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!
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.
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]
- Electrical current flows easily in conductors.
- Conductors can be grounded.
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.
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.”
- 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.
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.
Per ESD Handbook ESD TR20.20-2008 Ionization, section 184.108.40.206 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.
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.