Method for depositing a bronze alloy on a printed circuit board and printed circuit board produced by this method

Description

Technical field

[0001] The invention relates to the field of printed circuits for connectors or antennas, for example for connectors and antennas of smart cards or for devices intended for medical applications (e.g. detection of glucose in the blood) or for the connection of objects via the internet (IoT or "Internet of Things" in English). State of the art

[0002] For example, printed circuits according to the invention may comprise conductive tracks and / or electrical contact pads etched in a sheet of electrically conductive material previously deposited on a dielectric substrate, or circuits comprising one or more connection grids, each consisting of a sheet of electrically conductive material cut, then co-laminated with a dielectric substrate. Such printed circuits are used for example for producing contacts for electronic smart card modules, antennas for smart cards, mixed circuits comprising both contacts and an antenna, etc.

[0003] As an illustration, if we take the example of smart cards, these are generally made up of a rigid support, for example made of plastic, constituting the main part of the card, in which is incorporated a separately manufactured electronic module. This electronic module comprises a printed circuit, generally flexible, equipped with a chip (integrated circuit) and means for connecting the chip to a device making it possible to read and / or write data in the chip. These connection means - or connectors - are for example formed of contacts made up of conductive metal tracks flush with the electronic module, on the surface of the support.In addition to the need for excellent mechanical strength and corrosion resistance of the contacts, as well as good electrical conduction between the chip and the contacts on the one hand and between the contacts and a reader / writer on the other hand, smart card manufacturers want to match the color of the contacts to the color(s) of the card. To this end, the contacts are usually coated with either a layer of gold, to obtain a gold finish, or a layer of silver or palladium, to obtain a silver finish. However, this type of finish poses problems.For example, palladium is a relatively expensive metal; gold, on the other hand, must be deposited on a layer of nickel which, on the one hand, has magnetic properties that present disadvantages for radiofrequency applications and / or for applications requiring an absence of magnetic properties, and on the other hand, is problematic in the medical field if it must be placed in contact or near the skin, etc. Document US2013 / 236742 discloses a method for depositing a bronze alloy on a printed circuit.

[0004] An aim of the invention is to produce flexible printed circuits in which palladium and / or nickel are not used or are used only to a small extent, while retaining electrical and mechanical properties suitable for their use, in particular in contact modules for smart cards.

[0005] For this purpose, a process for depositing a white bronze alloy on a printed circuit board is presented below. This includes in particular: providing a dielectric substrate comprising a first and a second main face, with at least a first sheet of a first electrically conductive material (for example copper, aluminum, or one of their alloys, steel, etc.) at least on the first main face, at least one operation of electrolytic deposition of at least one layer of at least a second electrically conductive material on at least one area of ​​the first sheet.

[0006] Furthermore, in this method, said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material comprises an operation of electrolytic deposition of a layer of bronze comprising after deposition 45 to 65%, advantageously 45 to 62% and preferably 45 to 50%, by weight of copper, 30 to 45%, and preferably 40 to 45%, by weight of tin and 2 to 11%, advantageously 6 to 11%, by weight of zinc.

[0007] This process (as well as all operations described in this document) can be implemented reel-to-reel.

[0008] The bronze layer advantageously replaces a palladium layer, for example, on a visible face of a contact pad. It can also possibly make it possible to avoid depositing nickel (whereas nickel is essential in the layer underlying a gold layer, for example). The bronze layer is more economical than a palladium layer. The absence of nickel is preferable for radiofrequency applications and for certain medical applications.

