Gold-silver-alloys as inner layers of multi-metal stacks in electrical connectors

Abstract

The present invention is directed to a use of an alloy layer as an inner layer of a multi metal stack in electrical connectors as well as the metal stack as such and an electrical connector. The alloy comprises mainly the metals Au and Ag.

Description

[0001] Gold-Silver-Alloys as Inner Layers of Multi-Metal Stacks in Electrical Connectors

[0002] Description

[0003] The present invention is directed to an use of an alloy layer as an inner layer of a multi metal stack in electrical connectors as well as the metal stack as such and an electrical connector. The alloy comprises mainly the metals Au and Ag.

[0004] Electrical contacts are used today in virtually all electrical appliances. Their applications range from simple plug connectors to safety-relevant, sophisticated switching contacts in the communications sector, for the automotive industry or for aerospace technology. Here the contact surfaces are required to have good electrical conductivity, low contact resistance with long-term stability, as well as good corrosion and wear resistance with insertion forces that are as low as possible.

[0005] In electrical engineering, plug contacts are often coated with a hard-gold alloy layer, consisting of gold-cobalt, gold-nickel or gold-iron. These layers have a good resistance to wear, a good solderability, a low contact resistance with long-term stability, and good corrosion resistance. Due to the rising price of gold, less expensive alternatives are being sought. In US20020106944A a gold-nickel layer is used as an intermediate layer in a metal connector stack.

[0006] In particular, mobile phone connectors like USB-C must be resistant against in particular NaCI-containing atmospheres while charging. Typically, Gold layers are used as inter- or as final layer. But as Gold layers will be dissolved easily by NaCI containing solutions, the final Au layer is mostly replaced by Rh, RhRu, Pt or PtRu as final layer. Gold as interlayer as activation of the base layer or to maintain solderability is still commonly used and causing poor corrosion performance especially under anodic polarization at charging of the electronic devices.In CN107146964A a multi-metal layer stack has been proposed. An electroplated coating for terminals, which comprises a copper plating electroplated on the surface of a terminal. The surface of the copper plating is sequentially plated with an inner nickel-tungsten plating, an inner gold plating, a palladium plating, an outer gold plating and a rhodium ruthenium plating.

[0007] EP3417089A discloses an article comprising: a substrate; a coating formed on the substrate, the coating comprising: a first metallic layer formed on the substrate; a second metallic layer formed on the first metallic layer; a third metallic layer formed on the second metallic layer; and a fourth metallic layer formed on the third metallic layer. Preferably, the third metallic layer comprises gold as an inner metallic layer.

[0008] In addition, EP4121285A deals with a multi-metal layer stack for electrical connectors. Proposed here is an article for providing an electrical contact, the article comprising: a substrate; a first layer over the substrate; and a second layer over the substrate, wherein the first layer comprises a nanocrystalline nickel-tungsten alloy, and wherein the second layer comprises a nanocrystalline palladium-based alloy comprising a second metal. In a preferred aspect, the article comprises a third layer, the third layer comprising Au and / or Pd. Also here, an inner metallic layer of gold may be used below the outer Pd or Pd-alloy layer.

[0009] Even if a gold layer is introduced into a metal stack used in electrical contact elements as an inner metal layer the corrosion resistance is often low, in particular, if the connector is used in NaCI-containing atmosphere, e.g. at the seaside. Furthermore, gold is rather expensive, so a less costly application is heavily appreciated.

[0010] The present invention is concerned with the objective to minimize costs of the electrical connectors and, in particular, increase the corrosion resistance of known.

[0011] These and other objectives are dispensed with by an use according to claim 1 - 8. Preferred uses of the present invention are identified in claimsdependent from claim 1. Claims 9 and 10 are directed to a respective stack and an electrical connector.

[0012] Through using an electrodeposited inner alloy layer comprising Au and Ag in a multi-layer metal stack in electrical contacts the corrosion resistance of this metal stack is greatly improved over those having pure gold platings instead. The multi-layer metal stack comprises in this sequence at least a first elec-trolytically deposited metal layer, the inner alloy layer and a third electrolyt-ically deposited metal layer. Advantageously, the enhancement of the corrosion resistance is achieved without losing other important characteristics like soldering behavior, scratch resistance or conductivity of the stack. It is very surprising that the advantages provided by the present invention are solely achieved by substituting in pure intermediate Au-layers silver for some of the gold. Furthermore, this inventive approach is making the Au-layer less costly through using more Ag.

