Electro-Static Discharge Prevention in the Home
by Mark W. Hibben
"Not My Fault"
When I'm shopping for a new electronic component or device, I often peruse the comment pages of a site such as Newegg.com to read what people think of it. I find this is useful feedback, especially for computer components such as motherboards and CPUs. Even when a product receives a good overall rating from a large number of reviewers, it's not unusual for a few people to complain of successive repeated failures of the same board or device. It never occurs to these folks that they might be doing anything wrong. Instead they complain about the poor quality of the product. Indeed they may be doing nothing wrong as far as they can tell, because the damage they are inflicting is often completely invisible: electro-static discharge (ESD). Although electronics manufacturers have understood the problem for many years, many home hobbyists don't take the necessary precautions to prevent it. Anyone who even opens a computer case to insert an add-in card or extra disk drive should understand the basics of ESD prevention, and computer hobbyists should invest in a few low-cost items that help prevent ESD damage.
What is Electro-Static Discharge?
Materials that are electrically non-conducting (insulators), can acquire a static electrical charge due to either an excess or lack of electrons compared to a normal electrically neutral state.
Because the materials are non-conducting (non-metallic), once the charge imbalance occurs, it can be retained by an object for many minutes.
Often, this is the result of physical contact or rubbing between different, non-conducting materials: rubber sneakers across a tile floor, a plastic comb through the hair, a fuzzy sweater against the body. This is the so-called triboelectric effect, in which one material transfers electrons to a dissimilar material through frictional contact. The very first observation of static electrical phenomenon was made by Thales of ancient Greece who rubbed a piece of amber with wool. The Greek word for amber is elektron. People can take on an electro-static charge through normal activities. Since hair and skin tend to be electron donors, that charge is usually positive (electrons being negatively charged by formal agreement). The strength of the charge is measured by the electric potential or voltage. The table below gives static voltages induced by common activities.
The reader may wonder why, if people are regularly carrying around thousands of volts of static charge, they don't hurt themselves or each other beyond the occasional sting of a static discharge. The reason is that people don't make very good storage devices for electrical charge, or, in electronic terms, the human body has very low capacitance, about 100 picoFarads. The discharge energy is typically in the milliJoule range. This is about the same amount of energy as you would receive if you dropped a small ball bearing a couple of feet into the palm of your hand. Usually people don't even notice discharges below a few thousand volts. For electrical components, it's a different story.
Because of the ever smaller feature size of devices and conductors, especially the cross sectional area of the conductive path, the current density becomes high enough to fuse or even vaporize the device or interconnect. The table on the right lists ESD sensitivities in terms of voltage, assuming discharge from a human body:
On the microscopic scale of integrated circuits, the damage can be quite spectacular, as shown in this before and after pair of scanning electron micrograph images on the right.
One of the really insidious characteristics of ESD is its sensitivity to humidity of the environment. Thus it’s possible for someone to not experience ESD problems because they happen to be in a high humidity environment, although this is by no means adequate protection. In any case, ESD events are inherently unpredictable and erratic. This may explain the frequent complaint of PC system builders who experience a rash of “bad boards” and invariably cite their extensive history of successful builds to support the notion that it’s the fault of the boards. The previous work may have taken place in a more humid part of the country or time of the year. Or they may simply have been lucky.
When I started working in the aerospace industry in California in the 1980s, concepts of ESD mitigation were just starting to take hold as semi-conductor electronics became more highly integrated, and thus more vulnerable. I can't say how many ESD training sessions I had to attend, but it was probably in the dozens. What I want to present here is a distillation of that training for home use, since I realize that electronics hobbyists and computer users don't have the resources or time to implement the industrial strength safeguards. It's important for the reader to realize that there is no perfect solution and no way to completely eliminate that possibility of a damaging ESD event. Likewise, if the reader is willing to accept a somewhat elevated but still very acceptable level of risk, then the mitigation approaches can be very simple.
Most ESD protective equipment works by electrically attaching a person or a piece of equipment to an electrical ground. In most of the ESD tutorials and training there is an assumption made that the individual hobbyist will not be able to make: the assumption of a reliable electrical ground. Most ESD training is oriented to the workplace, where modern electrical building codes specify grounded three wire outlets and wiring. In a home or apartment, a reliable ground may not always be available. Having a reliable ground is absolutely essential for ESD mitigation as nothing really works without it. In this section, I'll explain the concept of the electrical ground and how to verify whether your building wiring is properly grounded. Many older homes and apartments don't have grounded three wire wiring, but I'll explain some workarounds that I've been able to use successfully.
