Silver coating for high temperature applications

Abstract

To provide silver coating for high temperature applications.SOLUTION: The present invention comprises an article including a series of metal layers arranged to suppress poor adhesion between the metal layers, and a producing method of the same, in which the metal layers include a silver top coat, a silver interlayer, a silver-tin alloy layer and a nickel layer, and the layers adhere to a substrate including copper or a copper alloy.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to high temperature resistant silver coated substrates for high temperature applications. More specifically, the present invention is directed to high temperature resistant silver coated substrates for high temperature applications comprising a series of silver, silver-tin and nickel or nickel alloy layers adjacent to a copper or copper alloy substrate having improved adhesion. [Background technology]

[0002] Silver finishes are widely used in electronic components due to their high electrical conductivity, good solderability, and corrosion resistance. In connector applications, copper alloys are first coated with nickel, followed by a silver finish layer. The nickel acts as a barrier layer, preventing copper diffusion in the silver and preserving the electrical properties of the silver functional layer. This combination has worked well in the electronics industry for many years, operating at low temperatures from about -10°C to about 35°C and moderate temperatures from about 36°C to about 100°C. Advances in technologies such as electric vehicles are significantly increasing the operating temperatures of some electrical connectors.

[0003] In recent years, various industries have shown increasing interest in using silver coatings in high-temperature applications, typically at temperatures exceeding 150°C. At such high temperatures, copper rapidly diffuses into the silver layer, making the use of a nickel barrier layer essential. Additionally, under such high-temperature conditions, oxidation of the nickel underlying the silver can easily occur, potentially resulting in poor adhesion between the nickel and silver. This is illustrated in both Figures 1 and 2. Figure 1 shows a cross-sectional view of a copper substrate with a roughly 3 μm electroplated nickel layer coated with a roughly 4 μm electroplated silver layer. The nickel layer is coated with a very thin, approximately 10–20 nm gold strike layer to inhibit oxygen diffusion into the nickel layer and prevent nickel oxidation. After 1,000 hours of exposure at 200°C, a significant gap developed at the interface between the silver and nickel layers, significantly compromising adhesion between the silver and nickel layers. Figure 2 shows another example of poor adhesion between a silver and nickel layer exposed to high temperatures for extended periods. Figure 2 shows a cross-sectional view of a copper substrate with an approximately 3 μm electroplated nickel layer coated with an approximately 4 μm electroplated silver layer. The nickel layer is also coated with a very thin palladium strike layer of approximately 10-20 nm in an attempt to inhibit nickel oxidation. After approximately 1000 hours of exposure at approximately 200°C, a significant gap developed at the interface between the silver and nickel layers, significantly compromising adhesion between the silver and nickel layers.

[0004] The accelerated oxidation of nickel beneath silver at high temperatures is not yet fully understood. It appears to be caused by oxygen diffusion through the silver grain boundaries. Conventional methods used to enhance the adhesion of silver to nickel, such as gold, palladium, or silver strikes and nickel surface activation, have not been able to overcome the oxidation and adhesion problems in high-temperature applications, as shown in Figures 1-2 for gold and palladium strike layers. Therefore, there is a need for improved metal articles with silver layers for high-temperature applications. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 3,041,255 Summary of the Invention [Means for solving the problem]

[0006] The article includes a substrate comprising copper or a copper alloy, a nickel or nickel alloy layer adjacent to the copper or copper alloy of the substrate, a silver undercoat layer adjacent to the nickel or nickel alloy layer, a silver-tin alloy layer adjacent to the silver undercoat layer, and a silver topcoat layer adjacent to the silver-tin alloy layer.

[0007] The method is: a) providing a substrate comprising copper or a copper alloy; b) depositing a nickel or nickel alloy layer adjacent to the copper or copper alloy of the substrate; c) depositing a silver undercoat layer adjacent to the nickel or nickel alloy layer; d) depositing a silver-tin alloy layer adjacent to the silver undercoat layer; and e) depositing a silver topcoat layer adjacent to the silver-tin alloy layer.

