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 184.108.40.206 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.
An ESD Protected Area (EPA) is a defined space within which all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential. This is achieved by simply using only ‘groundable’ materials, i.e. materials with an electrical resistance typically of less than 109 ohms, for covering of surfaces and for the manufacture of containers and tools. All surfaces, products and people are bonded to Ground. Bonding means linking, usually through a resistance of between 1 and 10 megohms. Movable items, such as containers and tools, are bonded by virtue of standing on a bonded surface or being held by a bonded person. Everything that does not readily dissipate charge must be excluded from the EPA.
Example of an EPA Area:
|1.||Bench Top Ionisers||8.||Ground Cords||15.||Floor Mats|
|2.||Packaging Containers||9.||Overhead Ionisers||16.||Wrist Straps|
|3.||Earth Bonding Point Bar||10.||Shielding Bags||17.||Foot Grounders|
|4.||Testers and Monitors||11.||Data Acquisition||18.||Lab Coats|
|5.||ESD Tape||12.||PCB Storage||19.||Floor Maintenance|
|6.||Worksurface Mats||13.||Signs and Labels||20.||Waste Bins and Liners|
|7.||Surface Resistance Meters||14.||Floor Marking Tape||21.||Document Handling|
|231726||Long Soft Rubber Nozzle, W/ Mini Crevice Tool||£78.72|
|231727||Long Soft Rubber Nozzle||£66.22|
|231728||Long Soft Rubber Nozzle, W/ Brush Head||£67.47|
|231729||ESD Micromotor Filter, Charged Filtermedia||£13.74|
231700 – Vacuum Cleaner
“The fundamental ESD control principles that form the basis of IEC 61340-5-1 [include]:
- Insulators in the environment cannot lose their electrostatic charge by attachment to ground
- Avoid a discharge from any charged, conductive object (personnel, equipment) into the device … by bonding or electrically connecting all conductors in the environment, including personnel, to a known ground”
[IEC 61340-5-1 Edition 1.0 2007-08 Introduction]
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As important as ESD control is, it is of secondary importance compared to employee safety.
When working with voltages over 250 VAC, ESD personnel grounding should not be used including Wrist Straps, ESD Footwear & Garments.
Ground fault circuit interrupters (GFCI) and other safety protection should be considered wherever personnel might come into contact with electrical sources.
The written ESD Control Plan should be in accordance with IEC 61340-5-1:5.1.1
ESD training should be repeated as specified in the company’s written ESD Control Plan.
For an introduction to ESD and to see how you can prevent your products being damaged watch our ESD Basics Presentation.