28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
Performance evaluation of the new Gold Capped
Silver (GCS) plating system for connectors contact
applications
Antoine Fares Karam
Doyle Anderson, Jeffrey Toran
Global Plating Engineering
Amphenol FCI, FCI Besancon SA
Besançon, France
antoine.fares-karam@fci.com
Global Engineering Services
Amphenol FCI, FCI USA LLC
Etters, PA, USA
jeffrey.toran@fci.com
The Gold Capped Silver (GCS) reel-to-reel plating system is
developed to replace gold and gold flash over palladium-nickel
(GF-PdNi or GXT) plating in connector applications to take
advantage of silver’s technical properties to enhance
performance of power connectors and/or provide an economical
advantage for signal connectors. GCS plating is a silver plating
based system that uses a matte hard silver as the foundation
layer. A gold flash layer protects the silver plating layer against
tarnishing, sulfidation and oxidation which may lead to increased
electrical contact resistance. The top layer is either a special
PolyAlphaOlefin (PAO) or a PerFluoroPolyEther (PFPE) highperformance lubricant to ensure good tribological performance.
The GCS plating system has improved performance compared to
conventional silver plating but with much lower silver plating
thickness and it can be applied to power and signal connectors.
Connectors plated with GCS are better for applications requiring
higher number of mating and un-mating cycles than typical soft
silver plating, which has serious limitations. This paper presents
detailed investigations and characterizations of the GCS plating
system and describes its application to several different types of
connectors. Thorough performance and surface analysis data are
provided to demonstrate the stability, reliability and robustness
of this plating system.
Keywords— GCS; silver; gold; electroplating;connectors
I.
INTRODUCTION
A. Bacgkround
“Gold plating” and “Gold Flash over Palladium-Nickel
alloy” (GF-PdNi or GXT®) are historically the most widely
used finishes in the electronic connector industry for the past
30 years, while the automotive connector industry uses
conventional Tin plating. Silver plating usage in this field has
been moderate for several decades, but since the early 1990’s
silver has become more favored by the increased use of
automotive power connectors.
Silver enjoys a number of technological benefits as it is one of
the best electrical and thermal conductors, which makes it the
“metal of choice” for a variety of electrical end-uses,
including switches and connector contacts. Consequently
silver has been used for years in both high and lower current
separable connector power applications. Nevertheless, the
placement of silver on electronic components and connector
Fares Karam
81
products has always been a source of concern due to silver
tendency to migrate between isolated conductors (silver
migration), leading to unintended electrical short-circuits [1],
as well due to silver sensitivity to sulfidation (formation of
Ag2S products on the surface) in the presence of sulfur
contaminant compounds, in a liquid or gaseous phase, in its
near environment.
The migration of silver is driven by its propensity to undergo
an oxidation reaction in the presence of moisture and an
electric field [2,3]. Silver-sulfides cause the typical ‘black’
color tarnishing of silver plated surfaces. This surface reaction
takes away from the beauty of the silvery colored surface,
however it does not severely degrade performance till failure,
as the tarnish compounds are electrically conductive [4].
Evidently this is still critical for power connectors as Silver
sulfide electrical resistivity is nearly 17.3 Ω.cm which is much
more resistive than Silver (1.59 mΩ.cm). Despite these two
drawbacks, more and more applications are using silver
plating as a lower or moderate cost connector finishing coating
in automotive connectors, electrical components and a limited
number of electronic connectors where Silver is very suitable
for signal connectors with higher normal force and lower
durability requirements. Plating chemical suppliers have
developed many anti-tarnishing agents to protect Silver
surfaces against corrosion, sulfidation and oxidation to
mitigate the aforementioned two risks. These chemical
solutions allow silver plated based applications to increase,
although they are not yet the best solutions and further
improvements need to be achieved [5,6] in order to make their
performance equivalent to inorganic anti-tarnish or protection
layers [7].