[0008] The articles of the present invention have good adhesive properties between the metal layers and good contact resistance even at high temperatures. [Brief explanation of the drawings]

[0009] [Figure 1] SEM cross section at 10,000x magnification of the silver layer on the gold strike coated nickel layer showing a large gap at the silver / nickel interface after 1000 hours of storage at 200°C. [Figure 2] SEM cross section at 10,000x of the silver layer on the palladium strike coated nickel layer showing a large gap between the silver and nickel interface after 1000 hours of storage at 200°C. [Figure 3]FIG. 1 is an example of a cross-sectional view of an article having a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel barrier layer, the nickel barrier layer being adjacent to a copper-based substrate. [Figure 4] FIG. 15 is a 15,000X SEM cross-section at room temperature of a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel layer adjacent to a copper alloy substrate. [Figure 5] 807x SEM of nickel coated copper substrate with silver layer showing adhesion failure of the silver layer from the nickel after 500 hours of heat aging at 200°C. [Figure 6] FIG. 15 is a 15,000x SEM cross-section of a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel layer adjacent to a copper alloy-containing substrate after storage at 200° C. for 1000 hours. [Figure 7] 1040x SEM of a nickel coated copper substrate with a silver layer showing adhesion failure of the silver layer from the nickel after 1000 hours of heat aging at 200°C. DETAILED DESCRIPTION OF THE INVENTION

[0010] As used throughout this specification, the terms "electroplating," "deposition," and "plating" are used interchangeably. The terms "composition" and "bath" are used interchangeably throughout this specification. The term "adjacent" means adjacent to or next to and bonded where adjacent layers have a common interface. The term "copper-based substrate" means that the substrate comprises at least copper or a copper alloy and may include other materials, such as additional metals, plastics, resins, or other organic and dielectric materials. The term "contact resistance" refers to the contribution to the total resistance to current flow in a system that can be attributed to the contact interfaces of electrical leads and connections. The "Newton" is the SI unit of force, equal to the force exerted by a one-kilogram mass at an acceleration of one meter per second, and corresponds to 100,000 dynes. The term "ohm" refers to the SI unit of electrical resistance and represents the electrical resistance in a circuit that carries one ampere of current when subjected to a one-volt potential difference. The term "aliquot" refers to a portion of a larger whole, especially a sample taken for chemical analysis or other processing. The term "normal force" refers to the force exerted by surfaces to prevent solid objects from passing through one another. The indefinite articles "a" and "an" are intended to include both the singular and the plural.

[0011] The following abbreviations have the following meanings unless the context indicates otherwise: °C = Celsius, g = grams, mL = milliliters, L = liters, ASD = A / dm 2 = amperes per decimeter squared, PVD = physical vapor deposition, CVD = chemical vapor deposition, PCB = printed circuit board or printed wiring board, SEM = scanning electron micrograph, EDX = EDS = energy dispersive X-ray spectroscopy, ASTM = American Standard Testing Method, mN = millinewtons, mOhms = milliohms, cm = centimeters, μm = microns, nm = nanometers, Ag = silver, Sn = tin, Ni = nickel, Cu = copper, EX = example, and NA = not applicable.

[0012] Unless otherwise specified, all percentages and ratios are by weight. All ranges are inclusive and combinable in any order, except where it is logical that such numerical ranges are constrained to add up to 100%.

[0013] 3 shows an article of the invention having a silver topcoat 1 adjacent to a silver-tin alloy layer 2 adjacent to a silver undercoat layer 3 adjacent to a nickel-containing barrier layer 4 adjacent to a copper-based substrate 5. Optionally, the silver topcoat layer can have an anti-tarnish layer (not shown). The article can be used as a component in various electronic devices, such as devices in which the article can be exposed to temperatures of 150° C. or higher, such as 200° C., and still retain good adhesion between the metal layers.

[0014] One or more layers of nickel or nickel alloy are deposited on a substrate containing copper. The nickel acts as a barrier layer, inhibiting copper diffusion into the top layer of silver. The substrate can be substantially all copper or can include one or more copper alloys, such as, but not limited to, copper-tin, copper-silver, copper-gold, copper-bismuth, copper-zinc, copper-nickel, copper-tin-silver, and copper-tin-bismuth. Preferably, the substrate is copper, copper-zinc, or copper-bismuth. The substrate can be a PCB or dielectric material, such as a plastic or resin material, having a copper or copper alloy layer. The nickel or nickel alloy is deposited adjacent to the surface of the copper or copper alloy layer of the substrate, forming an interface with the copper or copper alloy surface. Preferably, the nickel or nickel alloy layer is at least 0.5 μm thick, or between 0.5 μm and 10 μm thick, or between 1 μm and 5 μm thick. The one or more nickel or nickel alloy layers can be deposited by conventional methods used in the art for depositing nickel or nickel alloys on substrates. Most preferably, a nickel layer is deposited adjacent to the copper or copper alloy layer of the substrate. Such methods include, but are not limited to, PVD, CVD, electrolytic, and electroless metal plating. Such methods are well known in the art and literature. Preferably, electrolytic metal plating is used to deposit the nickel or nickel alloy adjacent to the copper or copper alloy of the substrate.

