©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das
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E.S.D ELECTRO-STATIC DISCHARGE - AMIT SHETH ( Fr C.R.C.E) According to recent studies conducted by the AT & T Bell labs, 25 % of all component failures today are related to E.S.D and out of all defective components that arrive 50%are damaged by E.S.D. the annual damage due to these failures is estimated at 25 Billion dollars! So it is not surprising that E.S.D is regarded as one of the biggest problems facing the electronics industry today. So what is E.S.D and why has it affected the industry so much?E.S.D or Electrostatic Discharge is defined as the transfer of electrostatic charge between bodies at different potentials caused by direct contact or induced by an electrostatic field. To understand E.S.D let us review a few concepts about static electricity. We know that when two bodies are rubbed against each other transfer of electrons takes place very rapidly. The body that loses electrons becomes positively charged while the one that receives it becomes negatively charged. Since charge is transferred one can say that electricity is generated. This generation of electricity due to transfer of charges is called Triboelectric effect. Static electricity can also be generated when two objects in contact are suddenly separated from each other. Even activities such as walking across the floor, standing up from a chair, movement of ICs in DIP sticks can generate static electricity. So how does this static electricity damage an Integrated Circuit? An Integrated Circuit (IC) consists of several transistors fabricated on one chip. Due to the advances in L.S.I and V.L.S.I thousands of transistors are crowded on a single chip. By decreasing the thickness of the gate oxides and interconnecting lines the manufacturers hope to achieve much higher speeds at very low power consumption. But under these conditions if the Electrostatic Discharge passes through an IC and the current that results is not diverted or diminished by a suitable protective mechanism, the discharge may raise the temperature of the junction inside the component to melting point which will cause damage to the junction or interconnecting lines. Since surface mount devices are smaller than conventional ICs they are even more susceptible to E.S.D damage. E.S.D causes two main types of failures: - 1. Immediate failure where the effect can be readily seen by the equipment manufacturer. 2. Delayed failure where the device is damaged only upto the point where it may pass quality control tests, but wears out sooner than its rated time. It is very difficult to determine the point at which the damage due to E.S.D occurs, since it can take place right from during the manufacturing process, assembly, testing to packing and shipping. So you need an E.S.D control program that protects the device right from the manufacturing plant to the retailer and the user. Some protective guidelines used in E.S.D control are: - 1. Most workstations are provided with measures like conductive table mats, wrist bands, conductive flooring since they protect the components from damage due to electrostatic charge on the body. 2. Air ionisers are used in E.S.D protective areas as these neutralise static charges on non-conductive materials used in the manufacture. 3. All test and soldering equipment are provided with ground potential and are checked periodically 4. Anti-static foams are used for protecting E.S.D sensitive devices instead of ordinary plastic foams during storage and transportation. 5. A number of monitoring devices like electrostatic alarms, electrostatic voltmeters and field meters are used to measure and control static charge on materials. Inspite of high initial costs most companies have profited considerably by implementing E.S.D preventive programmes. So the next time you buy an IC make sure your retailer has taken adequate E.S.D preventive measures.
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©1999 IEEE VESIT Student Branch Hits :
Webauthor : Saumitra M Das |