Watery Grave
A menacing thunderstorm catches you out on the lake in an aluminum canoe. As you paddle briskly toward shore, sensing the catastrophic danger of a direct strike by lightning, you face three choices: remain seated in the canoe; abandon the canoe and swim for shore; or invert the canoe and dive under it for protection, making a sort of crude Faraday cage.
Todd Hubing, past president of the IEEE EMC society, presented his solution to this puzzle at a lecture for the society. Lightning, Hubing points out, is so powerful that a direct blow kills you regardless of what you do. A full-sized bolt delivers something on the order of 300 MJ, enough to boil 1000 kg of water, explode the trunk of an oak tree, or fuse silica into solid glass. You will not survive a direct hit.
Your only options concern the "radius of survivability." If you improve your ability to withstand near-miss situations, you improve your ability to survive.
Therefore, Hubing assumes that lightning strikes somewhere nearby but not directly on your canoe. When it strikes, megajoules of energy pass through the water. If your body is immersed in the water at that time, part of the lightning's energy passes through you on the way to its final destination. Because it takes only a minute fraction of the bolt's total energy to kill you, Hubing suggests that your best option is to remain seated in the canoe. The passing current then diverts through the hull of your boat, not your body. You may lose your hearing if it strikes close by, but not your life.
Inverting the boat provides no useful protection against large transient currents flowing through the water. This scenario might work in a severe hailstorm, but not against lightning. Stay in the boat.
ESD protection for digital products involves a similar philosophy. You cannot avoid damage caused by large ESD discharges directly applied to your chips, but you can design your system to survive near-miss situations. The most common near-miss scenarios include discharges to your product chassis, the wires leading into or out of your chassis, or metallic objects near those wires.
Continuing the water analogy, if your processor chip sits "in the water"—that is, in the path of an ESD transient current—your system will fail when struck. You must remove your processor from the water by diverting the ESD transient current around the external shell of your product to ground (or wherever it is going), rather than passing it through your digital-logic ground plane.
Tough, reliable products incorporate multiple layers of ESD protection. Each layer attenuates the effects of transient ESD currents, protecting the processor and other internal circuitry from failure. The layers employ three main ideas:
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Shunt elements, such as spark gaps, varistors, and capacitors that divert high-voltage transients to the shell of your product;
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Series impedances formed by transformers, optocouplers, common-mode chokes, GMR (giant-magnetoresistive) couplers, or thick insulation that block the ingress of fast transient currents; and
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Topological changes to your product architecture, such as moving all of the connectors to one side of the product, that encourage currents arriving on one wire to quickly leave on another wire without traversing your system.
If you have not performed ESD testing on your product, find someone who is good at it, and ask his or her advice. Your local IEEE EMC chapter (http://www.emcs.org/) is an excellent place to start looking for an ESD guru who knows how to protect your hardware.
To do otherwise risks sending your project to a watery grave.