Neutralization

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.

Good morning everyone – how is your Thursday going so far?
Over the next couple of posts, we’ll tackle another important aspect of any ESD Control Program: Ionization. But before we dig into the nitty gritty and explain the different types of ionizers, we’ll have to cover a bit of theory and discuss the different types of materials that can be found in an ESD Protected Area: conductors and insulators. But don’t worry – we’ll keep it brief!

Conductors
Materials that easily transfer electrons (or charge) are called conductors and are said to have “free” electrons. Some examples of conductors are metals, carbon and the human body’s sweat layer. Grounding works effectively to remove electrostatic charges from conductors to ground. However, the item grounded must be conductive.

The other term often used in ESD control is dissipative which is 1 x 104 to less than 1 x 1011 ohms and is sufficiently conductive to remove electrostatic charges when grounded.

When a conductor is charged, the ability to transfer electrons gives it the ability to be grounded.

 

Per ESD Handbook ESD TR20.20-2008 section 2.5 Material Electrical Characteristics – Insulative, Conductive and Static Dissipative: ” A conductive material allows electrons to flow easily across its surface. Conductive materials have low electrical resistance. If the charged conductive material makes contact with another conductive material, the electrons will transfer between the materials quite easily. If the second conductor is a wire lead to an earth grounding point, the electrons will flow to or from ground and the excess charge on the conductor will be “neutralized”. Static dissipative material will allow the transfer of charge to ground or to other conductive objects. The transfer of charge from a static dissipative material will generally take longer than from a conductive material of equivalent size.
There is no correlation between resistance measurements and the ability of a material to be low charging. Static dissipative material shall have a surface resistance of greater than or equal to 1.0 x 10^4 ohms but less than 1.0 x 10^11 ohms. Conductor less than 1.0 x 10^4, and non-Conductor or Insulator 1 x 10^11 ohms or higher.” [ANSI/ESD S541 section 7.2]

Take-away:

  • Electrical current flows easily in conductors.
  • Conductors can be grounded.

Insulators
Materials that do not easily transfer electrons are called insulators and are by definition non-conductors. Some well-known insulators are common plastics and glass. An insulator will hold the charge and cannot be grounded and “conduct” the charge away.

Both conductors and insulators may become charged with static electricity and discharge. Grounding is a very effective ESD control tool; however, only conductors (conductive or dissipative) can be grounded.

Insulators like this plastic cup will hold the charge and
cannot be grounded and “conduct” the charge away.

Per ESD Handbook ESD TR20.20-2008 section 2.5 Material Electrical Characteristics – Insulative, Conductive and Static Dissipative: “Virtually all materials, including water and dirt particles in the air, can be triboelectrically charged. An insulator is a material that prevents or limits the flow of electrons across or through its volume is called an insulator. A considerable amount of charge can be formed on the surface of an insulator.

Take-away:

  • Electrical current does not flow easily in insulators.
  • Insulators cannot be grounded.

Insulators are non-conductors and therefore cannot be grounded. Insulators can only be controlled by doing the following within an EPA:

  • Always keep insulators a minimum of 12 inch from ESDS items or
  • Replace regular insulative items with an ESD protective version or
  • Periodically apply a coat of topical Antistat.

All nonessential insulators such as coffee cups, food wrappers and personal items shall be removed from the workstation or any operation where unprotected ESDS items are handled.” [ANSI/ESD S20.20-2007 section 8.3]

“Process essential” Insulators
When none of the above is possible, the insulator is termed “process essential” and therefore neutralization using an ionizer should become a necessary part of the ESD control program.

Examples of some common process essential insulators are:

  • PC board substrate,
  • insulative test fixtures and
  • product plastic housings.

An example of isolated conductors can be conductive traces or components loaded on a PC board that is not in contact with the ESD worksurface.

Reduction of charges on insulators does occur naturally by a process called neutralization. Ions are charged particles that are normally present in the air and as opposite charges attract, charges will be neutralized over time.

A common example is a balloon rubbed against clothing and “stuck” on a wall by static charge. The balloon will eventually drop. After a day or so natural ions of the opposite charge that are in the air will be attracted to the balloon and will eventually neutralize the charge. An ionizer greatly speeds up this process.

A balloon “stuck” on a wall by static charge.

Ionizers and Neutralization
An ionizer creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionization can neutralize static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.

An ionizer creates positively and negatively charged ions.

Per ESD Handbook ESD TR20.20-2008 Ionization, section 5.3.6.1 Introduction and Purpose / General Information “The primary method of static charge control is direct connection to ground for conductors, static dissipative materials, and personnel. A complete static control program must also deal with isolated conductors that cannot be grounded, insulating materials (e.g. most common plastics), and moving personnel who cannot use wrist or heel straps or ESD control flooring or footwear. Air ionization is not a replacement for grounding methods. It is one component of a complete static control program. Ionizers are when it is not possible to properly ground everything and as backup to other static control methods.

Note: Ionizers require periodic cleaning of emitter pins and the offset voltage must be kept in balance. Otherwise, instead of neutralizing charges, if it is producing primarily positive or negative ions, the ionizer will place an electrostatic charge on items that are not grounded.

Summary
The 2nd of the three fundamental ESD Control principles is to neutralize process essential insulators with ionizers:
Per ANSI/ESD S20.20-2007 Foreword “The fundamental ESD control principles are:

  • All conductors in the environment, including personnel, must be attached to a known ground
  • Necessary non-conductors in the environment cannot lose their electrostatic charge by attachment to ground. Ionization systems provide neutralization of charges on these necessary non-conductive items (circuit board materials and some device packages are examples of necessary non-conductors).
  • Transportation of ESDS items outside of an ESD Protected Area requires enclosure in static protective materials… Outside an EPA, low charging and static discharge shielding materials are recommended.

In addition, if a conductor is not grounded, it is an isolated conductor, and an ionizer is the only means to neutralize ElectroStatic charges on it.

Now that you know what conductors and insulators are, how to treat them in an EPA and when to use ionization, the next step is to learn about the different types of ionizers available. Stay tuned for next time.