GB2138025A - Silver-coated electric materials and a method for their production - Google Patents

Abstract

A silver-coated electric material is described wherein a partial or total surface of an electroconductive or non-electroconductive substrate is provided with a coating of silver or silver alloy, having a first intermediate coating layer made of Ni, Co, Cr, Pd or their alloys and the second intermediate coating layer made of Sn, Cd, Pd, Ru or their alloys between the silver coating and the surface of the substrate. A method for the production of the material is described which comprises successively depositing the layers by electroplating.

Description

SPECIFICATION This invention relates to silver-coated electric material which has excellent electrical connecting and metallurgical joining properties (soldering and bonding property) and corrosion-resistance.

A silver-coated material when in the partial or total surface of an electroconductive substrate (such as copper, copper alloys, nickel, nickel alloys, iron and iron alloys, aluminium, aluminium alloys) or of a non-electroconductive substrate (such as ceramics or plastics) is provided with a coating of silver or silver alloy, (for example, Ag-Au, Ag-Sb, Ag-In, Ag-Cu, Ag-Se, Ag-Pt) is widely available as an electric or electronic material.

Silver is excellent in electroconductivity and corrosion-resistance, but is high-priced because it is a precious metal. Therefore, there is a demand that silver should be employed only in parts where excellent electric characteristics are particularly needed, and the silver-coated electric material appeared according to this demand. It is obvious from an economical stand-point that the silver coating should be as thin as possible.

As an example of this electric material, composed of a substrate coated with silver, being employed as an electric and electronic material, the lead frame of semi-conductors is provided with a silver coating only at the mounting part where at least a silicon chip is attached or at a part where an Au wire is subjected to wire-bonding. The silver-coated electric material is also used for electric contacts such as switch, relay and connector, lead wires and terminals for various electronic parts, furthermore electrical cable conductors for electric instruments and aeroplanes.

Silver-coated materials, in which only a part of the substrate surface is provided with silvercoating by means of selective plating, are widely used for various applications. In other words, the electric material comprising a substrate with a silver coating which is the object of this invention, is widely used for applications in which excellent metallurgical joining property such as soldering property and bonding property is demanded, and for applications, such as electric contacts, in which excellent electrical connecting property is demanded, by utilizing the physical and chemical properties peculiar to silver.

Such applications naturally demand that the properties of silver-coated electric material do not deteriorate over a long period of practical use of lose its characteristics through thermal and chemical treatments to which it is subjected in the production of parts using it. It is said that in order for the silver-coated electric material to exhibit its orignal properties to meet such requirements, the thickness of the silver coating must be at least 1 ,um, usually 3-6 item.

As silver is expensive, however, the demand for thinner silver coating has become stronger.

However, when a silver coating is made thin for cost reasons, the following problems arise.

(1) Depending on the methods and conditions of preparing the silver-coated electric material, the silver coating becomes porous and therefore the substrate is liable to emerge from under the coating, because of the occurrence of so-called pin-holes.

(2) In the case of electroconductive substrates, less noble metals contained in the substrate play a solid phase diffusion-reaction into the silver coating and reach the outer most part of the silver coating to be oxidized. As a result, the resulting corrosive products easily accumulate on the surface of the electric material. This phenomenon appears remarkably where the electric material is exposed to a high temperature, because the solid phase diffusion-reaction proceeds according to exponential kinetics velocity as to the temperature.

Although the above two problems also appear more remarkably in gold-coated electric material wherein gold which is more expensive than silver is employed, they can be solved by providing an intermediate layer of Ni between the substrate and the gold coating.

In the silver-coated electric material also, the making of a Ni intermediate layer is practically used for semi-conductive lead frames and electric contacts. The occurrence of pin-holes and solid phase diffusion of the substrate into the silver coating is prevented by making the Ni intermediate layer generally 0.5 to 3 ym in thickness. However, even if the Ni intermediate layer like this is provided, it can not be a solution to the problem when the silver coating of the silvercoated electric material is made still thinner and where the electric material is exposed to high temperature in the process of making parts.

