ESD Testing Procedures/Methods Explained (Air & Contact Discharge)
- 10 Dec, 2018
ESD - Overview, Testing Methods, & Common Questions
There are two main methods of testing for electrostatic discharge or ESD that are in common practice today. We'll talk briefly about this type of testing generally, then go into both air and contact discharge testing. When talking about this type of electrical event it is important to remember that these methods are attempting to duplicate a real world event.
The focus on this post is to provide the basic information on this commonly occurring electrical event. For additional information see the references/additional resources below.
This type of testing is crucial given how commonly this event occurs and impact it can have on the function of electrical systems.
Above you will find a picture of how energy can buildup on person walking across a surface and discharge on a computer. You will notice the buildup and potential difference that exists between the person and the electrical equipment. This real world example shows the air discharge method, which will be the first method we discuss.
ESD Testing Categories
Electrostatic discharge testing can be broken down into three main categories, human body model (HBM), machine model (MM), and charge device model (CDM). Given the number and vast amount of standards that reference these types of testing the main focus of this discussion will be on the most common commercial IEC 61000-4-2 standard. The below table, from an EDN Network, access 8/2/19 (1) (extracted 8/5 www.edn.com). It provides a basic overview of all three types and provides detailed information on voltages and waveforms.
Waveform & Voltages
Given the different standards, tolerances, and resistance capacitance networks we will focus our discussion around the commercial based IEC 61000-4-2 standard. The below waveform is one that is provided by the Teseq NSG 435 manual which can be accessed by clicking here.
Given the nature of this event, there are many different ways that standards view the voltage associated with it. Taking a look at the chart above, you will notice that the HBM testing method is commonly done to 15kV whereas, machine model testing is a fraction of that voltage. Some applications, specifically automotive and military testing require up to 30kV.(3)
The test voltage can vary both because of application (or application environment) and by standard requirements for compliance, or conformity to a particular standard. This type testing is most often done in steps allowing for any deterioration of function or test category to be noted.
Air and contact discharge
There are a variety of different manufacturer's that make equipment that simulate and ESD event. However, the vast majority of them offer both testing solutions for positive and negative, as well as air and contact discharge.
Air and contact discharge tip requirements can be found directly in the associated standard (most often IEC 61000-4-2). However, when looking at a choosing a simulator these requirements are fairly standardized. This allows for the same discharge tips to be used in a variety of applications.
Below is a picture of both the air and contact discharge tips for the Teseq NSG 435 ESD simulator.
What is Air Discharge Testing?
This discharge method is tested to the way that it occurs naturally, moving the discharge object/generator with a surplus of energy towards the equipment under test or EUT. With this method however, there are a variety of factors that come into place given the environment in which the discharge occurs.
Common Environmental Effects:
Humidity - The lower the humidity the higher probability of a electrostatic discharge event occurring.
Temperature - The lower the temperature often leads to an increase in ESD events.
Speed of Approach - Attempting to duplicate required speed can be difficult and can also have an impact on the event.
The benefit of this testing method is that it most accurately replicates how a discharge would occur.
When conducting air discharge testing to immunity based standards it is important to pay special consideration to the humidity and temperature requirements. Once you determine which standard you are attempting to test to it is always best practice to ensure your within the tolerances.
However, given the many factors that can have an impact on the outcome of testing many standards preferred method is contact discharge.
What is Contact Discharge Testing?
IEC 61000-4-2 states "Contact discharge is the preferred test method. Air discharges shall be used where contact discharge cannot be applied." This standard also places specific requirements on the setup required for testing. Below you will find an example of the setup required.
This method of testing is where contact between the ESD generator and the equipment under test is maintained during the entire testing. This method of testing helps eliminate many environmental factors that can often have large impacts on test results.
However, for field testing this method may need to be supplemented with air discharge given the nature and large impact that environmental factors can have.
Contact discharge would rarely occur in a normal environment. Take the example above, a person would likely rarely maintain contact with an electrical equipment while generating energy to desired levels without a discharge.
Time Between Discharges?
