ESD Models  


Electrostatic discharge (ESD) occurs in a variety of ways, depending on where and how the static charge is accumulated and how the charge build-up is dissipated.  There are, however, three industry-standard ESD models that define how semiconductor devices are to be tested for ESD sensitivity under different situations of electrostatic build-up and discharge.  These are the Human Body Model (HBM), the Charged Device Model (CDM), and the machine Model (MM). 


It is highly recommended for every device to undergo testing against each of these ESD models so that it can be classified in terms of its ESD sensitivity levels.


The Human Body Model (HBM)


The Human Body Model simulates the ESD phenomenon wherein a charged body directly transfers its accumulated electrostatic charge to the ESD-sensitive (ESDS) device.  A common example of this phenomenon, and from which the name of this model was derived,  is when a person accumulates static charge by walking across a carpet and then transferring all of the charge to an ESDS device by touching it.  Of course, other 'non-human' materials that accumulate and transfer charge in a similar manner are also covered by the HBM.


Dating back to the 19th century when it was used to investigate gas explosions in mines, the HBM is the oldest and most commonly used model for testing the ESD sensitivity of a device.  The general ESD testing set-up for this model consists of a 100 pF capacitor that can be charged to a certain voltage, and then discharged by a switching component into the device being tested through a 1.5 K-Ohm resistor.  Figure 1 shows a basic HBM test circuit.


Figure 1. Basic HBM Test Circuit


Three examples of industry standards that define HBM ESD testing are JEDEC's JESD22-A114, the  MIL-STD-883 Method 3015 and  ESD Association's ESD STM5.1: Electrostatic Discharge Sensitivity Testing -- Human Body Model.  


The Machine Model (MM)


Originated in Japan as a result of investigating worst-case scenarios of the HBM, the Machine Model simulates a more rapid and severe electrostatic discharge from a charged machine, fixture, or tool.  The MM test circuit consists of charging up a 200 pF capacitor to a certain voltage and then discharging this capacitor directly into the device being tested through a 500 nH inductor with no series resistor.  Figure 2 shows a basic MM test circuit.


Figure 2. Basic MM Test Circuit


Two examples of industry standards that define MM ESD testing are JEDEC's JESD22-A115 and  ESD Association's ESD STM5.2: Electrostatic Discharge Sensitivity Testing -- Machine Model.


The Charged Device Model (CDM)


Not all ESD events involve the transfer of charge into the device.  Electrostatic discharge from a charged device to another body is also a form of ESD, and a quite commonly encountered one at that. 


A device can accumulate charge in a variety of ways, especially in situations where they undergo movement while in contact with another object, such as when sliding down a track or feeder.  If they come into contact with another conductive body that is at a lower potential, it discharges into that body.  Such an ESD event is known as Charged Device Model ESD, which can even be more destructive than HBM ESD (despite its shorter pulse duration) because of its high current.


There are currently two widely-used models for CDM testing: 1) the Socketted Discharge Model (SDM); and 2) the Real-world Charged Device Model (RCDM).  SDM simulates a device inserted in a socket, then charged from a high voltage source, and then discharged through a 1-ohm resistor.  SDM is easy to conduct but is not always replicating real-world CDM ESD events.


RCDM testing consists of putting the DUT in deadbug position on a thin dielectric (FR4), which is then placed over a ground plate.  The DUT is then charged either directly by a charging probe or indirectly by field induction.  Each pin is then discharged through a 1-ohm resistor to ground.


Two examples of industry standards that define CDM ESD testing are JEDEC's JESD22-C101 and  ESD Association's ESD STM5.3.1: Electrostatic Discharge Sensitivity Testing -- Charged Device Model.


See also:   What is ESD? ESD Test Waveforms ESDS LevelsESD FailuresESD Standards

ESD Controls ESD Audit ChecklistThe Triboelectric Series


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