Silver News
Silver Brazing Alloys Essential for Today's
Products
By Samuel Etris, Senior
Technical Consultant to The Silver Institute
Over six million troy ounces of silver are
used each year for brazing and soldering alloys essential
to the production of automobiles, refrigerators, aircraft,
air conditioners, electronic microcircuits, cutting and grinding
tools, electrical appliances and thousands of highly engineered
products. These all depend upon several of silver's properties
to provide high-performance joints that hold their components
together.
The concern for the health effects of lead
in households led to the development of a lead-free, silver
bearing solder. Research conducted by the Engelhard Corp.,
of Iselin, NJ, resulted in a tin-copper-silver solder that
not only provides a stronger bond for household drinking water
pipes than lead containing solders, but the addition of silver
provided an important sanitation effect. Silver also is present
in millions of homes preventing the buildup of bacteria in
water filters. Furthermore, silver-containing solder found
immediate acceptance in the plumbing trade because it required
no change in plumbing practices.
While the use of silver solders in households
began a new role for silver, the use of silver brazes (a higher
temperature application) has long been standard practice in
the assembly of top-of-the-line household refrigerators and
air conditioners. Silver-tin brazes provide a permanent, ductile
bond for the copper evaporator/condenser coils that are subject
to almost continuous thermal and physical stress as these
products heat up and cool down.
High-speed mass production of electronic
products has changed the face of the industry as well. The
individual tediously soldering multitudinous connections of
one component after another has been replaced by a wave of
molten solder washing over a printed circuit board onto which
electronic components are dropped by automated arms. Billions
of soldered connections such as this are made every day for
the production of telephones, personal computers, television
sets and other devices. Small components with 100 to 200 connector
pins are dropped into the fluid solder as fast as a production
line can move. Known as surface mounting, this high-speed
manufacturing technique has made possible a size reduction
in parts of microcircuit chips which allows contacts to be
placed closer together. This markedly reduces the size of
computers and electronic devices while silver solders ensure
perfect electrical connections.
A new high-speed technique, called ball grid
arrays, requires a silver-containing solder paste to be stenciled
on a printed circuit board forming a pattern of solder balls.
The electrical connections are set underneath the electronic
component to touch specific solder balls. A flash of heat
melts the solder ball making an instantaneous, permanent,
high-performance electrical connection. Richard Bell, Applications
Engineer, Indium Corp., in Clinton. NY, notes that the addition
of silver increases the adhesion and strength of these solders,
adding to the reliability of the connections during high-speed
electronics assembly.
Silver also has helped solve one of the most
difficult brazing problems: bonding metal to ceramic materials.
Vacuum tubes for microwave ovens and power-line
switches, and surge arresters to protect telephone and computer
power lines use a ceramic tube brazed to a metal base. Traditionally,
grinding and nickel plating of the ceramic had to be done
first. Research at WESGO, Inc., in Belmont, CA found that
an alloy of silver with titanium and copper would allow a
one-step brazing of a metal onto the ceramic surface. This
has resulted in considerable savings for the manufacturers
of high-power vacuum tubes.
With the advent of difficult-to-cut, high-strength,
high-performance composite aircraft, structural components
such as Kevlar, carbon-graphite and high-molecular polyethy]ene
fiber sheets, new cutting tools became necessary. For this
demanding task, diamond abrasive wheels are required to provide
the sharp efficient grinding and cutting required by production-line
industries. For such uses as this, Norton Abrasives Co., in
Northboro, MA developed a silver-copper-titanium alloy braze
that permanently bonds industrial diamonds to grinding wheels.
Silver, the most efficient heat conductor, draws away the
heat of grinding to reduce overheating of the grinding wheel.
The silver-alloy bonded diamond wheels sharpen
cutting and grinding tools that mill and machine the parts
for automobile engines, transmissions, and other moving- parts
which require close tolerances.
They also find important use in shaping the
high-strength composites used for aircraft landing gear brakes.
According to Howard Mizuhara, Director of
Research at WESGO, silver meets almost every brazing need.
Its alloys whet most commercial metals and ceramics at temperatures
low enough to avoid excessive thermal expansion mismatch between
the componenls. Also, during cool down, silver changes shape
to overcome any differences in the contraction between the
components. Thus, when the assembly reaches room temperature
very little residual stress remains. In addition, silver provides
the highest electrical and thermal conductivity with complete
resistance to corrosion, making silver vital for bonding together
today's sophisticated products.
Silver News - April/May 1995
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