For example, electronic parts for which silver-coated electric materials are used most as component materials are assembled by soldering such electric materials, and these electronic parts themselves are generally mounted on printed circuit boards by soldering.

Therefore, in these cases the electric material is exposed to high temperature. It is further exposed to high temperatures in resin-moulding and curing, and in the processes to control its properties, such as aging, drying, evaporation and spattering. In many cases, these processes are usually carried out at, temperatures of 1 50 C to 400"C.

One of the merits in the use of a silver-coated electric material is that it can be given various processes in a high-temperature atmosphere. The presence of oxygen is even mandatory in part of a resin-curing process. When a silver-coated electric material is subjected to treatments in the presence of such a high temperature oxygen atmosphere, the Ni intermediate layer in the electric material can not solve the problems, and the soldering property of the electric material is extremely depressed, and, in some cases, the silver coating comes off. This is the problem proper to the silver-coated electric material which can not be recognised in Au-coated electric material, and as the result of our elaborate study we have found that it is due the following reasons.

(1) At temperatures of 1 50 C to 400 C or higher, oxygen in the surrounding atmosphere penetrates the silver coating layer rapidly, reaches the surface of the nickel intermediate layer under the silver coating, and causes an oxidation-reaction with Ni. The penetrating oxygen is assumed to be in the atomic state, and therefore, particularly active. As a result, the surface of the nickel intermediate layer is covered with nickel oxide (NiO) to cut off the metallic bond between silver and nickel, and then the close adhesiveness of the silver coating and nickel intermediate layer is lost, thus causing the silver coating to come off.

(2) Silver is a metal that dissolves into the soldering bath most rapidly, and it dissolves by 2-3 ,um in thickness per second under normal soldering conditions. Therefore, where the silver coating is thin, nickel oxide, formed as above-mentioned, emerges from the surface of silver-coated electric material and deteriorates the soldering property of the electric material.

This invention concerns silver-coated electric material and a method for their production developed in the above circumstances.

The silver-coated electric material should desirably have the following characteristics.

1. Sufficient corrosion resistance to prevent effective corrosion of the substrate even in the case of thin silver coating.

2. Superiority in corrosion-resistance compared to conventional silver-coated electric materials in an environment of sulfides or the like.

3. Excellent adhesion of the silver coating and no separation of the coating when exposed to high temperature.

4. No diffusion of the substrate into the silver coating even at high temperatures and no deterioration of the properties of the coating such as conductivity and corrosion-resistance.

5. A high level of metallurgical joining properties such as soldering property retained not only in a non-oxidizing atmosphere but also in an oxidizing and high-temperature atmosphere.

6. A high level of electric connecting properties such as contact resistance maintained even in storage for a long period.

7. Low susceptability to silver migration which is one of the main defects of silver.

(Note): Silver migration is a phenomenon in which Ag ions migrate frorn silver conductor of a positive side to deposit and grow on a negative side between which insulating material intenvenes under direct electrical field, so to cause a short circuit.

8. High mechanical strength of the silver coating and excellent wear-resistance.

As to the method of preparing the same, we have found a method for efficient, commerical production of silver-coated electric materials which satisfy the above-mentioned characteristics.

Thus, according to a first aspect the invention provides a silver-coated electric material wherein a partial or total surface of an electroconductive or non-electroconductive substrate is provided with a coating of silver or silver alloy, characteristised in that a first intermediate coating comprising at least one member selected from the group consisting of Ni, Co, Cr, Pd and the alloys thereof and a second intermediate coating comprising at least one member selected from the group consisting of Sn, Cd, Pd, Ru and the alloys thereof are layered between the substrate and the silver coating in that order from the substrate to the silver coating.

According to a second aspect the invention provides a method of preparing the silver-coated electric material as just defined wherein in case the substrate is particularly electroconductive, three layers of the first intermediate coating, the second intermediate coating and the silver coating on the substrate are formed successively by electro-plating.

