Jeremy M. Smallwood - The ESD Control Program Handbook

If a conductive electrode approaches a charged insulating surface, a “brush” discharge can occur. Several contributory discharges occur on the insulating surface, radiating from a central spark channel – the whole looks rather like an old‐fashioned twig brush.

Brush discharges are less well documented than spark discharges. They typically have a lower peak discharge current than sparks (0.01–10 A) and unidirectional waveforms with fast rise and quasi‐exponential decay ( Figure 2.12) (Norberg et al. 1989; Norberg 1992; Norberg and Lundquist 1991; Smallwood 1999; Landers 2018). The power dissipation and energy of a brush discharge is not easy to calculate.

Figure 2.11The relationship between breakdown voltage and spark gap Pd (Paschen curve).

Figure 2.12Discharge from negatively charged (>20 kV) insulating surface.

Very high electrostatic fields can occur at sharp edges or points on conductors in an electrostatic field. When this field reaches or exceeds a threshold, ions can be sprayed from the point or edge into the air, as a small continuous ion current. This effect is used in ionizers to create a source of ionized air for neutralizing electrostatic charges.

Where an insulating surface is backed by a conducting material, and high charge levels can be generated, a strong propagating brush discharge can occur. This type of discharge is not usually of concern in electronic component handling, but it can be of concern as an ignition source in industrial processes.

Any object that is at a different voltage from an ESDS device can be a source of ESD if the object can touch the device or come close enough for a discharge to jump a small air gap between them. The ESD that occurs may be more or less damaging or problematic according to its characteristics. Different ESD sources produce waveforms with very different characteristics in terms of parameters such as peak current, duration, energy and charge transferred to the device, and frequency spectrum. Even an apparently similar source can give widely different ESD waveforms under different circumstances. Some examples of real ESD waveforms are given next – these may or may not be representative of ESD produced from similar sources in other real situations, which may be highly variable.

The charged human body is an important source of ESD, both in device damage in manufacturing processes and in electromagnetic susceptibility of working systems. The body is a conductor in electrostatic terms and can have a variable capacitance up to about 500 pF, although considerably higher capacitance has been measured under some circumstances (Jonassen 2016c; Barnum 2015b). The capacitance of the human body is dependent on its proximity to other objects such as furniture and walls. When standing, the characteristics of footwear and the nature of the floor are important factors.

Although the body is a conductor, it has significant resistance, and this limits the current flow and causes ESD waveforms from the human body charged to higher voltages (more than a few kV) to have a characteristic unidirectional wave shape ( Figure 2.13). The peak discharge current is typically in the range 0.1–10 A with duration of around 100–200 ns. Discharges from the human body at lower voltages can have highly variable waveform and current characteristics (Kelly et al. 1998; Bailey et al. 2015a; Viheriäkoski et al. 2012). This can significantly affect related risks of ESD damage.

Figure 2.13Example of waveform of discharge from the author charged to 500 V and discharging via skin of a finger (above) and small metal object (coin, below).

When a highly conductive (e.g. metal) object is not grounded, it can gain a high voltage either through triboelectrification or through induction in an electrostatic field. If this conductor now touches another grounded conductor or device, an ESD event will occur.

The waveform of real‐world ESD of this type can be highly variable depending on the characteristics of the source and discharge path. Typically, with low resistance source and discharge path materials, a high discharge current reaching tens of amps can occur. The waveform is often oscillatory, with the frequency determined mainly by capacitance and inductance of the source and discharge circuit. The waveform duration may be from a few nanoseconds to hundreds of nanoseconds.

If there is significant resistance in the discharge circuit, the peak ESD current and duration of the discharge are reduced. (For small ESD sources, the effective resistance of the discharge can be significant.) The number of oscillation cycles is also reduced. Eventually with sufficient circuit resistance, a single peak may occur. In practice, discharges from small metal items can look like charged device ESD ( Figures 2.14and 2.15).

If the resistance of the discharge circuit is sufficiently high, the peak ESD current is further reduced, and a unidirectional waveform with fast‐rising edge but long decay may occur.

Figure 2.14ESD waveform from screwdriver blade charged to +530 V. Charge transferred 0.03 nC.

Figure 2.15ESD waveform from a160 × 180 mm metal plate charged to 550 V. Charge transferred 2.5 nC.

When a component touches a highly conductive object (e.g. metal) at a different voltage, a very short duration high discharge current ESD event occurs. The voltage difference may occur if the component is charged or the object is charged, or both. The same type of discharge will occur if either the component or the object is grounded.

The voltage on the device may arise from tribocharging or induced as a result of nearby electrostatic field sources. Often field‐induced voltages can give the highest voltages arising on the device. Some examples of field‐induced charged device ESD obtained in a laboratory experiment are given in Figure 2.16. In this experiment, the devices were slid down a charged PVC tube onto a 1.7 Ω target plate connected to a fast digital storage oscilloscope (500 MHz bandwidth, 2 Gs s −1sample rate).

The fast high current peak typical of charged device ESD can be seen. The indicated peak current and rise and fall times of the waveform peaks are probably under‐represented, as these waveforms are typically faster than the measurement system used here.

Figure 2.16ESD waveforms from charged integrated circuits: (above) 32‐pin plastic‐leaded chip carrier and (below) 24‐pin dual‐inline package.