Printed circuit board and method of manufacturing printed circuits

Abstract

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

Description

Title of the invention: Printed circuit and method of manufacturing printed circuits Technical field

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

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

[0003] To be able 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] An aim of the invention is to contribute to the design and manufacture of water analysis devices which do not have the aforementioned drawback.

[0005] To this end, a printed circuit 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 in 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 layer of heating ink is at least partly deposited opposite the layer of carbon nanotubes.

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

[0008] The printed circuit according to the invention further comprises one of the following characteristics, considered in isolation and independently of one another, 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 may 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 advantages of being compatible with the use of flexible substrates to form flexible circuits (particularly usable in reel-to-reel manufacturing processes), being a good thermal conductor, being relatively inexpensive and being relatively easy to implement (in a reel process, with electrochemical etching, etc.);

[0010] - the copper layer of the sensor electrodes is covered with a nickel layer 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 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 avoid 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 may 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 the nickel while maintaining efficient electrical conductivity and optimized cost in relation to the performance obtained;

[0012] - the layer of carbon nanotubes (single-channel or multi-channel) deposited between the two sensor electrodes has a thickness 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 between 100 and 800 nanometers allows to obtain an active 3D surface and makes it possible to detect free chlorine with concentrations in the order of ppm;

[0013] - the layer of carbon nanotubes deposited between the two sensor electrodes occupies an area 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 have better electrical conductivity than multi-walled carbon nanotubes; this advantage makes it possible to obtain more amplified electrical signals; in addition, they have 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 environments; it also withstands temperatures up to 200°C; the thickness range mentioned above allows polyimide to be used as a flexible substrate compatible with reel-to-reel processing (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 occupied by the heating ink is between 20 and 80 millimeters squares; with this range of thicknesses, a good superposition of the sensor surface located on the opposite face of the substrate is obtained; thus, when the heating ink is heated, the heat transfer is done homogeneously and allows the free chlorine (for example) adsorbed on the carbon nanotubes to be desorbed efficiently.

[0018] According to another aspect, the invention relates to a method for 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 makes it possible to carry out selective deposition and guarantees good control of the thickness deposited.

[0019] Optionally, according to this method, the layer of carbon nanotubes is deposited by inkjet printing. This technique makes it possible to deposit a small amount of material selectively. It is also suitable for the deposition of inks whose viscosity is very low (close to water). This is particularly the case for carbon nanotubes, the deposition of which by screen printing is complicated or even impossible. Dispensing (micro or nano-dispensing), whose deposition principle and advantages are similar to those of inkjet, can also be considered to form the layer of carbon nanotubes. Brief description of the figures

[0020] Other aspects, aims and advantages of the invention will appear on reading the detailed description which follows, as well as with the aid of the appended drawings, given as non-limiting examples and in which:

[0021] - [Fig.l] is a schematic section of a portion of an example of a mode of production of a printed circuit according to the invention,

[0022] - [Fig.2] schematically represents, seen from its second main face, a example of a printed circuit according to the invention,

[0023] - [Fig.3] schematically represents a portion of the second main face of the printed circuit board of [Fig.2],

[0024] - [Fig.4] schematically represents, seen from its first main face, a example of printed circuit board of Figures 2 and 3,

[0025] - [Fig.5] schematically represents a portion of the first main face of the printed circuit board of [Fig.4],

[0026] - [Fig.6] schematically represents the heat transfer through a portion, sectional view of an exemplary embodiment of a printed circuit according to the invention, and

[0027] - [Fig.7] schematically represents several stages of a process for putting into work of an exemplary embodiment of a printed circuit according to the invention. Detailed description

[0028] The invention is exemplified below in the context of a printed circuit intended for the manufacture of a sensor for the analysis of a liquid such as water. However, the invention may also relate to a printed circuit intended for the manufacture of a sensor for the analysis of a gas.

[0029] According to the example of the embodiment of a printed circuit shown in Figures 1 to 5, this is a “double-sided” type printed circuit.

[0030] This circuit is therefore in the form of a multilayer structure comprising a dielectric material in the form of a sheet with two main faces (a first main face and a second main face). Electrical circuits are then formed on each of the two main faces. For example, these circuits comprise conductive tracks 20, contacts 21, electrodes 2, 3, etc. formed in a layer of electrically conductive material. The sheet of dielectric material is therefore used as substrate 1 for the electrical circuits formed on each of the two main faces. In the figures, a portion of circuit is shown essentially comprising electrodes: two sensor electrodes 2 on the first main face and two heating device electrodes 3 on the second main face.

[0031] For example, the sheet of dielectric material comprises at least one of the following materials: polyimide, glass-epoxy, polyethylene terephthalate, etc. In the exemplary embodiment illustrated in [Fig.l], the sheet of dielectric material constituting the substrate 1 is a 75 μm thick polyimide sheet.