The silver-coated electric material according to the invention has improved metallurgical joining properties (soldering property and bonding property), electrical connecting property, corrosion resistance and further wear-resistance thereof, and the susceptibility to silver migration thereof is also reduced.

Where the substrate is made of Ni, Co, Cr, Pd or the alloys thereof or in case the layer of Ni, Co, Cr, Pd or the alloys thereof is preformed on the surface of the substrate, the first intermediate coating according to the invention can be replaced by the substrate.

The reason why the above constitution is selected as the first intermediate coating is that Ni, Co, Cr and Pd are metals having high melting points and they either do not react with silver at all or not substantially and that these metals are difficult to react with the substrate materials such as copper and copper alloys which are most conventional and are used most as substrates.

Particularly, Cr acts effectively as a barrier to the copper-substrate, but it is generally hard and brittle, and therefore it should not be used for applications which require an excessive thickness or workability.

A preferable first intermediate coating is one that contains at least 10% in total of one or more than one of four metals--Ni, Co, Cr and Pd. A content of less than 10% does not exhibit any substantial effect for preventing the substrate to diffuse into the silver coating. Examples of the alloys thereof are Ni-Co, Ni-Pd, Ni-Co-Pd, Co-Pd, Ni-Cr, Ni-Zn, Ni-Fe, Co-Zn, Ni-Cu, Co-Sn, Ni-P and Co-B. The preferable thickness of the first intermediate coating is 0.1 to 5 ,um for many practical applications, because the thickness of the less than 0.1 lim does not fulfil the function of the first intermediate coating, that is, the effect for preventing the substrate to diffuse into the silver coating, and the function of the second intermediate coating.On the other hand, a thickness exceeding 5 um is not economical, because it does not further enhance these functions.

The second intermediate coating is made of Sn, Cd, Pd, Ru or the alloys thereof and it prevents the first intermediate coating from being oxidised at high temperatures. However, it is believed that the mechanism of preventing the oxidation at high temperature differs according as the second intermediate coating is made Sn or Cd or their alloy on the one hand or of Rd or Ru or their alloy on the other hand.

This is, because Sn and Cd are metals having low melting points and are soluble in silver, and have strong affinity with oxygen, it is considered that they diffuse rapidly into the silver coating to prevent oxygen from reaching the surface of the first intermediate coating by combining with oxygen which enters the silver coating from the atmosphere or by depressing the rate of oxygen entry.

On the other hand, because Pd and Ru are metals having high melting point and have an extremely poor affinity with oxygen, it is considered that they become a barrier against oxygen which enters the silver coating from the atmosphere to prevent oxygen from reaching the surface of the first intermediate coating.

The preferable thickness of the second intermediate coating is usually 0.01-2 sssm in practical applications. A thickness of less than 0.01 ,um does not show any substantial effect to prevent oxygen from reaching the surface of the first intermediate coating, and a thickness exceeding 2 jtm does not exhibit increased effect, so it is economically unfavourable.

Although Pd and Ru are precious metals belonging to the platimum metals group, they are comparatively low-priced noble metals, costing about ten time as much as silver. Therefore, if the thickness of the silver coating can be greatly reduced by employing them in a thin film having such a thickness, particularly of 0.01-0.1 ym, another economical effect of reducing the silver amount can be achieved.

The alloys of Sn, Cd, Pd and Ru may also be used as the second intermediate coating and sometimes the alloys are more effective.

Sn and Cd are active metals. Therefore, sometimes they partially dissolve out into the electroplating bath, or they replace silver to dissolve out, at the time of electroplating silver thereon, and this causes lowering of their adherence with the silver coating. In these cases, Sn-Pb, Sn-Bi, Sn-Cu, Sn-Ni, Sn-Zn, Sn-Co, Cd-Cu and other alloys are effective as the second intermediate coating.

