Electro Static Discharge (ESD)

(C)opywrite 1996, may be used freely in any non-profit publication as long as credit is given to the author and you just drop me a note (garycox@netten.net) to let me know that you used it.

Updated October 1997

By Gary W. Cox

Electro Static Discharge is a subject that technicians are becoming increasingly aware of in the computer/electronics industry. Electro Static Discharge or ESD is the shock that is felt when a person walks across a carpet and touches a door knob and a discharge of electricity occurs between that person and the door! That shock was actually a discharge of electricity in the range of 2000 to 3000 volts as the shock voltage has to be at least 2000 volts before the human body can feel it... The reason the 2000 volt ESD discharge does not kill someone is that the current is very low but that same voltage can kill electronic components or even worse damage the components to where the a chip acts erratic or fails months later. Furthermore did you know that studies have shown that a static charge built up by scuffing shoes on a carpet can exceed 20,000 volts?

When working on electronic equipment proper ESD precautions need to be taken to ensure the safety of electronic components! Today's low power consumption chips (such as CMOS chips) are even more susceptible to ESD damage, more so than chips made many years ago. Some chips can even be damaged by voltage as little as twice their operating voltage so if you have a chip which operates on a 5 volt power supply a 10 volt spike could damage or destroy the chip provided that the chip does not have sufficient built in ESD protection.

So what exactly is ESD? ESD is the same type of electricity that you feel when a person is shocked by walking across a carpet and touching certain objects or another person who has an opposite charge. ESD is the same type of electricity produced by lightning. A static charge is created when electrons are stripped from one object (creating a positive charge) and deposited on another object (resulting in a negative charge). In a conductor these positive and negative charges recombine nearly instantly. In an insulator these charges can continue to build consequentially building up a significant charge until a recombination of charges occur in the form on a discharge from the object holding the positive charge to an object holding a negative charge. If the voltage becomes high enough the discharge can occur through the air creating a spark.

Humidity itself has an effect on Electro Static Discharges as the higher the humidity the fewer problems that occur with ESD. Studies have shown that in an area with more than 50% humidity it is more difficult for a discharge from a human to exceed 2000 volts but at 5% humidity a discharge can easily exceed 15,000 volts! Therefore, humidity doesn't prevent ESD it just reduces the voltage level in a discharge and allows for materials to more easily discharge themselves through the atmosphere rather than through another object.

What problems doESD cause? The most obvious problem with ESD is damage to an electronic component. A static charge built up on a human that is in turn discharged to an electronic component by the human touching the component can either destroy that component or damage it. However, an ESD damaged chip can be even worse than a complete failure of a chip as a wounded chip still might test as good but cause intermittent problems or erratic behavior. The damaged or wounded chip in a PC can cause intermittent failures or erratic behavior that may become worse in time until the component completely fails. So ESD can not only destroy a component but ESD can shorten a component's life and cause a lot of headaches for a technician trying to troubleshoot a intermittent failure oran erratic behavior...

How can you protect against ESD? While storing electronic components or boards they should be stored in a anti static shielded bag. Anti static shielded bags can be recognized as they are usually semi clear with a silvery tent to the package. Anti static shielded bags prevent static from entering the bag whereas anti static unshielded bags can allow static to enter the bag thus potentially damaging the components. However, it should be noted that the anti static shielded bags should be in good condition as if the bags are badly crinkled or have holes in them they are no longer any good. I have seen tests done with a static meter where the meter was placed into a static shielded bag in good condition and a static charge was created outside of the bag yet inside the bag no charge was detected. However, a static shielded bag which was in poor condition showed a voltage on the meter where static voltage leaked into the bag.

If the components are stored in a plastic bin the bin should be made of a conductive plastic as a non conductive plastic bin will build up a charge since it is a non conductor. When chips are setting on a bench do not set a non conductor such as a styrofoam cup near the chip. The cup can contain a charge which can in turn be induced onto the chip since the chip is a conductor of electricity thus potentially destroying or damaging that chip. Also do not store boards or components in the open air. Air is a generator of static (just look at how lightning is formed in the air). The board or component should either be stored in a anti static shielded bag or in a conductive plastic storage bin.

Be aware of synthetic clothing as it can contain static charges. Long hair can really retain a charge and a grounding strap on a persons body will not discharge your hair. So if you have long hair be sure not to rub it across any components.

An ionizer in the room which produces both positive and negative ions in the air can help objects in that room slowly dissipate their charge thus reducing the chance of an ESD discharge.

When working on electronic equipment a static mat grounded to a known good ground should be used for your work area. The person working on the equipment should wear a grounded static strap thus eliminating the static charge on that person. The ground that the static mat and static strap is attached to should be checked for proper grounding. In most modern buildings the third prong in an electrical outlet is a good ground which can be tested by a mini tester (cost less than $5). The static strap itself costs around $10 with mats costing from about $20 and up. Consult the manual with your static strap or mat for proper grounding procedures...

WARNING: When working on monitors different precautions are in order. Do NOT use a static strap when working on a live monitor! If a portion of the monitor is touched and a voltage is sent through your body the charge will go directly to ground. With the static strap on one hand and touching the monitor with the other hand the path to ground is through your heart which will KILL you! Even without power applied do NOT touch the monitor until the CRT is discharged as the monitor will retain a very high voltage charge even without power! Additional precautions are in order as well...

Your first reaction to all of this is probably that you have never had a problem before with static electricity. However, it may not be evident that you have had a problem as the equipment may have only been wounded and did not fail until months later. Furthermore, plenty of places on a circuit board exist that a charge can be dissipated and not damage the circuit as some ESD protection is built into many boards. Older boards and components from many years ago are not as susceptible to ESD as today chips which operate at lower voltages and low power consumption levels not to mention today components are much smaller and thus internal traces are more susceptible to damage from ESD.

Also working equipment can even be effected by ESD where the equipment is not damaged but undesired behavior may result as ESD can float grounds or flip memory circuits or cause electronic switches to not operate properly. These issues are more or less design problems that the designer of the equipment needs to deal with in shielding the electronic components from outside interference. However, ESD can cause poorly ESD protected equipment to malfunction and the operator should be aware that it may be operator himself that is causing the equipment malfunction. Available for computer users is anti static mats for keyboards as well as small "touch me" grounded pads where the user discharges himself before touching the computer. However, this issue is an article in itself describing how the designer should design his circuits to protect against ESD or EMI interference and how the user of equipment should be aware of the possible problems that he may cause because of ESD.

Products for static prevention is available from many different sources. One source I have found that sells just static prevention products is a company called ESD Systems located at 261 Cedar Hill Street, Marlboro, MA 01752-3004, phone: (508) 485-7390.

They also have a WEB sight at: www.esdsystems.com