B. Current approaches for application of Ag as plating for
connectors
Beside the usage of the organic or inorganic anti-tarnishing
systems, other alternative approaches, such as alloying silver,
are used to make Silver plating acceptable in specific
applications. We can find Silver-Palladium alloys where it is
demonstrated that a minimum of 10% to 15% Pd is sufficient
to greatly retard silver migration and inhibit sulfidation
reaction for a long time [8,9,10]. Silver-Tin alloys (Sn content
between 10% and 30%) were also developed with the same
objectives [11,12,13], but most of these alloys, including
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28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
recently developed and commercialized chemistries, are not
really convenient for high speed reel-to-reel connector
terminals plating. These alloyed silver plating solutions have
not shown, yet, wide commercial use due to their complex
plating baths formulations, the difficulties to achieve
reasonable plating line speed, the weak stability of the plating
process itself or the cost issues in some cases.
Other (simpler) approaches have been utilized such as using
lubricated conventional Silver coatings [14] for power
connectors, or a lubricated special nanocrystalline hard silver
system [15] for power and signal connectors. The objective of
this paper is to present a new approach to enlarge the usage of
silver based coatings for electrical contact terminals, the
Amphenol-FCI developed Gold Capped Silver or GCS™
plating system.
II.
GOLD CAPPED SILVER (GCS™)
A. Brief description of the GCS™ plating system
We have developed the GCS™ plating system to replace
Gold and Gold Flash over Palladium-Nickel (GF-PdNi or
GXT®) plating in some connector applications to take
advantage of Silver’s technical properties to enhance
performance of power connectors and/or provide an
economical advantage especially for signal connectors. As
represented in Fig. 1, the GCS™ plating system is a silver
plating based system that uses a matte hard silver replacing
conventional soft silver. A gold flash layer protects the silver
plating layer against tarnishing and operates partially as a solid
lubricant layer. It avoids silver layer oxidation and sulfidation
which usually leads to increased electrical contact resistance.
The top layer is a special polyalphaolefin (PAO) or a
PerFluoroPolyEther (PFPE) high-performance lubricant and is
conferring a self healing function to this protection layer which
ensures that the wear track during mating/un-mating cycles is
always lubricated and protected. The Nickel underplating is a
nanocrystalline highly corrosion resistant layer.
B. Technical Background of the GCS™ plating systems
Plating gold over silver has been known for decades in the
jewelry industry and it is even regulated in many countries for
quality purposes [16]. In the field of passive electronic
components, plating gold over silver has been mainly used for
switches [17,18] where Gold, Silver and Gold over Silver are
the three most common contact materials. Gold plating is
usually dedicated for low current and/or low voltage switches
where any surface oxidation or tarnishing cannot be tolerated.
Conventional soft Silver plating is usually used for switches
and circuits where an electrical arc is expected. Gold plating
over Silver is typically used in double pole switches
applications in order to obtain some of the advantages of both
materials; added to the fact that gold will remain intact and
will provide tarnish free contacts for high reliable long term
switching [19,20].
In summary, Gold plating over Silver plating finishes have
been used for several years in some passive electronic
components [21] but not yet used for separable interfaces such
as connectors which have completely different performance
requirements in terms of durability cycling. Transposing the
“Gold over Silver” process to connectors, as used currently in
switches, does not work if it is not optimized and adapted
properly. GCS™ is a performance optimized multi-layer
plating system and the four layers are required to make the
contact interface resistant to corrosion, sulfidation and ensure
durability. Hereafter several detailed experimentation results,
demonstrating the suitability of using the GCS™ system for
both Power & Signal connector applications, are shown.
III.
EXPERIMENTAL
Several characterization techniques are used to assess the
properties of the GCS™ plating system. Fretting-Corrosion
and Durability testing with continuous low-level electrical
contact resistance (LLCR) measurements were made using
Bruker UMT3 universal mechanical testers equipped with a
mechanical reciprocating module and a separate piezzo
module. A flat real connector header contact or a flat plated
specimen were fixed on these lower modules. The upper
module has a ball holder or a special hemispherical coupon
with variable radius (between 0.8mm and 1.6mm) allowing to
simulate different connector terminals receptacle geometries
under different contact pressures. SIMS (Secondary Ion Mass
Spectrometry) analyses were performed as qualitative and
semi-quantitative composition elemental profiles in depth
from the top surface down to the bulk of the silver plated
layer. The used instrument is TOF-SIMS V equipped with one
Bi gun (25 KV) and another Cs gun (2 KV). All the metallic
hemispherical caps and flat substrates were made of the same
phosphor-bronze copper base metal or the same high
conductivity copper base metal as the tested real connectors.