Going to Ground
The best electrical ground can be obtained by driving a metal stake or pipe literally into the ground, thus the name. The ground behaves like a nearly infinite reservoir of electrons, a vast ocean of electrical charge. If a positively charged object (an object missing some electrons) comes in contact with the ground, electrons will flow to the object to neutralize the charge. If a negatively charged object (one with too many electrons) comes in contact with the ground, electrons will flow into the “reservoir”, once again neutralizing the object.
Because the “ocean” of electrons is so vast, a flow of a few electrons to or from the reservoir doesn’t really change the net charge of ground, and thus the potential energy of the ground. Thus, grounding an object (or yourself) is the best way to neutralize any excess static charge that may have accumulated. Because of the reservoir property of ground, any two true grounds (such as two metal stakes driven into the ground, separated by some distance) look like a dead short to each other. An ohm meter will measure no resistance between two perfect grounds. Of course, household grounds are not perfect, because of the resistance of the electrical wiring, but this is usually no more than a few ohms, which is low enough for our purposes.
Testing for Ground
Just because your home may have three wire outlets, don't assume they are properly grounded. In older homes, three wire outlets have often been installed merely for the convenience of being able to plug in three prong appliances, and the ground wire is not even connected. If your outlets are of the two prong variety, you can safely assume that you lack grounding capability for the outlet and your wiring is two conductor lacking a third ground wire. Use a ground fault tester or a surge protector or ups system that has ground fault detection to verify the ground if you have three prong outlets. In the United States, 3 wire outlets and plugs have a standard form factor specified by the National Electrical Manufacturers Association (NEMA).
Ground fault testers like the Greenlee are available at home supply stores for under $10. If testing shows that your ground is good, then you're ready to set up your ESD safe workspace (either temporary or permanent). If your ground tests faulty, you still have some more work to do.
First inspect the outlet (with the power turned off, of course) to verify that indeed you lack a ground wire in your household wiring. If the problem is not a missing ground wire, then your wiring has some other problem such as the ground wires not being properly grounded. Here I recommend that you hire a licensed electrician to perform any necessary repair, since improper grounding is a basic electrical safety issue that should be addressed in any case.
More often, the ground fault is due to a simple lack of a grounding wire. You can obtain a ground by running a heavy gauge wire to a metal water pipe. I recommend an American Wire Gauge of 12 or lower (AWG lower than 12 is actually thicker wire). If the metal pipe runs through the ground, it will provide a good electrical ground. You can attach the wire to the pipe using a simple metal hose clamp. The arrangement is a bit ugly, but it works electrically and it's reasonably safe, assuming your pipe is really grounded. You still need to test for this, using your Ground Tester, which I'll explain in more detail below.
I recommend this only for purposes of attaching grounding straps for ESD mitigation and not as a method for obtaining a ground for your household appliances and electronics. If you need grounded three wire outlets installed in your home, you should hire an electrician and have them professionally installed. If your household wiring is two wires with a metal shield or conduit, you may be tempted to use the shield as ground. This is usually not a good idea. Generally, these shields are not grounded, but allowed to “float”. For older homes, especially homes built before WWII, this is safer than grounding the shield. Once again, if you contemplate any significant changes to your household wiring, consult a licensed electrician.
Attaching to Ground
Now that you've got a good ground, you'll need to attach your ESD protective gear (described below) to it safely. Surprisingly, much of the guidance available for the consumer is poorly thought out or just plain unsafe. Once again, this stems in part to the industrial heritage of ESD prevention programs where attaching to a ground is trivial. Usually, an exposed metal ground strap attached to a wall is available to clip or clamp a grounding wire to. Most residences don't come equipped with exposed copper grounding straps attached to the walls. Even if you have modern grounded three wire outlets, the ground is still buried in the outlet, and it's right next to high voltage AC. I've seen some instructional videos where a banana plug with a ground wire was directly inserted into the grounded part of a three wire NEMA receptacle of a surge suppressing outlet strip. Theoretically, this is safe, if you know what you are doing, but actually, I don't recommend it, especially if small children are in the house. I believe a better method is to use a surge suppressing outlet strip or Uninterruptible Power Supply (UPS) with an external grounding lug. This lug is specifically there for the purpose of attaching grounding wires, and there's no danger of accidentally plugging your ground strap into a high voltage AC current source. If your surge suppressing outlet strip lacks a grounding lug (and many do) but provides surge protection for Coax RF cables (used for TV antennas and Cable TV service), you can use the threaded part of the RF connection as a safe attachment point for your ground.