[0013] The composition of the inner alloy layer comprising Au and Ag can be achieved by those skilled in the art. Electrolytes for electrolytically depositing Au and those for depositing Ag are known in the art (Technologies for the Electrodeposition of Metals and Alloys: Electrolytes and Processes, > Gam '

[0014]

[0015] < & Gm .mari. Published: 11 June 2011, Publisher Name: Springer, New York, NY). These are normally basic electrolytes comprising cyanides. These electrolytes can be mixed in the required ratio to electrolytically deposit the AgAu-alloy layer on a respective substrate. The substrate serves as a cathode in an electrolytic cell (https: / / www.91 lmetallur-gist.com / wp-content / uploads / 2016 / 10 / Non-Cyanide-Electrolytes-for-Gold-Plating.pdf). Besides these cyanide electrolytes for Ag and Au, also there are known cyanide-free electrolytes which can be used as mentioned above (https: / / link.springer.com / article / 10.1007 / sll581-020-03541-5). In principal these alloys are well known already in the area of jewelry (US4487664, GB2046794A).

[0016] The alloy can contain minor amounts of other metals. However, other metals are present in the inner alloy layer to not more than 1 wt.-%, more preferred less than 0.5 wt.-%, and most preferred less than 0.1 wt.-% of the alloy. Ina very preferred aspect the electrolyte used according to the invention comprises other metals selected from the group consisting of Sb, Se, Te, Bi. These other metals can be added to the electrolyte used according to the invention in the respective amounts. The skilled person knows which compounds can come into consideration here (EP3870739A). In a very preferred aspect the inner alloy layer is substantially consisting of Ag, Au and Te.

[0017] The desired ratio of Au to Ag substantially depend upon the Au to Ag-ratio in the electrolytic bath. It is advantageous from a cost point of view to exchange as much Au by Ag in the inner alloy layer. However, the Au content of the inner alloy layer should not drop below 20 wt.-%, better not below 25 wt.-% of the layer in order not to jeopardize the positive characteristics of an intermediate gold layer. In a preferred aspect, the Au is present in that alloy in a range of 20 - 80 wt.-%, more preferably 30 - 70 wt.-% and most preferred 40 - 60 wt.-%. Here, costs, corrosion resistance and the positive characteristics of an intermediate gold layer form the best compromise. In a further preferred aspect the rest of the alloy substantially is Ag except for some impurities as mentioned above. A very preferred disclosure for an electrolytic deposition of this inner alloy layer can be found e.g. in: Minimizing gold amount by alloying silver in hard gold, Son et al. in Sustainable Materials and Technologies, Vol. 39, 2024.

[0018] As already mentioned, the inner alloy layer is deposited on first metallic layer. This first metallic layer is preferably a metal or metal alloy selected from the group of CuSn(Zn), Ni, NiPd, NiW, NiP, NiWP, Pd or Pd-alloy, Ag or Ag-alloy https: / / koreascience.kr / article / JAK0201020451739721.pdf. Layers of CuSn(Zn), Ni, NiPd, NiW, NiP, NiWP, Pd or Pd-alloy, Ag or Ag-alloy and the way they are electrolytically deposited onto e.g. electrically conductive substrates are known to those skilled in the art (https: / / link.springer.com / arti-cle / 10.1007 / sl0800-006-9162-7). The substrates can be of any kind, but for the purpose of electrical connectors the substrate most likely will be a copper or copper-alloy material as well as stainless steel. It is also conceivable that the preferably electrically conductive substrate may be covered with a Cu-layer. Onto this substrate a first layer of a metal or metal alloy is deposited electrolytically.

[0019] As an example, a Ni-layer can electrolytically deposited onto copper (https: / / link.springer.com / article / 10.1007 / s00339-018-1861-5). Once the Ni-layer has been created as a next step the inner alloy layer comprising Au and Ag is electrolytically deposited onto the Ni-layer as mentioned above. It should be considered that between the Ni-layer and the inner alloy layer a further layer can optionally be present that can enhance the adhesion of the inner alloy layer to the first metal layer if desired. Such layers for increasing the adhesion are preferably those selected from the group consisting of Au, Au-alloys other than AuAg (e.g. AuCo, AuNi, AuFe, AuCu, AuFeln), Ag, Ag- alloys other than AgAu (e.g. AgSb, AgBi, AgGe, AgPd, AgPdTe), Pd, Pd-alloys (e.g. PdNi, PdCo, PdFe, Pdln, PdSn). The skilled person knows howto produce them (https : / / books. google. com / books / about / Electroplating. html?id =Ii le- pplpqOC).

