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November 2005

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Subject:
From:
John Burke <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, John Burke <[log in to unmask]>
Date:
Tue, 29 Nov 2005 09:37:55 -0800
Content-Type:
text/plain
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You mean apart from the one where my daughter ran into the ocean with a
mobile phone in her pocket??........................8-)

John

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Eric CHRISTISON
Sent: Tuesday, November 29, 2005 9:34 AM
To: [log in to unmask]
Subject: Re: [TN] Humidity and enclosed RF board

Agree with Richard about the environment. Letting the enclosure breathe
is
also a good plan; how many mobile phones fail due to humidity related
failures?

You might also consider adding a breathing desiccator to the enclosure
although they tend to be expensive and need replaced every six moths or
so.

Regards, 
  
  

Eric  Christison 
Mechanical Engineer 
Home, Personal, Communication Sector - Imaging Division 
STMicroelectronics 
33 Pinkhill 
Edinburgh 
EH12 7BF 

Tel:     +44 (0)131 336 6165 
Fax:    +44 (0)131 336 6001 



> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
> Sent: Tuesday, November 29, 2005 5:25 PM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity and enclosed RF board
> 
> 
> It also depends on the environment where the box will live. 
> If it is near the sea, there is a much higher probability of 
> failure if any moisture does enter from outside. 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls
> Sent: Tuesday, November 29, 2005 11:01 AM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity and enclosed RF board
> 
> Well, it depends.
> 
> You say that the casing seals the electronics from the 
> external atmosphere. How well?  Is it a hermetic seal?  Are 
> you using an elastomeric gasket? Do you package with an inert 
> cover gas?  What is the design life of the product and what 
> are the consequences of failure?
> 
> In my experience, even those enclosures assumed to be 
> "hermetic" are not, and that usually means moisture will 
> eventually get in and will have a hard time getting back out. 
>  So it all depends on how well insulated your case keeps the 
> electronics throughout the design life. Elastomeric gaskets 
> willl wear out.  Field repair is often less than optimal.
> 
> If moisture gets inside the case, it will condense on your 
> boards when you go through dew point.  Can it grow dendrites? 
>  Yes.  What is the risk?  It Depends.  Dendrites have been 
> observed to grow in as little as 3.5 volts DC.  There is no 
> "safe" level of humidity because the degree of risk is 
> dependent on both humidity and the amount of ionic soil.  The 
> dirtier it is, the less humidity it takes.  Some papers have 
> suggested 70% RH as a threshold, but this is usually SIR type 
> of testing, not field trials.  In addition, if your product 
> is functioning in some of the more polluted areas of the 
> world, and the enclosure is not hermetic, then the pollutant 
> gasses also get into the enclosure and do nasty things.
> 
> Doug Pauls
> 
> 
> 
> 
>              "Tempea, Ioan"
>              <itempea@POSITRON
>              .QC.CA>
> To
>              Sent by: TechNet          [log in to unmask]
>              <[log in to unmask]>
> cc
> 
>  
> Subject
>              11/29/2005 10:43          [TN] Humidity and enclosed RF
> board
>              AM
> 
> 
>              Please respond to
>               TechNet E-Mail
>                    Forum
>              <[log in to unmask]>
>              ; Please respond
>                     to
>               "Tempea, Ioan"
>              <itempea@POSITRON
>                   .QC.CA>
> 
> 
> 
> 
> 
> 
> Hi Technos,
> 
> application: one RF board and one antenna, no conformal 
> coating, both sealed inside of a casing; outdoor unit, will 
> see temp and humidity variations.
> 
> question: assuming that the boards are well insulated from 
> the exterior, should I worry about long term rel of the 
> boards? Otherwise said, due to temp variations, can the 
> residual humidity inside the casing condensate on the boards 
> and cause damage? Or can it make grow dendrites?
> 
> And a general question:  For dendrites we need a potential 
> difference, humidity and dirt; is there any threshold for the 
> humidity level, below which dendrites will not grow?
> 
> Thank you,
> Ioan
> 
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