What is Ideal Humidity for Final Assembly?

What is Ideal Humidity for Final Assembly?
What is ideal humidity within a final assembly facility? No soldering involved. The facility is used for final box build. Phil Zarrow and Jim Hall, The Assembly Brothers, answer this question and offer their own insights.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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Phil Zarrow
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.


And welcome to the Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, coming to you from high atop Mount Rialto, which is Board Talk's version of reality, such as it is.

And we're here to talk about soldering and assembly processes, materials, and all kinds of fun things like that. And I believe today, Jim, we have a climate question.

This question comes from Craig. The question is what is ideal humidity for final assembly? No soldering.

We're basically talking about box build. I'm interpreting that as saying not attaching components.

Not attaching components, not putting on conformal coating,everything's done. It's time to go in the box.

What is the ideal humidity, Phil?

As we know at this point, we're not really material-sensitive, per se. What's done is done. So, essentially, our biggest concern at this point in time is electrostatic discharge, ESD.

Isn't generally 50% relative humidity a desired norm, minimum for ESD protection?

I almost committed you to living here and you don't live here anymore, Phil. But here in the Northeast, where us rugged men live, often when the heat comes on in the winter, like it is now, the air gets very dry.

You get static electricity walking across the carpet, touch the door knob - classic examples. The materials, once they're done soldering they in their lifetime are going to get saturated with ambient moisture anyway, so there's no concern about that in final assembly.

But anybody who has listened to us before knows, if you are going to solder them again, as in rework, repair, field repair or something like that, the books are open again.

And what do we always recommend? We always recommend baking out the board before you rework it to take care of the MSD situations and accumulated moisture in the circuit board.

If you are going to solder again, yes, everything is back to zero in that respect, and you have to be aware of the humidity. But from final assembly, the ESD is the big question.

Anything to add to that, Uncle Jim?

So, moving along, we pounded the hell out of that question, didn't we? We hope Craig and anybody else listening is more informed than confused.

I don't think there's any confusion here - 50% is the target. And always take the proper precautions, too. We've had cases where - don't leave anything for granted. Everybody should be properly grounded.

You spent all that good time not soldering like my brother, so don't mess it up now. And you sure wasted enough time listening us tackle this very easy question, didn't you?

So whatever you, don't solder like my brother.


The experts are partially correct here. Final Assembly is performed at literally thousands of CEMs under 30% to 70% RH, but as Doug Pauls always says, it depends. Not all Final Ass'y is the same. Some do final assembly soldering of the chassis connections to various internal CCAs. Some do conformal coating or potting processes in Final Assembly areas, and they may need a relatively dry environment to keep the CC or potting surfaces dry. So...it depends. But usually these are exceptions to the rule, and the biggest concern IS in fact ESD.

The attitude that "humidity is not a concern if you follow good grounding and insulator control" is not acceptable; both EOS/ESD 2020 and the ESD MIL-STDs and other standards all state that if the RH drops below 20% in any portion of the facility where ESD handling is required, then all-hands notification is given as a precaution. It is NOT redundant. Consider all of the ESD control measures (proper personal grounding of the product handlers, wearing of ESD smocks, dissipative sole grounders/ESD shoes, dissipative floor, dissipative mats/benches, proper ionization for tape dispensers in coating areas, an internal ESD audit system, and especially the relative humidity, these are all lug nuts on the ESD control wheel.

Do you drive your car around with one or two lug nuts missing or loose because the rest are "good enough"? No? Well, don't do that with your ESD control system either! If you have ALL of the lug nuts present and working, should one (or even two) come loose such as a broken ground wire found on a bench, and/or an operator working with no wrist strap on, and on top of all that your humidity levels were only 15 to 20% during January when this happened, then how can you exonerate all product produced on that bench and by that operator during that month?

It is all suspect, and you have some containment work to do. If all of the systems are working, then there is little danger to the product, and it is much easier to disposition any affected product accordingly, and your customer has a much better feeling about how you operate.
Richard Stadem, General Dynamics Mission Systems
I assume that there will be employees present. Humidity should be 40% to 60% for ergonomic reasons. I just read that the Mayo clinic says 30-50%. In the past I have seen nylon parts get brittle in low humidity conditions.
Stephen Olan, Stim Canada
There's no need to control humidity for ESD control in most cases, providing you have specified your ESD control measures well. ESD control equipment specified according to IEC 61340-5-1 and ANSI/ESD 20.20 is tested to operate down to 15 %rh. There are some types of material you might have to be careful with, notably "pink polythene", which depends on moisture from the air for its low charging action. If there's no moisture, it isn't low charging. Occasionally there might be reasons that standard ESD control is inadequate and humidity might be used as an ESD control, but they are unusual.
Jeremy Smallwood, Electrostatic Solutions Ltd
If it is desired to keep boards and components dry during reflow soldering, wave soldering or storage times, inert atmosphere may help. I mean N2 as inert gas protection. Inerting the components surroundings, it is possible to eliminate not only moisture (< 5 ppm H2O), other contaminants present at air ambient (or oven ambient) too. They are replaced by inert gas. Measuring the contaminant levels and adjusting N2 flow may provide nice economical and good quality results.
Luiz Felipe Rodrigues, Air Liquide
I suspect that you must be aware that if products are assembled at 50%RH at room temperature into, sealed containers, that they can become flooded with moisture when the temperature falls. Wet products can fail for a number of reasons. Our customers want their parts dry.
Tom, Salzer
Please allow me to add my two cents about humidity control from an ESD control perspective in an electronics assembly environment. Per the ESD Associations' ANSI/ESD S20.20-2014 Standard, there is no "requirement" for humidity control. There has only long been a "recommendation" for 30% to 70%. Some level of humidity between those two points is beneficial to limiting static generation but humidity is not the best solution to static control. If you need ESD control, use approved methods and techniques for proper grounding and insulator control.
Gregg Heckler, Desco Industries, Inc.

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