Method for depositing a bronze alloy on a printed circuit and printed circuit obtained by said method

Abstract

Disclosed is a method for depositing a bronze alloy on a printed circuit (5). Said method comprises an operation of electrolytically depositing at least one layer of bronze (12) on a copper sheet (10). The bronze layer (12) comprises, after deposition, 45-65% by weight of copper, 35-45% by weight of tin and 2-11% by weight of zinc. Also disclosed is a printed circuit (5) obtained by this method. (Most illustrative figure: Fig. 5)

Description

Description Title of the invention: Method for depositing a bronze alloy onto a printed circuit board and printed circuit board obtained by this method 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. Blood glucose detection) or for the connection of objects via the internet (LoT or "Internet of Things"). State of the art

[0002] For example, printed circuit boards according to the invention may include conductive traces and / or electrical contact areas etched into 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 and then co-laminated with a dielectric substrate. Such printed circuit boards are used, for example, for manufacturing contacts for electronic modules of smart cards, antennas for smart cards, mixed circuits comprising both contacts and an antenna, etc.

[0003] For example, smart cards typically consist of a rigid substrate, for instance made of plastic, which forms the core of the card. This substrate contains a separately manufactured electronic module. This electronic module includes a printed circuit board, usually flexible, equipped with a chip (integrated circuit) and means for connecting the chip to a device that reads and / or writes data to the chip. These connection means—or connectors—are, for example, contacts made of conductive metallic tracks that are flush with the surface of the substrate on the electronic module.Besides the need for excellent mechanical strength and corrosion resistance of the contacts, as well as good electrical conductivity between the chip and the contacts on the one hand and between the contacts and a read / write device on the other, smart card manufacturers want to match the. The color of the contacts matches the color(s) of the card. For this purpose, the contacts are generally coated with either a layer of gold, for a gold finish, or a layer of silver or palladium, for a silver finish. However, this type of finish presents problems. For example, palladium is a relatively expensive metal; as for gold, it must be deposited on a layer of nickel which, on the one hand, has magnetic properties that are disadvantageous for radio frequency applications and / or for applications requiring the absence of magnetic properties, and on the other hand, is problematic in the medical field if it has to be placed in contact with or near the skin, etc.

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

[0005] To this end, a process for depositing a white bronze alloy onto a printed circuit board is presented below. This process includes, in particular:

[0006] - the supply of a dielectric substrate comprising a first and a second principal face, with at least a first layer of a first electrically conductive material (for example copper, aluminium, or one of their alloys, steel, etc.) at least on the first principal face,

[0007] - at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material on at least one area of ​​the first sheet.

[0008] Furthermore, in this process, said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material comprises an electrolytic deposition operation of a bronze layer 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.

[0009] This process (as well as all the operations described in this document) can be implemented from roll to roll (“reel-to-reel” in English).

[0010] The bronze layer is an advantageous replacement for a palladium layer, for example, on a visible face of a contact area. It can also potentially eliminate the need for nickel deposition (whereas nickel is (essential as an underlying layer beneath a gold layer, for example). The bronze layer is more economical than a palladium layer. The absence of nickel is preferable for radio frequency applications and certain medical applications.

[0011] The process mentioned above advantageously includes one or more of the following characteristics, considered independently of each other or in combination with one or more others:

[0012] - it includes 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 Organic Solderability Preservative in English) or a self-assembled monolayer (“SAM” or Self-Assembled Monolayer in English);

[0013] - 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;

[0014] - alternatively, if said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material also includes 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 may be carried out directly on at least a portion of this surface layer;

[0015] - said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material also includes 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.

[0016] - said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material includes an electrolytic deposition operation of the bronze layer, as well as an electrolytic deposition operation of a nickel layer and a nickel-phosphorus layer, prior to the deposition of the bronze layer.

[0017] In another respect, a printed circuit board is described below. It includes contact areas configured to form contacts for at least one smart card module. This printed circuit board then includes

[0018] - on one of the main faces of the dielectric substrate, a sheet of the first electrically conductive material, at least part of whose surface is covered with a stack of layers comprising at least one nickel layer, one nickel-phosphorus layer and the bronze layer;

[0019] This printed circuit board may also include one or more of the following features, considered independently of each other or in combination with one or more others:

[0020] - it includes connecting 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:

[0021] - 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;

[0022] - 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 layer of nickel, a layer of nickel-phosphorus, a surface metal layer comprising at least one of the following elements: gold, silver, palladium, rhodium and ruthenium;

[0023] - the bronze layer has a thickness greater than or equal to 150 nanometers and less than or equal to 600 nanometers.