IV.
Fig. 1
Fares Karam
RESULTS
A. Electrical Contact Resistance
Tarnishing is a long–established challenge in the silver
plating industry. Tarnishing is predominantly caused by
airborne sulfur, which reacts with the surface of silver and
Schematic representation of GCS plating system.
82
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28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
produces black silver sulfide (Ag2S). This sulfide layer affects
the plating’s excellent electrical properties. Unlike most other
corrosion films, the growth of the silver sulfide layer is linear
over time. When there is a thick enough layer of Ag2S and an
enough high temperature, an additional process of thin
filament growth (whiskers) begins [22]. In the GCS™ plating,
the Gold flash layer is used to act primarily as an inorganic
anti-tarnish film replacing organic post-treatments which
cannot withstand 105°C or 125°C temperature ageing tests,
nor typical three lead-free reflow cycles. In fact, it’s extremely
detrimental for electrical contact resistance to use organic antitarnish agents beyond their operating thermal range as this will
lead to decomposition of the organic layer and significant
increase of resistance (Rc) at the terminal-contact interface
[14]. Taking into consideration the typical electrical resistivity
values for the mostly used plated deposits in connectors (see
Table I) it is evident that both Gold and Silver exhibit
excellent electrical conductivity and we would not expect, in
theory, any electrical drawback when super-imposing these
two layers.
TABLE I. ELECTRICAL RESISTIVITY OF TYPICAL MATERIALS AND
PLATED DEPOSITS ON CONNECTORS CONTACTS
Material
Resistivity
(mΩ.cm)
1.68
1.59
2.82
10.78
Copper (Cu)
Silver (Ag)
Aluminum (Al)
Palladium (Pd)
Material
Gold (Au)
Tin (Sn)
Nickel
PdNi (80/20)
Resistivity
(mΩ.cm)
2.44
10.91
6.99
~9.5 to 11.5
Technical literature [23,24] demonstrates clearly that
interdiffusion between Gold and Silver occurs, which will
change the top layer composition and thus change the
resistivity. Fig. 2 shows the Low level Contact Resistance
(LLCR) variation according to normal load for a system
consisting of a bronze hemisphere (see Fig. 3), coated with
0.76µm hard Gold, against a power type connector flat contact
coated with 2.5µm freshly plated GCS™. The second flat
contact was also plated with 2.5µm GCS but was subject to 12
days of thermal treatment at 105°C to accelerate silver-gold
(AuAg) interdiffusion. Values are given for measurements
without any “wipe” between hemispherical caps and flat
samples to avoid removing any specific top layer formed due
to interdiffusion, although in reality there is always at least
one mating cycle, which is a more favorable case.
5
Varying the normal force between 0.2N and 5N for this
Sphere-Flat system corresponds to varying the contact
pressure between 300 MPa and 1000 MPa according to Hertz
stress calculations.
Fig. 3
Hemispherical caps and flat contact used for ECR testing
It is important to note that reported electrical contact
resistance (ECR) values include bulk-copper resistance due to
the wiring configuration in the tribology equipment. The
curves show that as plated GCS™ is slightly less resistive than
the thermally aged GCS™ for the lower normal loads
corresponding to lower contact pressures. These results are in
agreement with technical literature describing that thermal
ageing induces a faster interdiffusion rate between Silver and
Gold leading to an alloy with higher resistivity [25].
Nevertheless, we can see clearly on Fig. 2 that both curves are
overlapping quite quickly. The fact that the two GCS™ curves
tend to have equivalent contact resistance values with
increasing normal load (so with increasing contact pressure),
suggests that interdiffused AuAg layer is very thin.
