Jeremy M. Smallwood - The ESD Control Program Handbook

Figure 2.7Faraday cage.

Figure 2.8Voltage 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 mis 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.

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 mby connecting a ground wire between the metal object and earth. The positive charges on the metal object flow to earth. The voltage V mis 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 2.9An earthed metal plate in an electric field becomes charged by grounding.

Figure 2.10Faraday 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.

If a charged object is placed within a closed hollow conducting object such as a box, then the field lines from the charge couple to the surrounding conductor ( Figure 2.10). The net charge contained within the conducting box induces an equal net charge on the box.

This principle is used to measure electrostatic charge on items by placing the charged item in a container, which is known as a Faraday pail .

If the container is grounded, then the outside world is shielded from electrostatic fields arising from the charges. If it is not grounded, then it is itself a charged object and can be a source of ESD.

Normally air is an excellent insulator. If, however, the electrostatic field strength exceeds about 3 MV m −1(3 kV mm −1), the insulating properties of air breaks down and ESD occurs. A large amount of stored charge can be rapidly dissipated by this event. The discharge may be sudden, as in sparks, or it may be gradual as in corona discharge.

An understanding of ESD is important in understanding the characteristics of ESD sources.

The spark discharge occurs between conducting electrodes that initially have a high voltage difference between them. Large energies (μJ to >1 J) may be dissipated in very short, or long, times (ns to >ms) depending on discharge circuit (including the load characteristics). Peak currents are typically greater than about 0.1 A and can exceed 100 A. The discharge waveform is highly dependent on the source and “load” circuit characteristics and can have unidirectional or oscillatory waveforms (see Section 2.7).

The energy E stored in a capacitor C charged to voltage V is easily calculated using this simple formula

In the absence of significant series resistance, it is often reasonable to assume that all this energy is transferred to the discharge.

The electrical breakdown field strength of about 3 MV m −1is valid for normal air pressure and rather large distances (e.g. for a gap of 10 mm and large diameter or flat electrodes, the breakdown voltage would be about 30 kV). The relationship between breakdown field strength and air pressure is given by Paschen's law (Kuffel et al. 2005) and is nearly linear for larger gaps and uniform fields. At smaller gap distances d the breakdown voltage reaches a minimum (known as the Paschen minimum ). As breakdown voltage V bis also dependent on atmospheric pressure P , the Paschen curve is usually plotted as breakdown voltage against the product Pd ( Figure 2.11). For air, according to Paschen's law, below about 350 V no breakdown occurs (minimum Pd 0.55 Torr cm, or 7 μm at 1 atm), and ESD can happen only with direct metal‐to‐metal contact. There is evidence that in practice discharges can occur through small gaps below the Paschen minimum voltage, possibly due to field emission (Wallash and Levitt 2003).