[0009] The above-mentioned method advantageously comprises one or other of the following characteristics considered independently of each other or in combination with one or more others: it comprises a finishing operation during which a surface treatment is carried out after the deposition of the bronze layer, to form for example a protective layer comprising an organic solderability preservative (OSP) or a self-assembled monolayer (SAM); this surface treatment is carried out directly (i.e. without any other material between the protective layer and the bronze layer) on at least a portion of said bronze layer; alternatively, if said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material also comprises an electrolytic deposition of a surface layer comprising at least one element included in the list consisting of gold, silver, palladium, rhodium and ruthenium;the surface treatment can be carried out directly on at least a portion of this surface layer; said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material also comprises an electrolytic deposition, in the form of a thin layer, less than 15 nanometers thick, of at least one element included in the list consisting of gold, silver, and palladium. said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material comprises an operation of electrolytic deposition of the bronze layer, as well as an operation of electrolytic deposition of a nickel layer and a nickel-phosphorus layer, prior to the deposition of the bronze layer. ;

[0010] In another aspect, a printed circuit is described below. It includes contact pads configured to form contacts for at least one smart card module. This printed circuit then includes on one of the main faces of the dielectric substrate, a sheet of the first electrically conductive material of which at least part of the surface is covered with a stack of layers comprising at least one layer of nickel, one layer of nickel-phosphorus and the layer of bronze;

[0011] This printed circuit optionally further comprises one or other of the following characteristics considered independently of one another or in combination with one or more others: it comprises connection wells at the bottom of which is arranged a stack of layers comprising at least: a nickel layer, a nickel-phosphorus layer, a surface layer comprising at least one of the following elements: gold, silver, rhodium, ruthenium and palladium: it comprises a thin layer of gold, silver or palladium with a thickness less than or equal to 15 nanometers, underlying the bronze layer and the surface layer; it comprises on the other of the main faces of the dielectric substrate, a second sheet of the first electrically conductive material of which at least part of the surface is covered with a stack of layers comprising at least: a nickel layer, a nickel-phosphorus layer, a surface metal layer comprising at least one of the following elements: gold, silver, palladium, rhodium and ruthenium;the bronze layer has a thickness greater than or equal to 150 nanometers and less than or equal to 600 nanometers.;

[0012] Other characteristics and advantages of the invention will appear on reading the detailed description and the attached drawings in which: FIG. 1 schematically represents in perspective a smart card comprising an example of a module according to the invention; FIG. 2 schematically represents a top view of a portion of an example of a printed circuit according to the invention, comprising several connectors for a smart card module; FIG. 3 represents in section, partially and schematically, an example of a single-sided printed circuit for a smart card module connector such as that shown in the figure 1 ; FIG. 4represents in section, partially and schematically, an example of a double-sided printed circuit for a smart card module connector such as that shown in the figure 1 ; FIG. 5 represents in section, partially and schematically, an example of a double-sided printed circuit such as that of the figure 4 , on which several layers are electrodeposited; as well as its single-sided variant, if we ignore the sheets and layers located under the dotted lines; FIG. 6 represents in section, partially and schematically, another example of a double-sided printed circuit such as that of the figure 4 , on which several layers are electrodeposited; as well as its single-sided variant, if we ignore the sheets and layers located under the dotted lines; FIG. 7 shows in section, partially and schematically, yet another example of a double-sided printed circuit such as that of the figure 4, on which several layers are electrodeposited; as well as its single-sided variant if we ignore the sheets and layers located under the dotted lines; FIG. 8 shows in section, partially and schematically, yet another example of a double-sided printed circuit such as that of the figure 4 , on which several layers are electrodeposited; as well as its single-sided variant if we ignore the sheets and layers located under the dotted lines; FIG. 9 represents in section, partially and schematically, an example of a single-sided printed circuit such as that of the figure 3 , on which several layers are electrodeposited; and FIG. 10 represents in section, partially and schematically, another example of a single-sided printed circuit such as that of the figure 3 , on which several layers are electrodeposited.

[0013] In this document, an example of application of the printed circuit according to the invention is taken from the field of smart cards, but the person skilled in the art will be able, without demonstrating inventive activity, to transpose this example to other applications of printed circuits (contacts for USB sockets, antennas, devices for medical applications such as pressure sensors in contact with the skin, strips for detecting glucose or other compounds in the blood, electrodes for producing encephalograms, etc.).

