Dehumidifier for SMT boards and Microphones

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camarada78

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I live in a very wet region (Rio de Janeiro, Brasil) and many repair problems I deal here regarding SMT boards are related to humidity. Sometimes i use a hairdryer, but most of the times i use an improvised DIY box with a dehumidifier inside it. where i put the SMT boards in need of drying. Usually they are audio interface boards, sometimes control panels of bigger devices, small amplifiers, microphones (when the condenser capsule is 'stuck', dont know the term in english, sorry) and so on...

My question is, is there a professionaly made solution for it? Like a lab-ready dehumidifier box? I thought of something like a food dehidrator (with drawers) but with bigger room for mics, boards and so on.

Thanks for the attention,

have a good day

Leandro
 

CJ

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you can buy a hot/cold box, they have a heating element precisely controlled for heat and use an external CO2 bottle for cold, used for testing circuits for thermal stability. you might pick up a used one for cheap. and you can spray boards with Krylon clear coat to protect hi-z areas from humidity.
 

camarada78

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you can buy a hot/cold box, they have a heating element precisely controlled for heat and use an external CO2 bottle for cold, used for testing circuits for thermal stability. you might pick up a used one for cheap. and you can spray boards with Krylon clear coat to protect hi-z areas from humidity.
Interesting, i will check it out.
 

Newmarket

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I suggest to 'bake' the problem assembly then conformal coat or 'varnish' to provided a barrier to moisture.
Bake at c. 105 deg C - so it's above the boiling point of water but below the pcb delamination temperature (assuming FR4 pcb).
Say 30 mins (attatched guide says not more than two hours). Then apply coating as soon as practicable. Note what drying times/conditions recommended for the coating.
Of course, assuming that everything on the pcb is okay to be at >100deg C. If not option is to remove then refit before coating.

Link is 'random' from the interweb and relates to starting with a bare pcb but similar considerations.
Recommendations for PCB baking - PCB Tech
 

Whoops

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food dehidrator (with drawers)

That seems to be a pretty good idea,
also the Varnish clear coat after getting rid of the humidity seems to be a good solution.

This food dehidrator seems to be pretty cool:

 

xeawr

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That seems to be a pretty good idea,
also the Varnish clear coat after getting rid of the humidity seems to be a good solution.

This food dehidrator seems to be pretty cool:

Question: Why doesn't the clear coat mess up the hi-z area at the FET input of a mic? This is something I've never understood...
 

Whoops

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The clear coat/varnish will protect the Hi-Z area components and tracks from dust, moisture or any various debris
 

xeawr

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The clear coat/varnish will protect the Hi-Z area components and tracks from dust, moisture or any various debris
I appreciate your response. I understand the purpose of the clear coat, I thought however that the hi-z area is sensitive to basically anything touching it (other cables or capacitors, flux residue, body oils from skin etc.). I understand that moisture, oils etc. reduce the resistance and thus cause problems, but I thought because the resistance is so high, one must prevent anything from touch this area because basically anything can reduce the resistance of that part of the circuit.

Thank you for your contributions, I love to learn and I really appreciate the time you people invest here!
 

Khron

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I appreciate your response. I understand the purpose of the clear coat, I thought however that the hi-z area is sensitive to basically anything touching it (other cables or capacitors, flux residue, body oils from skin etc.). I understand that moisture, oils etc. reduce the resistance and thus cause problems, but I thought because the resistance is so high, one must prevent anything from touch this area because basically anything can reduce the resistance of that part of the circuit.

Thank you for your contributions, I love to learn and I really appreciate the time you people invest here!

Lacquer(?) seems to work well enough for Rode, who make some of the quietest(?) condenser mics, so... Can't be all that bad, i guess.
 

ccaudle

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Houston
but I thought because the resistance is so high, one must prevent anything from touch this area
The coating is high enough resistance that it does not change the electrical properties, but an uncoated PCB can absorb atmospheric moisture and become lower resistance over time. You have to clean the PCB surface very thoroughly before applying to be fully effective.
 

Newmarket

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The coating is high enough resistance that it does not change the electrical properties, but an uncoated PCB can absorb atmospheric moisture and become lower resistance over time. You have to clean the PCB surface very thoroughly before applying to be fully effective.

+1. It's important to get virtually all the moisture out pf the assembly before coating. If it gets 'sealed' in there then tries to 'escape' when it expands due to temperature then you can get more problems than you were trying to solve.
For cleaning - ultrasonic cleaning recommended.
 

dom

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I would just use a small temperature regulated oven. No need for a dehumidifier. Bake your device in the oven for a day or so at a reasonable temperature to remove moisture. A reasonable temperature for a full mechanical assembly (like a microphone) may be around 40 °C or so. For a bare PCB assembly the temperature can usually be substantially higher; the vast majority of TH and SMD parts are rated to at least 70 °C.
 
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