[0015] Preferably, nickel or nickel alloy electroplating is carried out at a current density of at least 0.01 ASD. More preferably, the current density is between 0.1 ASD and 15 ASD, and even more preferably between 0.5 ASD and 6 ASD. Minor experimentation can be used to adjust the current density for a particular substrate. The electroplating process used can be conventional.

[0016] The nickel ions in the plating composition can be provided by using any suitable solution-soluble nickel compound, preferably a water-soluble nickel salt. Such nickel compounds include, but are not limited to, nickel sulfate, nickel chloride, nickel sulfamate, and nickel phosphate. Mixtures of nickel compounds can be used in the plating composition. Preferably, the nickel compound is added to the plating composition in an amount sufficient to provide a nickel ion concentration in the plating composition of 0.1 g / L to 150 g / L, or 0.5 g / L to 100 g / L, or 1 g / L to 70 g / L, etc.

[0017] A wide variety of electrolytes, including acids and bases, can be used in nickel plating compositions. Acidic electrolytes include, but are not limited to, alkane sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, and propanesulfonic acid; arylsulfonic acids such as alkylolsulfonic acid, toluenesulfonic acid, phenylsulfonic acid, and phenolsulfonic acid; amino-containing sulfonic acids such as amidosulfonic acid; sulfamic acid; mineral acids; carboxylic acids such as formic acid and haloacetic acids; hydrohalic acids; and pyrophosphoric acid. Salts of acids and bases can also be used as electrolytes. Furthermore, the electrolyte can include a mixture of acids, a mixture of bases, or a mixture of one or more acids and one or more bases. Such electrolytes are generally commercially available from various sources, such as Aldrich Chemical Company, Milwaukee, Wisconsin.

[0018] A wide variety of surfactants can be optionally used in the nickel plating composition. Anionic, cationic, amphoteric, and nonionic surfactants can be used as long as they do not interfere with nickel plating performance. Surfactants can be included in conventional amounts well known in the art.

[0019] Optionally, the nickel plating composition may include one or more additional components, including, but not limited to, brighteners, grain refiners, and ductility enhancers. Such additional components are well known in the art and are used in conventional amounts.

[0020] The nickel plating composition may optionally contain a buffer. Examples of buffers include, but are not limited to, borate buffers (such as borax), phosphate buffers, citrate buffers, carbonate buffers, and hydroxide buffers. The amount of buffer used is an amount sufficient to maintain the pH of the plating composition at a desired level, and such amounts are well known to those skilled in the art.

[0021] The nickel plating composition can include one or more alloying metals. Such alloying metals include, but are not limited to, tin, copper, and bismuth. Nickel-phosphorus is a preferred alloy. Such metals are provided as their solution-soluble salts, as is well known in the art. Conventional amounts can be included in the nickel plating composition to provide a nickel alloy deposit. Preferably, the alloying metal is removed from the nickel plating bath.

[0022] Suitable electrolytic nickel plating baths are commercially available, as are many disclosed in the literature. Examples of commercially available electrolytic nickel baths are NICKEL GLEAM™ electrolytic nickel products and NIKAL™ SC electrolytic nickel products, both available from Rohm and Haas Electronic Materials, LLC, Marlborough, MA, USA. Further examples of electrolytic nickel plating baths are the Watts-type baths disclosed in U.S. Patent No. 5,629,999.

[0023] The electroless nickel plating composition can include a reducing agent. Preferably, the electroless nickel plating composition includes a reducing agent. Such reducing agents include, but are not limited to, sodium hypophosphite, potassium hypophosphite, thiourea and thiourea derivatives, hydantoin and hydantoin derivatives, hydroquinone and hydroquinone derivatives, resorcinol, formaldehyde and formaldehyde derivatives, DEA (n-diethylamine borane), sodium borohydride, and hydrazine. Such reducing agents can be used in conventional amounts, such as from 0.1 g / L to 40 g / L. Examples of commercially available electroless nickel compositions include DURAPOSIT™ SMT88 electroless nickel and NIPOSIT™ PM980 and PM988 electroless nickel, all available from Rohm and Haas Electronic Materials, LLC.

[0024] The nickel plating composition may have a pH in the range of 1 to 14, preferably 1 to 12, more preferably 1 to 8. The working temperature of the nickel plating composition during plating may be from 10°C to 100°C, or such as from 20°C to 50°C.