Pd and Ru are precious metals and high-priced, the cost can therefore be reduced by employing Pd-Ni, Pd-Co, Pd-Ni-Co, or Ru-Ni alloy containing more than 40% of Pd.

Among metals employed as the second intermediates coating, Sn is particularly effective in practical applications, because it is less toxic than Cd and has an excellent reactivity with silver, but excessive thickness of Sn induces deterioration in function of the silver coating. Therefore, the desirable thickness of Sn is about 1/500-1/10 of that of the silver coating.

Because Ni, Co, Cr, Pd or the alloys thereof which constitute the first intermediate coating of this invention are difficult to react with Sn or Cd which constitutes a part of the second intermediate coating, Sn or Cd of the second intermediate coating does not react with the substrate and stably diffuse into the silver coating, owing to existence of the first intermediate coating, so it can exhibit the oxygen-attracting effect in the silver coating as stated above.

When the first intermediate coating is not provided, Sn or Cd comes to react with copper constituting the substrate made of copper or copper alloy, and the effect of the second intermediate coating as beforementioned can not be effectively obtained.

The effect of the first intermediate coating on this particular function of the second intermediate coating is particularly noticable when the first intermediate coating is made of Ni-Zn alloy.

As obvious from the above explanation, the intermediate coating should be provided to the substrate in the order of the first intermediate coating and then the second intermediate coating.

As to the method of preparing the silver coated electric material of this invention, it is industrially possible to form each coating by mechanical cladding, adhering through evaporation and spattering, but an electroplating process is most practical, because each of the first and second intermediate coatings and the silver coating can be effectively formed by the conventional electroplating process, and these three coatings are successively formed in that order.

Furthermore, the important point is to control each coating, particularly a thin layer like the second intermediate coating to the desired thickness. This control can be easily realized only by supplying electricity according to Faraday's law.

This invention is illustrated by examples as follows.

Example 1 Ag-plated Cu wire (0.6 mm dia.) is provided as a lead wire for diode. Si chip is soldered to a point of Cu wire subjected to headering with high Pb solder and cured with shielding resin to complete a diode. As the soldering and curing are done at 350 C for 15 min. (in H2) and at 21 5 C for 10 hrs. (in atmosphere), respectively silver-coated lead wire is required to maintain solderability after treatments.

In this example, Cu wires of 0.6 mm dia. were subjected to conventional electrolytic degreasing and then pickling, and thereafter subjected to electroplating in the following electrolytic baths to obtain Ag plated Cu wires of various intermediate layers indicated in Table 1.

(a) Ni plating bath: NiSO4 250 g/l pH 3.0 NiCl2 25 450 C H3BO3 30 DK = 3.0 A/dm2 (b) Ni- 1 0 h Co plating bath: NiSO4 240 g/l NiCI2 40 pH 3.0 CoSO4 20 45 C H3BO3 20 DK = 2.5 A/dm2 (c) Pd plating bath: Pd (as P-salt) 10 g/l NHSO3(NH2)2 100 pH 7.5, 30 C DK = 0.5 A/dm2 (d) Pd-45% Ni plating bath: PNP-50 by Nisshin Kasei Co. in Japan Pd 10/g, Ni 10 g/l pH 5.0, 30 C DK= 1A/dm2 (e) Pd-15% Ni plating bath: PNP-80 by Nisshin Kasei Co. In Japan Pd 20 g/l pH 8.9 Ni 10 g/l 30 C DK = 0.5 A/dm2 (f) Cr plating bath: CrOs 250 g/l 40 C H2SO4 25 g/l DK = 20 A/dm2 (g) Sn plating bath: SnSO4 100 g/l 15 C H2SO4 50 DK = 1.5 A/dm2 Glue 5 g/l ss-naphthol 5 g/l (h) Sn-60% Cu plating bath: CuCN 15 g/l 65 C Na2SnOs 100 DK= 2.5 A/dm2 NaCN 20 NaOH 10 (i) Cd plating bath: Cd(CN)2 35 g/l 30 C NaCN 100 DK=2.5A/dm2 NaOH 40 (j) Ru plating bath: Ruthenex by Tanaka Precious Metals Co. in Japan 65 C DK = 0.5 A/dm2 (k) Ag-strike bath: AgCN 3 g/l 20@C KCN 30 DK = 5 A/dm2 x 5 sec.

