A GUIDE TO CORRECT USE OF ANTISTATIC FOAMS

History

Pink ESD foam has been ubiquitous in ESD packaging since its creation in the seventies. It is a fantastic product and solves many requirements in many situations. Pink foam has regularly been used as a cushioning aspect of packaging or work surface and has become a "fix” for many ills. There are misconceptions about the life span of the anti-static properties. Pink anti-static foams have a shelf life. Once the shelf life has expired, the foam can be quite dangerous sensitive components and the manufacturing environment. Upon further inspection of an EPA (ESD Protected Area), the most common broken standard ESD practices is the misuse of these foams.

As it relates to ANSI/ESD S20.20, ANSI/ESD S541 tells us in 6.1 and 6.2 that packaging (pink foam) used both inside and outside an EPA is required to have certain characteristics.

6.1 Inside an EPA

Packaging used within an EPA (that satisfies the minimum requirements of ANSI/ESD S20.20) shall be:

• Low charge generation.
• Dissipative or conductive materials for intimate contact.
• Items sensitive to < 100 volts human body model may need additional protection depending on application and program plan requirements.

6.2 Outside an EPA

Transportation of sensitive products outside of an EPA shall require packaging that provides:

• Low charge generation.
• Dissipative or conductive materials for intimate contact.
• A structure that provides electrostatic discharge shielding.

Low charge generation is one of the characteristics of the materials. S541 also states that for "intimate" or direct contact of sensitive products, it must be dissipative or conductive. Foam is used for "intimate contact" in most applications. Which is why it's so important to understand what makes Pink ESD foam static dissipative and why it has a shelf life. When that shelf life has been reached, you just have regular foam. So we can better understand the “shelf life,” we first need to look at regular foam in general, ESD foam, and where it can and cannot be used.

Foam Is Great For Cushioning

Regular foam as a substrate offers wonderful padding to protect items from physical impact. The concern with regular foam is that it sits very high on the triboelectric scale and has a high likelihood to produce a static charge. In fact, all materials, even conductors, can be tribo-electrically charged. How much charge is affected by material type, speed of contact and separation, humidity and several other factors. As a result, regular (non ESD) foam is cannot be used in an EPA (ESD Protected Area) environment. Remember, after the ESD properties disappear from the Pink ESD Foam, you just have regular foam left.

Pink Anti Static Foam

By the nature of its chemical composition and high surface area, flexible foam is ideal for static charge accumulation. This is fixed with the addition of anti-static chemical additives or anti-static surfactants. These additives are usually applied to the foam during manufacture. The color pink is just the color the industry decided on to help identify the foam belonging to “ESD” or “Antistatic” materials.

The surfactants used are low molecular weight fatty acids commonly based on amides or amines. Surfactants are mobile (blooming) surface modifiers that temporarily change the properties of friction between mating surfaces (tribo charging). This is a pretty bold statement. Let’s break it down for a better understanding. The molecules (in surfactants) in their initial state are unsaturated.

These molecules have unsaturated bonds that want to absorb moisture. In their unsaturated state, they lower the coefficient of friction of the foam which helps its Antistatic properties. The challenge of unsaturated molecules is their main goal in life is to become saturated. Once a molecule is saturated, they are considered expired and their antistatic properties are gone. How long until these molecules to become saturated? That can change depending on several factors. Humidity and the environment play a vital role.

Handling the foam also plays a vital role. The truth is, there is no actual guideline for how long this takes. Many industry experts consider one year as the critical date to start testing while others recommend testing earlier and some later. It boils down to your understanding and your procedures. What works for some might not work for all.

Shelf Life

Now we know why foam has a shelf life. Once foam's shelf life has passed, foam won't appear any different, but its ESD protective properties will be gone. We also understand that, based on many factors, shelf life can be short or long. So what to do?

Fortunately, the ESD Association has provided us a guide to help navigate this problem. 6.1 and 6.2 of ANSI/ESD S541 tells us, as it relates to ANSI/ESD S20.20, that packaging (in our case, pink foam) used in- and outside an EPA has to have certain characteristics. One of the characteristics is that materials must be low charge generating. It also states that for intimate contact of sensitive products, it must be dissipative or conductive.

Foam is usually used for intimate contact of sensitive products. You see foam lining racks and shelves, in bottom of drawers, inserted in containers and as separators between stacks of circuit boards or assemblies. This is pretty self-explanatory. We can't have any charge generating packaging material in an EPA or in certain circumstances outside of an EPA.

Now we have an issue. We've identified and explained how these foams may or may not be static safe. If the foam still meets its material specifications, we are all set. If the properties of pink foam are gone, what should you do? Fortunately for us, we again can turn to the ESD Association for guidance. Specificially, ANSI/ESD S541. In section A.6:

The static control properties of some packaging materials can deteriorate with time and use. Compliance Verification of static control packaging properties should be part of the ESD control compliance verification plan.

A6 Compliance Verification

This is a vital statement. It not only validates that material can deteriorate over time, it also tells us that we must develop a verification process to ensure the properties are still good to go. Another reference to validate this is ESD TR53-01-06. This document covers compliance verification of ESD protective equipment and materials.

Package Compliance Verification

Periodically confirm packaging materials as recommended in ANSI/ESD S541 (Packaging Materials for ESD Sensitive Items). Because of the big variety of packaging materials in use, the users should develop their own plan for verification of packaging compliance.

FOD (Foreign Object Debris)

One more item for your consideration when using foam for intimate contact with electronics is concerns about FOD – foreign object debris. All foams, regular non-ESD foams, antistatic foams, and conductive foams will shed particulates to varying degrees. Some are much better than others, but foam will generate some FOD. If you’re wondering about FOD when handling devices, select alternate options to eliminate contact with foam.

Summary

So here we are. We’ve learned that you might have problems if you are using Pink ESD Foam.

• Pink ESD Foam has a shelf life
• The shelf life of that foam is variable
• When it loses its ESD properties, it's unacceptable in, and potentially out, of an EPA
• If you’re going to use Pink ESD Foam, it should be a short term solution
• Consider the potential FOD issues with foam
• Have a quality program in place if you are going to use foam long term

Thanks to the ESD Association, there is a potential solution to the challenge of limited shelf life. Compliance verification can make an antistatic foam viable. If you don't have compliance verification, or if it isn't practical, then other more permanent options should be considered.