Printed circuit board and printed circuit board manufacturing process

Abstract

Printed circuit board comprising a substrate (1) comprising a dielectric material in sheet form. The substrate (1) comprises two main faces. Each of the main faces supports electrodes (2, 3) formed from an electrically conductive material. At least two sensor electrodes (2) are made on one main face and at least two heating electrodes (3) are made on the other main face. A layer of carbon nanotubes (7) is deposited between the two sensor electrodes (2) and a layer of heating ink (6) is deposited between the two heating electrodes (3). This heating ink layer (6) is at least partially deposited opposite the layer of carbon nanotubes (7). Abstract figure: 1

Description

Title of the invention: Printed circuit board and method for manufacturing printed circuit boards technical field

[0001] The invention relates to the field of printed circuits and printed circuit board manufacturing processes. For example, it relates to the field of flexible printed circuits for sensors in devices for analyzing a gas or a liquid such as water. State of the art

[0002] Water analysis devices with sensors comprising two metallic electrodes deposited on a dielectric substrate and a layer of carbon nanotubes deposited between these electrodes are already known. Chlorine present in the water can be physisorbed onto the carbon nanotubes, thereby changing their electrical resistance. In particular, the resistance value of the carbon nanotube layer increases as the chlorine concentration decreases. It is thus possible to determine the chlorine concentration in the water. This type of sensor has applications in monitoring swimming pool water or drinking water.

[0003] In order to reuse such sensors, it is necessary to rinse them thoroughly and dry them between each use. These operations make the use of this type of sensor impractical. Summary of the invention

[0004] One object of the invention is to contribute to the design and manufacture of water analysis devices that do not have the aforementioned drawback.

[0005] To this end, a printed circuit board is proposed comprising a substrate itself comprising a dielectric material in the form of a sheet. This substrate therefore comprises a first main face and a second main face. The distance between the first main face and the second main face determines the thickness of the substrate. Each of the first and second main faces supports, in particular, electrodes formed from an electrically conductive material.

[0006] Thus, the first main face comprises at least two sensor electrodes between which a layer of carbon nanotubes is deposited, and the second main face comprises at least two heating device electrodes between which a layer of heating ink is deposited. This heating ink layer is at least partially deposited opposite the layer of carbon nanotubes.

[0007] Thanks to these arrangements, it is possible to apply an electric current between the heating device electrodes. This electric current passes through the heating ink. The resistivity of the heating ink is such that its temperature increases when the electric current passes through it. This temperature increase propagates through the thickness of the substrate to heat the layer of carbon nanotubes. This results in the desorption of chlorine (for example) adsorbed onto the carbon nanotubes, thus returning the carbon layer to its initial condition, ready for a new measurement. Consequently, a sensor comprising a printed circuit according to the invention has the advantage of not needing to be rinsed and dried between each use. Applying a current between the heating device electrodes is sufficient to reactivate the sensor.

[0008] The printed circuit board according to the invention further comprises one of the following characteristics, considered in isolation and independently of each other, or in combination with one or more others:

[0009] - the sensor electrodes are formed in a copper layer having a thickness between 10 and 50 micrometers; other metals and metal alloys can also be used; for example, these other metals and alloys may include silver, aluminum and / or steel; copper with a thickness between 10 and 50 micrometers has the particular advantages of being compatible with the use of flexible substrates to form flexible circuits (particularly usable in coil-to-coil manufacturing processes), being a good thermal conductor, being relatively cheap and being relatively easy to implement (in a coil process, with electrochemical etching, etc.);

[0010] - the copper layer of the sensor electrodes is coated with a layer of nickel whose thickness is between 1 and 10 micrometers; other metals and metal alloys can also be used; nickel with a thickness between 1 and 10 micrometers has the particular advantage of forming a good protective layer to isolate the copper from corrosion phenomena; it also serves as a barrier layer between the copper and the gold (to prevent diffusion phenomena); in the thickness range mentioned above, good "encapsulation" of the copper is obtained;