(I) Ag plating: AgCN 30 g/l 20 C KCN 45 DK = 1.5 A/dm2 K2CO3 10 The above Ag-plated Cu wires were subjected to the same consecutive heating processes of 350 C X 15 min. (in H2) and of 215 C X 10 hrs. (in atmosphere) as are done in the above diode manufacture, and then dipped into eutectic solder bath of 235 C for 5 seconds according to MIL Standard. Thereafter, the percentages for the wetted areas of solder were measured as indicated in Table 1.

TABLE 1 First inter- Second inter mediate layer mediate layer Ag Coating Wettability Compo- Thickness Compo- Thickness Thickness of Soldering Sample No. sition ( m) sition ( m) ( m) (%) 1 Ni 0.75 Sn 0.05 1.0 93 2 Ni 0.75 Cd 0.05 1.0 90 3 Ni 0.75 Pd 0.08 1.0 98 Sample 4 Ni 0.75 Ru 0.08 1.0 95 of 5 Ni 0.75 Pd-45Ni 0.08 1.0 95 this 6 Ni-1OCo 0.75 PD-15Ni 0.03 1.0 93 Inven- 7 Ni-1OCo 0.75 Sn-60Cu 0.05 1.0 95 tion 8 Pd 0.1 Sn 0.01 1.0 95 9 Pd 0.1 Substituted by 1st 1.0 90 intermediate layer 10 Pd-45Ni 0.1 Pd-15Ni 0.06 1.0 97 11 Cr 0.25 Sn 0.03 1.0 93 12 Cr 0.25 Pd 0.05 1.0 95 13 Pd-45Ni 0.1 Cd 0.02 1.0 90 4 Conven- 14 - - - - 3.5 90 tional 15 - - - - 1.0 15 sample 16 Ni 0.75 - - 3.5 40 17 Ni 0.75 - - 1.0 10 As evident from Table 1, Nos. 1-13 samples of this invention with a 1.0,um-thick Ag coating passed 90% wettability, which is the minimum target of their solderability while Nos. 14 and 15 comparative conventional samples barely attained 90% wettability with a 3.5 m-thick Ag coating and Nos. 16 and 17 comparative conventional samples had only 40% wet ratio even with a 3.5 m-thick Ag coating.

Pd and Ru metals used for the second intermediate layer are expensive, costing about 10 times as much as Ag. However, when Pd and Ru metals are used in a thickness of 0.08 m for the second intermediate layer, Ag coating thickness can be reduced by 2.5,um. Therefore, it is found that the use of Pd and Ru metals is economical.

Example 2 Fe-14Cr alloy strip (0.32 mm thick) is used as a lead frame of semi-conductor and subjected to usual press forming to obtain 16-pin frames which is then Ag-plated overall in a thickness of 7 m. In tests, the present invention was applied to reduce Ag plating thickness to 3.5 m. Si integrated circuit was soldered to a central tab portion of the lead frame and supersonic-welded to an inner lead portion of the frame with Au wire of 30 ym dia. by means of electrode on the element. The soldering must endure against a heating condition of 400 C X 5 min. and the latter supersonic welding must endure against that of 200 C X 15 min. in atmosphere.

Various Ag-plated lead frames of Table 2 were made according to the plating method of Example 1 and the below-mentioned plating method, and respective lead frames were subjected to the following tests.

Test 1-Solderability Test: After 400"C X 5 min. heating in the air, the lead frames were dipped into 350"C bath of 95% Pb-5% Sn, and then their wetted area was measured and recorded as a percentage.

