[AMPS] Silver Solder - Some General Information

[Dave Bowker]

I posted this to the Tower Talk reflector earlier in response to a thread
there.  Since many amp. builders use the silver soldering process in
construction, especially at VHF/UHF, I thought it would be of interest here.

73, Dave, K1FK
~~~~~~~~~~~~

Silver solders are generally used in applications requiring more than
ordinary strength obtainable from tin-led solders, or to provide a specific
color match, electrical conductivity, or corrosion resistance of the joined
materials. Like tin-lead solders, silver solders come in numerous different
alloys, each with it's specific purpose, and specific melting temperatures,
which can vary anywhere from around 1200*F to over 1800*F, depending on the
composition of the alloy. Tensile strengths of silver soldered joints can
vary from a low of less than 5000 psi up to approximately 65,000 psi, again
dependent upon the alloy used and type of material being joined.

Some of the more common and economical silver solders are copper-zinc alloys
with small amounts of silver added, and may contain anywhere from 9 to 80%
silver, depending upon the intended application, desired melting point, and
have a color varying from silver to a brass-yellow. They are generally
applied using a brazing torch rather than soldering iron, although a few do
lend themselves to soldering iron application. Some silver solders have a
small amount of cadmium or indium added to lower the melting point.

Most silver solders are corrosion-resistant, malleable, ductile, and have
high strength, but care in preparing the materials to be soldered affect the
solderability and ultimate strength. Lead and iron make them difficult to
work with and any presence of tin makes the joint brittle and weak.

A commonly available general purpose silver solder used for soldering of
refrigeration equipment and electrical work can be found in the plumbing
department of larger hardware stores, building supply outlets, and some
larger hobby shops. It is an alloy of 50% silver, 34% copper, 16% zinc, has
a melting point of ~1280*F, and meets the specifications of ASTM No. 5
Silver Solder. This is best applied to larger tubing and lap joints using a
small brazing torch, while solid copper wire up to 12 gauge may be
effectively soldered with large-wattage soldering guns or large soldering
irons having large-mass tips.

There are some lead-silver solders available to replace regular tin-lead
solders. They contain a very small percentage of indium and have melting
points as low as 310*F. These generally flow better in prepared joints than
regular tin-lead solders and produce joint tensile strengths approaching
4900 psi. Most low-melting point silver solders fail in tensile strength at
elevated temperatures (<900*F) and should be used with caution in such
applications.

A common flux used with silver solders is hydrous sodium borate (commonly
known as Borax and obtainable in grocery and drug stores) which acts as a
deoxidizer during the soldering process, and is non-corrosive to the
adjoining metals (as well as the silver-soldered joint). This is most
commonly used in the powder form by welders when brazing ferrous metals
using brass/bronze uncoated rods and oxyacetylene torch brazing methods or
available as coated rods for stick welding (brazing) using electric welding
methods.

Silver solders which do not require application of an external flux
generally contain a small amount of phosphorus in the alloy which acts as a
deoxidizer during application, and are generally used in brazing brass,
bronze, and nickel alloys, but these are not easily obtainable in the retail
market.

73, Dave, K1FK
Fort Kent, ME



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