Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 66

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 2.13 Example 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).