Hi folks! I would like to back up a step before we start worrying about the
solder joint (I'm with you Bev - I am a bit confused as to what solder
joint Jeff is describing). If you plate gold over silver you may not get
the chance to form a solder joint. Gold and silver form a continuous solid
solution alloy - in simple terms no intermetallic phases form. Once the
silver diffuses to the surface it will become very unsolderable due to the
formation of silver sulfide (other surface reactions are possible but due
to sulfur dioxide in the air the silver sulfide seems to win the race of
who forms first). This diffusion reaction will occur at room temperature.
Sorry Jeff but putting gold over silver will cause you more headaches than
its worth. Do you know what is causing you silver to tarnish? The immersion
silver pwbs I deal with also accumulate some tarnish but I have had little
solderability impact.  Hope that helps - time for a Coke!

Dave Hillman
Rockwell Collins
[log in to unmask]



                      Bev Christian
                      <[log in to unmask]        To:       [log in to unmask]
                      ET>                      cc:
                      Sent by: TechNet         Subject:  Re: [TN] Gold thickness
                      <[log in to unmask]>


                      02/21/2003 06:32
                      AM
                      Please respond to
                      "TechNet E-Mail
                      Forum."; Please
                      respond to Bev
                      Christian






Jeff,
It is more complicated than that.  First, you talk about contacts and then
you talk about solder joints, so I am a little confused.  I presume you
will have lots of pads and some will be the basis for solder joints and
some will be physical contact points, but will not be soldered. Is that a
correct interpretation?  If that is so, then you have to worry about not
only the gold limit for your solder joints, but the effect of the silver!
Werner has said more than once that he believes that silver is more
detrimental to solder joint reliability than gold.  Now if you overplate a
silver pad with gold and then cover with eutectic tin/lead solder paste and
reflow the gold will surely dissolve and I would expect a significant
amount of the silver as well.  This will mean that your "noble metal"
content could be quite high.

In addition, we need to know how high a temperature your system will see,
because I would expect the metal boys may have something to say about
silver/gold diffusion if you get high enough in temp.

Have fun!

regards,
Bev Christian
Research in Motion

-----Original Message-----
From: Jeff Brown [mailto:[log in to unmask]]
Sent: February 20, 2003 7:05 PM
To: [log in to unmask]
Subject: [TN] Gold thickness


Dear Technetters,

We want to protect silver plated contacts on solar cells from tarnishing by
overplating with a thin layer of gold.

I believe that the gold content of the final solder joint shouldn't exceed
3% (by volume of metal) to avoid embrittlement, but that means our gold
thickness will be 4 microns or less.

Is 4 microns thick enough to provide a non-porous gold layer and prevent
the
silver from tarnishing?

Thanks in advance.

Jeff Brown
Research Engineer
Centre for Sustainable Energy Systems

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