Technical Paper
`
`Overview of the Use of Silver in
`Connector Applications
`
`Marjorie Myers
`
`Interconnection & Process Technology
`Tyco Electronics
`Harrisburg, PA
`
`February 5, 2009
`503-1016
`Rev. O
`
`Feinmetall Exhibit 2026
`FormFactor, Inc. v. Feinmetall, GmbH
`IPR2019-00082
`
`Page 1 of 14
`
`

`

`Introduction:
`
`The separable signal connector industry uses predominantly electrodeposited gold (hard gold) and tin based
`finish systems for separable interface signal contact interfaces. These electrodeposition processes are
`widely available, well understood, fast, stable, and easily controlled.
`
`Electrodeposited pure silver contact finishes are favored for higher current power transmission and often
`lower current separable power connector applications. Silver has the highest electrical and thermal
`conductivity of any metal leading to Contact Resistance (CR) values in the range of 0.1 - 1 mΩ at higher
`forces. The higher current power transmission connectors typically are designed to have very high normal
`force (10 – 100 N) that preferably incorporate wipe, have silver thicknesses greater than 2 and up to 20 μm
`and commonly do not have a nickel underplate. Lower current power connectors typically have a minimum
`of 200 cN of normal force with wipe, a nickel underplate (minimum 1.25 μm) with a minimum of 2 μm
`silver, and have low durability requirements.
`
`Silver also functions well as a connector finish in many appropriate higher normal force/lower durability
`signal applications. Most other signal connectors operate at a significantly lower normal force with higher
`durability requirements. The fact that silver will tarnish in most connector environments and is not a
`durable finish may present a problem if used for these signal connector applications. Another factor
`complicating the application silver in many signal applications is that there is no accelerated testing
`correlation for silver finishes. The combination of potential corrosion mechanisms and application
`environments is complex and not fully understood [1, 2, 3, 4].
`
`Two recent industry wide changes have driven connector interests to explore expansion of the use of silver
`as a signal separable contact interface finish. For gold finish applications, the significant increases in the
`cost of gold metal (Figure 1) provide the motivation to seek alternate finishes.
`
`
`35.04 $/g
`
`32.51 $/g
`
`Palladium
`
`10.42 $/g
`
`Gold
`
`Silver
`
`0.34 $/g
`
`2008
`2007
`2006
`2005
`2004
`2003
`2002
`2001
`2000
`1999
`1998
`
`Year
`
`503-1016, rev. O, 5feb09
`
`1
`
`40
`
`30
`
`20
`
`10
`
`0
`
`Metal cost in $/gram
`
`
`
`
`Page 2 of 14
`
`

`

`Figure 1: 10 year illustration of connector finish metal cost fluctuations (www.kitco.com).
`
`For tin finish applications, it is the conversion to ‘lead free’ tin deposits. An effective way to minimize the
`risk of tin whisker related failures for most tin finished contact designs is to follow recommended
`design/application criteria and only use currently accepted whisker mitigating tin plating chemistries. In
`connector designs where these recommendations cannot be met, a cost effective alternative to tin may be
`required. Figure 2 shows examples of tin whiskers.
`
`
`
`Figure 2: Examples of tin whisker formation.
`
`
`
`
`
` A
`
` large part of what drives making contact finish choices is cost. What limits contact finish choice is
`performance requirements and manufacturing concerns. The finish appropriate for any connector
`application is determined not only by performance requirements, but also lifetime exposures under which it
`will have to function. Silver has a long history of performing well in the tarnished state in power
`applications and certain appropriate signal applications.
`
`Characteristics of silver as a contact finish:
`Positives:
`Silver has a unique combination of material properties such as the highest thermal and electrical
`conductivity of any metal and a relatively low hardness. Theory and experience show how these aspects
`lead to very low CR values for mated clean silver surfaces [5, 6, 7, 8]. Current passing through a clean
`silver-to-silver contact interface sees a relatively large conducting area (less constriction) made up of
`adhesively bonded metal-to-metal asperity junctions. This unique combination of properties results in
`relatively low CR, superior thermal-rise performance, and excellent vibrational stability. The attributes
`make it attractive for use in power applications. Figure 3 shows CR readings taken from several common
`connector finish surfaces compared to silver.
`
`Silver also has good solderability characteristics, even if the silver is somewhat tarnished. If the level of
`tarnish is excessive, a more active flux may be required. Immersion silver is widely used as a solderable
`finish on board applications but can have limited shelf life if the silver is exposed to the environment.
`
`With any changes away from traditional finish choices comes the risk of unintentional finish combinations.
`Generally, it is recommended to mate ‘like on like’, that is to say, mating a silver plug to a silver receptacle.
`Fortunately, silver can be an appropriate choice for mating to either gold based or tin finishes. Performance
`levels and costs of such combinations will fall somewhere between the two different finishes. For example,
`if you are mating silver to tin, you will not appreciably improve durability and fretting failure could still be
`a risk; all at the higher cost of silver [9]. General rankings of the galvanic corrosion susceptibility of
`different contacting material combinations show that silver-to-gold and silver-to-tin are satisfactory
`combinations in many environments; with silver-to-tin presenting the most risk in harsher environments [10,
`11, 12]. Therefore, a combination may have to be tested to determine its viability in more severe
`environment applications.
`
`
`503-1016, rev. O, 5feb09
`
`2
`
`Page 3 of 14
`
`