[0025] Following the deposition of the nickel or nickel alloy, one or more silver undercoat layers are deposited adjacent to the one or more nickel or nickel alloy layers. The silver can be deposited by conventional methods used in the art, such as electrolytic, electroless, or immersion silver plating. Preferably, the silver is electroplated or electroless plated adjacent to the nickel or nickel alloy. More preferably, the silver is electroplated adjacent to the nickel or nickel alloy layer. The silver undercoat layer preferably has a thickness of at least 0.01 μm, or from 0.05 μm to 2 μm, or from 0.1 μm to 2 μm, or from 0.1 μm to 1 μm.

[0026] Conventional electroplating silver compositions can be used. The silver composition can be a cyanide-containing or cyanide-free silver composition. When a cyanide-containing silver composition is used to plate silver, the silver composition is preferably alkaline. Sources of silver ions include, but are not limited to, potassium silver cyanide, silver nitrate, sodium silver thiosulfate, silver gluconate, silver-amino acid complexes such as silver-cysteine complex, and silver alkylsulfonates such as silver methanesulfonate. Mixtures of silver compounds can be used. The concentration of silver ions in the composition is preferably 2 g / L to 60 g / L. Such silver compounds are commercially available from various sources, such as Aldrich Chemical Company, Milwaukee, Wisconsin. Examples of commercially available silver plating compositions are SILVER GLO™ 3K silver electroplating bath, SILVERJET™ 300 silver electroplating bath, SILVER GLEAM™ 360 silver electroplating bath, and ENLIGHT™ silver plating 600 and 620, all from Rohm and Haas Electronic Materials, LLC.

[0027] A wide variety of conventional surfactants, such as anionic, cationic, amphoteric, and nonionic surfactants, can be used in the silver plating composition. The surfactants can be included in conventional amounts. The silver plating composition can include one or more additional conventional components. Such additional components include, but are not limited to, electrolytes, buffers, brighteners, grain refiners, chelating agents, complexing agents, reducing agents, levelers, and ductility enhancers. Such additional components are well known in the art and are used in conventional amounts.

[0028] The silver plating composition may have a pH in the range of 1 to 14, preferably 1 to 12, and even more preferably 1 to 10. The working temperature of the silver plating composition during silver plating is, for example, 10 to 100°C, or 20 to 60°C. The preferred current density is 0.1 ASD to 50 ASD, and more preferably 1 ASD to 20 ASD.

[0029] Preferably, the silver undercoat layer has a thickness equal to or greater than that of the subsequent silver-tin alloy layer. The silver undercoat layer inhibits the formation of nickel-tin intermetallic compounds from the subsequent silver-tin alloy layer. Furthermore, the silver undercoat layer inhibits the formation of undesirable Kirkendall voids at the nickel interface, which may cause adhesion failure of the article after long-term use. Preferably, the thickness ratio of the silver undercoat layer to the silver-tin alloy layer is within the range of 1:1 to 20:1 or more, more preferably within the range of 1:1 to 40:1.

[0030] The deposited silver-tin alloy layer adjacent to the silver undercoat layer is rich in silver content, being at least 60% by weight silver, with the remainder being tin. The silver-tin alloy layer is ductile and can inhibit oxygen diffusion to the nickel surface, preventing or reducing corrosion. Preferably, the silver-tin alloy is electroplated on the silver undercoat layer. Conventional silver-tin alloy electroplating baths can be used. An example of a commercially available silver-tin alloy electroplating bath is SILVERON™ GT-820 silver-tin alloy electroplating bath, available from Rohm and Haas Electronic Materials LLC.

[0031] The silver-tin alloy electroplating bath contains one or more sources of silver ions. Sources of silver ions include, but are not limited to, water-soluble silver salts such as silver halides, silver gluconate, silver citrate, silver lactate, silver nitrate, silver sulfate, silver alkanesulfonates, and silver alkanolsulfonates. When silver halides are used, the halide is preferably chloride. Preferably, the silver salt is silver sulfate, silver alkanesulfonate, or a mixture thereof, more preferably silver sulfate, silver methanesulfonate, or a mixture thereof. Silver salts are generally commercially available or can be prepared by methods described in the literature. The amount of one or more silver salts used in the bath depends, for example, on the desired alloy composition to be deposited and the operating conditions. Preferably, the silver salt in the bath is present at a concentration of 1 g / L to 100 g / L, preferably 10 g / L to 80 g / L.

[0032] Water-soluble tin ion sources include, but are not limited to, tin halides, tin sulfates, tin alkanesulfonates, tin alkanolsulfonates, and acids. When tin halides are used, the halide is preferably a chloride. Preferably, the tin ion source is tin sulfate, tin chloride, or tin alkanesulfonate, more preferably, the tin ion source is tin sulfate or tin methanesulfonate. Tin compounds are generally commercially available or can be prepared by methods known in the literature. The amount of tin salt used in the bath depends on the desired composition of the alloy being deposited and the operating conditions. Preferably, the tin salt ranges from 0.01 g / L to 80 g / L, more preferably from 0.5 g / L to 40 g / L.

