Introduction
Welcome back to our series on ESD mitigation! In our last post, we introduced the basics of ESD and highlighted the limitations of traditional measurement techniques. Today, we'll delve into the details of electrostatics, focusing on the crucial difference between voltage and charge.
But first – what does electrostatics mean? Well, the charge and electric fields involved are not completely static, it simply means they’re moving at time scales much slower than those required for electromagnetism, so there is no discernable interaction with magnets and magnetic fields. This means that the important metrics for electrostatics are electric potential or voltage, and electrostatic charge.
Voltage and Charge
Electric Potential / Voltage | Electrostatic Charge |
Defined as the potential difference between two points, requires a reference | Defined as an absolute quantity in space, with no external reference |
Equal everywhere throughout a conductor | Redistributes on the surface of conductors so that the internal field is zero |
Poorly defined on insulators | Tends to stay in place on insulators |
Can be measured relative to a ground potential | Historically difficult to measure outright |
Logical metric for defining Standards and thresholds in the EPA (ESD Protected Area) | Varies spatially and is difficult to predict analytically |
Voltage: The Traditional Metric
Voltage, or the potential difference between two points, is the metric most commonly used in ESD mitigation standards because voltage is constant across a conductor and straightforward to measure. However, voltage doesn't provide a complete picture of the electrostatic landscape.
Charge: The Real Culprit
Electrostatic charge is the real root cause of ESD events. Charge is distributed nonlinearly across surfaces and can vary greatly depending on the object's shape and material properties. While more challenging to quantify, understanding charge distribution is essential for effective ESD mitigation.
Unexpected Charge Effects
Numerical methods are necessary to fully predict how charge is distributed in space, but it is possible to build an intuition on its distribution by following a few trends, as shown in the figure above.
One unexpected feature of charge is that it concentrates at sharp points, as demonstration by the middle shape. This is why most ionizers use sharp needles to ionize the air, and why a static shock is more painful when it occurs at the tip of your finger than at the back of your hand.
The scenario on the right highlights the effect of induced charge. Even if an object has zero net charge, or when it is grounded at 0V, a nearby charged object can create non-zero charge on a particular surface. This explains why the hairs on your arm stand on end when you approach a highly charged object, and how you can sometimes anticipate when you are about to get shocked. It’s also how the floating PCB in the last post experienced an ESD event from a grounded operator!
Why Charge Matters
When it comes to ESD, it's the movement of charge that causes damage. Traditional voltage measurements can miss these subtle charge distributions, leaving electronics vulnerable. IONA Tech’s cutting-edge wearable devices use a miniaturized electric field mill to measure local surface charge on the human body. By shifting the focus from voltage to charge, IONA Tech's wearable measurement equipment offers a more effective solution for ESD mitigation. In our next post, we'll explore the technology behind the StatIQ Band and how it works in real-world scenarios. Stay tuned!
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