Pressure sensor and process for manufacturing same
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-16
- Publication Date
- 2026-04-08
AI Technical Summary
The existing pressure sensors for air conditioning systems in motor vehicles require additional conductive elements for immunity to magnetic fields, increasing size, cost, and manufacturing complexity due to the need for extra components and steps.
A simplified pressure sensor design integrates a grounding zone on the substrate's external face, electrically connected to the printed circuit, eliminating the need for a separate conductive element between the electronic card and metal body, thereby reducing parts and manufacturing steps.
This configuration results in a more cost-effective and streamlined manufacturing process with fewer components, maintaining immunity to magnetic fields and ensuring accurate pressure measurement without increasing the sensor's size.
Smart Images
Figure EP2024063630_05122024_PF_FP_ABST
Abstract
Description
[0001] DESCRIPTION
[0002] TITLE OF THE INVENTION: Pressure sensor and manufacturing method thereof
[0003] The invention relates to the field of pressure sensors. It relates more particularly to the field of pressure sensors for air conditioning or cooling systems of motor vehicles and a method of manufacturing such a sensor.
[0004] Pressure sensors are commonly used in motor vehicles to measure the pressure of fluids such as gases, liquids or a gas / liquid mixture in an air conditioning system intended to regulate the temperature inside the passenger compartment or a cooling system of an electric or thermal engine, for example.
[0005] The pressure sensor traditionally consists of a metal body, a plastic connector, and a pressure-sensitive element comprising a substrate secured to the metal body. The substrate forms a membrane that deforms under pressure.
[0006] To manufacture the pressure sensor, a first dielectric layer is deposited over the entire upper face of the substrate.
[0007] Four resistors are then printed and etched on the first dielectric layer and interconnected in series to form a loop called a Wheatstone bridge. This Wheatstone bridge forms, between each adjacent resistor, four poles each having their own electrical potential.
[0008] Four connection areas commonly called “pads” are then printed on the four poles allowing access to the four electrical potentials of the four poles.
[0009] The pressure sensor also includes a circuit board on which at least one signal processing unit is arranged. Four connection areas are also printed on the circuit board near the substrate and electrically connected to the signal processing unit.
[0010] An electrical connection is provided between the connection areas of the electronic board and the connection areas of the substrate.
[0011] Pressure applied to the underside of the membrane induces a deformation of the membrane, the four resistors and consequently a variation of the resistances. Variations in the value of the resistors induce an imbalance of the Wheatstone bridge. A voltage, an image of the pressure due to the imbalance of the Wheatstone bridge, is measured via the connection areas and processed by the signal processing unit.
[0012] The substrate is formed from a metal alloy. The substrate is welded to the metal body. The interface between the substrate and the metal body is watertight and electrical continuity between the two components is ensured.
[0013] In order to ensure immunity to magnetic fields of the pressure sensor, it is necessary to add a conductive element with a certain elasticity between the electronic card and the metal body to make an electrical contact (or coupling) between the electrical mass of the pressure sensor and the metal body.
[0014] The conductive element is a thin, formed metal plate or a metal spring.
[0015] However, this state-of-the-art solution poses a problem of size and cost, because it is necessary to add an additional conductive element during an additional step, lengthening the manufacturing time and making the manufacturing process more complex.
[0016] The aim of the invention is therefore to overcome the drawbacks of the prior art by proposing a method for manufacturing a simpler and faster pressure sensor making it possible to obtain a simplified pressure sensor because it comprises fewer parts and is less expensive.
[0017] To this end, the invention thus relates, in its broadest sense, to a pressure sensor comprising a connector, a printed circuit, a metal body and a pressure-sensitive element comprising a substrate comprising an internal face connected to the metal body and an external face covered by a dielectric layer on which are positioned four resistors arranged to form four electrical poles. Each electrical pole is formed between two resistors and is connected to a conductive zone extending over the dielectric layer. Each conductive zone is electrically connected to a connection zone formed on the printed circuit by a connecting element. According to the invention, the external face of the substrate comprises at least one grounding zone electrically connected to the printed circuit by a connecting means.
[0018] The invention thus provides a simplified pressure sensor because it includes fewer parts and is less expensive. It is no longer necessary to add a conductive element with a certain elasticity between the electronic card and the metal body.
[0019] According to one variant, the grounding zone is positioned between two conductive zones.
[0020] This configuration allows the size of the pressure-sensitive element not to be increased.
[0021] According to another variant, each connecting means is a connecting wire comprising a first end connected to the grounding zone and a second end connected to a connection zone formed on the printed circuit.
