|Ask the Experts|
October 8, 2020
ESD and Humidification
In the handling of circuit board assemblies placing them into systems, is ambient air humidification necessary if other means of protection are enforced including ESD mats/table tops, ESD wrist and ankle grounding straps, conductive wax flooring and protective clothing?
|Expert Panel Responses|
The main ESD items for the ESD protected area are ESD tables, ESD mats, Wrist straps ESD floor. Control air humidity is an additional possibility for better control. You cannot discharge enough charge in the EPA without the ESD items. The problem is you need an air humidity from more as 75 % or higher,it's not good for your PCB.
In the last time I have often read, that some companies offer devices for air humidity systems instead of ESD items. This is not true. Air humidity control can support the other ESD items and the personal grounding.You cannot reduce ESD charge without ESD tables, ESD mats, Wrist straps ESD floor etc.
Air humidity control will be used in chemical industry or paper industry, plastic factories etc.
If all ESD precautions being taken care as well if shop-floor having ANSI ESD S 20:20 qualified, ambient air humidification requirement should be based on product requirement. On ESD risk, ANSI S20:20 std takes care.
Supplier Quality Leader
Let's do a thought exercise. Assume we have a workstation located in a very dry area, but otherwise configured with dissipative mats. Workers are trained to use wrist/ankle straps, and a good maintenance program ensures that the floor surface is maintained.
If the assemblies are packaged in ESD-safe bags or totes and only removed by protected personnel, and if the system itself is on an ESD-safe surface, we can assume that any charges that might be present on assemblies or the system chassis are safely bled off.
The risks that the dry environment poses are as follows:
Companies that are highly sensitive to ESD will typically require a minimum of 30 to 40% RH, regardless of ESD protection.
CEO & Managing Partner
Elevated relative humidity (RH) is known to help reduce the concentration or buildup of electric charge (potential difference). This can result in electro-static discharge (ESD) when a conductive path (such as assemblies) is introduced between the potential differences, resulting in a sudden flow of electrons which can cause damage to components.
As with many preventative systems, more than one method is often employed to ensure destructive ESD events cannot occur. Conductive mats, straps and floor treatment are good systems for reducing ESD in what are called ESD protected areas (EPAs). However, no single protection method can provide complete ESD protection throughout your facility. Maintaining the ambient RH between 30% and 60% should be one of several systems used to prevent ESD damage to components and assemblies.
It is also important to maintain a record of all the systems used to control the environment in your manufacturing facility. Solid data records offer the only proof that the systems are in place and operating correctly. A system that will measure the temperature and RH in your facility at various locations and securely record the information can provide a more automated, error-free approach.
Factory-wide automatic monitoring eliminates the unreliable pen and paper method of manual recording and will issue a warning if dangerously low RH levels are reached. This is a superior method to verify a facility's control of ambient conditions.
Senior Project Engineer
Electronic Controls Design Inc
Additional humidification of an assembly area is not necessarily required when other measures are in place. Obviously, it is imperative that the other methods utilized are verified for compliance to proper ESD standards on a regular basis.I assume that the reference to "ankle" grounding is being used in the same context as "foot" ground. Remember that 2 foot grounds are required.
If mobile operators are transporting unprotected product, not in a closed shielding bag, the resistance to ground via the floor/foot ground system must be 3.5 x 10^7 or less.
I am glad you made the distinction of handling completed circuit card assemblies (CCAs) into systems (box level), as opposed to assembly of components onto CCAs.
During assembly of components to circuit boards, the humidity needs to be controlled in order to provide optimum solder paste performance and conformal coat adherence, yet not so dry as to cause a rise in ESD events.
For box build, if the humidity level can be increased without having to worry about any effect on solder paste rheology or coating adhesion, then it should be, provided the CCAs being assembled into the box are already conformal coated.
For box build, I would think that about 45-55%RH is ideal.
Is humidification necessary if the other ESD controls are in place? It depends.
There are at least 7 spokes considered to be present on the ESD-control wheel. They are:
The philosophy is that all 7 spokes of the ESD-control wheel are preferred over 5 or 6 lug nuts, with one or two missing altogether.
The idea is to have all 7 systems working together for the good of the whole. Here is an example:
So it begs the question, what kind of product is being builtin your program? High-reliability avionics controls? Nuclear missile guidance and control systems? Traffic control systems? Consumer products such as keychain fobs with LED illuminators and a little LCD watch?
Well for the first three products described, you had better have all 7 lugnuts attached securely and being monitored at least once a week,right? Perhaps you could get by with lesser controls for the key fobs.
But if you are producing 10 million key fobs per week for a margin of $5 each, and your customer finds out they don't work due to ESD damage and cancels all orders, you could still be kicking yourself because you skimped on the ESD controls.
So only you and your customer can determine what ESD control methodology is appropriate for the product being built. What is the cost of a single ESD failure in the field? The rework? Or the lost customers? Or the collective lawsuit from the families of those who perished in the plane crash?
Maybe it's a good thing to have that humidification system,eh? So the wheels don't come off?
Only you can decide.
Good luck with it or without it.
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