Test II-Bonding Strength Test: The lead frames were heated at 400 C for 5 min, cooled, and then heated at 200 C for 15 min. The Au wire was given supersonic-welding under a bonding pressure of 45 gr. The average tensile strength of 20 samples was obtained.

Test III-Silver Migration Test: Two lead portions cut from the lead frame were subjected to the above heating treatment and fixed on constant filter paper with 2 mm interval, and applied with DC voltage of 25 V at 60"C x 95% RH. After shelf test of 24 hrs, interpole resistance was measured.

Ni-15% Zn plating bath: Table 2 First intermediate Second intermediate Aglayer layer Coating Test I Test II Test III Thickness Thickness Thickness Wet area Tensile Interpole Sample No. Composition ( m) Composition ( m) ( m) (%) strength gr. resist.M# 1 Ni 0.1 Sn 0.01 3.5 90 10.2 5 2 Ni 0.1 Sn 0.05 3.5 95 11.1 100 Sample 3 Ni 0.1 Sn 0.1 3.5 93 10.0 100 of this 4 Ni 0.1 Sn 1.0 3.5 30 4.5 500 Inven- 5 Pd-45Ni 0.1 Pd-15Ni 0.05 3.5 90 12.0 500 tion 6 Pd-45Ni 0.1 Pd 0.1 3.5 95 12.5 1,000 7 Pd-45Ni 0.1 Ru 0.08 3.5 95 12.2 100 8 Ni-15Zn 0.1 Sn 0.01 3.5 93 11.0 100 Conven- 9 - - + - 3.5 60 7.8 < 1 tional 10 - - - - 7 95 11.9 < 1 Sample NiSO4 200 g/l pH 3.0 ZnS04 5 50"C Na2SO4 50 DK = 2.5 A/dm2 H3BO3 30 As distinct from Table 2, it is found that the lead frames Nos. 1-8 of this invention are all superior in metallurgical solderability (Test I and II) even with 3.5,um-thick Ag-coating and have a high interpole resistance (Test Ill), with little danger of silver migration.

On the other hand, the conventional lead frame No. 9 is not adqueate in solderability and has enhanced danger of silver migration. Further, the conventional lead frame No. 10 is in danger of silver migration while it is good in solderability.

Example 3 As spring contact for key board switch, phosphorous bronze strip (0.08t, Sn = 8.0%) with a 0,5 5,um-thick Ag-plating was used. For applying the strip to samples of this invention, the plating methods of Examples 1 and 2, and the below-mentioned plating method were used to make various samples of Table 3. These contacts were subjected to the aging treatments under the following two conditions in order to confirm a long performance as contact and measured for contact resistance.

The measurement was carried out by pushing an Ag rod probe with semi-sphere shape (R = 4.0 mm) to the spring contact under a 75 gr. load and applying a current of 100 mA. The results were indicated in Table. 3. ) Aging I: Kept in humidity chamber of 60'C and relative humidity 95% for 1,000 hrs.

Aging II: Treated in atmosphere of 200"C for 10 hrs. Co plating bath: CoSO4 400 g/l pH 3.5 NaCI 25 50"C H30B4 45 DK = 1.0 A/dms TABLE 3 First inter- Second inter- Ag mediate layer mediate layer Coating Thick- Thick- Thick Compo- ness Compo- ness ness Sample No. sition m) sition m) ( m) Sample 1 Ni 0.25 Sn 0.02 0.5 of this 2 Co 0.25 Sn 0.02 0.5 inven- 3 Pd-45Ni 0.25 Sn 0.01 0.5 tion 4 Pd-45Ni 0.25 Pd-15Ni 0.02 0.5 Conven- 5 - - - - 0.5 tional 6 Ni 0.25 - - 0.5 Sample TABLE 3 (Contd) Aging I Aging.II later later contact contact resistance resistance Sample No. (m#) (mS3) Sample of 1 9.2 10.0 this 2 7.9 1.5 Invention 3 5.0 7.8 4 4.5 4.7 Conven- 5 39.0 > 100.0 tional 6 13.0 18.0 Sample As evident from Table 3, it is found that spring contacts given an Ag-coating of this invention are small in contact resistance or reduced in deterioration of contact resistance in comparison with the conventional ones.

