Grounding of conductive and static dissipative materials, personnel, and equipment is the primary method used to limit static charge for the protection of electrostatic discharge (ESD) susceptible items in the work environment. A static control program may include air or nitrogen ionization techniques to mitigate charge on isolated conductors (conductors that are not grounded), and insulating materials (e.g., most common plastics) that cannot be grounded.
The ionization standard test method, ANSI/ESD STM3.1: Ionization, defines test methods and instrumentation (the charged plate monitor or CPM) for making discharge time (charge neutralization) and offset voltage (ion balance) measurements of air ionization equipment in defined environments [1]. These standard test methods are applicable for product qualification, for selecting an air ionizer in a specific application, as well as subsequently determining that the incoming product meets the selection criteria.
The test instrumentation and methods of ANSI/ESD STM3.1 or ANSI/ESD SP3.3: Periodic Verification of Air Ionizer Performance, are usable for compliance verification of ionizer performance per ESD TR53: Compliance Verification of ESD Protective Equipment and Materials [1][2][3]. However, there is a lack of measurement methods for newer ionizer designs which are not covered by the ionization test methods in STM3.1, SP3.3 and TR53.
This article defines ionization qualification and periodic verification test procedures for ionizers which are not addressed in STM3.1 or SP3.3, including air-assist bar ionizers, soft x-ray ionizers, an alternative method of room ionization, and non-airflow alpha ionizers. New test locations and measurement distances are provided. The test procedures describe test conditions such as compressed air pressures, air volume, interlock systems and laminar flow. Discharge time and offset voltage testing contained in existing ionization standard test methods are used with these new test procedures. Test points and distances from ionizers are similar in format to STM3.1 and use the same measurement equipment, the CPM.
The measurement techniques, under the specified conditions, can be used to determine ion balance and charge neutralization time for ionizers for qualification, and for periodic verification tests in production locations. The test methods and conditions proposed may be used by manufacturers to provide performance data describing their ionizer product performance. The end user may also use these methods to select an ionizer for a specific application, and then verify continued compliance to expected ionizer performance.
Measurement Techniques
The CPM should be used in accordance with STM3.1 to make performance measurements of new types of air ionization equipment. The isolated conductive plate of the CPM is 15 cm by 15 cm (6” x 6”), and the total capacitance of the test circuit, with the plate, while the instrument is in its normal operating mode, is 20 picofarads ± 2 picofarads. The CPM is used to perform two primary tests for ionizers, discharge time and offset voltage.
In the discharge time test, the isolated conductive plate of the test fixture is charged to an initial test voltage (typically 1000 volts) and allowed to discharge to 10% of the initial test voltage. The time required is monitored and recorded for both polarities of initial charge. This time is referred to as the discharge time. In the offset voltage test, the isolated conductive plate is momentarily grounded to remove any residual charges and to verify zero on the voltage monitoring device. The plate is then monitored within the ionized environment, and the resulting observed voltage is referred to as the offset voltage.
Air Assist Bar Ionizers
1. Description
Air assist bar ionizers are ionizing bars with emitter points mounted in nozzles supplied with a compressed air supply to enhance the operating range. A typical example is shown in Figure 1. The length of the bar is usually determined by the application and typically ranges from 600 mm to 3000 mm.
Figure 1: Example of an Air Assist Bar
2. Procedure
The ionizer should be installed 1200 mm (48”) from the test surface. The space between the ionizer and the test surface should not contain airflow obstructions. Unless otherwise specified, the test surface should be static dissipative or conductive and properly grounded. The test technician should be properly grounded. Unless otherwise specified, the input pressure should be 207 kilopascals (30 psig). End users can follow these suggestions, or they can test air assist bar ionizers in the same configuration, input pressure, and distance as they intend to use. Air consumption for each air-assist bar ionizer should be recorded as it may vary depending on product configuration and bar length.
With the ionizer installed 1200 mm (48”) from the test surface, measurements should be performed at 300 mm (12”), 600 mm (24”), and 900 mm (36”) vertically from the test surface. Test locations
should be no more than 600 mm (24”) apart and spaced evenly along the bar. See Figure 2 for an example of the test locations for a 1200 mm (48”) bar ionizer length. Shorter or longer bars may have different numbers of test locations. Discharge time and offset voltage should be measured at specified test locations.