[0014] According to an example of application of the printed circuit according to the invention, illustrated by the figure 1, a chip card 1 comprises a module 2 with a connector 3. According to this example, the chip card 1 is a bank card in ID-1 format. The module 2 is for example a banking type module (also called an “EMV” module for Europay Mastercard Visa) meeting the ISO 7810 standard. The module 2 is generally produced in the form of a separate element which is inserted into a cavity provided in the card. This element comprises a dielectric substrate 4 (see Fig. 2 ), whose thickness is for example between 25 and 150 micrometers (it is therefore generally flexible) of PET, polyimide, glass-epoxy, etc. On the dielectric substrate 4 is made the connector 3, to which a chip (not shown) is subsequently connected, by the face of the substrate opposite that comprising the connector 3.

[0015] There figure 2thus illustrates an example of a portion of printed circuit 5, with six connectors 3. Each connector 3 comprises a contact pad 8 formed of conductive pads 6. In the example illustrated here, eight of the conductive pads 6 are intended to form the electrical contacts 7 (identified from C1 to C8 as defined by the ISO 7816-2 standard)

[0016] The connector 3 may be formed from a single-sided structure (with a sheet of conductive material on only one of the main faces of a dielectric substrate 4) or a double-sided structure (with a sheet of conductive material on each of the two main faces of a dielectric substrate 4).

[0017] An example of a single-sided structure is illustrated in the figure 3This single-sided structure is for example produced according to the following method: a dielectric substrate 4 is provided, which is coated on one of its main faces with a layer of adhesive 9; then, the dielectric substrate 4 provided with the layer of adhesive 9 is perforated in order to produce connection wells 14 and possibly a cavity 15 in which a chip will subsequently be housed; the dielectric substrate 4 provided with the adhesive layer 9 is then complexed (laminated) with a first sheet 10 of a first conductive material such as a sheet of copper, aluminum, or one of their alloys, or even steel, etc., before possibly undergoing hot crosslinking of the adhesive layer 9. Alternatively, it is possible to directly use a laminate (“clad” in English), but in this case, the connection wells 14 and / or the cavity 15 are formed for example using a laser configured to perforate only the dielectric substrate 4.In all cases, the bottom of the connection wells 14 and / or of the cavity 15 is thus made up of an electrically conductive surface, on which layers of conductive material may possibly be electrodeposited, with a view to an electrical connection, for example using a wire bonding technology.

[0018] An example of a double-sided structure is illustrated in the figure 4. This double-sided structure is for example produced according to the following method: a dielectric substrate 4 is provided already supporting, on a first of its main faces (which will correspond to the rear face), a second sheet 11 of a first conductive material such as a sheet of copper, aluminum, or one of their alloys, or even steel, etc.; it is then a laminate for example; the other of its main faces (which will correspond to the front face) is coated with a layer of adhesive 9; then, this laminate provided with the layer of adhesive 9 is possibly perforated in order to produce connection wells 14 and possibly a cavity 15 in which a chip will subsequently be housed; the laminate provided with the layer of adhesive 9 is then complexed (laminated) with a first sheet 10 of conductive material (for example also made of the same conductive material as the first conductive material, even if the respective thicknesses of the first 10 and second 11 sheets may be different; it will be noted however that the first 10 and second 11 sheets may be made of different electrically conductive materials).The bottom of the connection wells 14 and / or of the cavity 15 is thus made up of an electrically conductive surface, on which layers of conductive material may possibly be electrodeposited, for the purpose of an electrical connection, for example using a wire bonding technology. Alternatively, it is possible to directly use a double-sided clad, but in this case, the connection wells 14 and / or of the cavity 15 are formed for example using a laser configured to perforate only the dielectric substrate 4 and the second sheet 11 of electrically conductive material.