`

`Contact Resistance (mΩ)
`
`20.0
`
`10.0
`
`5.0
`
`3.0
`
`1.5
`
`1.0
`
`0.5
`
`0.3
`
`
`
`Loading to 200 cN
`various platings in the as-plated state
`
`Wipe
`
`mate Sn
`hard Au
`matte Ni
`immersion Ag
`electroplated Ag
`
`10
`
`15
`
`25
`
`50
`Load (cN)
`
`75
`
`100
`
`150
`
`0
`
`200
`
`100
`
`200
`300
`400
`500
`Wipe (microns)
`
`600
`
`20
`
`10
`
`35
`
`1.5
`
`1
`
`0.5
`
`0.3
`
`Contact Resistance (mΩ)
`
`
`Figure 3: Contact Resistance (median with standard deviation) data for four standard contact
`finishes loaded to 200 cN, with wipe.
`
`Potential Issues:
`Unfortunately, silver has some less favorable contact performance attributes. Silver does not have the
`‘noble’ character of gold and will form surface tarnish films when exposed to some reducible sulfur bearing
`atmospheres. Silver has a high Coefficient of Friction (COF) (high insertion forces) and poor wear
`characteristics (poor durability). Silver may be susceptible to electro-migration failures under specific
`conditions.
`
`Silver tarnish films:
`Silver tarnish films can be many colors, anywhere from yellow to tan to blue to black. Customers may
`object to this corroded appearance on contact interfaces because similar dark features on a tin surface (e.g.
`fret spots) or gold plated surface (e.g. corrosion product creeping from pore sites) generally lead to CR
`instability at the contact interface. A silver plated contact surface can appear discolored and still function
`very well if used correctly in the application.
`
` A
`
` related potential visual issue with tarnished silver films has to do with the process control systems
`designed to visually image/locate the position of contacts during assembly. If this system requires (or is
`programmed for) ‘shiny’ or ‘white’ silver contacts and the silver contact surface has tarnished, the system
`may reject the part erroneously. Because the visual appearance of the tarnish films that can form on
`exposed silver surfaces can be so variable, it may be difficult to compensate for automatically.
`
`In most stable connector application environments, the growth of silver tarnish has been reported to be
`linear and not self-limiting [1, 13, 14, 15, 16, 3, 17, 2, 4]. Tarnish films generated on silver finished
`surfaces exposed to most connector field applications are predominantly covalently bonded semi-
`conducting α silver sulfide (Ag2S) and to a lesser extent, small amounts of insulating and harder to displace
`silver chloride (AgCl). Film morphologies are typically non-uniform and possibly greater around surface
`features where water can collect. These predominantly silver sulfide tarnish films are semi-conductive at
`ambient temperatures, inherently soft, and relatively easily displaced with contact interface wipe at
`sufficient normal loads (Figure 5). If substrate material corrosion products (e.g. copper) are incorporated
`into the silver sulfide film, CR issues will likely occur [18, 19]. This is one reason why a nickel underplate
`is recommended when possible, and thicker silver plating is used when a nickel underplate is not used.
`
`
`503-1016, rev. O, 5feb09
`
`3
`
`Page 4 of 14
`
`

