The likelihood that ESD occurs to an ESDS device.
Most devices have many possible ESD entry points. Each of these will typically have a different ESD susceptibility.
The likelihood that the ESD strength will exceed the ESD damage threshold of the device at the point of discharge.
Considering these conditions shows why ESD damage is a probabilistic phenomenon. Only a small fraction of ESD events may cause ESD damage, because
The capacity of the ESD source to produce damage varies with the characteristics of the source.
The magnitude of the source voltage and energy will vary tremendously with the conditions that cause it.
The variation in discharge paths will cause great variation in characteristics of the discharge such as peak current, waveform, rise and fall times and duration, and power and energy deposited in the ESDS device.
The susceptibility of the ESDS device to peak current, power, energy, waveform, and other parameters of the discharge through it varies between devices.
The ESD current path through the ESDS component may be through parts that have different damage thresholds.
The ESD event may not be sufficiently strong to exceed the damage threshold of its path through the component.
So, ESD may occur many times during handling or processing a component, without ESD damage occurring. The probability of ESD damage occurring may be one in hundreds or thousands of ESD events. Combine this with the fact that damage will be discovered often at a much later manufacturing stage, and may not be identified as due to ESD, it is small wonder that many people get the impression that ESD damage is something that does not happen to them!
Nevertheless, each time that a conductor contacts an ESDS component is an opportunity for ESD to occur. Given sufficient ESD opportunities and insufficient ESD control, ESD damage becomes inevitable.
2.10.3 The Principles of ESD Control
The principles of ESD control are remarkably simple. Each principle is aimed at reducing a particular ESD risk.
ESDS are handled only within an electrostatic discharge–protected area (EPA; see Chapter 4) or under ESD‐controlled conditions. This ensures that when ESDS devices are handled, the ESD risks are controlled to an acceptable level.
Outside the EPA in unprotected areas (UPAs), ESD protective packaging is used to protect the ESDS device. The packaging is designed to prevent ESD sources outside the package from having a significant effect on the ESDS device within the package. It also provides a safe region within the package in which ESD risks are controlled.
Within the EPA, ESD risks are controlled by eliminating as far as is possible the sources of ESD likely to damage the ESDS.
Conductors that may touch an ESDS, especially metal items and people, are grounded wherever possible. This ensures that as far as possible, electrostatic voltages on conductors are the same and near zero. This is to prevent them from becoming charged and a source of significant ESD to ESDS devices.
Where conductors cannot be grounded and might contact an ESDS, the voltage difference between the conductor and the ESDS device must be reduced to a sufficiently low level to prevent significant ESD risk.
Electrostatic fields that may occur near ESDS devices are eliminated or reduced to a low level. Insulators that may become charged and the source of electrostatic fields are, where possible, removed from the vicinity of ESDS devices. This is to reduce the risk of induced charging of isolated ESDS devices or other ungrounded conductors.
Items that may contact an ESDS device are preferably made from materials that have appreciable electrical resistance. If ESD occurs, this helps to reduce discharge current to a safe level and absorb much of the discharge energy within the material rather than in the ESDS device.
These measures do not eliminate ESD but help reduce the numbers of ESD occurring, the magnitude of any ESD that occurs, and the likelihood of it damaging the ESDS. Reducing the number of times contact is made with an ESDS device can also help reduce the likelihood of ESD damage occurring. So, the simple measures of not handling the ESDS device any more than necessary and reducing to minimum contact between the ESDS device and other conductors that might be at different voltage can make a useful contribution to reducing ESD risk. If the materials are resistive rather than low resistance where they contact ESDS devices, then a further reduction of risk of ESD damage is made. This is due to reduction of peak current in the discharge and absorption of energy by the resistance of the material.
Suitably specified ESD protective packaging can reduce the risk of ESD damage occurring in the UPA to an insignificant level. The packaging must be specified to address the ESD risks appropriate to the ESDS component or item.
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