All that Glitters is not Gold… Interest in using Silver as a Connector Finish Continues to Grow
by Vince Azzano, Precision Plating Company, Inc.
On June 6th Engelhard fabricated gold moved above $1,668 per troy ounce. The next day Engelhard fabricated palladium prices rose to $910 per troy ounce. The recent rise in gold and palladium prices have once again encouraged renewed interest in finding a lower cost alternative connector finish. Due to its "near noble" nature, high conductivity and relatively low cost (less than $50 per troy ounce), many of our customers are finding that, for many of their applications, silver may be that alternative finish.
Silver, an Introduction
Silver has been used for decades in both high and lower current separable connector power applications. Because silver has the highest electrical and thermal conductivity of any metal in the periodic table, very low contact resistance values can be obtained when silver is used as a power contact in connector systems applications with relatively high contact forces [1.2]. In these applications silver's relatively low hardness results in a large conducting contact area composed of adhesively bonded metal-to-metal junctions allowing the contact to efficiently conduct current while at the same time helping to dissipate heat [1.2].
On the other hand, silver does not have the same "noble" characteristics that gold and palladium have. While silver is stable in pure air and water, silver reacts readily with sulfur or hydrogen sulfide to produce a silver sulfide film that is familiar to all of as tarnish [1.2]. Silver tarnish films can appear anywhere from light tan to a brownish, blue or even a black appearance in severe cases. These tarnish films have been found to be semi-conductive, soft and easily displaced in contact systems with a wiping action and relatively high normal contact forces [1.2]. That is why silver does not pose any significant problems in power applications [1.2].
Silver also exhibits good solderability characteristics, even in cases where the surface is lightly tarnished. Silver is used widely as a solderable finish in printed circuit board applications.
When it comes to separable connector signal applications, however, the superior material characteristics that make silver work so well electrically can translate into reduced performance in signal applications [1.2]. Silver has a high coefficient of friction (COF). In densely packed connector applications with multiple contacts, this high COF can result in unacceptably high connector insertion forces [1.2].
Silver's high COF, together with the relative softness of the material, can cause the silver-to-silver contract interface to become mechanically bonded together [1.2]. These surface bonds in silver to silver connection systems can be so strong that multiple mating cycles can lead to galling and premature wear [1.2].
If you've been reading carefully, you now understand that a connector engineer that is considering silver as a replacement for hard gold or palladium in a separable connector application will find themselves between the preverbal rock and hard place.
Connector engineers find themselves faced with two competing or conflicting design considerations when considering silver. The first is that engineers using silver would like to be able to use a high normal force and a wiping connector design to maintain low contact resistance, overcome any of the potential negative effects of tarnish and take advantage of silver’s high electrical conductivity. However, higher normal forces and a wiping action in silver to silver connection designs can result in high insertion forces and galling wear. Consequently, connector designers are faced with trying to balance normal force, and low contact resistance against high insertion force and premature wear.
Thankfully there are some answers to this design quandary; high technology contact lubricants and next generation silver tarnish inhibitors.
Contact Lubricants and Tarnish Inhibitors
The application of contact lubricants and tarnish inhibiting surface treatments for silver can provide an effective means of mitigating the potential negative effects of tarnish formation and high insertion forces, and can lead to lower contact wear, lower contact resistance and improved contact durability [1.2]. There are a wide variety of lubricants and tarnish inhibitors that are available for silver and choosing the right material is application dependant.
The tarnish inhibitors that are available can also have the effect of reducing the COF, but they are not as effective at reducing the COF as contact lubricants. Conversely, the available contact lubricants can also provide some level of tarnish protection but none of them are as effective as are the tarnish inhibitors that are available. Other considerations are the choice of a surface treatment that will be appropriate for the lifetime of the connection system, operating temperature and environment of the specific connector application.
As you can see there are very likely some existing hard gold and palladium connection systems that could take advantage of the significant cost saving offered by silver. While silver is not without its challenges, by properly matching design characteristics with the application requirements, engineers may find that the advantages of silver can be successfully employed.
The appropriateness of the potential use of silver in your application should be determined through design and application analysis as well as through function testing.
