Conductors & Insulators
We’ve previously published a post that explains when you need ionisation. However, following this post, we got a number of questions that prompted us to dive a bit deeper into the whole subject of ionisers. Basically with this post we’re starting right at the beginning so stay tuned…
Before talking about ionisers in more detail, we need to have a little chat about the types of materials that can be found in an EPA – conductors and insulators:
• Electrical current flows easily
• Can be grounded
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.
• Electrical current does not flow easily
• Cannot 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.
Insulators like this plastic cup will hold the charge and cannot be grounded and “conduct” the charge away.
Insulators, by definition, are non-conductors and therefore cannot be grounded. Insulators can be controlled by doing the following within an EPA:
• Keep insulators a minimum of 31cm from ESDS items at all times or
• Replace regular insulative items with an ESD protective version or
• Periodically apply a coat of topical antistat
“Process essential” Insulators
When none of the above is possible, the insulator is termed “process essential” and therefore neutralisation using an ioniser should become a necessary part of the ESD control programme.
Examples of some common process essential insulators are a 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 neutralisation. Ions are charged particles that are normally present in the air and as opposite charges attract, charges will be neutralised 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 neutralise the charge. An ioniser greatly speeds up this process.
A balloon “stuck” on a wall by static charge.
What is an ioniser?
An ioniser creates great numbers of positively and negatively charged ions. Fans help the ions flow over the work area. Ionisation can neutralise static charges on an insulator in a matter of seconds, thereby reducing their potential to cause ESD damage.
An ioniser creates positively and negatively charged ions.
Note: Ionisers require periodic cleaning of emitter pins and the offset voltage must be kept in balance. Otherwise, instead of neutralising charges, if it is producing primarily positive or negative ions, the ioniser will place an electrostatic charge on items that are not grounded.
This citation from the ESD handbook provides an excellent summary:
“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 and footwear. 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 188.8.131.52 Introduction and Purpose / General Information)
Now that you know what conductors and insulators are, how to treat them in an EPA and when to use ionisation, the next step is to learn about the different types of ionisers available. However, as this post is already quite long, we will save that part for next week so stay tuned…. Click here to read the follow-up post.
Posted on 2016-03-03, in Articles, ESD Information, How-To, Ionisation, Question & Answers and tagged Conductor, Electrostatic Discharge, ESD, ESD Control, ESD Control Plan, ESD Control Programme, ESD Damage, ESD protected area, ESD Protection, Insulator, Ionisation, Ionisers, Ionization, ionizers, Neutralisation. Bookmark the permalink. 7 Comments.