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Frequently Asked Questions


How does humidity affect static charge?

The moisture content in the air affects the conductivity of certain insulating materials and their ability to hold static charge. The higher the relative humidity (>50%), the higher the conductivity of these materials. Conversely, the lower the humidity (<30%), the more insulative these materials become and the more charge they hold.

Logically, it would follow that high humidity would be an effective means of controlling static. However, even under high relative humidity, unacceptable levels of static charge can be generated and remain for long periods of time. Additionally, high humidity can contribute to other problems including oxidation and soldering difficulties. Using high humidity as a means to control static charge is slow, uncomfortable, expensive, and often ineffective.

How can I control electrostatic charge in my facility?

No single method exists for controlling all static problems. The proper use of equipment and remedial procedures help cure most static problems.

Grounding:
Static on a conductor can be easily controlled if the object is grounded. Grounding provides a path so that charge can migrate to ground, effectively neutralizing the charge. However, grounding an insulator does not work, because charges do not migrate on insulators.

Antistatic or static dissipative materials:
Insulative materials, usually plastics, that are made conductive with the addition of carbon or metal fillings. The conductive dispersion can be adjusted depending on the amount of fillings added to provide resistivity ranging from fully conductive to dissipative.

Ionization:
Air ionizers work by flooding the atmosphere with positive and negative ions. These ions are attracted to ions of the opposite polarity on a charged surface. As a result, the static electricity that has built up on products, equipment and surfaces is neutralized.

Education:
Training personnel and making them aware of electrostatic issues and the need for antistatic gloves, suits, smocks, and wrist/heel straps can make a large difference in the amount of problems that occur in a production facility.

How is static charge created?

The most common method of charge generation is triboelectric charging. Whenever materials are brought into close contact, a charge exchange may occur between the surfaces of the two materials. The magnitude of this charge exchange will depend on a number of factors, but the result is two oppositely charged objects when the materials are separated.

The second common method of creating static charge is known as inductive charging. This occurs when a charge is "induced" on an isolated conductive object that is brought into the field created by the charge on another object.

What is an ion?

Air ions are molecules of one or more of the gases that make up air (nitrogen, oxygen, carbon dioxide, etc.) that have gained or lost electrons. If they lose electrons the molecules have a net positive charge, and are "positive ions". If they gain electrons, the molecules have a net negative charge, and are negative ions. Air ions are a normal constituent of outdoor air, but natural ion levels are too low to be used to neutralize static charge in indoor environments.

Are there different types of ionization methods?

Yes. The most common methods to artificially increase the level of air ions are corona discharge and ionizing radiation from nuclear or soft x-ray sources.

Corona Ionization:
Corona ionizers produces ions by applying high voltage to a sharp point, comes in three different forms: alternating current (AC), steady-state direct current (DC), and pulsed DC. AC ionizers are typically used in heavy industrial applications, while steady-state DC and pulsed DC corona ionization are the methods most often found in Class 10,000 and better cleanrooms.

Nuclear Ionization:
Nuclear ionizers use radiation sources such as polonium-210 to produce alpha particles. Alpha particles collide with surrounding air molecules, creating pairs of positive and negative ions. As there is no electrostatic field present, airflow is generally used to move the ions to where they are needed. The drawback to nuclear ionizers is that they generally require replacement every 12-15 months and are subject to government regulations.

Photon Ionization:
Photon ionizers uses a soft x-ray energy source to displace electrons from the gas molecules. The advantage of photon ionization over alpha is that photon ionizers can be turned off and shipped over national borders without constraint. Photon ionizers are regulated in the U.S. by the Food and Drug Administration, and require safety interlocks and shielding from human exposure.

How do corona ionizers work?

Corona ionizers work by applying high voltage to the tip of a sharp point. The geometry of the sharp point creates a high electrostatic field in the vicinity of the point, establishing what's known as the corona region. The high electrostatic field interacts with the electrons in the gas molecules of the air in the corona region. With positive high voltage applied to the point, electrons are pulled out of the gas molecules and are attracted to the point. The gas molecules that have lost electrons now have a net positive charge, making them positive ions. The positive ions are in a region of high positive electrostatic field and are repelled away from the point into the work area.

All types of corona ionization use high electrostatic fields to create air ions. The electrostatic field can move the ions to the work area, or some type of airflow can be used to move the ions more efficiently. Corona ionization is commonly found in 3 different forms, alternating current (AC), steady state direct current (DC), and pulsed DC. Each of these methods is used in a variety of ionization products and applications.

Which type of ionizer do I use in my application?

There is no "best ionizer technology" for all applications. The application will determine the appropriate type to use. You may want to begin by reviewing the ESD Association advisory document ADV3.2-1995 that covers many of the issues involved in the selection process. Topics cover:

  • Charge Neutralization. How effective is the ionizer at reducing static charge?
  • Impact on static problem. Does the ionizer help reduce or eliminate the static problem?
  • Environmental considerations. Does the environment use laminar, turbulent, or even no airflow?
  • What class cleanroom do you have?
  • Intallation considerations: Distance, power distribution and control.
  • Operation: safety considerations, particle emissions, maintenance, reliability, and warranty, cost.

What type of problems are associated with electrostatic charge?

Electrostatic charge results in electrostatic attraction (ESA), electrostatic discharge (ESD), and electromagnetic interference (EMI). The presence of these problems in the production environment results in destruction to sensitive devices, lock-up or malfunction of microprocessors, product flow or machinery operation problems, and particle contamination.

 

 

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