Fundamentals of Electrostatic Discharge Part 3: How Devices Fail
This article is based on an original publication by ESDA.
An electronic device is damaged from electrostatic discharge when it no longer meets one or more specified parameters. ESD damage, categorized as either catastrophic failure or latent defect, typically occurs from mishandling during manufacture or at any time through field service.
Catastrophic failure is permanent damage to a device's circuitry, causing either total or partial malfunction, and may include a metal melt, junction breakdown or oxide failure. Depending on when the ESD event takes place, a failure of this nature may not be detected until after the part goes into production but typically prior to the device's shipping.
Whereas catastrophic failures are relatively easy to identify during testing, latent defects, or malfunctions that occur following a period of normal operation, are controversial and elusive, especially after the device has been assembled. A latent failure may be attributable to a previous ESD event that results in partial degradation of the device while its primary function is unaffected. A latent defect may also reduce the operating life of the device. Systems or products in which such defects have been incorporated are commonly expensive to repair and sometimes hazardous to the workforce.
The three main ESD events include direct electrostatic discharge to the device, electrostatic discharge from the device and field-induced discharges. The actual damage inflicted on a device is dependent upon its ability to withstand the voltage from an event, known as ESD sensitivity or susceptibility. An ESD sensitive item is commonly referred to as an ESDS.
Discharge to a device occurs when any charged conductor discharges to it. A charged conductor can be the human body. When you walk across the floor, an electrostatic charge builds up. When a finger contacts, or nearly contacts, the leads of an ESDS, the body may discharge, which may cause ESD damage. This model is known as the Human Body Model (HBM). When discharge to a device occurs from a charged conductive object, such as a metal tool, this is referred to as the Machine Model (MM).
Electrostatic discharge from a device usually takes place when a static charge accumulates on the ESDS through contact with packaging materials or machine or work surfaces when the device moves across the surface or vibrates in its package. This transfer of charge is called the Charged Device Model (CDM). Devices that have discharged typically experience different types of failures than devices receiving a charge as the capacitances, energies and current waveforms vary by ESD event.
While automated assembly reduces the number of HBM events, it does not reduce the amount of damage inflicted on sensitive components. A device sliding down a feeder, for example, may be more likely to become damaged by firing a rapid discharge when it comes in contact with a conductive surface.
A field-induced discharge is the final ESD event that can cause an electronic device to fail. When an object becomes electrostatically charged, an electrostatic field is created. If an ESDS is placed in the field, a charge may be induced on the item. A transfer of charge may take place as a CDM event if the item is grounded while within the field. If the item is removed from the field and grounded again, another CDM event may be transferred from the device.
Test procedures based on ESD event models can help determine the amount of static protection a component requires. Its withstand voltage is the highest voltage level that does not cause device failure. A number of electronic components are susceptible to damage at less than 100 volts. Miniature devices, such as disk drive components, are increasingly susceptible to ESD as product designs include more circuitry.
In summation, most materials and conductors can be triboelectrically charged. Speed of contact and separation, humidity, material type and more determine the amount of charge. When an object is charged, an electrostatic field is created. A discharge can occur at any time between manufacturing and field service operations, causing either immediate damage or premature failure from a latent defect. The ensuing damage is dependent upon the device’s ability to withstand the voltage. The three main ESD events include direct electrostatic discharge to the device, electrostatic discharge from the device and field-induced discharges. ESD damage can be mitigated by effective control and training programs.