The ESD Control Program Handbook. Jeremy M. Smallwood

(e.g. track or component pin) on the charged board touches a highly conductive machine part (e.g. stop pin), a charged board ESD event can occur (Figure 2.17). The PCB can have high effective capacitance, so this type of discharge can be quite energetic.

      2.6.5 ESD from a Charged Module

      Many products, modules, or subassemblies have an insulating plastic housing containing a circuit board. The connections to this may be brought out to terminations at flying leads or a connector.

The housing can become highly charged, e.g. by rubbing or removal from packaging, inducing a high voltage on the PCB within the housing (Figure 2.18). If a connection is made to the module in this state, a discharge can occur at the termination at which contact is made.

Figure 2.17 ESD waveform from a printed circuit board (above) charged to 1 kV (below) field induced charged by insulator 40 mm away.

Figure 2.18 ESD waveform from a charged automotive module taken out of a polythene bag. Charge transferred 35 nC.

Figure 2.19 Voltage on an automotive cable core as polythene packaging is removed.

      2.6.6 ESD from Charged Cables

Cables and wiring looms can have significant capacitance between the wires in the cable and between the wires and ground. This can be of the order of 100 pFm⁻¹. Wires in the cable can become charged by various means such as by movement of the cable or by removal of the cable from packaging (Figure 2.19). If the cable is connected to equipment in this state, a charged cable ESD event can occur to the first terminal to make a connection (Figure 2.20).

2.7 Electronic Models of ESD

Many ESD sources can be simply modeled using a simple R‐L‐C circuit (Figure 2.21). The values of each component vary widely between different sources and help to explain the different types of waveforms observed.

      At the heart of any ESD source is charge build‐up and storage. This is represented by the capacitance in the model C. In many cases in real life, this charge storage may be on a conductor (e.g. metal item).

Figure 2.20 ESD waveform from a charged automotive wiring loom cable lying against an earthed metal plate. Positive (above) and negative (below) charging polarity.

Figure 2.21 Electronic model of a simple ESD source.

      The discharge is usually initiated by a breakdown of an air gap or some other insulating medium. At low voltages, it can also be initiated by contact or near‐contact between two conductors. The discharge can itself have significant impedance R_ESD that can affect the waveforms produced and the energy delivered into the victim device. Often, however, this is negligible compared to the other impedances in the circuit, especially for larger ESD events.

After the discharge commences, the current flows through some circuit that includes some elements of resistance R_s and inductance L_s. These are normally due to the resistance and electrical properties of the materials in the current path.

      In the case of ESD to a victim device, the device also has impedance, modeled in this simple circuit by a resistance R_d. In practice, a nonlinear impedance would be more typical of a semiconductor device. The impedance of the spark channel is highly variable and nonlinear.

      For simplification, the total circuit resistance R is assumed to be linear and is the sum of the circuit resistances.

      The discharge current I_ESD of this circuit has the form

      For derivation of the equations for this and the following equations, the reader is referred to other texts (e.g. Agarwal and Lang 1987, https://en.wikipedia.org/wiki/RLC_circuit). This equation has two roots α, β given by

The waveform shape takes very different forms depending on the circuit component values. If the total circuit resistance is large and dominates the discharge path impedance, the waveform has a unidirectional shape, simulated in Figure 2.22 using model component values given for human‐body model ESD (see Table 3.12). This occurs when

Figure 2.22 Simulated overdamped device current waveform IESD for dominant circuit resistance: R_s = 1500 Ω, R_d = 10 Ω, R_ESD = 0 Ω, L_s = 10 000 nH, C_s = 100 pF, V_ESD = 500 V.

The discharge current rises rapidly to a peak I_p that, when inductance is small, approaches the value and polarity near that predicted by Ohms law.

      Thereafter, the current drops nearly exponentially with decay time approaching R_sC_ESD.

      At the other extreme, if the circuit resistance is insignificant compared to the inductive and capacitive impedance, the waveform is quite different. This occurs when

The waveform rises to a peak