[0022] This solution provides a solution that simplifies the manufacturing process.
[0023] According to another variant, the grounding zone is formed by a spared portion of the external face of the substrate. The spared portion is directly connected to the printed circuit.
[0024] This solution allows for a reduction in manufacturing steps.
[0025] According to another variant, the grounding zone is formed by a portion of a first metal layer covering a spared portion of the external face of the substrate. The four resistors are formed by other portions of the first metal layer. The portion of the first metal layer is connected to the printed circuit.
[0026] According to another variant, the grounding zone is formed by a metallic conduction zone covering a portion of the first metallic layer itself covering a spared portion of the external face of the substrate. The metallic conduction zone is connected to the printed circuit.
[0027] This solution allows the same steps to be used as those carried out to form the conductive zones, simplifying the manufacturing process.
[0028] The invention also relates to an air conditioning system for a motor vehicle comprising a pressure sensor as defined above. The invention also relates to a method of manufacturing a pressure sensor comprising:
[0029] - a step of depositing a dielectric layer on an external face of the substrate,
[0030] - a step of depositing a first metallic layer on the dielectric layer,
[0031] - a step of depositing a second metal layer on the first metal layer,
[0032] - an etching step on the first metal layer to form four resistors on the dielectric layer arranged to form four electrical poles, a conductive zone being formed on a portion of each electrical pole from the second metal layer, and
[0033] - a connection step between the conductive zone and a connection zone formed on the printed circuit by a connection element.
[0034] According to the invention, the manufacturing method comprises an operation of grounding the pressure-sensitive element comprising a step of forming at least one grounding zone on the external face of the substrate and a step of electrical connection between the grounding zone and the printed circuit by a connecting means.
[0035] The invention thus provides a method for manufacturing a pressure sensor that is simpler and faster because it includes one less step compared to known solutions consisting of mounting a resilient conductive element between the metal body and the electronic card.
[0036] According to a variant, during the step of depositing the dielectric layer on the external face of the substrate, at least one spared portion is formed on the external face of the substrate by masking a portion of the external face during the deposition of the dielectric layer.
[0037] The grounding area is made at the same time as the deposition of the dielectric layer, without any additional step, simplifying the manufacturing process.
[0038] According to another variant, the external face of the substrate is entirely covered by the dielectric layer during the step of depositing the dielectric layer on the external face of the substrate. A laser ablation operation is then applied to at least a portion of the dielectric layer to form at least one spared portion on the external face of the substrate.
[0039] According to another variant, the spared portion forms the grounding zone. The method comprises an electrical connection step during which the connection means connects the spared portion to a connection zone formed on the printed circuit.
[0040] This electrical connection is made at the same time as the connection between the conductive area and the connection area of the printed circuit.
[0041] According to another variant, the first metal layer covers the portion spared during the step of depositing the first metal layer. The grounding zone is then formed by a portion of the first metal layer covering the portion spared and at the same time as the conductive zones.
[0042] The grounding area is made at the same time as the resistors, without additional steps.
[0043] According to another variant, a portion of the first metal layer covers the portion spared during the step of depositing the first metal layer. A second metal layer is deposited on the portion of the first metal layer covering the portion spared to form a metal conduction zone. The metal conduction zone then forms the grounding zone.
[0044] The grounding area is made at the same time as the conductive areas of the pressure sensitive element, simplifying and reducing the duration of the manufacturing process.
[0045] Embodiments of the present invention will be described below, by way of non-limiting examples, with reference to the appended figures in which:
[0046] [Fig.1] schematically illustrates a pressure sensor, according to one embodiment of the invention;
[0047] [Fig.2] schematically illustrates a pressure sensitive element connected to a printed circuit of the pressure sensor of Figure 1;
[0048] [Fig.3] schematically illustrates a pressure-sensitive element, according to another embodiment of the invention; [Fig.4] schematically illustrates a pressure-sensitive element, according to another embodiment of the invention.
[0049] The invention relates to a pressure sensor 1, as illustrated in FIG. 1, and a method of manufacturing the same.
[0050] This pressure sensor 1 can be used in motor vehicles to measure the pressure of fluids such as gases, liquids or a gas / liquid mixture in an air conditioning system intended to regulate the temperature inside the passenger compartment or a cooling system of an electric or thermal engine, for example.
[0051] The pressure sensor 1 comprises a metal body 4 and a connector 2 comprising a plastic protective casing connected to the metal body 4.
[0052] The pressure sensor 1 comprises a pressure-sensitive element 5 comprising a substrate 6 secured to the metal body 4. The substrate 6 forms a membrane deformable under pressure. The substrate 6 is formed from a metal alloy and comprises an internal face connected to the metal body 4 and an external face 7.
