Relative humidity (RH) directly affects the ability of a surface to store an electrostatic charge. As RH increase, the time a surface will hold a charge will decrease and the dissipation rate will increase. As the RH increases, so does the natural conductivity of the air, but even at 100% RH the increase in the natural neutralization rate cannot replace proper ESD control practices.
There is a noticeable difference in the ability to generate static electricity when the air gets dryer (the % RH decreases). 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. A normal range for humidity in an electronics manufacturing environment is be between 30%RH and 70%RH. Some facilities try to maintain a constant moderate RH (~50%), whereas other environments may want lower %RH due to corrosion susceptibility to humidity sensitive parts. The recommended humidity range is usually determined by the specs of the devices and components being assembled.
The ESDA’s TR20.20-2008 discusses RH in couple of different places. Some significant statements are listed below. We recommend purchasing the ESD Handbook – ESD TR20.20-2008 from the ESD Association.
ESD Handbook ESD TR20.20-2008 section 2.3 Nature of Static Electricity:
The amount of charge created by triboelectric charging is affected by the area of contact, the speed of separation, relative humidity, material work function, surface energy and other factors.
The moisture level in the air, or relative humidity in the environment, are important considerations in the liberation and accumulation of static electricity. It is well known that static electricity in the form of static cling and static shocks are more prevalent when the air is dry. Heating interior air in the winter months dries out the already dry air in the higher latitudes. Static charge accumulation is easier on dry materials since moisture on surfaces tends to allow charges to slowly dissipate or recombine.
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. For most situations, 30 to 70% RH is considered the appropriate range. Special areas, such as wafer fabrication, may require lower humidity control for processes that are affected by moisture (e.g., photoresist application). Soldering is known to be affected by high relative humidity conditions (>70%). For areas that have low ambient humidity, ionization is an important consideration to aid in reducing charge accumulation levels and provide neutralization of charges after they are developed but before they can cause difficulties.
ESD Handbook ESD TR20.20-2008 section 5.3.16 Humidity:
Humidity is beneficial in all ESD control program plans. Contact and separation of dry materials generates greater electrostatic charges than moist materials because moisture provides conductivity that helps to dissipate charge. For this reason, ESD effects are most noticeable in the winter since heating systems reduce building environment moisture. Geographic location (desert vs. coastland) is also a major contributor to ambient conditions inside buildings. Any circumstance that results in a low relative humidity will permit a greater accumulation of electrostatic charges. 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.
Besides the increasing propensity to generate electrostatic charges on dry materials in general, performance of many ESD protective materials degrade. In fact, when exposed to low humidity conditions, some ESD protective materials become totally ineffective or become sources of electrostatic charges. Therefore, evaluation of ESD control materials should include performance testing in controlled environments at the lowest expected operating relative humidity level. Manufacturers of ESD protective materials should be able to provide performance data in regards to relative humidity. Likewise, materials should be tested in moderate humidity conditions as well to ensure they do not become “too conductive” and present a potential safety hazard to personnel working with substantial voltages. See the Personnel Safety section of this handbook for further guidance in this area.
Humidity control in factories or physically large areas or buildings can be difficult and expensive. In smaller rooms or areas, it may be possible to use portable humidifiers to raise the immediate area humidity. However, in large facilities and factories the environmental systems many need to include steam generation and monitoring equipment to control humidity. This type of equipment is expensive to install and purchase especially in pre-existing facilities. To reduce the total cost impact, companies should consider the need for humidification equipment when planning new facility construction.