A GUIDE TO CORRECT USE OF ANTISTATIC FOAMS

History

Pink ESD foam has been common in ESD packaging since its inception back in the 1970's. It's a great product that meets many requirements for many uses. Pink foam has regularly been used as padding for packaging or work surface and became a "fix” for many problems. That said, there is confusion about the ant-static nature of the foam and how long it lasts. Pink anti-static foams have a shelf life. Once the shelf life is gone, pink foam can be very dangerous sensitive components and the manufacturing environment. Upon further inspection of an EPA (ESD Protected Area), the most common abused standard ESD practices is the use, or misuse of these foams.

As it relates to ANSI/ESD S20.20, 6.1 and 6.2 of ANSI/ESD S541 tells us that packaging (in our case, pink foam) used in- and outside an EPA must meet 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.

One of those characteristics states that materials must be low charge generating. For most applications, foam is being used for “intimate contact”. That is why it's so important to understand what makes Pink ESD foam static dissipative and why it has a shelf life. Once the shelf life has been reached, you're left with regular foam. So we can better understand the “shelf life,” first we must look at at regular foam in general, ESD foam, and appropriate applications of it.

Foam Is Great For Cushioning

Regular foam as a substrate gives good cushion to protect products from physical damage. The challenge with regular foam is that it is very high on the triboelectric scale and has a high likelihood to produce a static charge. All materials, even conductors, can be tribo-electrically charged. The extent of the charge is impacted by type of material, velocity of contact and separation, relative humidity and several other factors. Therefore, regular (non ESD) foam is is not valid to use in an EPA (ESD Protected Area) environment. Remember, when the ESD properties dissipate from Pink ESD Foam, you're left with regular foam.

Pink Anti Static Foam

By the nature of its chemical composition and high surface area, flexible foam is ideal for the build-up of static charges. This is overcome by the addition of anti-static chemical additives or anti-static surfactants. These additives are usually applied to the foam during the manufacturing process. The color pink is just the color the industry came up with to help identify the foam as a category of “ESD” or “Antistatic” materials.

The surfactants used are low molecular weight fatty acids commonly developed from amides or amines. Surfactants are mobile (blooming) surface modifiers temporarily changing the coefficient of friction between mating surfaces (tribo charging). This is a pretty bold statement. Let us take a look for a better understanding. The surfactant molecules are unsaturated in their initial state.

These unsaturated molecular bonds want to absorb moisture. In this unsaturated state, they work well to lower the friction of the foam which helps its Antistatic properties. The challenge of unsaturated molecules is they want to become saturated. Once a molecule is saturated, the game is up and the antistatic properties are gone. How much time until these molecules to become saturated? That varies depending on several factors. Humidity and the environment play a vital role.

Handling the foam also plays a vital role. Sadly, there is little real guideline for how long this takes. Many industry experts look at one year as the critical date to start testing while others recommend testing earlier and some later. It all comes 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 its shelf life has passed, foam won't appear any different, but the foam's ESD protective properties will be gone. We also know that, based on many factors, shelf life can vary. So what to do?

Luckily, the ESD Association has given us a guideline to help address this problem. As it relates to ANSI/ESD S20.20, ANSI/ESD S541 tells us in 6.1 and 6.2 that packaging (in our case, pink foam) used both inside and outside an EPA has to meet certain characteristics. One of those characteristics is that materials must be low charge generating. It also points out that for intimate contact of sensitive products, it must be dissipative or conductive.

Foam is often used for intimate contact of sensitive products. You see it lining racks and shelves, in bottom of drawers, inserted in totes and as separators between stacks of circuit boards or assemblies. This makes sense. We cannot have any charge generating packaging material in an EPA or in certain scenarios outside of an EPA.

Now we have a problem. We have outlined and explained why these foams might or might not be static safe. If the foam still meets its material specifications, we are all set. If the ESD of pink foam have expired, what can you do? Conveniently for us, we 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 an important statement. It both validates that material can deteriorate over time, it also tells us that we must develop a verification process to ensure the properties are still present. ESD TR53-01-06 is another reference that validates thos. ESD TR53-01-06 covers compliance verification of ESD protective equipment and materials.

Package Compliance Verification

Make sure to validate packaging materials as recommended in ANSI/ESD S541 (Packaging Materials for ESD Sensitive Items). Because of the wide variety of packaging materials, the users should develop their own compliance verification plan for packaging.

FOD (Foreign Object Debris)

One more item for your consideration in the use of foam for intimate contact with electronics is concerns of foreign object debris (FOD). All foams, standard non-ESD foams, antistatic foams, and conductive foams will shed particulates to varying degrees. Some are much better than others, but foams will generate some FOD. If you are worried about FOD when handling devices, choose alternative options to eliminate contact with foam.

Summary

So we've done it. We’ve learned that you might have problems if you're using Pink ESD Foam.

• Pink ESD Foam has a shelf life
• The shelf life of that foam is unknown
• When foam loses its ESD properties, it's unacceptable in, and potentially out, of an EPA
• If you're using 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, we have a potential solution to the issue 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 used.