At 10:31 AM 5/22/96 CDT, "Tippit, Jack" <[log in to unmask]> wrote:
>
>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 ).

Hi Jack,

I've had a lot of headaches with sockets over the years, most of them
involving tin-lead interfaces.  Problems can be difficult to track down
because they may be intermittent or the users may "fix" the problems by
re-seating or pressing down on the socketed component.  The socket styles we
have had issues with were PLCC: tin-lead on tin-lead, DIP: rectangular
tin-lead terminal in a gold socket, PGA: round 0.018 inch tin-lead terminal
in gold sockets.

We definitively traced our PGA/DIP problems to fretting corrosion and most
likely our PLCC problems also were fretting.  Fretting corrosion of
tin/tin-lead contact interfaces occur best under low level vibration
conditions, such as vibration generated by cooling fans.  Under vibration,
micro-movement of the contact interface occurs.  Every time the contact
moves, fresh tin is exposed.  This exposed tin oxidizes so when the contact
wipes back again some of the oxides get trapped at the contact interface.
When enough of the oxide builds up, the contact resistance increases.  If
the fretting is severe enough, the oxide breaks all electrical contact and
an open connection occurs.  Reseating the socketed component temporarily
fixes the problem.

I have some papers on fretting corrosion and can dig them out and cite them
if desired.

Contact lubricants have shown promise for preventing fretting corrosion of
tin-lead/tin-lead interfaces but not tin-lead on gold.  I'd stay away from
tin-lead on gold for hi-rel applications which may see some low level
vibration.  We are trying to eliminate sockets here.

I've also seen problems with fretting corrosion of miniature incandescent
lamp contacts used in push-button switches.

Write if you'd like more info.

Regards,
Gregg Klawson
GTE Government Systems Corp
Taunton, MA ,USA
telephone: +1.508.880.1822, email: [log in to unmask]

my views are my own