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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, discuss this question and offer their suggestions.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
With over 35 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.

Transcript


Phil
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.

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

Phil
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.

Jim
What is the ideal humidity, Phil?

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.

Jim
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.

Phil
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.

Jim
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?

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

Comments

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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