Further, in Example 3, the surfaces of the spring contacts subjected to the aging treatment I were preformed on analysis for the corrosive products by the cathodic reduction and resulted in detecting Cu oxide and Ag sulfide. Quantities of electricity necessary for the reduction are indicated in Table 4.

TABLE 4 Quantity of electricity for reduction Sample No. coulomb/cm2 Sample of 1 9.1 x 10-3 thislnven- 2 9.2X10-3 tion 3 11.0X10-3 4 14.0 x 10-3 Conventional 5 32.0 x 10-3 Sample 6 15.0X10-3 In other words, the surfaces of the spring contacts with Ag coating according to this invention make less corrosion product than the conventional samples. This is particularly so in the case of Nos. 1, 2 and 3 samples using Sn in the second intermediate layer. It is pressumed that a very small quantity of Sn was dispersed into the Ag coating to form an alloy, thereby improving corrosion-resistance of the Ag coating.

As mentioned above, the Ag-coated electric materials of this invention are superior in metallurgical solderability, electric connection and corrosion resistance, and also high in manufacturing precision and easy to obtain an efficient Ag-coating, thereby providing a conspicuous effect to the electric and electronic industries.

Claims (12)

1. A silver-coated electric material wherein a partial or total surface of an electroconductive or non-electroconductive substrate is provided with a coating of silver or of a silver alloy, wherein a first intermediate coating disposed between the substrate and a second intermediate layer comprises one or more of Ni, Co, Cr, Pd and alloys thereof and a second intermediate coating disposed between the first intermediate layer and the coating of silver or silver alloy comprises one or more of Sn, Cd, Pd, Ru and alloys thereof.

2. A silver-coated electric material according to claim 1, wherein Ni, Co, Cr, Pd or an alloy thereof serves simultaneously as the first intermediate coating and the substrate, and the first intermediate coating being preformed on the surface of the substrate.

3. A silver-coated electric material according to claim 1, wherein the first intermediate coating comprises an alloy containing not lower than 10 wt% in total of one or more of Ni, Co, Cr and Pd.

4. A silver-coated electric material according to any of claims 1 to 3, wherein the thickness of the first intermediate coating is 0.1 to 5 clam.

5. A silver-coated electric material according to any of claims 1 to 4, wherein the thickness of the second intermediate coating is 0.01 to 2 ilk.

6. A silver-coated electric material according to any of claims 1 to 5, wherein the second intermediate coating is made of Sn or an alloy of Sn.

7. A silver-coated electric material according to any of claims 1 to 5, wherein the second intermediate coating is made of Sn and the thickness thereof is 1 /500 to 1/10 of that of the coating of silver or silver alloy.

8. A silver-coated electric material according to any of claims 1 to 5, wherein the second intermediate coating is made of Pd-Ni, Pd-Co or Pd-Ni-Co alloy containing not less than 40 weight % of Pd.

9. A silver-coated electric material according to claim 1, wherein the first intermediate coating is a Ni-Zn alloy.

10. A silver-coated electric material according to claim 1, wherein the substrate being made of Cu or an alloy of Cu.

11. A method of preparing a silver-coated electric material wherein a partial or total surface of an electroconduct;ve substrate is provided with a coating of silver or silver alloy, which method comprises applying to a substrate successive layers consisting of a intermediate coating made of one or more of Ni, Co, Cr, Pd and alloys thereof, a second intermediate coating made of one or more of Sn, Cd, Pd, Ru and alloys thereof and a coating of silver or silver alloy, the layers being applied by means of electroplating.

12. A silver-coated electric material according to claim 1 substantially as herein described with reference to any of the specific Examples.