Brass is zinc and copper so if you suck out the zinc -dezincification - you
will end up with just the copper, and that is what you are seeing. As you
say the mechanism is diffusion of Zinc into the tin of the solder so is not
flux related (save that the solder has to wet the brass for it to happen,
which it won't do in the absence of flux, at least not very well).
Potentially there are structural problems in the joint area associated with
this and you will also be dumping zinc into your solder pot if you're using
one, not desirable. The zinc will make oxide skin and can also cause grainy
or lumpy joints if allowed to accumulate.
You should specify "immunised" brass, this means the brass is plated with
copper or nickel before tinning, so the solder does not see the brass. This
is a standard procedure/finish and should not present any supply issues.


Regards

Mike Fenner
Indium Corporation

T: + 44 1908 580 400
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-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Friday, August 18, 2006 3:45 PM
To: [log in to unmask]
Subject: [TN] Soldering to brass

A metallurgist I am not....

We have brass eyelets that we solder wires into.  I am unable to obtain
detailed information on the specific alloy, but the vendor told me it was
70% Copper, 30% Zinc.  We are soldering using 63/37 solder and RMA flux.
During soldering, the eyelet appears to change to copper in the non-soldered
areas.  A brief search of some Tech-Net archives has told me that the Zn
will diffuse out and create solderability problems.  But is the end result
that the material appears to be copper as in my case?

As a side note, I tried a water sol flux and observed the same condition.  I
then tried simply heating an eyelet with an iron - no flux or solder - and
there was no change, so it appears to be flux related.  I also cut a piece
up to verify it is solid brass, not plated.  We tried brushing the eyelet
after soldering and it appears that you can restore the brass finish to some
extend.

Anyway, I have confirmed with the supplier these should have been hot solder
dipped, and we have already ordered replacements.  But I am now faced with a
question I cannot answer - what is the reliability concern of this
phenomenon, if any?  These solder joints get conformal coated, and then the
entire PCB is RTV'd into a case, so it is well sealed.  Can I continue to
use the existing stock, or is this a serious reliability issue?

Thanks!




Kevin Glidden

Manufacturing Engineer



Astronics Luminescent Systems Inc.

4 Lucent Drive

Lebanon, NH 03766



P: 603-643-7766 x3152

F: 603-643-5947



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