Antoine Fares Karam

Anthony Franchini

Ana Torrealba

Damien Comte

Aurore Brézard-Oudot

Sophie Noël

Doyle Anderson

Jeffrey Toran

Sophie Noel

Compatibility of Gold, Palladium and hard Silver based plating systems for connector applications

come

Introduction

Electronic connectors and overall interconnect gold plating male/female applications have historically used (Au/Ni) as the main plating for interfaces. Gold presents the advantages of being an excellent conductive material, of being a noble metal with excellent corrosion resistance and of presenting naturally a very low coefficient of friction making it an excellent solid lubricant. Nevertheless, the economical constraints and sometimes, some technical limitations have lead, widely, to the development and use of alternative platings. Gold Flash over Palladium-Nickel (GF-PdNi) was developed in the late 1970's [1] and commercialized starting 1986 [2] in order to obtain a much lower cost plating as well as a higher durability (resistance to mating and unmating cycles) metallic layer. GF-PdNi presented also the advantage of being a lower porosity deposit showing better resistance to corrosion and showed better compatibility for high temperature applications. All these advantages offset its lower resistance to fretting corrosion compared to gold and final connector users had the choice of these two platings according to their technical requirements. These two platings are considered fully compatible: This means that when a connector is plated with Au/Ni, it can be mated to its counterpart which is plated with GF-PdNi, without excessive wear-through precious metal layer. In the mid 1990's the Automotive industry started developing conventional silver plating especially for some power applications. Although conventional silver is known to have a low hardness, low young modulus and a poor coefficient of friction, its excellent conductivity (it has practically the lowest resistivity among

Compatibility of various plating systems

Now that all these different plating systems are on the market, we are facing many cases where these plating systems are mated to each other on the same type of connectors. As already mentioned, conventional gold plating (Au/Ni) is known to be compatible with GF-PdNi and it has been produced and used since nearly 35 years for billions of connectors. Thus, it was important to study further the compatibility of these different platings especially when Au/Ni and GF-PdNi are mated with GCS (Au/Hard Pure Silver/Nickel) and GCSb (Gold over Hard Silver-Antimony Alloy/Nickel). In the remainder of this article, GCS and GCSb acronyms will be used to designate these alternative plated stacks. From one side, silver based platings were historically used in Automotive connectors, where we have usually relatively large connector contacts with relatively high normal forces. From another side, Au/Ni and GF-PdNi are historically mainly used for relatively mid-size and low size connectors contacts with relatively moderate or much lower normal forces. We have examined the contact pressure (Hertz Stress) for 88 connector families commercialized by the top twenty connector manufacturers. These 88 connectors families mean in practice thousands of connector models as usually every connector family could cover up to twenty+ versions of connectors which all use the same contact system and the same contact constriction model between male and females. The output of this preliminary statistical examination, shown in Table 1, should not be considered exhaustive, but simply to identify trends. The result was very interesting as most (> 75%) of automotive connectors are using electrical contacts with a Hertz stress ranging between 200 MPa and 700 MPa while the absolute majority of electronic connectors (> 90%) are using contacts with Hertz stress ranging between 600 MPa and 1400 MPa. So, we need to validate that the plating systems which are proven systems in automotive, are compatible, in terms of resistance to wear, with electronic applications where contact pressures are quite different. Although some scientists believe that contact pressure complexity is a barrier to its usage as a parameter in mated, avoiding excessive precious metals wear of plated headers or receptacles.

Connector contact pressure calculation background

In practice, the huge development of digital electronic applications coupled to miniaturization requirements, have lead connector designers to reduce regularly the geometrical dimensions of electrical contacts while keeping constant the normal forces which are recommended by international standards. Most of these normal forces were specified in 1980's when there was much less miniaturization. The electronic connectors application went through highly accelerated miniaturization in the past 15 years while for automotive industry, this miniaturization was much slower. The current development of autonomous vehicles will probably change this quite quickly. In all cases, we can say that we are using today electronic connectors with much higher contact pressures compared to the 1970's and 1980's, while the trend in plating is to reduce the plating thicknesses for precious metals. Gold thickness of 1.27µm (50µin) and 2.0µm (80µin) were a standard in the early 1990's while today we use gold thicknesses ranging between 0.25µm (10µin) and 0.76µm (30µin) most of the time. So, this is another reason to validate compatibility between different plating systems under different contact pressures. This work shows the compatibility between Au/Ni, GF-PdNi, GCS and GCSb to ensure up to 250 mating cycles of intermateability, without excessive wear, under Hertz Stress ranging between 400 and 1300 MPa.

Contact pressure calculation

The Hertz theory is used to calculate the apparent contact area when two solids are pressed one against the other, provided the part geometries as well as the Young modulus and the Poisson coefficient are known. The Hertz theory assumes that the material deformations are elastic, and the contact area is small compared to the size of the solids. Without going deeply into Hertz theory it is important to note the fundamental equations. If we have two solid spheres having an elastic contact under a normal force (F), the spheres are characterized by their radii (R 1 ) and (R 2 ), their Young moduli (E 1 ) and (E 2 ) and Poisson coefficients (ν 1 ) and (ν 2 ). The contact area is a disc of radius "a". From a mathematical point of view, the problem is equivalent to the case of a sphere of radius Fig. 1 : Distribution of electrical contacts Hertz stress for 88 connectors families (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) contact design [8,9], today the digital computing and numerical simulation capabilities are enormous and allow us to bypass these limitations. We also need to determine the conditions where these different plating systems perform the best (compatibility) when cross-R and Young modulus E pressing onto infinite rigidity, R and E being given by: a plane of (EQ1)

The calculation was published by Hertz in 1882 where "a" is :

(EQ2)

The local contact A 4-wire system was used to measure 'dry circuit' contact resistance (Rc). A typical plated hemisphere and a schematic representation of the Rc measurement principle are shown in Figure 2. All durability tests were done according to EIA-364-09C standard. A stroke length of ±2mm and a sliding speed of 25mm/min were used. All platings were deposited using reel-to-reel plating lines including the flat and the hemispherical lab samples.

pressure at a distance (EQ3) "r" from the centre of the disc is given by: The contact sure is:

average pres- (EQ4)
The maximum stress within the material is in the range of:

(EQ5)

Those calculations assume that the deformation of the materials are purely elastic, which is not always the case in real world applications, especially for normal forces and contact geometries generating very high contact pressures. These equations consider that the surface state is perfect while in real contacts we have always certain level of roughness although it could be extremely low. Plated layers could also go through partial plastic deformation during mating and unmating cycles [10]. Thus, it is commonly noted that a contact pressure calculation for is valid for a certain specific moment and certain surface state. In other words, the contact pressure we are calculating is between male and female contacts before durability testing. We call that the nominal initial contact pressure. During durability cycles, wear occurs in the contact area resulting in Hertz stress variation, probably due to modifications in contacts spot size because of elasto-plastic or, eventually, plastic deformation [10,11]. For simplification, we always refer to the initial nominal contact pressure. Finally, Hertz stress equations consider only the 2 solids which are homogeneous material with well-defined Young Modulus.

For connectors, we use plated base material and to be more representative of reality, we measure and calculate the composite Young Modulus of the total plated stack using a nanoindentation technique [11]. This methodology is commonly used as it gives more accurate Hertz stress values, which takes into account the barrier underlayer plating (nickel in our case) as well as the final precious metal(s) layers.

Fig.2 : Typical plated hemipshere used for durabllity simulations and wiring setup

The base material used was a commonly used C70250 (CuNiSiMg) alloy with 43% IACS conductivity. All samples and contacts went through electrochemical alkaline degreasing followed by electropolishing before plating a ductile (26%) 1.5µm ±0.25µm nanocrystalline nickel. All gold layers used an electroplated cobalt-hardened gold. Thick gold (Au/Ni) layer thicknesses were nominally 0.76µm ±0.1µm while flash gold layer thicknesses were 0.07µm ± 0.015µm. The Palladium-Nickel (PdNi) layer was plated using a low ammonia plating bath to ensure low stress PdNi layer, containing 80%Pd and 20%Ni nominal. The PdNi layer thickness was 0.69µm ± 0.16µm on contact area. The hard pure silver layers had a thickness of 3.2µm ± 0.7µm. The silver-antimony (AgSb) layers thicknesses were 2.9µm ±0.4µm and were plated at 10 A/dm² current density. They contained ~3% of Antimony (Sb). A Fischer XDV-µ polycapillary X-Ray fluorescence spectrometer (XRF) with 25µm spot diameter, was used to measure all plating thicknesses and verify base material composition. All samples were lubricated using a PerFluoroPolyEther (PFPE) lubricant. Scanning Electronic Microscopy (SEM) morphological analyses were performed using a ZEISS EVO MA-10 SEM at 15 kV operating voltage, equipped with an Oxford Instruments XMax EDS probe fitted with a 50mm² detector. Plated stack composite Young modulus and composite hardness were determined using an Anton Paar NHT3 nanoindenter.

