ANSI/ESD S20.20 Humidity Requirements
Humidity control in relation to an ESD control program continues to be misunderstood across the industry. Humidity does help in the reduction of charge accumulation, but it does not control charge accumulation to reduce the risk to sensitive items. The differences in electrostatic charge accumulation on insulators from 20% to 30% RH at room temperature are minor. As a result, the EOS/ESD Association, Inc. does not rely on humidity as part of a control program in
ANSI/ESD S20.20.
If a company meets the requirements of ANSI/ESD S20.20 or IEC 61340-5-1, the ESD control items used to mitigate risks of ESD shall be qualified prior to their initial use within the ESD control program. Most of the ESD control items require environmental conditioning during the product qualification testing process. This is typically conducted at 12% ± 3% RH and 23°C ± 3°C. If the ESD control item being qualified meets the requirements of ANSI/ESD S20.20 or IEC 61340-5-1, then humidity controls are not required as the item has been shown to function in a worst-case environment.
In the most recent revisions of ANSI/ESD S20.20 (-2021) and IEC 61340-5-1 (edition 3, to be published soon), organizations are allowed to complete product qualification at the lowest annual humidity the facility experiences. This lowest annual humidity may be due to ambient conditions where the facility is located or may be due to other factors, such as the environmental conditioning of the facility. For example, if the lowest humidity level in the facility is 30% RH, then any ESD control items that are used within the facility may be qualified at or below 30% RH. ESD control items that leave the facility, for example, packaging, must still be qualified per the standard test method as the environment the ESD control item may be exposed to is unknown.
Product Qualification During Higher Humidity Seasons
Product qualification must be completed per the requirements of ANSI/ESD S20.20. This allows for two approaches, either you qualify per the standard test method, or you qualify at the facility’s lowest annual humidity. If you plan to qualify per the facility’s lowest annual humidity during the summer when the relative humidity is high and in the winter the relative humidity drops to a much lower level, an acceptable approach would be the creation of a testing plan where product qualification measurements are completed over a period of time until the data is captured at the lowest annual humidity.
Departures from a Facility’s Lowest Annual Humidity
If a facility experiences a humidity level that is below the product qualification level, the amount of change in relative humidity and the length of time are crucial factors in determining risk. In general, a low risk would be determined by a small change in humidity and a short time period.
Product qualification test methods typically have a ±3% RH tolerance to the environmental conditioning requirements, this tolerance could be applied to the facility’s minimum humidity limit.
When humidity changes, it can take time for the materials in the environment to acclimate to the new environment. If, for example, the humidity dropped below the organization’s limit by 3% RH or more for only a few minutes, this would be low risk. However, if this occurred for multiple hours, it may be considered non-conformance.
In either situation, the organization should gather additional objective evidence to determine whether it is deemed safe to handle ESDS items, including increased compliance verification of ESD control items or process assessment during this period. If it is determined that the program limits are being exceeded, the handling of ESDS items shall be stopped. If the objective evidence indicates handling of ESDS items is acceptable (for example, ESD control items meet program requirements), this information shall be captured and stored as a record.
Humidity Levels Below the Environmental Conditioning Requirements of 12% ± 3% RH
All ESD control materials and items used in your facility must work at the lowest humidity level experienced at the facility or at least 9% to 15% relative humidity. If your facility experiences humidity levels lower than 9%, it is recommended to verify that the ESD control items still function properly at the lowest level experienced at the facility. Materials such as flooring, worksurfaces, garments, wrist straps, packaging, etc., that meet EOS/ESD Association, Inc. and IEC standard test methods should work at even lower humidity levels, but it is best to verify. This can be done with random resistance to ground testing on workstation items during extremely dry days.