[0024] Other features and advantages of the invention will become apparent upon reading the detailed description and accompanying drawings, in which:

[0025] - FIG. 1 schematically represents in perspective a smart card comprising an example of a module according to the invention;

[0026] - FIG. 2 schematically represents, viewed from above, a portion of an example of a printed circuit board according to the invention, comprising several connectors for a smart card module;

[0027] - FIG. 3 shows in cross-section, partially and schematically, an example of a single-sided printed circuit board for a smart card module connector such as the one shown in figure 1;

[0028] - FIG. 4 shows in cross-section, partially and schematically, an example of a double-sided printed circuit board for a smart card module connector such as the one shown in figure 1;

[0029] - FIG. 5 represents in cross-section, partially and schematically, an example of a double-sided printed circuit board such as that of figure 4, on which several layers are electrodeposited; as well as its single-sided variant, if we disregard the sheets and layers located under the dotted lines;

[0030] - FIG. 6 represents in cross-section, partially and schematically, another example of a double-sided printed circuit board such as that of figure 4, on which several layers are electrodeposited; as well as its single-sided variant, if we disregard the sheets and layers located under the dotted lines;

[0031] - FIG. 7 represents in cross-section, partially and schematically, yet another example of a double-sided printed circuit board such as that of figure 4, on which several layers are electrodeposited; as well as its single-sided variant if we disregard the sheets and layers located under the dotted lines;

[0032] - FIG. 8 represents in cross-section, partially and schematically, yet another example of a double-sided printed circuit board such as that of figure 4, on which several layers are electrodeposited; as well as its single-sided variant if we disregard the sheets and layers located under the dotted lines;

[0033] - FIG. 9 shows a partial and schematic cross-section of a single-sided printed circuit board like the one in Figure 3, on which several layers are electrodeposited; and

[0034] - FIG. 10 shows in cross-section, partially and schematically, another example of a single-sided printed circuit board such as that of figure 3, on which several layers are electrodeposited.

[0035] In this document, an example of an application of the printed circuit board according to the invention is taken from the field of smart cards, but a person skilled in the art will be able to transpose this example to other applications of printed circuits (USB connectors, antennas, devices for medical applications such as skin pressure sensors, etc.) without having to demonstrate inventive step. glucose or other blood test strips, electrodes for performing electroencephalograms, etc.).

[0036] According to an example of an application of the printed circuit board according to the invention, illustrated in Figure 1, a smart card 1 comprises a module 2 with a connector 3. In this example, the smart card 1 is a bank card in ID-1 format. The module 2 is, for example, a bank card-type module (also called an "EMV" module for Europay Mastercard Visa) conforming to the ISO 7810 standard. The module 2 is generally made as a separate component that is inserted into a cavity in the card. This component comprises a dielectric substrate 4 (see Fig. 2), the thickness of which is, for example, between 25 and 150 micrometers (it is therefore generally flexible), made of PET, polyimide, glass-epoxy, etc. The connector 3 is formed on the dielectric substrate 4, to which a chip (not shown) is subsequently connected, on the side of the substrate opposite the side bearing the connector 3.

[0037] Figure 2 illustrates an example of a portion of a printed circuit board 5, with six connectors 3. Each connector 3 comprises a contact area 8 formed of conductive areas 6. In the example shown here, eight of the conductive areas 6 are intended to form the electrical contacts 7 (identified from C1 to C8 as defined by ISO 7816-2).

[0038] The connector 3 can 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).

[0039] An example of a single-sided structure is illustrated in Figure 3. This single-sided structure is for example made according to the following process: 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 with the layer of adhesive 9 is perforated to make connection wells 14 and possibly a cavity 15 in which a chip will later be housed; the dielectric substrate 4 with the layer of adhesive 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 steel, etc., before possibly undergoing hot curing of the layer of adhesive 9.Alternatively, it is possible to use a laminate directly ("clad" in English), but in this case, the connection wells 14 and / or cavity 15 are formed, for example, using a laser configured to only perforate the. dielectric substrate 4. In all cases, the bottom of the connection wells 14 and / or the cavity 15 is thus made up of an electrically conductive surface, on which layers of conductive material may be electrodeposited, possibly for the purpose of an electrical connection, for example using a wire bonding technology.

