Abstract

Current Charged Plate Monitor (CPM) technologies have certain technical limitations when dealing with voltage-switching static eliminators such as AC, pulsed AC and high-frequency AC types. These limitations mainly stem from the insufficient response speed of charged plate monitors and other simple portable static eliminator testers. Both ANSI/ESD S20.20-2021 and IEC 61340-5-1-2024 standards require testing the peak ion balance voltage performance of static eliminators, while some charged plate monitor manufacturers only provide average test data, resulting in discrepancies in measurement results. Since certain CPM technologies struggle to accurately assess the actual risks of various static eliminators, such devices may not only fail to neutralize static electricity effectively but also trigger Electro-Static Discharge (ESD) events. This paper discusses the above issues by studying static eliminators adopting pulsed AC modulation technology and comparing measurement data from charged plate monitors and high-speed oscilloscopes.

1. Introduction

In many industrial fields, such as integrated circuit manufacturing and packaging, PCBA production, lithium battery processing, MOS component assembly, as well as the processing of digital electronic equipment and Flat Panel Display (FPD) glass substrates, users deploy static eliminators mainly to effectively neutralize static charges on insulators and isolated conductors. As a critical tool for evaluating the performance of static eliminators, the Charged Plate Monitor (CPM) test serves as a core part of the ESD control program. Relevant standards require that the peak balance voltage of static eliminators under both positive and negative polarities must be lower than ±35V.

It is worth noting that traditional AC static eliminators, due to their high-voltage switching characteristics, have been proven to cause ESD damage to ultra-high-sensitivity components (such as magnetoresistive heads in the disk drive industry). To overcome this limitation, a new ionization technology combining pulsed AC high-voltage power supplies and compressed air has been developed over the past decade. Compared with traditional AC static eliminators relying on fixed high voltage (approximately 4-7kV) and power frequency (50/60Hz) transformers, this new type of equipment adopts single-point emission, polarity switching and multi-nozzle design, with adjustable output voltage, frequency and duty cycle flexibly, representing a significant advancement in ion generation technology.

Nevertheless, despite the advantages of pulsed AC static eliminators in adjustable parameters, the readings from existing charged plate monitors often only "appear" to meet the ±35V peak requirement specified in ANSI/ESD S20.20 standard due to insufficient measurement response speed of CPM instruments. This result is largely caused by the inherent technical limitations of current CPM devices. Notably, current specifications including ANSI/ESD STM 3.1 do not clearly define the response speed of charged plate monitors, which may lead to measurement data failing to truly reflect the ion balance performance and potential risks of the equipment under actual operating conditions.

2. Compliance Study of Ion Balance Measurement Required by ANSI/ESD S20.20 Standard

When static eliminators are used in an Electrostatic Protected Area (EPA), ion balance measurement is one of the key parameters of the ESD control program. ANSI/ESD STM 3.1 specifies various test setups and charts for static eliminators. The document describes the measurement methods for static eliminators and recommends peak measurement of ion balance voltage for static eliminators to avoid ESD risks.

As part of the ESD control program, ANSI/ESD S20.20-2021 sets clear limits for ion balance (offset voltage) measurement, requiring ion balance within ±35V. Both AC and pulsed AC static eliminators claim to meet this requirement, but this conclusion is constrained by the technical limitations of charged plate monitors and lacks key measurement information. Multiple experiments show that ion balance can be expressed in various forms, and its practical significance needs further clarification.

Figure 1: Ion Balance (Offset Voltage)

Testers generally believe that the measured value of a static eliminator should remain stably below the ±35V peak (Case shown in Waveform A of Figure 1). If the ion balance voltage (offset voltage) occasionally exceeds the limit only for an extremely short time (as shown in Waveform B), it is usually considered acceptable; continuous offset values exceeding the specified limit are deemed non-compliant.

Figure 2: Ion Balance (Offset Voltage) of AC Signal

Another key issue is that when the measured signal is a fast AC signal, the test result of offset voltage may deviate significantly. For example, if a low-frequency AC signal shown as the green waveform in Figure 2 is measured with an offset voltage of +30V, it indicates that the ESD risk of the device is only 30V. However, for high-frequency AC signals (such as peaks reaching ±65V), the theoretically calculated offset voltage may be close to 0V — this makes the static eliminator "compliant" with ANSI/ESD S20.20 standard in terms of data. In fact, this calculation method completely conceals the real risks brought by the fast voltage switching of the static eliminator and fails to truthfully reflect whether it can effectively neutralize charge energy on Electro-Static Discharge Sensitive Devices (ESDS).

3. Measurement Results of Charged Plate Monitors

A. Specifications of Charged Plate Monitors

Three different models of charged plate monitors were selected for comparative testing in this study: CPM374 from Kleinwachter, MC158Pro from Tronovo, and Model 157 from Trek.

B. Test Setup for Static Eliminator and Charged Plate Monitor

The pulsed AC static eliminator was installed in an enclosed cleanroom environment to minimize the impact of external factors on measurement errors of charged plate monitors. In accordance with ANSI/ESD SP3.5 (the new test standard for air-assisted bar ionizers), the Tronovo TB5303B air-assisted bar static eliminator was mounted 250mm away from the 15cm×15cm ion collecting plate (See Figure 3).

