Garry,
You need not be a PhD to know. However, your docs are right. Copper
atoms are known to be rather stable, but in some cases they can start
mass 'evacuation', so called electromigration, either as surface
diffusion or grain boundary diffusion or bulk diffusion. I think surface
diffusion is the most known for Copper. If you have exposed copper on
you conductors, a voltage potential difference, and also have humidity
plus some mobile ions, like Chloride ions, you will get an electrolysis
effect, that can cause shorts between your you conductors. This happens
with semiconductor chips, I have had some cases to investigate, but can
possibly happen with PWBs as well. In one such case that I looked at,
the whole corrosion mess started with copper that diffused and was
changed to copper salts, that stared other reactions, which finally
caused all available metals to start moving and corrode. The blackish
mess was Silver, Copper, Nickel, Tin, Lead and some more constituents.
When the area dried, the process stopped occasionally, but went on as
soon as humidity was generated again. When the copper has a Nickel
barrier, this won't happen, because Nickel seems less apt to let atoms
move away. It's many years ago, when I worked with the cases, but I
think there was a additional explanation why small spots of exposed
copper was problematic: you get something called anode magnification,
which amplifies the corrosion in that specific copper spot. You may need
talk with your 'PhD's for a better explanation (one page is not enough).
So, I have said it before, we don't ignore exposed Copper on the
vertical sides of the lines, especially not when used for RF, or high
voltages combined with humid environment. If, on the other hand, the
boards are properly coated, I think the described problem will not
likely occur.
(There is another type of electromigration caused by the electron wind
in a high current copper conductor, but I'm not sure that kind of
diffusion is actual in your case.)
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Dehoyos, Ramon
Skickat: den 6 juni 2007 18:18
Till: [log in to unmask]
Ämne: Re: [TN] Exposed copper (space applications)
The Maytag man is very expensive here on earth. It cost me
almost as a new machine to replace three boards that burned on a three
year old washer. I told the repair man no can do. It ended up on the
curve. By the way, that repair man was very busy, unlike the commercial.
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Burke
Sent: Wednesday, May 30, 2007 12:08 AM
To: [log in to unmask]
Subject: Re: [TN] Exposed copper (space applications)
Gary I am sure someone on the forum will reply but I just wanted to
comment on those PhD's............
Scared? Might be more like cautious - they perhaps know what it would
cost to get the Maytag repair man to do a site visit in deep
space...................
John
John Burke
(408) 515 4992
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gary Bremer
Sent: Tuesday, May 29, 2007 8:29 PM
To: [log in to unmask]
Subject: [TN] Exposed copper (space applications)
Where is it stated that the conductive pattern can not have any exposed
copper on the vertical surfaces? Is that any empirical data that
supports a
failure mode if there is exposed copper on the vertical surfaces of a
conductive pattern? I have searched MIL-STD-2000, NASA-STD-8739-2 and
NASA-STD-8739-3 but can only find that it is allowed. Where I work those
with PhD's worry about exposed copper on the vertical edges of the
conductive patterns and are scared to allow to move forward because of
possible failure modes with a deep space launch. For my limited
knowledge of
metallurgy: after copper is exposed to moisture and oxygen, corrosion
takes place only on the surface and does not migrate any further. I do
have access
to NHB5300 to see if exposed copper was not allowed.
Gary Bremer
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