Hi Neil - the world of alternative finishes is so much fun! Because of your
different finishes let me break my response into two sections:

#1"Material: CuZn37-F37 Finish:  2 microns Silver Plated.  Nominal 0.2
microns gold."
* One problem you will have with this finish stackup is that the silver
will quickly diffuse into the gold and then tarnish to a point were the
solderability is lost. The gold/silver stackup was used briefly on
connectors but then discontinued due to the diffusion problems. Check out
this paper for more info: "Connector Reliability in the Military
Environment", Connection Technology, May 1989, pages 19-22. The paper
contains some good references too.

#2 "Material: CuZn37-F37  Finish :  Nominal 0.2 microns gold"
* There are a couple of issues with this stackup. The copper will diffuse
into the gold over time and become oxidized resulting in the loss of
solderability. The second problem (which could also be occurring with
finish stackup #1) is the diffusion of zinc to the copper/gold interface. A
loss of solderability results from this mechanism too. Take a look at Klein
Wassink's book, "Soldering In Electronics", ISBN # 0-901150-14-2, section
6.3.1.3 for more info.

Many component suppliers put a barrier layer of copper to prevent the zinc
diffusion but these components are intended for JIT operations due to the
copper diffusion issues. Other suppliers use a nickel barrier which is much
more robust. As for why the supplier is giving you two non-typical surface
finishes on the same component- that is a mystery! A more standard finish
such as tin/lead or gold/nickel would be more solderable than the finishes
you have now. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]





Neil Atkinson <[log in to unmask]> on 10/01/98 02:00:51 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Neil Atkinson <[log in to unmask]>

To:   [log in to unmask]
cc:
Subject:  [TN] Solderability Problems




Technetters,

This is the second time I have asked this question but so far I haven't
had a reply.  Maybe I'll ask in a different way this time!

We are trying to solder a microswitch to a PCB and are having serious
problems with dry joints (i..e de-wetting of the legs).

Having spoken with the supplier of the switch he has informed me that
the switch legs(there are two of them) have different platings these
are:

Material: CuZn37-F37
Finish:  2 microns Silver Plated.  Nominal 0.2 microns gold.

and

Material: CuZn37-F37
Finish :  Nominal 0.2 microns gold

My questions are:
What does the silver do to solderability?
Why would a switch manufacturer use these finishes?
Should the second leg have a barrier layer (Nickel?) between the brass
and the gold?
My swich supplier is willing to listen but what would be the best finish
for us to solder whilst meeting the switch manufacturers requirements?

HELP!!!!

Neil Atkinson

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