[0019] For example, as shown in section on the figures 5 to 10 , a connector 3 (i.e. essentially a module 2 without a chip) has a multilayer structure formed from the dielectric substrate 4, an adhesive layer 9 (optional and not shown in the figures 5 to 10), and a first 10, and optionally a second 11, sheets consisting of a first electrically conductive material on which is electrochemically deposited at least one layer 12 of at least one second electrically conductive material. For example, the first electrically conductive material consists of copper or a copper alloy. The layer of second electrically conductive material 12 consists of bronze comprising, after deposition, 45 to 65%, advantageously 45 to 62% and preferably 45 to 50%, by weight of copper, 30 to 45%, and preferably 40 to 45%, by weight of tin and 2 to 11%, advantageously 6 to 11%, by weight of zinc. The respective compositions of the bronze layer 12 are not necessarily the same on the first 10 and the second sheet 11. The composition of the bronze layer 12 deposited on the second sheet 11 can in fact be determined by the desire to obtain better weldability thereon.The bronze layer 12 is deposited for example from a Miralloy ® bath marketed by the company Umicore ®, at a temperature close to or equal to 60°C, with a current density of 4A / dm 2. Other materials can be deposited electrochemically between the first electrically conductive material and the bronze layer 12, or even above it.

[0020] The bronze layer 12 can be used, for example, either to replace, at least on one side, noble or precious metals (gold, silver, palladium) in a multilayer structure such as that used for the production of smart card modules, or to replace, at least on one side, nickel in a multilayer structure such as that used in devices aimed at medical or radiofrequency applications for example.

[0021] In the example illustrated on the Figure 5, the multilayer structure is a double-sided structure. It comprises a dielectric substrate 4 whose nature has already been mentioned above. This dielectric substrate 4 is perforated for example to form connection wells 14 allowing to connect, each respectively, electrically a chip located on a rear side ("rear side" or "bonding side" in English) to contact pads 8 located on the front side ("front side" or "contact side" in English). This dielectric substrate 4 comprises on each of its main faces, respectively a first 10 and a second 11 sheets both formed of a first electrically conductive material, for example copper or a copper alloy (alternatively, the first electrically conductive material can be aluminum, or one of its alloys, steel, etc.).

[0022] Several layers of electrically conductive materials are deposited electrochemically on at least certain areas of the free surface of each of the two sheets 10, 11 of first electrically conductive material. In the example illustrated by the Figure 5 , the rear face thus receives a layer of nickel 16, a layer of nickel-phosphorus 17, a thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, and finally a surface layer 19 comprising at least one of the metals chosen from gold, silver, palladium, rhodium and ruthenium. On the front face, the printed circuit successively receives a layer of nickel 16, a layer of nickel-phosphorus 17, a thin layer 18 in the form of a “flash” or primer of gold and a bronze layer 12 whose composition is mentioned above. Optionally, the front face undergoes a protective treatment and is therefore covered with a protective layer 20.

[0023] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the Figure 5 . Front face Back side Adhesive 9: 10 to 25 µm Copper foil 10: 12 to 70 micrometers Copper foil 11: 12 to 70 micrometers Nickel layer 16: 0.5 to 6 micrometers Nickel layer 16:1 at 15 micrometers Nickel-Phosphorus layer 17: 0.05 to 0.6 micrometers Nickel-Phosphorus layer 17: 0.05-0.6 micrometers Thin layer 18: Gold or Silver or Palladium: 0 to 15 nanometers Thin layer 18: Gold or Silver or Palladium: 0 to 15 nanometers Bronze layer 12: 100 nanometers to 3 micrometers Surface layer 19 containing at least one of the elements among: Au, Ag, Pd, Rh or Ru: 10 nanometers to 1 micrometer (up to 3 micrometers for Ag) Protective layer 20 (optional)

[0024] According to this example, the stacking of the layers on the rear face is the same whether at the bottom of the connection wells 14 or on the second main face of the substrate.