[0053] The substrate 6 is welded to the metal body 4. The interface between the substrate 6 and the metal body 4 is sealed and electrical continuity between the two components is ensured.
[0054] The pressure sensor 1 comprises a printed circuit 3 (or electronic card). The pressure-sensitive element 5 is housed in a circular hole 42 provided in the printed circuit 3, as illustrated in Figure 2.
[0055] The method of manufacturing the pressure sensor 1 comprises a step of depositing a dielectric layer 8 on the external face 7 of the substrate 6. The dielectric layer 8 is formed of a silicon oxide, for example.
[0056] The manufacturing method then comprises a step of depositing a first metal layer on the dielectric layer 8. The first metal layer is more precisely printed on the dielectric layer 8. The first metal layer is formed from a nickel / chromium alloy, for example.
[0057] Four resistors 9, 10, 11, 12 are etched from the first metal layer and interconnected in series to form a loop called a Wheatstone bridge 43. This Wheatstone bridge 43 forms, between each adjacent resistor 9, 10, 11, 12, four poles 13, 14, 15, 16 each having their own electrical potential. Each electrical pole 13, 14, 15, 16 is formed between two resistors 9, 10, 11, 12.
[0058] The manufacturing method comprises a step of depositing a second metal layer on the first metal layer and the dielectric layer 8 to form a conductive zone 17, 18, 19, 20 on a portion of each electrical pole 13, 14, 15, 16.
[0059] The four conductive areas 17, 18, 19, 20 are commonly called “pads” and extend over the dielectric layer 8. Each conductive area 17, 18, 19, 20 has a general shape of a triangle, for example. Other shapes are possible.
[0060] The four conductive zones 17, 18, 19, 20 provide access to the four electrical potentials of the four poles 13, 14, 15, 16. The four conductive zones 17, 18, 19, 20 are formed from a nickel / gold alloy, for example.
[0061] Preferably, the Wheatstone bridge 43 and the four conductive zones 17, 18, 19, 20 are obtained using the thin-film printing process which is well known.
[0062] The manufacturing method then comprises a step of connection between the conductive zone 17, 18, 19, 20 and a connection zone 21, 22, 23, 24 formed on the printed circuit 3 by a connection element 25, 26, 27, 28.
[0063] Four connection areas 21, 22, 23, 24 are formed on the printed circuit 3 near and around the substrate 6.
[0064] Four connecting elements 25, 26, 27, 28 are thus formed to electrically connect the four conductive zones 17, 18, 19, 20 of the pressure-sensitive element 5 to the four respective connection zones 21, 22, 23, 24 of the printed circuit 3.
[0065] This connection is made using bridging technology, also called wire bonding.
[0066] The four connecting elements 25, 26, 27, 28 are conductive wires in this example.
[0067] Alternatively, each conductive area 17, 18, 19, 20 is connected to each connection area 21, 22, 23, 24 by a flexible circuit (not shown). The printed circuit 3 comprises a signal processing unit 29. The four connection areas 21, 22, 23, 24 of the printed circuit 3 are electrically connected to the signal processing unit 29.
[0068] Thus, a pressure applied to the internal face of the pressure-sensitive element 5 (or membrane) induces a deformation of the pressure-sensitive element 5, of the four resistors 9, 10, 11, 12 and consequently a variation of the resistors 9, 10, 11, 12.
[0069] Variations in the value of resistors 9, 10, 11, 12 induce an imbalance of the Wheatstone bridge 43. A voltage, an image of the pressure due to the imbalance of the Wheatstone bridge 43, is measured via the connection zones 21, 22, 23, 24 and processed by the signal processing unit 29.
[0070] According to the invention, the method for manufacturing the pressure sensor 1 comprises an operation of grounding the pressure-sensitive element 5 comprising a step of forming at least one grounding zone 30, 31 on the external face 7 of the substrate 6.
[0071] The external face 7 of the substrate 6 therefore comprises at least one grounding zone 30, 31 electrically connected to the printed circuit 3 by a connecting means 32, 33 in order to ensure immunity to the magnetic fields of the pressure sensor 1 and more precisely of the pressure-sensitive element 5.
[0072] In the example of Figure 2, the pressure sensitive element 5 comprises two grounding zones 30, 31 which are each positioned between two conductive zones 17, 18, 19, 20.
[0073] A first grounding zone 30 is positioned between a first conductive zone 17 and a second conductive zone 18. A second grounding zone 31 is positioned between a third conductive zone 19 and a fourth conductive zone 20.
