Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 53

Water is an electrically conducting material. Moisture from the air forms a thin layer on the surface of many materials and can contribute to their apparent electrical conductivity. Some materials, especially natural materials such as paper, reduce by orders of magnitude in their resistivity as relative humidity increases from dry conditions.

As material surface resistance is increased under dry conditions, electrostatic charge build‐up is often greatly enhanced. Some ESD control materials use additives to attract moisture to a polymer surface and provide static dissipative behavior. These materials may not work well at low humidity. As a rule of thumb, electrostatic charge build‐up is generally increased for humidity less than about 30% rh.

The external atmospheric humidity varies daily with the climate and weather, in a range from below 10% rh (cold and dry winter conditions) to 100% rh (fog). The atmospheric relative humidity often has a large effect on material resistance, especially for materials that have resistance above about 1 MΩ. The effective resistance and charge decay times can be reduced over several orders of magnitude with increasing relative humidity for some materials.

Air relative humidity is a strong function of temperature and reduces as temperature increases for a given moisture content. Relative humidity is approximately halved by a 10 °C rise in temperature, if no moisture is added or removed. If, as in winter, cold air is brought indoors and heated, very low relative humidity can result. Hence, ESD problems can be seasonal and occur often in winter. Even in a room where the relative humidity is controlled, dry local microclimates can form where there are heat sources such as equipment, especially if air circulation is restricted.

Table 2.3 The effect of humidity on typical electrostatic voltages (MIL HDBK 263).

Action	Voltage observed
@ 10–20% rh	@ 65–90% rh
Person walking across carpet	35 000	1 500
Person walking across vinyl floor	12 000	250
Person working at bench (not grounded)	6 000	100
Vinyl envelope	7 000	600
Polythene bag picked up from bench	20 000	1 200
Chair padded with polyurethane foam	18 000	15 000

A view of the effect of relative humidity on static electricity in daily life is indicated by the following typical voltages (Table 2.3) given by MIL HDBK 263 as observed at different ambient humidities. These are indicative and cannot be used to predict voltages occurring in real situations.