Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 55

For a small point or spherical charge with the balancing charge a long way distant, the strength of the electric field falls off rapidly with distance r, as it is proportional to 1/r². The field line spread out radially (Figure 2.4). In many practical situations the object presenting an electrostatic field source is too large to be considered a point source.

Figure 2.4 Field lines (shown dashed) emerging from a small point or spherical charge.

Figure 2.5 Electrostatic field between parallel plates.

For larger and different shaped objects, the field line pattern can be very different and the fall‐off in field strength can be much less rapid. In practice, the field lines start and finish on conductors at different voltages in the region, and field lines may be more or less curved at any region in space between them.

The electrostatic field between two large flat parallel plates, well away from the plate edges, is uniform (Figure 2.5), and the field lines are parallel between the electrodes. Away from the plate edges, the field E is uniform and is easily calculated from the voltage difference between the plates V and the distance between them d.

If a conductor is placed in an electrostatic field, it has the effect of drawing the field to itself with field lines always emerging at right angles to the conductor surface. In response, charges on the conductor are redistributed until the voltage is the same all over the conductor surface. One of the consequences of this is that any instrument we use to measure an electrostatic field inevitably changes the field it is measuring. Figure 2.6 shows how this happens with the electrostatic field between a metal plate at voltage V and a grounded electrostatic field meter. The field meter actually sees a field higher than the V/d value. The same effect happens of course for any component, PCB or any other electrostatic discharge–sensitive (ESDS) device brought into the field. The density of the field lines at the surface are related to the field strength and surface charge density induced at the surface. A high concentration of field lines indicates a high field strength.

Figure 2.6 Electrostatic field between a field meter and metal plate at voltage V.

Field lines tend to congregate at the tip or edge of an object, and the electrostatic field strength becomes more intense in these regions. Discharges tend to occur preferentially from high field strength regions at sharp edges on objects. This is used to an advantage, for example, in using sharp pins to produce intense fields and corona discharges as a source of ions in an ionizer for charge neutralization.

For a charged insulating surface, the situation is even more complicated. A charged insulator will normally have a highly variable charge density over its surface. The surface voltage is highly dependent on the surface charge density and presence of other materials nearby.