ESD Products and Equipment
An ESD Packaging Compendium
by Gene Chase
Mar 15, 2007

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Improper packaging of ESD sensitive (ESDS) components, assemblies and equipment resulting in hard and soft failures has cost both manufacturers and users millions of dollars, maybe even billions. This is because most suppliers and customers do not understand how (ESDS) items can fail.

In the late 1970s when informed of the ESD issue, most suppliers and users were skeptical that ESD could damage electronic devices or assemblies (I recall a device engineer telling me, in the same time frame, that there was no way static electricity could damage a bipolar transistor). One of the Bell System’s first experiences with ESD was in 1980 at a Western Electric Company assembly facility that was using 8Kb-EPROMs (electrically programmable random access memories) costing $32.00 apiece. A call was received at Bell Laboratories for help because many of these EPROMs were failing in the reprogramming operation.

One of the workers noticed that most of the EPROMs that were failing were stored and transported in white styrofoam trays. However, EPROMs stored and transported on black conductive foam sheets had fewer failures. The EPROMs were piggy-backed with leads soldered, one on top of the other with the top EPROM’s chip select pin soldered to a 4-inch long dangling wire. This wire was a great antenna for EMI and ESD and, because styrofoam can very easily accumulate large amounts of static charge, it presented a classic ESD problem. Styrofoam can typically charge to thousands of volts and, being an insulator, can hold this charge for long periods of time.

In this situation, it was likely that just waving the EPROM across the charged styrofoam tray or taking it in and out was enough to damage it. That’s because the most likely EPROM ESD failure mechanism would have been attributable to the charged device model (CDM). Charge residing on the styrofoam tray was induced into the EPROM. Damage occurred when one of the EPROM pins touched grounded. Even if the operator wore a wrist strap, the problem would not have been solved.

As a result of the visit to this facility, operators were instructed to dispose of all styrofoam from the work areas. Unfortunately, even now styrofoam is still sometimes used allegedly to protect an integrated circuit or transistor pins from mechanical damage. In addition, you often see a styrofoam coffee cup in a work area where it does not belong. A styrofoam cup, even full of coffee, can be a significant charge generator because the charge that is on the outside of the cup stays, regardless of whether the person holding the cup is grounded or how much coffee was left.

When Unger, et. al. [1] demonstrated that devices could fail from sliding in a plastic shipping tube, dropping with pins hitting a grounded metal table, some were in disbelief. How could this happen? This was the CDM in action. What made the CDM so devastating was the speed of the discharge,
sub-nanoseconds, and therefore the peak current, often amperes. As a matter of fact, when the component’s pins touched the metal table, only the charge on the metal leads was discharged. The trapped charged on the insulating parts of the package remained and could re-induce charge to the metal parts of the component. So, if the component was not damaged the first time its pins touched the metal table, it could still be damaged when picked up and dropped a second time, even by a person holding the plastic shipping tube wearing a grounded wrist strap. (Appendix B of EIA Standard-541 shows a method to measure the charge on an item exiting from a shipping tube [2].)

Plastic shipping tubes with slippery static dissipative inside coatings help to limit the charging of devices while sliding in the tubes. Conductive tubes can be risky, particularly if held by an ungrounded person.

What Are the ESD Damage Models?
In order to understand the importance of packaging to protect an item from ESD, manufacturers and users need to understand the ESD models. For years, many thought that all ESD damage came from charged persons touching and inducing charge in ESDS items. This is the Human Body Model (HBM) and, unfortunately, even with the knowledge of how to control it, it is still a major cause of ESD failures. However, in today’s manufacturing environment, machines do a lot of the handling of ESDS items and their role in causing ESD failures is much more complicated than the HBM, and needs to be understood.

Most testing of even complicated digital signal processors is done with automatic test handlers. Every time an item slides, contacts and separates from a surface, it can become charged. This is called triboelectric charging and its theory is not well understood, in particular for organic materials such as plastic packaging [3]. In the past there were serious problems with components sliding on insulating (anodized) surfaces in an automatic test handler, becoming charged and being discharged in the test head by the CDM. Similarly, contact electrification or tribocharging must be considered when choosing packaging. (Donn Bellmore presented his work at the 2001 EOS/ESD Symposium on determining the ESD characteristics of anodized aluminum surfaces used in automatic handling equipment [4].)

Every time a component or assembly is placed-in or removed from a package, whether slid into a static shielding bag, placed in a plastic clamshell or a corrugated container it can become charged. If already charged, it also can become discharged. At one time it was thought that a conductive container was the final answer to ESD control. It would be the perfect shield from direct ESDs and the ultimate EMI protector.

No one understood that, if this conductive container resided on an insulating surface and became charged, the first pins of a neutral item touching the charged container could have this charge induced into them possibly causing CDM ESD damage.

Conversely, if a charged item was placed into a grounded conductive container, again CDM damage could occur. In addition, an isolated conductive package could be triboelectrically charged, induce charge into an assembly, plug-in card, and cause a CDM when the card was inserted into a grounded equipment rack. A package was needed that was not conductive or insulating. The answer was a partially resistive package that slowed the charging and discharging process, so the static dissipative package was developed.

How Did ESD Packaging Evolve?
Early ESD packaging consisted of pink-poly plastic bags and carbon loaded plastic bags. Neither of these packaging schemes was always reliable. Both types of bags were useful because they were usually sealed and helped protect components or assemblies from a direct human body discharge and contamination.

Unfortunately, some early antistats used in bags and foams had products that caused corrosion. The issue of contaminants from antistats has not been totally gone away. Koyler, et. al. [5] reported cases of corrosion of open devices by surfactant droplets while in antistatic plastic container. Assuming there is some air gap from the inside of the bag to the assembly or device, bags can protect items from a direct contact ESD that may occur when a charged person touches it (HBM). However, the pink-poly bags used an internal antistat and became insulating at low humidities or after time. Insulating pink-poly bags then became hazardous charge accumulators and generators. The bags would share their charge with their static sensitive contents by induction that could be damaged later if grounded.

Grounded carbon-loaded bags that were conductive placed charged components and assemblies in jeopardy of the CDM. Conversely, components or assemblies in charged carbon-loaded bags could be damaged by the CDM if removed and placed on a grounded conductive surface. Because the carbon loaded bag was conductive, it protected the component or assembly from any kind of radiated energy, whether a direct ESD or electromagnetic induction (EMI). Sometimes, the carbon would actually come out of the bag leaving the device with areas having carbon deposits.

Packaging Standards Introduced
The ESD Association-Glossary [6] defined the static dissipative range of 1 x 10⁴ ohms to 1 x 10¹¹ ohms surface resistance and volume resistivity from of 1 x 10³ ohm to 1 x 10¹⁰ ohms. Packaging materials with surface and volume properties in this range were then thought to protect from all ESDs. Unfortunately again, this proved to not always be the case unless the package was capable of keeping itself and its contents equipotential at all times. In the case of a direct discharge to the package, the static dissipative container may not protect at all. However, it has been shown that sufficient air gaps between the surface of the package contents and its outside walls can attenuate a direct ESD discharge [7].

In the case of a static dissipative container such as a plastic bag, incorporation of a metal layer in the bag can attenuate direct ESD discharges. The static shielding plastic bag has become a popular way to package assemblies and components. Since it has limited mechanical protection it must be transported in a corrugated box. Corrugated and plastic clamshell manufacturers have also incorporated metal layers in their containers that are effective shields. Today there are many new ways to make plastics static dissipative and even conductive. These include the incorporation of powdered metals, mixed-metal oxides, polymers and permanent coatings [8].