ESD & How It's Tested
Electrostatic discharge or ESD can be thought of as the build up of potential charge between to different objects which can be transferred through contact or by air. This post will provide a brief overview of ESD from a broad EMI compliance stand point, including testing overview, methods of discharge, and brief walk through of common standards.
This type of EMI event is common and effects a variety of different electrical and electric products. The image to the right helps illustrate the build up extra electrons as the individual walks across the floor, increasing the negative charge. The contact of the person to the equipment with a difference in potential causes a rapid discharge of electrons to the electrical equipment to reach a balanced state.
When talking about this type of electrical event it is important to remember the associated standards and methods are attempting to duplicate a real-world event, providing reproducible testing criteria. Many immunity standards go into great detail about exact criteria, test equipment, and setups, to ensure compliance reference the associated standard.
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 EN/IEC 61000-4-2 standard. The below table, from an EDN Network, access 8/2/19  (extracted 8/5 www.edn.com). It provides a basic overview of all three types and provides detailed information on voltages and waveforms.
When evaluating the different methods, it is also important to take understand that often times charges are created but instead moved by "carriers" and transmitted. In the situation above, the person creates the charge, however in many cases humans and objects can transfer a charge from one object to another. This plays a large role in the number of discharge events that happen to electrical and electronic equipment.
What are Contact & Air Discharge Methods?
The contact discharge test method is when contact is maintained from the ESD simulator to the equipment under test (EUT) while the discharges are being applied. This method of testing helps eliminate many environmental factors that can often have large impacts on test results, because of this EN/IEC 61000-4-2 states that contact discharge is the preferred test method.
Air discharge testing is the other main method of testing for immunity to ESD commonly required. It involves moving ESD generator (surplus of energy) towards the equipment under test (EUT) until the potential becomes large enough to over come the gap and discharge occurs. This method does more accurately replicate the event in a field environment, making it ideal for simulating real world failures.
There are many environmental factors that have the potential to impact testing with the air discharge method, below you will find a few of the more substantial.
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 - Attempt to duplicate the recommended speed ensuring compliance.
Current Waveform & Voltages
Like many other immunity-based EMC events ESD includes both current and voltage requirements. Depending upon the level of testing will determine how closely and the methodology by which both current and voltage evaluated. For many situations, monitoring the voltage level for air discharge and current waveform is sufficient. The below waveform is one that is provided by the Teseq NSG 435 manual which can be accessed by clicking here.
Current waveforms are required by the underlying test standard, commonly IEC 61000-4-2. This is typically done with an ESD current target, associated cabling, attenution, and oscilliscope. For compliance to a particular standard, it is required that the simulator meet both resistance and capacitance combination and associated waveform.
Voltages measurements for verification are typically done at 2kV, 4kV, 8kV, and upwards of 15kV depending upon the capabilities of the simulator. This is commonly done with an external voltmeter designed for higher voltage measurements, such as the ES105. Given that some test requirements go as high as 30kV(3), it is ideal to have capabilities to measure at least that level
Test & Verification Equipment
As mentioned previously our focus on electrostatic discharge is centered around an EMI and product reliability. The exact requirements for equipment is often times determined by the underlying standard, as with the resistance capacitance combination, waveform, and voltages. Typically what is minimally required is an ESD simulator and test station, or setup designed for compliance with appropriate grounding and planes. The below menu offers some of the most common equipment and verification accessories required for testing.
The devices associated with replicating the associated voltage, current, and waveform requirements with an ESD event are referred to as ESD simulators or ESD guns. There are a variety of different manufacturers, however typically simulators are broken down by max discharge voltage (~15kV-16kV & 30kV) as well as the resistance capacitance combination. The max values are typically relayed for air discharge given it has the highest voltage requirements.
Typically contact tips, resistance capacitance networks (also called RC networks) can be switched out to meet different testing criteria. The networks themselves can typically be purchased separately in a variety of common and custom combinations. The below video offers a comparison of the most common simulators used for compliance to EN/IEC 61000-4-2.
Air & Contact Discharge Tips
The majority of major simulators today include both air and contact discharge tips with the simulator. The method by which they attach to the simulator is somewhat different for each unit, some slide on and some turn on. Regardless of how they are attached they are all required to be compliant to the associated test standard, typically IEC 61000-4-2.
The image on the right shows the Teseq NSG 435 ESD simulators contact and air discharge tips and how the attach to the simulator. There is also additional tips available with many common simulators allowing for modifications of risetime and testing methodology. These are typically used for special applications or standards, not typically required for immunity testing.
Testing both positive and negative polarity of pulses is typically required from a compliance and product reliability standpoint. This is often done throughs selection on the menu screen on the simulator itself, however some models require switching a connection on the side of the equipment. This capability is included with simulators that are available.
ESD Current Targets & Voltmeters
Verification and calibration of simulators, and the associated current waveform they produce, is done with an ESD target. These are specially designed devices have set criteria, defined by associated standards, the provide resistance, insertion loss, and test level amongst others. It is recommended that the standard is followed including the plane used to hold the target while pulse is monitored.
When attempting to capture pulses with the ESD target, attenuator (typically 20dB), and associated oscilloscope (>1GHz) it is crucial to be sure there is limited noise surrounding the test environment or area. Should the oscilloscope trigger it will be necessary to either move the setup and test area or use a faraday cage to limit noise.
Common ESD Standards
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.
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)
- 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.
Testing for military based applications and standards are also different from both purely automotive or commercial based requirements. A major consideration for the majority of military requirements, including MIL-STD-461 CS118 is the requirements to very discharge methods current and voltage. This is often done through a voltmeter for voltage, and a target, attenuator, cabling and capable oscilloscope. Each testing can also require different resistance and capacitance requirements, making sure the ability to purchase these networks crucial.
- MIL-STD-461 CS118 - Given the wide range of equipment that falls under the category and scope of "personnel borne electrostatic discharge" this standard is very commonly used. It is also relatively new as it was released December of 20159(7). This standard, as a part of the broader military standard 461 provides great details about the equipment required for testing. It also mandates that the common 150pF/330Ohm resistance capacitance configuration be used, the same requirement as used in IEC 61000-4-2.
- MIL-STD-883 - This standard is based on a more specific application for microelectronic devices for use within military and aerospace systems. This standard also specifies that ESD simulator current waveform verification be done, however specifies only current be done. It also requires substantially less voltage then other standards, the final or fifth step requiring only 4,000 volts. A major consideration for this standard is that it requires specific waveforms and associated 100pF/1500Ohm resistance capacitance configuration.
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.
How long between ESD discharges?
The exact time between discharge depends upon the standard that the testing is being conducted to. However, in most situations time between successive discharges should be at least 1 second.
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
6. Wikipedia - IEC 61000-4-2
7. Interference Technology - Review of MIL-STD-461 CS118