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May 2003

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Wed, 28 May 2003 09:32:14 +0800
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Thanks, Tim - that'll teach me to go stirring pots with a touch of
facetiousness. Aluminiium, of course, is a case in point. It just struck me
as something of a contradiction that oxides should form on an alloy that
had earlier been described as uncorrodable. I'll go back to my corner now.

Peter


Tim Reeves <[log in to unmask]>  28/05/2003 01:28 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to Tim Reeves

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: Re: [TN] Sn-Ni PCB plating








Big mistaken assumption: uncorrodable = no oxide formation
Actually, the presence of a tough oxide film on the surface is what makes
certain metals corrosion resistant. Virtually all metals form oxides in the
presence of oxygen. It's the ability of that oxide film to "seal off the
surface" from further corrosion that's key.
Tim Reeves


-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Sunday, May 25, 2003 9:15 PM
Subject: Re: Sn-Ni PCB plating


Happy,

As I read your post, plating Gold, tin, tin-lead, silver, Pd, etc would be
to provide a solderable surface, not to prevent oxides forming (since
tin-nickel is uncorrodable, there should be no oxides, right?). Gold is not
a finish that most people would want to solder to either as it causes
solder embrittlement. The others are OK. Gold is normally only used to
cover and protect another, solderable plated finish that DOES oxidise, but
it's so thin that it doesn't impact on solder joint quality. I've not come
across Gold as the material to actually solder to. Or have I missed a big
something?

Peter



Happy Holden <[log in to unmask]> 23/05/2003 10:40 PM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to HTH

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: Re: [TN] Sn-Ni PCB plating









Good question Jack,
The property that makes it uncorrodable (a very thick-non porous oxide
film) also makes it very difficult to solder.  In the old days we could use
an active acid flux but that is no longer true.  The current non-clean type
of flux would not even penetrate the oxide on a tin-nickel pad and it would
end up non-wetted (not even de-wetted).  Gold, tin, tin-lead, silver, Pd,
etc all solder very nicely, so they need to be put over the tin-nickel to
prevent the oxides from forming.

The other factor is that the plating bath is difficult to control and you
have to watch it to make sure it does not plate stressed.  I once saw the
traces 'jump' off the boards after etching, the tin-nickel was so stressed.
But like I said, the sail boat metal parts are remarkable!

Happy



   "Jack C. Olson"
   <[log in to unmask]>                       To:        TechNet
                                        <[log in to unmask]>
                                                cc:        [log in to unmask]
   05/23/2003 09:32 AM
                                                Subject:        Re: Sn-Ni
                                        PCB plating







Ok, I'm probably showing my ignorance here,
but with a stable non-corrodable finish like
Sn-Ni, why do you need the gold?

Jack (a PCB layout guy)







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