Lightning is a massive electrostatic discharge (ESD) when the dielectric of air between clouds and earth breaks down.
I just moved to the arid west of the US after a couple of years in the south. I grew up here, but I’d forgotten how common it is to get little static shocks. The high humidity index in the south makes the air more conductive. However, here the dry, and thus more insulating, air means that the shocks are much bigger when you get them. As kids, we used to scoot across the carpet in our socks and try to shock each other (this is highly frowned upon at weddings - you’ve been warned).
While I’m still surprised by every shock, I’m not damaged. This is not true for electronics, which can be destroyed by voltages so small that you’d never feel them. As a result, it’s important to plan for mitigation and protection in your PCB esb designs.
It’s easy to generate enough voltage differential for a shock just by walking, even if you aren’t doing it intentionally.
What is ESD?
Electrostatic discharge (ESD) occurs when two objects with different charges get close enough, or charged enough, to break down the dielectric between them. For consumer products, that breakdown usually occurs over the air, with voltages of over 40 kV/cm.
Lightning is the form of ESD that people are usually most familiar with when clouds and the earth form a giant capacitor. Less dramatic is when you shake a fleece or wool blanket at night and see the little sparks jump.
How does this affect my PCB?
Any printed circuit board may be subject to an ESD if it’s touched or comes close enough to people, packaging, cables, furry pets, or any other object that might contain an opposite charge. When they do touch, that voltage discharges and creates a comparatively massive voltage spike. As the voltage spike dissipates, the discharge current generates electromagnetic fields across the PCB. The goal of ESD protection is to minimize any impact or effects from the discharge and resulting EM.
In particular, many modern chipsets are made using such small lithography features that they have little or no tolerance for high voltage, even DC values above their operating voltage of 3.3V. The result of an ESD event directly reaching one of these components is usually disastrous, completely ruining the IC.
Nearly every element of your PCB design (traces, routing, layers, component placement, and spacing) can affect the ESD protection on your board. That means you need to consider ESD early in your design process; otherwise, you’re likely to require major PCB redesign to fix routing and component placement issues.
What causes ESD in products?
Even if you aren’t fending off giant cousins with a pterodactyl wingspan and huge static collecting feet, you can still have an ESD events from a mundane activity. Frequently, walking alone collects enough charge to damage common components.
On a PCB, a discharge usually originates at the user interface, or at another input. Any activity, like plugging in cables, or pushing buttons, keys, or screens can, and often does, result in a discharge. Also, touching components themselves can be really bad news. I lost a lot of RF modules that way.
Plugs and connectors are the most common source of ESD into consumer products.
How do I protect my board?
The first line of defense for your PCB is to protect the board from any external connections. Since this is how users will most often interact with your product, it deserves serious consideration. Every time someone plugs something in pulls a cable out, or pushes a button, they’re introducing ESD risk to the device.
On your PCB, make sure any and all connectors are attached using a copper land, or pad. You should keep the pad separate from the PCB ground so that any shock at the input isn’t immediately routed to every other component on the board.
Instead of connecting to ground, you should use a TVS (transient voltage suppressor) at each external connection to protect the board and its sensitive components. The TVS is a small circuit of diodes that blocks most voltage input, but breaks down under high voltage conditions, helping to protect the rest of the PCB.
There are a number of other design practices you should utilize:
Reduce circuit loops
Minimize parasitic inductance
Limit trace length
Stay tuned, we will be discussing these points in greater detail over the next few weeks.
It’s true that tracking voltage tolerances of your parts, checking trace widths, and keeping on top of all your other requirements can be an ordeal, but it’s a necessary part of designing a well-protected board. Great PCB tools, like Altium Vault and the features in Altium Designer, can make tracking your requirements and design rules easier. Then you can focus on the fun parts of your PCB design. Altium representatives are available to help you now… or after you’ve finished home-testing ESD by rubbing balloons on your dog to make his fur stand up.
Have a question about ESD? Contact a representative at Altium.
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