Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

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.,
Northbrook, IL

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. We can:

1. Reduce the potential for generating static discharges by eliminating static generating material.
2. Shield static-sensitive parts and assemblies in uncontrolled environments.
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 characteristics.

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 models (b).

Electrical Characteristics

A suitable container must offer the right combination of grounding, attenuation ad static-generation characteristics.

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 as follows:
Electrostatic shielding materials: <104 W/square

Conductive materials: <105 W/square

Static-dissipative materials: 105 <1012 W/square

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)

Contamination

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 W/square.

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, 2.2.2.1 (Resistivity Property: Electrostatic shielding materials):

"[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 shielding.

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 areas.

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 generate charge.

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).

 

Chemical Characteristics

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.-- Ed.)

Mechanical Characteristics

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.

Summary

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 W/square

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 program.

The ESD Journal is not affiliated with any trade organization, Association or Society

ESD Journal & esdjournal.com are Trademarks of Fowler Associates, Inc. - All Rights Reserved

The content & Look of the ESD Journal & esdjournal.com are Copyrighted by Fowler Associates, Inc. - All Rights Reserved Copyright 2011

The YouTube name and logo are copyright of YouTube, LLC.