Figure 2: Examples of Air Assist Bar Test Locations
Soft X-ray (Photon) Ionizers
1. Description
Soft x-ray, or photon ionizers use a low power (less than 10 kiloelectron volts) x-ray source to produce intrinsically balanced positive and negative ionization. X-ray ionizers usually consist of a high voltage power supply and an x-ray (tube) source. Examples of x-ray ionizers are shown in Figure 3.
Air ions are produced along the path of the x-ray (approximately 1000 mm [40”] in air) and do not require electric field or airflow to distribute the ions. For radiation safety, testing of x-ray ionizers should be performed inside an enclosure which prevents accidental x-ray exposure to personnel. Typically, this will include some type of safety enclosure for the x-ray source and electrical interlocking to turn the x-ray source off when the enclosure is opened. See Figure 4 for an example of the dimensions of this safety enclosure. Figure 4 illustrates only the dimensions of the x-ray test enclosure. Information on construction techniques, materials to be used, and operating procedures should be obtained from manufacturers of x-ray ionizers or appropriate standards [4].
Figure 4: X-ray Ionizer Test Enclosure
2. Procedure
Install the x-ray source at the top center of the safety enclosure, following the manufacturer’s instructions. With the enclosure closed, turn on the x-ray ionizer. The test technician should first check for x-ray leakage of the safety enclosure with a radiation survey meter before performing any tests. Assure that the leakage level, if any, is within safety requirements [4].
Test locations for the CPM are shown in Figure 5. The CPM plate is placed on the bottom of the safety enclosure so that the plate is 150 mm (6”) from the bottom of the enclosure. Discharge time and offset voltage should be measured at specified test locations.
Figure 5: X-ray Ionizer Test Locations
Alternate Room Ionization
1. Description
ANSI/ESD STM3.1 provides test procedures for several methods of room ionization. Different test setups are used depending on the type of room ionization. A new room ionization method uses ionizing bars, that face each other on, or near the ceiling of the room. They may be of different lengths or spacing depending on the application. There is currently no test method for this type of room ionizer in STM3.1. An example of an installation of this type of room ionization is shown in Figure 6.
Figure 6: Example of New Room Ionization Method
2. Procedure
The ionizer should be installed on or near the ceiling of the test area. The ionizer bars should be located 2000 mm (80”) apart. The CPM plate should be located at the test positions shown in Figure 7, at a height of 1500 mm (60”) below the installation of the ionizing bars. Airflow, laminar or otherwise, should be recorded at the CPM height in the middle of the test area. The space between the ionizer bars and the CPM should not contain airflow obstructions. The test technician should be properly grounded and stand outside of the test area. Discharge time and offset voltage should be measured at the specified test locations. End users can follow these suggestions, or they can test the room ionization bar ionizers in the same configuration and distance they intend to use.
Figure 7: Alternate Room Ionizer Test Locations
Non-Airflow Alpha Ionizers
1. Description
Alpha ionizers come in many forms. Some are designed to work with existing airflow and others have self-contained fans or use gas flow to move the ions to the charged objects. The alpha ionizer does not use emitter points, allowing it to be mounted extremely close to charged objects, at 4 cm or greater distances. However, without airflow, the typical operating distance is 7.5 cm and the typical maximum operating range is limited to 15 cm. The test methods contained in STM3.1 all make measurements at distances 15 cm or greater from the ionizer. These test methods are unsuitable for characterizing non-airflow alpha ionizers meant to be operated at a 7.5 cm distance from charged objects. Examples of non-airflow alpha ionizers are shown in Figure 8.
Figure 8: Examples of Non-Airflow Alpha Ionizers
2. Procedure
The CPM plate should be installed 15 cm (6 inches) from the test surface. Unless otherwise specified, the test surface should be static dissipative or conductive and properly grounded. The test technician should be properly grounded. The non-airflow alpha ionizer should be installed 7.5 cm (3 inches) above the CPM plate. The space between the ionizer and the CPM plate should not contain airflow obstructions. End users can follow these suggestions, or they can test non-airflow alpha ionizers in the same configuration and distance as they intend to use.