[0011] - the nickel layer is covered with a gold layer whose thickness is between between 10 and 300 nanometers; other metals and metal alloys can also be used; for example, these other metals and alloys may include silver, platinum and / or palladium; a gold deposit with a thickness of 10-300 nanometers allows good encapsulation of nickel while maintaining high electrical conductivity and an optimized cost relative to the performance obtained;

[0012] - the layer of carbon nanotubes (single channel or multi channel) deposited between the two sensor electrodes has a thickness of between 100 and 800 nm; other carbon-based materials such as graphene, graphite or carbon black can be used; the use of carbon nanotubes (single-channel or multi-channel) with a thickness of between 100 and 800 nanometers makes it possible to obtain an active 3D surface and to detect free chlorine with concentrations on the order of ppm;

[0013] - the layer of carbon nanotubes deposited between the two sensor electrodes occupies an area of ​​between 15 and 40 square millimeters; with such an area it is possible to obtain optimal coverage of the sensor electrodes (interdigitated);

[0014] - the layer of carbon nanotubes deposited between the two sensor electrodes is of the single-walled carbon nanotube type; single-walled carbon nanotubes exhibit better electrical conductivity than multi-walled carbon nanotubes; this advantage allows for more amplified electrical signals; in addition, they exhibit better compatibility with a flexible substrate; multi-walled carbon nanotubes form more rigid deposits;

[0015] - the dielectric material is polyimide, the thickness of which is between 50 and 100 micrometers; polyimide has the advantage of being a stable substrate and resistant to organic and inorganic media; it also withstands temperatures up to 200°C; the range of thicknesses mentioned above allows the use of polyimide as a flexible substrate compatible with a roll-to-roll implementation (which is itself fast, reproducible and stable);

[0016] - the thickness of the heating ink is between 10 and 60 micrometers; this range of thicknesses allows for a homogeneous heating system;

[0017] - the surface area occupied by the heating ink is between 20 and 80 millimeters squares; with this range of thicknesses we obtain a good superposition of the surface of the sensor located on the opposite face of the substrate; thus, when the heating ink is heated, the heat transfer is homogeneous and allows to efficiently desorb free chlorine (for example) adsorbed on the carbon nanotubes.

[0018] According to another aspect, the invention relates to a method of manufacturing a printed circuit according to the invention, in which the heating ink is deposited by screen printing or by dispensing; Screen printing or dispensing allows for selective deposition and guarantees good control of the deposited thickness.

[0019] Optionally, according to this process, the layer of carbon nanotubes is deposited by inkjet printing. This technique allows for the selective deposition of a small amount of material. It is also suitable for the deposition of inks whose Viscosity is very low (close to that of water). This is particularly true for carbon nanotubes, whose deposition by screen printing is complicated or even impossible. Dispensing (micro- or nano-dispensing), whose deposition principle and advantages are similar to those of inkjet printing, can also be considered for forming the carbon nanotube layer. Brief description of the figures

[0020] Other aspects, objectives and advantages of the invention will become apparent from the following detailed description, as well as from the accompanying drawings, which are given by way of non-limiting examples and on which: [Fig. 1] is a schematic cross-section of a portion of an example embodiment of a printed circuit board according to the invention, [Fig.2] schematically represents, viewed from its second main face, an example of a printed circuit board according to the invention. [Fig.3] schematically represents a portion of the second main face of the printed circuit board of [Fig.2], [Fig.4] schematically represents, viewed from its first main face, an example of a printed circuit board from Figures 2 and 3, [Fig.5] schematically represents a portion of the first main face of the printed circuit board of [Fig.4], [Fig. 6] schematically represents the heat transfer through a cross-sectional portion of an example embodiment of a printed circuit board according to the invention, and [Fig. 7] schematically represents several steps of a method for implementing an example embodiment of a printed circuit board according to the invention. Detailed description

[0021] The invention is exemplified below in the context of a printed circuit board for manufacturing a sensor for analyzing a liquid such as water. However, the invention may also relate to a printed circuit board for manufacturing a sensor for analyzing a gas.

[0022] According to the example of the embodiment of a printed circuit board shown in Figures 1 to 5, this is a "double-sided" type printed circuit board.

