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First Published in EOS/ESD Technology Europe Spring, 1990 The Basics of ESD-Protective Containers Properly selected shipping and handling containers can limit ESD damage, both to electronic devices and to your budget. by:
Raymond P. Becker 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 (i.e., bins, tote boxes, and PCB racks) for ESD-sensitive parts. This article offers a base upon which you can build a set of specifications suitable to your application for ESD-protective containers. For any container to be effective, it must offer certain electrical, chemical, mechanical, and value-related characteristics.
Electrical Characteristics A suitable container must offer the right combination of grounding, attenuation, and static-generation characteristics. Grounding is a connection to earth to establish zero potential relative to ground so that charge can be conducted away or dissipated. Decay rate is the time required for charge to dissipate almost completely. Charge accumulated on a highly conductive object will rapidly dissipate when the object is grounded. Insulative objects, by contrast, can hold charge for 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. The surface reistivity of a material is a measure of the surface resistance of a square section of that material, and it is given in ohms per square. The accepted method of testing for surface resistivity is ASTM MD 257-78. The United States' Electronic Industries Association (EIA) categorizes materials by surface resistivity as follows:
In general, the higher the resistivity of a material, the slower the decay rate. Capacitance relative to ground also 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-- in this case, the (grounded) surface upon which a tote box or bin might rest. For a fixed charge (voltage), higher contact resistance translates into slower decay rates and is affected by three factors:
[Note: Charge on materials is 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.] 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 contaminating particles such as dust or debris also tends to increase contact resistance. The pressure a container presents to the are upon which it rests affects contact resistance by forcing more area into contact with a grounded surface. In the case of softer material's, 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 of >1010 W/sq. would have too slow a decay rate. Ideally, surface resistivity should range between 10 5 and 1010 W/sq. 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-541, Paragraph 2.2.2.1 (Resistivity Property: Electrostatic
shielding material): Although the amount of ESD protection required will vary with the sensitivity 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 in which static is uncontrolled, use a container that incorporates electrostatic shielding. Note that in situations requiring parts to travel between static-controlled and nonstatic-controlled environments, non-shielding containers may not be useful, even part of the time. Although it might seem less costly to use nonshielding containers in ESD-controlled areas, some risk is always involved. Parts and assemblies must be transferred to shielding containers when material is moved into uncontrolled areas, and such repackaging increased both risk and expense. Increased cost, risk, and handling inconvenience are the disadvantages of using both shielding and nonshielding containers. Furthermore, the cost of a shielding container is far less than the cost associated with damaged parts of extra handling. Charge generation is the result of two surfaces coming in contact with and separating from each other. Since containers rub against various other materials, a package's tendency to charge is an important characteristic. Unfortunately, little work has been done on the charge-generation characteristics of 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," (Ref. 1,2). Until a specific method is developed for repeatably measuring container charge generation, a simple comparison method might be used. Rubbing a sheet of synthetic material, 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 charge. In summary, a container's preferred electrical characteristics would include the following:
Chemical Characteristics Two basic concerns relate to the chemical composition of a container: the composition's effect on the product and its effect on the environment. Product compatibility means that a container should not alter the performance or the reliability of the components it houses, nor should it have an adverse effect on personnel or the environment. Fortunately, an acidity/alkalinity specification calling for neutral pH will eliminate most concerns, although testing of individual package types is recommended to gauge the full environmental effect of a container material. External Mechanical Characteristics Size, style, and durability are the primary mechanical characteristics of a container. Although they sound straightforward, these characteristics involve more than just picking a container big and sturdy enough to hold the devices in question. Size and style impact the efficiency with which plant space is used, as well as compatibility with automated handling equipment. A container significantly larger than the device(s) it must hold during transport or storage can increase labor costs and reduce available work area. So too can a container that is too small to hold a reasonable number of items. Thus, a container should be sized for the items it will contain and the handling system in use. Style is determined by the are of use and also by the handling system. In a static-controlled environment a cover may be optional, while in an area without static control, it is a necessity. A cover is also 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 to allow automatic insertion and removal of parts. In others, where assured closure is needed, an attached or hinged 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. Durability requirements are also determined by the application. Passive applications such as storage require less durability than intraplant movement and shipment of parts. A one-way shipping container must be inexpensive enough to make its disposal affordable. Internal Mechanical Characteristics As important as the container itself are the internal components of the container. Using materials that are not compatible with the container can, and probably will, defeat the protective features of the container. The same considerations used to select the container should be used to select the internal components, with a few exceptions. Since ESD-sensitive parts can be in intimate contact with the internal surfaces of the container, special consideration must be given to electrical, chemical, and mechanical characteristics. Assuming a shielding container is used, it is not longer necessary to use additional shielding inside of the container. In fact, there is a consensus that relatively high resistivity on the inside of the container is preferable. The view is supported by the capacitive probe test indicating that containers with lower resistivity on the outside than on the inside have greater attenuation. The theory is that the static charge takes the path of least resistance around the outside, to ground. Therefore, partitions and cushioning foam are preferred with resistivities in the static-dissipative range. Partitions with static-dissipative film covering their conductive surfaces have recently been introduced to the market. While still somewhat expensive, such partitioning does, in fact, raise the resistivity of the partition set, and in large enough quantities can be less costly than individually bagging sensitive components. In addition, the coating increases the life of a partition, enhancing multiple use when the partition is coated on all exposed sides. This overall coating can be accomplished inexpensively by using a "rolled over" partition instead of a "straight up" or "standing" partition that otherwise might tend to tear and shed particulates. Summary From the foregoing, we can draw some general conclusions about the characteristics of effective ESD-protective containers. An effective ESD-protective container should: 1. Have
a surface resistivity between 105 and 1010
W/sq., A container's structure must also conform to the environment, to the devices carried within it, and to the handing equipment with which it will be used. Finally, the container selected must offer an adequate ration 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. References 1. BA Unger, "Evaluation of Integrated Circuit Shipping Tubes," 1981 EOS/ESD Symposium Proceedings, EOS/ESD Association, Rome, NY. 2. BA Unger, "Triboelectric Characterization of Packaging Materials," 1981 EOS/ESD Symposium Proceedings, EOS/ESD Association, Rome, NY. |
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Electrostatic
shielding materials: < 104 W/sq.
A
surface resistivity of 105 < 1010
W/sq.,