[0033] The silver-tin alloy bath can be acidic or alkaline. Preferably, the silver-tin alloy bath is acidic. Acidic electrolytes include, but are not limited to, alkane sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, and propanesulfonic acid; arylsulfonic acids such as phenylsulfonic acid, phenolsulfonic acid, and toluenesulfonic acid; sulfuric acid; sulfamic acid; hydrochloric acid; hydrobromic acid; fluoroboric acid; and mixtures thereof. Such acid electrolytes can be included in conventional amounts. Preferably, the acid electrolyte is included in an amount of 10 g / L to 400 g / L, more preferably 50 g / L to 400 g / L. Such silver-tin alloy baths have a pH of less than 2, preferably less than 1.

[0034] Conventional surfactants, such as anionic, cationic, amphoteric, and nonionic surfactants, can be included in the silver-tin alloy plating composition. The surfactants can be included in conventional amounts. The silver-tin alloy plating composition can include one or more additional conventional components. Such additional components include, but are not limited to, electrolytes, buffers, brighteners, grain refiners, chelating agents, complexing agents, reducing agents, levelers, and ductility enhancers. Such additional components are well known in the art and are used in conventional amounts.

[0035] Preferably, the current density for depositing the silver-tin alloy is 0.05 ASD or more, more preferably 1 ASD to 25 ASD. The silver-tin alloy can be electroplated at a temperature of room temperature to 55°C, room temperature to 40°C, or room temperature to 30°C.

[0036] The baths can be used to deposit silver-rich silver-tin alloys of various compositions. Preferably, the silver-tin alloys have a silver content of 60% to 95% by weight, with the remainder being tin, more preferably, the silver content ranges from 70% to 90% by weight, with the remainder being tin, and most preferably, the silver content is 75% to 85% by weight, with the remainder being tin.

[0037] A silver topcoat layer is then deposited adjacent to the silver-tin alloy layer. The silver topcoat layer is preferably at least 1 μm thick, such as from 1 μm to 20 μm thick, such as from 1 μm to 10 μm thick, such as from 1 μm to 5 μm thick. The silver topcoat layer enables good electrical conductivity of the article with low contact resistance, even after exposure to high temperatures of 200° C. or higher for long periods of time, such as 1000 hours or more.

[0038] The conventional silver deposition compositions and methods described above for depositing a silver undercoat layer can be used to deposit a silver topcoat layer adjacent to the silver-tin alloy layer. Preferably, the silver topcoat layer is deposited adjacent to the silver-tin alloy layer by electroplating silver using the silver electroplating composition described above. The silver composition can be a cyanide-containing silver composition or a cyanide-free silver composition. Silver electroplating is performed under the same temperature and current density parameters described above for the silver undercoat layer. Silver electroplating is performed until the desired silver thickness is achieved in the topcoat layer. Preferably, silver electroplating is performed until the thickness ratio of the silver topcoat layer to the silver-tin alloy layer is within the range of 2:1 to 100:1, more preferably within the range of 3:1 to 30:1, and even more preferably within the range of 6:1 to 12:1.

[0039] Optionally, an anti-tarnish layer can be deposited on the silver topcoat layer. Conventional anti-tarnish compositions can be used. Commercially available examples of such anti-tarnish materials are NO-TARN™ PM3 anti-tarnish formulation, PORE BLOCKER™ 100 anti-tarnish formulation, and PORE BLOCKER™ 200 anti-tarnish formulation (available from Rohm and Haas Electronic Materials, LLC).

[0040] Preferably, the silver topcoat layer of the article of the present invention is free of gold, palladium, or other metal layers, such as flash layers. The silver topcoat eliminates the need for such additional metal or flash layers. More preferably, the article of the present invention comprises a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel-containing barrier layer adjacent to a copper-based substrate, and optionally, an anti-tarnish layer adjacent to the silver topcoat layer. More preferably, the article of the present invention comprises a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel-containing barrier layer adjacent to a copper-based substrate, and optionally, an anti-tarnish layer adjacent to the silver topcoat layer, wherein the silver topcoat layer is thicker than the silver-tin layer and the silver undercoat layer is equal to or thicker than the silver-tin alloy layer. Even more preferably, the article of the present invention comprises a silver topcoat layer adjacent to a silver-tin alloy layer adjacent to a silver undercoat layer adjacent to a nickel-containing barrier layer adjacent to a copper-based substrate, and optionally, an anti-tarnish layer adjacent to the silver topcoat layer, wherein the thickness ratio of the silver topcoat layer to the silver-tin alloy layer is 2:1 to 100:1, and the thickness ratio of the silver undercoat layer to the silver-tin alloy layer is preferably 1:1 to 20:1 or more, more preferably 1:1 to 40:1.