Consequently, and except for the very low contact pressure
interfaces, the superficial thin interdiffusion layer has minor
influence on the system ECR. In other words, the AuAg
interdiffusion layer is protecting the silver layer against
tarnishing while having minimal impact on its ECR. In all
cases we have always low ECR values which are suitable for
all electronic connector applications and the slight increase in
GCS™ resistivity is not significant.
B. Validation of Gold and Silver interdiffusion on GCS™
plated power connectors
We performed interdiffusion evaluations on High Power
Edge Card connector (HPCE®). The connector contacts are
GCS™ plated using a reel-to-reel plating line under industrial
conditions and assembled. Connectors were mounted on test
printed circuit boards and thermal ageing was conducted at
105°C up to 2000 hours. LLCR measurements were made
every 500 hours. Fig. 4 shows the LLCR variations for the
twelve different rows of the connector. We can see the
maximum average LLCR increase is 0.3mΩ after 2000h while
the maximum LLCR increase among all the individual values
measured does not exceed 0.47 mΩ.
4.5
0.6
GCS 12 days 105 C
Contact Ressistance Variation
(mΩ)
Contact Resistance (mΩ)
4
3.5
GCS as plated
3
2.5
2
1.5
1
0.5
0
0
1
2
3
4
5
0.5
0.4
0.3
0.2
0.1
0.0
0
6
GCS contact resistance variation acording to normal force
Fares Karam
1000
1500
2000
Thermal Ageing of HPCE at 105 C (h)
Contact Force (N)
Fig. 2
500
83
Fig. 4
R01
R02
R03
R04
R12
R11
R05
R06
R07
R08
R09
R10
Contact resistance variation of GCS™ plated HPCE®
connector according to thermal ageing
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28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
It is important to note that for such type of power connector,
the maximum allowed LLCR increase is 0.6mΩ. These low
LLCR values prove the thermal stability of the GCS™ system
from an electrical point of view and confirm that interdiffusion
of Silver and Gold has minor impact on electrical
characteristics of the GCS™ interface.
C. Quantitative investigation of Gold and Silver interdiffusion
in GCS™ plating system
One of the most interesting methods to study the
interdiffusion of Au and Ag is SIMS given its sensitivity and
very low limits of detection. SIMS spectra were taken before
heat treatment and after 24, 48, 120, 500, 1000, 1500 and 2000
hours to study the evolution of interdiffusion. Fig. 5 shows the
SIMS elemental profiles in depth for Au, Ag and Ni taken on
the HPCE® connector after 2000h of thermal ageing, so
representative of an extreme case. This figure is a wide-range
spectrum and shows the top gold layer (Au- and Au3- ions)
with two intense peaks followed by, a small, very thin
interdiffusion layer of (AgAu- ions) and the thick Ag layer
(Ag- ions). The vertical Y axis scale was modified for the
different species to better identify the Gold-Silver interface
and show the interdiffusion areas.
Corrosion Resistance of GCS™ plating system
The pertinence of using standard corrosion tests for Ag
based coating is a commonly debated subject [14,26,27] in the
connector industry. Consequently, for GCS™ plating we
decided to cover all aspects and to conduct Standard Bleach
test (Sodium hypochlorite vapors) based on ASTM B92001(2011), Standard EIA specified Mixed Flowing 4 Gas
(MFG) corrosion tests and Standard IEC high concentration
SO2 (10 ppm) and H2S (1 ppm) corrosive gas exposure tests.
The MFG and IEC high-SO2/H2S corrosive gas tests are
performed after 100 mating/un-mating cycles. The Bleach
tests are conducted on laboratory connector samples without
durability cycling in order to understand the impact of
lubricant and AuAg interdiffusion. Fig. 6 shows the number of
pores observed on GCS™ plated terminals (12contacts) after a
105°C thermal ageing followed by the Bleach test. The results
confirms that without a final lubricant layer, PAO lube in this
case, we have higher porosity. This demonstrates the
importance of stable and neutral lubricant usage. A direct
comparison of corrosion resistance to this bleach test is made
with 30GXT (0.76µm GF-PdNi) and 30Au (0.76µm Full
Gold) platings for reference. We can note that GCS™ has
similar corrosion resistance to GF-PdNi and Full Gold.