[0025] According to a variant of the embodiment illustrated by the Figure 5, the rear face of the dielectric substrate 4 is left bare (without the second sheet 11 of the first electrically conductive material and the layers (16 to 19) electrodeposited thereon); on the other hand, at the bottom of the connection wells 14, there is a stack comprising a nickel layer 16, a nickel-phosphorus layer 17, a thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, and finally a surface layer 19 comprising at least one of the metals chosen from gold, silver, rhodium, ruthenium and palladium. This is then a single-sided structure.

[0026] In the example illustrated on the figure 6, the multilayer structure is a double-sided structure. It comprises a dielectric substrate 4 as mentioned above. As in the previous example, the dielectric substrate 4 is perforated and comprises on each of its main faces, respectively a first 10 and a second 11 sheets, both formed of a first electrically conductive material consisting, for example, as previously of copper or a copper alloy. The first 10 and second 11 sheets are attached to the dielectric substrate 4 in one of the ways described above for example.

[0027] Several layers of electrically conductive materials are deposited electrochemically on at least certain areas of the free surface of each of the two sheets 10, 11 of first electrically conductive material. In the example illustrated by the figure 6, the front and rear faces receive a layer of bronze 12. Optionally, the front and rear faces receive a protective layer 20. The protective layer 20 is optional and can be deposited on only one of the two faces or on both faces. This embodiment is particularly advantageous for replacing nickel and palladium on the front face. It is also advantageous in terms of production costs because there is a reduced number of electrolytic deposition operations.

[0028] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the figure 6 . Front face Back side Adhesive 9: 10 to 25 micrometers Copper foil 10: 12 to 70 micrometers Copper foil 11: 12 to 70 micrometers Bronze layer 12: 100 nanometers to 3 micrometers Bronze layer 12: 50 nanometers to 6 micrometers Protective layer 20 (optional) Protective layer 20 (optional)

[0029] According to a variant of the embodiment illustrated by the figure 6, the rear face of the dielectric substrate 4 is left bare and at the bottom of the connection wells 14, there is a stack comprising the bronze layer 12 and the optional post-treatment layer 20. This is then a single-sided structure.

[0030] In the example illustrated on the figure 7 , the multilayer structure is a double-sided structure. It comprises a dielectric substrate 4 as mentioned above. As in the previous examples, the dielectric substrate 4 is perforated and comprises on each of its main faces, respectively a first 10 and a second 11 sheets, both formed of a first electrically conductive material consisting, for example, as previously of copper or a copper alloy. The first 10 and second 11 sheets are attached to the dielectric substrate 4, in one of the ways described above for example.

[0031] Several layers of electrically conductive materials are deposited electrochemically on at least certain areas of the free surface of each of the two sheets 10, 11 of first electrically conductive material. In the example illustrated by the figure 7 , the front and rear faces receive a layer of bronze 12. Optionally, the front face receives a protective layer 20. This embodiment is particularly advantageous for replacing nickel and palladium on the front face. Optionally, the rear face receives, on the bronze layer 12, at least one layer comprising at least one metal from the following list: gold, silver, palladium, rhodium, ruthenium.

[0032] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the figure 7 . Front face Rear connection face Adhesive: 10 to 25 micrometers Copper foil 10: 12 to 70 micrometers Copper foil 11: 12 to 70 micrometers Bronze layer 12: 100 nanometers to 3 micrometers Bronze layer 12: 50 nanometers to 6 micrometers Protective layer 20 (optional) Thin layer 18 Gold or Silver or Palladium: 0 to 15 nanometers (Optional) Surface layer 19 containing Au, Ag, Pd, Rh or Ru: 10 nanometers to 1 micrometer. (up to 3 micrometers for Ag) (Optional)

[0033] According to a variant of the embodiment illustrated by the figure 7 , the rear face of the dielectric substrate is left bare (without the second sheet 11 of the first electrically conductive material and the layers (18 to 19) electrodeposited thereon); on the other hand, at the bottom of the connection wells, there is a stack comprising a bronze layer 12, an optional thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, and finally at least one layer 19 comprising a metal or a compound from the following list: gold, silver, palladium, rhodium ruthenium. This is then a single-sided structure.

