Jack Tippit asked:

>We have instances where tin/lead coated pins are inserted into gold plated
>sockets, etc.  Components even have gold/tin-lead or gold/tin interfaces,
>such as an IC socket with a gold contact with a tin sleeve.  Mil-Std-889B
>indicates gold as being incompatable with tin or tin/lead.  Has anyone
>looked into this issue and is there a potential for corrosion formation and
>associated problems?  Again, I just refereing to mechanical contact ( not
>soldering ).

Gold and tin-lead are far enough apart on the electronegativity series to
create a slight Galvanic couple. Gold is more noble, and tin is the anode.
In time, the effect would be anodic corrosion at the contact interface.

Bellcore and others have long ago studied the effect and specified that no
dissimilar metals should be used in separable connectors. Nevertheless, you
see it done all the time. That's one reason so many PC type connectors
become flaky, IMO. Many design engineers have minimal awareness of this
type materials issue, and insist that since it works at plug-in, there is
no big deal.

This is a long range reliability concern which should be addressed.
Wherever possible, our Company uses gold-gold for pin and socket
connections, as it is the best in the long term. There are other design
issues such as normal force, quality of the plating, voltage, etc. to look
at, but it is widely accepted that gold and tin/lead should not be mated in
contacts.

Somewhere in the AMP website (homepage http://www.amp.com ) is there is a
good article on this.


cheers,

Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com