by Rohit Ramnath, Master Bond Inc.
Electrostatic charge discharged through an IC can damage the IC. But protective coatings can deliver needed static protection without adding physical components that do not have any other purpose beyond ESD protection on the chip.
Electromagnetic interference (EMI) and radio frequency interference (RFI, so called because the offending source is within the frequency range dedicated to radio frequency transmission) are important issues in the world of electronics. EMI/RFI is energy that unintentionally affects an electrical circuit and causes it to degrade its operation or, in the worst cases, malfunction. This energy is emitted from various sources, such as radios or appliances, and can be present whenever current is interrupted through an inductive load. For example, electromagnetic interference at 2.4 GHz can be caused by 802.11b and 802.11g wireless devices, Bluetooth devices, baby monitors and microwave ovens.
Because electronic devices, components and the ICs that enable them are all getting smaller, EMI/RFI is more of an issue today. As more mobile electronics, such as cell phones and phablets (cell phone-tablets), use higher-output-power and higher-chip-speed/higher-frequency CPUs, leakage of extraneous electronic signals results in unintentional EMI/RFI interference.
EMI/RFI protection also has become a more significant issue due to the increased use of plastic enclosures and housings for manufacturing digital electronic devices, which has increased the need for shielding components from this insidious and stray electromagnetic radiation.
EMI/RFI shielding can be defined as the process of blocking interfering signals from one circuit to another by separating them with a barrier made of conductive material. Coatings can be applied to protect the devices from these types of interference, providing a sort of “immunity” for sensitive components and preventing excessive emissions of EMI to other susceptible equipment. We’ll shortly see why coating is an effective and reliable solution for circuitry being protected in such critical applications as military, aerospace and medical equipment.
But first, to best apply the concepts of shielding effectively requires an understanding of the source of the interference and the environment surrounding the source, as well as the receiver (the components and boards requiring protection). Here’s a brief review of what, why and how EMI/RFI occurs.
There are two possible scenarios. Electromagnetic source pulses may be generated either internally from the affected equipment itself, or externally. In either case, interference occurs when there is a source, a receptor, and a coupling between the source and the receptor.
There are four basic coupling mechanisms between components, circuits, or equipment: conductive, capacitive, magnetic (also sometimes referred to as inductive), or any combination of these, and radiative. Conductive coupling occurs when the coupling path between EMI sources and EMI-susceptible equipment is formed by direct contact with a conducting body; for example, a transmission line, wire, cable, PCB trace, antenna leads or a ground return used in common by two circuits. Typical sources of conducted interference include switching power supplies, ac motors and microprocessors.
Capacitive coupling occurs when a varying electrical field exists between two adjacent conductors usually less than a wavelength apart (which is only 30 cm for a 1 GHz wave, one reason why this must be accounted for in particular by designers in high frequency applications), inducing a change in voltage across the gap. Capacitive coupling must be taken into consideration not just when circuits are involved; it can happen when two cables of a system are routed close to each other, in effect forming a capacitor.
Inductive coupling is basically broken down into magnetic or electrical. These occur when a short distance separates the source and the receiver. Electrical induction occurs when a varying electrical field exists between two adjacent conductors, inducing a change in voltage across the gap (and without physical contact of the conductors). Magnetic induction can exist when a varying magnetic field exists between the source and the receiver. In common usage, electrical induction is referred to as capacitive coupling (as just described), and magnetic induction as inductive coupling.
Radiated EMI coupling is normally experienced when+ the source and victim are separated by a large distance — typically more than a wavelength. Radiated RFI is most often found in the frequency range from 30 MHz to 10 GHz.
Interference potential for critical applications
Military equipment presents particularly challenging situations. The ambient conditions can be extreme; invariably there are severe vibration and shock issues and the environment itself is subject to a great deal of electromagnetic noise. Simply put, shielding against EMI/RFI is a must here. Additionally, with the advent of electromagnetic and electro-optical sensors in modern military platforms, the suppression of EMI/RFI is also considered a vital element of survivability, not just to ensure optimal performance but to prevent detection by opposing forces. EMI also can be intentionally used for obstructing communication, as in some forms of electronic warfare. In all of these applications, a highly conductive coating can shield against EMI/RFI effects and attenuate the electronic signatures.
What’s more, with lighter weight plastics having made substantial inroads into the electronic battlefield, there is an ever-increasing need to protect the sensitive digital electronics with coatings that guard against EMI/RFI and thereby ensure that components are fit for duty. Among the many real-world military applications affected by EMI/RFI are sensing devices on weapons systems, vehicle electronics and aircraft systems.