[0074] Alternatively, the pressure-sensitive element 5 may comprise a single grounding zone 30, 31.
[0075] The two grounding areas 30, 31 have a general triangular shape of the same shape and approximately the same size as the four conductive areas 17, 18, 19, 20 of the pressure-sensitive element 5. The two grounding areas 30, 31 are positioned opposite each other. The Wheatstone bridge 43 has a general elongated shape with two opposite longitudinal sides 44, as illustrated in FIGS. 3 and 4. Each grounding area 30, 31 has a vertex 45 positioned opposite one of the two longitudinal sides 44.
[0076] According to an embodiment shown in Figure 3, during the step of depositing the dielectric layer 8 on the external face 7 of the substrate 6, a first spared portion 38 and a second spared portion 39 are formed on the external face 7 of the substrate 6. The spared portions 38, 39 can be obtained by masking two portions of the external face 7 during the deposition of the dielectric layer 8. The spared portions 38, 39 are in fact areas on the external face 7 of the substrate 6 not covered by the dielectric layer 8.
[0077] Each spared portion 38, 39 has a generally triangular shape with a main vertex 46 positioned opposite one of the two longitudinal sides 44 of the Wheatstone bridge 43.
[0078] The spared portions 38, 39 can be produced by means of a physical mask or photolithography. The spared portions 38, 39 are produced during the printing of the dielectric layer 8.
[0079] The spared portions 38, 39 are positioned opposite each other and separated by the Wheatstone bridge 43.
[0080] Each spared portion 38, 39 extends from the main vertex 46 to an opposite side 47 flush with a peripheral edge 48 of the substrate 6. The spared portion 38, 39 comprises a first edge 49 and a second edge 50 which are rectilinear. The first edge 49 runs along a first side 51 of the first conductive zone 17. The second edge 50 runs along a second side 52 of the second conductive zone 18.
[0081] The first spared portion 38 forms a first grounding zone 30 and the second spared portion 39 forms a second grounding zone 31.
[0082] The operation of grounding the pressure-sensitive element 5 then comprises a step of electrical connection between the grounding areas 30, 31 and the printed circuit 3 by a connecting means (not shown in this example). Each connecting means comprises a first end connected to one of the spared portions 38, 39 and a second end connected to a connection area 36, 37 formed on the printed circuit 3.
[0083] According to another embodiment shown in Figure 4, the external face 7 of the substrate 6 is entirely covered by the dielectric layer 8 during the step of depositing the dielectric layer 8 on the external face 7 of the substrate 6. This deposition is followed by a laser ablation operation applied to at least a portion of the dielectric layer 8 to form at least one spared portion 40, 41.
[0084] In the example of Figure 4, the laser ablation operation is applied to a first portion of the dielectric layer 8 to form a first spared portion 40 and to a second portion of the dielectric layer 8 to form a second spared portion 41.
[0085] Each spared portion 40, 41 has a generally triangular shape with a vertex 45 positioned opposite one of the two longitudinal sides 44 of the Wheatstone bridge 43.
[0086] The spared portions 40, 41 are positioned opposite each other and separated by the Wheatstone bridge 43.
[0087] Each spared portion 40, 41 extends from the apex 45 to an opposite side 53 along a peripheral edge portion 48 of the substrate 6.
[0088] The spared portion 40, 41 comprises a first edge 54 and a second edge 55 which are rectilinear. The first edge 54 runs along a first side 51 of the first conductive zone 17. The second edge 55 runs along a second side 52 of the second conductive zone 18.
[0089] The first spared portion 40 forms a first grounding zone 30 and the second spared portion 41 forms a second grounding zone 31.
[0090] The operation of grounding the pressure-sensitive element 5 then comprises a step of electrical connection between the two spared portions 40, 41 and the printed circuit 3 by a connecting means (not shown in this example).
[0091] Each connecting means comprises a first end connected to one of the spared portions 40, 41 and a second end connected to a connection zone 36, 37 formed on the dielectric layer 8 of the printed circuit 3.
[0092] According to another embodiment (not shown), the first metal layer covers the two spared portions 38, 39, 40, 41 during the step of depositing the first metal layer to form two grounding zones 30, 31.
[0093] The two grounding zones 30, 31 are formed by a portion of the first metal layer covering the spared portion 38, 39, 40, 41 and at the same time as the four conductive zones 17, 18, 19, 20.
[0094] Alternatively, a single spared portion 38, 39, 40, 41 may be formed regardless of the method used by masking according to the embodiment of FIG. 3 or by ablation according to the embodiment of FIG. 4. A single portion of first metal layer may also be produced to form a single grounding zone 30, 31.
