David D Hillman wrote:
>
> 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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I am in full agreement with what Dave Hillman has said about this
solderability problem and I simply would like to reinforce it using my
layman's terminology.
To my understanding, the reason for the different layers of plating, as
it was explained to me by some "experts" in the field many years ago is
the following. And if somebody knows a better explanation, I would not
mind to be corrected.
The problem with soldering any metal is oxydation of the metal. If the
metal has oxydized or tarnished, then it is difficult to have the solder
adhere to it. The metal should be cleaned before soldering or should be
kept clean and be protected and prevented from oxydizing until it is
ready to be soldered. Another problem with solderability, is the long
term effect of having intermetallic compounds which could make the
solder joint brittle and would fail prematurely.
One way of preventing the base metal of the leads from oxydizing is to
plate it with gold. Gold does not oxydize readily. However, copper
(and silver) diffuse into gold, and ultimately the difused copper could
oxydize and we're back to square one. So, a barrier is needed to
prevent this difusion. Nickel was found to be a good barrier material.
So, the nickel is used as a good "barrier" or some people call it an
"adhesive" layer between the solder and the base metal of the leads/legs
of the package, almost irrelevant of what that base metal is. However,
nickel oxidizes very quickly and once it is oxydyzed, it too becomes
very difficult to solder. So, to prevent the nickel from oxydizing, it
is plated with gold, almost immediately after the nickel is plated on
the leads.
So back to what Dave has said, a good idea is to use the nickel layer
under the gold.
Gabe Cherian
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