Experimental 3 Results

Durability testing was simulated in the laboratory using a tribometer and validated, in most of cases, on real connector contacts. The tribometer with continuous low-level electrical contact resistance (LLCR) measurements is a Bruker UMT3 universal mechanical tester equipped with a mechanical reciprocating module. A flat real connector header contact or a flat plated specimen were fixed on this lower module. The upper module has special hemispherical coupon with different radii (between 0.8mm and 1.6mm) allowing to simulate different connector terminals receptacle geometries under different contact pressures.

In this paper we have included the compatibility between Au/Ni and GF-PdNi as reference systems. We have performed 250 mating/unmating simulation cycles on hemisphere/flat setup under different contact pressures. The compatibility between these two plating systems with GCS and GCSb plating systems were studied. We provide typical observations, analysis and SEM micrographs to illustrate the methodology of investigation, but we cannot show each individual result in this short article, given the large number of plating combinations evaluated.

GF-PdNi and Au/Ni platings

we are starting to expose the nickel underlayer that is quite visible with EDS mapping. A quick examination of the flat samples shows that we have gold transfer from the hemisphere to the flat GF-PdNi surface which shows gold over-thickness in some areas on the wear track. The exposed nickel underlayer, calculated using an image treatment software, is less than 10% of the total contact area. This value of 10% of exposure is the maximum limit defined by Telcordia GR-CORE-1217 standard concerning the wearthrough of contacts noble metallization of separable contact interfaces [12].

Typical results under 700 MPa are shown in table 3.1 where we can see the optical images of the interface surfaces as well as the SEM/EDS elemental mapping of gold and palladium. After 250 mating cycles, when 0.76µm of Au/Ni plated on flat, we can see gold transfer to the 0.76µm GF-PdNi plated hemisphere. This local over-thickness of gold explains the dark area in the palladium elemental map at the center of hemisphere where Pd is masked by Au. We can see clearly that no excessive wear is observed and no nickel underlayer is exposed, demonstrating in this case the full compatibility and intermateability of the these two plating systems.

Mechanically speaking, we can say that at 1300MPa of nominal Hertz stress, the 0.76µm Gold plating on female contact is incompatible with 0.76µm GF-PdNi for 250 mating/unmating cycles. Indeed, we are starting to have excessive wear. One could argue that remaining excessively thin gold layer could be acceptable, because if we examine the evolution of contact resistance (Rc) during these 250 mating cycles, shown in Figure 3, we can notice that Rc is extremely stable and quite low during the entire experiment. This is a positive element in favor of accepting a final residual gold layer of a few nm of thickness. Nevertheless we can also estimate that such final thin gold layer is not enough to protect the electrical contacts against corroding and aggressive atmospheric contaminants or pollution gases. Consequently, we can consider that under 1300MPa of Hertz stress, these two platings are incompatible, or, 1300MPa is the maximum contact pressure for compatibility. To re-establish this compatibility, we should either reduce the maximum In practice, we always notice that, when hemispheres are plated with 0.76µm GF-PdNi and flats are plated with 0.76µm of Au/Ni, these two systems are perfectly compatible even at extremely high contact pressure such as 1300 MPa or even 1400 MPa. This is what is expected of such high durability GF-PdNi plating. To illustrate an incompatible case, table 3.2 shows the surface wear after 250 durability cycles for a hemisphere plated with Au/Ni and a flat sample plated with GF-PdNi, under 1300 MPa, which is a very high nominal initial contact pressure. Visually speaking, we could say that surfaces are acceptable as optically we still observe the golden color on the hemisphere. But we start seeing some greyish shades. That indicates that we are close to the nickel underlayer. EDS elemental mapping shows that the remaining yellow golden color is the result of an extremely thin layer of gold, probably less than 15~20nm thickness. In fact, number of mating/unmating cycles (durability limits), change connector contact design to reduce nominal contact pressure, reduce normal loads if this is possible or increase the precious metal layer thicknesses.

Au/Ni and GCS platings

To illustrate the wear behavior of a hard pure silver based plating system, we used the GCS plating which has a gold flash layer on top surface to protect silver.

It is important to note that the Young modulus for a GCS plating is around 110~115 GPa which is much higher than the Young modulus of pure soft silver plating which is in the range of 85~90 GPa. We expect better shear resistance of GCS plating during durability cycling. But GCS still has a lower Young modulus than Au/Ni plating which is in the range of 160~170 GPa. It is also much lower than the modulus of GF-PdNi plating which is between 245 and 255 GPa. Consequently, for a constant normal force, when using a gold plated hemisphere, replacing the GF-PdNi on the flat surface by GCS plating will reduce the contact pressure by 200~300 GPa. This means, that it is extremely difficult to reach, in real GCS plated connectors, such 1300 MPa of Hertz stress. In practice, a contact pressure of 1000 MPa is considered as extremely high for a silver based plating system. This is also what explains that for automotive silver plated connectors, we use high normal forces which are practically not allowed to be used for gold plating. These high normal forces provide enough contact pressure to break the blackish silver sulfides (Ag 2 S), formed on silver surface, and retrieve the excellent electrical conductivity of silver plating. Table 3.3 shows the results, after 250 durability cycles, between a hemisphere plated with 0.76µm of Au/Ni and a flat plated with GCS, at 700 MPa. Visually we see significant wear tracks and quite large contact area on the gold plated hemisphere. When performing elemental mapping on the hemisphere, we can notice silver transfer from the flat surface, which is expected knowing that silver plating is usually sub-This limited worn gold zone constitutes less than 5% of the total measured contact area and it was not systematically observed. We have shown, on purpose, the worst case for pedagogical illustration. A quick examination of the flat sample shows that we are still on the hard silver layer, in all cases, and there is never wear-through to the nickel underlayer. A silver layer thickness measurement using micro-XRF shows that only 60% to 70% of silver was worn after 250 mating cycles at 700 MPa. Using GCS and Au/Ni plating seems to be a robust option provided that we respect certain Hertz stress rules. In other experiments, mating GF-PdNi to GCS has shown also to give excellent results, even better than traditional Au/Ni plating, ensuring a very high durability plating couple. In practice, we notice that mating a silver plating contact to a non-silver plated contact gives better durability results than when it is mated to itself because we have less galling and more abrasion [13], which is the most frequent wear mechanism of silver.

Au/Ni and GCSb platings

GCSb in this study is a variant of GCS plating. We use a silver-antimony (AgSb) alloy instead of pure hard silver. This AgSb alloy is known to have much higher composite hardness up to 180 Vickers compared to hard pure silver limited to 120~135 Vickers.

We measured its Young modulus and it is in the range of 150 GPa. The presence of an alloying element will slightly increase the silver electrical resistivity but we still observe very low electrical contact resistance between hemisphere & flat, as shown in Fig. 4 under different contact pressures. Rc remains stable and very low at 900 MPa while at 400 MPa and 700 MPa, we notice a 3 mΩ average increase, as we have fewer contact spots when we reduce Hertz stress. These 3mΩ could be significant for some power connector applications. This is an additional reason to study and select the appropriate contact pressure when using different plating systems on male and female contacts.

Unfortunately, even when alloying silver (Ag) with antimony (Sb), we still observe silver galling occurring, if we try to work in such high Hertz stress range. Table 3.5 shows the SEM micrograph of the GCSb surface after 250 mating cycles under nominal contact pressure of 1200 MPa.

Fig.4 : Contact resistance evolution vs. durability cycles between Au/Ni hemispheres & GCSb flats

This GCSb variant indeed provides better durability in several cases given that we have better resistance to shear vs. pure silver, as well as higher hardness silver.

It has been proven that the ratio between hardness and Young Modulus is much more correlated to wear extent than hardness alone [14,15] and consequently this is an important difference with GCSb. Table 3.4 presents the surface wear after 250 durability cycles, at 900 MPa of Hertz stress between a hard Au/Ni plated hemisphere and a GCSb plated flat. The optical high resolution photo of hemisphere shows that gold plating is almost not scratched and the little whitish color is the result of silver transfer from flat to hemisphere. Concerning flat GCSb plating, the elemental analysis micrograph shows that gold is removed but the AgSb layer is practically intact and no wear-through is observed to the nickel underlayer. We may think, at this stage, that with a harder Silver alloy with superior Young modulus, we may be able to use very high contact pressures, observed in many miniaturized connectors nowadays.

