Damages to Electrostatic Discharge (ESD) are often caused by one of the following events: field-induced discharges, electrostatic discharge from the device, or direct electrostatic discharge to the device. Whether or not damage occurs to an ESD sensitive item (ESDS) by one of the mentioned ESD events is determined by the ability of the device to dissipate the energy of the discharge or withstand the levels of voltage involved. The device’s ESD sensitivity or ESD susceptibility is known as the level at which a device fails.
This article will discuss on the different events that cause ESD device failure.
The process of electrostatic charging that can directly or indirectly damage devices is also known as Field Induction. In this event, whenever an item becomes electrostatically charged, there is an electrostatic field related to that charge. In case an ESDS is placed in that electrostatic field, a charge may be induced on the object. If it is then grounded while within the electrostatic field, a transfer of charge from the device occurs as a Charged Device Model (CDM) event. If the item is removed from the electrostatic field region and grounded again, another CDM event will happen as the charge (of opposite polarity from the first event) is transferred from the device.
Electrostatic Discharge from the Device
Another ESD event is the transfer of charge from an ESDS to a conductor. Static charge may accumulate on the ESDS itself through contact, handling and separation with worksurfaces, machine surfaces, or packaging materials like cleanroom supplies (cleanroom bags and moisture barrier bags). This may happen frequently when a device vibrates or moves across a surface in a package. CDM is the model used to simulate the transfer of charge from an ESDS. The current waveforms, energies, and capacitances involved are totally different from those of a discharge to the ESD sensitive item, which result to different failure modes.
Direct Electrostatic Discharge to the Device
When any charged conductor such as the human body discharges to an object, an ESD event can happen. A cause of electrostatic damage could be the direct transfer of electrostatic charge from the human body or a charged material, like ESD mat to the ESDS. An electrostatic charge accumulates on the body when one walks across a floor. A close proximity or simple contact of a finger to ESDS' leads or assembly which is typically on a different electrical potential can permit the body to discharge, possibly causing damage to ESD. The Human Body Model or HBM is the model used to simulate this event. The same discharge can happen from a charged conductive item, such as fixture or metallic tool. This time, the model used to describe the happening is identified as the Machine Model (MM).
How to Determine the Required Degree of ESD Control Protection
To help define the sensitivity of components to ESD, test procedures based on the models of ESD events can be conducted. The ESD withstand voltage is the highest voltage level that does not cause device failure. Many electronic components are susceptible or sensitive to ESD damage at relatively low levels of voltage.