Factbase: Understanding Electrostatic Discharge
or How I Learned to Stop Worrying and Love Static Electricity
Electrostatic Discharge is the phenomenon where electricity suddenly flows from one object to another. For our purposes, the worry is that electricity might flow from you to your Mac mini; you might feel only a shock while your mini is permanently damaged. Why does this happen, and how do we prevent it from zapping our hardware?
What’s going on with electrons?
All objects want to keep the number of electons and protons they contain in balance. In this state, we say that they are electrically neutral. All objects can also transfer electrons to other objects; this transfer is the flow of electricity. Sometimes, this transfer leaves one object with a shortage of electrons and the other with a surplus. These objects are then considered either positively or negatively charged.
Objects that allow electrons to move easily are called conductors; those that do not are called dielectrics, or insulators. ESD becomes a problem when objects become highly charged and, because they are insulators, are unable to neutralize themselves. At some point, they may find a means to do this suddenly; this discharge, or rapid exchange of electrons, is ESD.
We say that objects with distinctly different charges have a ‘high potential’—while a lot of electrons need to transfer from one object to another, they are prevented from doing so. Conversely, two objects with large but equal charges have a low potential—though each carry an excess or lack of electrons, transferring electrons helps neither object become neutral, so nothing happens. When we refer to electric potential, the term only describes the size of the difference in charges carried by two objects; the term does not address the strength of each object’s charge as measured in absolute terms. (This concept tends to elude people at first.)
So, how do charge imbalances occur?
People are most familiar with static electricity as a mechanism for building up a charge. Static electricity occurs when two insulators of different materials—such as your foot and the carpet—rub together and an electron is transferred from one is to the other. This exchange leaves one object with a negative charge, and the other with a positive charge; there is now a small potential between the two objects. If you were to walk around your house all day without returning those excess electrons, that potential might grow quite large.
In this scenario, air is the insulator that prevents your charge from equalizing with other objects. Water molecules are good at picking up excess electrons, and their free motion in the air makes them very good transport mechanisms; this results in few charged objects in a humid climate. Dry air is a much better insulator than humid air, so if you live in a dry climate you’re much more likely to have static issues. In a dry climate, an object (usually you) repeatedly picks up electrons through friction until the electron imbalance becomes so large that some event causes it to suddenly equalize—electrons leave you, rapidly, and move by way of a spark to an object with a shortage of them. This spark is ESD.
Why do sparks occur?
As two objects with high potential get close to each other, one object’s surplus electons are attracted to the other oject. As this occurs, the electrons are no longer evenly distributed across the object’s surface; they pool up where the two objects are closest. As they pool up, they may cause the air near them to convert to a plasma, a form of matter where the electrons are no longer attached. Whereas the air was an insulator, the plasma created from the air is a conductor; electrons very rapidly flow from one object to another through this new conductive path. Now the two objects are joined by a conductor and—zap—electrons flow between them, equalizing the charge. The spark you feel is the tranfer of energy.
More interesting is that, because the creation of the plasma strips electrons from neighboring air molecules, those electrons join in; electrons from the air actually increase the spark’s total energy. In fact, the electrons from spark’s formation may increase the delivered energy by a hundred-fold, or more.
Caution: ESD Has Other Dangers
Sparks hurt because they are extremetly hot and cause a small burn.
If you get yourself charged up in your car and then stop to buy gas, you may discharge near the gasoline fumes; the spark can be sufficient to start a fire at the end of the gasoline nozzle. Needless to say, this will completely screw your day, not to mention your car.
Always touch a large metal object (your car door, the gas pump itself, etc.) before starting the gas pump, and don’t get back in your car while the gas is pumping.
How do I prevent this from harming my equipment?
When dealing with electronics, remember that they are composed of insulators with veins of very fine conductors and semiconductors going through them. Amidst this web of pathways are delicate components that are easily melted by a large surge of electricity. (After all, that’s how an arc welder works.) So, if you quickly discharge to your mini, well, it breaks. Forever.
Two tools for dealing with ESD are anti-static bags and ESD wrist-straps. Anti-static bags are designed to keep the object inside from building up a charge. While some are in fact anti-static, most are actualy slightly conductive and static dissipative. The bag not only prevents the object inside from building up a charge, but the bag can slowly dissipate the charge as well. The bag solves a big problem with shipping components; most components are packed with polystyrene (styrofoam), which loves giving up electrons. Without a static dissipative bag, after 15 rounds with the UPS guy your Mac mini could emerge from its box as a single-use defribullator.
The ESD wrist-strap has a metal button which comes in contact with your skin and connects by wire to your computer. This creates a circuit so that electrons can flow smoothly from you to the computer, without creating a spark through the air—it’s correct to say that you are ‘grounded’ to the computer. (In most straps, a resistor in the wire prevents any flow of electrons from being a sudden surge.) The ESD strap works by allowing excess electrons to flow gently and safely to your computer, while you go about the business of dismantling your fine computer. However, the ESD strap will not help if you, the strap and your mini are all neutrally grounded, and then you grab a highly charged hard drive sitting on the table. You’ll simply discharge from the hard drive and fry that instead.
So, how do you make sure you never blow up one of your components, ESD wrist-strap or not? The key to the whole thing is recognizing that it is the sudden release of energy, especially through a spark, that causes the problem.
Since air is not a perfect insulator, the simple solution is to move slowly, giving time for any excess electrons to flow between objects through the air or some other resistive surface (e.g. a wooden table top). Simply put your hand an inch or so away from a component (too far for a spark to occur) and leave it there for 10 or 15 seconds; this will give the electricity enough time to flow slowly from you to the component. Remember, each object that you encounter might have a significant charge difference from you; give the object time to come into balance with yourself, without it happening suddenly. Do this consistently and you’ll never need an ESD strap and never blow up an electrical component.