ANSI/ESD S541: ESDA Releases Critical New Packaging Standard

David E. Swenson

The new packaging standard updates test methods and harmonizes specifications with globally accepted standards.

One of the most important standards in the static control world was EIA 541-1988, "Packaging Material Standards for ESD Sensitive Items." Please note the word was in the previous sentence. This venerable and often-referenced standard was canceled by ANSI in 1999 because it had not been updated since its introduction in 1988. ANSI documents are supposed to have a review every five years at which time they are either revised or formally reissued. Neither happened with EIA-541 through two complete official review cycles.

In 1998, to address the need for an updated standard, the ESD Association decided to develop a new packaging material standard. The result in final form is ANSI/ESD S541-2003, "ESD Association Standard for the Protection of Electrostatic Discharge Susceptible Items–Packaging Materials for ESD Sensitive Items."

History

In the late 1980s, the Electronics Industry Association (EIA) task team responsible for EIA-541 asked the ESD Association (ESDA) for assistance in developing necessary new test methods to support the changes that were already known and understood for the future revision of EIA-541. Of significance is that this request came only one year after the release of the original EIA-541 document.

Test methods and explanations in EIA-541 were lacking in several areas, notably surface and volume resistance of items, understandable electrostatic shielding, and a technical as well as practical discussion of antistatic behavior. The team recognized that application guidance was needed in any new standard. The original document provided only definitions, with no information relevant to the use of materials in any particular protection scheme.

ESDA formed a Packaging Program Development Team under the auspices of the Association Standards Committee in 1990. The first efforts were dedicated to development of test methods to determine surface resistance and volume resistance of materials and items fabricated from those materials. The test methods in Table I taken from ANSI/ESD S541 are a result of this sustained effort over a 12-year period.

The introduction of ANSI ESD S20.20, "ESD Association Standard for the Development of an Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices)" presented an opportunity to introduce a supporting ESD packaging material standard. Packaging is discussed in general in S20.20, but detail is intentionally omitted because options are acceptable in any given ESD control program plan. One of the goals for ANSI/ESD S541 is to provide guidance on when and where to use various types of packaging material.

The Fundamentals of Packaging

ANSI/ESD S20.20 states that ESD protective packaging must be defined for movement of ESD-susceptible items within a protected area, between job sites, and in field service operations. The mission of ANSI/ESD S541 is to assist in the determination of the type of package to use in each of the situations described in S20.20.

At the time of introduction, the original EIA-541 document provided much-needed definitions for material types. Of particular importance is the definition for antistatic, which appears in the foreword of the document: Antistatic no longer refers to a resistivity range. In this standard, antistatic refers to a material's ability to resist triboelectric charge generation. Even though more than 15 years has passed, many practitioners continue to misuse the term antistatic. ANSI/ESD S541 reinforces the original definition, and everyone involved in the creation of the new standard now hopes the definition will finally stick after all these years.

Dissipative and conductive materials are defined in EIA-541 using ASTM-D-991 and ASTM-D-257 as appropriate for the type of material. Materials with bulk conduction properties used D-991, and materials with surface conduction properties generally used D-257. Numerous papers, magazine articles, and telephone and hallway discussions have occurred over the years because of confusion over test methods for the dissipative range of materials.

The latter method has shortcomings that are well documented. These shortcomings will not be discussed further in this article except to say that they are the main reason for the work that led to the development of ANSI/ESD S541 and precipitated the development of all the additional new test methods.

EIA-541 only defines material and packaging forms, and it provides little application assistance. ANSI/ESD S541 offers considerable assistance in the use of various packaging material forms. Several figures in the S541 document aid in understanding sensitive material movement within any conceivable work environment. Figure 1 shows the application of ESD packaging materials within a generic manufacturing process.

Figure 1. Application of ESD packaging properties throughout the packing and shipping process.  (Click on image above to enlarge it)

Packaging within areas that meet the requirements of an ESD protected area, as defined by S541, includes low charging (a new and better way to describe antistatic) and dissipative. Shielding materials are considered optional. For movement of sensitive items outside of an ESD protected area, these materials and electrostatic discharge shielding are required.

Figure 2 describes the ESD protected area options in a bit more detail. Within a protected area, limited protective packaging may be required depending on the actual handling practices and materials present in that area. Moving ESD-susceptible parts outside of a protected area requires the development of a packaging scheme that protects parts to an appropriate level. The end-user must determine that level, but the guidance offered in ESD S541 should help.

Figure 2. An example of ESD protected area (EPA) configurations. UPA represents unprotected areas.  (Click on image above to enlarge it)

The most important ideas behind the state-of-the-art packaging scheme defined by ANSI/ESD S541 are the following excerpts from paragraphs 6.1 and 6.2:

• The surfaces that contact sensitive items shall be low charge generating and dissipative or conductive –required inside an EPA as well as outside of an [ESD protected area].

• For movement outside an [ESD protected area], the packaging structure must provide electrostatic discharge shielding.

With these simplistic concepts in mind, it should be relatively easy to understand the packaging concepts required to satisfy both ANSI/ESD S20.20 and ANSI/ESD S541.

Application Assistance

The appendices of ANSI/ESD S541 provide considerable guidance that should aid in choosing packaging materials for a given application. Having an idea of the environment that susceptible parts will travel through is very important. In addition, understanding the sensitivity level of the parts helps determine the type of packaging required for the shipping or storage application.

