Does ESD-Control Packaging Provide Adequate Protection for Telecommunications Equipment?

Proper packaging of electrostatic discharge sensitive (ESDS) printed circuit boards (PCBs) is an undisputed requirement for any ESD-control program. With a myriad of packaging products on the market, what do customers need to consider when selecting ESD packaging?

This was the question that confronted the Bell Operating Companies when they got together in early 1992 to formulate a packaging wish list. At that meeting, eight items relating to packaging were deemed high-priority: closed container, scanability, transparency, flame retardancy, stackability, mailability, reusability, and recycleability. This wish list was published by the Bellcore ESD Team.

As a follow-up to that meeting, a seminar was held at Bellcore TEC in April 1995. Invited were Bell Operating Company ESD Team experts, telecommunications equipment suppliers, packaging manufacturers, and ESD testing and certification companies. The 40 attendees were given the opportunity to voice their ESD packaging concerns. Many of the conclusions of this forum were used as a basis for the reissue of Bellcore GR-1421-CORE.¹

Discussion Topics

This is an abbreviated list of some of the hot topics raised at the seminar.

Is a totally enclosed container really required?

Totally enclosed containers for ESDS PCBs are necessary for three reasons:

To protect them from direct-contact ESD.

To provide physical protection.

To protect the circuitry from outside contamination.

Open-ended corrugated sleeves are not sufficient. Containers must have inner and outer surfaces that are static dissipative to prevent triboelectric charging of the container and assembly.

The static-dissipative properties of the container should remain active at low humidity (I2% RH) for long periods of time in dry climates. Some plastic clamshells and bags do not meet this requirement.

Should a package be sealed?

Containers should have a tamper-proof seal. An unsealed container is an unknown entity. It could contain a good repaired assembly or a defective one.

The contents of an unsealed container may have been handled by personnel who did not use ESD protection. The PCBs may be contaminated or have sustained physical damage.

Often, containers are opened by untrained storeroom personnel to verify that the outside symbols on the package match the product inside. No package should be opened by anyone without ESD-control training. Packages must be resealed with a new seal indicating that the storeroom personnel or technicians have opened them.

Can PCB bar codes be scanned reliably through a transparent window or package?

Bar codes were not designed to be scanned through a transparent medium.² The original wand-type LED scanners were meant to be used in contact with the bar- code over-laminate. As scanners improved, they worked when scanned in proximity to the bar code, but not through a window. With the introduction of laser scanners, some could read the bar code through a plastic package in 1 s to 3 s about 89 to 93% of the time.³

The real question is not whether bar codes can be decoded through a window, but can they be read according to a specified ANSI grade?⁴ The Bellcore standard is an academic “C” grade for a specified wavelength and aperture number to read a code 39 symbol.⁵ Reading the small code, 39 symbols can be difficult because of the minimum spacing of 0.010 inch. However, this grade guarantees that the bar code will be read correctly 100% of the time.

The ANSI bar-code grading system consists of four parameters: decodability, contrast, modulation, and defects. Eighty percent of the plastic clamshell packages tested failed modulation or defects, and consequently the “C” grade of the ANSI guidelines.³ Improvements are needed in the light-transmission properties of the plastic package.

Flammability?

Packaging flammability is important to the telephone companies whenever packaged PCBs are not stored in closed metal cabinets or separate areas away from active equipment.

What about over-packs for shipping individual containers?

Over-packs may be necessary for shielding and security reasons when shipping individual plastic containers. Also, some plastic containers are not rugged enough to withstand shipment in the mail.

Recycleability?

The issue of recycleability mostly involves laminating different materials. Some of the problems stem from the use of paper labels on plastic packages or the use of plastic windows in corrugated packaging. In the case of plastic containers with paper labels, removing the adhesive may be difficult and costly.

This problem could be solved by using plastic labels or designing the plastic container with a slot for a nonadhesive label. In the case of corrugated containers, openings can be fabricated that do not require an adhesive to hold the window in place.

Should packaging be static shielding?

Users usually do not know how sensitive a PCB may be to ESD when exposed to a changing electric field. Assume all electronics packaged in static-shielding containers are sensitive to varying electric fields and use the necessary ESD precautions. The supplier must identify the most cost-effective packaging with the best shielding properties.

Which is the package of choice: The corrugated container or plastic clamshell?

In some cases, plastic containers obviously are not rugged enough to package very heavy, bulky electronics. In other cases, either plastic clamshells or corrugated boxes can be used. Plastic containers may be less expensive than their corrugated equivalents, but is packaging cost an issue for a $1,000+ PCB?

Some end users insist on seeing the contents of containers. Others do not care or are concerned about security during shipping and do not want to expose the contents. Plastic or corrugated containers with transparent windows to provide reliable bar-code scanning reduce PCB handling by personnel.

A corrugated container, because of its ruggedness, may have more reuse cycles. An all-plastic container may be easier to recycle.

Future Packaging Requirements?

Packaging surveys done within the Bell Operating Companies showed that one half of today’s ESD packaging fails a three-parameter test:

Is the package totally enclosed?

Does the surface resistivity fall in the static-dissipative range?

Does the package have an ESD warning label?

This survey is of serious concern. It demonstrates that some engineers do not specify static-control packaging that meets current industry guidelines, and that some packaging is being sold to equipment suppliers who assume that it meets ESD requirements without testing it.⁶

Telephone-company procurement organizations tell us that equipment contracts will include GR-1421-CORE requirements for packaging. They are willing to pay for packaging quality, reliability, recycleability, and reusability.

References

1. GR-1421-CORE, Generic Requirements for ESD-Protective Circuit Pack Containers, Issue 2, June 1995, Bellcore Documents, (800) 521-2673.

2. Private communication with A. Gilligan, AT&T, developer of the bar code.

3. Bellcore and Applied Image, a study funded by Bell Atlantic.

4. American National Standard for Information Systems-Bar Code Print Quality-Guideline, ANSI X3.182-1990, American National Standards Institute.

5. Generic Requirements for Common Language^®, Bar Code Labels, Figure 4, Bellcore, TR-STS-000383, Issue 5, January 1991.

6. ESD ADV-2.0-1994, ESD Association Advisory for Protection and Sensitivity Testing of Electrostatic Susceptible Items-Handbook, Section 6, ESD Association.

About the Authors

Gene Chase is a technical consultant for Electro-Tech Systems. During his career, he also was affiliated with Bell Laboratories and Bellcore, serving as chairman of the Bellcore ESD Team. Mr. Chase received B.S.E.E. and M.S. degrees from the New Jersey Institute of Technology. Electro-Tech Systems, 3101 Mt. Carmel Ave., Glenside, PA 19038, (215) 887-2196.

Art Varga is manager of static-control and flame-retardant products at Century Container. He is a NARTE-certified electrostatic discharge control engineer and was a founding member of the Bell Communications Research ESD Control Team. Mr. Varga is an active member of the ESD Association and president of the association’s Midwest Chapter. Century Container, 300 N. Mannheim Rd., Hillside, IL 60162, (708) 450-0600.

June 1997