The RF connection lugs are a little large in diameter for an alligator clip, but you can buy RF connectors at your local building supply store and use one of these to attach your ground.
One method of grounding attachment that I don't recommend is to attach your grounding wire to the center screw of a wall outlet plastic cover. I have seen this shown in ESD pamphlets, but it really isn't safe, especially if you have small children in the home. It is true that for a grounded outlet, the screw is also grounded. However, having grounded wires in close proximity to high voltage AC power is not a good idea. If line cords are plugged into the outlet, it's possible for the ground wire to slip underneath the plug and come into contact with the bare metal terminal, possibly causing a short circuit.
If you're using an external ground wire that you attached to a water pipe, you can attach that temporarily to the grounding lug of your outlet strip. This provides a convenient way to verify the ground, using either a ground fault tester inserted into one of the outlet strip outlets, or using the outlet strip fault detection, if available. This should not be used as a permanent way to provide grounded three wire outlets in homes or apartments that lack grounded wiring. The ground wire may fall off or become disconnected or severed, since it is exposed and unprotected. Permanent household wiring is either in the wall or covered by protective electrical conduit for this reason. If the ground tests good, it should be safe to use the outlet strip with its ground on a temporary basis for the length of your electronics related activity. Unplug the grounded outlet strip when you are finished.
ESD Protective Gear
Now that you have a reliable ground and a safe way to attach to it, the rest is easy, because it's just shopping, and using what you've bought. At the very least, you should have a static safe wrist strap. The strap goes around your wrist or bare ankle if you need both hands free. The wrist strap provides a path for the excess static charge you might be carrying around to be neutralized. The other end attaches to your ground. This end is usually a banana plug or alligator clip over a banana plug. Always put on your wrist strap prior to touching any electronic components, boards, or assemblies, or opening the case of a personal computer.
An alternative way of attaching your wrist strap to ground is to use the case of the computer or other electronics as the point of attachment. For PCs that connect through grounded three wire outlets and that have power supplies that can be switched off, this is very convenient. Simply switch the power supply off, but leave the power cord plugged into a grounded outlet. The power supply housing and any metal in contact with the power supply will be grounded. For other electronics, this may not be safe or convenient, since most home electronics really never power completely off unless the power cord is unplugged. In that case, attach your wrist strap to your verified ground, and unplug the power cord of the device before opening it. Always follow manufacturers' safety directions when it comes to servicing your home electronics.
For occasional use, the ESD wrist strap and a reliable ground are sufficient. You will be able to open your computer safely and perform upgrades of hard drives, memory, and expansion boards. If you ever plan to do these things, it's worthwhile to invest the minimal cost and time to be ESD safe. If you're a hobbyist or avid gamer who builds PCs, you should also buy a static safe mat and a pair of static safe work gloves.
The mat is plastic, and usually can be rolled up after use, but it's also conductive enough to dissipate static charges of objects placed on the mat, but not so conductive that it might short out an electronic circuit that was powered up. The mat also needs to be attached to ground, just like your wrist strap. You really need the mat to have a safe surface on which to place a motherboard that you are working on. It also provides an added margin of protection whenever you are working on your PC or other electronics by placing them on the mat.
The static safe work gloves are really more for cleanliness, but they have conductive threads sewn into the fabric in order to prevent static charge build up. Of course, you need to wear your wrist strap when using the gloves. I use the gloves mainly when installing processors into motherboards in order to avoid touching contact surfaces with my fingers. The gloves can feel a little clumsy at first, but of all the things you want to protect from ESD, a computer processor is probably the most valuable. By the way, in order to avoid getting thermal paste on the gloves, I apply the paste after the processor is installed in the motherboard. I use a chisel-bladed X-acto knife to spread the paste and find I can do this without getting paste all over the place, even when the motherboard is already installed in the PC case.
It's also very useful to have a few ESD safe plastic bags handy when removing ad-in cards or other electronics. I've accumulated a stockpile over the years just by buying stuff, but you can also buy these if you don't have any.
They are really optional if you already have an ESD mat, but more important if you don't, since you need some place to put electronics you have removed or are handling.
The ESD shopping list is shown below. Note that if you don't need the mat, you can get everything for under $100. Prices are based on on-line stores. Your prices for these items can vary quite a lot. I happen to live close to a Fry's Electronics where many of these items are even less expensive, such as the ESD mat. As always, it pays to shop around. Here's some links to on-line ESD supply houses. Please note that I don't vouch for their business ethics or the reliability of their service or products.