Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 82

The occurrence of ESD to a susceptible component does not necessarily cause a failure. Indeed, it is likely that ESD happens before detectable damage occurs. So, ESD damage is perhaps best thought of as a risk with a probability that may depend on many factors.

 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.