History
Pink ESD foam has been a staple in the ESD packaging world since its inception back in the 70’s. It is a great product that fulfills many needs in many applications. Typically used as a cushioning aspect of a package or work surface, pink foam has been a “solution” for many problems. However, there has been a misconception of the anti-static properties and how long they last. Pink anti static foams have a shelf life. Once that shelf life is gone the foam can become extremely dangerous to the manufacturing environment and sensitive components. When walking through an EPA (ESD Protected Area) the most common violation of standard ESD practices is the use, or misuse of these foams.
ANSI/ESD S541 tells us in 6.1 and 6.2 as it relates to ANSI/ESD S20.20 that packaging (in this case pink foam) used inside and outside an EPA must 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.
History
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. For most applications foam is being used for “intimate contact”. That is why it is so important to understand what makes Pink ESD foam static dissipative and why it has a shelf life. Once that shelf life has been reached you just have regular foam. In order to better understand the “shelf life” we first need to look at regular foam in general, ESD
foam, and applications where it can an cannot be used.
Foam is Great for Cushioning
Regular foam as a substrate provides great cushioning to protect products from physical damage. The problem with regular foam is that it is very high on the tribo-electric scale for producing a static charge. In fact all materials, even conductors, can be tribo-electrically charged. The level of charge is affected by material type, speed of contact and separation, humidity and several other factors. Thus regular (non ESD) foam is not acceptable in an EPA (ESD Protected Area) environment. Remember, once the ESD properties disappear from the Pink ESD Foam you just have regular foam left.
Pink Antistatic Foam
By the nature of its chemical composition and high surface area, flexible foam is prone to the build-up of static charges. This characteristic is overcome with the addition of anti-static chemical additives or antistatic surfactants. These additives are usually incorporated into 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 typically based on amides or amines. Surfactants are mobile (blooming) surface modifiers that temporarily change the friction properties between mating surfaces (tribo charging). That’s a pretty big statement. Lets break it down to help understand it better.
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 work well to lower the friction rate of the foam and help in its Antistatic properties. The problem with unsaturated molecules is their main goal in life is to become saturated. Once the molecule is saturated the game is up and the antistatic properties are no longer present. How long does it take for these molecules to become saturated? That depends
on many factors. Humidity plays a vital role. The environment plays a vital roll. The handling of the foam plays a vital role. In reality there is no real guideline for how long it will take. Most people in the industry look at one year as the magic date to start testing while others test earlier and some later. It all comes down to your understanding and your procedures. What works for some might not work for others.
Shelf Life
So now we know why foam has a shelf life. Once the shelf life has expired, foam will not appear to be any different, but its ESD protective properties will be gone. We also know that based on many factors that shelf life can be short or long. So what do we do?
Fortunately the ESD Association has provided us a guideline to help navigate this problem. ANSI/ESD S541 tells us in 6.1 and 6.2 as it relates to ANSI/ESD S20.20 that packaging used inside and outside an EPA must have 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. We already have identified that foam is generally 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.
That’s pretty self-explanatory. We cannot have any charge generating packaging material in an EPA or in certain scenarios outside of an EPA.
But now we have an issue. We have identified and explained how these foams may or may not be static safe. If the foam still meets its material specifications we’re all good to go. But what happens if the properties are gone? Fortunately for us we again can turn to the ESD Association for guidance. More specifically ANSI/ESD S541. In section A.6:
A6 Compliance Verification:
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
This is an important statement. Not only does it validate that material can deteriorate over time it tells us that we must create a verification process to assure 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
Packaging Compliance Verification
The test procedures referenced in ANSI/ESD S541 (Packaging Materials for ESD Sensitive
Items) can be used to periodically verify packaging materials. Due to the wide variety of packaging materials in use, the users should develop their own compliance verification plan for packaging.
FOD
One final consideration in the use of foam for intimate contact with electronics is the concern about FOD – foreign object debris. All foams, standard non ESD foams, antistatic foams and conductive foams will shed particulates to varying degrees. Some are much better than others but foam will produce some FOD. If FOD is a concern for the devices to be handled then it is best to consider other options to eliminate contact with foam.
Summary
So there you have it. We’ve identified that you may have a problem if you are using Pink
ESD Foam.
- Pink ESD Foam has a shelf life
- The shelf life of that foam is unknown
- When it loses its ESD properties it is unacceptable in and potentially out of an EPA
- If Pink ESD Foam is going to be used as a solution it should be short term
- Consider the potential FOD issues with foam
- A quality program should be instituted if the foam is going to be used long term
Thanks to the ESD Association, have there is a potential solution to the problem of
limited shelf life. Compliance verification can make an antistatic foam viable. In the
absence of compliance verification, or if it is not practical then other more permanent
options should be considered.
