Dry air and ESD control
The specifications and devices being assembled determine the recommended humidity range for a manufacturing facility. When the air is dry, static control becomes much more challenging. Although ESD events may be reduced by increasing the humidity, unfortunately that same humidity may cause a reduction in the quality of moisture-sensitive devices because of corrosion, soldering defects and the popcorn effect.
Ionizer choices for reducing ESD
Although ionizers cannot replace grounding or shielding of ESD work surfaces, ionization can lessen ESD events in areas where dry air is normal. Desco has number of ionization products available through All-Spec.
- Benchtop and overhead Ionizers – work surface ionizers create positively and negatively charged ions that are moved to the controlled area with fan-driven airflow.
- Point-of-use air ionizers – use compressed gas to combat electrostatic attraction neutralizing charges on particles causing contamination or visual defects on products.
- Laminar flow ionization bars – used with laminar flow hoods, controlled chambers and other contained applications.
- Room Ionization – reduces electrostatic discharge (ESD) and electrostatic attraction (ESA) in cleanroom environments.
For more information, consult the ESDA’s TR20.20-2008 RH information. Below find some of the significant statements about dry air and static electricity.
- 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.
- Humidity control alone cannot 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%).
- Ionization is an important consideration in areas with low ambient humidity to aid in reducing charge accumulation levels and provide neutralization of charges after they are developed but before they can cause difficulties.
- 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.
- ESD effects are most noticeable in the winter since heating systems reduce building environment moisture.
- Geographic location (desert vs. coastland) is 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.
- In general, an upper limit of 70% is desirable to prevent corrosive effects on the metal portions of electronic devices and assemblies.
- Dry air increases the tendency to generate electrostatic charges on dry materials and performance of many ESD protective materials degrade.
- When exposed to low humidity conditions, some ESD protective materials become totally ineffective or become sources of electrostatic charges.
- 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.
- 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 alternatives
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 many large facilities and factories the environmental systems 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.