[0020] Once the first metal layer has been electrolytically deposited onto the substrate and the inner alloy layer comprising Au and Ag has been established as mentioned above, a further third layer is electrodeposited onto the inner alloy layer preferably comprising Rh or Rh-alloy or Pt or a Pt-alloy. As for the first metal layer and the inner alloy layer also between the inner alloy layer and the third metal layer an intermediate layer may optionally be positioned to enhance the adhesion between both layers (see intermediate layer as mentioned above). Metallic layers of comprising Rh or Rh-alloy or Pt or a Pt-alloy and the way they are electrolytically deposited onto the inner alloy layer are known to those skilled in the art (Three new platinum (alloy) electrolytes in the Umicore Electroplating News at a glance Metal Deposition

[0021]

[0022] The multi-layer stacks in which the inner alloy layer is used can comprise the following composition:

[0023] First underlayer Inner alloy layer Third outer layer Ni AuAg Rh

[0024] NiPd AuAg Rh

[0025] NiW, AuAg Rh

[0026] NiP AuAg Rh

[0027] NiWP AuAg Rh

[0028] Pd AuAg Rh

[0029] Pd-alloy AuAg Rh

[0030] Ag AuAg Rh

[0031] Ag-alloy AuAg Rh

[0032] Ni AuAg Rh-alloy

[0033] NiPd AuAg Rh-alloy

[0034] NiW, AuAg Rh-alloy

[0035] NiP AuAg Rh-alloy

[0036] NiWP AuAg Rh-alloy

[0037] Pd AuAg Rh-alloy

[0038] Pd-alloy AuAg Rh-alloy

[0039] Ag AuAg Rh-alloy

[0040] Ag-alloy AuAg Rh-alloy

[0041] Ni AuAg Pt

[0042] NiPd AuAg Pt

[0043] NiW, AuAg Pt

[0044] NiP AuAg Pt

[0045] NiWP AuAg Pt

[0046] Pd AuAg Pt

[0047] Pd-alloy AuAg Pt

[0048] Ag AuAg Pt

[0049] Ag-alloy AuAg Pt

[0050] Ni AuAg Pt-alloy

[0051] NiPd AuAg Pt-alloy

[0052]

[0053] NiW, AuAg Pt-alloy NiP AuAg Pt-alloy

[0054] NiWP AuAg Pt-alloy

[0055] Pd AuAg Pt-alloy

[0056] Pd-alloy AuAg Pt-alloy

[0057] Ag AuAg Pt-alloy

[0058] Ag-alloy AuAg Pt-alloy

[0059]

[0060] The layer as presented above have certain thicknesses. Preferably, the inner alloy layer has a thickness of 0,01 - 5 pm, more preferably of 0,05 - 3 pm and most preferably 0,05 - 2 pm. Also the first layer preferably has a thickness of 1 - 10 pm, more preferably of 1 - 8 pm and most preferably 2 - 5 pm. The third layer has a thickness of 0,1 - 5 pm, more preferably of 0,2 - 3 pm and most preferably 0,3 - 2 pm.

[0061] Preferably, it can be that between the layers exemplified above a further thin intermediate layer can be placed which enhance the adhesion of the first to third layers. These intermediate layers can be selected from the group consisting of (see above)v These intermediate layers are preferably rather thin and normally have a thickness of 2 - 500 nm, more preferably 20 - 300 nm and most preferably 50 - 200 nm. The skilled reader knows how to galvanically deposit these intermediate layers on the first and / or inner alloy layer (see above).

[0062] An inner alloy layer comprising Au and Ag as mentioned above serves in a multi-metal layer stack for electrical connectors as a means to enhance the corrosion resistance of such a layer. In a preferred aspect of the invention the corrosion resistance is enhanced by a factor of >2, more preferably by a factor of >3 and most preferably by a factor of >5 compared to an otherwise equal inner layer of pure AuCo of the same thickness.

[0063] In a further aspect, the present invention is directed to a metal stack comprising at least three layers and having a first electrodeposited metal layer comprising CuSn(Zn), Ni, NiPd, NiW, NiP, NiWP, Pd or Pd-alloy, Ag or Ag-alloy(preferably without Au), onto that layer an electrodeposited inner metal layer comprising Au and Ag and onto that layer a third electrodeposited layer comprising Rh or Rh-alloy or Pt or a Pt-alloy. The preferred aspects mentioned for the inventive use of the inner alloy layer apply mutatis mutandis also for the metal stack.

[0064] In a further embodiment, an electrical connector comprising a metal stack as mentioned above is claimed. These electrical connectors are well known as e.g. USB-C-ports or Pogo Pins (https: / / en.wikipe ia.org / wiki / PO' w-l). The connectors are used in female ports as well as male connectors to load electrical devices or to transmit data.

