Hi Phil! Cool - a true metallurgy question (sorry but I have had a week on
Mondays, not enough Diet Coke and very little exercising of the
metallurgical portion of my brain cells)! Soldering to a zinc casting is
possible but there are some issues to consider:

a) Oxides - Zn forms a pretty ugly oxide which interferes with the
soldering process. The surface oxides will need to be adequately removed
prior to the soldering operation. You can use either chemical or mechanical
means to accomplish the oxide removal depending on what impact either
method has on the pwa.

b) Corrosion - in some corrosive environments the solder alloy elements
(Sn, Pb, Bi, Cu, etc) can lead to intergranular attack of the zinc alloy. A
well documented example of this type of phenomena  is "dezincification" of
brass alloys with solder joints. You would want to test the use environment
conditions to insure that a corrosive attack would not take place.

c) Intermetallics - zinc and antimony for an intermetallic phase which is
very brittle and can degrade the solder joint. Also because zinc solder
alloys are typically low melting you could cause the formation of other low
melting solder phases (say you had Sn/Pb/Bi in your solder joint - the
formation of the ternary eutectic at 98C may be an issue).

There are some industry uses of zinc solder alloys - the Sn91/Zn9 solder
alloy has been used in some segments of the industry on specific
applications and the use of aluminum/zinc solder alloys is routinely used
on aluminum alloys in some cases. If you decide to use a zinc
casting/soldering I recommend you fully investigate your use environment to
see if it is a practical choice. An alternative avenue of pursuit is to
copper plate/alkaline tin plate the zinc casting similarly to how aluminum
casting are processed today.  Good Luck.

Dave Hillman
Rockwell Collins
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Phil Bavaro <[log in to unmask]>@ipc.org> on 08/30/2002 10:59:16 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
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Subject:    [TN] Zinc casting and solderability


Here is a question that I do not recall having come across ever before.

A mechanical designer has posed the question of substituting zinc in place
of a copper as a raw material.  This part will be required to be soldered
down to the circuit card it interfaces with as this is an RF assembly.

Since Zinc alloys are very good for casting definition, is it also possible
to use Zinc as the base metal.

Or does it require sealing with nickel and then tin plated in order to be
used for soldering applications?

I realize that some of the new leadfree alloys contain low percentages of
zinc but this question is oriented towards high contents of zinc in the
alloy.

I will keep researching this topic but my first thoughts were that the
experts on this panel should be able to point me in the right direction at
least.

Phillip A. Bavaro
QUALCO/\/\/\/\  Incorporated
CCA Technical Support
Engineer, Senior
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Pager (858) 271-3640
Tel  (858) 658-2542 Voice mail
Cell (858) 682-6434 (offsite, after hours)
Fax (858) 658-1584

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