`

`
`
`
`
` A
`
` history of using silver finished contacts has shown that low and stable CR is maintained in many
`applications using tarnished silver contact surfaces – even with films on the order of a thousand angstroms
`in thickness [17]. If the quality and level of silver tarnish is not excessive and wipe and sufficient normal
`loads are incorporated into the connector design, silver tarnish typically will not cause contact performance
`problems (Figure 5). The films can even lead to durability and insertion force improvements.
`
`Silver sulfide:
`Silver sulfide (Ag2S) forms when silver atoms react with reduced sulfur (HS-) dissolved in the water film
`found on silver surfaces in typical connector environments. The primary source of this reduced sulfur is
`dissolved gaseous hydrogen sulfide (H2S), or to a lesser extent (less prevalent in the atmosphere)
`hydrolyzed carbonyl sulfide (COS) [20, 3, 4, 21]. Hydrogen sulfide comes from processes such as organic
`decay, combustion processes, volcanic activity, and manufacturing sources such as paper mills, sewage
`plants, and high sulfur packaging materials. Silver tarnish can become excessive if used or stored
`unprotected in environments with localized source of hydrogen sulfide [14, 15, 2].
`
`Silver chloride:
`To a lesser extent, silver chloride (AgCl) has been detected in some field exposed silver tarnish films.
`Silver is sensitive to the presence of chloride (Cl-) and will react to form silver chloride [20, 14, 2, 4].
`Chloride can come from species such as dissolved hydrochloric acid (HCl) gas or other chloride containing
`particulates (e.g. NaCl). Hence, silver chloride has been found in some field exposed silver tarnish films.
`The higher the level of harder insulative silver chloride in the tarnish film (relative to the level of softer
`semi-conductive silver sulfide), the more insulating and harder to displace the film becomes upon wipe
`leading to CR issues at thinner tarnish film levels [13, 20, 16, 14, 2, 3]. The reality is that in most field
`
`503-1016, rev. O, 5feb09
`
`4
`
`As-stored silver plated part
`
`
`
`As-stored silver plated part
`
`Class IIa Mixed Flowing Gas exposed silver
`24 hours
`
`Figure 4: Examples of tarnished silver surfaces.
`
`
`
`Class IIIa Mixed Flowing Gas exposed silver
`24 hours
`
`Page 5 of 14
`
`

`

`applications environments, a predominantly silver sulfide film forms with sometimes minor amounts of
`silver chloride.
`
`
`Ag As-received vs. tarnished
`Normal Loading cap/flat data
`Mixed Flowing Gas Exposure
`24 hours CIIIa
`
`100
`
`50
`
`10
`
`235
`
`1.5
`1
`
`0.5
`
`Contact Resistance (mΩ)
`
`Wipe
`
`Ag: As-received
`Ag: plus 24 hours CIIIa MFG test 1
`Ag: plus 24 hours CIIIa MFG test 2
`
`100
`
`50
`
`Contact Resistance (mΩ)
`
`2351
`
`11
`
`.5
`
`0
`
`250
`
`0
`200
`400
`600
`Wipe Distance (microns)
`
`0.5
`
`
`
`0
`
`50
`
`100
`
`150
`
`200
`
`
`
`
`
`Load (g)
`
`
`
`
`
`
`
`
`
`
`
`As-received silver over nickel plated coupon
`
`24 hours CIIIa Mixed Flowing Gas exposed silver
`over nickel plated coupon
`
`
`Figure 5: Contact resistance (median with standard deviation) data comparing as-plated and
`tarnished silver surfaces, with wipe.
`
`Silver sulfate:
`There is the possibility of forming silver sulfate (Ag2SO4) in the presence of sulfur dioxide (SO2), but only
`appreciably in the presence of artificially high levels of sulfur dioxide that are two and three orders of
`magnitude higher than found in typical ambient environments [13, 14, 4]. Silver sulfate have not been
`found in tarnish films that have been exposed to typical connector environments.
`
`Potential tarnish accelerating factors:
`Chlorine gas:
`Even though silver does not react directly with chlorine gas (Cl2), its presence has a synergistic effect on
`silver tarnish formation when combined with hydrogen sulfide gas. This is especially evident on silver
`surfaces exposed to accelerating environments containing both hydrogen sulfide and chlorine gas. If the
`ratio of hydrogen sulfide to chlorine is great enough, the film growth rate begins to deviate from linear and
`approach parabolic. These artificial environment exposed samples develop tarnish films faster, and with a
`greater level of silver chloride than is found on most field tarnished samples [13, 20, 14, 16, 2, 3]. The
`reality is that chlorine gas, commonly used in mixed flowing gas testing, is virtually non-existent in the
`atmosphere. This synergy may account for silver chloride level and CR discrepancies between field and
`
`503-1016, rev. O, 5feb09
`
`5
`
`Page 6 of 14
`
`