[0040] An example of a double-sided structure is illustrated in Figure 4. This double-sided structure is for example made according to the following process: a dielectric substrate 4 is provided which already supports, on one of its main faces (which will correspond to the back 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 with the layer of adhesive 9 is possibly perforated in order to make connection wells 14 and possibly a cavity 15 in which a chip will later be housed; the laminate 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 can be different; it should be noted however that the first 10 and second 11 sheets can be made of different electrically conductive materials).The bottom of the connection wells 14 and / or the cavity 15 is thus formed of an electrically conductive surface, onto which layers of conductive material may optionally be electrodeposited for electrical connection, for example using wire bonding technology. Alternatively, a double-sided clad can be used directly, 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 and the second layer 11 of electrically conductive material.

[0041] For example, as shown in cross-section in Figures 5 to 10, a connector 3 (i.e., essentially a module 2 without a chip) has a multilayer structure formed by the dielectric substrate 4, an adhesive layer 9 (optional and not shown in Figures 5 to 10), and a first 10, and possibly a second 11, sheet made 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 is copper or a copper alloy. The layer of the second electrically conductive material 12 is made 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 may, in fact, be determined by the desire to obtain better weldability on it.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.

[0042] 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 for medical or radio frequency applications, for example.

[0043] In the example illustrated in Figure 5, the multilayer structure is a double-sided structure. It comprises a dielectric substrate 4, the nature of which has already been mentioned above. This dielectric substrate 4 is perforated, for example, to form connection wells 14, each allowing for the electrical connection of a chip located on a rear side (or "bonding side") to contact areas 8 located on the front side (or "contact side"). This dielectric substrate 4 has, on each of its main faces, respectively, a first 10 and a second 11, both made of a first electrically conductive material, for example, copper or a copper alloy (alternatively, the first electrically conductive material could be aluminum, or one of its alloys, steel, etc.).

[0044] Several layers of electrically conductive materials are deposited electrochemically on at least some areas of the free surface of each of the Two sheets 10, 11 of a first electrically conductive material. In the example illustrated in Figure 5, the back side 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 side, the printed circuit board 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, the composition of which is mentioned above. Optionally, the front side undergoes a protective treatment and is therefore covered with a protective layer 20.

[0045] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 5.

[0046] According to this example, the stacking of layers on the back face is the same whether at the bottom of the connection wells 14 or on the second main face of the substrate.

[0047] According to a variant of the embodiment illustrated in 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); however, 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 selected from gold, silver, and palladium, and finally a surface layer 19 comprising at least one of the metals selected from gold, silver, rhodium, ruthenium, and palladium. This is then a single-sided structure.

[0048] In the example illustrated in 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 has, on each of its principal faces, respectively a first 10 and a second 11 layer, both formed of a first electrically conductive material consisting, for example, as before, of copper or a copper alloy. The first 10 and second 11 layers are attached to the dielectric substrate 4 in one of the ways described above, for example.

[0049] Several layers of electrically conductive materials are electrochemically deposited on at least some areas of the free surface of each of the two sheets 10, 11 of the first electrically conductive material. In the example illustrated in Figure 6, the front and back faces receive a layer of bronze 12. Optionally, the front and back faces receive a protective layer 20. The protective layer 20 is optional and can be deposited on one or 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 it reduces the number of electrolytic deposition operations.

[0050] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 6.

[0051] According to a variant of the embodiment illustrated by 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.

[0052] In the example illustrated in 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 has, on each of its principal faces, respectively a first 10 and a second 11 layer, both formed of a first electrically conductive material consisting, for example, as before, of copper or a copper alloy. The first 10 and second 11 layers are attached to the dielectric substrate 4 in one of the ways described above, for example.

[0053] Several layers of electrically conductive materials are electrochemically deposited on at least some areas of the free surface of each of the two sheets 10, 11 of the first electrically conductive material. In the example illustrated in Figure 7, the front and back 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 back 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.

[0054] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 7.

[0055] According to a variant of the embodiment illustrated in 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); however, 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 compound from the following list: gold, silver, palladium, rhodium, ruthenium. This is then a single-sided structure.

[0056] In the example illustrated in Figure 8, the multilayer structure is a double-sided structure. It differs from that described in relation to Figure 7 primarily in that, on the front face, the bronze layer 12 is covered with a thin optional 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.

[0057] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 8.

[0058] As before, a single-sided structure is obtained as an alternative 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 on it.

