Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 76

Ionizers are devices used to generate air ions for neutralization of surface charges on charged materials and objects (Jonassen 2016a, 2016b). These are available in various types based on passive, radioactive, electrical, and other principles of operation.

Passive ionizers rely on high electric fields developed around sharp points or edges on earthed conductors to generate air ions by corona discharge. These will always generate ions of the opposite polarity to the voltage producing the field at the point or edge. Unfortunately, corona discharges do not occur below a threshold field strength, and this means there is a threshold voltage, known as the corona inception voltage, below which ions are not produced, and neutralization does not occur. This threshold voltage may be several kilovolts (kV). This means that passive ionizers cannot be used to reduce voltages to below this threshold, and they are seldom useful in ESD control in electronics manufacture. They do find major applications in industrial processes such as plastic film manufacturing, printing, copiers, and other processes involving insulating materials.

Electrical ionizers also use the high electrostatic fields at points, usually needles, to generate air ions. In this case, however, a power supply is used to raise the needle to a voltage above the corona inception voltage to ensure that sufficient ions are produced. A balanced ion source with equal numbers of positive and negative ions is usually required. This can be produced either by using an AC driving voltage or by using two sets of needles, one at positive voltage and the other at negative voltage. This can produce an ion stream that is nearly balanced, but it is difficult to produce an exact balanced ion stream by this method.

Radioactive ionizers use a radioactive source to ionize the air by impact of radioactive particles with molecules in the air. Exactly balanced ion streams are produced by this means, as the molecule splits into two ions, one positive and one negative ion. The rate of production of ions is small and limited by the level of radioactivity of the ionizing material.