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Conductors & Insulators

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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:

Conductors

• 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.

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

Insulators

• 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.

InsulatorInsulators 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.

BalloonA 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.

IoniserAn 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.

Summary

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 5.3.6.1 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.

Flexibility of EN 61340-5-1 ESD Control Plans

To have an ESD control programme conform to EN 61340-5-1 does the programme have to use all the ESD protected area ESD control items listed in Table 3? These are: working surfaces, storage racks, trolleys, flooring, ionization, seating, and garments.

Answer:
No, you can decide which ESD control items to use.

Per EN 61340-5-1 clause 5.2.1 “ESD control program plan, The organization shall prepare an ESD control program plan that addresses each of the requirements of the program. Those requirements concern:

  • training,
  • compliance verification,
  • grounding/bonding systems,
  • personnel grounding,
  • EPA requirements,
  • packaging systems,
  • marking.”

Each company has flexibility designing its programme as EN 61340-5-1 Introduction states: “Each company has different processes, and so will require a different blend of ESD prevention measures for an optimum ESD control program. It is vital that these measures are selected, based on technical necessity and carefully documented in an ESD control program plan, so that all concerned can be sure of the program requirements.”

The ESD control programme plan is to be written. We recommend starting by reviewing Annex A of User guide CLC/TR 61340-5-2:2008 “Example ESD Control Document based on IEC 61340-5-1”. It notes “The following document demonstrates the flow and required sections for an ESD control program as defined by IEC 61340-5-1. This program is based on one of the most basic ESD programs that can be implemented. In most cases, an actual ESD program will utilize more ESD control elements. Personnel are grounded by a wrist strap. Handling operations are performed at a grounded worksurface and ESD sensitive devices are moved from operation to operation inside a metallized shielding bag.”

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High Voltage and ESD Control

When working with voltages over 250 VAC, ESD personnel grounding should not be used, rather ionization may be required to neutralize electrostatic charges. “Wrist straps should not be worn by personnel where they could come into contact with voltage over 250 V.” [CLC/TR 61340-5-2:2008 User guide wrist strap clause 4.7.2.6]

Per CLC/TR 61340-5-2:2008 User guide wrist strap clause 4.7.2.5 “For personnel safety, wrist straps incorporating a 1.0 x 10^6 ohm resistor should not be used in situations where there is an exposed electrical circuit of 250 V or higher. Some additional options for high voltage operations are as follows:

  • Select a wrist strap system that utilizes a higher value resistor
  • Intentionally isolate the operator from ground through the use of high resistance floor mats or gloves.

NOTE: Several publications exist that provide varying limits of allowable current for personnel safety. For example, most power supplies are current limited to 5 mA. For this reason, the user should check with local safety personnel to determine requirements for their particular area.”

Per CLC TR 61340-5-2 2008 Ionization clause 4.7.6.1 “The primary method of static charge control is direct connection to ground for conductors, static dissipative materials and personnel. However, a complete static control programme must also deal with isolated conductors that cannot be grounded as well as insulating materials (e.g., most common plastics). … Air ionization can neutralize the static charge on insulated and isolated objects by charging the molecules of the gases of the surrounding air. Whatever static charge is present on objects in the work environment, this will be neutralized by attracting opposite polarity charges from the air.”

As important as ESD control is, it is of secondary importance compared to employee safety. Questions regarding safety should be answered by the facility’s safety officer. The safety officer and ESD Coordinator should be aware of this information

EN 61340-5-1 clause 4 “Personnel safety, The procedures and equipment described in this standard may expose personnel to hazardous electrical conditions. Users of this standard are responsible for selecting equipment that complies with applicable laws, regulatory codes and both external and internal policy. Users are cautioned that this standard cannot replace or supersede any requirements for personnel safety.
Electrical hazard reduction practices should be exercised and proper grounding instructions for equipment must be followed.”

Per EN 61340-5-1 wrist strap product qualification is to be tested per ANSI/ESD S1.1 which includes “Current-Limiting Resistance, A resistance value incorporated in series with the wrist strap’s electrical path to ground. This resistance limits electrical current that could pass through the ground cord in the event of inadvertent user contact with electrical potential.” [Section 3 Definition of terms]

When equipment voltage exceeds 250 volts, guidance should be obtained from the appropriate Safety Officer if inadvertent user contact is possible. If so, Wrist Straps and ESD footwear, as well as Static Control Garments, should not be used. If the Safety Officer determines that the high voltage is adequately insulated and isolated, then personnel grounding, which is more effective and less expensive, can be used.

Vermason Wrist Straps [and many Foot Grounders] include a 1 megohm current limiting resistor (or resistance). At 250 volts, a 1 megohm [1,000,000 ohms] resistor limits current to 0.25 milliamp. These products are not recommended for use on equipment with operating voltage exceeding 250 volts. Do not remove the resistor. If it becomes damaged, replace the product immediately.

These products are for ElectroStatic control. They will not reduce or increase your risk of receiving electrical shock when using or working on electrical equipment. Follow the same precautions you would use without ESD grounding devices, including:

  • Make certain that equipment having a grounding type plug is properly grounded.
  • Make certain that you are not in contact with grounded objects other than through the ESD grounding device.

Per CLC/TR 61340-5-2:2008 User guide garment clause 4.7.7.5 Other considerations “Personnel safety should be considered before allowing static control garments to be worn where there is exposure to high voltage.”

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