On Sat, 30 Jul 2016 14:00:33 -0700, you wrote:
>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.
It's possible that the amount of tin available may have something to
do with the possible whiskering. I don't remember that there was a
problem with tin plating on PC boards (not that I ever heard), but
once we went to environmentally safe mostly tin solder, we had a
problem with whiskering. Since NASA was required by congressional
edict to be be environmentally sensitive and reflect the current
administrations's environmental goals (yes, that's cynical), NASA of
course used the almost completely tin solder. I am rather curious of
how many orbital satellites ended up in a landfill, but then
again.....
From what I know, regular tin plating is not a problem. Mostly tin
solder is. How long this whisker growth takes I do not know, a rough
suspicion (and that's all it is) says 3 to 5 years regardless of what
you do.
For a home project, you just have to take it apart and figure out what
the problem is. For something in orbit, you're pooched....
For consumer electronics, you just buy a new one, after all, you
didn't think that your 1000 dollar TV would last more than 3 to 5
years, did you?
I use lead solder.....
Harvey (the other one, who's watched too many political commercials
lately)
<grin>
>
>Harvey
>
>
>On 7/30/2016 12:08 PM, Brad Thompson brad.thompson@...
>[Homebrew_PCBs] wrote:
>>
>>
>>
>> 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
>>
>>