[0041] A silver topcoat layer having a thickness ratio of 2:1 to 100:1 relative to the thickness of the silver-tin layer provides low contact resistance for the article, thereby improving electrical conductivity. A silver undercoat layer having a thickness ratio of preferably 1:1 to 20:1 or greater, more preferably 1:1 to 40:1 relative to the thickness of the silver-tin layer, suppresses the formation of Kirkendall voids and NiSn intermetallic compounds, improving adhesion of the metal layers of the article. Furthermore, a silver-tin alloy layer having a thickness of 0.05 μm or greater is believed to prevent undesirable oxygen diffusion into the nickel or nickel alloy layer. Oxidation of the nickel or nickel alloy layer can be measured using standard EDS or EDX analysis methods known in the art.

[0042] The articles of the present invention can be used in low and high temperature environments with minimal concern for adhesion failure of the metal layer, and can be used as parts or components in PCBs, electrical connectors, light emitting diodes (LEDs), electric vehicles, and other applications where the silver layer may be exposed to temperatures of 150°C or greater.

[0043] The following examples are included to illustrate the invention, but are not intended to limit the scope of the invention. [Example]

[0044] Examples 1 to 7 The thickness of the electroplated silver and silver-tin alloy layers was measured using a Fisherscope model XDV-SD X-Ray Fluorescence Spectrometer (XRF) available from Fischer. The XRF was calibrated using Bowman silver and tin pure elemental thickness standards, and the composition and thickness of the silver and silver-tin alloy were calculated using a combination of the pure elemental standards and the fundamental parameter (FP) calculations in the XRF instruction manual.

[0045] The crosshatch adhesion test was performed according to ASTM D3359, a standard test method for measuring tape adhesion. This test evaluates the adhesion of a film coating to a metal substrate by applying and removing a pressure-sensitive tape across a cut made in the film with a suitable tool, such as a knife or razor blade. A non-standard pin bending method (frequently applied on production lines) was also used to evaluate the adhesion of the metal layer. This method involved bending a pin to an approximately 90-degree angle and then observing the outside (where material expansion occurs) and inside (where material compression occurs) of the bent pin with an optical microscope to identify any delamination of the layer.

[0046] Contact resistance was evaluated before and after thermal aging using a KOWI 3000 commercial contact resistance measurement device available from WSK Mess-und Datentechnik GmbH, Germany. The digital force gauge was equipped with a gold-plated copper alloy probe with a 2.5 mm diameter hemispherical tip. Using the KOWI 3000 integrated current source, the electrical resistance of the contact between the gold-plated copper alloy probe and several brass panels (available from OSSIAN Lagerqvist AB, Sweden) electroplated with a metal layer of silver topcoat / silver-tin alloy / silver undercoat / nickel was automatically measured as a function of contact force. Contact resistance was measured at 10 mN, 20 mN, 30 mN, 40 mN, 50 mN, 60 mN, 70 mN, and 80 mN. Thermal aging was performed at 200 °C for 500 hours and 200 °C for 1000 hours. A conventional convection oven was used for heat aging.

[0047] Cross sections of the panels were photographed and examined at 10,000x or 15,000x magnification in a ZEISS SEM scanning electron microscope equipped with EDX.

[0048] Each brass panel was coated with 2-3 μm of nickel. The silver undercoat and silver topcoat were electroplated using a SILVERJET™ 300SD silver electroplating bath. The silver-tin alloy layer was electroplated using a SILVERON™ GT-820 silver-tin alloy electroplating bath.

[0049] Comparative Example 6 was electroplated with a silver strike layer of approximately 20 nm in place of the silver undercoat layer using a potassium cyanide silver strike bath containing 2 gm / L of silver ions and 100 g / L of free potassium cyanide, and Comparative Example 7 was electroplated with a silver strike layer of approximately 20 nm in place of the silver undercoat layer using a cyanide-free silver plating bath, SILVERON™ GT-101 strike bath. All baths are available from Rohm and Haas Electronic Materials, LLC.

[0050] The brass panel was connected to a rectifier, and the counter electrode was a platinum-plated titanium electrode. The temperature of the silver and silver-tin alloy baths was 45°C during electroplating. The baths were stirred during electroplating using a conventional stirring device. The current density ranged from approximately 1 to 5 ASD. Electroplating was continued until the desired thickness of the silver and silver-tin alloy was achieved. Table 1 discloses the thickness of the silver and silver-tin alloy electroplated on the nickel-coated brass panel. The panel was removed from the plating bath and rinsed with deionized water at room temperature.

