Electrostatic Discharge or ESD is a well-known cause of failure for electronic and electro-optical components. The most commonly referenced test standards address simulation of the ESD event that can be produced by handling or touching of the parts by a person who has an electrostatic charge on his or her body. This type of ESD simulation testing is referred to as HBM or Human Body Model testing. This Newsletter will discuss some of the differences among the various ESD (HBM) standards, but the standards themselves should be reviewed thoroughly to get a good understanding of the requirements.
Other ESD testing standards have been developed around a Charged Device Model (CDM), a Machine Model (MM) and a Field Induced Model (FIM). These are also important considerations within the industry, but are not discussed in this newsletter.
Human Body Model (HBM) testing is performed with an RC (Resistor/Capacitor) circuit to produce current pulses which simulate ESD strikes on the components. The shape of the current pulse (rise time, ringing, etc.) is tightly specified by the standards to assure repeatable results that can be used to compare the relative sensitivity levels of different products.
The primary purpose of ESD (HBM) testing is to determine the threshold level of voltage that will cause failure of a component, or alternately, the “withstand voltage” (the highest voltage that can be applied without causing failure). Some of the standards use the withstand voltage to classify components according to their relative sensitivity levels (Class 0, Class1, etc.). Be careful in quoting ESD classification levels, because the various standards do not agree on the voltage levels that fall within each class. Any reference to ESD classification must also refer to the standard that was used. While the standards provide methods to determine the threshold level, ESD testing is also sometimes used as a “go / no go” test to determine if a component will pass a predetermined stress level.
All the standards listed below specify the same basic current pulse for testing, with only minor differences in tolerances. However, the test methodology varies considerably from standard to standard, and can have a significant impact on the complexity and cost of testing.