Questions about ESD Control?

Question: 1) In the scope, 200 volts CDM & 35 volts on isolated conductors are added. What is the intention to add them? Why are both of them determined on the value of 200 volts and 35 volts, respectively? 

Question: 2) As defined in clause 8.3.1, “If the field measured on the process required insulator is greater than 125 volts/inch and the process required insulator is less than 2.5 cm (1 inch) from the ESDS item, steps shall be taken to either A)… or B)…”, is there any relation between 125 volts/inch in here and 200V CDM in scope? If there is ESDS items whose sensitivity level is lower than CDM 200 volts (e.g., 100 volts) to be handled, which additional control elements or adjusted limits should be required? 

For answers to these questions and more answers about ESD control see the ESDA’s Threshold March/April edition HERE.

2 Comments

  1. Kevin Dudley
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    Is there a defined process or best practice to follow for determining risk to built product if there is a issue found during ESD testing of benches, carts, tools, etc? For example, if the testing of a work surface fails, how do we determine if product built on that work surface since the previous passing test is compromised?

    Reply
    • Desco
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      Grounding works great and a properly grounded ESD worksurface meeting the ANSI/ESD S20.20 required limits will reliably remove electrostatic charges to ground. The very best practice is to inspect all product manufactured during a period that an EPA ESD protective product was out of the required limit of ANSI/ESD S20.20. For example, per ESD TR 12-01 Section 4.2 “A broken wrist strap may expose products to ESD over an entire shift if it is checked only at shift change with wrist strap checker.”

      However, if product is tested and passes, there still may be ESD damage.

      Per ESD Handbook ESD TR20.20 section 2.7 Device Damage – Types and Causes “Electrostatic damage to electronic devices can occur at any point, from the manufacture of the device to field service of systems. Damage results from handling the devices in uncontrolled surroundings or when poor ESD control practices are used. Generally damage can manifest itself as a catastrophic failure, parametric change or undetected parametric change (latent defect).”
      2.7.1 Catastrophic Failures “When an electronic device is exposed to an ESD event it may no longer function. The ESD event may have caused a metal melt, junction breakdown, or oxide failure. The device’s circuitry is permanently damaged, resulting in a catastrophic failure.”
      2.7.2 Latent Defects “A device that is exposed to an ESD event may be partially degraded, yet continue to perform its intended function. However, the operating life of the device may be reduced dramatically. A product or system incorporating devices with latent defects may experience a premature failure after the user places them in service. Such failures are usually costly to repair and in some applications may create personnel hazards.” It is easy with the proper equipment to confirm that a device has experienced catastrophic failure or that a part is degraded or fails test parameters. Basic performance tests will substantiate device damage. However, latent defects are virtually impossible to prove or detect using current technology, especially after the device is assembled into a finished product. Some studies claim that the number of devices shipped to users with latent defects exceeds the number that fail catastrophically due to ESD in manufacturing.”

      So to avoid ESD damage, particularly Latent Defects, it is best to have a robust and disciplined ESD control program.

      Reply

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