[0051] [Table 1]

[0052] The room temperature contact resistance, measured in milliohms, was evaluated for plated brass panels under various normal forces, as shown in Table 2.

[0053] [Table 2]

[0054] As shown in Table 2, the contact resistance of brass panels plated with the metal layer of the present invention in Examples 1-4 and stored at room temperature for 500 or 1000 hours decreased with various applied normal forces. The contact resistance of Comparative Examples 5 and 6 was also low because those panels had a silver top layer. In contrast, Comparative Example 7, which had a silver-tin alloy as the top layer, had a high contact resistance after storage at room temperature due to oxidation of the tin present in the alloy.

[0055] Aliquots of the plated brass panels of Examples 1-4 were cut and embedded in epoxyamine molding polymer. The aliquot samples were polished and etched using conventional polishing and etching processes known in the art to obtain the desired cross sections. The cross sections and metal layers were examined with a ZEISS SEM scanning electron microscope. Figure 4 shows an SEM of one of the plated brass panels at room temperature. The metal layers are clearly visible: the silver topcoat (1) adjacent to the silver-tin alloy layer (2) adjacent to the silver undercoat layer (3) adjacent to the nickel barrier layer (4) adjacent to the copper alloy-containing substrate (5). No separation of the metal layers was observed at the nickel interface. The results for panels stored at room temperature for 500 hours were identical to those for panels stored at room temperature for 1000 hours.

[0056] Crosshatch adhesion tests and bend tests were performed on aliquots of the plated brass panels of Examples 1 to 7 after 500 or 1000 hours of storage at room temperature. No separation of the metal layer was observed in the aliquots.

[0057] Aliquots of the plated brass panels from Examples 1 and 3-7 were then heated for 500 hours at 200° C. After 500 hours, the contact resistance of the panels was measured and the results are shown in Table 3 below.

[0058] [Table 3]

[0059] The contact resistance of brass panels plated with the metal layer of the present invention in Examples 1, 3, and 4 had low contact resistance at various normal forces. Comparative Example 6, which had a pure silver finish, maintained low contact resistance even after annealing. In contrast, Comparative Examples 5 and 7, which included a thick silver-tin alloy interlayer, exhibited high contact resistance after annealing at 200°C for 500 hours. Examples 1, 3, 4, and Comparative Example 5 demonstrated that to maintain low contact resistance after annealing, the thickness of the silver top layer must be thicker than the silver-tin alloy interlayer.

[0060] Crosshatch adhesion and bend tests were performed on aliquots of plated brass panels from Examples 1-4 and Comparative Example 6 after 500 hours of heat aging at 200°C. All samples from Comparative Example 6 failed both tests, indicating poor adhesion of the silver layer to the nickel after heat aging. Figure 5 is an SEM of one of the plated panels from an aliquot of Example 6 at 807x magnification, showing the silver layer separated from the nickel layer in the crosshatched area. All plated panels from the aliquots of Examples 1-4 had good adhesion in both the crosshatch and bend tests. There were no observable adhesion failures for any of the samples.

[0061] Several electroplated brass panels and copper pins (available from OSSIAN Lagerqvist AB, Sweden) having a 2-3 μm thick nickel layer and the metal plated layers in Table 1 for Examples 1-4 and 7 were stored in a conventional convection oven at 200°C for 1,000 hours. After 1,000 hours, the brass panels and pins were removed from the oven and allowed to cool to room temperature. Crosshatch adhesion tests and bend tests were performed on aliquots of Examples 1-4. There was no visible separation of the layers on the brass panels and copper pins.

[0062] Aliquots of the copper pins were cut, and the metal layers were inspected for adhesion performance and voids in the metal layers. No separation of the metal layers was observed. Figure 6 is an SEM at 15,000x magnification of one of the plated copper pins from Example 1. All metal layers appear bonded together without any separation observed, and no Kirkendall voids were observed. The silver-tin alloy layer (not distinguishable in the figure) reacted with portions of the silver topcoat layer (1) and silver undercoat layer (3), resulting in an indistinct boundary between the silver-tin layer. The boundary between the nickel barrier layer (4) and the copper alloy-containing layer (5) is clearly defined. The dark gap at the interface with the silver topcoat (1) at the top of Figure 6 is due to poor adhesion of the epoxyamine molding polymer to the silver at that particular location.

