Introduction to ESD
Electrostatic Discharge (ESD) interference is a specialized type of ambient interference that results from the extremely rapid equalization of charges between conductive surfaces. One common type of ESD is static generated by friction between two insulating materials. Humans experience ESD as a brief electric shock.
Electronic parts straddling a charged and uncharged plastic surface experience ESD as a current flow that can be large enough to instantly cause damage or destruction.
ESD produces heat, light, sound, and electromagnetic radiation throughout the entire spectrum. A simple ESD spark can set fires, fog film, shock personnel and ignite explosives. The failure of only one semiconductor junction in an electronic device can render the device useless. Worse, ESD can cause latent failures so that components can pass testing, but later fail in field use. Modern military and commercial electronic devices can have millions of semiconductor junctions. The effects of ESD are cumulative and progressive degradation that is not readily apparent can occur over time.
Electrical and electronic devices have been plagued by ESD problems ever since the semiconductor was invented. In the days of electron tubes, ESD was of little concern because the tubes were constructed of metals and glass that were highly immune to ESD interference.
In semiconductor devices, which have low voltaic potentials for operation, the effects of ESD began to show up first as mysterious failures of field effect transistors. As the operating current requirements of semiconductor devices decreases, their susceptibility to ESD damage increases. Conductive chairs, floors, garments, workstations, protective packaging, and ionizing systems all offer the same simple goal: to ensure that conductors have the same charge level when they meet. This is the essence of ESD prevention.
No precise model exists for replicating the ESD effects of the human body on an intended environment.
However, it is possible to simulate these effects using an ESD gun and measure the results. Typically the human body model consists of a highly charged capacitor of 100-250 pF in series with a resistor of 100-1500 ohms. The current waveform resulting from an ESD discharge is normally depicted as a rapidly rising 5 nS pulse with an exponentially falling tail whose half current point is 30nS. Many studies have been conducted on what constitutes an ESD event, in particular, the act of a person’s finger touching a device. The most recent and respected standard is EN61000-4-2, in which the ESD probe resembles a human finger and is imparted a potential of up to 25,000 volts. The waveshape is very carefully controlled to simulate this event.
Dielectric Breakdown Phenomena of ESD
Dielectric breakdown occurs in insulators when an induced electric field exceeds the electric field between nuclei and the electron, which bond the nuclei together. In conductors not all of an atom’s electrons are needed to create a chemical bond. The “leftover” electrons are free to move under the influence of an external electric field without damaging the bonds. In an insulator, however, all the electrons of each atom are necessary to form the bonds that hold the material together. Consequently, when an induced internal field “wins the tug of war” over the nuclei for the bonding electrons, some electrons breaks loose from their atoms. These initially freed electrons create an internal current, causing an avalanche effect as they move through the insulator. The material, in effect, “falls apart” in the region of the insulator where the breakdown occurs, often creates a channel through the insulator. If a charged conductor contacts a MOSFET device’s lead, the conductor’s charge will transfer to the conductive areas of the chip, creating very high localized electric fields because of the small capacitance (1 pF or less) of these internal areas. After breakdown is initiated the conductor’s charge will add to the induced internal current, thereby causing substantial heating of the conducting path. Often there is sufficient heat to melt some of the metallization and spew it along the surface of the breakdown channel, causing what is referred to as a “gate short” of the semiconductor substrate in a MOSFET.
ESD problems are prevented by insulating devices or by providing extremely low impedance paths for conduction of the current back to the source. Current typically flows through the power mains because they are at earth potential. Insulation is one very effective way of dealing with ESD. In this method, the device employs a high dielectric insulator such as Mylar, which makes it difficult for the ESD event to occur. Usually the ESD pulse will find a random way to travel across the insulator to some grounded surface. A disadvantage to using insulation is that the surface itself may become charged if its resistivity is too high. The ESD pulse will always seek an air path to return to the source if insulation is used and a key ingredient of protection is removing that path to avoid direct discharge. Conduction is an equally effective way of dealing with ESD. Lowering the RF impedance of the case material will improve ESD performance. Providing a Faraday shield structure (a continuous shield around the device) is another effective method for preventing ESD. Low impedance rubber or plastic impregnated with carbon filler may prove effective because the pulse may be absorbed and turned into heat energy.
It has been found that electrostatic discharge (ESD) is a major source of factory and field failures. Preventive engineering using the appropriate shielding has found to significantly reduce field complaints. European requirements (EN 61000-4-2) require ESD testing. It is particularly important to perform this test on medical equipment as it is related to fail-safe design. Midwest EMI Associates has the apparatus to perform ESD testing to EN 61000-4-2 standards.
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