`

`accelerated environment exposed silver surfaces. This illustrates the risk of applying silver in particular
`environments that may have a local source of chlorine gas.
`
`Water:
`The presence of moisture is needed for silver to tarnish in response to corrosive environments. This
`moisture allows for the dissolution of corrosive elements leading to dissolution of metallic silver. Surface
`water films can either form as monolayers generated by humidity, or in a condensed form [2, 4]. The
`tarnishing of silver has been reported as both positively dependant on [22, 4], and independent of [14, 15],
`increasing relative humidity levels depending on the exposure environment.
`
`Nitrogen Dioxide:
`Though silver does not react with nitrogen dioxide (NO2), it has also been shown that the rate of silver
`sulfide tarnish formation can be somewhat accelerated by the presence of nitrogen dioxide [20, 22, 2, 3, 4]
`– though the mechanisms are not well understood. It is not considered to be a dominant factor in silver
`corrosion.
`
`Ozone/photocorrosion:
`Another synergistic accelerant would be the presence of ozone (O3) [3, 23]. The ozone does not react
`directly with the silver but if it is prevalent in an environment, the resulting accelerated formation of the
`tarnish film could become a problem for contact interface electrical performance. In fact, silver oxides
`(AgO and Ag2O) are formed which are very insulating and difficult to displace if trying to make electrical
`contact [13, 4]. Ozone is not found at significant levels in most connector application environments.
`
`Another related potential accelerant of tarnish film formation is photons capable of driving photocorrosion.
`This phenomenon is not well understood in connector environments, but data suggests it would be a
`possible cause of failure in some cases [4]. It would not be a factor in an enclosed connector system that
`would block light from the contact surfaces. It could possibly be a factor in light exposed applications.
`Initial data from another study has shown that both ozone and Ultra Violet (UV) exposure are required for
`the silver oxides to form [24].
`
`Silver tarnish creep across gold surfaces:
`Silver tarnish films have a tendency to creep/migrate across any adjacent gold surfaces [25, 26, 27, 15, 28,
`21]. This can lead to high CR values because it is much harder to displace the tarnish films formed across
`the harder gold sub-surface. This is also why silver is generally not used as an over plate for gold finishes.
`This risk can be avoided in most designs.
`
`Galling - adhesive bonding and wear:
`Unfortunately, part of what makes silver contact finishes work so well electrically (relatively soft => large
`contact area) also contributes to its inherently poor mechanical/durability performance. Clean silver has a
`relatively high Coefficient of Friction (COF) and is not a durable finish (Figure 6).
`
`Upon mating of two clean silver surfaces, the surface material supporting the load is plastically deformed.
`The material supporting the load between the surfaces is plastically deformed and work hardened
`(deformation zones) to form a contact interface of multiple metallically bonded (e.g., cold welded) metal-
`to-metal junctions with a relatively large total contact area. If clean, the adhesive bonds can be as strong as
`an inter-crystalline grain boundary. Because the material around the original asperity junction interface has
`been work hardened, any relative motion (i.e., sliding) at that interface will cause sub surface material
`shearing as the asperity junctions are broken. Where ever the weakest juncture in the composite structure is
`located is where the damage occurs. The severity of wear increases with increased normal forces.
`
`Poor wear durability is evidenced by clean silver’s inherently high COF (Figure 7). Clean silver surfaces
`that have been pressed together require some force to pull apart resulting in material being ripped out of
`either surface. Because silver contacts are typically used at relatively high normal loads for electrical
`reliability reasons (relative to hard gold), the durability of silver finishes is further limited. These factors
`contribute to the high insertion forces found with clean silver contacts. Silver is usually inappropriate for
`high durability applications.
`
`503-1016, rev. O, 5feb09
`
`6
`
`Page 7 of 14
`
`