Environmental considerations are the purview of the packaging engineer. Selection of packaging materials with an understanding of the electrostatic issues adds another facet to what a packaging engineer must understand. Physical considerations such as cushioning, puncture and tear resistance, and moisture transmission rate, to name a few, may have an effect on ESD protection because of the materials involved. Often the packaging engineer may have to give up some specific ESD protection capability to maintain a physical protection attribute that seems more important. For example, perhaps an application requires a specific dynamic cushioning profile and the designer cannot locate a low-charge generating cushioning material with the same specifications.

For the most part, the physical concerns will outweigh the ESD concern because the perceived potential for damage is greater from physical trauma.1 As long as the material used does not contribute to excessive charge generation, it is likely that everything will be fine in terms of shipping the sensitive part.

Understanding the sensitivity level of the handled parts is also very important. Adhering to the concepts of ANSI/ESD S20.20, an ESD control practitioner can design a safe handling program for 100-V human body model (HBM) sensitive parts (as a minimum). The packaging concepts of ANSI/ESD S541 should protect any conceivable part made today if used correctly. If the part sensitivity is not known, the most often used scenario is to assume 100-V HBM sensitivity as a starting point, remembering the state-of-the-art concepts listed earlier for packaging design.

Looking at a specific example, a common packaging material today is tape and reel. A base plastic has formed pockets that hold parts in a serial fashion. The stored parts are held in place with a cover tape. S541addresses material selection. The pocket tape component is readily available in a conductive form. This is necessary to provide some level of dissipation and perhaps even shielding (if conductive enough).

Of concern is the often-used clear tape that covers the pocket tape to hold the parts within the pockets. This material needs to have low-charge-generation characteristics during the application to the pocket tape as well as during the removal in automated handling equipment. It is well known that poor ESD performing cover tape can cause parts to jump out of the pockets during removal of the tape (very high triboelectric charge generation) and perhaps even contribute to direct ESD damage. At a minimum, the cover tape must have low-charging characteristics to prevent part movement and to reduce any chance of inductive charging of the parts.

Although not directly a packaging concept, the idea of equipotential bonding has applicability in the packaging of components. Conductive foam or other structures are available that can be used to shunt leads of components together. Shunting of component leads in this way can significantly reduce ESD susceptibility. Although not a perfect protection scheme, shunting can often provide a significant level of protection that is easy and inexpensive to apply. It may also open up other packaging options when ESD control materials are insufficient for cushioning, vibration protection, or other physical considerations.

It is worth repeating that the material that contacts an ESD- sensitive item needs to be low charging and dissipative–at least dissipative–to allow charge to spread out. In a practical sense, reduction of high-charge concentration limits the possibility of a sudden discharge, so this is a very important concept.

Device Damage Considerations

ANSI/ESD S541 includes a short but important discussion regarding device damage. Two scenarios exist that must be considered: discharge to a device and discharge from a device. This elegantly simple approach should actually assist many of those tasked with making packaging decisions in understanding what they have to do to protect parts.

To discharge to a device (using HBM or machine model), a charged conductor must be involved. The charged conductor may be a person, package, conveyor, cart, or some sort of fixture or tool. Preventing the discharge is the defined goal of procedural standards such as S20.20. Protection from an external discharge is a defined goal of an appropriate packaging scheme.

Discharge from a device is a potentially serious problem if a device is allowed to get charged up. Addressing this problem (not charging up devices) should also be within the defined goals of an ESD control program. If devices get charged, it is very difficult to remove the charge without damage. There are several schools of thought on this area and plenty of controversy. From many perspectives, the use of ionized air may be the best means available to safely neutralize the charge on a charged device. Grounding a charged device, even through high resistance, is problematic at best. A small capacitance device with a high charge (regardless of how it was obtained) brought into contact with a grounded surface (regardless of surface resistance and resistivity) will not lose the charge.2,3,4 Therefore, it is imperative in any static control program to reduce the likelihood of charging parts in the first place. Fortunately, devices can withstand a relatively high level of charging and subsequent discharge without sustaining damage. There are several (ancient) papers that discuss this area in depth that an interested reader might reference.3,4

ESD Packaging and Material Types

ANSI/ESD S541 Appendix E contains a discussion covering many of the packaging forms or related process items that may find application in an ESD control program plan. This section should aid program designers by providing general guidance in selecting materials or at least assist in understanding what might be available in a generic sense.

Conclusion
ANSI/ESD S541-2003 is the much-anticipated new ESD packaging standard. The new standard provides much guidance for selecting packaging materials for the appropriate applications. The new ANSI/ESD S541 document is available free from ESDA at http://www.esda.org. It is an important document for anyone interested in ESD and ESD protective packaging. The concepts established in the original EIA-541 are supported, test methods are updated, and specifications are harmonized with globally accepted standards.

References

1. DE Swenson, "Improved Standards for Specifying Static Control Materials," Compliance Engineering 17, no. 4 (2000): 40-44.

2. JR Huntsman, "Triboelectric Charge: Its ESD Ability and a Measurement Method for Its Propensity on Packaging Materials," in Proceedings of the EOS/ESD Symposium, Rome, NY: ESD Association, 1984.

3. DE Swenson and NP Lieske, "Triboelectric Charge-Discharge Damage Susceptibility of Large Scale IC's," in Proceedings of the EOS/ESD Symposium, Rome, NY: ESD Association, 1987.

4. DE Swenson and R Gibson, "Triboelectric Testing of Packaging Materials: Practical Considerations--What Is Important? What Does It Mean?" in Proceedings of the EOS/ESD Symposium, Rome, NY: ESD Association, 1992.

David E. Swenson recently retired from 3M after 35 years. He and his wife Geri have established a new company, Affinity Static Control Consulting LLC (Round Rock, TX). Swenson can be reached at 512-244-7514 and via e-mail at static2@swbell.net.