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May 1, 2017

ESD Grounding - 1 Meg Ohm Resistor

Why do we need to use a 1 meg-ohm resistor for ESD grounding?


Experts Comments

This is a good question.

A 1 megohm resistor allows any static charge, whether from the bench through the product and then through the grounded person or operator or vice-versa, to discharge completely over time, typically less than 1 second.

Without the 1-meg resistance, the static discharge would be an instantaneous drain directly to ground through the product. Let's take the case of a highly charged operator who is not wearing any dissipative footwear and is not grounded with a wrist strap as an example: If the ESD mat or a conductive metal surface that the sensitive circuit card assembly (CCA) is resting on is grounded with just an electrical wire connection directly to ground, there is nothing to slow down the discharge of the operator through the CCA and through the conductive mat to ground.

The CCA takes the full brunt of the electrical discharge (the ESD event) instantaneously, and that instantaneous discharge is what can damage the components on the CCA.

If a 1-megohm resistor is in series with that wire, the charge bleeds off over a few milliseconds and this reduces the shock to the CCA. ESD wrist straps typically have a 1-meg resistor built into them. This is to protect against a highly charged mat or ungrounded surface from discharging instantaneously through the CCA and then through the operator to the ground connection that the wrist strap is plugged in to.

Foot straps and ESD shoes are designed to be dissipative in this manner also. ESD floors are dissipative, mats are dissipative, ESD tools are dissipative, all of these things work to allow built up electrical charges to accomplish equilibrium (no difference of electrical potential) within a few milliseconds, but never instantaneously.

Engineering is nothing more than the management of forces trying to reach equilibrium, whether it is temperature, gas, chemistry, electrical or fluid forces. It does not work for spiritual or political forces, however. Those defy logical or scientific reaction. So may the Force be with you.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
The short answer to your question is, "For operator safety".

Think about it... what is the purpose of the grounding strap? It's to provide a path to ground for electrical energy. The problem is that we work around sources of electricity all the time in the electronics industry.

If an operator had just a plain wire from his or her wrist the operator would definitely be grounded for ESD purposes. However, he or she would also be grounded if that person comes across a bare wire carrying 110V or, in many cases 220V or more. In those cases, the wrist strap would go from a simple relief of ESD to a potential life threatening ground.

The electrical energy, the full force of the 110, 220 or more is now coursing through the operator's body with an easy path to ground. A typical household electrical circuit may have 15 to 20 amps of current. As little as 0.25amps traveling across a person's chest can cause a potentially lethal heart condition.

By placing the 1 meg ohm resistor in the grounding wire the operator is protected from electrical shock, injury, or even death.
Kris Roberson
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
To prevent the user from being electrocuted should their wrist strap come into contact with a live mains voltage. 1 meg Ohm is more than enough to carry ESD potentials safely to earth and protect the user as well.
Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
To prevent the electrostatic charge build-up for ESD protection, everything which can acquire a charge is grounded including people working there. The workers use conductive shoes and a grounded band on their wrist to keep themselves grounded.

However, if someone is well grounded and touch something with some high voltage on it, they can easily get a very large and dangerous electrical shock. To prevent this, people working in this kind of environment are not directly grounded but through a resistor which limits the current flowing through them to safe value. Generally this current limiting is done using 1megaohm resistor in the grounding wire.

This 1 megohm resistor limits the current to much less than 1mA, if someone accidentally touch a wire with mains potential (230V).
Santosh Kumar
R&D Manager
MK Electron Co. Ltd
Santosh Kumar is R&D Manager at the MK Electron Co. Ltd., Korea and engaged in the electronic interconnect materials development and technical marketing. His key focus is novel lead-free solder materials, electronics packaging, wire bonding materials and process.
The 1-megohm resistance is used for two reasons:
  1. It limits the speed that charge is passed, avoiding rapid discharge of an item, which is one way to induce ESD damage to the item (the other being a rapid discharge from a charged object to the item)
  2. It provides for operator safety by limiting current in the event that the wearer of a grounding device comes in contact with high voltage.
Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
A resistor is used as a safety precaution. Since electrical equipment is also connected to ground ... (some companies do not have a separate electrical ground from their ESD ground) there is a chance that electrical equipment could short-back into the electrical ground and thereby transmit large currents into the ESD straps of individuals.

That could be catastrophic. Follow the EOS/ESD standards and all will be good.