[0032] For example, each layer of electrically conductive material is formed from 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 exemplary embodiment illustrated in [Fig.l], each of the first and second main faces of the substrate 1 comprises electrodes 2, 3 made in copper layers 18 μm thick. For example, the sensor 2 and heating 3 electrodes are made in the copper layers using photolithography and electrochemical etching techniques. For example, as illustrated in FIGS. 2 to 5, the sensor 2 and heating 3 electrodes 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, according to variants other shapes of electrodes can be envisaged and these can be identical or different for the sensor electrodes 2 and the heating electrodes 3.

[0033] 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.

[0034] For example, at least one layer of electrically conductive material is optionally at least partially covered with one or more layers of metal (for example, nickel, gold, etc.). The deposition of this, or these, layers of metal is for example carried out electrochemically. In the exemplary embodiment illustrated in [Fig.l], a layer of nickel 4 4 micrometers thick is deposited electrochemically on each of the sensor electrodes 2, and a layer of gold 5 200 nanometers thick is deposited electrochemically on the layer of nickel 4.

[0035] A layer of adhesive material (not shown) may optionally be used to adhere a layer of electrically conductive material to at least one of the two main faces of the sheet of dielectric material. For example, the layer of adhesive material is made of an epoxy resin. The multilayer structure is then made 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, of one or the other, or of both main faces of the sheet of dielectric material.

[0036] A layer of heating ink 6 is deposited on the second main face, between the heating device electrodes 3 and at least partly on them. For example, this heating ink 6 is deposited by screen printing 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 only slightly, or not at all, overflows onto the rest of the printed circuit).

[0037] The heating ink 6 is for example composed of a substance included in the following list: Polymer binder, carbon black, silver, ethyl acetate.

[0038] 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 at least a surface area of ​​15 square millimeters on the sensor formed by the sensor electrodes 2.

[0039] As illustrated by [Fig.6], after using the circuit according to the invention to determine the concentration of a substance physisorbed on the layer of carbon nanotubes 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 layer of carbon nanotubes 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 layer of carbon nanotubes 7.

[0040] [Fig. 7] illustrates an example of implementation of a sensor comprising a printed circuit according to the invention: an initial measurement of the resistance is carried out between the sensor electrodes 2, before bringing the sensor into contact with the substance to be detected, then the resistance is again carried out between these same sensor electrodes 2 while the sensor is immersed in a liquid 10 containing the substance to be detected (for example example of chlorine in a 5 ppm chlorinated aqueous solution). The difference in the resistances thus measured makes it possible to determine the concentration of this substance. Heating is then carried out using the heating electrodes 3, to desorb the substance whose concentration has been measured (for example the 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 measurement of the resistance is carried out between the sensor electrodes 2, before bringing the sensor into contact with a substance to be detected again, then the resistance is again carried out 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 makes it possible to determine the new concentration of this substance.

[0041] The printed circuit according to the invention has the advantage of being able to be 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 produce. Indeed, the flexibility of the substrate 1 and of the different conductive layers forming the printed circuit according to the invention is suitable for implementation in roll-to-roll manufacturing processes. This allows implementation of processes rapidly producing large volumes of printed circuits.

Claims

Claims

1. Printed circuit comprising a substrate (1) comprising a dielectric material in the form of a sheet, 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, characterized in that the first main face comprises at least two sensor electrodes (2) between which a layer of carbon nanotubes (7) is deposited and the second main face comprises at least two heating electrodes (3) between which a layer of heating ink (6) is deposited, this layer of heating ink (6) being at least partly deposited opposite the layer of carbon nanotubes (7).

2. A printed circuit according to claim 1, wherein the sensor electrodes (2) are formed in a copper layer having a thickness of between 10 and 50 micrometers.

3. Printed circuit according to claim 2, wherein the copper layer of the sensor electrodes (2) is covered with a nickel layer (4) whose thickness is between 1 and 10 micrometers.

4. Printed circuit according to claim 3, in the nickel layer (4) is covered with a gold layer (5) whose thickness is between 10 and 300 nanometers.

5. Printed circuit according to one of the preceding claims, in which the layer of carbon nanotubes (7) deposited between the two sensor electrodes (2) has a thickness of between 100 and 800 nanometers.

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

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

8. Printed circuit according to one of the preceding claims, in which the dielectric material is polyimide, the thickness of which is between 50 and 100 micrometers.

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

10. Printed circuit according to one of the preceding claims, in which the surface occupied by the heating ink (6) is between 20 and 80 square millimeters.

11. A method of manufacturing a printed circuit according to one of the preceding claims, in which the heating ink (6) is deposited by screen printing.

12. A method of manufacturing a printed circuit according to the preceding claim, wherein the layer of carbon nanotubes (7) is deposited by inkjet printing.

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