CLICK THE PLAY BUTTON TO WATCH THE LESSON.
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.
If the electrostatic field source is negatively charged, the side of the neutral conductor closest to it will become positively charged, while the opposite side will acquire a negative charge. Importantly, this process does not alter the overall amount of charge contained within the neutral conductor; instead, it changes how this charge is distributed across the conductor's surface.
If we remove the charged object, the charges in the neutral conductor will redistribute, allowing the conductor to revert to a neutral state. However, if the source of the electrostatic field remains close, the charges will continue to remain separated, with positive charges accumulating on one side and negative charges on the other.
Now, if we connect this polarized conductor to earth ground, this connection allows the excess charges on the negatively charged side of the conductor to flow into the ground, effectively neutralizing that side of the conductor. The charges will seek a balanced state between the external charge from the electrostatic field source and the charges present in the conductor.
Should we suddenly remove this ground connection while the source of the electrostatic field remains nearby, the conductor will retain a charge. The charges that flowed to the earth ground will be lost, leaving the conductor charged with the remaining charges.
This scenario illustrates how charges bound in an insulator can influence the movement of charges in a nearby conductor, ultimately resulting in the creation of additional charged objects.
In the table below, we can observe how certain common daily activities can generate significant electrostatic voltages. Additionally, it highlights how these voltages are highly influenced by the humidity level in the surrounding air.
Being aware of this relationship allows us to leverage it for our benefit and helps us understand the environmental implications on our devices and their users regarding how ESD can affect circuit performance.
Attraction of charges
We previously noted that charges in a conductor spread across its surface when subjected to an external charge. But why does this occur?
The answer lies in the attraction between opposite charges and the repulsion between like charges. This interaction causes charges to redistribute, effectively moving apart from each other.
However, this movement is confined to the surface of the object; once the charges reach the surface, they cease to move inward. This indicates that no accumulation of charges will occur within the conductor itself.
The intriguing part arises when we provide a pathway for the surface charges to flow. Given that the mobility of charges within a conductor is high, these surface charges can easily move away. A common example of this is when we connect a conductor to an earth ground.
So does this mean conductors are inherently more dangerous than insulators, leading us to disregard the risks associated with insulators when discussing ESD?
Not quite. The potential danger of an insulator lies not in the charges it contains, but rather in its ability to induce a charge in a nearby conductor. This induced charge can lead to the occurrence of an electrostatic discharge event.
Everything begins with these charges and the electrostatic fields that surround them. Understanding this is crucial, as it lays the groundwork for the concept of free space capacitance, which I will explain next.
Free Space Capacitance
Now, let’s delve into how free space capacitance differs from the more traditional concept of parallel plate capacitance that many of us are already familiar with. Free space capacitance can be conceptualized as the capacitance of an object relative to the space surrounding it. To calculate this, we start by comparing the surface area of the object to the surface area of a hypothetical sphere.
To calculate the free space capacitance of an object, we first determine its surface area and then create a sphere with the same surface area. The free space capacitance is the capacitance between this sphere and another sphere with an infinite radius that surrounds it. We use a specific formula, derived from the capacitance formula for spheres, to find this value.
🔓 A key point to grasp is that every object has an associated free space capacitance.
The formula provided in Figure 8 can be used to calculate this capacitance in picofarads, where 𝑅 represents the radius of the sphere that shares the same surface area as the object being measured.
As a reference, we can approximate that the typical capacitance of a human is around 50 picofarads. Of course, this is just an approximation to illustrate a point. For comparison, the Earth has an approximate capacitance of about 700 microfarads.
Capacitance between parallel plates
In addition to free space capacitance, we also need to account for the capacitance between two metal plates, which is essential for understanding how discharge events can occur between objects that are not physically connected.
The formula to calculate this capacitance is derived from classical physics, where:
𝜖 is the dielectric constant of the medium between the plates,
𝐴 is the area of the plates,
𝐷 is the distance separating the plates.
Both the formulas for free space capacitance and parallel plate capacitance are integral in determining the total capacitance of an object.
Now that we have established this foundational concept, we can apply it to calculate the total capacitance of a person, allowing us to model the electrostatic discharge effects that users can exert on our devices. This understanding is essential because humans can accumulate electrical charges, and due to our conductive nature, we have the potential to transfer these charges during an electrostatic discharge event to sensitive equipment or electronic circuits, leading to possible damage.
Human Body Model
In modeling the human body for ESD effects, we typically consider the equivalent area of a human as a sphere with a diameter of one meter. This approximation gives us a free space capacitance of approximately 50 picofarads, as previously mentioned. However, in addition to this free space capacitance, we must also account for the parallel plate capacitance. This means we need to evaluate how the capacitance of our body is influenced by our surrounding environment.
For instance, consider the soles of our feet in contact with the earth ground and our hands, or fingers touching walls, or other structures. There will be a certain amount of parallel capacitance between these various elements, and it’s important to include all these variables when calculating the overall model of our body.
The total equivalent capacitance typically used for this human body model ranges from about 50 picofarads to 250 picofarads. Once we establish this total capacitance, we must also consider the electrical resistance of the body. This aspect is significant because as our bodies accumulate charge, a voltage potential builds up, and resistance plays a critical role in determining how the current flows through us and eventually discharges onto another object. Notably, the resistance can vary based on contact points; for example, the resistance of our fingertip differs from that of the palm of our hand.
