Check out the following article by AMP.

http://www.amp.com/product/articles/dd68_1.html

For those of you without internet access, here is their "Tin Commandments":

From AMP:
   1.Contacts should be mechanically stable in the mated condition.
   2.Tin-plated contacts need at least 100 grams normal contact force.
   3.Tin-plated contacts need lubrication.
   4.Tin plating is not recommended for continuous service at temperatures
above 100 degrees C.
   5.The electrical performance of contacts is not strongly affected by the
choice of btight tin, matte tin, or tin-lead alloy platings.
   6.Electroplated-tin coatings should be at least 100 microinches thick.
   7.Mating tin-plated contacts to gold-plated contacts is not recommended.
   8.Sliding or wiping action during contact engagement is recommended with
tin-plated contacts.
   9.Tin-plated contacts must not be used to make or break current.
  10.Tin-plated contacts can be used under dry-circuit or low-level
conditions.

I wish I had more experience in the long term effects here.  But, maybe this
will help.  I have purchased edge connectors that have gold plating only in
the area of contact and tin coating elsewhere.  We use the connectors
internally in applications up to 125C.  However, we've only had experience
with them for about two years...not what I would consider long term.

Glenn

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