In this tutorial we are going to learn about What is ESD & How to Protect it?
ESD – Electro Static Discharge is the transfer of electrical charge between 2 bodies at different potentials, either through direct contact or through an induced electrical field. This definition is observed as a HV pulse that may cause damage or loss of functionality to susceptible devices. It is the phenomenon that gives you a mild shock when you walk across a carpeted floor & then touch a doorknob. If it discharge gives a harmless shock to humans, it is lethal to sensitive electronics. For example, the simple act of walking across a vinyl floor can generate up to 12 KV of static electricity. Generally many times the charge needed to ruin a standard Shottky TTL component. The process of moving around can generate static electricity. The term ESD refers to a charged person, or object, discharging static electricity. Although the current associated with such a static charge is low, the electric potential can be in the millions of volts & can severely damage electronic components. CMOS devices are particularly prone to damage from static electricity.
Common Causes of ESD
Electrostatic generation arising from friction between 2 materials is called triboelectric charging. It occurs when two materials are separated or rubbed together. Examples include:
- Opening a common plastic bag.
- If removing adhesive tape from a roll or container then it will not work proper.
- Walking across a floor.
- When movement of computer boards or components around in their trays on carts.
- Sliding circuit boards on a work bench.
- When human use hands to pick parts or their containers, ungrounded personnel can transfer high static charges. As per these static charges are slowly dissipated, ESD event can inflict damage to the devices. Electrical fields can penetrate electrical devices.
- If ungrounded person handling a component or computer board in non-static shielding container can inadvertently transfer an electrical charge through the container into the sensitive electronic device.
For preventing ESD can be relatively easy and inexpensive. 2 areas of focus are:
• Eliminating static charges from the workplace.
• Properly shielding components and assemblies from static fields.
Eliminating static electricity in the workplace is accomplished by grounding operators, equipment, & devices (components & computer boards). Grounding prevents static charge buildup & electrostatic potential differences. Generally electrical field damage is averted by transporting products in special electrostatic shielding packages.
Design techniques for ESD protection.
1. Use filters to attenuate the energy at the source.
2. Use filters to attenuate energy received at the load.
3. Reduce coupling by separating components from each other.
4. Reduce coupling efficiency between source and load.
5. Reduce coupling efficiency by orienting susceptible devices perpendicular to areas susceptible to ESD corruption.
6. Install a metal shield over components and assemblies properly grounded.
7. Reduce electric field coupling by decreasing the impedance of the transmit & receive antenna.
8. Reduce magnetic field coupling by increasing the impedance of the antenna.
9. Couple signals lines to a ground structure
Best workplace should be outfitted with the following items:
ESD protective clothing/smocks. Formal cloths must not come in contact with components or computer boards since the various materials in clothing can generate high static charges. ESD protective wearing cloths and wrist bands, manufactured with conductive fibers, are recommended. Electrostatic shielding containers or totes. These containers like bags, boxes, etc. these bags and boxes made of materials that is protect sensitive devices during transport and storage. Antistatic or dissipative carriers. These provide ESD protection during component movement in the mfg process and only antistatic materials alone will not provide complete protection.
They must be used in conjunction with other methods such as totes or electrostatic shielding bags.
Dissipative table mat. The mat should provide a controlled discharge of static voltages & must be grounded. For surface resistance is designed such that sliding a computer board/component across its surface will not generate more than 100 V.
Personal grounding. A wrist strap or ESD cuff is kept in constant contact with bare skin & has a cable for attaching it to the ESD ground. The purpose of the wrist band is to drain off the operator’s static charge. The wrist band has a current-limiting resistor for personnel safety. Wrist straps must be tested frequently to ensure that they are undamaged & operating correctly. When a wrist band is impractical, special heel straps or shoes can be used. These items are useful only when used in conjunction with a dissipative floor.
ESD protective floor or mat. The flooring mat must be grounded through a current-limiting resistor. The flooring mat dissipates the static charge of personnel approaching the work bench. There should be Special conductive tile or floor treatment can be used when mats are not practical or cause a safety hazard and also Lab Chairs should be grounded with a drag chain to the flooring.