Figure 3: Installation Diagram of Pulsed AC Static Eliminator and CPM 374

Since the temperature and humidity of the test area differed from the factory setting environment, fine-tuning was performed on the static eliminator to finally make its average ion balance lower than ±35V.

C. Charged Plate Monitor Test Results of Pulsed AC Bar Static Eliminator. CPM374 was adopted for the initial test. The pulsed AC static eliminator was set to 12Hz output, and the test result showed the ion balance voltage was -45V without peak data (See Figure 4).

Figure 4: Test Result of CPM374

CPM374 is equipped with dedicated software for display and remote control of static eliminator tests. Waveform images captured from the computer screen show that when the frequency of the static eliminator was adjusted from 12Hz to 88Hz, the waveform captured by CPM software was closer to 0V, and the measured ion balance voltage was +5V.

It can be seen that no peak fluctuation occurred when CPM374 measured ion balance. This means peak monitoring of the static eliminator will be missed when CPM374 is used for measurement.

Figure 5: Waveform Change Measured by CPM374 When Static Eliminator Frequency Adjusted from 12Hz to 88Hz

Subsequently, the Trek 157 charged plate monitor was replaced at the same position to test the pulsed AC bar static eliminator under identical test conditions (See Figure 6). When the static eliminator maintained an output frequency of 12Hz, the test result of Trek 157 showed an average offset value of +6.3V, with peaks of +130.5V and -144.8V respectively (See Figure 7).

Figure 6: Installation Diagram of Pulsed AC Static Eliminator and Trek 157

Figure 7: Test Result of TREK 157 at 12Hz Frequency

When the frequency of the static eliminator was adjusted to 88Hz, the measurement data of TREK 157 showed an average offset value of +11.4V. (See Figure 8)

Figure 8: Waveform Change Measured by TREK 157 When Static Eliminator Frequency Adjusted to 88Hz

To further verify the consistency of test data and instrument adaptability, Tronovo MC158Pro was used to test the pulsed AC bar static eliminator (See Figure 9). When the static eliminator was set to 12Hz output, the measurement result of MC158Pro showed that the average ion balance voltage was +12.0V, with peaks of +162.8V and -174.0V (See Figure 10).

Figure 9: Installation Diagram of Pulsed AC Static Eliminator and MC158Pro

Figure 10: Test Result of MC158Pro

When the static eliminator frequency was adjusted to 88Hz, the average value measured by Tronovo MC158Pro charged plate was -5.9V, with peaks of -19.0V and +6.4V (See Figure 11). Benefiting from its optimized high-frequency sampling algorithm and wide measurement bandwidth design, the waveform captured by MC158Pro was smoother than that of Trek 157, with only weak signal fluctuations at the moment of voltage switching. This phenomenon originates from the instantaneous instability of corona discharge of the static eliminator rather than instrument limitations.

Figure 11: Waveform Change Measured by MC158Pro When Static Eliminator Frequency Adjusted from 12Hz to 88Hz

4. Measurement Results of Static Eliminator with Ion Collecting Plate Combined with Oscilloscope

Given that the response speed of charged plate monitors is insufficient to capture complete high-frequency voltage fluctuations, a RIGOL oscilloscope was used in this study to replace the charged plate monitor. The isolated plate of the charged plate monitor was connected to the oscilloscope to determine the frequency measurement accuracy of the pulsed AC static eliminator.

A. Oscilloscope Test Results of Pulsed AC Static Eliminator

The measurement setup of the pulsed AC static eliminator was consistent with the charged plate monitor test, except that the ion collecting plate was connected to the oscilloscope input channel (See Figure 12). The results show that the oscilloscope reading at 12Hz was 12.044Hz (See Figure 13); when the static eliminator frequency was adjusted from 12Hz to 88Hz, the measured frequency was 88.248Hz (See Figure 14)

Figure 12: Installation Diagram of Pulsed AC Static Eliminator and Oscilloscope

Figure 13: CPM Plate - Oscilloscope Measurement Result of Pulsed AC Static Eliminator at 12Hz Output

Figure 14: CPM Plate - Oscilloscope Measurement Result of Enhanced Pulsed AC Static Eliminator at 88Hz Output

The measurement results indicate that the test data of MC158Pro charged plate monitor is highly consistent with the oscilloscope, and the changing trends of voltage signals presented by both are basically matched.

5. Conclusion

All experimental results of this study show that the widely used charged plate monitors (CPM) are limited by response speed and cannot fully capture the dynamic voltage range, so they cannot accurately evaluate the real performance and potential risks of pulsed AC static eliminators. Such instruments can usually only measure low-frequency signals, with very limited reflection of balance values and residual decaying voltage changes on the CPM plate; their test results tend to mislead users into believing that the relevant static eliminators meet the ESD control requirement of ±35V peak specified in ANSI/ESD S20.20-2021 standard. However, when instruments with higher response speed (such as MC158Pro and Trek157) are adopted for testing, their dynamic response capability can effectively match the voltage variation characteristics of pulsed static eliminators, thus achieving more accurate and reliable measurement and evaluation of equipment performance.