[0059] In the example illustrated in 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 layer 10, made of the first electrically conductive material, is attached to the dielectric substrate 4 in one of the ways described above.

[0060] Several layers of electrically conductive materials are deposited electrochemically on at least some areas of the free surface of the first sheet 10. In the example illustrated in Figure 9, the front face receives a layer of bronze 12, then optionally 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 a 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.

[0061] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 9.

[0062] In the example illustrated in Figure 10, the multilayer structure is a single-sided structure. It differs essentially from the one previously described by the fact 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.

[0063] However, as an option, the front face can then receive a protective layer 20.

[0064] The table below summarizes examples of characteristic thicknesses of each of the layers of the structure illustrated by Figure 10.

[0065] In the embodiments presented above with their variants, when a protection treatment 20 is carried out, this may correspond, in a non-exhaustive manner, to the passage through;

[0066] - a bath of organic weldability preservative, such as benzotriazole or an imidazole (for example an alkyl benzimidazole, an aryl benzimidazole, etc.);

[0067] - a bath suitable for forming a self-organizing monolayer, such as a mixture of polyethylene glycol ether and propylene glycol, or a mixture of octylphenoxyethanol and octadecane-1-thiol, or sorbitan polyoxyethylene monooleate (Polysorbate 80, CAS number 9005-65-6), or a mixture of propoxylated ethoxylated (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)

[0068] For example, a smart card module comprising a stack consisting of a dielectric 4 covered with a copper foil 10 on which are electrodeposited a layer of nickel 16, a layer of nickel phosphorus 17, a flash of gold 18 and a layer of bronze 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 Figure 5), has a contact resistance (CRM) of less than 500m0hm before and after being subjected to a 24h salt spray test, in accordance with ISO 10373.

Claims

Demands 1. A method for depositing a bronze alloy onto a printed circuit board (5), comprising: the provision of a dielectric substrate (4) comprising a first and a second principal face, with at least a first sheet (10) of a first electrically conductive material at least on the first principal face, at least one electrolytic deposition operation of at least one layer (12) of at least a second electrically conductive material on at least one area of ​​the first sheet (10), characterized in that said at least one electrolytic deposition operation of at least one layer (12) of at least a second electrically conductive material comprises an electrolytic deposition operation 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. Method according to claim 1, comprising a finishing operation in which a surface treatment is carried out after the deposition of the bronze layer (12).

3. A method according to claim 2, wherein the finishing operation comprises the application of a protective layer (20) comprising an organic weldability preservative.

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

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

6. A method according to any one of claims 1 and 2, wherein said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material also comprises an electrolytic deposition of a surface layer (19) comprising at least one element included in the list consisting of gold, silver, palladium, ruthenium, rhodium. A method according to any 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). A method according to claim 6 or 7, wherein said at least one electrolytic deposition operation 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 (18) less than 15 nanometers thick, of at least one element from the list consisting of gold, silver, and palladium. A method according to any one of the preceding claims, wherein said at least one electrolytic deposition operation of at least one layer of at least one second electrically conductive material comprises, prior to the deposition of the bronze layer (12), an electrolytic deposition operation of a nickel layer (16) and a nickel-phosphorus layer (17).Printed circuit board (5) obtained by the method according to any one of the preceding claims, comprising contact areas (7) configured to form contacts for at least one smart card module (2), said printed circuit board (5) comprising on one of the principal faces of the dielectric substrate (4), the first sheet (10) of a first electrically conductive material, at least a portion of whose surface is covered with a stack of layers comprising at least: a nickel layer (16), a nickel-phosphorus layer (17), and a bronze layer (12). Printed circuit board (5) according to claim 10, comprising connection wells (14) at the bottom of which is disposed a stack of layers comprising at least: a nickel layer (16), a nickel-phosphorus layer (17), and a surface layer (19) comprising at least one of the following: gold, silver, rhodium, ruthenium, and palladium.Printed circuit board 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, underlying the bronze layer (12) and the layer (19). Printed circuit board (5) according to any one of claims 10 to 12, comprising on the other of the principal faces of the dielectric substrate a second sheet (11) of a first electrically conductive material, at least a part of whose surface is covered with a stack of layers comprising at least: a nickel layer (16), a nickel-phosphorus layer (17), a layer. 18 surface (19) comprising at least one of the following elements: gold, silver, rhodium, ruthenium and palladium. Printed circuit board (5) according to any 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.