`

`
`
`
`
`
`
`
`
`Severe galling wear of silver – substrate exposure
`
`Galling wear of a silver surface mated to silver
`
`Figure 6: Images illustrating the poor wear characteristics of silver plated surfaces.
`
`The combination of higher normal forces and silvers’ high COF promotes vibrational stability of a contact
`interface by preventing mechanically or thermally driven relative micro-motion (e.g. fretting) from being
`transferred to the contact interface. Silver itself is not susceptible to fretting oxidation in typical connector
`environments. If conditions are severe enough for fretting motion to occur, silver is susceptible to severe
`adhesive fretting motion wear (galling). Fretting motion could quickly lead to exposure of nickel and/or
`copper substrate materials which are susceptible to fretting oxidation failures and unacceptable CR
`increases.
`
`
`COF values - 10 forward and back cycles @ 200g
`
`Ag-received Ag
`24 hours CIIIa MFG exposed Ag
`lubricated Ag
`
`1.6
`
`1.4
`
`1.2
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`Coefficient of Friction (COF)
`
`1.0
`
`1.5
`
`2.0
`
`2.5
`
`3.0
`
`3.5
`
`4.0
`
`4.5
`
`5.0
`
`6.5
`
`7.0
`
`7.5
`
`8.0
`
`8.5
`
`9.0
`
`9.5
`
`10.0
`
`10.5
`
`6.0
`5.5
`Stroke
`
`
`Figure 7: Coefficient of Friction (median with standard deviation) data for 10 forward and back wear
`passes comparing as-plated, lubricated, and tarnished silver surfaces.
`
`The presence of even minor amounts of silver tarnish at the surface can weaken the adhesive metal-to-
`metal contact interface bonds by providing a shearable layer at the asperity junctions. The existence of
`even minimal storage generated silver tarnish on mating silver surfaces has a tendency to initially reduce
`
`503-1016, rev. O, 5feb09
`
`7
`
`Page 8 of 14
`
`

`

`COF (Figure 7 strokes 1.0 and 1.5). It can reduce the level of galling/wear and insertion force and may
`improve durability performance with little or no impact on CR performance.
`
`Electromigration:
`Electro-migration shorting failures can occur when metallic bridges form between closely spaced metallic
`current paths under certain conditions. Many metals have been shown to be susceptible to electro-
`migration under particular conditions, (e.g. copper, tin, lead, etc.), but silver is considered to be the most
`active. Several conditions need to exist simultaneously for electro-migration failures to happen: an
`electrolyte path for the metal ions to flow (e.g., water plus ionic contamination), a voltage sufficient to
`drive the process, and enough time at potential/path conditions to complete the formation of the metallic
`bridge (dendrite or ‘stain’).
`
`In the past, silver electro-migration failures had primarily been a potential issue for electrolytic silver plated
`small centerline Direct Current (DC) board applications where there was a chance to form a relatively short
`water based ionic electrolytic path in the presence of a sufficient driving voltage [31, 32]. In the printed
`circuit board industry today, where the problem is most likely to occur, the use of thin immersion silver and
`improvements in design, processing, testing, and applications requirements have led to the wide spread use
`of silver in board applications without such failures [33, 28].
`
`Electromigration failures generally do not occur in separable connector interface applications because the
`combination of geometries and application conditions are rarely susceptible. For the very limited
`design/applications that might have all the qualities required to potentially lead to electromigration, there
`are several ways to mitigate the risk - the first one being to avoid the use of electroplated silver. Others
`would be to maximize line spacing, minimize differential voltages (driving force), use hydrophobic
`materials, limit features that could trap water (e.g., scratches, cracks, board crevices, etc.), apply
`hydrophobic and/or conformal coatings, overplate the silver with a less active metal, limit or eliminate the
`chance of ionic contamination (e.g., avoid no-clean flux residue, exclude paper fibers, etc.), and otherwise
`minimize water film formation. In a situation where silver is mated to a gold plated board pad
`configuration and the combination of design and conditions could make electro-migration a possibility,
`testing may be warranted [32].
`
`Recommendations for use of silver in typical connectors:
`Normal force and wipe:
`Since silver has poor durability and the tarnish films are unpredictable, a design needs to be able to
`wipe/displace the films away from the contact interface upon mating. Therefore, it is recommended that
`silver finished contact interface designs have a relatively high normal force and incorporate wipe whenever
`possible. Typical silver plated contact designs use 200 cN of normal force.
`
`Durability considerations:
`To compensate for silvers’ poor durability characteristics, silver is recommended for use in low durability
`applications (e.g. < = 10 cycles). The actual appropriate number of durability cycles a particular connector
`could tolerate would be design and application dependant. The use of a wear reducing surface treatment
`(e.g. lubricant) could increase the number of durability cycles that a silver finish could tolerate and still
`function properly for a particular application (Figure 7).
`
`Centerline considerations:
`Most connector designs would not have a problem with this failure mode. Historically, electromigration
`failures between relatively thick electroplated silver finished board traces were an issue in certain smaller
`centerline board applications using hygroscopic board materials operating at relatively high voltages.
`Improved board manufacturing has led to the wide spread use of immersion silver in board applications
`without such failures. As the connector industry is migrating to smaller and smaller form factors,
`electromigration may need to be tested for in very specific situations.
`
`Nickel underplate:
`It is recommended to use a nickel underplate (minimum of 1.25 microns) whenever possible. The nature of
`the tarnish film will change significantly if copper alloy elements from the substrate reach the surface of the
`
`503-1016, rev. O, 5feb09
`
`8
`
`Page 9 of 14
`
`

