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September 9, 2016

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?

B.K.

Experts Comments

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.
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Hartmut Berndt
President
B.E.STAT group
Mr. Burndt has been in Electronics & Semiconductor Technology since 1980 and work in the area of Electrostatic (ESD) and electronic devices. He is President of the B.E.STAT group (Germany); expert in ESD audits, trainings, failure analysis and ESD control programs.
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.
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Subrat Prajapati
Supplier Quality Leader
Ge Healthcare
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
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:
  1. Any article that is not touching an ESD-safe surface or handled regularly by the ESD-protected operator can build up a charge. Incidental contact between assemblies and these items can result in the charge being rapidly dissipated (an ESD event). An example of this would be air-flow over a piece of paper building  charge over time. Controlling the humidity is often easier than ensuring that no static-generating materials ever enter the workspace.
  2. If any of your other systems fail, humidity control provides additional safety margin. We must expect that grounds will fail, operators will forget to wear (or test) straps, straps will be worn improperly, Mat grounds will be compromised, etc.
In some environments, it is difficult to humidify. It may be expensive to add humidification to existing HVAC systems, for example. In these cases, consider local ionization. Bench-top ionizers, when configured, set up and maintained properly, will provide protection within their effective area.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Companies that are highly sensitive to ESD will typically require a minimum of 30 to 40%RH, regardless of ESD protection. 
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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
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. A brand new technology, SensorWATCH, is designed to deliver this kind of functionality and more. 
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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
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.
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Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
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:
  • A dissipative floor
  • Operators wearing dissipative footwear
  • Operators wearing ESD-safe smocks correctly
  • Operators wearing wrist straps when handling ESD product
  • A humidification system in place including charting of the relative humidity and alarms
  • All ESD benches identified as such and properly connected, each with its own connection to ground
  • An audit system in place to monitor all of the above, with sufficient frequency for each method to ensure compliance and effectiveness  

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:
  • Three ESD bench surfaces were inadvertently daisy-chained together to a single ground point in violation of EOS/ESD 20.20, a commonly used ESD standard.
  • One of the wires broke, leaving two bench surfaces ungrounded. But because there was an ESD floor in place, a requirement that the operators wear dissipative shoes or footstraps, dissipative smocks buttoned up, a wrist strap being worn when handling ESD-sensitive product outside of its protective Faraday cage,  an ionizer present and turned on if the product was Class 0, and the wrist strap connectors had their own wire to ground (not in the same line as the ESD surface), it could be argued that in all probability no damage was done. In this case all of the other ESD prevention methods were in place, and because a floor check proved the floor was dissipating properly, there were no failed wrist strap checks, no written violations of ESD footwear or smocks, no record of any other ESD failures at test, and the charts showed that the HUMIDITY was between 40 and 65%RH between the passing bench audit and the failing bench audit, the company was able to successfully argue no product containment action was necessary. The audit records showed the benches passed the ground check the week prior, and only 6 days elapsed before the broken connection was discovered in the subsequent check.  
Now, the other side of that coin is that there is a cost to install and maintain an ESD floor. There is a cost for a weekly audit to catch things like the broken ground wire, a cost to buy smocks and test them once a year, and a cost to provide ESD footwear, heel straps, and wrist bands, etc.  

So it begs the question, what kind of product is being built in 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.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
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