We notice significant silver wear on the track. EDS mapping shows that nickel underlayer is exposed.

Given the limited number of wear debris around the wear track, we are probably again under a galling wear mechanism coupled to limited abrasion. We have flakes of Silver detached from the surface of flat. Unfortunately, alloying Ag with Sb and applying a lubricant does not completely offset the galling mechanism. Under such high contact pressures, the lubricant layer between male and female contacts becomes extremely thin and inefficient to reduce the intrinsically high coefficient of friction. The flash gold on top of AgSb is playing the role of a solid lubricant, same as for GF-PdNi, but it is gradually worn during the 250 durability cycles. The maximum durability cycles before wear-through, under different contact pressures, will be published separately.

tion point where we should not mate these plating systems together. We have significant silver transfer from flat to hemisphere. What was not expected is the transfer of gold from hemisphere to GCS plated flat and we observed the first significant nickel underlayer exposure of Au/Ni layer at 800 MPa. Lubricant is known to reduce galling and flaking extent for metallic coatings, but it seems that gold and silver solid solution is formed under the pressure accelerating the transfer (wear) of gold on hemisphere. When plating GCSb, these two systems are compatible till 900MPa. Nevertheless, we observe in this case more conventional debris on both sides, showing that we have different wear mechanisms between GCS and GCSb. Alloying silver with 3% of Sb (Antimony) is generating quite a different plating system. When examining Table 3.7, we immediately notice that when using a GF-PdNi plated hemisphere vs. Au/Ni plated flat, we have an improvement in wear compatibility between these

Compilation of results & discussion

After showing above several examples of the investigation methodology, tables 3.6, 3.7 and 3.8 show the compilation of all obtained results from a wear compatibility point of view. When the intermateability result case is highlighted in green, this means that no wear-through of noble metal layer down to the nickel underlayer was observed on both flat and hemisphere contacts. When the case is highlighted in red, it is not advised to mate these 2 plating systems together under the specific mentioned contact pressure given we have significant wear down to non-noble barrier underlayer. When the intermateability case is highlighted in dual green-orange color or only in orange, it means that we have slight wear, less than or close to the tolerable 10% of contact area, specified by the Telcordia standard. We consider that this is the maximum contact pressure which we could adopt according to application specifications. Table 3.6 shows that 0.76µm Au/Ni plating is fully compatible with GF-PdNi up to 1200 MPa of contact pressure, and we start seeing wear-through down to the Ni layer of Au/Ni plated hemisphere starting 1300 MPa.

Table 3.7 : Compatibility of GF-PdNi when plated on hemisphere with 3 other plating systems according to various contact pressures Table 3.8 : Compatibility of GCS when plated on hemisphere with 3 other plating systems according to various contact pressures platings. GF-PdNi vs. GF-PdNi shows in this case its higher durability, technically speaking, compared to other plating systems. Using GF-PdNi plating on hemispheres allows us to use GCS plating on flats up to 1000 MPa before we observe adhesive wear on these flats. When plating the flat with GCSb, compatibility with GF-PdNi is up to 1100 MPa and then we start observing abrasive wear. Above 1200MPa, we observe additional oxidation wear beyond abrasion debris which may be the result of localized frictional heat generation [13] at such high Hertz stress for these two high stiffness platings. Nevertheless, this remains a hypothesis as durability tests were conducted at moderate sliding speed of 25mm/min and frictional heat generation is theoretically minimized. Further investigations are ongoing to understand the mechanisms of such oxidation. The results in table 3.8 show that when using GCS on the hemisphere, the maximum contact pressure which can be used is 1000MPa when mating to a 0.76µm Au/Ni plating or GCSb on flat samples. This is already a very high contact pressure for a silver based plating. It is interesting to notice that when using GF-PdNi flat samples vs. GCS In practice, this value shows also to be the limit for 250 mating cycles intermateability for Au/Ni plating with itself. It is interesting to note that when the flat is plated with GF-PdNi, this plating did not show any damage as we have gold transfer from the 0.76µm thick layer on hemisphere to the flat. When the flat is plated with similar Au/Ni plating, the gold transfer is mutual and we could observe Nickel exposed spots randomly on both hemisphere and flat. This is a known electroplated gold wear mechanism but in our case, the transfer phenomena seems to be significantly increased under high contact pressures. When mating a Au/Ni plated hemisphere to a GCS plated flat, the compatibility of these two systems is limited up to 700MPa only and 800MPa is considered as the inflec- on hemisphere, we have much less wear, for GCS evidently, compared to the opposite configuration discussed above. When the flat is plated with GF-PdNi, the maximum contact pressure to be used is 800MPa while it was 1000MPa in the opposite case. The continuous solicitation of GCS plating, during 250 durability cycles, when plated on hemisphere (simulating female contacts), leads to continuous galling and transfer of silver from hemisphere to flat, resulting in faster nickel underlayer exposure which is not desirable for corrosion resistance issues. Another important observation is the maximum 700MPa contact pressure when GCS is mated to itself. We have in this case maximum galling, although attenuated by lubrication. Nevertheless, 700MPa with both parts plated with GCS is obtained with normal forces which could generate a contact pressure higher than 1000MPa if the plating would have been GF-PdNi or Au/Ni on both parts. In other words, using GCS as a plating system is still very interesting from an electrical and economical point of view, provided that connector contact designer would forecast this since the beginning. All the plating systems investigated above have various hardness and Young moduli and many results were unexpected. All this suggests and confirms that hardness is not the main major parameter influencing wear evolution [16]. Again, it seems that the ratio of hardness to Young's modulus correlates much more accurately to the wear behaviour on multi-layered coatings. The existing mathematical relationship between this ratio (hardness/Young Modulus) and the wear observed on

[2]

[3]

[4]

[5]

[6]

[7]

[8] our real connectors and the tribological sphere/flats will be presented elsewhere.

hemi-[9]

[10]

4 Conclusions [11] We have presented in this investigation work, the compatibility ranges for different plating systems according to the nominal initial contact pressure of sphere/flat contact configuration, up to 250 durability (mating/unmating) cycles. The objective was to offer connector designers some guidance when developing a connector with several plating options, as compatibility, in terms of wear resistance, will vary according to defined normal force and thus contact pressure. This work covered traditional gold plating, the historical GF-PdNi alternative plating, as well as 2 other silver based platings which are used now in the industry. Using asymmetrical platings (different platings on male and female contacts) allows higher performance and in some cases it can simultaneously reduce plating costs.

[12]

[13]

[14] Abrasion.

Tribology: Fundamentals and Advancements; Chap. 7, Wangyang N.; Grummon D.: Effects of the ratio of hardness to Young's modulus on the friction and wear behavior of bilayer coatings. App. Phys. Lett., 2004, Vol.85, No.18, pp. 4028-4030 Aadarsh M.: Analysis of relation between friction and wear. Int. J. Mech. Eng. & Rob. Res. 2014, Vol. 3, No. 3, pp. 603-606 5 Further inquiries [16] Antoine FARES KARAM Amphenol FCI, 2 Rue LaFayette, Besançon, France. E-Mail: antoine.fares-karam@fci.com

Fig. 3 :

 3 Fig.3 : Contact resistance and coefficient of friction evolution during 250 durability cycles between Au/Ni hemisphere and PFPE lubricated GF-PdNi flat under 1300 MPa of contact pressure.

  Au/Ni compatibility with GCSb (Au/AgSb/Ni) at 900 MPa of contact pressure

Table 3 .

 3

	1300	0.76µm Au/Ni	0.76µm GF-PdNi
	MPa	Hemisphere	Flat
	Optical
	Au EDS Mapping
	Ni or PdNi EDS	Mapping

1: GF-PdNi compatibility with Au/Ni at 700 MPa of contact pressure

Table 3 .

 3 The presence of a contact lubricant has significantly reduced such galling in this case, compared to what is historically observed with nonlubricated silver plating. This silver transfer on the hemisphere partially masks the signal of the gold layer where we can see a non-yellow (black) spot. We say partially, because EDS mapping of elemental nickel shows few nickel spots from the nickel underlayer

2 : GF-PdNi compatibility with Au/Ni at jected to galling.

Table 3 .

 3

3 : Au/Ni compatibility with GCS at

Table 3 .