[0063] The tin from the silver-tin alloy layer partially reacted with the silver layers (1) and (3) to form a silver-tin intermetallic compound. Because the grain boundaries of the silver-tin alloy layer were not aligned with the grain boundaries of the silver layers (1) and (3), it is believed that the silver-tin intermetallic compound blocked the oxygen diffusion pathway through the grain boundaries of the silver layer (1), preventing oxygen from migrating to the nickel surface where oxidation could occur. Without being bound by theory, there was no direct channel or grain boundary from the silver topcoat to the silver underlayer and nickel. This was because the grain orientation and grain size of the silver-tin alloy were different from the silver in the silver layer. This lack of a direct pathway prevented oxygen atoms from migrating to the nickel surface.

[0064] In contrast to the aliquots of Examples 1 to 4, Kirkendall voids were observed in the cross section of Comparative Example 7, which contained only a very thin silver layer of about 20 nm between the silver-tin alloy layer and the nickel layer. Kirkendall voids can cause adhesion failure over long-term use. The present invention increases the thickness of the silver underlayer to prevent such void formation after the reaction of the tin in the silver-tin alloy layer with the nickel layer.

[0065] Crosshatch adhesion and bend tests were performed on brass panels and copper pins from an aliquot of Comparative Example 6 after 1000 hours of storage at 200° C. in a conventional oven. All samples failed both tests, showing poor adhesion of the silver layer to the nickel after 1000 hours of heat aging at 200° C. Figure 7 is an SEM at 1040x of one of the plated panels, showing the silver layer separated from the nickel layer.

[0066] The contact resistance of aliquots of plated brass panels of Examples 1-4 and 7 that were heated for 1000 hours at 200° C. was measured, and the results are shown in Table 4 below.

[0067] [Table 4]

[0068] The contact resistance of brass panels plated with the metal layer of the present invention in Examples 1-4 decreased at various applied normal forces, in contrast to Comparative Example 7. The contact resistance was significantly higher in Comparative Example 7, which included a silver strike layer adjacent to the nickel and a top silver-tin layer adjacent to the silver strike layer but removed the silver topcoat. The high contact resistance was due to oxidation of the tin present in the silver-tin layer. This also demonstrates that a thick pure silver top layer is necessary to maintain low contact resistance at a temperature of 200°C for 1000 hours.

Claims

1. An article for use at temperatures exceeding 150°C, a substrate comprising copper or a copper alloy; a nickel layer or nickel alloy layer adjacent to the copper or copper alloy of the substrate; a silver undercoat layer adjacent to the nickel layer or nickel alloy layer; a silver-tin alloy layer adjacent to the silver undercoat layer; and a silver topcoat layer adjacent to the silver-tin alloy layer; the thickness of the silver undercoat layer is equal to or greater than the thickness of the silver-tin alloy layer; the thickness of the silver topcoat layer is greater than the thickness of the silver-tin layer; The article wherein the thickness ratio of the silver topcoat layer to the silver-tin layer is from 3:1 to 30:

1.

2. 10. The article of claim 1, wherein the thickness ratio of the silver topcoat layer to the silver-tin layer is from 3:1 to 12:

1.

3. 10. The article of claim 1, wherein the thickness ratio of the silver undercoat layer to the silver-tin layer is from 1:1 to 40:

1.

4. 10. The article of claim 1, wherein the silver undercoat layer has a thickness of 0.5 μm or greater.

5. The article of claim 1, wherein the silver-tin alloy layer has a thickness of 0.1 μm or more.

6. 10. The article of claim 1, wherein the silver topcoat has a thickness of 1 μm or greater.

7. A method for producing an article for use at temperatures above 150°C, comprising: a) providing a substrate comprising copper or a copper alloy; b) depositing a nickel or nickel alloy layer adjacent to the copper or copper alloy layer on the substrate; c) depositing a silver undercoat layer adjacent to the nickel or nickel alloy layer; d) depositing a silver-tin layer adjacent to the silver undercoat layer; e) depositing a silver topcoat layer adjacent to the silver-tin layer; Including, the thickness of the silver undercoat layer is equal to or greater than the thickness of the silver-tin alloy layer; the thickness of the silver topcoat layer is greater than the thickness of the silver-tin layer; The method wherein the thickness ratio of the silver topcoat layer to the silver-tin layer is from 3:1 to 30:

1.

8. 8. The method of claim 7, wherein the thickness ratio of the silver topcoat layer to the silver-tin layer is from 3:1 to 12:

1.

9. 8. The method of claim 7, wherein the thickness ratio of the silver undercoat layer to the silver-tin layer is from 1:1 to 40:1.

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