operating in 100% RH, non-condensing

[zappedagain]

I have a customer who is requesting operation at 100% relative humidity, non-condensing, over a 35-120F (2-49C) temperature range. This is going to be in a controlled environment, so I expect this customer has valid reasons for requiring this range (exotic processes). My design is presently spec'd for operation over 10-90% RH, -10C to 70C.

If he said condensing then I would have to conformal coat my PCBs and go down that path. What concerns are there for non-condensing RH?

Z

 

[IRstuff]

I think that you'd have to conformal coat regardless. Water vapor is actually more active than liquid water, so uncoated boards still have the potential of corrosion as well as moisture damage to plastic parts.

TTFN

FAQ731-376

 

[zappedagain]

Thanks. That's a good start. I'll post other info that I find.

I came across an interesting 'ruggedization' paper at Curtis Wright Controls where they talk about 'Level 50' ruggedization (hee hee, my spell checker wants to change that to bastardization). Do you recognize that as a standard or is that their nomenclature? I'm off to search the MIL-STDs...

Z

 

[CharlieGill]

I would think that 100% RH non-condensing would be difficult to maintain. There's bound to be some condensation as you are right at the saturation point. I agree, conformal coating.

Charlie

 

[itsmoked]

I done stuff in places that were 100% non-condensing. Insane places to stand and work BTW. When you come into the space you go blind as the "non-condensing" atmosphere promptly condenses onto your "cold" eyeballs. After a minute or three that stops happening as you eyeballs warm up.

So the point is, when your equipment is brought in it will get soggy immediately. Once it warms up to the space temperature it will dry off. If it gives off heat while operating it will probably stay even dryer.

You can run without conformal coatings on most things. But this is because the room is 'controlled'. If the temperature was allowed to drift or cycle then you would have the standard problem of your equipment cyclically being below the dewpoint and getting wet.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[zappedagain]

I read the specification again and it states that the unit must "withstand ambient temperatures of 2C to 49C degrees F and relative humidity’s of 10 to 100% (non condensing) without permanent damage". Does that mean operation or storage? I'm going back to the customer for clarification.

IR - I'm unfamiliar with moisture damage to plastic parts. What types of damage to what types of plastic are you referring to?

Thanks all,

Z

 

[IRstuff]

Back in the "old" days, military parts were partly controlled via MIL-STD-883, and the temp/humidity tests would clobber plastic IC packages, so almost all MIL-STD-883 parts were either ceramic, metal, or a combination thereof.

The moisture in the air can get through the plastic and start to create corrosion products at the wirebonds and leadframe inside the package. There were, supposedly some plastic parts that were built with overcoats internally to prevent that, but I would imagine that the majority of COTS plastic ICs don't have any overcoating.

The conformal coating is required for both protection during the exposure, as well as preventing intrusion from the condensation after exposure. There are very few military systems that get to remain in a controlled and constant environment. We had a shipboard system that was able to extract water from the air every night, until the shroud was partly filled with liquid water, and the components inside the shroud were rusted/corroded or otherwise degraded.

TTFN

FAQ731-376