Note that the grounding of racks and equipment needs to be checked with a resistance meter as well. You want the equipment and work surfaces to have "single point grounding" ... this means that the feet of the equipment should be electrically isolated from conductive floors.

Otherwise, the effective resistance of parallel circuits follows this formula ...1/Rt = 1/R1 + 1/R2 + 1/R3 + ...   I can explain in more detail is you wish to contact me directly.
Rick Perkins
Chem Logic
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
The resistor is a safety feature to limit the current in the event the operator touches an energized conductor. A 1 meg-ohm resistor is typically used for protection from normal mains supplies of 110 to 220 VAC.
Richard Henrick
Quality Assurance/Regulatory Compliance Manager
Sanmina Corporation
Richard has 18 years experience in the medical electronics industry at both a contract manufacturer and OEM. His experience includes PWA and finished device manufacturing as a Manufacturing Engineer and during the past 7 years as plant Quality Assurance/Regulatory Compliance Manager. He holds 5 American Society for Quality Certifications and is a Certified IPC 610 Trainer.
A high resistance, such as 1 MΩ, is used to discharge static slowly.  Discharging static electricity quickly results in a spark and a spark at or near the source of discharge is results in electrostatic overstress (damaging high voltage) to ICs and sensitive circuitry.

Grounding sensitive equipment or personnel through a 1 MΩ resistor is a universally accepted practice and prescribed in a variety of ESD standards.
Gary Freedman
Colab Engineering
A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.
simply to protect the personnel from being electrocuted in event any electrical equipment on conductive table top gets short circuited. The current in case of short circuit takes shortest resistance path, one meg makes circuit through human body more resistive, thereby preventing persons being electrocuted. For ESD precaution, it is essential to hold equipment, personnel and ESD assemblies grounded at same potential.
KN Murli
Astra Microwave Products, Hyderabad, AP India
Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.
The resistor protects the operator from becoming the path to ground in the event there is contact with live voltages. Have you gained access & downloaded the current EOS/ESD ST 20.20?

The use of a 1 meg ohm resistor has been a part of EOS/ESD safety for at least 25 years. I recommend that you familiarize yourself with the standards that have been implemented for years. I have no idea what country you are located in but the usage of 1 meg ohm protection is the norm in any quality wrist strap sold currently on the market.
Jerry Karp
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
Reader Comment
If the question is about wrist straps, no question it requires 1M resistor (for the dual wristsraps two 1M resistors in each half). Regarding reducing strength of discharge, lets stress-test this theory - what if this resistor is 10M? 100M? 1G?  What about the proverbial doorknob that has infinite impedance to ground,yet one can be zapped on contact by walking on the carpet? As evident, the theory of 1M resistor used to "slow down" or reduce discharge strength doesn't hold proverbial water.

Metal objects, i.e. workbenches and others, have capacitance to ground, big or small. Charged metal object coming into galvanic contact with such workbench or other conductor, grounded or not, would rapidly equalize voltage between the two objects whether any resistor at all is present anywhere.

If you want to reduce discharge current on contact, reduce metal-to-metal contact (dissipative tweezers come to mind) or assure that the contacting objects have the same voltage, preferably both equal to ground. This is the entire purpose of grounding workbenches and other metal objects.

1M resistor performs no utility in reducing discharge strength but it does increase possibility of induced voltage on the objects from radiated sources such as fluorescent lights, power lines, operation of tools and such.
Vladimir Kraz, OnFILTER, USA
Reader Comment
If the resistivity of the floor covering is too low, then its recommended to install a dissipating floor mat connected to ground via 1 M ohm in the area to be protected.
Swaroop Pawar, Schneider Electric
Reader Comment
Vladimir Kraz covered it but I will restate it.

Do you need to have 1 MΩ resistor in the ground path? Not necessarily, either for ESD risk or for standards compliance. Some people have earthed metal benches with no problem. 

If you are worried about charged device ESD risk, a resistor in the ground path won't help. What you need in this case is to prevent contact between the ESD sensitive device (ESDS) and conductive materials (<10 kΩ surface resistance). Or, before contact, the ESDS and the material must be at nearly the same voltage.

If you are concerned about personal grounding in the presence of power line or other high voltages, you need a protective resistance in line with the grounding appropriate to the power voltage present and maximum electrostatic voltage you can tolerate on the person's body. 1 MΩ is usually a good solution for this. Look at ANSI/ESD S20.20 and ESD S541 or IEC61340-5-1 and IEC 61340-5-3.
Jeremy Smallwood, Electrostatic Solutions Ltd
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