[0023] This circuit is therefore in the form of a multilayer structure comprising a dielectric material in the form of a sheet with two principal faces (a first principal face and a second principal face). Electrical circuits are then formed on each of the two principal faces. For example, these circuits include conductive tracks 20, contacts 21, electrodes 2, 3, etc., formed in a layer of electrically conductive material. The sheet of dielectric material is thus used as a substrate 1 for the electrical circuits formed on each of the two main faces. The figures show a portion of a circuit consisting mainly of electrodes: two sensor electrodes 2 on the first main face and two heating device electrodes 3 on the second main face.

[0024] For example, the dielectric material sheet comprises at least one of the following materials: polyimide, glass-epoxy, polyethylene terephthalate, etc. In the example embodiment illustrated in [Fig. 1], the dielectric material sheet constituting the substrate 1 is a polyimide sheet 75 pm thick.

[0025] For example, each layer of electrically conductive material is formed of one or more metals. For example, this or these metals are included in the following list: copper, aluminum, copper alloy, aluminum alloy, steel, etc. The electrically conductive material of the first main face is not necessarily the same as that of the second main face. In the example embodiment shown in [Fig. 1], each of the first and second main faces of the substrate 1 has electrodes 2, 3 made in copper layers 18 µm thick. For example, the sensor electrode 2 and the heating electrode 3 are made in the copper layers using photolithography and electrochemical etching techniques. For example, as illustrated in Figures 2 to 5, the sensor electrode 2 and the heating electrode 3 are made in the form of interdigitated combs.For example, the distance between two fingers of these combs is between 50 and 200 micrometers. In the example illustrated by Figures 2 to 5, the interdigitated comb-shaped patterns defining the sensor electrodes 2 and the heating electrodes 3 are identical. However, depending on variations, other electrode shapes can be considered, and these can be identical or different for the sensor electrodes 2 and the heating electrodes 3.

[0026] A measuring track 8 is connected to each of the sensor electrodes 2. A current supply track 9 is connected to each of the heating electrodes 3.

[0027] For example, at least one layer of electrically conductive material is optionally at least partially coated with one or more layers of metal (e.g., nickel, gold, etc.). The deposition of this or these metal layers is, for example, carried out electrochemically. In the example embodiment illustrated in [Fig. 1], a nickel layer 4, 4 micrometers thick, is electrochemically deposited on each of the sensor electrodes 2, and a gold layer 5, 200 nanometers thick, is electrochemically deposited on the nickel layer 4.

[0028] A layer of adhesive material (not shown) may optionally be used to allow a layer of electrically conductive material to adhere to at least one of the two main faces of the dielectric material sheet. For example, the layer The adhesive material consists of an epoxy resin. The multilayer structure is then achieved by laminating together a sheet of dielectric material and a layer of electrically conductive material on each of the first and second main faces, with optionally a layer of adhesive material between the sheet of dielectric material and the layer of electrically conductive material, on one, or the other, or both main faces of the sheet of dielectric material.

[0029] A layer of heating ink 6 is deposited on the second main face, between the heating device electrodes 3 and at least partially on them. For example, this heating ink 6 is screen-printed with a thickness of 20 micrometers. As can be seen in [Fig. 3], the area covered by the heating ink 6 essentially corresponds to an area comprising the interdigitated combs (in other words, the area covered by the heating ink 6 extends only slightly, or not at all, onto the rest of the printed circuit board).

[0030] The heating ink 6 is for example composed of a substance included in the following list: Polymer binder, carbon black, silver, ethyl acetate.

[0031] A layer of carbon nanotubes 7 is deposited on the first main face, between the sensor electrodes 2. For example, this layer of carbon nanotubes 7 is deposited by inkjet printing, with a thickness of 5 micrometers. This layer of carbon nanotubes 7 at least partially fills the space between the fingers of the interdigitated combs defining the sensor electrodes 2. The carbon nanotubes occupy a minimum surface area of ​​15 square millimeters on the sensor formed by the sensor electrodes 2.