 3

	6 : Compatibility of Au/Ni when plated on
	hemisphere with 3 other plating systems according to
	various contact pressures


A Torrealba

S Noël

A Brézard Oudot

A Fares Karam

D Comte

A Franchini

J Toran

Durability of some asymmetrical contact pairs for connector application

Reliability of connectors is still a major concern for end users in many applications. One way to improve it is to optimize the surface coatings deposited on base substrates. Although gold remains the most widely used contact plating material, many alternative coatings have been developed in the past or recent years. Among these alternatives are silver final coatings and gold flash over palladium-nickel plated alloy. The properties of these two platings have been mainly investigated in symmetrical configurations where male and female contacts have similar deposits. In this work, asymmetrical configurations -where platings on male and female contacts are dissimilarare studied. Wear behaviour was investigated with friction and fretting tests. The aim was to characterize the role of the various layers on the wear performances of contacts. Contact resistance behaviours were measured and correlated to the wear tracks investigated with various techniques such as SEM, EDS and 3D-Profilometry. It is shown that in some cases dissimilar final coatings on male and female contacts can significantly improve durability. resistance of PdNi is improved when in an asymmetric Au (1.2 µm) versus PdNi contact.

Introduction

In this paper the wear and fretting resistance of two multilayer plating systems are studied: hard pure silver plating on nickel and a gold flashed palladiumnickel on a nickel layer. The four possible configurations were tested for friction and fretting properties since the degradation modes are known to be different. The aim of the study is to bring some insight on the wear mechanisms and to determine if it is possible to improve the performance of each coating by using asymmetrical contacts.

The development of electrification and of connected consumer devices is causing an exponential rise of the number of connectors used and produced worldwide. Historically gold has been the best metal to meet all connector needs. However in view of its cost other alternatives have been developed, such as various palladium alloys [START_REF] Antler | Fretting of electrical contacts: An investigation of palladium mated to other materials[END_REF] [2] in the 80s and hard silver [3]. The concerns were mainly frictional polymer and tarnish film formation for these two types of coatings respectively.

Numerous studies have been done to determine the performance in terms of fretting and wear resistance of all the coatings used in the connector industry, such as the study of the fretting and wear resistance of nickel [4], tin coatings [5] or a new alternative, gold capped silver (GCS) [6]. Gold flash over palladium nickel and the hard silver with (or without surface post-treatment and lubrication) have been used successfully for particular applications. Literature shows that in general palladium nickel alloy has good wear resistance [7] but low fretting resistance [8], while silver coatings have good fretting resistance [9] but low wear resistance. Antler's seminal work on connectors has shown in [10] that dissimilar contact pairs could be very detrimental to wear and fretting behaviour. More recently in [11] Lin et al. have shown that the fretting

Samples and experimental set-ups 2.1 Samples

A hemisphere (cap) of 1.4 mm radius was stamped on CuSn6 substrates before the electrodeposition of the coating. All the samples (caps and flats) were plated on industrial reel-to-reel plating lines in order to obtain a better reproducibility and all had a 1.3-1.6 µm nanocrystalline Ni first coating. The multilayers studied are described in Table 1. The PdNi alloy coating is called GXT TM ; it consists of 80% w/w Pd and 20% w/w Ni deposited from a low ammonia plating bath. A cobalt hardened gold flash of about 50 to 80 nm was deposited on the top. The hardness was measured with a nanoindentation tester at 0.1 gf and found to be between 400 HV and 420 HV. The GXT TM coatings will be referred to as GXT for simplicity. Care was taken in the measurement with the X Ray fluorescence spectroscopy (XRF Fisher XDVµ model) of the Pd and Ni concentrations in the PdNi coatings. A low free cyanide silver plating bath with no alloying was used for the silver coatings. Plating parameters were adapted to obtain hard silver deposit with average hardness between 120 and 130 HV. These AGT® coatings will be referred to as nAGT because they are not lubricated. All the hardness measurements were carried out under 0.1 g load with a NHT 3 Anton mean values of 500 measurements per cycle were calculated. All experiments were done at room temperature (23°C) and 55% relative humidity.

Contacts will be referred to as coating1/coating2, the first being the cap and the second the flat through all the paper.

Characterizations

3D images, profiles and wear volumes were measured by a Bruker Contour GT-X 3D Optical Profiler. The wear volumes calculations took into account positives volumes due to transfer and negative ones due to deformation and particles ejection; the following formula was used:

Paar nanoindenter. The CuSn6 substrates hardness was 230 HV with a Young modulus of 120 GPa.

= | -- + | -- | + | = = | -- | + | -- + + |
Scanning electron microscope (SEM) images were obtained with a ZEISS EVO MA10 microscope fitted with an Oxford Instruments Xmax probe for Energy Dispersive X-ray Scanning (EDS) analysis for the identification of the chemical elemental composition of selected surfaces. The analyses were done at 15 KeV, the depth of analysis was therefore less than 1 µm.

Experimental set-ups

Contacts were submitted to durability tests. The evaluation of a coating pair was based on the electrical properties (contact resistance Rc), the friction properties (friction coefficient µ not shown here) and wear properties after several numbers of cycles.

Results and discussion

Durability experiments

Symmetrical contacts

Durability and fretting tests

The evolution of Rc and of the wear were studied as a function of number of friction cycles n. Figure 1 shows Rc(n) curves for the symmetrical contacts nAGT/ nAGT and GXT/GXT up to 1200 cycles. All the experiments were performed several times and were repeatable; a representative curve is giving in the following text for all the runs.

The durability testing was done with a Bruker UMT3 universal tester fitted with a mechanical reciprocating module. The stroke was set at 2 mm (peak to peak) under a 1 N normal load and a speed of 125 mm/min (0.53 Hz). The contact resistance and the tangential force were measured continuously every 0.1 s. The mean values of 10 measurements were calculated.

The fretting tests were performed with a dedicated device (electro-dynamic shaker [4]) working at constant displacement. The conditions were: d=50 µm peak to peak displacement, 10 Hz and normal load 1 N. The contact radius calculated with the Hertz formulation is 25µm for the studied substrate. The tests were carried out at ±25µm, in order to be in fretting gross slip mode and not in reciprocating sliding mode. During the test, the voltage drop in the contact was measured with an acquisition card (333 10 3 samples/s), the DC current value set at 20 mA and voltage limit at 250 mV. The

CuSn6

The behaviour of the nAGT/nAGT contacts is described first. An initial contact resistance (Rc1) of 2.4 mΩ is measured, the values then remain low and stable during the first 400 cycles (~ 2 mΩ). After these 400 cycles there is a rapid increase of Rc to a value of about 30 mΩ at 480 cycles. This indicates the beginning of a degradation of the silver layer; after a decrease Rc drastically increases over a value of 1 Ω.

worn out and at 1200 friction cycles, the wear depth reach 4 µm for the flat and 10 µm for the cap.

The composition of the wear tracks were analysed for different numbers of cycles 150, 250, 430 and 1200 cycles. The beginning of the increase of Rc at 430 cycles corresponds to the thinning of Ag and exposure of Ni. The experiments show that as long as the surface of contact involves an average composition of 57 at% of Ag, the contact resistance remains stable.

The second multilayer studied is a symmetrical GXT/GXT contact. Figure 1 shows the variation of Rc as a function of durability friction cycles. Rc values are low and stable (~ 2.5 mΩ) till 500 cycles, they start to increase slowly up to 850 cycles (10 mΩ). After 850 cycles the values vary between 5 mΩ and 40 mΩ. The durability properties of GXT/GXT is thus observed to be better than that of nAGT/nAGT

3.1.2

The SEM images in figure 4 show the composition of the wear tracks for the friction tests of GXT/ GXT contacts stopped at different numbers of cycles.

Asymmetrical contacts

For an applicative point of view it was important to investigate the behaviour of contacts with dissimilar platings involving the two described above. During the first 150 cycles Au is not removed. As the number of cycles increases, the percentage of gold decreases ([Au]=7 and 10 at% at 600 cycles for the cap and the flat respectively) and PdNi is more exposed ([Pd]=26 and 41 at% for the cap and the flat respectively (figure 5). The behaviour of GXT/nAGT is described first.

Figure 6 shows the initial Rc value is 2.9 mΩ, it stays stable for the first 120 cycles (~ 2 mΩ) and then increases drastically after 300 cycles (Rc=1 Ω at 320 cycles). Comparing the size of the wear tracks on the caps after 150 cycles for the two types of contacts shows that the nAGT/nAGT track is 14 times larger than the GXT/GXT one due to the difference in hardness (125HV/ 420 HV respectively) and the following wear modes.

