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Brad,
You are correct about the whiskers, but the conformal coat will not stop them. The whiskers will punch through the coating, and if two conductors are close enough, their respective whiskers can contact. I would stay away from tin, if at all possible.
The conditions that lead to whisker growth are not understood. They don't need extreme outdoor conditions, nor moisture, nor even air to initiate or grow. A lot of money has been thrown at this problem, and still there is no understanding of the basic physics of this phenomenon. Unfortunately, a lot of DIP package ICs were manufactured with tin platings on their leads. Solder dipping right up to the body eliminates the threat, but may cause other damage. Newer packages use other platings that are not susceptible to whiskering.
Harvey
On 7/30/2016 11:43 AM, Rich Osman lists@... [Homebrew_PCBs] wrote:
>
> On 7/30/2016 10:34 AM, alan00463@... [Homebrew_PCBs] wrote:
>>
>> In searching for a vendor of Liquid-Tin I ran across
>> this waterproofing spray:
>> http://bamtech.com/product/acrylic-coating/
>>
>> Since I live in the RustBelt and know what oxidized
>> copper looks like, I wonder if I should use this on
>> my PCB to keep the copper from contacting moist air?
>>
> Some protection will be smart.
>>
>>
>> My PCB will be used outside in a plastic box. The
>> plastic box will be waterproof, but not airtight.
>> I am guessing the Liquid Tin and leaded solder ap-
>> plied to the traces will prevent oxidation. I just
>>
> No, tin will just change the type of oxide. Tin oxidizes too.
Hello--
Worse yet, tin can grow conductive whiskers that can cause short
circuits and intermittents:
http://nepp.nasa.gov/whisker/reference/tech_papers/2012-Panashchenko-IPC-Art-of-Metal-Whisker-Appreciation.pdf
There's information on what conformal coatings NASA found that would
resist whisker growth.
The waterproof box will collect moisture inside due to infiltration of
moist air and temperature cycling.
About all one can do is to create an air- and water-tight box that has a
moisture-accumulating
dryer cartridge venting the box. You'd have to periodically replace the
cartridge.
73--
Brad AA1IP