Here at Precision Plating Company we are uniquely positioned to leverage our 107 years of experience in the metal finishing industry to support you and your organization in your efforts to reduce costs while maintaining the functional performance of your connection systems. Please visit us at ppc1904.com for more information.
Fun Historical Facts
Silver is a soft, white, lustrous metallic element with the chemical symbol Ag and the atomic number 47. The chemical symbol Ag comes from the Latin word for silver argentum .
Silver has been known to man since ancient times and is mentioned in the book of Genesis.
Slag heaps found in Asia Minor and on the islands of the Aegean Sea indicate silver was being separated from lead as early as 3000 BC using surface mining .
For thousands of years silver has been used for jewelry, ornaments and utensils, for trade, and as the basis for many monetary systems .
Silver coins are the oldest mass produced form of currency. Silver has been used as a coinage metal since the times of the Greeks. Around 1200 BC silver mines near Athens provided silver for the burgeoning Greek civilization 
The stability of the ancient Roman currency relied to a high degree on the supply of silver bullion. At the peak of their production Roman miners produced more than 200 tons per year, and an estimated 10,000 tons of silver currency circulated in the Roman economy by the middle of the second century AD .
In 1792, Alexander Hamilton, then the U.S. Secretary of the Treasury, proposed the adoption of a gold and silver based monetary system . Silver remained in circulating U.S. coins until the supply of silver could not meet the demand for coins and the face value of the coin fell below its bullion, or meltdown value. The U.S. government eliminated silver from quarters and dimes in 1965 .
In 1980, the silver price rose to a peak for modern times of $49.45 per troy ounce due to market manipulation of Nelson and Herbert Hunt . Some time after Silver Thursday, the price was back to $10 per troy ounce By December 2001, the price had dropped to $4.15 per troy ounce and in May 2006, it had risen back as high as $15.21 troy ounce . In March 2008, silver reached $21.34 per troy ounce and in late April 2011, silver reached an all-time high of $49.76 per troy ounce .
Surprisingly, in earlier times, silver has commanded much higher prices. In the early 1400's the price of silver is estimated to have surpassed an equivalent of $800 per troy ounce in today's dollars . At an April 2011 price of about $49.76 per troy ounce, silver is about 1/30th the price of gold . The silver to gold price ratio has varied from 1/15 to 1/100 in the past 100 years .
Supply and Demand
Total silver fabrication demand grew by 12.8 percent to a 10-year high of 878 million ounces in 2010 . This surge was led by the industrial demand.  In 2010 silver’s use in industrial applications grew by over 20% to 487 million ounces. Here in 2011 silver is again seeing pronounced advances in industrial demand .
Silver metal occurs naturally in its pure form and as an alloy with gold and other metals . Today most silver is produced as a byproduct of copper, gold, lead, and zinc mining and refining .
Interesting Physical Properties
Silver has a density of 10.5 g/cm3 and a melting point of 962 degrees C .
Silver has the highest electrical conductivity and the lowest contact resistance of any metal .
Among metals, silver has the highest thermal conductivity and an extremely high optical reflectivity .
Various silver compounds are used as disinfectants and antimicrobials. Recorded use of silver to prevent infection dates back to ancient Greece and Rome times . In the Middle Ages it was used disinfect water and food during storage . Nineteenth century sailors on long ocean voyages would put silver coins in barrels of water and wine to keep the liquid potable . Pioneers in America used the same idea as they made their journey from coast to coast and silver solutions were approved in the 1920s by the US Food and Drug Administration for use as antibacterial agents .
1] Overview of Silver in Connector Applications, Marjorie Myers, Tyco Electronics, 2/2009
2] The Performance of Silver as a Contact Finish in Traditionally Gold Finished Contact Applications, Marjorie Myers, Tyco Electronics, 9/2009
3] The Mateability of Tin to Gold, Palladium and Silver, Edward Bock, AMP Incorporated, 4/1990
4] The Silver Institute, http://www.silverinstitute.org/old_world_silver.php
6] Silver coin, http://en.wikipedia.org/wiki/Silver_coin