[0065] Figures:

[0066] Fig. 1: Exemplified is a process for manufacturing a multi-layer metal stack for electrical connectors.

[0067] Fig. 2: Shows a multi-layer metal stack of the present invention, with:

[0068] 1: being the substrate

[0069] 2: being the first layer

[0070] 3. being the inner alloy layer

[0071] 4: being the third layer.

[0072] Fig. 3: Shows an electrical connector (10) in the form of a fire-wire connector; (11) shows the plastic cover of the connector; (14) shows the part that is inserted into the port with its front side (15); (12) the plastic holder for the metal stacks (13).

[0073] Fig. 4: Shows a schematic sketch of a 0.2 dm2brass sheet for the ASET with 2 possible spot areas / measurement points indicated as crosses.Experimental Report:

[0074] This experimental report is directed towards a corrosion test protocol in which different multi-metal stacks are subjected to a corrosive environment. The test is called the ASET (Artificial Sweat Electrochemical Test).

[0075] An Artificial Sweat Electrochemical Test is used to simulate the effects of sweat on electronic devices and components. This test is particularly important for wearable electronics such as smartwatches, fitness trackers and other devices that frequently come into contact with human skin and human sweat. These devices should be resistant to sweat and water to prevent corrosion and malfunctions, especially during the charging process of this devices.

[0076] Such an ASET test uses an artificial sweat solution that mimics the composition of real human sweat, in this case a sodium chloride solution. This solution is applied to the contact materials of multi-metal stacks from which the electronic components are made, to test their resistance to corrosion, discoloration and other possible damage under the influence of electric current.

[0077] The Artificial Sweat Electrochemical Test is performed in several steps to test the resistance of electroplated contacts or their layers / stacks to sweat, simulating the contacts of electronic devices. Here is a general overview of the process.

[0078] 1. ASET sample preparation:

[0079] 1.1. Substrate Preparation

[0080] As a substrate for the multi-metal stacks to be tested in the ASET 0,2 dm2polished brass sheets with 0,2 dm2area are used. It is best to choose sheets with a surface that is as flawless as possible.

[0081] 1.2. Coating processEach ASET sample is individually pre-treated and coated. Always do the plating processes wet-on-wet and run the sheets into the plating processes under current.

[0082] 1 Ultrasonic degreasing 2 Min - 2 Cathodic degreasing 1 Min 5 - 6 V

[0083] 3 Acid dip 30 Sek - 4 Nickel electrolyte 15 Min 5 A / dm2(~ 10 pm) 5 Cathodic degreasing 1 Min 5 - 6 V

[0084] 6 Acid dip 30 Sek - 7 Electrolyte to be tested (top-layer) - -

[0085]

[0086] Between the individual steps always rinsing takes place.

[0087] 1.3. Layer thickness measurement with X-Ray

[0088] The thickness and alloy composition of all coated samples must be determined by XRF measurement before the ASET test.

[0089] The coating thickness used for the evaluation is the mean value obtained from a measurement at each of the two possible ASET measuring points on the sample, i.e. the mean value obtained from two measured values. For alloys, the composition must also be measured and specified.

[0090] Fig. 4 shows a sketch of a sample plate with the 2 possible spot areas / meas-urement points.

[0091] Tolerance range of the coating thickness at 0.8 pm target coating thickness is ±0.05 pm. If possible, measure the coating thickness on the same X-ray device and with the same measurement program, as significant differences can already occur here.2. Examination of ASET samples:

[0092] The examination of the ASET samples is done via optical comparison of the surfaces using a microscope and photographic documentation:

[0093] At 0 cycles of ASET (initial state), take 4 images with centered alignment (at 20x, 200x, 500x and lOOOx magnification).

[0094] Then the samples are then exposed to the artificial sweat solution under the influence of an electrical charge with 5 voltage for a certain time to simulate the effect of sweat on the contacts during the charging process. The artificial sweat solution is composed as follows:

[0095] 225 ml of the ASET solution with a sodium chloride concentration of 50 g / l NaCI (p.a. quality), is placed in a 250 ml beaker at 40 °C, and stirred with 50 - 60 rpm (2 cm stirring rod).

[0096] Now, set the rectifier to a voltage of 5 V. Use platinized titanium as a cathode on one side and on the opposite an anode holder made of titanium with a clamp to hold the ASET sample in place.

