I came across this manufacturer's white sheet today, and thought the
reflector could use some new material. I thought it might be of interest to
those considering silver plating or those considering different contact
metals. It does not appear to be copyrighted. I've edited the company name,
manufactured products, and impertinent materials.
I hope this is useful to someone.
Jim Smith, KQ6UV
The word "corrosion" is defined as the destructive alteration of a metal
by reaction to its environment. An example would be the effect of removing
rust from an iron bar that has developed over an extended period of time
from the exposure to oxygen and water vapor. When the rust is removed, the
surface of the iron is pitted and deformed to the point that the structural
integrity is compromised. The term corrosion is sometimes used
interchangeably with the term tarnish. However, it should be remembered that
tarnish is not destructive to the silver plating and the use of the term
tarnish means to discolor or to loose luster.
It is true that tarnish can form on the silver plating of contacts. But
since it is not destructive, the action is inhibited once the tarnish
completely covers the exposed silver, thereby acting as a protective coating
over the base silver. In addition, tarnish is not mechanically bonded
through the tarnish process and can be removed with a minimal amount of
mechanical pressure.
Fortunately, there is more oxygen in the environment than there is
sulfur. Tin reacts with oxygen while silver, under ordinary circumstances,
does not oxidize. It is true that silver reacts with sulfur compounds, and
due to environmental pollution, hydrogen sulfide (H2S02) readily exists,
thereby causing silver to tarnish. But tin also reacts with sulfur dioxide
under moist conditions.
Electrically, silver has several unique properties over other materials
used commercially for plating.
The resistivity factor of silver is superior to most metals used for
plating. It can be easily seen from the following data, the electrical
superiority of silver over other common metals.
Silver 1.63 X 10-6 ohms/centimeter
Copper (pure) 1.70 X 10-6 ohms/centimeter
Gold 2.44 X 10-6 ohms/centimeter
Tin 1.15 X 10-5 ohms/centimeter
Lead 2.20 X 10-6 ohms/centimeter
The low resistivity of silver has another advantage in relation to the
skin effect at higher frequencies. High frequency signals transmitted
through a conductor do not travel through the center of the conductor, but
instead travel along the outside surface "skin" of the conductor.
Another example of the superiority of silver over tin, is the work
function potential difference that exists when either of the two metals are
connected with a copper conductor. The work function difference between
silver and copper is 0.21 volts. The work function difference between tin
and copper is 0.24 volts.
If the tin plating on a contact happens to be tin/lead (which is very
common), this value increases to 0.29 volts. To prevent destructive
corrosion, the recommended work function potential between any two
dissimilar metals should always be equal or less 0.40 volts. Both silver and
tin when joined with copper are under this value, but this is another
indication of the superiority of silver over either tin or tin/lead plating.
Because silver has a lower resistivity than tin, silver can carry a higher
level of current. For example, a wire made from 22 AWG silver has a fusing
point of 21 amps, whereas a 22 AWG wire made of tin has a fusing point of
only 6.61 amps.
From the mechanical aspect, silver is the most optimum choice to use for
contact plating.
The following table will show the mechanical difference between the
three most common metals used in contact plating.
METAL RELATIVE HARDNESS
TENSILE STRENGTH kg/mm2
Gold 2.5
11.5
Silver 2.7
15.1
Tin 1.8
1.4
The relative hardness of a metal is its resistance to scratching by 1 of
15 materials (from talc to diamond) according to the Mohs scale
The tensile strength of a material is a measure of how much stress can
be applied before shearing occurs, Silver has the obvious advantage over
tin. Tin has a low tensile strength which would indicate that it's
durability would be much lower the design that utilizes silver plating.
It has been our experience, that there has never been a case where
silver tarnish has caused any kind of detriment in the product performance,
regardless of the system application.
SUMMARY OF MOST SIGNIFICANT ADVANTAGES OF SILVER (X = SUPERIOR
PERFORMANCE)
METAL GOLD
SILVER TIN
RESISTIVITY
X
CURRENT CAPACITY
X
RELATIVE HARDNESS
X
TENSILE STRENGTH
X
Bibliography:
The Encyclopedia of Chemistry, Van Nostrand Reunhold, 1973
Buschaum's Complete Handbook of Practical Electronic Reference Data, Walter
Buscham So. D 1980
Reference Data for Radio Engineers, ITT, 1975
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