Design Tips and Articles

Failure Modes from an ESD Event

Most ESD problems found on a printed circuit board falls within two categories–component damage and operational disruption.

• Component damage: Occurs whether or not the component is installed in a circuit. A semiconductor element affected by an ESD event fails because of junction puncture burn-through or fusing. This type of damage is permanent and is difficult to detect visually.
• Operational disruption: Caused by either direct or indirect injection of a high-speed, transient energy event. Direct discharge occurs when ESD current finds its way to circuits through ports: power, ground, input, or output. When a sufficient amount of current is present, circuits will react in some manner. Permanent damage is possible however, operational errors may occur.

There are four basic failure modes from ESD-related events.

1. Upset or damage caused by ESD current flowing directly through a vulnerable circuit. This relates to any current discharged directly into the pin of a component that causes permanent failure. This condition can occur when handling a PCB or physical device. Through this mode, direct discharge to the component from the outside environment, i.e., a keyboard can carry damaging ESD energy. Even a small amount of series resistance or shunt capacitance will limit any ESD current present, although the acceptance value is specific to each component type.
   
   
2. Upset or damage caused by ESD current flowing in the ground circuit. This situation typically occurs in systems where chassis ground is directly connected to circuit ground. Most designers generally assume that all circuit grounds have a low-impedance path to chassis ground. Once ESD is injected into the circuit ground, problems may be observed anywhere within the PCB. A discharge to a metal structure results in transient currents traveling in unpredictable paths, often upsetting circuits, but not necessarily destroying them. Current distribution does not follow intended circuit paths with fast rise-time transitions. A small amount of stray capacitance somewhere in the network becomes a low-impedance path to a ground system because wires are too inductive to pass transient current to a chassis ground. Transient current travels through a lower impedance ground system rather than a higher impedance signal returnpath.
   
   
3. Upset caused by electromagnetic field coupling (indirect discharge). This effect usually does not cause damage, although damage to very high-impedance components can occur. Damage is rare because only a small fraction of the ESD energy is coupled directly into the vulnerable circuit. The induced voltage is usually not enough to do more than upset the logic.
   
   
4. Upset caused by a pre-discharged (static) electric field. This failure mode is not as common as the other modes. This mode appears in very sensitive, high-impedance circuits. A pre-discharged (static) electric field is caused by stripping electrons from one object (resulting in a positive charge) and depositing these electrons on another object (resulting in a negative charge). In a conductor, charges recombine almost instantly, whereas in an insulator, the charges remain separate. In an insulator, it may be a long time before significant charge recombination occurs and, consequently, a voltage builds up. If the voltage becomes large enough, a rapid breakdown occurs through the air or insulator, creating the familiar ESD arc or spark.
   
   
   

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