[0034] In the example illustrated on the figure 8 , the multi-layer structure is a double-sided structure. It differs from that described in relation to the figure 7essentially by the fact that, on the front face, the bronze layer 12 is covered with an optional thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, itself covered with at least one layer 19 comprising a metal from the following list: gold, silver, palladium, ruthenium, rhodium. Optionally, the front face then receives a protective layer 20.

[0035] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the figure 8 . Front face Back side Adhesive: 10 to 25 micrometers Copper foil 10: 12 to 70 micrometers Copper foil 11: 12 to 70 micrometers Bronze layer 12: 100 nanometers to 3 micrometers Bronze layer 12: 50 nanometers to 6 micrometers Thin layer 18: Gold or Silver or Palladium 0 to 15 nanometers (Optional) Thin layer 18: Gold or Silver or Palladium 0 to 15 nanometers (Optional) Surface layer 19 containing Au, Ag, Pd, Rh or Ru: 10 nanometers to 1 micrometer Surface layer 19 containing Au, Ag, Pd, Rh or Ru: 10 nanometers to 1 micrometer (up to 3 micrometers for Ag) (Optional) Protective layer 20 (optional)

[0036] As previously, a single-sided structure is obtained as a variant by not covering the second main face (rear face side) of the dielectric substrate with a second sheet 11 made up of the first conductive material and any layers deposited thereon.

[0037] In the example illustrated on the figure 9 , the multilayer structure is a single-sided structure. It comprises a dielectric substrate 4 as mentioned above. On the front side, the first sheet 10 made of the first electrically conductive material is attached to the dielectric substrate 4 in one of the ways described above.

[0038] Several layers of electrically conductive materials are deposited electrochemically on at least certain areas of the free surface of the first sheet 10. In the example illustrated by the figure 9, the front face receives a layer of bronze 12, then possibly a thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, and finally at least one surface layer 19 comprising a metal from the following list: gold, silver, palladium rhodium, ruthenium. Optionally, the front face then receives a protective layer 20.

[0039] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the figure 9 . Front face Adhesive: 10 to 25 micrometers Copper foil 10: 12 to 70 micrometers Bronze layer 12: 100 nanometers to 3 micrometers Thin layer 18: Gold or Silver or Palladium: 0 to 15 nanometers (Optional) Surface layer 19 containing Au, Ag, Pd, Rh or Ru: 10 nanometers to 1 micrometer (up to 3 micrometers for Ag) Protective layer 20 (optional)

[0040] In the example illustrated on the figure 10, the multilayer structure is a single-sided structure. It differs essentially from that previously described in that it does not include a thin layer 18 in the form of a “flash” or primer of one of the metals chosen from gold, silver and palladium, nor a surface layer 19 comprising a metal from the following list: gold, silver, palladium, rhodium, ruthenium.

[0041] On the other hand, optionally, the front face can then receive a protective layer 20.

[0042] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by the figure 10 . Front face Adhesive: 10 to 25 micrometers Copper foil 10: 12 to 70 micrometers Bronze layer 12: 100 nanometers to 3 micrometers Protective layer 20 (optional)

[0043] In the embodiments presented above with their variants, when a protective treatment 20 is carried out, this may correspond, in a non-exhaustive manner, to the passage in; an organic solder preservative bath, such as benzotriazole or an imidazole (e.g., an alkyl benzimidazole, an aryl benzimidazole, etc.); a bath suitable for forming a self-organized monolayer, such as a mixture of polyethylene glycol ether and propylene glycol, or a mixture of octylphenoxyethanol and octadecane-1-thiol or polyoxyethylene sorbitan monooleate (Polysorbate 80, CAS number 9005-65-6), or a mixture of ethoxylated propoxylated (C12-18) alcohols (CAS number 69227-21-0) with poly(oxyethylene) lauryl ether (CAS number 9002-92-0) and 1-octadecanethiol (CAS 2885-00-9)

[0044] For example, a smart card module comprising a stack consisting of a dielectric 4 covered with a copper sheet 10 on which are electrodeposited a layer of nickel 16, a layer of nickel phosphorus 17, a gold flash 18 and a bronze layer 12 of 0.5 micrometer comprising 45 to 50% by weight of copper, 40 to 45% by weight of tin and 6 to 11% by weight of zinc (structure of the Figure 5 ), has a contact resistance (CRM) of less than 500mOhm before and after being subjected to a 24h salt spray test, in accordance with ISO 10 373.

