The failures caused by static electricity events can be generalized as catastrophic failures, latent failures, and upset failures.
Electrostatic shielding forms a barrier that isolates the sensitive circuit inside from the intense external electric field.
The inclusion of small apertures is allowed in electrostatic shields, as the electric field inside the cavity is arbitrarily small for such openings.
Faraday’s cage is an example of electrostatic shielding in electronic circuits
Static charges are present everywhere. In electronic circuits, the accumulation of static charges induce currents when it comes to physical contact with oppositely charged objects. The presence of static charges makes electronic circuits vulnerable to damages. The vulnerability or sensitivity towards static electricity (otherwise called electrostatic discharge, ESD) in high-speed, high power density electronic circuits is considerably high compared to its low-speed, low-power dense counterparts.
The effect of static charges in electronic circuits can result in either catastrophic or latent failures. The usual static damages seen in electronic circuits are leakages, variations in the functional characteristics of components, and performance degradation. To protect sensitive electronic circuits from static damages, electrostatic shielding can be used.
So, what is electrostatic shielding? Sensitive electronic circuits enclosed in electrostatic shields are alienated from static charge accumulation. In this article, we will discuss the effect of static charges on sensitive electronic circuits, what electrostatic shielding is, and how it is implemented.
Static Electricity Events in Electronic Circuits
The term static charge means electric charge at rest, but constantly looking for a path to flow. The flow of static charge produces current, which is called a static electricity event or electrostatic discharge (ESD). The ESD Association (ESDA) is dedicated to studying the ESD phenomena, and defines the cause of ESD as the imbalance in electrical charges internal or external to the electronic component or circuit.
According to the ESDA, billions of dollars are wasted yearly in the electronic industry due to static electricity damage. Electronic systems subject to static charges are at risk. Static electricity can cause a sensitive electronic system to fail or be dysfunctional. Static charge flow or ESD can affect semiconductors, ICs, internal components, USB ports, etc. Let’s take a look at some of the effects of static electricity in the upcoming section.
The Effect of Static Electricity on Electronic Circuits
About one-third of field failures in semiconductors are due to ESD. In semiconductors, static electricity events cause leakages, contact damages, burnouts, resistor-metal interface damages, or gate-oxide ruptures. Bipolar and MOS technology-based semiconductors are static sensitive, and failure modes in these devices include insulation breakdowns, thermal overstress, metallization failures, and the migration of materials.
The effects of static electricity in ICs are very serious, as they form holes in the chips. Static electricity discharge events increase the heat generation in ICs to above their temperature limits and cause thermal damage to all components and systems in their vicinity.
The internal components in an electronic device can withstand the consequences due to static charge for one or two events. However, repeated static electricity events degrade the internal components of the electronic circuits over time and result in equipment operational disruption. There can be contact loss or burning off of internal components under static electricity events.
Types of Failure
ESD failures can be generalized as catastrophic failures, latent failures, or upset failures. Catastrophic failures make the device completely non-operational, whereas latent and upset failures make the system go for more rounds of operation until the repeated subjection to static electricity events damage it. The impact of static charge on electronics is aggravated as the speed and size of the circuit decreases. The static sensitivity of high power density circuit designs is greater than its low power-dense counterparts. The demand for high power density circuits calls for static electricity protection, otherwise called electrostatic shielding.
What Is Electrostatic Shielding?
So, what is electrostatic shielding, really? Electrostatic shielding is a method used in the electronics industry to protect sensitive components or circuits from static electricity events. In electrostatic shielding, the circuit or component to be protected is placed inside an enclosure. The enclosure is grounded, and there are zero fields inside the cavity. The space or region inside the enclosure remains unaffected by the external field produced by static charges.
Shielding in Co-Axial Cables
A similar electrostatic shielding method is used in co-axial cables, where the outer conductor is grounded and encloses the central conductor. The outer conductor acts as an electrostatic shield to the central conductor that carries signals.
Using a Faraday cage is the oldest electrostatic shielding technique used in electronic circuits. This technique is also called volumetric shielding, where a metallic screen surrounds the sensitive circuit. The metallic screen prevents external fields from reaching sensitive electronics. Volumetric shielding is expensive, as poor air circulation inside the enclosure demands excellent thermal management.
Apertures in Electrostatic Shields
As closed cavity-based electrostatic shielding is costly, small slots, holes, or apertures are introduced into the conductive enclosures surrounding the sensitive electronic circuit. The apertures in the metallic enclosures make the electric field inside the cavity non-zero. However, small apertures are allowed in electrostatic shields, as the electric field inside the cavity is arbitrarily small for such openings. It also reduces the expenditure on ventilation. These apertures are sometimes utilized for cable or wire harnessing and access points to the internal circuit.
Considering all the merits of small apertures, Faraday’s cages are implemented with metallic grid walls for electrostatic shielding. Cadence software offers tools that assist you in implementing electrostatic shields with or without apertures.