The three mechanisms through which ESD is generated in electronic circuits are triboelectrification, conduction, and inductive charging.
The primary and foremost principle of ESD control in an electronic circuit is to design an ESD-resistant system.
For ESD control, electronic components and circuits must be handled only in an electrostatic protected area.
To control ESD, electronic components and circuits must be handled in an electrostatic protected area
Electrostatic discharge (ESD) is regarded as one of the most expensive and detrimental events affecting the electronic industry. Generally, the environment where circuits or devices are placed is prone to static electricity, and under the influence of ESD, electronic circuits or devices can malfunction.
The impact of ESD on electronic components varies. In integrated circuits, ESD may cause contact damages, burnouts, interface damages, or ruptures. The effects of ESD on capacitors or resistors can be a variation in the tolerance values.
Irrespective of the component type, ESD is harmful to the life and performance of electronic components. Sometimes, the consequences of ESD are catastrophic. Other times they cause parametric failures or latent damages. Hardware engineers need to be aware of the basic principles of electrostatic discharge control to prevent these damages. In this article, we will discuss some fundamentals of ESD as well as basic principles of ESD control.
What Is Electrostatic Discharge?
Electrostatic discharge is the release of static electricity between two electrically charged objects in contact. In ESD, there is a build-up of charges between two surfaces. When the surfaces rub against each other, the charges move, causing excess charges on one surface and deficiency on another. The movement of charges due to electrostatic discharge causes current, called ESD current, to flow in objects.
Besides ESD currents being caused by contact with charged conductors, there are a few other ways in which ESD-sensitive components can be affected by ESD. The human body or packaging materials may discharge through electronic components and cause ESD. In rare cases, field-induced ESD also occurs in electronic circuits.
How Electrostatic Discharge Is Generated
The three mechanisms through which ESD can be generated in electronic circuits are:
Triboelectrification is a contact charging process. There is a transfer of charges from one object surface to another in triboelectrification. The ESD resulting from triboelectrification depends on the area of contact, friction, and pressure, and the material property influences the polarity of charges on the contact surfaces.
When two charged bodies of different electric potentials come in contact, ESD is generated by conduction. The charge flows from the body at the higher potential to the one at the lower potential, continuing until the two bodies become equipotential.
Inductive charging occurs in the presence of induced electric fields. Even though the net charge on a conductor placed in an electric field is zero, there are charges of opposite polarities on either side of the conductor. The relocation of the opposite polarity charges forms a capacitance divider, and a voltage is established across the conductor. The voltage level developed in the conductor is dependent on the capacitance value.
When there is a conducting path, the capacitors formed through inductive charging in the conductor discharge, and the accumulated charges decrease. Usually, ESD due to inductive charging is characterized by high current pulses lasting up to microseconds. This high level of current is capable of damaging sensitive electronic circuits.
The Principles of Electrostatic Discharge Control
Controlling ESD in electronic circuits is a very challenging task, especially with circuit miniaturization. The task of ESD control needs to begin right from circuit design. Here are some basic principles for ESD control.
- Design ESD-resistant circuits: The primary and foremost principle of ESD control in an electronic circuit is to design an ESD-resistant system. Reducing ESD-sensitive devices or introducing input protection on sensitive components, boards, and assemblies can help control ESD. However, as the size of the devices and circuits diminish, they become more prone to ESD.
- Plan ESD control as required: Understand the circuit thoroughly and classify circuit components in the order of their susceptibility to ESD. Identify the component with the highest ESD sensitivity and evaluate its withstand voltage. With these inputs, plan an ESD control program by following standards such as IEC 61350-5-1 or ANSI/ ESD s20.20.
- Set up an electrostatic protected area: For ESD control, electronic components and circuits need to be handled only at a specific location, called an electrostatic protected area, where all conductive or dissipative materials, as well as personnel, are grounded properly. This grounding makes the electrostatic protected area a static charge-free zone.
- Prevent ESD generation: ESD generation can be prevented in electronic circuits by reducing the physical contact and separation of dissimilar materials using ESD control tools such as wrist wraps or ESD footwear. Equipotential surfaces are free from static discharge and prevent ESD generation and charge accumulation. The goal is to make conditions unfavorable for triboelectrification, conduction, and inductive charging.
- Use grounding and dissipative materials: It is impossible for humans to completely wipe out all static charges from electronic circuits and the areas handling them. So, as a protective measure, grounding and conductive or dissipative materials can be used for neutralizing the existing charges and thereby making them inactive from ESD generation.
- ESD protection for electronic components and circuits: This ESD control principle deals with the prevention of discharge by using proper grounding and shunting to keep sensitive parts shielded from ESD. Proper packaging and handling procedures are clearly mentioned on packages to prevent mishandling and to control ESD.
The principles of electrostatic discharge control can be customized depending on the susceptibility of the electronic circuit to ESD. As the number of ESD-sensitive components increases with circuit miniaturization, more care needs to be given to preventing ESD.
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