As we have noted before, humidity is not an effective way to control ESD in an electronics production environment. While the ability to generate static electricity increases when the air gets dryer (the % RH decreases), increased humidity does not eliminate the ESD event entirely or provide protection to ESD susceptible devices . As an example, walking across a carpet can yield a charge of 35kV at 10% RH (very dry air), but will drop significantly to 7.5kV at 55% RH. However, even the reduced charge at 55% RH would not be safe to most modern electronic devices that are susceptible to 100 volts or less.
The Circuit Insight – Ask the Experts blog post titled “ESD and Humidification” provides good feedback from professionals at electronics manufacturing companies and other industries with regards to humidity and ESD control.
The ESD Association also covers the importance of RH in a few documents. These include:
- ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity: However, it is impractical to use humidity control alone to provide static control since static charges are developed even at relative humidity levels of 90% and greater.
- ESD Handbook ESD TR20.20-2008 section 5.3.16 Humidity: Relative humidity above 30% in ESD protective areas is desirable as long as other adverse conditions are not created as a result of humidity levels. Generally speaking an upper limit of 70% is desirable to prevent corrosive effects on the metal portions of electronic devices and assemblies.