Fig. 7: EDS composition images of GXT/nAGT contacts for different numbers of friction cycles. Left GXT caps, right nAGT flats

For the two symmetrical contacts studied we have two totally different wear modes. The multilayer nAGT/nAGT shows adhesive wear with a more severe degradation. When the silver in the wear track is removed and displaced to the edges of wear area, the nickel underlayer is exposed and the contact resistance increases from 2 mΩ to 1 Ω after nearly 480 cycles. For the GXT/GXT multilayer there is abrasive wear, with a much smaller worn surface with a contact resistance that gradually increases but without ever reaching 50 mΩ. The beginning of the increase of Rc at 300 cycles corresponds to the thinning of Ag and exposure of Ni on the flat. Experiments show that as long as the contact surface involves an average composition of at% Ag higher than that of Ni, the contact resistance remains stable. the number of cycles on the cap and flat. After 850 cycles [Cu] reaches 11 and 6 at% for the cap and flat respectively. An analysis focussed on the centre of the cap wear scar did not show any O. It can be observed that at all the stages the GXT flat wear tracks show some Au (probably located on the side on the wear tracks). These two observations are correlated to the stable and low values of Rc measured during the 850 cycles.

in Rc has started but the values are still below 10 mΩ. For higher numbers of cycles less Ag is observed inside the wear tracks of the cap and flat. Rc values are high and fluctuate. After 60 000 cycles severe wear is observed and both cap and flat tracks are composed of O, Ni, Cu and small amounts of Ag.

Fretting experiments

Fretting behaviour of Ag/Ag contacts has been described in several works [9] [12] as well as that of Au flashed PdNi [8] [1] [13]. The first type of contacts (soft/soft) shows minor fretting degration while the second one (hard/hard) shows a severe one; the severity of this latter one depends of the thickness of the gold flash and the contact pressure.

The same four pairs have been submitted to fretting tests (described above) in order to investigate the

soft/hard

Comparing the wear scars after 10 000 fretting cycles (corresponding to a distance of 1m) to those after 300 friction cycles (distance 1.2m) it can be seen that the wear of the fretting test is more severe. Cu starts to be exposed for the fretting test, while for the friction test flat and cap are still coated with Ag at more than 25 at%.

Dissymmetric contacts

Figure 12 shows the electrical behaviour during 150 000 cycles for the two asymmetrical pairs.

Fig. 12: Fretting behaviours of nAGT/GXT and GXT/nAGT contacts

The behaviour of the GXT/nAGT contacts is described first. For 600 cycles the Rc values stay constant and low (~ 3mΩ); they then increase and vary strongly. Finally the most interesting case is the nAGT/GXT contact. Figure 12 shows that during the whole test the contact resistance values never reach 10 mΩ. Rc starts at 2.1mΩ, then during 1 400 cycles there is a plateau at 1.5mΩ, then an increase to about 2 mΩ and finally a plateau of stable values lasting 150 000 cycles.

Figure 17 displays the 2D profiles of the nAGT cap and the GXT flat after 150 000 cycles. The cap has been worn by the hard GXT flat and the remaining scar is 5 µm deep. Transferred material 5µm high can be seen on the flat. Oxygen is detected in rather high quantities on the cap and the flat due to the presence of Cu and Ni. Nevertheless at this stage the measured contact resistance is 2.8 mΩ probably due to the presence of Ag on the periphery of the wear scars. "noble" metal such as silver on a hard surface acted as a solid lubricant diminishing fretting degradation. because of the beneficial transfer of the silver from the cap to the flat.

Durability tests of four configurations of contacs were investigated. Soft (AGT) cap on soft (AGT) flat, hard (GXT) cap on hard (GXT) flat and mixing the two for disymetric contacts. It was shown that:

[2]

[3] -With the soft/soft configuration strong wear of the soft Ag coating occurred, leading to an Ag/Ni contact interface, the beginning of Rc increase and finally to a Cu/Ni interface giving high Rc.

[4] -With the hard/hard configuration 50 nm flashed Au remained long enough in the the interface for the Rc to increase slowly with Rc < 10mΩ after more than 800 cycles.

[5] -the hard cap on soft flat strong wear is recorded, Ag transferred to the cap and finally the contact interface becomes Ni/Cu leading to a very fast Rc increase. This is the worse case of the study.

-Finally the soft/hard configuration gave very long lasting low Rc because of low wear with some transfer of the flashed Au on the cap and some tranfer of the Ag to the flat. An unexpected "alloytype" Ag-Au-Cu acted as a third body during friction.

[6] 28 th contact applications. Proc.

ICEC2016 Wear, 162-164, 1993, pp.339-346.

[9]

Fig. 1 :

 1 Fig. 1: Durability of nAGT/nAGT and GXT/GXT contacts

  Figure 2 shows the EDS chemical maps of the wear tracks pasted over the SEM images of the contact. The compositions calculated for the depicted wear tracks are plotted in figure 3. At 150 friction cycles the wear track is still fully covered with silver ([Ag]=96 and 88 at% for the cap and flat). At 250 cycles Ni starts to be exposed ([Ni]=21 and 46 at% for the cap and flat) and becomes more predominant at 430 cycles ([Ni]=25 and 55 at% for the cap and the flat). The atomic percentage of O is also more important (32 and 14 at% for the cap and flat), which correlated to the beginning of the electrical disturbances. Finally, at 1200 cycles the cap and flat are worn and oxidized through to the copper substrate ([Cu]=14 and 7 at%; and [O]=38 and 19 at% for the cap and flat respectively) while Rc values are very high (~ 100 mΩ).

Fig. 3 :

 3 Fig. 3: Chemical composition (%at) of the wear tracks showed in figure 2 : caps A) and flats B)

Fig. 2 :

 2 Fig. 2: EDS composition images of nAGT/nAGT contacts for different numbers of friction cycles. Left nAGT caps, right nAGT flats. The white line shows the region used for the composition calculation. Fig. 4: EDS composition images of GXT/GXT contacts for different numbers of friction cycles. Left GXT caps, right GXT flats Profilometric measurements and wear volume calculations showed there was a small transfer of silver from the flat to the cap at 150 friction cycles. Beyond that, the cap and the flat are more and more

  Durability of nAGT/GXT and GXT/nAGT contacts umb cycles

Fig. 5 :

 5 Fig. 5: Chemical composition (%at) of the wear tracks shown in figure 4: caps A) and flats B) The composition graphs of figure 5 can be correlated to the Rc evolution: Rc values remain stable and low as long as the mean interface composition involves more than 20 at% of Au. As this percentage decreases, PdNi and Ni are exposed and Rc values start to increase.

Figure 7

 7 Figure7shows the composition maps of nAGT flats and GXT caps. After 30 cycles Ag is transferred from the flat to the cap while the flat remains covered with 57 at% Ag. At 300 cycles the flat is mostly composed of Ni which is also detected on the cap in an oxidised form. Figure8confirms that there is 18 at% of Ni and 52 at% of O on the cap and 60 at% of Ni and 8 at% of O on the flat. At that stage a drastic change of Rc is observed. The percentage of gold on the cap

Fig. 8 :

 8 Fig. 8: Chemical composition on the wear tracks shown in figure 7: caps A) and flats B) Fig. 10: EDS composition images of nAGT/GXT contacts for different numbers of friction cycles. Left nAGT caps, right GXT flats Figure 9 shows the 2D profiles of the flats at the corresponding wear stages and the maximum wear depths dz. After 30 cycles dz. = 2.8 µm which is about the thickness of the Ag layer; at 300 cycles dz. =3 µm The compositions maps of figure 10 shows that at 90 cycles Ag the cap is worn and that Ni is exposed on the cap. At 850 cycles a zone in the middle of the cap seems to be composed of Ni on the outside and a mixture of Cu, Au and Ag in the middle.and for 600 cycles dz. = 4 µm. The wear volumes calculated from the 3D measures (not shown here) showed that the wear volume on the caps were always positive.

