Techies
Keep-in-mind that a conformal coating will delay these effects it will not
prevent them. Ionics under the coating will induce osmosis as temperatures
and humidity levels rise and fall.
Also Eric, given that the life expectancy of a mobile is presently less than
18 months, they hardly have time......mark you, as Motorola informed me, the
heat generated is usually enough to ensure that moisture does not remain a
problem for long.
--
Regards Graham Naisbitt
[log in to unmask]
CONCOAT SYSTEMS LIMITED
Unit B2 Armstrong Mall
Southwood Business Park
Farnborough, Hampshire GU14 0NR
Phone: +44 (0)12 5252 1500
Fax: +44 (0)12 5252 1112
www.concoatsystems.com
On 29/11/05 17:34, "Eric CHRISTISON" <[log in to unmask]> wrote:
> 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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