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ESD or Electrostatic Discharge is something that affects all modern electronics. It is the flow of charge between two objects. The best example of this is when you walk across the carpet in sox, wool sox are best, and then touch a metal doorknob. The spark that jumps between your finger and the doorknob is ESD. This, dielectric breakdown, occurs when the charge buildup exceeds the dielectric strength of the air between the doorknob and your finger. Because there is a buildup of charge on the two objects there is an electric field between you and the metal doorknob. That field wants to move electrons from a high potential energy location to a lower potential energy location. The air acts like a resistor because of its intrinsic dielectric strength. Once the air becomes ionized, due to the strong electric field created by the displacement of electrons, the resistance between the two objects drops and electrons begin to flow, pop!
This is great fun when we are children, but as adults trying to manufacture a delicate electrical device, well not so much. Electric sparks require more than 100,000 volts to jump a one-inch gap. I have worked in a variety of locations where the treatment of ESD varied from working on a carpet and a poly-wool covered seat that required me to touch my ESD mat while I sat down to wearing heal straps, wrist straps, working with ion generators, conductive flooring and more. My favorite was when handing electronics between people we had to have skin to skin contact first. I don’t know how effective it truly was but it was a rather intimate way of working with electronics.
The fact is that is does not require much voltage to destroy modern electronics. But destroying a product you are working on is easier to deal with than merely damaging it. When the device is destroyed we call that catastrophic failure while slightly damaging it is called a latent defect. If the product is damaged but not destroyed, it will likely work, sort of. And that is the problem. You might spend a great deal of time and money with a damaged circuit that work most of the time. An even worse scenario is if you manage to ship this product to your customer and they experience an unreliable product.
I mentioned some of the ways to prevent ESD damage above. It is important to know that you do not have to touch a product to damage it. You can transfer enough charge from a foot away to damage some circuits. Manufacturers do testing to determine at what voltage level their devices will be damaged. ESD protection can be incorporated into a chip or it can be added to a circuit board as a separate chip.
TVS (transient voltage suppressors) diodes are devices that will provide some protection to your electronics. They are designed to conduct charge rapidly from a signal line to ground. They are rated to protect different voltage levels, e.g. a 3.3V or 5.0V lines. They are also rated to handle up to a certain maximum ESD event, e.g. 15kV (air) or 8kV (contact). It is recommended that any lines that leave your board be protected. Some devices such as an RS232 serial chip may come with its own protection. But buttons used for user interface and connected directly to the MCU are very vulnerable and should have a TVS diode attached to the line as close to the connector as possible.
This newsletter is sponsored by Celtic Engineering Solutions LLC, a design engineering firm based out of West Jordan, Utah, which can be found on the web at www.celticengineeringsolutions.com. You can find the newsletter on the company blog, LinkedIn or by subscribing. Send your emails to The Celtic Engineer at [email protected].