`

`silver. This can occur through mechanisms such as diffusion or corrosion creep at breaks in the silver
`electrodeposit (similar to what happens with gold). This will most likely lead to an increase in CR if these
`more tenacious films get into the contact interface [18, 31]. At higher temperatures, oxygen will diffuse
`through silver to the copper alloy interface at a relatively fast rate and could lead to blistering if no nickel
`underplate is used. A nickel underplate also will prevent a relatively weak layer of silver-copper
`intermetallics from forming at temperatures greater than 150°C which could lead to adhesion problems [30].
`
`Silver Thickness:
`What silver thickness is appropriate is dependant on application factors such as environmental severity,
`time at temperature considerations, durability requirements, nickel underplate, and surface treatment.
`Silver plating thicknesses are typically in the range of 2 µm or greater in most separable contact interface
`applications that use a nickel underplate. If no nickel underplate is used, a greater thickness of silver may
`be required to prevent substrate corrosion products from getting to the surface [18]. These higher
`thicknesses also provide more silver material between the atmosphere and the substrate material(s),
`possibly leading to more wear cycles before any substrate material is exposed.
`
`Higher reduced sulfur and/or chlorine and/or ozone containing environments:
`Silver finished contacts are generally not used in applications where they would be openly exposed to
`outdoor or industrial environments due to concerns with excessive reduced sulfur exposure and tarnish
`levels. Exposure to environments with high levels of hydrogen sulfide (e.g. paper plant), chloride (e.g. salt
`spray), chlorine gas, and/or ozone should also be avoided for silver plated contacts. If silver is to be used in
`such environments, environmental sealing may be required to avoid excessive tarnishing.
`
`Managing Silver Corrosion:
`Surface treatments can be an effective way to attenuate tarnish formation, reduce insertion forces,
`improving durability, and minimize the risk of electro-migration where functionally appropriate. They
`come with their own set of possible issues. Liquid based lubricants and other surface treatments (e.g., Self
`Assembled Monolayers (SAM)), are commonly used. There are many commercial versions available.
`
`Coating silver surfaces with a thin solid top layer to prevent tarnish has been done. Any solid layers on the
`relatively soft silver will no longer provide protection in the contact region if they are wiped away upon
`mating. Therefore, this approach is probably only potentially effective in keeping silver surfaces that are
`not disturbed ‘tarnish’ free and will have limited effect in a contact area. Attempts have been made to use a
`gold flash to protect the surface of the silver from tarnishing. There are three reasons that this approach is
`risky. The facts that silver and gold will readily inter-diffuse, silver sulfide will creep/migrate across gold
`surfaces, and gold is very susceptible to arc erosion leads to this approach having marginal value [34].
`Such considerations have to be taken into account when considering these types of coatings.
`
`For a surface coating to be effective, it has to perform without losing functionality or causing an
`unacceptable increase in CR. They generally do not interfere with any subsequent soldering operations, but
`they can be rendered ineffective or harmful if removed during assembly and use, or exposed to
`temperatures above or below their proper operating range. If excess surface treatment material can migrate
`to other regions of the connector or system and is a problem, the use of such a surface treatment may not be
`acceptable. Whether or not a surface treatment can be used effectively for a silver finished connector is
`dependant on the performance requirements, application exposures, design limitations, visual requirements,
`and customer perception.
`
`There are non surface treatment methods to shield silver surface atoms from environmental corrosive
`elements (e.g. hydrogen sulfide). Anything that limits the ingress of sulfur to the contact area has an effect.
`These methods are also known as physical blocks. Two metallic surfaces pressed together helps limit the
`flow of corrosive elements into and around a formed contact interface, leading to less corrosion in the
`immediate area. In fact, formed functioning contact interfaces will remain electrically stable if not
`disturbed mechanically. The shielding effect of a connector housing (closed or actively sealed) can be
`quite dramatic, as well as the effect of any equipment enclosure or environmental controls (e.g., air
`conditioned, filtered) [17]. Additionally, sometimes greases or gels are used in conjunction with a
`
`503-1016, rev. O, 5feb09
`
`9
`
`Page 10 of 14
`
`

