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ESD and Testing Automated Equipment
December 31, 1969 |Estimated reading time: 5 minutes
By Donn Bellmore, Universal Instruments Corporation
Equipment manufacturers constantly are asked what electrostatic voltage levels are present in their equipment during operation. Customers often provide specifications to benchmark acceptable or unacceptable equipment. Recently, I have received acceptance criteria and specifications for equipment referencing ANSI/ESD SP10.1-2007.1 While Appendix A of the standard provides guidance in best practices, the standard itself does not provide a range of acceptability and should not be used for this purpose.
There is reason for concern; the technology level of modern devices makes ESD sensitivity much more severe. In automated handling equipment (AHE), predominant failure types include charged device model (CDM) and machine model (MM). A properly grounded machine using practices and procedures specified in ANSI/ESD S6.1-20052 should not have MM failures. Unless there are numerous operator interventions, the human body model (HBM) should not be a concern. HBM failure possibilities are mitigated with a good ESD control program as specified in ANSI/ESD S20.20-2007.3 The remaining failure, CDM types, are caused by charged devices discharging to grounds during handling. If a device is charged by a nozzle or guide and then placed on a PCB with a ground plane, a discharge will occur.
As seen in the ESD Association Technology Roadmap, device sensitivities are becoming more severe. ANSI/ESD SP10.1-2007 is used to evaluate the electrostatic environments in AHEs and processes by using a standard practice to measure the voltage level on a device as it is processed through the system. Some function speeds may be faster than the voltage sensors, so correlation of measurements to known values is critical. ANSI/ESD SP10.1-2007 provides standard practices for making and correlating ESD measurements.
Understanding ANSI/ESD SP10.1-2007There are several factors to review when discussing ANSI/ESD SP10.1-2007. These include the resistance measurement apparatus and procedures, voltage measuring apparatus and procedures, and correlation of the measured voltage levels and known voltages.
Resistance Measurements Apparatus. The resistance measurements in SP10.1-2007 are straightforward; various meter manufacturers satisfy the standard's requirements. Preferably, the meter is capable of measuring 0 to 1 × 106 Ω with an open circuit voltage of 10 V and 1 × 106 Ω to 1 × 1011 Ω with an open circuit voltage of 100 V.
Charge Accumulation Measurements. Electrostatic charge is difficult to measure in an AHE running production material, so voltage measurements often are made as an indirect method of evaluating charge. Electrostatic voltmeters and voltage sensors are used since their response times typically are faster than field-strength meters and because their smaller apertures can resolve smaller surface areas.
Follow the manufacturer's specifications in selecting and using the correct sensor with aperture size and distance requirements. Measurement applications dictate the size and configurations of the probe.
Correlation. In most of these measurements, as seen in Figures 1 and 2, the velocity of the device moving past the probe may be faster than the meter display response time. When that happens, the meter will not display the correct level of voltage. Consequently, a meter with a voltage output connected to an oscilloscope should be used. The voltage output circuit of the meter has a faster response time than the display, allowing the oscilloscope to capture the complete waveform. This is more representative of the actual voltage present.
Another factor is the distance and velocity of the device relative to the probe. As the device's distance and velocity change, the measured voltage also will change. The voltage measurement, therefore, must be correlated to a known value throughout the application. A device comparable in size to the one used in the actual application should be selected. To ensure a good charge, place a piece of foil on an isolated surface of the test device. This area should pass closest to the aperture. Charge the foil and device with a known voltage. The charge should not be dissipated by contact with a grounded surface, such as a dissipative nozzle or other guide, as it passes over the probe aperture. The pass should be made at the same velocity and distance as the original measurement application. The resulting difference in the measurement and known value is the correlation factor. This should be repeated several times at different voltages and the data compared. Once this is completed for each function of interest, the measurements of the applications can occur, with results compared to the correlation voltages.
In measuring machine elements heads, grippers, belts, guides, and nozzles operators may be able to run the application for a period of time and then step the AHE through the process, making electrostatic measurements of the machine elements in a stationary mode. Correlation may not be needed with these steps.There are numerous probe types available. Familiarity with available probes and their configuration, such as where the aperture is located, will help with probe positioning to surfaces being measured with the optimum clearance range.
Always review the manufacturer's documentation closely. Some probes may have a voltage equal to the measurement on the body of the probe and thus must be isolated when mounted. The voltage on the probe will affect the mounting mechanism used.
ConclusionThe standard practice, ANSI/ESD SP10.1-2007, is a fairly easy procedure to incorporate into AHE and process verification. The evaluations can be accomplished efficiently with the proper selections of probes, meters, and process familiarization, along with proper correlation of measurements.
REFERENCES:1. ANSI/ESD SP10.1-2007, Automated Handling Equipment (AHE), ESD Association, Rome, N.Y.
2. ANSI/ESD S6.1-2005, Grounding, ESD Association, Rome, N.Y.
3. ANSI/ESD S20.20-2007, Protection of Electrical and Electronic Parts, Assemblies and Equipment (Excluding Electrically Initiated Explosive Devices), ESD Association, Rome, N.Y.
Donn Bellmore, corporate ESD specialist and reliability analyst in Quality and Reliability, has worked in the ESD field for 23 of his 28 years with Universal Instruments Corporation. He has been responsible for the Corporate ESD Control Program for the last 15 years and is lead technical advisor to the Corporate ESD Committee. He developed and taught numerous courses and tutorials such as "Controlling ESD by Design." Donn Bellmore also is senior vice president of the ESD Association and has been an active member since 1995. He is chair of the Automated Handling Equipment Working Group; participates in the Garments, Ionization Committees; and is a member of the Standards Committee. Donn Bellmore is a member of the American Society for Quality (ASQ) and ASM International. Contact him at (607) 779-7471; bellmore@uic.com.