Ionisation and ESD Control

The primary function of an ioniser is to neutralise electrostatic charges on insulators and isolated (non-grounded) conductors to reduce the magnitude of electrostatic discharges in the ESD protected area

 

Per EN 61340-5-2 User guide clause 4.7.6.1 The primary method of static charge control is direct connection to ground for conductors, static dissipative materials and personnel. However, a complete static control program must also deal with isolated conductors that cannot be grounded as well as insulating materials (e.g., most common plastics). …Air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment, this will be neutralized by attracting opposite polarity charges from the air.

 

Per Table 3 ESD protected area requirements of EN 61340-5-1Edition 1.0 2007-08, the test method for product qualification and compliance verification of Ionisation is per ANSI/ESD STM3.1 (product qualification being performed at 12 % RH and 23 degrees C; compliance verification being the basic test procedure).  The limit for decay time is less than 20 seconds and the limit for offset voltage balance is less than +/- 50 volts.

 

RS supplied ionisers primarily use corona discharge.  The positively and negatively charged ions are moved to the work area via airflow.  The bench top and overhead ionisers should direct the ionised air onto where the ESD sensitive items are handled.  Zero Volt ionisers produced nearly an equal amount of positive and negative charged ions and provide a very low offset voltage balance (typically +/- 3 volts).  The low offset voltage is controlled by a patented auto-balancing sense feedback consistent with EN 61340-5-2 User guide clause 4.7.6.5.4.4 Various feedback and auto-balance methods are available to assure ionizer performance. … Providing a sensor for each ionizing device can improve feedback control. Also, an effective monitoring and control method can reduce the amount of maintenance required for proper operation of the ionizer device or system.

 

Per User guide CLC/TR 61340-5-2:2008 clause 4.7.6.7 “All ionization devices will require periodic maintenance for proper operation. Maintenance intervals for ionizers vary widely depending on the type of ionization equipment and use environment.”  Ionisers should be pieces of equipment that have serial numbers and are included in the company’s maintenance and calibration schedules.  This is particularly critical to ensure that the offset voltage or balance is within acceptable limits.  Otherwise, instead of neutralising charges, the out of balance ionizer will charge insulators and isolated conductors.  The user, depending on the value and function of their products, must determine the appropriate frequency of maintenance and calibration.

 

Most RS supplied ionisers use steady state DC ionisation.  Experience has found that steady state DC overhead and some benchtop ionizers works effectively with a modest air flow suitable for sensitive items, soldering operations, and where operator comfort is a concern. 

 

Steady state DC Systems consist of separate negative and positive ion emitters connected by a pair of high voltage cables to their respective high voltage power supplies. The spacing between emitters will vary depending on the design, and DC power is constantly applied to the emitter points.  Emitters of opposite polarities are spaced farther apart in the DC systems than in the AC systems. Ion recombination occurs at a lower rate and steady state DC systems will operate at a lower airflow than AC systems. In some situations, it may not be desirable to place sensitive components close to the emitter points. The electric field of the ionizer is used to move ions in the absence of high airflow. [EN 61340-5-2 User guide clause 4.7.6.5.1.3 Steady-state DC ionizers]

 

AC systems utilize emitters that are switched rapidly between positive and negative high voltage, usually at the power line frequency. Ion recombination is high, as both polarities are produced in rapid succession at each emitter point. The electrostatic field from the emitter points also changes direction rapidly. In some situations, it may not be desirable to place sensitive components close to the emitter points. For these applications, moving ions away from the emitter points will require airflow. AC systems are often mounted at the output of an air delivery system. [EN 61340-5-2 User guide clause 4.7.6.5.1.2 AC ionizers]

 

Per EN 61340-5-2 User guide clause 4.7.6.1 Air ionization is not a replacement for grounding methods. It is one component of a complete static control program. Ionizers are used when it is not possible to properly ground everything and as backup to other static control methods. In clean rooms, air ionization may be one of the few methods of static control available.