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Is Air Humidification A Necessity For ESD Control
Is air humidification a necessity for ESD Control?
Have you ever walked across the car park on a bright cold winter’s day only to get zapped by your car’s door handle? Yup, we’ve all been there and it’s commonly known that these ‘zaps’ are much more frequent in cold dry weather. It begs the question: esd magazine if I don’t get zapped when the air is moist, will using air humidifiers in a manufacturing environment prevent ESD damage of sensitive components? Let’s find out!
Humidity
Humidity describes the amount of water vapour in the air. There are 3 main measurements of humidity with the most common one being the relative humidity (RH). It is expressed in percent and describes \"how much humidity there is in the air, compared to how much there could be. Meteorologists often use the relative humidity as a measurement to describe the weather at various places.\" [Source]
At 0% the air is completely dry; at 100% it is so moist that mist or dew can form. The optimum relative humidity level is somewhere between 40% and 60%:
• A lower relative humidity increases charge generation as the environment is drier.
• If the humidity level is too high, condensation can form on surfaces.
Charge Generation and ElectroStatic Discharge (ESD)
The simple separation of two surfaces generates an ElectroStatic charge. Examples:
• Unwinding a roll of tape
• Gas or liquid moving through a hose or pipe
• A person walking across a floor with heels and soles contacting and separating from the floor
The amount of static electricity generated varies and is affected by materials, friction, area of contact and the relative humidity of the environment. A higher charge is generated at low humidity, in a dry environment.
Once an item has generated a charge, it will want to come into balance. If it is in close enough proximity to a second item, there can be a rapid, spontaneous transfer of electrostatic charge. This is called discharge or ElectroStatic Discharge (ESD).
Going back to our earlier example of getting a zap from your car’s door handle:
1. Charge generation: you walk across the car park with your soles contacting and separating from the floor. A charge is built-up on you.
2. ElectroStatic Discharge (ESD): you touch the door handle. Charges move from your body to your car until both are balanced out.
Impact of relative humidity on ESD
Many people will notice a difference in the ability to generate static electricity when the air gets dryer (relative humidity decreases). Relative humidity (RH) directly affects the ability of a surface to store an electrostatic charge. \"With a humidity level of 40% RH, surface resistance is lowered on floors, carpets, table mats and other areas. … the moisture in the air forms a thin protective \"film\" on surfaces that serves as a natural conductor to dissipate electric charges. When humidity drops below 40% RH, this protection disappears and normal employee activities lead to objects being charged with static electricity.\" [Source]
In an electronics manufacturing environment lower humidity may result in lower output from production due to an increase in ESD events during manufacturing processes.
Air Humidification and ESD
Air humidifiers are used to add moisture to the air and are commonly used in drier environments to keep humidity at a constant (optimum) level. Given that a lower humidity level increases the risk of ESD events, the obvious questions are:
1. Can air humidifiers replace normal ESD Control measures?
2. Are air humidifiers required for complete ESD protection?
Let’s address both of these questions:
• Let’s be very clear about one thing here: air humidifiers cannot replace ESD Control measures.As explained further above, ESD is caused by two items that are at a different electrostatic equipotential and want to equalise their charges. Adding moisture to the air using humidifiers will not stop this discharge from happening. The only thing you may achieve is a reduction in the number of ESD events. BUT: they will still happen; just walking across a carpet will generate a charge on an operator. If they then touch an ESD sensitive component, discharge will still occur and may damage the component. No humidifier in the world will prevent this.
The only way to control electrostatic charges on a person or object is through ESD grounding – this will ensure any charges generated dissipate to earth:
• Operators need to be grounded using wrist straps or a footwear/flooring system.
• Surfaces need to grounded using appropriate matting and ground cords.
For more information on how to create a ESD workstation and how to correctly ground all elements, have a look at this post.
• Low air humidity can increase the number of ESD events so it may make sense to keep a factory at a higher humidity level. However, there are many other factors that come into play when choosing the ‘right’ humidity for a manufacturing environment. The recommended humidity range is usually determined by the specifications of the devices and components being assembled. Increasing the humidity in an electronics manufacturing facility can help to reduce ESD events but increased humidity can lead to other unwanted quality issues in an electronics manufacturing environment such as corrosion, soldering defects and the popcorn effect on moisture sensitive devices.
A normal range for humidity in electronics manufacturing is 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.
And remember: you will not eliminate ESD by using humidifiers and keeping humidity levels at a higher humidity level. You need an ESD Control Programme in place to avoid ESD and associated damages.
Conclusion
A lower relative humidity level increases charge generation as the environment is drier. This will result in more ESD events which can potentially damage sensitive components. The only way to protect sensitive components from ESD damage is by having proper ESD control measures in place and connecting operators, objects and surfaces to ground. This will ensure each element is kept at the same electrical potential and any electrostatic discharge is being removed to ground.
Humidification can help reduce the number of ESD events in an electronics manufacturing environment but at the same time there are other factors (e.g. moisture sensitivity of components) that need to be considered, too.
2018-10-20 02:05:36 BY 游客   查看:238 次   以下共有回复:0 篇  
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