Electrostatic dissipation protection
A related application is for electrostatic dissipation. The requirements for ESD are similar to those of EMI, but the level of conductivity needed is lower — resistivities of 102 to 104 ohm-cm are adequate. (Note: The acronym ESD can and is used to refer to both Electrostatic Discharge and Electrostatic Dissipative techniques. To prevent confusion going forward, this article will refer to Electrostatic Dissipative techniques as “static protection”—a means of preventing unwanted charge build-up on the surface of parts that could otherwise transfer to sensitive components. And this article will use the acronym ESD to refer to discharge events.)
Basically, ESD can happen any time a charged conductive object approaches another conductive object. Initially, a strong electric field forms between the objects, then an arc can occur; in 0.7 to 10 ns, the current in this arc can sometimes exceed 100 A. The arc continues until the objects touch or until the current drops too low to sustain the arc. Current injected by the arc can penetrate the thin insulating layers inside components, damaging the gates of MOSFETs or CMOS components and triggering latch-up in CMOS devices.
However the situation is made more difficult since the trends in semiconductors include shrinking geometries, thinner gate oxides, higher-speed circuit operation and an increasing number of I/O pins. These scenarios just about guarantee that electronic components will have increased susceptibility to damage due to electrostatic discharge.
Electrical overstress (EOS) is another important issue here. EOS is a term used to describe the thermal damage that may occur when an IC is subjected to a current or voltage that is beyond the datasheet specification limits of the device. EOS is a much slower phenomenon than ESD, but the associated energy is very high. The thermal damage is the result of the excessive heat generated during the EOS event.
In a sense, electrostatic discharge (ESD) is a special type of EOS. While ESD is a very high-voltage (more than 500 V) and moderate-peak-current (about 1 to 10 A) event that occurs in a short time, EOS is a lower-voltage (less than 100 V) and large-peak-current (more than 10 A) event that occurs over a longer time frame.
A discharge of static electricity can be viewed as a miniature lightning bolt. And while a human being cannot feel the ESD shock until it reaches several thousand volts, a component’s rated ability to withstand ESD may be 50 V or lower.
Static protection materials are generally subdivided into three categories: conductive, dissipative and anti-static. Materials are divided into these groups based on their individual surface resistance, which is a measurement of how easily an electric charge can travel across a medium.
An efficient way to prevent ESD is to use materials that are not too conductive but will slowly conduct static charges away. These dissipative materials—the most important category for the purposes of discussing ESD protection for electronic circuitry—allow the charges to flow to ground more slowly and in a more controlled manner than with conductive materials.
Anti-static materials are generally referred to as any material that inhibits triboelectric charging. This is commonly seen when substrates become electrically charged by the rubbing or contact with another material. A material’s anti-static characteristic is not necessarily correlated with its resistivity or resistance.
Products for ESD include coatings to protect against static charge buildup. These formulations provide dependable protection by allowing static to dissipate safely without wearing off or losing resistivity. It is important that the coatings used can be easily applied to most surfaces and resist weathering, abrasion, humidity, and corrosion. If the coating can be readily removed, it vastly diminishes the protection.
Coatings: an effective shielding solution
Returning to EMI/RFI, a range of compounds using different conductive fillers have been developed to absorb and attenuate electromagnetic energy, shielding susceptible electronics. These provide numerous resistance and attenuation levels and can be applied by brushing and conventional spray techniques.
It is also important to note that coatings can lower system costs by reducing the amount of filtering needed to pass certain emissions standards. And unlike a snubber network — a resistor in series with a capacitor across a pair of contacts — it doesn’t come with additional component cost and board space penalty. What’s more, while snubbers may offer modest EMI reduction at very low currents, they generally do not work at currents over 2 A with electromechanical contacts.
One of the ways shielding effectiveness is measured is by attenuation. This refers to the differences of an incoming intensity of a source and its decrease after it passes through the coating. The actual unit of measurement is decibels (dB), and the scale is logarithmic. One of the test methods used for measuring the effectiveness is IEEE 299, and a reference material like aluminum or copper can be used for comparison.
In conclusion, stray energy emitted from various sources can interfere with the internal circuitry of a device, degrading its operation or in the worst cases, causing a malfunction. To guard against this possibility, conductive coatings for EMI/RFI shielding applications feature high electrical conductivity and offer resistance to heat and humidity.
Similarly, when electrostatic charge is discharged through an IC it can create a large current flow and energy dissipation, which can damage the IC. Here, too, protective coatings are a proven way of ESD control and, with the shrinking size of ICs and less and less real estate available, coatings provide a needed static protection solution without adding physical components that do not have any other purpose beyond ESD protection on the chip.
Three examples of Master Bond coatings for EMI/RFI shielding
Master Bond Inc.