We would like to seek your technical guidance regarding the walking test requirement in relation to a <50V CDM requirement.
Previously, when our applicable CDM requirement was <200V, as per the ESD S20.20 based requirements, we maintained the walking test (combination of ESD flooring and footwear) peak body voltage at <100V.
Now, we have a customer-specific requirement of <50V CDM.
In view of this tighter CDM requirement, we would like to clarify the following…
Does the reduction in the CDM requirement from <200V to <50V require any corresponding change in the walking test requirement?
Can the existing walking test criterion of <100V peak body voltage still be considered acceptable for a <50V CDM requirement, or should a more stringent walking test limit be established?
If a revised walking test limit is recommended, please advise the technically appropriate peak voltage limit and the applicable standard/guideline supporting the same.
The walking test for qualifying a footwear flooring system is to ensure personnel grounding which is more applicable to the HBM limits of an ESDS item. So, I think you can keep the 100 volts limit for the walking test as long as your HBM limits stay at over 100 volts.
The biggest limit in ANSI/ESD S20.20 that is affected by lower CDM limits is the limit for insulators. The best way to figure this new limit is by doing a good process assessment to see how much any insulators in the area may induce on an ESDS item. If you are unfamiliar with how to do this, you could hire a consultant to come and help you.
There is a discussion in the working group for ANSI/ESD S20.20 to add an additional way to figure out the contribution to the ESD risk from insulators that is more directly correlated to the 200 volts CDM limit. The draft of this section currently says “Measure the voltage induced on the ESDS item at the location where the ESDS item is handled with a high impedance contact voltmeter contacting a conductive part of the ESDS item or a non-contact electrostatic voltmeter. The voltage shall be less than 200 volts.” If you can do this yourself, then you would use your new CDM limit of 50 volts instead of 200 volts.
Another possibility is for you to reduce the limits for insulator in ANSI/ESD S20.20 periportally (by a quarter). Although the induced voltage is not in a linear relationship with the field, this may at least give you some reasonable approximation of the new limits. Another issue with this approach is that some meter’s resolution at this low of a value may not give the best results.
You may also consider lowering the limits of other optional ESD control items like worksurfaces but I don’t have any information on how much to lower it by.