Abstract

Current Charge Plate Monitor (CPM) technology has 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 charge plate monitors and other simple portable static eliminator test instruments. 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 CPM manufacturers only provide average value test data, resulting in discrepancies in measurement results. Since some CPM technologies cannot accurately assess the actual risks of various static eliminators, these instruments may fail to neutralize static electricity effectively and even 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 charge 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 digital electronic equipment and Flat Panel Display (FPD) glass substrate processing, the main purpose for users to use static eliminators is to effectively neutralize static charges on insulators and isolated conductors. As a critical tool for evaluating the performance of static eliminators, the Charge Plate Monitor (CPM) test is 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 have been proven to cause ESD damage to ultra-high-sensitivity devices (such as magnetoresistive heads in the disk industry) due to their high-voltage switching characteristics. To overcome this limitation, a new ionization technology combining pulsed AC high-voltage power supply and compressed air has been developed over the past decade. Compared with traditional AC static eliminators that rely on fixed high voltage (about 4-7kV) and power frequency (50/60Hz) transformers, this new type of equipment adopts single-point emission, polarity switching and multi-nozzle design, and can flexibly adjust output voltage, frequency and duty cycle, representing a significant advancement in ion generation technology.

However, although pulsed AC static eliminators have advantages in adjustable parameters, due to the insufficient measurement response speed of existing charge plate monitors, their test readings often only "appear" to meet the ±35V peak requirement in the ANSI/ESD S20.20 standard. This result is largely caused by the technical limitations of current CPM instruments. It should be noted that current specifications such as ANSI/ESD STM 3.1 do not clearly define the response speed of charge plate monitors, which may lead to measurement data failing to truly reflect the ion balance performance and potential risks of the equipment under actual working conditions.

2. Study on Compliance 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. The ANSI/ESD STM 3.1 document specifies various test setups and charts for static eliminators, describes the measurement methods for static eliminators, and recommends peak measurement of ion balance voltage of static eliminators to avoid ESD risks.

As part of the ESD control program, the ANSI/ESD S20.20-2021 standard sets a clear limit 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 limited by the technical limitations of charge plate monitor technology and omits key measurement information. Multiple experiments show that ion balance can be expressed in various ways, 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 be stably lower than the ±35V peak (corresponding to waveform A in Figure 1). If the ion balance voltage (offset voltage) occasionally exceeds the limit only for a very short time (as shown in waveform B), it is usually still acceptable; if the offset value continuously exceeds the specified limit, it is deemed non-compliant.

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

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

3. Measurement Results of Charge Plate Monitors

A. Charge Plate Monitor Specifications

Three different models of charge plate monitors were selected for comparative testing in this study: CPM374 from Kleinwachter, TS7508Pro from Tronovo, and Model 157 from Trek.

B. Test Setup for Static Eliminator and Charge Plate Monitor

The pulsed AC static eliminator was installed in a closed cleanroom environment to minimize the influence of external factors on CPM measurement errors. 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 installed 250mm away from the 15cm×15cm ion collector 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 adjustments were made to the static eliminator so that the average ion balance was finally lower than ±35V.

C. CPM Test Results of Pulsed AC Bar Static Eliminator. CPM374 was used for the initial test. The pulsed AC static eliminator was set to output at 12Hz. The test result showed that the ion balance voltage was -45V with no peak data (see Figure 4).

Figure 4: Test Result of CPM374

CPM374 is equipped with dedicated software for displaying and remotely controlling static eliminator tests. The waveform captured from the computer screen shows that when the frequency of the static eliminator is adjusted from 12Hz to 88Hz, the waveform captured by the CPM software is closer to 0V, and the measured ion balance voltage is +5V.

It can be seen that no peak fluctuation occurs when CPM374 measures ion balance. This means that using CPM374 will miss the peak monitoring of the static eliminator.

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

Subsequently, the Trek 157 charge 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 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

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

Figure 8: Change of TREK 157 Measured Waveform when Static Eliminator Frequency Adjusted to 88Hz

To further verify the consistency of test data and instrument adaptability, Tronovo TS7508Pro was used to test the pulsed AC bar static eliminator (see Figure 9). When the static eliminator was set to output at 12Hz, the measurement result of TS7508Pro showed an average ion balance voltage of +12.0V, with peaks of +162.8V and -174.0V (see Figure 10).

Figure 9: Installation Diagram of Pulsed AC Static Eliminator and TS7508Pro

Figure 10: Test Result of TS7508Pro

When the frequency of the static eliminator was adjusted to 88Hz, the average value measured by Tronovo TS7508Pro charge 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 bandwidth design, the waveform captured by TS7508Pro is smoother than that of Trek 157, with only weak signal fluctuations at the moment of voltage switching. This phenomenon comes from the instantaneous instability of corona discharge of the static eliminator rather than instrument performance limitations.

Figure 11: Change of TS7508Pro Measured Waveform when Static Eliminator Frequency Adjusted from 12Hz to 88Hz

4. Measurement Results of Static Eliminator with Ion Collector Plate and Oscilloscope

Given that the response speed of charge plate monitors is insufficient to capture complete high-frequency voltage fluctuations, a RIGOL oscilloscope was used to replace the charge plate monitor in this study. The isolated plate of the CPM 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 for the pulsed AC static eliminator was consistent with the CPM test, except that the ion collector 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 for Pulsed AC Static Eliminator at 12Hz Output

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

The measurement results show that the test data of TS7508Pro charge plate monitor is highly consistent with the oscilloscope, and the changing trend of voltage signals presented by both is basically matched.

5. Conclusion

All experimental results of the study show that the widely used Charge 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 decay voltage changes on the CPM plate; their test results are easy to mislead users into believing that the static eliminators have met the ±35V peak ESD control requirements specified in ANSI/ESD S20.20-2021 standard. However, if instruments with higher response speed (such as TS7508Pro and Trek157) are adopted for testing, their dynamic response capability can effectively match the voltage variation characteristics of pulsed static eliminators, so as to achieve more accurate and reliable measurement and evaluation of equipment performance.