Claims

1. Process for depositing a bronze alloy on a printed circuit (5), comprising: providing a dielectric substrate (4) comprising first and second main faces, with at least a first sheet (10) of a first electrically conductive material on at least the first main face, at least one operation of electrolytic deposition of at least one layer (12) of at least one second electrically conductive material on at least one zone of the first sheet (10), characterized in that said at least one operation of electrolytic deposition of at least one layer (12) of at least one second electrically conductive material comprises an operation of electrolytic deposition of a bronze layer comprising, after deposition, 45 to 65% by weight of copper, 30 to 45% by weight of tin and 2 to 11 % by weight of zinc.

2. Process according to claim 1, comprising a finishing operation during which a surface treatment is carried out after the bronze layer (12) has been deposited.

3. Process according to claim 2, in which the finishing operation comprises the application of a protective layer (20) comprising an organic preservative of solderability.

4. Process according to claim 2, wherein the finishing operation comprises the application of a protective layer (20) comprising a self-assembled monolayer.

5. Process according to one of claims 2 to 4, in which the surface treatment is carried out directly on at least a portion of the said bronze layer (12).

6. Process according to one of claims 1 and 2, in which said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material also comprises electrolytic deposition of a surface layer (19) comprising at least one element from the list consisting of gold, silver, palladium, ruthenium and rhodium.

7. Process according to one of claims 2 to 4, each taken in combination with claim 6, wherein the surface treatment is carried out directly on at least a portion of said surface layer (19).

8. Process according to claim 6 or 7, in which said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material also comprises electrolytic deposition, in the form of a thin layer (18), less than 15 nanometers thick, of at least one element included in the list consisting of gold, silver and palladium.

9. Process according to one of the preceding claims, wherein said at least one operation of electrolytic deposition of at least one layer of at least one second electrically conductive material comprises, prior to the deposition of the bronze layer (12), an operation of electrolytic deposition of a nickel layer (16) and of a nickel-phosphorus layer (17).

10. Printed circuit (5) obtained by the process according to one of the preceding claims, comprising contact areas (7) configured to form contacts for at least one chip card module (2), this printed circuit (5) comprising, on one of the main faces of the dielectric substrate (4), the first sheet (10) of a first electrically conductive material, at least part of the surface of which is covered with a stack of layers comprising at least : a nickel layer (16), a nickel-phosphorus layer (17), the bronze layer (12).

11. Printed circuit (5) according to claim 10, comprising connection wells (14) at the bottom of which is arranged a stack of layers comprising at least: a nickel layer (16), a nickel-phosphorus layer (17), a surface layer (19) comprising at least one of the following elements: gold, silver, rhodium, ruthenium and palladium.

12. Printed circuit according to claim 11, further comprising a thin layer (18) of gold, silver or palladium with a thickness less than or equal to 15 nanometers, between the bronze layer (12) and the layer (19).

13. Printed circuit (5) according to one of claims 10 to 12, comprising, on the other of the main faces of the dielectric substrate, a second sheet (11) of a first electrically conductive material, at least part of the surface of which is covered with a stack of layers comprising at least: a nickel layer (16), a nickel-phosphorus layer (17), a surface layer (19) comprising at least one of the following elements: gold, silver, rhodium, ruthenium and palladium.

14. Printed circuit board (5) according to one of claims 10 to 12, wherein the bronze layer has a thickness greater than or equal to 150 nanometers and less than or equal to 600 nanometers.