D.
60
Number of pores (> 20µm)
2 days @ 105 C
50
5 days@ 105 C
12 days@ 105 C
40
30
20
10
0
GCS / PAO
Dynamic SIMS profile of GCS plated layers on HPCE contact
We can notice a very thin layer (approximately 3~6 nm) on
the surface which is relatively rich in Ag, then the Ag
concentration drops drastically and there is a layer
(approximately 35nm to 40nm) which is extremely rich in Au.
When we approach the thick Ag layer, we can notice a
separate peak indicating the simultaneous presence of Au and
Ag together and the thickness of such layer is estimated
between 25nm and 35nm according to analyzed samples.
Then, the Au diffusion into the thick Ag layer drops quickly
and becomes insignificant.
It can be concluded that, after this severe thermal treatment to
reach the worst interdiffusion case, the Gold layer of the
GCS™ plating system is performing its protective function,
for silver layer, very well. This interdiffusion AuAg layer on
the top surface explains that, from an aesthetic point of view,
the terminals shows a color which is slightly less yellowish
than the conventional aspect of Gold.
Fig. 6
84
30Au
ASTM Bleach porosity tests after thermal ageing. Comparison
of GCS™, GXT® and Full Gold plating performances
1
30GXT
GCS (PAO Lube)
0.8
0.6
0.4
0.2
0
As Plated
Fig. 7
Fares Karam
30GXT
The impact of MFG 4 gas tests on GCS™ electrical contact
resistance are shown in Fig. 7 where tests were performed on
an assembled power type connectors. GCS™ is using PAO
lube and a direct comparison is made to 0.76µm GF-PdNi
(30GXT) plated connectors.
Contact Resistance (mΩ)
Fig. 5
GCS / No Lube
Durability 200x
MFG : 10 days
Unmated
MFG : 10 days
mated
Standard MFG 4 gas test impact on low level contact
resistance of GCS™ plated connectors
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28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
Before introduction into MFG chamber, 200 durability mating
and un-mating cycles were performed. The LLCR values show
the stability of GCS™ plating and its high corrosion
resistance. The ECR values are evidently lower for GCS than
for GXT® due to the intrinsic silver electrical properties. It is
important to note that 30GXT is a standard non lubricated
deposit. The GCS plated (PAO lube) HPCE® connectors went
through IEC high-SO2 (10ppm) and high-H2S (1ppm)
corrosive gas tests after 100 mating & un-mating cycles and
during 21 days for each gas. The LLCR variation results are
given in Table II. We can notice that the maximal increase in
ECR is very low.
TABLE II. ELECTRICAL CONTACT RESISTANC VARIATION DURING GAS
SULFIDATION TESTS
Sulfidation Sequential Tests
Description on HPCE connectors
After 100 mating cycles
After 21 days H2S exposure
After 21 days SO2 exposure
Maximum  ECR
Mated Connectors (mΩ)
0.04
0.01
0.02
Fig. 9 shows similar curve but for GCS™ plating which was
thermally treated at 105°C (12 days) to provoke maximal
AuAg interdiffusion (worst case simulation). The applied
normal load was 1.2N corresponding to a contact pressure of
950 MPa, so this is representative of most of Power and Signal
connectors applications. The frequency used is 10Hz and the
selected stroke is ±25µm. This stroke amplitude is quite severe
for electronics applications (usually we use a stroke between
±5µm and ±10µm [28,29]) but we kept evaluating under
severe testing conditions. If we use 10mΩ contact resistance
as the upper (maximum) threshold limit, we will notice that
conventional soft silver plating alone can withstand 41K
cycles before the complete wear-through of the silver layer.
The fretting-corrosion curve for the GCS™ plating shows that
we can reach 200K cycles. Consequently the GCS™ system is
expected to have seriously improved performance in vibration
tests, much better than conventional Silver plating.
In summary, GCSTM plating system can withstand very harsh
corrosion tests. The interdiffusion Gold-Silver layer is remains
a very efficient corrosion protector for the thick silver layer.