[0032] As illustrated by [Fig. 6], after using the circuit according to the invention to determine the concentration of a physisorbed substance on the carbon nanotube layer 7, a current is sent through the heating electrodes 3, so that the resistivity of the heating ink 6 produces a temperature rise which propagates through the substrate 1 to cause the desorption of this substance and the drying of the carbon nanotube layer 7. For example, for a heating ink 6 having a resistivity of 12 Q / m2, deposited between fingers of the sensor electrodes 2 spaced 150 micrometers apart, and deposited to a thickness of 50 nanometers, a current of 2 amperes is applied for 3 minutes, to obtain a reactivation of the carbon nanotube layer 7.

[0033] Figure 7 illustrates an example of the implementation of a sensor comprising a printed circuit board according to the invention: an initial resistance measurement is taken between the sensor electrodes 2, before the sensor is brought into contact with the substance to be detected; then the resistance is measured again between these same sensor electrodes 2 while the sensor is immersed in a liquid 10 containing the substance to be detected (for example, chlorine in a 5 ppm chlorinated aqueous solution). The difference in The resistances thus measured allow the concentration of this substance to be determined. Heating is then applied using the heating electrodes 3 to desorb the substance whose concentration was measured (for example, 5 ppm of chlorine) and to dry the layer of carbon nanotubes 7, in order to reactivate it. The sensor is then available again for a new measurement. For example, a new initial resistance measurement is taken between the sensor electrodes 2, before the sensor is brought into contact with a substance to be detected again. The resistance is then measured again between these same sensor electrodes 2 while the sensor is immersed in another liquid 11 containing the substance to be detected (for example, chlorine in a 2 ppm chlorinated aqueous solution). The new difference in the resistances thus measured allows the new concentration of this substance to be determined.

[0034] The printed circuit board according to the invention has the advantage of being easily integrated into a portable sensor device (its dimensions, and in particular its thickness, are relatively small). It can be used for various applications. It is inexpensive to manufacture. Indeed, the flexibility of the substrate 1 and the various conductive layers forming the printed circuit board according to the invention is suitable for implementation in roll-to-roll manufacturing processes. This allows for the implementation of processes that rapidly produce large volumes of printed circuit boards.

Claims

Demands

1. Printed circuit board comprising a substrate (1) comprising a dielectric material in sheet form, the substrate (1) comprising a first main face and a second main face, the distance between the first main face and the second main face determining the thickness of the substrate, each of the first and second main faces supporting electrodes (2, 3) formed in an electrically conductive material, in which the first main face has at least two sensor electrodes (2) between which is deposited a layer of carbon nanotubes (7) and the second main face has at least two heating electrodes (3) between which is deposited a layer of heating ink (6), this layer of heating ink (6) being at least partially deposited opposite the layer of carbon nanotubes (7),in which the sensor electrodes (2) are formed in a copper layer having a thickness of between 10 and 50 micrometers, characterized in that the copper layer of the sensor electrodes (2) is covered with a nickel layer (4) having a thickness of between 1 and 10 micrometers and the nickel layer (4) is covered with a gold layer (5) having a thickness of between 10 and 300 nanometers.

2. Printed circuit according to claim 1, wherein the layer of carbon nanotubes (7) deposited between the two sensor electrodes (2) has a thickness of between 100 and 800 nanometers.

3. Printed circuit according to any one of the preceding claims, wherein the layer of carbon nanotubes (7) deposited between the two sensor electrodes (2) occupies an area of ​​between 15 and 40 square millimeters.

4. Printed circuit according to any one of the preceding claims, wherein the layer of carbon nanotubes (7) deposited between the two sensor electrodes (2) is of the single-walled carbon nanotube type.

5. Printed circuit board according to any one of the preceding claims, wherein the dielectric material is polyimide, the thickness of which is between 50 and 100 micrometers.

6. Printed circuit according to any one of the preceding claims, wherein the thickness of the heating ink (6) is between 10 and 60 micrometers.

7. Printed circuit according to any one of the preceding claims, wherein the area occupied by the heating ink (6) is between 20 and 80 square millimeters.

8. A method for manufacturing a printed circuit according to any one of the preceding claims, wherein the heating ink (6) is deposited by screen printing.

9. A method for manufacturing a printed circuit according to any one of claims 1 to 7, wherein the layer of carbon nanotubes (7) is deposited by inkjet printing.

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