View Full Version : Electrostatic Discharge (ESD)

May 15th, 2014, 04:25 PM
Electrostatic Discharge (ESD)
Electrostatic Discharge (ESD) is the sudden flow of electricity between two electrically charged objects caused by contact, an electrical short, or dielectric breakdown. A buildup of static electricity can be caused by tribocharging or by electrostatic induction.

ESD can create spectacular electric sparks (thunder and lightning is a large-scale ESD event), but also less dramatic forms which may be neither seen nor heard, yet still be large enough to cause damage to sensitive electronic devices.

ESD can cause a range of harmful effects of importance in industry, including gas, fuel vapour and coal dust explosions, as well as failure of solid state electronics components such as integrated circuits. These can suffer permanent damage when subjected to high voltages. Electronics manufacturers therefore establish electrostatic protective areas free of static, using measures to prevent charging, such as avoiding highly charging materials and measures to remove static such as grounding human workers, providing antistatic devices, and controlling humidity.

Causes of ESD
One of the causes of ESD events is static electricity. Static electricity is often generated through tribocharging, the separation of electric charges that occurs when two materials are brought into contact and then separated. Examples of tribocharging include walking on a rug, rubbing a plastic comb against dry hair, rubbing a balloon against a sweater, ascending from a fabric car seat, or removing some types of plastic packaging.

Another cause of ESD damage is through electrostatic induction. This occurs when an electrically charged object is placed near a conductive object isolated from ground.

Types of ESD
The most spectacular form of ESD is the spark, which occurs when a heavy electric field creates an ionized conductive channel in air. This can cause minor discomfort to people, severe damage to electronic equipment, and fires and explosions if the air contains combustible gases or particles.

Many ESD events occur without a visible or audible spark. A person carrying a relatively small electric charge may not feel a discharge that is sufficient to damage sensitive electronic components. Some devices may be damaged by discharges as small as 30V. These invisible forms of ESD can cause outright device failures, or less obvious forms of degradation that may affect the long term reliability and performance of electronic devices. The degradation in some devices may not become evident until well into their service life.

Damage prevention in electronics
Prevention of ESD bases on Electrostatic Protective Area (EPA). EPA can be a small working station or a large manufacturing area. The main principle of an EPA is that there are no highly charging materials in the vicinity of ESD sensitive electronics, all conductive materials are grounded, workers are grounded, and charge build-up on ESD sensitive electronics is prevented.

ESD prevention within an EPA may include using appropriate ESD-safe packing material, the use of conductive filaments on garments worn by assembly workers, conducting wrist straps and foot-straps to prevent high voltages from accumulating on workers' bodies, anti-static mats or conductive flooring materials to conduct harmful electric charges away from the work area, and humidity control. Humid conditions prevent electrostatic charge generation because the thin layer of moisture that accumulates on most surfaces serves to dissipate electric charges.

Ion generators are sometimes used to inject ions into the ambient airstream.

Manufacturers and users of integrated circuits must take precautions to avoid ESD. ESD prevention can be part of the device itself and include special design techniques for device input and output pins. External protection components can also be used with circuit layout.
Due to dielectric nature of electronics component and assemblies, electrostatic charging can not be completely prevented during handling of devices. Most of ESD sensitive electronic assemblies and components are also so small that manufacturing and handling is done with automated equipment. ESD prevention activities are therefore important with those processes where components come into direct contact with equipment surfaces. In addition, it is important to prevent ESD when an electrostatic discharge sensitive component is connected with other conductive parts of the product itself. An efficient way to prevent ESD is to use materials that are not too conductive but will slowly conduct static charges away. These materials are called static dissipative and have resistivity values in the range of 105 to 1012 ohm-meters. Materials in automated manufacturing which will touch on conductive areas of ESD sensitive electronic should be made of dissipative material, and the dissipative material must be grounded.



May 15th, 2014, 04:56 PM
??? What precipitated this? :-)

May 15th, 2014, 05:12 PM
actually has been on my mind a lot lately and i'll have to be extra sensitive to practicing this on used equipment as I teach the young minds everyday.
i've been meaning to contribute (and plagiarize) since February.. some recent posts i invoked inspired me.
EDIT: keep in mind, this gives google one more source to index & promote VCF.

May 16th, 2014, 07:54 AM
The tough thing about ESD damage is it doesn't have to be felt by you to occur. To feel a static discharge (feeling a shock from static electricity) it tends to be around 3000volts. Anything under and you likely won't even notice it but it only takes a few rogue volts to be able to short out a component. Volts don't kill, they just hurt. It only takes 1 Amp to kill someone which is the amount of electricity flowing through the "stream". High amperage circuitry is in several places but monitors (flyback transformer) are a dangerous culprit that could actually kill you from the shock. This also brings up that you should NOT wear a grounding strap while working on a monitor as this could actually increase the attraction of the voltage from those components.

Here's a neat chart from Cisco on day to day activities and what voltage they can generate: Cisco How Much Static (http://www.cisco.com/web/learning/le31/esd/HowMuchStaticP.html)

May 16th, 2014, 10:39 AM
Static is more likely to build up in dry air. Over here the air is typically very humid, so the only time we get noticeable ESD is during winter time.

May 16th, 2014, 12:36 PM
Indeed, if you want to avoid ESD handling issues, do your foolin' around underwater. Wear SCUBA gear. Soldering is a bit of a problem, however. :)

Next best thing is to work on a concrete slab floor with no carpeting.

Ole Juul
May 16th, 2014, 02:20 PM
Static is more likely to build up in dry air. Over here the air is typically very humid, so the only time we get noticeable ESD is during winter time.

Indeed. In Vancouver, which is also West Coast Marine, it is almost impossible to get a balloon to stick to the ceiling and you can scuff around on a synthetic carpet all day without building up enough static to make a spark. I think that people who don't live in coastal climes assume that the whole world is full of static. It isn't.

Now that I live on the leeward side of the coastal mountains, I've made a conscious effort to treat my electronics carefully because we get humidity levels which routinely drop below 50%. Where you live makes a huge difference.

PS: Regarding "amps to kill someone", the usual number that is used for regulatory purposes is 18ma through the heart. That, of course is difficult to achieve without good skin conduction, well chosen body path, and high voltage. Still it makes a good benchmark for designing maximum limits in various situations.