The time between discharges is most often determined by the standard being used a guide for testing. However, in field testing or product development the time can also be determined by the application and electrical environment. Often times 1 second is an ideal baseline to which you can shorten or lengthen the intervals as required. For military testing MIL-STD-883E Section states the a minimum of one second must be left between pulses. (5)
Common ESD Standards
Commercial based testing for electrostatic discharge, as mentioned above, can be divided into categories based upon testing categories (HBM, MM, etc.) The focus on the standards below is on the most common standards used. Many of these, including IEC 61000-4-2, has the higher limits of testing at 15kV(6).
- EN/IEC 61000-4-2 - International Electrotechnical Commission standard currently most recent version 2008. One of the most commonly used main stream commercial testing standards for electrostatic discharge testing. This standard specifies a variety of criteria including waveform, test setup, voltages, etc. The website for the IEC (https://webstore.iec.ch/publication/4189) states (retrieved 8/12/19) "he object of IEC 61000-4-2:2008 is to establish a common and reproducible basis for evaluating the performance of electrical and electronic equipment when subjected to electrostatic discharges."(2)
- ANSI C63.16 - This standard surpasses some testing parameters of other standards (EN IEC 61000-4-2) allowing for an increased immunity of products to this type of event. The goal of ANSI C63.16 is to lower customer complaints by identify failures related to ESD events that may not have presented themselves with testing during prescribed existing international standards.
- CE Mark - This series of standards covers a variety of EMC testing including immunity and emission, encompassing electrostatic discharge testing. As the CE implies these testing standards comparable to the International Electrotechnical Commission IEC series of standards.
Electrostatic discharge testing for automotive applications is different from commercial based in a variety of ways. The largest variations stem from voltages, test setups, standards, number of discharges, and resistance/capacitance (RC) networks. The voltages of these applications can be in excess of 25kV and often require several RC networks.
- ISO 10605 - This standard is one of the most common international standards and is referenced often by the manufacture based standards. It also requires testing criteria to 30kV as opposed to the lower voltage requirements of IEC 61000-4-2 and other commercial based standards. Texas Instruments, (http://www.ti.com/lit/an/slva954/slva954.pdf) provides an excellent article comparing these two basis for testing. This standard requires RC Networks:
- Manufacturer OEM - The large role that Manufacturer OEMs play in the automotive ESD makes it unique. The requirements of the associated standards often times reference the more common international standard (IE ISO 10605) however, provide unique requirements. These requirements often present themselves in the form of specific resistance/capacitance networks.
What does ESD Stand for?
ESD stands for Electrostatic Discharge.
What is Electrostatic Discharge
Electrostatic Discharge is defined by IEC 61000-4-2 by the IEC Webstore preview as "a transfer of electric charge between bodies of different electrostatic potential in proximity or through direct contact." (2)
What is ESD testing?
ESD testing, or ESD immunity testing, is designed for ensure "an electronic or electrical product shall work as intended in its environment."(4) The exact criteria is most often determined by first establishing either a standard or testing plan by which evaluation and testing criteria are determined.
Is ESD dangerous to humans?
Electrostatic discharge in most situations would be viewed as uncomfortable rather dangerous. However, given the impact that it can have on electrical equipment it is possible to have a negative impact on medical devices associated with humans.
Why does ESD Matter?
The electrical occurrence matters in the impact it can have on electrical circuits. Given the nature of electrical based devices in our daily lives ESD has increased in importance. With the increase in devices there has also been an increase in the amount of testing required.
Mueller Electric - Introduction to Electrostatic Discharge
EC&M - Electrostatic Discharge: Causes, Effects, and Solutions
Desco - Electrostatic Discharge (ESD) A Person Can Feel
Desco Europe - The impact of relative humidity on ESD
InCompliance - Static Electricity and People
ANSI Webstore - IEEE C63.16
Texas Instruments - ISO 10605 Application Note
1. EDN Newtork - Understanding and comparing the differences in ESD testing
2. IEC Webstore - IEC 61000-4-2 Preview
3. Teseq - ESD Testing with precision and convenience
4. EM Test - By the way - What is EMC?5. Custom MMIC - Mil-std-883