative
objectives, by contrast, can hold charge for very long periods.
On materials that have electrical characteristics lying between
those of conductors and insulators, charge will decay according
to the material's surface resistivity, capacitance relative to ground
and contact resistance relative to ground.
The surface resistivity of a material
is a measure of the surface resistance of a square section of that
material, and it is translated in ohms per square, or W/square.
The accepted method of testing for
surface resistivity is ASTM MD257-78. The Electronic Industries
Association (EIA) categorizes materials by their surface resistivity
Electrostatic shielding materials: <104 W/square
Conductive materials: <105 W/square
Static-dissipative materials: 105 <1012
Insulative materials: > 1012 W/square
In general, the higher the resistivity
of a material, the longer will be the time required for a charge
to decay, and therefore, the slower the decay rate.
Capacitance relative to ground will
affect decay rate because high capacitance can increase the charge
a device sees, as shown by the formula: Q = CV.
Contact resistance is the resistance
between a container and ground; the (grounded) surface upon which
a tote or bin might rest, for example. For a fixed charge (voltage),
higher contact resistance translates into slower decay rates and
is affected by three factors:
Surface asperities (or roughness)
Pressure (or weight).
(Note that charge on materials in the
lower dissipative range can drop very quickly upon grounding because
the surface current is nonlinear when charge avalanche occurs due
to high E-fields.-- Ed.)
Contact area, and therefore contact
resistance, between a container and a grounded surface is usually
a function of a container's shape and size, but it also varies with
the quality of the container's surface.
Asperities, or surface roughness, make
it impossible for the entire underside of a container to contact
the surface upon which it rests. Thus, large surface-contact areas
are somewhat unusual. The insulative effect of particles such as
dust also tends to increase contact resistance.
The pressure a container presents to
the area upon which it rests affects contact resistance by forcing
more area into contact with a grounded surface. In the case of softer
materials, increased pressure will tend to make the container's
surface flow around smaller contaminate particles. Thus, higher
pressures or weights generally mean lower contact resistance and
faster decay rates.
In practice, the interplay of these
factors means that containers with a resistivity greater than 1010
W/square would have too slow a decay rate. Ideally, surface resistivity
should range between 105 W/square and 1010
Attenuation is the degree of
electrostatic shielding afforded by a container. It is defined as
the strength of field allowed to enter a container according to
EIA IS-5, 22.214.171.124 (Resistivity Property: Electrostatic shielding
"[Such materials] are capable
of attenuating an electrostatic field so that its effects do not
reach the stored or contained items and produce damage. An electrostatic
shielding material shall have a conductive layer with a surface
resistivity less than 1 X 104 W/square or a volume resistivity
of less than 100 W-cm."
Although the magnitude of the ESD protection
required will vary with the sensitive of the devices being protected,
play it safe: If there is the slightest possibility that a container
holding ESD-sensitive items could enter an environment where static
is uncontrolled, use a container that incorporates electrostatic
Also, it is important to note that
in situations where parts travel between controlled and uncontrolled
environments, nonshielding containers are not necessarily useful,
even part of the time. Although it might seem less costly to use
nonshielding containers in areas where ESD is controlled, some risk
is always involved, and parts and assemblies will have to be transferred
to shielding containers when material is moved into uncontrolled
Increased cost, risk and handling inconvenience
are the disadvantages of using both shielding and non-shielding
containers. Furthermore, the cost of a shielding container is far
less than the cost associated with damaged parts or extra handling.
Charge generation is the result
of two surfaces coming in contact and separating from each other.
Since containers rub against various other materials, charge generation
is an important issue.
Unfortunately, very little work has
been done on charge generation characteristics in containers. Among
the best references on charge generation are B.A. Unger's "Evaluation
of Integrated Circuit Shipping Tubes" and "Triboelectric
Characterization of Packaging Materials," from the Proceedings
of the 1981 EOS/ESD Symposium.
Until a specific method is developed
for repeatably measuring container charge generation, a simple comparison
method might be used. Rubbing a sheet of synthetic materials, such
as polyvinyl, against a variety of containers can give a rough,
subjective feel (literally) for the containers' charge-generation
properties. Ideally, a container should have a low propensity to
In summary, a container's preferred
electrical characteristics would include:
A surface resistivity of 105 < 1010 W/square
Inclusion of a layer with surface resistivity of < 104
W/square for electrostatic field attenuation
A limited tendency toward charge generation.
|Fig. 2. Anti-ESD boxes for semiconductor devices
come in sizes small enough to hold a single integrated circuit
(a), and with enough length and volume for volume shipment or
storage of ICs in their storage tubes (b).
Two basic concerns relate to the chemical
composition of a container: its effect on the product and its effect
on the environment.
Product compatibility means that a
container should neither alter the performance or reliability of
the components it houses nor have an adverse effect on personnel
or the environment.
Fortunately, a specification calling
for neutral pH will eliminate most concerns, although individual
testing is recommended to measure the full environmental effect
of a container material.
(Chemical effects can be subtle; for
an example, see "Here's an Irony: Protective Bags Contaminated
Circuits," EOS/ESD Technology, Dec/Jan 1988, p. 15.--
Size, style and durability are the
primary mechanical characteristics of a container. Determining these
characteristics is more than just a matter of selecting a container
big and study enough to hold the devices in question.
Size and style affect the efficiency
with which plant space is used, as well as compatibility with automated
handling equipment and other factors. A container significantly
larger than the device(s) it must hold during transport or storage
can increase labor cost and reduce work area. So too can a container
that is too small to hold a reasonable number of items.
A container should be designed for
the items it will contain as well as for the handling system in
use. Style is determined by the area of use and also by the handling
system. In a static-controlled environment a cover may be optional,
while in an uncontrolled area it is a necessity. Also, a cover is
required by the military and by many manufactures when the item
contained is not being worked on. This is good practice in general,
and prevents damage that might arise out of casual contact.
In some handling situations, a removable
cover may be necessary for automatic insertion and removal of parts.
In others, where assured closure is needed, an attached cover would
be the better choice. It is also important to take into account
whether a container is going to be used vertically, horizontally
or both ways.
Durability requirements are also determined
by the application. Passive applications such as storage require
less durability than intra-plant movement and shipment of parts.
A one-way container will have a short, intense lifetime, but must
be inexpensive enough to make its disposal affordable.
Generally, mechanical requirements
are a function of manufacturing, shipping and storage environments,
and containers should be selected to conform with them.
||Fig. 3. These printed circuit board
boxes illustrate a space-saving approach to ESD-protected storage
and shipment. The PCBs in the photo above are stored on one
of their shorter edges, minimizing the amount of floor or shelf
space required to hold them.
From the foregoing, we can draw some
general conclusion about the characteristics of cost-effective ESD-protective
containers. A cost-effective ESD-protective container should:
Have a surface resistivity between 105 and 1010
Offer an electrostatic-field attenuation through inclusion of a
layer with surface resistivity < 104 W/square
Have a limited tendency toward charge generation
Be chemically inert (i.e., pH about 7.0)
In addition, the containers' structures
must conform to the environments in which they will be used, to
the devices that will be carried within them and to the handling
equipment with which they will be used. Finally, the containers
selected must offer an adequate ratio of price to performance. With
this guidance and intelligently defined criteria, you can ensure
that the containers you purchase conform to the needs of your ESD-control