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In this lesson, we will discuss electrostatic discharge, or also known as ESD, and explore how to mitigate the EMI issues this phenomenon creates in our electronic products and printed circuit boards.
Understanding Static Electricity in Circuit Board Design
One of the significant phenomena we must consider when designing printed circuit boards is the occurrence of static electricity. Specifically, we need to focus on the discharge event commonly referred to as electrostatic discharge (ESD). ESD can be viewed as a particular case of immunity to transient interferences.
To fully grasp this phenomenon and its implications, we first need to examine how static electricity is generated. Static electricity forms when two objects accumulate an electric charge through processes such as induction or friction. This can happen, for instance, when two materials come into contact and rub against each other, resulting in the transfer of electrons between them. The underlying cause of this transfer lies in the differences in energy levels between the two materials.
When a charged material interacts with another, it can either attract or repel the electrons of the second material, leading to the creation of an induced charge. As a result, a positively charged area is produced in the second material, which in turn causes an attraction between the two.
There are various methods through which static electricity can be generated. Some common examples include the triboelectric effect, the piezoelectric effect, and induction charging, among others. Focusing on the triboelectric effect, this phenomenon occurs when two materials with differing electrical properties are rubbed together, leading to the generation of electrostatic charges.
But what do we mean by "electrical properties"?
These properties are outlined in what is known as the triboelectric series. This series ranks materials based on their tendency to either give up electrons or acquire them. Essentially, the triboelectric series serves as a guide to help us understand how different materials interact with one another in terms of charge transfer.
The materials at the top of the triboelectric series will tend to transfer charge to those at the bottom. However, it’s important to note that just because a material is ranked higher doesn’t necessarily mean it will create a greater amount of charge transfer. The actual transfer of charge is influenced by various factors, including the smoothness of the surfaces in contact and the speed at which the two materials are separated.
For us to effectively understand the ESD phenomenon, we must recognize that many issues arise from materials with low charge mobility. This low mobility allows charges to accumulate on the surface of the materials, leading to a higher potential difference. For instance, if we take two conductors with high charge mobility and separate them, we won't observe any triboelectric charging. In this case, as soon as we begin to separate the materials, the charges will not accumulate but will return to their original state.Thus, our focus should shift to situations where an insulator comes into contact with a conductor.
We previously mentioned that static electricity arises from low charge mobility in materials, which means that the charges primarily exist at the material's surface rather than throughout its interior. In insulators, this localized charge does not disperse through the material; instead, it remains concentrated where it was generated. This behavior contrasts sharply with conductors, where charges can move freely across the surface. Consequently, when we ground an insulator, nothing occurs because the charges cannot migrate from their initial location. In contrast, grounding a conductor will effectively remove excess charges.
🔓 A key point to remember is that the charge typically originates in the insulator, where it remains stationary, and can then be transferred to a conductor either through direct contact or induction.
Once the charge is present in the conductor, where it is free to move, the risk of electrostatic discharge increases, especially if this conductor is brought near another metallic object. In such cases, the potential for ESD becomes a significant consideration in circuit board design, and understanding these interactions is essential for preventing issues related to static electricity.
This process can happen even when the metallic object in question is not directly connected to an earth ground. The current can still flow through the parasitic capacitance that exists between the metal and the earth ground. This mechanism is essentially how transient events occur, operating on principles similar to those found in alternating current (AC) systems within a capacitor.
During an ESD event, the current does not travel through the protective earth connection, which is often represented by the green and yellow wire. This wire presents a high impedance to the ESD discharge. Instead, the ESD charge flows through the parasitic capacitance formed between the metallic object and the earth ground, effectively acting like a capacitor.
This explains why we can sometimes feel an ESD event, for example, when we touch a door handle that is not directly connected to earth ground. The sensation arises due to the capacitance between the door handle and the earth ground.
When we consider a charged element, whether it is an insulator or a conductor, it will create a static electric field around it due to the presence of these charges. If this charged object is brought near another neutral conductor, it induces polarization or displacement of charges within the neutral conductor. This means that the charges in the neutral conductor will begin to align in a specific direction to counterbalance the applied charge from the charged object, effectively creating its own electrostatic field.