Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 58

In Figure 2.8, we saw that a voltage was induced on the metal object when a charged object was brought nearby. In that situation, we can discharge the capacitor C_m by connecting a ground wire between the metal object and earth. The positive charges on the metal object flow to earth. The voltage V_m is then zero (Figure 2.9). Note that at the time of connection, an ESD occurs as the charge flows to earth. This is an important phenomenon in ESD control – an ESD occurs when two conductors at different voltages make contact, e.g. when grounding a conductor in the presence of an electrostatic field.

If the earth wire is then removed, the metal object remains at zero volts. However, it has a net negative charge Q, as the balancing positive charge Q has flowed away. The metal plate is now charged although the voltage on it is zero! If the charged field source object is then taken away, the voltage on the metal object rises to a negative voltage due to its negative charge.

This process is called charging by induction. It can happen in practice if an object, tool, device, or person becomes grounded temporarily when in an electrostatic field.

If a person or object can become charged and can act as a source of electrostatic field, then a nearby device can be subjected to that field. If the device is momentarily grounded, ESD occurs at that time, and it can become charged by induction. This can leave it in a charged state, at risk of ESD occurring on subsequent contact with another conductor at different voltage – grounded or not. If a grounded person moves to pick up a sensitive device within an electrostatic field, they may cause an ESD event when they touch the ESD‐sensitive device. Practical demonstrations of these processes are given in Section 12.7.10.

Figure 2.9 An earthed metal plate in an electric field becomes charged by grounding.

Figure 2.10 Faraday pail.

Induced voltage differences can also lead to breakdown over small gaps between nearby conductors in a field, if the voltage difference exceeds the gap breakdown voltage. This can also lead to ESD risks.