Fig. 9 :

 9 Fig. 9: 2D profiles of the nAGT flats after 30, 150, 300, 600 cycles (GXT/nAGT contacts)

Fig. 11 :

 11 Fig. 11: Chemical composition on the wear tracks shown in figure 10: caps A) and flats B) From the compositions of figure 11 a wear mechanism can be proposed. At the very beginning of the friction test (6 cycles) some Au of the GXT flat is transferred to the cap but mainly Ag form the cap is transferred to the flat ([Ag]=36 at% at 90 cycles). Then the concentration of Ag decreases with

Fig. 13 :

 13 Fig. 13: EDS images of GXT/nAGT for different numbers of fretting cycles. Left GXT caps, right AGT flats

Fig. 14 :

 14 Fig. 14: Chemical composition (%at) of the wear tracks shown in figure 13: caps A) and flats B)

Fig. 17 :

 17 Fig. 17: 2D profiles of the nAGT/ GXT contact after 150000 fretting cycles Figure 18 shows the wear volumes of the cap and flat. Negative volumes represent displaced matter and ejected matter and positive ones represent displaced matter and transfer. These wear volumes are almost constant during the first 100 000 cycles. After that a very large increase of the cap wear volume (900 10 3 µm 3 ) is observed. It is due to the filing of the Ag top of the cap by the hard flat. Simultaneously some transfer of silver on the flat occurs as seen in figure 17.

Fig. 15 :

 15 Fig. 15: EDS images of nAGT/GXT for different numbers of fretting cycles. Left nAGT caps, right GXT flats. The EDS images in 15 show that Ag from the cap is transferred to the flat, exposing Ni. The compositions in figure 16 indicate that th e percentage of Ag remains between 36 and 52 at% on the cap and between 40 and 76 at% on the flat during 21 000 cycles. After 150 000 cycles the wear tracks of both the cap and the flat are composed of Cu, O and some Ag.

Fig. 18 :

 18 Fig. 18: Wear volumes of the caps and flats for nAGT/GXT contacts

Fig. 16 :

 16 Fig. 16: Chemical composition (%at) of the wear tracks shown in figure 15: caps A) and flats B)

Table 1 :

 1 Thickness and composition of the GXT and nAGT plating systems.

Table 2 :

 2 Number of friction and fretting cycles for Rc>10 mΩ.Friction of GXT/GXT contacts lasts twice longer than nAGT/nAGT. Using a GXT cap on nAGT (asymmetrical GXT/AGT) degrades the behaviour as compared to the symmetrical contacts. Using a nAGT cap on GXT flats ensures low Rc values. Fretting duration of GXT/GXT is 1.8 smaller than that of nAGT/nAGT. Using a GXT cap on AGT multiplies by 1.3 the fretting duration as compared to GXT/GXT contacts. Finally using a nAGT cap on GXT (asymmetrical nAGT/GXT) no Rc increase due to fretting was recorded for 150 000 cycles. This study has shown that the transfer of a soft relatively

	[10]
	[11]
	26 th
	[12]
	[13]


A Torrealba

S Noël

A Brézard-Oudot

A Fares Karam

D Comte

J Toran

Analysis of Fretting Behavior of Silver and Gold Flashed Palladium-nickel Dissimilar Coatings for Connectors

Keywords: fretting, silver, dissimilar contacts, gold flash, palladium nickel. I

these problems [START_REF] Antler | Fretting of electrical contacts: An investigation of palladium mated to other materials[END_REF][START_REF] Antler | The application of palladium in electronic connectors[END_REF][START_REF] Bare | Connector resistance to failure by fretting and frictional polymer formation[END_REF][START_REF] Antler | Contact fretting of electronic connectors[END_REF][START_REF] Lees | Fretting behaviour of gold flashed palladium, palladium nickel and palladium silver contact materials[END_REF][START_REF] Morse | A statistical comparison of gold and palladium-nickel plating systems for various fretting parameters[END_REF][START_REF] Aukland | An examination of the metallic bonding of a clad material and two gold plating systems under contact force fretting conditions[END_REF]. After numerous studies to determine the performances of these coatings, gold flashed palladium nickel and hard silver have been used successfully for particular applications, often with lubrication or anti-tarnish top layers. In a work on palladium nickel [START_REF] Myers | Comparison of hard Au versus hard Au flashed PdNi as a contact finish[END_REF] it is emphasized that the results in literature are difficult to use because the deposition processes have evolved and that the measurement of the accurate composition of alloys is difficult.

Palladium nickel alloy is known for good wear properties [START_REF] Myers | The performance implications of silver as a contact finish in traditionally gold finished contact[END_REF] but low fretting resistance [START_REF] Morse | A statistical comparison of gold and palladium-nickel plating systems for various fretting parameters[END_REF] although some authors find a gold flash -when thick enough -to be beneficial [START_REF] Antler | The application of palladium in electronic connectors[END_REF][START_REF] Morse | A statistical comparison of gold and palladium-nickel plating systems for various fretting parameters[END_REF]. Silver coatings have good fretting properties [START_REF] Antler | Contact fretting of electronic connectors[END_REF][START_REF] Antler | Electrical effects of fretting connector contact material: a review[END_REF][START_REF] Song | Correlation between friction and wear properties and electrical performance of silver coated electrical connectors[END_REF] but low wear resistance.

Antler's seminal work on connectors has shown in [START_REF] Antler | Electrical effects of fretting connector contact material: a review[END_REF] that dissimilar contact pairs could be detrimental to wear and fretting behavior. It is shown that load and motion cause wear which alters the composition of the contact surface, which is the key issue in the stability of the contacts resistance. In the case of dissimilar metal pairs the type of wear (adhesive, abrasive) and the direction of transfer (if any) are permanently changing and make any modelization attempt complex or illusory.

A thin low shear film on hard contacts is known to be favorable but the wear off of such thin film is usual. Most of the reports on the good fretting properties of gold flashed palladium nickel alloys emphasize the role of the gold flash thickness.

More and more interest is shown on the subject of compatibility of dissimilar coating. The nature of the layer that predominates on the surface is crucial for the electrical behavior of the dissimilar contacts. In [START_REF] Lin | Fretting property of asymmetrical metal contact pairs[END_REF] it was shown that the fretting resistance of gold flashed palladium nickel finish is improved when in contact with a 1.2 µm thick gold one because of the transfer of gold.

In [START_REF] Torrealba | Durability of some asymmetrical contact pairs for connector application[END_REF] the durability behavior of contacts with hard pure silver and gold flashed palladium-nickel finishes was Abstract-It is well known that good wear performance can be obtained with Au-flashed palladium-nickel sphere on flat contacts simulating various connector applications. It has also been shown that these good wear properties could be recorded for asymmetrical coatings such as a noble metal sphere on Auflashed palladium-nickel. In this paper we investigate the fretting properties of such asymmetrical coatings particularly of Ag versus Au flashed PdNi. The contacts are submitted to severe fretting wear by applying imposed sine displacements of amplitude similar to the Hertz contact radius. The contact resistance, the friction force and wear evolutions were analyzed and correlated to 3D profilometry measurements allowing wear volumes quantification and EDS elemental identification of the wear tracks.

Very different behaviors are observed whether the silver coating is on the sphere or on the plane though in both case the embedding of the oxidized wear debris by silver improves the fretting behavior. The transfer of silver from the sphere contact onto the palladium nickel flat is seen to be beneficial to the electrical stability of the interface for a long time, leading to more stable results than even silver/silver contacts. Nevertheless an unusual phenomenon of the galling type is observed for which more investigations are needed. INTRODUCTION The development of connected consumer devices is causing an exponential rise of the number of connector with at the same time a necessity for reliability and cost reduction. Though gold has been the favored finish for many years other alternatives have been developed, such as palladium alloys in the 80s and hard silver. The concerns were mainly frictional polymer and tarnish film formation for these two types of coatings respectively but different types of processes (alloying palladium, anti-tarnish top coat), have been developed to avoid investigated both in symmetrical and in dissimilar configurations. The main results involve large amplitude friction simulating connector's insertion/extraction. The aim of this study is to describe and bring understanding on the contact behaviour of dissimilar pairs during fretting experiments which are known to involve more complex mechanisms. The chosen contact materials are hard silver and gold flashed palladium nickel for which no studies were found in the literature.

The questions we want to address are:

• what are the wear mechanisms involved when such dissimilar contacts are fretted?

• if transfer occurs what is its direction and what is the composition of the contact interface and its resulting effect on the contact resistance?

II. SAMPLES AND EXPERIMENTAL TECHNIQUES

A. Samples

Contacts will be referred to as coating1/coating2, the first being the cap and the second the flat through all the paper.

Contacts are of the sphere on flat type: the spheres are stamped in strips of CuSn6 substrate and have a radius of 1.4mm. A nano-crystalline nickel under layer (1.3 to 1.5µm thick) was first plated on the substrates. Various coatings were then electroplated on industrial reel-to-reel plating lines to allow a good reproducibility.

A palladium nickel alloy 80%wt Pd and 20%w Ni was deposited from a low ammonia plating bath. A cobalt hardened gold flash 50 to 80nm thick was then electrodeposited on top. This coating is referred to as GF-PdNi.