[0097] One cycle of the ASET corresponds to 10 minutes and is monitored by a timer. After 1 cycle, the sample is rinsed, dried and again and 4 images are taken at 20x / 200x / 500x / lOOOx magnification.

[0098] After that, the components are examined for corrosion, discoloration and other possible damage via optical comparison of the surfaces using a microscope and photographic documentation.

[0099] If no corrosion is visible, continue with another cycle and images until the first significant corrosion occurs. The maximum number of cycles until the test is cancelled is 7 cycles or 70 minutes. If corrosion is visible before 7 cycles are reached, test is stopped and images are taken at 20x, 200x, 500x and lOOOx magnification.3. Determination of corrosion

[0100] The corrosion is indicated in minutes, when the first signs of corrosion occur.

[0101] To determine the corrosion, the images of a sample at 0 cycles and X cycles are compared. A distinction must be made between what constitutes a cor-rosion pore and whether this pore was already present at 0 cycles, in which case it must not be counted. The main focus is on the start of corrosion. If you are unsure whether i.e. a black dot or a small crack on the surface represents a corrosion product, you should continue the test cycles until it can be clearly stated that corrosion is occurring at this point.

[0102] The results are documented and analyzed to determine whether the layers meet the requirements and are usable for electronic devices. This test is particularly important for portable electronics that often come into contact with human skin, such as smartwatches and fitness trackers.

[0103] 4. Results of ASET tests:

[0104] Stack No. Layer sequence ASET result 1 23,7 ct AuCo - 0,1 pm 20 min Pure platinum - 0,8 pm

[0105] 2 12 ct AuAg - 0,1 pm 56 min Pure platinum - 0,8 pm

[0106] 3 Pure palladium - 0,5 pm 68 min

[0107] 12 ct AuAg - 0,1 pm

[0108] Pure platinum - 0,8 pm

[0109] 4 12 ct AuAg - 0,5 pm 56 min Platinum ruthenium alloy 90:10 - 1 pm

[0110] 5 23,7 ct Au Co - 0,5 pm 10 min Platinum ruthenium alloy 90:10 - 1 pm

[0111]

[0112] 5. Summary

[0113] An Artificial Sweat Electrochemical Test (ASET) is used to simulate the effects of sweat on electronic devices and components. This test is particularly important for wearable electronics such as smartwatches, fitness trackers and other devices that frequently come into contact with human skin and human sweat. These devices should be resistant to sweat and water to prevent corrosion and malfunctions, especially during the charging process of this devices. Such an ASET test uses an artificial sweat solution that mimics the composition of real human sweat. This solution is applied to the contact ma-terials from which the electronic components are made to test their resistance to corrosion, discoloration and other possible damage under the influence of electric current. As can be seen from the above table, the results with having an AuAg-inner alloy layer are superior to the layers most often used in the prior art (AuCo).

Claims

Claims1. Use of an electrodeposited inner alloy layer comprising Au and Ag for enhancing the corrosion resistance of a multi-layer metal stack in electrical contacts comprising in this sequence at least a first electrolytically deposited metal layer, the inner alloy layer and a third electrolytically deposited metal layer.

2. Use according to claim 1,characterized in that,Au is present in that alloy in a range of 20-80 wt.-%.

3. Use according to claim 1 or 2,characterized in that,the inner alloy layer is electrolytically deposited on a first layer of CuSn(Zn), Ni, NiPd, NiW, NiP, NiWP, Pd or Pd-alloy, Ag or Ag-alloy.

4. Use according to one of the preceding claims,characterized in that,a third layer of Rh or Rh-alloy or Pt or a Pt-alloy is electrolytically deposited onto the inner alloy layer.

5. Use according to one of the preceding claims,characterized in that,the inner alloy layer has a thickness of 0,05 - 5 pm.

6. Use according to one of the preceding claims 3 - 5,characterized in that,the first layer has a thickness of 1 - 10 pm.

7. Use according to one of the preceding claims 4- 6,characterized in that,the third layer has a thickness of 0,1 - 5 pm.

8. Use according to one of the preceding claims,characterized in that,the corrosion resistance is enhanced by a factor of >2 compared to an otherwise equal inner layer of pure AuCo.

9. Metal stack comprising at least three layers and having a first electrodeposited metal layer comprising CuSn(Zn), Ni, NiPd, NiW, NiP, NiWP, Pd or Pd-alloy, Ag or Ag-alloy, onto that layer an electrodeposited inner metal layer comprising Au and Ag and onto that layer a third electrodeposited layer comprising Rh or Rh-alloy or Pt or a Pt-alloy.

10. Electrical connector comprising a metal stack of claim 9.

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