Setting up an ESD-safe workstation is often more challenging than it first appears. There are many methods of controlling ElectroStatic Discharge (ESD), and typically, it requires a combination of these to curb all static problems. Unfortunately, there is no single method that will fill all requirements.
Wrist straps and work surface mats are probably the most familiar to everyone, draining charges from operators as well as from the product being worked on. But what if the static charge in question is on an insulator? Electronic products, by nature, will normally consist of conductors and insulators. Insulators at the workstation can be found on the product itself, tools being used, tapes for masking, even circuit boards. A static charge on an insulator cannot be drained by grounding, as you could with a conductive material.
Ionization
To effectively remove charges from insulators, we need to make the surrounding air more conductive. We have all seen a balloon cling to a wall because of a static charge, and we know that, after a period of time, it will drop. That is because the air is somewhat conductive and the charge eventually drains off. The problem with this concept is that it takes too long. The more conductive the air is, the faster the charge will be neutralized.
A balloon “stuck” on a wall by static charge.
The method most frequently used to increase the conductivity of the air is ionization.
Ionizers are useful in preventing electrostatic charge generation, ElectroStatic Discharge, ElectroStatic Attraction, as well as preventing equipment latch-up. Per ANSI/ESD S20.20 section 6.2.3.1. Protected Areas Requirement states: “Ionization or other charge mitigating techniques shall be used at the workstation to neutralize electrostatic fields on all process essential insulators if the electrostatic field is considered a threat.”
How do Ionizers work?
Most ESD workstations will have some insulators (e.g. product plastic housing) or isolated conductors (e.g. PCB board components not in contact with ESD worksurface) that cannot be removed or replaced. These should be controlled using ionization.
Ionizers create great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. If there is a static charge present on an item in the work area, it will be reduced and neutralized by attracting opposite polarity charges from the air.
Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
Electronic enclosures are process-essential insulators
The charged ions created by an ionizer will:
- neutralize charges on process required insulators,
- neutralize charges on non- essential insulators,
- neutralize isolated conductors and
- minimize triboelectric charging.
How does Ionization fit into an ESD Control Program?
Ionization is just one component of your ESD Control Program. Before utilizing ionization, you should follow the fundamental principles of ESD Control:
- Ground all conductors (including people) using conventional grounding methods (e.g. wrist straps or footwear/flooring system).
- Remove all insulators, e.g. coffee cups, food wrappers etc.
“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.” (ESD Handbook ESD TR20.20 Ionization, section 5.3.6.1 Introduction and Purpose / General Information)
- Ionizers can be critical to reduce induction charging caused by process necessary insulators
- Ionizers can be critical in eliminating charges on isolated conductors like devices on PCBs
- Ionization can reduce ElectroStatic Attraction (ESA) and charged particles clinging and contaminating products.
The SCS Ionizer 9110-NO in Use
It is recommended to use ionizers with feedback mechanisms, so you’re notified if the offset voltage is out of balance.
Ionizers 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 neutralizing 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.
Summary
The best way to keep electrostatic sensitive devices (ESDs) from damage is to ground all conductive objects and remove insulators. This is not always possible because some insulators are “process-essential” and are necessary to build or assemble the finished product. The only way to control charges on these necessary non-conductive items is the use of ionization systems. Applications include:
- eliminating charges on process essential insulators,
- neutralizing workstations where ESDS are handled,
- removing charged particulates to create a static free work area.
For more information and to select the right ionizer for your application, check out our Ionizer Selection Guide.