`

`connector housing (e.g. the housing is ‘filled’ with a grease style lubricant) to further mitigate flow of
`corrosive elements to the contact interface.
`
`Most packaging cardboard and interleaving paper contain and release high enough levels of sulfur to cause
`accelerated tarnishing on packed silver plated parts. Therefore, ‘low sulfur’ products are usually used to
`package silver plated parts. Products such as Silver Saver paper are commonly packed with silver plated
`parts and serve to absorb environmental sulfur limiting the amount that can reach and react with the plated
`silver.
`
`Common connector testing methods associated with silver finished product:
`
`Due to the poor durability and high insertion force characteristics of silver finished connectors, durability
`and insertion force testing are widely used. What specific level and type of testing is depends on all the
`characteristics of the connector as well as the intended application. Therefore, the parameters of such
`testing are product and application specific.
`
`Chloride exposure (e.g. salt spray) can lead to tenacious and resistive tarnish films containing excessive
`levels of silver chloride. Therefore, occasionally salt spray (fog) [35, 36] testing is done on silver finished
`connectors if there is a risk. Silver is not generally used as a contact finish in these types of environments
`without some form of environmental shielding/sealing. This type of testing is also used to determine the
`susceptibility of a given dissimilar metal connection to galvanic corrosion conditions.
`
`Electromigration testing [32] is rarely done for separable contact interface designs. There are some cases
`where it may be warranted depending on the application.
`
`Mixed Flowing Gas (MFG) testing is done by exposing contact surfaces to elevated levels of multiple
`corrosive gases under specific temperature and humidity conditions. Typically for specific time intervals as
`part of testing sequences prescribed by product qualification testing requirements. These MFG tests were
`developed after a long process of comparing field trial data taken from connectors with a variety of finishes
`to corresponding laboratory tested sample data in an effort to correlate lifetime exposure performance to
`accelerated testing results. In the end, no industry accepted accelerated MFG laboratory vs. field life
`correlations could be developed for silver due to the complexity of the silver corrosion process, the synergy
`between differing levels of multiple gases, and the fact that silver is so sensitive to typically erratic field
`fluctuations in hydrogen sulfide gas levels [1, 13, 20, 16, 2, 3, 4, 21]. Alternate single gas testing
`environments (e.g. hydrogen sulfide, flowers of sulfur) were also determined to be misleadingly benign and
`not representative of silver field exposures [1, 13, 2, 3].
`
`MFG testing was ultimately developed primarily for copper and gold surface finishes on nickel plated
`copper alloy substrates [37, 38]. There are several versions in use, but the most common are four gas Class
`IIa (Indoor) and Class IIIa (Outdoor). Generally, they are not used for silver finished connector
`qualification. Because these standard sulfur bearing gas environments are repeatable, available and will
`generate some level of silver sulfide and/or silver chloride containing tarnish films on silver surfaces,
`occasionally they are used for testing. Their use is customized to a particular applicat