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Questions And Answers

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Question

What changes should be made to the ESD control program to handle Class 0 ESD sensitive items?

Answer

The ESD Handbook ESD TR20.20-2008 does not have advice regarding Human Body Model Class 0 devices. It does have a graph that shows the relationship between resistance to ground and voltage on a person’s body.  See Figure 14 “Relationship between Body Voltage and Resistance to Ground” on page 50.  Based upon this one might consider reducing the upper resistance limit for personnel grounding.


Human Body Model Class 0 is less than 250 volt withstand voltage.  ANSI/ESD S20.20 covers to Human Body Model 100 volts.  So the first question about a Class 0 device should be what exactly is the withstand voltage.  If it’s 150 volts, the regular S20.20 program should be OK.  If less than 100 volts then ANSI/ESD S20.20 section 2.0 states “Activities that handle items that are susceptible to less than 100 volts HBM may require additional control elements or adjusted limits.”

 

Adding Ionization is likely a good idea.  The Required Limit for Ionization (other than Room Systems) per ANSI/ESD S20.20 Table 3 is < +/- 50 volts Offset Voltage, and “User Defined” for Discharge Times. One might consider reducing the required limit for offset voltage (balance), and/or require faster discharge times.  

 

One might consider requiring ionizers with an automatically self-balancing offset voltage feature.  The ESD Handbook ESD TR20.20 does include in Ionizer section 5.3.6.5.3 “Selection of Air Ionization Equipment” “high value products may warrant ongoing monitoring of offset voltage by ionizers with sensors and auto-balance feedback mechanisms with or without alarms.”

 
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