ESD Tests and ESD Precautions

In our daily life we often face to Electrostatic Discharge as the rapid transfer of static electricity on the human body to another object in the form of electric current. This discharge generally do not create a negative impact on human health. However, if the object that receives the current is a sensitive equipment or dangerous material, for example a flammable substance, then the result is not so much innocent.

Electrical and electronic devices may show susceptibility against this instant and rapid discharge. ESD in the order of kilovolts, can very easily affect circuits and devices running on the order of millivolts or volts. Therefore, especially the electronic devices restricted to some specific rules are expected to show an immunity at certain level against ESD. The most widely used standard in electrical and electronic equipment, is the European Union standardEN 61000-4-2.ESD Immunity Standardorganzied under Electromagnetic Compatibility (EMC) group mainlydescribes the basic pulse shape of ESD, voltage level to be applied, application method and the test setup simulating the ESD  resulting from of the operators of electrical and electronic equipment.

Waveform of an ESD Current (EN 61000-4-2)

Waveform of an ESD Current (EN 61000-4-2)

In the previous article entitled “What is ESD?” it was mentioned that ESD can occur by direct contact discharge and also by air discharge caused by breakdown of the air between two different potential objects approaching each other. Accordingly ESD tests of the standards explain to perform two seperate tests as contact (touching) or air (approaching) discharge. In these tests, ESD generator simulates the body of an operator charged with static electricity and the rounded tipof the generator represents operators finger.

Portable ESD Generator

Portable ESD Generator

Today, considering that many electronic devices have touch screens, it is more clear to understand the importance of ESD tests, especially for end users. So how does ESD affect the electronic device? In fact, the effect can manifest itself in several different ways. First, if we consider ESD as on the order of kilovolts therefore break the insulation of air, we would not be difficult to predict the mechanical and thermal effects. Especially, in the organizationsmanufacturing electronic device or a card, the components on the card may face physical and thermalstresses and burn out if an operator touches them without any measures. ESD, by generating an electromagnetic energy around the circuit may cause sensitive circuits and components to fail by electromagnetic effects. Besides all these, ESD causing unwanted currents to flow and voltage to be induced, may directly affect the sensitive circuits. This will result as breakdown of the components, reset of the device, display faulty values or maybe completely go off.

ESD precautions should be evaluated in two categories :

Precautions to be taken in the source device or operator: The main way to prevent ESD is to prevent static electricity forming. Antistatic materials may be preferred for this. For example, to cover the floor of the working area with material that does not cause static charging, in the same way operator shoes, garments, gloves made ​​of material which does not create static electricity is a solution. Furthermore, grounding the operator or the device potential to cause static electricity will prevent to be charged and lead the charge to flow to ground under control. The ESD wrist strap which is mandatory to wear while working is used for this purpose…

Precautions to be taken on the victim device or circuits: First, if possible, covering the vulnerable devices with antistatic material can be considered as a solution. However, it is unlikely to perform this practically.So, ESD resistant component selection, designing circuit according to the rules of EMC, efficient and proper grounding, designing filters to regions likely to face ESD or before sensitive components, and most common using varistors (MOV especially) that cuts the high voltage or current wave will lead to solutions in general.

What is ElectroStatic Discharge, ESD?

Electrostatic Discharge, ESD, is the very fast electricity flow caused by apporaching or contacting to each other of two oppositely charged object. Besides the charged objects, the materials whose charges are seperated or polarized can also be the source of ESD. The cause of ESD is the static electricity charge. For a certain polarized object, electricity remains as potential, stationary. In case of contact with an object at different potantial, the stationary charges start to flow from one object to the other, and this is known as “discharge”. If the potential difference is too high, the discharge process doesn’t need a contact. Depending on the environment (for example air), the medium loses its insulation and becomes conducting when the sufficient distance is achieved by approaching (for air, 1 cm for a potential of 30 kV). This is called “electrical breakdown”.  What actually happens is increasw of the electrical field between the objects, inversely proportional to the distance, and to cause loose of insulation of the media.

The comb rubbing to hair, or the ballon to sweater…

So how does the static electricity that causes ESD occur? The most common form is by friction, as we know from primary school times…all of us probably have seen the experiments, electrical charging of a balloon or a comb when rubbed to hair or sweater…Or we have experienced the crackling when taking the sweater off. A sort of charging of different chemical structures by contact, also known as “triboelectric”. It can be explained as charging one object positively and the other negatively after contacting them and then seperating. The seperation of charges under high electric field can also be the base to source of ESD.

In fact, we all meet each other with ESD in our daily life. After walking on a carpet floor and reaching the door to handle, the “click” we hear and the startle response we give…we feel the same thing when we touch the metallic body of the car after we get out, or touching the metal side arms of a treadmill after running…It is undrestandable how the events occur, from the information I have outlined above. The friction of our shoes with the carpet or the clothes we wear with the seat of the car causes our body to be charged with static electircity, and when these charges find a conductive medium (the car’s chasi, door’s handle, treadmill’s arms) they immidiately flows to that object. The volume of the voice we hear or if we’re in a dark environment the intensity of the light we see depends on the amount of charge we have.

Oh My God..! I Crush..!

After the information I gave and the summary of the event, we understand that the first sentence we have after the “click” and the startle response,  “I crush..!”, is not correct J In fact the only thing is the discharge of the stationary potential we have in our body.

The other familiar event for ESD is the thunder and the lightning. The charge exchange of the static electricity charged  clouds between each other or with the ground, or say discharge. Just much more severe of the one we experience on our body, depending on the amount of the charge displaces. The charge potential of static electricity in our bodies is in the order of 20-30 thousand volts. It is important to know that this amount is hundreds of million volts in lightning..!

Although the ESD we face everyday doesn’t have a considerable threat to human health, it is source of fault especially for sensitive electronic devices, and there must be some precautions. Some sufficient countermeasures must be taken to remain the immunity of the devices working in the orders of millivolts, against voltages in the range of kilovolts. The damaging effect of the ESD may simply be a distortion on a screen, but may also cause misfiring of an explosive battle assembly. Now it must be more clear to understand the importance of an ESD formed between your finger and the doorknob, if you are in a place where the flammable and explosive materials are waiting just for a small spark for ignition.