Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 34

Surface resistivity is defined as a material surface property. It is based on the theoretical resistance of a square of material surface with sides of unit length, with a voltage applied to two opposing sides of the square (Figure 1.2). In theory, the current flows across the surface of the square. For a material of surface resistivity ρ_s with linear electrodes of width w placed parallel on the surface a distance d apart, the surface resistance R_s measured between the electrodes is

where d = w, which reduces to ρ_s = R_s.

The unit of surface resistivity is ohms (Ω). In some industries, it is quoted as ohms per square (Ω/sq). This reflects the property that the value of the surface resistance measured with a square electrode pattern (d = w) is the same, no matter what the dimension of the side of the square is.

In practice, standards exist for measuring surface resistivity using concentric ring electrodes (IEC 62631‐3‐2 (International Electrotechnical Commission 2015), IEC 16340‐2‐3, ANSI/ESD STM 11.11 (EOS/ESD Association Inc. (2015a)). This is further discussed in Chapter 11.

Figure 1.2 Surface resistivity definition.

Surface resistance is a resistance measured between two electrodes on the surface of a specimen. The electrodes may be of any convenient form. Sometimes this measurement is made using electrodes designed so that conversion from surface resistance to surface resistivity is a simple calculation. In ESD control practice, conversion to surface resistivity is often not needed, and the surface resistance result obtained with defined standard electrodes is used directly.