A low free cyanide silver plating bath with no alloying was used for the silver coatings. Plating parameters were adapted to obtain hard silver deposits which are referred to as HAg. They are without any top coating of any kind.

The symmetrical and dissymmetric contacts studied are described in table1.

B. Characterization techniques and experimental test

Fretting tests were performed with a dedicated device involving an electro-dynamic shaker working at constant displacement. The conditions were: d=+/-25µm displacement, 10Hz and normal load 1N. The contact radius calculated with the Hertz formulation is 25µm for the studied substrate. The tests were carried out at ±25µm, in order to be in fretting gross slip mode and not in reciprocating sliding mode. During the test, the voltage drop in the contact was measured with an acquisition card (333 10 3 samples/s) with a DC current value of 20mA and voltage limit at 250mV. The friction force was simultaneously recorded and a "maximum friction force per cycle" was calculated. The experiments were done at room temperature (23°C) and 55% relative humidity.

The displaced matter due to deformation, transfer, particle ejection was evaluated with a Bruker Contour GT-X 3D Optical Profiler. 3D images, profiles and wear volumes were measured thanks to the dedicated software. The wear volumes were calculated for the cap 𝑉 𝑐𝑐𝑐 and the flat 𝑉 𝑓𝑓𝑐𝑓 , taking into account positives volumes due to transfer and negative ones due to deformation and wear; a total wear volume for the contact was then calculated as the sum 𝑜𝑜 𝑉 𝑐𝑐𝑐 and 𝑉 𝑓𝑓𝑐𝑓 .

Scanning electron microscope (SEM) images of the wear tracks were obtained with a ZEISS EVO MA10 microscope fitted with an Oxford Instruments Xmax probe for Energy Dispersive X-ray Scanning (EDS). Element mapping was performed to analyze the composition of the wear tracks and calculation of the compositions in %at was performed for selected zones. The analyses were done at 15keV, the depth of analysis was therefore less than 1µm. Careful calibrations were performed to evaluate the accuracy of the composition calculations. For example for the 50 to 80nm of gold on the palladium nickel surface the compositions were: gold: 36%at, palladium: 35%at, nickel: 26%at and oxygen: 4%at.

The thicknesses of the various layers were measured by Xray fluorescence. Care was taken in calibration. The roughness was Sa=93nm and Sa=95nm, for HAg and GF-PdNi samples respectively.

The mechanical properties of the layers (hardness and Young modulus) were measured with a NHT 3 Anton Paar nanoindentor at a load of 1mN; a large matrix of measurements was performed for each type of sample to avoid roughness influence.

The Young modulus and hardness values were measured to be: 118GPa and 230H V for the CuSn6 cuprous substrate, 194GPa and 330H V for GF-PdNi, 190GPa and 560H V for nano crystalline nickel and 107GPa and 125H v for hard silver.

Resistivity values were 10 10 -8 , 1.6 10 -8 and 14 10 -8 Ω.m for CuSn6, HAg and GF-PdNi respectively.

III. RESULTS AND DISCUSSION

The electrical behaviour of symmetrical and dissimilar contacts of Table I was assessed in long run tests; they were stopped when the resistance kept very high values for several thousands of cycles. All experiments were repeated several times; after observing the different behaviours the choice was made to compare contacts with the same cap finish. Fig. 1 shows the long run behaviour of HAg/HAg contacts compared to HAg/GF-PdNi contacts and Fig. 2 compares HAg/GF-PdNi contacts to GF-PdNi/HAg ones.

Figure 1 Mean contact resistance per cycle versus the number of cycles for HAg/HAg and HAg/GF-PdNi contacts during fretting (20µm, 10Hz, 1N)

The behaviour of the HAg/HAg contacts is described in Fig. 1 over 200 000 cycles. This is a typical curve obtained several time but note that silver friction and fretting is not perfectly repeatable as it has already been reported [START_REF] Song | Correlation between friction and wear properties and electrical performance of silver coated electrical connectors[END_REF]. Small increases in contact resistance (Rc) to about 10mΩ are recorded repeatedly but on the whole very little electrical degradation is recorded. The behaviour of the HAg/GF-PdNi contact does not show these small peaks of resistance and Rc remains constant during the 150 000 cycles.

Figure 2 Mean contact resistance per cycle versus the number of cycles for GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts during fretting (20µm, 10Hz, 1N)

The electrical behaviour of the GF-PdNi caps on GF-PdNi or HAg flats is described in Fig. 2. Fretting of GF-PdNi/GF-PdNi contacts is characterized by a very fast and irreversible increase of the contact resistance value. The dissimilar GF-PdNi/HAg contact evolution during fretting is characterized by fast increases of Rc followed by quick returns to lower values. The values are mostly higher than 10mΩ but the behaviour is not as catastrophic as for GF-PdNi/GF-PdNi.

To evaluate how the electrical and mechanical mechanisms take place, both the contact resistance values and the friction force ones are plotted with a zoom on the beginning in Fig. 3 and4.

Figure 3 Mean contact resistance per cycle versus the number of cycles for

HAg/HAg, HAg/GF-PdNi, GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts at the beginning of the fretting test (20µm, 10Hz, 1N) Fig. 3 shows the evolution of the contact resistance between 1 and 10 000 cycles of fretting. The values of Rc at different stages of fretting are summarized in Table II. The first observation concerns the initial values of the contact resistance which are: 3.1mΩ for GF-PdNi/GF-PdNi, 3.1mΩ for GF-PdNi/HAg, 2.4mΩ for HAg/HAg and 2.3mΩ for HAg/GF-PdNi contacts.

A quick way to evaluate the contact resistance is with the simple ρ/2a equation. The usual calculation of the contact radius a is done with the Hertz equation with the substrate parameters. Here this would give the right order of magnitude (~2mΩ) but does not account for the observed experimental values. Equation Rc= ρ(πH/4F) 1/2 where H is the hardness of the softer material can be used and gives different values.

Taking the hardness value of the softer material as silver (H=1.2GPa) gives R=3.1mΩ. This value is in agreement with the value for the GF-PdNi/HAg contact where the flat is HAg. When the hemisphere is HAg coated the experimental values are lower (~2.3mΩ) which could be explained by the 5µm thickness of silver on the cap. Finally the all GF-PdNi contacts have a value of 3.1mΩ (similar to the GF-PdNi/HAg) The way in which a very thin film participates to the contact physics is complex and gathering experimental data is a first step to gain some insight. Fig. 4 shows the maximum friction force per cycles: at the beginning of the test a very strong and sharp friction peak (2N) is observed for HAg/HAg contacts while for the GF-PdNi/GF-PdNi contact no peak is recorded and the friction force is very low friction (0.4N) and almost constant. The dissimilar contacts display a smaller peak (~1N) that decreases after about 1000 cycles. The peak corresponding to the HAg cap on GF-PdNi flat seems to be sharper than the GF-PdNi/HAg one probably due to the larger amount of silver that can be deformed.

Figure 4 Maximum friction force per cycle versus the number of cycles for HAg/HAg, HAg/GF-PdNi, GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts at the beginning of the fretting test (20µm,10Hz, 1N)

Both the friction and electrical behaviours of the dissimilar contacts are different whether the cap or the flat is HAg.

Many fretting runs were performed and stopped at various numbers of fretting cycles so that the friction scars could be observed by profilometry, SEM and EDS.

In the next section we investigate the topographies and the compositions of the caps and the flats after 1000 fretting cycles in order to understand the beginning of the evolution of the contact resistance values. Fig. 5 shows the 3D images after 1000 fretting cycles for the four configurations, the first line for HAg/HAg and HAg/GF-PdNi, the second one for GF-PdNi/GF-PdNi and GF-PdNi/HAg. Wear can be characterized by removal of material from the contact area or by addition of transferred material into the contact area.

Both cap and flat are shown for a configuration; the caps have been flattened with the dedicated software. -GF-PdNi/GF-PdNi contacts display barely any scars and GF-PdNi/HAg contacts show transfer onto the cap and wearing of the flat.

We observe that dissimilar contacts of HAg and GF-PdNi behave very differently whether the cap is a GF-PdNi coating or an HAg one. This is correlated to the Rc values: Rc(1000)=1.6mΩ for HAg/GF-PdNi and Rc(1000)=7.6mΩ for GF-PdNi/HAg.

