Electrostatic Discharge Protection

Published:2011/4/22 2:30:00 Author:Phyllis From:SeekIC

Integrated circuits without ESD protection will suffer from electrostatic discharge in the forms of dielectric failure, ruptured passivation, electrothermal migration, contact spiking, and splattered aluminum. If you want to have a well understanding of ESD, you should get to know about what is the voltage level, the voltage and current waveform, IC-protection structure, test method, and application circuit.

The process of generating electrostatic discharge is like this: two different materials come together, and transfer charge, then move apart, finally produce a voltage. For example, wearing a leather shoe and walking on a rug can produce as high as 25kV voltage. The electrostatic voltage inducing level is affected by factors like the relative charge between leather shoe and rug, and humidity.

The test methods of ESD are usually composed of two ways. The first one is MIL-STD-883 Method 3015.7. This method makes each package pin different from others, and classifies the device by its lowest failure occurring voltage. This method is helpful in the packaging and handling of ICs. The layout of circuit is important as the actual waveform delivered at the IC depends on parasitic inductance and capacitance associated with the test connections and PC board.

The other way, called IC-121, is different from the first way in the value for resistance and capacitance. The two ways are complementary, thus one do not have to worry about the problem of choosing one over the other. As ESD problems can appear at any time, maybe during manufacturing, or when PC board assembly, even after the final product is putting into service. There are exceptions, of course, for those ICs whose pins are exposed to the outside world through connectors, the first method mentioned above does not meet the requirement of providing an adequate representation of ESD susceptibility for the input/output pins.

How to distinguish whether an IC is with high resistance to ESD or not? First is knowing the level of ESD voltage that the IC can withstand. And second is finding out if the ESD can cause latchup problem and if the ESD protection will affect normal operation of IC. Thirdly, observing the special precautions when applying the IC.

Recently, Toshiba Corp announced to offer bidirectional diodes for ESD protection for mobile phones, portable audio applications and LED matrices. A bidirectional ESD diode has a first device and a second device which include a first region and a third region respectively with a P-type conductivity formed by doping a semiconductor substrate; and a second and a fourth one with a N-type conductivity. The first region of the first device are connected to an anode terminal of the bidirectional diode. The fourth region of the first device and the first region of the second device are connected to the cathode terminal. The anode terminals is connected to a signal terminal of the circuit when the cathode terminal is connected to a positive power line of the circuit, and the cathode terminal is for connection to the signal terminal when the anode terminal is connected to a ground line or a negative power line of the circuit. The related integrated circuit is IT8702.

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