Electrostatic Dissipative Coatings (ESD) Explained

By Marty Korecky

Electrostatic Dissipative Coatings (ESD coatings) have been available in the marketplace for many years. Information regarding their performance may be confusing and therefore often misunderstood. We hope to clarify how these “work-surface coatings” prevent the discharge of static electricity, which when released, leads to the damage of sensitive electronic components used in the electronics industry.

Packaging electronic components in ESD plastic bags protects them from being damaged by an electrostatic discharge. The bag has been designed to allow the static charge to bleed away, protecting the electronic component from taking a direct hit.

Static electricity happens when there is an imbalance of positive and negative charges. At rest, most items are electrically balanced and have a neutral electrical charge. As you sit in a comfortable chair reading this article, you are electrically neutral. Since you are an avid reader of PCT, and this article is in the January/February issue, it is winter, it’s cold outside, and the humidity is low. Your doorbell rings just as you get interested in this article. You stroll across your carpeted room and reach for the doorknob. ZAP! You get shocked as you reach out and touch the doorknob.

You received a shock because as you walked across the carpet in your socks, the dissimilar materials (your socks and the rug) changed the balance between the positive and negative charges in your body. You picked up electrons from the carpet, making you negatively charged. You became a capacitor by storing electrons. When you touched the doorknob, the extra electrons you collected as you walked across the carpet moved all at one time to the doorknob and—ZAP! Once that occurred, you went back to a neutral electrical state. While still standing there, you can touch the doorknob again, but the energy is gone—no shock. While uncomfortable for you, this type of discharge is what damages electronics. The rapid discharge of electricity through an electronic circuit board may degrade sensitive components. This degradation will reduce the life of the element and lead to the circuit board’s eventual failure.

We utilize ESD materials and practices to reduce the chance of damage to electronic components. ESD does this by slowing the flow of electrons—or static electricity—to electrical ground. The higher the resistance, the slower the flow. If the electrical resistance is increased high enough, we insulate the electrical transfer process and stop the flow of electricity altogether.

Electronic component manufacturers put safeguards in place to reduce electrostatic exposure. Humidity controls lessen the chance for static buildup while ESD floor mats, workstations, and furnishing accessories help bleed the electrical charge to electrical ground. Operators wear grounding straps before working with sensitive electronics, all of which must comply with international standards.

There are two tests for electrostatic dissipative materials. Surface resistivity, Rs, (Ohms/square or Ohms/m2) is the resistance to leakage current along the surface of the insulating material. Volume resistivity is the resistance to leakage current through the body of the insulating material. Testing uses special instrumentation and concentric ring probes capable of measuring 103 to 1013. There are several test methods in use, including ASTM D-257. Below is a depiction of the terms we use for surface resistivity, Rs. To help understand the values, 100 is equal to 1, 103 is 1,000, and 109 is 1,000,000,000. Resistance units are ohms. For clarity, 100 is equal to 1 ohm, 103 is 1,000 ohms or (1 kilo- ohm), and 109 is 1,000,000,000 ohms or (1 giga-ohm).

Powder coatings are by nature insulative products. Typical powder coatings consisting of the resins and curing agents, pigments, and additives found in most coatings provide an electrically insulative coating. It is for that reason the electrical bus bar industry uses powder coatings. These coatings can have ratings of greater than 1,000 ohms/mil, keeping the products electrically safe.

One of the key attributes of the ESD coating is that it must be consistent in its ability to bleed a static charge to electrical ground. Early attempts to make ESD coatings included the addition of carbon black to the powder coating formulation. The ESD results were varied, and the carbon black limited the coating colors to blacks and dark colors. Today, many powder manufacturers have patents protecting their ESD formulations and inventions.

Before you begin a project requiring an ESD coating, make sure you get a copy of the specification. Talk with your powder supplier to ensure the powder you are purchasing will meet the specification. If required to test the ESD of the coating, make sure you know what equipment is needed and which test method you must use. You must provide training for your employees on the correct test method and how to use test equipment properly.

Marty Korecky is business specialist and commercial quality for AkzoNobel Coatings, Inc.