The typical resistance of the human body is generally measured between 500 ohms to 10 kilo ohms. However, if we are in contact with another conductive material, such as holding keys, this resistance can drop as low as 50 ohms. Thus, incorporating resistance into the human body model is essential for simulating scenarios where users touch a device or merely approach it. This understanding allows us to establish testing procedures to evaluate our devices against these ESD phenomena effectively.
Testing
With an equivalent model in place, we can create instruments for testing. A typical, straightforward electrical model used during such tests is the ESD gun, which consists of a discharge capacitor and an equivalent series resistance. The component values can vary based on specific standards, but common values, such as those specified in the EN 61000-4-2 standard, are around 150 picofarads and 330 ohms.
Let’s examine the waveform of the ESD discharge applied during standard ESD tests. The ESD discharge waveform comprises two distinct waveforms.
The first waveform exhibits a sharp rise time attributed to the free space capacitance of the probe's tip, while the second, slower waveform results from the 150 picofarad capacitance in series with the tester's ground strap inductance.
Typically, the ESD waveform features a rise time ranging from 0.7 nanoseconds to 1 nanosecond, although these parameters may vary based on the applicable testing standards, which should be referenced accordingly. The discharge voltage can vary, typically ranging between 4 kV with a current of 15 A and 8 kV with a current of 30 A.
🔓 It's worth noting that some devices are already sensitive to discharges of only a few hundred volts.
The challenge arises when the discharge voltage is low, as we often cannot sense it, making debugging ESD failures particularly difficult. In contrast, when we encounter electrostatic discharges with voltages exceeding 20 kV, the effects are readily noticeable, often resulting in painful experiences.
Discharge through the door handle
Now that we have established a foundation for understanding electrostatic discharge (ESD) events, let’s examine one of the most common scenarios we often encounter: the ESD discharge that occurs when we touch a door handle. This example is helpful for illustrating the underlying principles at work during such events.
The soles of our shoes are typically made of insulating materials, which can become charged through contact with carpets or other materials. This phenomenon relates to the triboelectric effect, where certain materials gain or lose electrons upon contact. As a result, the charge builds up on the insulative soles of our shoes.
Although these charges are trapped in the shoes and cannot move freely due to their insulating properties, they can still influence the charges in our bodies, which are conductive. When we approach a conductive object, like a door handle, the accumulated charges on our body tend to redistribute themselves, causing an accumulation of charge at the extremities of our skin. If we touch the door handle, grounding that point, for instance at the tip of our finger, the charges can discharge, resulting in an ESD event.
The intriguing part of this phenomenon is that the door handle, while a conductor, does not need to be directly wired to an earth ground for the discharge to occur. This happens because of the parasitic capacitance between the door handle and the earth ground. Since ESD is a transient event, the charge will find this capacitance and behave as if it is a short circuit. Thus, the door handle effectively appears to be connected to ground, allowing the ESD event to take place. This situation is analogous to a capacitor connected to an alternating current (AC) source.
Migration of charges
Next, let’s discuss how charges can leave a charged body or object. There are two primary mechanisms: arcing and leakage. Leakage can occur in a few ways, not just through direct contact with a conductor. For example, when touching the door handle, the amount of humidity in the air also plays a significant role. Higher humidity increases the number of water vapor molecules in the air, which serves as charge carriers, making the air more conductive and facilitating charge recombination.
Another method of charge leakage occurs when a charged conductor is grounded. This principle is particularly important in electronic labs, where ESD wrist straps connect to grounding systems to allow for the safe leakage of charges from the body, acting as a conductor, to the ground.
However, this connection is still limited by the resistance of the bracelet. A direct connection without any resistance could pose dangers, particularly if a fault occurs in the grounding system. Thus, the bracelet not only aids in the discharge of accumulated charges but also limits the discharge current, which could otherwise be excessively high.
In addition to the charges escaping conductors, we must also be aware of the issues created by the electromagnetic fields generated by ESD events. These fields can disrupt circuits even when they are located at a distance from the site of the ESD event and are not physically connected. This is possible because the sharp and rapid rise time of the discharge event generates electromagnetic fields with sufficient energy to affect sensitive electronic devices.
These electromagnetic fields become especially problematic in the case of plastic enclosures, which offer little to no shielding against such fields. Therefore, it is essential to consider both the direct effects of ESD events and the potential impact of the electromagnetic fields they produce when evaluating device vulnerability.
Methods of ESD protections
Now that we have a grasp on how ESD can affect devices, let’s delve into the methods we can implement to protect our circuits from such disturbances.
These methods generally fall into several categories:
we can limit or avoid charge buildup,
insulate devices,
provide alternative discharge paths,
shield devices from electromagnetic fields,
and reduce the loop area to minimize the effects of magnetic fields.
The more we can integrate these protective measures into our design, the better we can safeguard our devices against the disturbances caused by ESD events.
Before we discuss these methods in detail, let’s briefly touch on the types of disturbances we aim to prevent.
ESD can lead to various issues classified by their disturbance level: