Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 57

If we consider the behavior of a conductive object as a capacitor, we can quickly understand that the voltage on an isolated conductive object will change under the influence of a nearby charged object and its electric field.

In Figure 2.8 an earthed electrostatic field meter is monitoring the voltage V_m of a metal plate. There is an effective capacitance C_m between the metal object and the field meter. The metal object has no net charge and is initially at zero volts.

Figure 2.7 Faraday cage.

Figure 2.8 Voltage developed on a metal plate in an electric field.

A positively charged object is then brought near. As it approaches, it couples to an increasing amount of negative charge Q on the metal object attracted to the side nearest the positively charged object. The same amount of positive charge is repelled and appears on the side of the metal object nearest the field meter, coupled to an equivalent negative charge on the (grounded) field meter. The field meter sees a positive voltage on the metal object, as voltage on the metal object increases (the capacitor C_m is charged) by an amount.

While the total amount of charge on the plate does not change, an amount –Q is attracted toward the positively charged object to charge C_g, and an amount + Q is repelled to charge C_m.

Note that the metal plate would have a capacitance and voltage with respect to ground, even if the field meter were not present. Although there is now a voltage on the plate, the net charge remains zero (+Q −Q), and it is not charged!

This changing voltage on an object happens in practice if any conductive object passes through an electric field. If, for example, an integrated circuit passed into an electric field arising from a charged garment surface, it could acquire a voltage in this way. If it were subsequently grounded in this state, an ESD event would happen.