E. Fretting corrosion performance of GCSTM
The lubricant enhances fretting-corrosion performance of
the deposit and this is crucial for applications where
connectors must withstand vibration conditions. Conventional
Silver (soft silver) has moderate fretting-corrosion resistance
compared to gold plating making its use very sensitive when
connectors are subjected to fretting wear or to high vibration
environment [28]. In general, to overcome these limitations,
conventional soft silver is mainly used in high contact normal
force systems with an anti-tarnish coating. 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 [14]. The GCS™ plating improves the overall durability
of the contact system by using hard silver plating. Fig. 8
shows the fretting corrosion curve of a system consisting of a
1.6mm diameter full Gold plated bronze hemisphere and a flat
CuSn8 bronze substrate which was plated with conventional
soft Silver.
Fig. 8
Fretting corrosion curve of conventioanl soft silver plating
Fares Karam
85
Fig. 9
Fretting corrosion curve of GCS plating
F. Current Carrying Capacity of GCSTM
Given the low electrical resistivity of Silver, it is expected
that the current carrying capacity would perform better with
GCS™, but with AuAg interdiffisuion, it was important to
carry detailed investigations. Fig. 10 shows a direct
comparison between the temperature rise curves of GCS™
and GF-PdNi (GXT™) plated power connectors. We can
clearly notice that GCS™, like most silver based plating, is
much more adequate for higher electrical currents. It is useful
to note that historically GF-PdNi temperature rise curves were
very close to Full Gold plating. We did not measure, in our
case, the full curve for the Gold version of the PowerBlade®
connector, but simply showed (point in yellow) that when
carrying 30A current, the temperature of a 0.76µm Gold
plated connector is ~32% higher compared to exactly similar
GCS™ plated contacts. Based on different types of tested
connectors, we have noticed 15% to 20% increase in current
carrying capacity in average. It is also important to compare
what is the maximum temperature increase after severe
environmental and corrosion tests of GCS™ connectors. Our
measurements indicate that 22°C is maximum rise of
temperature when contact are powered with 30A current after
being submitted to thermal shocks (36 cycles, -55°C to +80
ºC) and 10 days damp heat tests (95% RH at 40°C). Even after
20 days MFG exposure, the temperature rise of GCS™ plated
connectors did not bypass 19.2°C for a maximal allowed
increase of 30°C.
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28th International Conference on Electric Contacts, 6-9 June 2016, Edinburgh
[5]
[6]
[7]
[8]
[9]
[10]
Fig. 10
Current carrying capacity curves of GCS™ and GXT® (GFPdNi) plated PowerBlade® connector. The yellow point
represents the performance of a Full Gold plated connector
[11]
[12]
[13]
V.
CONCLUSIONS
The GCS™ plating system has improved performance
compared to conventional silver plating systems. Even after
mating/un-mating cycling, the silver layer remains protected
from oxidation and sulfidation keeping its excellent electrical
performance for connector usage. Connectors with GCS TM
plating have a stable low coefficient of friction which makes it
more adequate for connectors, requiring higher number of
mating and un-mating cycles, compared to typical soft silver
plating system. The GCSTM plating allows connectors to
achieve high performance with lower Silver plating thickness
compared to conventional Silver plating. Conventional Silver
has moderate fretting-corrosion resistance compared to gold
and in laboratory simulated fretting tests, however the GCS™
plating system performed almost similar to gold plating and
successfully passed different vibration tests. The GCS™
plating shows a 15-20% T-Rise performance improvement
compared to GXT® or Gold plating which makes it a very
good plating for higher current capable power connectors. The
GCS™ plating uses PAO or PFPE advanced lubricants to
ensure good tribological performance without the formation of
silver oxides or silver sulfide precipitation. The Gold flash
layer keeps acting as an intrinsic inorganic protection for the
silver layer and the lubricant is an organic self-healing
protection layer. GCSTM plating can be applied to Power and
Signal connectors whenever we need to benefit from the
higher electrical conductivity of Silver or the economical
advantage of Silver deposit.
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