To assess the evolution with the number of cycles the wear volumes have been measured on both the caps and the flats. They are displayed in Fig. 6 for the HAg/HAg and HAg/GF-PdNi contacts. For the HAg/HAg contacts the caps always lose material; the flats show positive wear (transfer) on the wear scar till 60 000 cycles. After that, both the cap and the flat lose material probably as ejected debris. At higher numbers of cycles the debris in the interface can become a harder less ductile third body. Note that at 200 000 cycles the wear is the same on the flat and on the cap. 150 000 cycles there is a very large difference in the amount of transferred material on the flat and the amount of lost material. Fig. 7 shows the evolutions of the wear volumes for the GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts. For the symmetrical GF-PdNi/GF-PdNi contact the caps always lose material. Some transfer is observed on the flats at 1 000 cycles and then the flats also lose material. Note that volume lost on the flat is much smaller than on the cap.

For the GF-PdNi/HAg contact the cap gains material during the first 10 000 cycles and then both sides of the contacts lose material. At 60 000 cycles the flat and the cap lose the same amount of material. This allows knowing the direction of the transfer and additional information is given by plotting the total volume of wear for the contact such as in Fig. 8 for contacts with an HAg cap. It can be seen that the total wear for HAg/HAg contacts is rather high (200 10 3 µm 3 ) at the beginning and then diminishes to 10 3 µm 3 (60 0000 cycles). During this period of time the Rc values increase intermittently to about 10 mΩ; after 60 000 cycles the Rc values increase slowly and the wear volume stays about constant over 200 000 cycles. For the HAg/GF-PdNi contact the total wear volumes has about the same value but increases by more than a factor of 5 between 100 000 and 150 000 cycles while the contact resistance remains the same (2.7mΩ).

At the same time there is no evidence of degradation of the electrical properties for HAg/GF-PdNi contacts. Fig. 9 displays the total wear volumes of GF-PdNi caps on GF-PdNi or HAg flats. Much smaller volumes of wear are recorded (from 10 to 100 10 3 µm 3 ); these values slowly increase with the number of cycles. The behaviour is about almost the same for both configurations of contacts while their electrical properties are very different. All the caps and flats at different stages of fretting were observed by SEM and analysed by EDS. Maps showing elements were acquired and composition was calculated on carefully selected areas corresponding to the wear tracks. From all these maps several were selected, described below and analysed in correlation to the wear volumes. The analysis performed after 10 000 cycles are different: the wear scars show less exposed nickel spots (Fig. 11). All the composition data (in %at) are shown in [START_REF] Torrealba | Durability of some asymmetrical contact pairs for connector application[END_REF]. We have chosen to show here the evolution of the silver % in the wear tracks as plotted in Fig. 12. The % silver decreases when the quantities of nickel, oxygen and eventually copper increase. For HAg/HAg the % of silver in the wear tracks remains above 20% for the cap and the flat till 200 000 cycles; though the Rc values rise slowly in this laboratory fretting test it is not indicative of drastic failure.

The fretting scars for HAg/GF-PdNi are shown in Fig. 13 for 1 000 cycles: the transfer of silver from the cap to the flat is well observed. After 10 000 cycles, Fig. 14, some silver is transferred back onto the cap. The % of silver is plotted in Fig. 16 and up to 60 000 it is around 40%. Finally at 150 000 cycles (Fig. 15) the silver % drops because the copper % is about 40%. This is not linked to a contact resistance increase because it is assumed that a mixture of gold copper and silver is formed following a phenomenon of "galling". Galling is known to be due to strong adhesion between surfaces; unlike cold welding which is often associated with vacuum or high loads, galling can occur at microscopic scale and involves mechanically assisted surface diffusion. It does not exclude motion as in the present case. The EDS maps of GF-PdNi/HAg contacts after fretting are shown below. Fig. 18 shows that after 1 000 fretting cycles the GF-PdNi caps wear scar is mainly silver from the flat. The % of silver at this point is 74% on the cap and 67% on the flat. The contact resistance value is higher (Rc1000=7.6mΩ) than at the beginning of the run but the high percentage of silver prevents it from rising quickly. Fig. 19 after 10 000 cycles shows the transfer of silver occurs in both directions from one part of the contact to the other. The composition of silver % plotted Fig. 21 shows a strong decrease of silver (~20% at 10 000 cycles) but enough is still inside the surface of contact to allow a value of resistance of 29mΩ. Large fluctuations of the contact resistance were shown in Fig. 2 but even after 60 000 cycles its value is about 68mΩ which is several orders of magnitude lower than for GF-PdNi/GF-PdNi contacts. Fig. 20 shows the elements inside the wear tracks: much copper is seen on the cap wear track and some on the flat on. Never the less the small quantity of silver plotted in Fig. 21 (3% for the cap and 12% for the flat) is enough to avoid a drastic increase by allowing some conducting paths through silver (the oxidized debris are embedded in some silver). When the flat coating is silver and the cap one is gold flashed palladium nickel reasonably low resistance increase is observed during fretting due to the embedding of the poorly conducting debris in silver.

When the cap coating is silver and the flat one is gold-flashed palladium nickel, strong adhesion occurs. Transfer of silver onto the flat is observed followed by the formation of an alloy type material composed of silver-gold and copper which remains in the contact interface during the fretting cycles until accumulated stresses cause a galling phenomenon exposing the copper substrate. This could be avoided for example by the use of a lubricating coating diminishing the adhesion: future work will investigate such path.

  -PdNi/GF-PdNi b) HAg/GF-PdNi c) GF-PdNi/HAg d) HAg/HAg maybe because of the influence of the gold layer on both the cap and the flat.

Figure 5

 5 Figure 5 3D profilometric images of the fretting scars after 1 000 cycles for: a) and b) HAg/HAg cap and flat, c) and d) HAg/GF-PdNi cap and flat, e) and f) GF-PdNi/GF-PdNi cap and flat, g) and h) GF-PdNi/HAg cap and flat. -After 1000 cycles the HAg (a) caps show deep hollow scars and transfer on the flat (b).-GF-PdNi/GF-PdNi contacts display barely any scars and GF-PdNi/HAg contacts show transfer onto the cap and wearing of the flat.

Figure 6

 6 Figure 6 Wear volumes for the caps and the flats versus the number of fretting cycles; HAg/HAg and HAg/GF-PdNi contacts.For the HAg/GF-PdNi contact the caps always lose material and the flat always gain material. Note that for

Figure 7

 7 Figure 7 Wear volumes for the caps and the flats versus the number of fretting cycles; GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts.

Figure 8

 8 Figure 8 Total wear volumes versus the number of fretting cycles for HAg/HAg and HAg/GF-PdNi contacts.

Figure 9

 9 Figure 9 Total wear volumes versus the number of fretting cycles for GF-PdNi/GF-PdNi and GF-PdNi/HAg contacts.

Fig. 10

 10 Fig. 10 shows the element maps (cap and flat) of the HAg/HAg contact after 1 000 fretting cycles. Both the cap and the flat show that nickel is present in the track -either due wear off (of the cap) or wear off and pile-up on the flat. Note that the nickel exposed on the cap is not yet oxidized while both metallic nickel and oxidised nickel are detected on the flat as shown in zone a) and b) respectively.

Figure 10

 10 Figure 10 EDS maps of HAg/HAg contacts after 1 000 fretting cycles (Rc=5.6mΩ).

Figure 11 EDS

 11 Figure 11 EDS maps of HAg/HAg contacts after 10 000 fretting cycles (Rc=2.52mΩ).

Figure 12 %

 12 Figure 12 % silver in the wear scar versus the number of fretting cycles for the cap and the flat of HAg/HAg contacts

Figure 13 EDS

 13 Figure 13 EDS maps of HAg/GF-PdNi contacts after 1000 fretting cycles (Rc=2.1mΩ)

Fig. 17

 17 Fig.17shows an element map for GF-PdNi/GF-PdNi after fretting for 30 000 cycles; it is typical of all the maps observed for this type of contacts. As abrasive wear occurs the

Figure 17

 17 Figure17EDS map of GF-PdNi/GF-PdNi contacts after 30 000 fretting cycles (Rc=2.7Ω)

Figure 18

 18 Figure 18 EDS map of GF-PdNi/HAg contacts after 1 000 fretting cycles (Rc=7.2mΩ)

TABLE I :

 I THICKNESS AND COMPOSITION OF THE HAg AND GF-PdNi SAMPLES.

			Ag (µm)	Ni (µm)
	HAg	Cap: 4.5-5.0	1.3-1.5
		Flat: 2.5-3.0	1.3-1.5
		Au (nm)	PdNi (µm)	Ni (µm)
	GF-PdNi	50-80	0.7-0.8	1.3-1.5

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