For a small form, or spot, alpha ionizer, the test location is shown in Figure 9. For a linear bar less than 37.5 cm (15”) in length, the test location is shown in Figure 10. For linear bars 37.5 cm to 56 cm (15” – 22”) there are two test points, each located 2.5 cm (1”) from the ends of the bar. This is shown in Figure 11. For linear bars greater than 56 cm (22”) in length, there are three test points. Two are located 2.5 cm (1”) from the ends of the bar, and the third is located at the center of the bar. This is shown in Figure 12. Discharge time and offset voltage should be measured at each specified test location.
Figure 9: Small Form Alpha Ionizer Test Location
Figure 10: Alpha Ionizer Bar (< 37.5 cm) Test Location
Figure 11: Alpha Ionizer Bar (37.5 cm< Length < 56 cm) Test Location
Figure 12: Alpha Ionizer Bar (Length >56 cm) Test Locations
Compliance Verification
The preceding sections have described test methods and test setups appropriate to the qualification and performance testing of air ionizers. Compliance verification of these ionizers in use will require a simplification of the test methods. The purpose is
to assure that ionizers continue to deliver the
required performance for a given end user application. This is accomplished by choosing one or two test locations, depending on the ionizer, that represent the end user application.
Consideration should be given to the location of critical processes and product placement relative to the ionizer when establishing the measurement location. The CPM plate location should be documented for future compliance verification testing. Discharge time for both polarities and offset voltage should be recorded at the test location.
For air-assist ionizing bars, testing should be done at the actual airflow and pressure of the application. The CPM should be placed at the center of the ionizing bar, or at any other repeatable location depending on the application. The height of the CPM plate should be chosen by the end user. A proposed test location for compliance verification of air assist bars is shown in Figure 13.
Figure 13: Air Assist Bar Compliance Test Location
For x-ray ionizers, the CPM plate should be placed inside the end user safety enclosure where the ionizer is operating. Safety procedures to verify x-ray leakage should be followed prior to compliance verification testing. The CPM plate should be located at the center of the enclosure, or at any other repeatable location depending on the application. The distance of the CPM plate from the x-ray ionizer should be chosen by the end user. A proposed test location for compliance verification of x-ray ionizers is shown in Figure 14.
Figure 14: X-ray Ionizer Compliance Test Location
For the alternate room ionization method, testing should be performed at the center of each ionizer bar (i.e., two test locations), or at any other repeatable location depending on the application. The distance of the CPM plate from the ionizer bars should be chosen by the end user. A proposed test location for compliance verification of alternate room ionization is shown in Figure 15.
Figure 15: Alternate Room Ionizer Compliance Test Locations
For non-airflow alpha ionizers, whether they are of small form or are linear bars, testing should be performed with the CPM plate centered below the ionizer, or at any other repeatable location depending on the application. The distance of the CPM plate from the ionizer bars should be chosen by the end user. A proposed test location for compliance verification of non-airflow alpha ionization is shown in Figure 16.
Figure 16: Non-Airflow Alpha Ionizer Compliance Test Location
Conclusion
This article has described test methods for new types of ionizers not contained in existing industry ionization standards. Based upon the existing standards, test locations and test methods have been suggested for air assist bar ionizers, x-ray ionizers, an alternate form of room ionization, and non-airflow alpha ionizers. Methods for qualification and acceptance testing as well as periodic compliance verification have been presented.
These methodologies are being discussed within the ESD Association Workgroup 3 and should be available for industry comment soon.
Acknowledgements
The test methods described in this article are part of the ongoing work of ESD Association Working Group WG-3 – Ionization. The members of WG-3 have contributed to the information presented in this article.
References
- ANSI/ESD STM3.1: ESD Association Standard Test Method for the Protection of Electrostatic Discharge Susceptible Items – Ionization
- ANSI/ESD SP3.3:
ESD Association Standard Practice for the Protection of Electrostatic Discharge Susceptible Items – Periodic Verification of Air Ionizers - ESD TR53: ESD Association Technical Report for the Protection of Electrostatic Discharge Susceptible Items –
Compliance Verification of ESD Protective Equipment and Materials - 10 CFR PART 20, (NRC) Standards for Protection against Radiation
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