Maker Blog - Protect your Product from ESD

2016-02-11 | By Maker.io Staff

Electrostatic discharge (ESD) refers to the time when static electricity stops being static and actually zaps. For people, this is painful and maybe amusing to friends but generally not a problem. With electronics, an ESD event can completely destroy a piece of electronics or, perhaps worse in some situations, cause intermittent, difficult-to-trace faults. These events can be controlled in the prototyping and production phases, but once a product is released into the wild, it needs to be robust enough to withstand anything your customers throw at it. While nothing will make a board completely impervious to ESD, there are ways to make it much less susceptible, using both well-thought-out board design and additional protection devices.

For the board layout, an important concept to remember is that inductance is not your friend when it comes to ESD. Longer traces—and traces that pass through multiple vias—have higher inductance, which generates its own voltages as ESD current flows through the traces, increasing the peak current. To reduce this issue, find out which lines are actually going to come in contact with the outside world, usually power connectors, communication lines, and certain touch interfaces. Those lines, which have the highest probability of being exposed to ESD, should have their lengths and usage of vias minimized as much as possible.

Two parts specifically designed to help reduce the effects of ESD are Transient Voltage Suppression (TVS) diodes and metal oxide varistors (MOVs). These specialized parts, ideal for placement on power lines where relatively high power is constantly on the line but high-speed communications are not expected, are designed to protect circuits against transients and overvoltage situations. As a simplified definition, TVS diodes are typically used for low-voltage devices while MOVs are used on line-powered devices. It is relatively simple process, yet extremely important, to choose the correct ratings for these devices—limits between the nominal and maximum voltage levels of the circuit.

Besides these specialty devices, resistors can be put inline with vulnerable traces. Because there will be negligible current on these lines, mostly just a changing voltage on high-impedance inputs, this resistor will not affect performance or power consumption. However, if a high voltage is introduced to the line, instead of a nearly zero-impedance line leading to the port, there will be a resistor to both reduce the current and absorb some of the power. This very low-cost method of protecting certain lines can be very effective; but for high-frequency lines the change in impedance can be enough to take the signals out of compliance.

ESD can make anyone’s day worse, but with proper planning, and discovering what protection devices and methods work best with your design, factoring in space and money, its effects can be greatly reduced. The first time you hear that tell-tale zap when someone is holding your product, you’ll be glad that you put in the effort of ESD-proofing.

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