[0095] The operation of grounding the pressure-sensitive element 5 then comprises a step of electrical connection between the two portions of the first metal layer and the printed circuit 3 by two connecting means (not shown in this example).
[0096] Each connecting means comprises a first end connected to one of the portions of the first metal layer and a second end connected to a connection zone 36, 37 formed on the dielectric layer 8 of the printed circuit 3.
[0097] According to another embodiment illustrated in FIG. 2, the first metal layer covers the two spared portions 38, 39, 40, 41 during the step of depositing the first metal layer. An etching step is carried out to form the two portions of the first metal layer covering the two spared portions 38, 39, 40, 41.
[0098] A second metal layer is then deposited on the two portions of the first metal layer. The second metal layer makes it possible to form two metal conduction zones 56, 57 superimposed on the two respective portions of the first metal layer by etching, for example. A first metal conduction zone 56 forms a first grounding zone 30. A second metal conduction zone 57 forms a second grounding zone 31. The two metal conduction zones 56, 57 are made of nickel / gold alloy and are printed at the same time as the conductive zones 17, 18, 19, 20 connected to the Wheatstone bridge 43.
[0099] The operation of grounding the pressure-sensitive element 5 then comprises a step of electrical connection between the first metal conduction zone 56 and the printed circuit 3 by a first connecting means 32, and between the second metal conduction zone 57 and the printed circuit 3 by a second connecting means 33.
[0100] Each connecting means 32, 33 comprises a first end 34 connected to one of the metallic conduction zones 56, 57 and a second end 35 connected to one of the two connection zones 36, 37 formed on the dielectric layer 8 of the printed circuit 3.
[0101] In the example of Figure 2, the connecting means 32, 33 are conductive wires 32, 33. The connection is made using bridging technology also called wire bonding. The conductive wires 32, 33 can be made of aluminum or gold.
[0102] Alternatively, the connecting means 32, 33 may be flexible circuits (not shown).
[0103] Alternatively, the pressure sensitive element 5 may comprise a single grounding zone 30.
[0104] Alternatively, the pressure-sensitive element 5 may comprise more than two grounding zones 30, 31.
Claims
CLAIMS 1. Pressure sensor (1) comprising a connector (2), a printed circuit (3), a metal body (4) and a pressure-sensitive element (5) comprising a substrate (6) comprising an inner face connected to the metal body (4) and an outer face (7) covered by a dielectric layer (8) on which are positioned four resistors (9, 10, 11, 12) arranged to form four electrical poles (13, 14, 15, 16), each electrical pole (13, 14, 15, 16) being formed between two resistors (9, 10, 11, 12) and being connected to a conductive zone (17, 18, 19, 20) extending over the dielectric layer (8), each conductive zone (17, 18, 19, 20) being electrically connected to a connection zone (21, 22, 23, 24) formed on the printed circuit (3) by a connecting element (25, 26, 27, 28), characterized in that the external face (7) of the substrate (6) comprises at least one grounding zone (30, 31) electrically connected to the printed circuit (3) by a connecting means (32,33)., 2. Pressure sensor (1) according to claim 1, characterized in that the grounding zone (30, 31) is positioned between two conductive zones (17, 18, 19, 20).
3. Pressure sensor (1) according to any one of claims 1 or 2, characterized in that each connecting means (32, 33) is a connecting wire (32, 33) comprising a first end (34) connected to the grounding zone (30, 31) and a second end (35) connected to a connection zone (36, 37) formed on the printed circuit (3).
4. Pressure sensor (1) according to any one of claims 1 to 3, characterized in that the grounding zone (30, 31) is formed by a spared portion (38, 39, 40, 41) of the external face (7) of the substrate (6), the spared portion (38, 39, 40, 41) being directly connected to the printed circuit (3).
5. Pressure sensor (1) according to any one of claims 1 to 3, characterized in that the grounding zone (30, 31) is formed by a portion of a first metal layer covering a spared portion (38, 39, 40, 41) of the external face (7) of the substrate (6), the four resistors (9, 10, 11, 12) being formed by other portions of the first metal layer, the portion of the first metal layer being connected to the printed circuit (3).
6. Pressure sensor (1) according to any one of claims 1 to 5, characterized in that the grounding zone (30, 31) is formed by a metallic conduction zone (56, 57) covering a portion of the first metallic layer, the portion of the first metallic layer covering a spared portion (38, 39, 40, 41) of the external face (7) of the substrate (6), the metallic conduction zone (56, 57) being connected to the printed circuit (3).
7. Air conditioning system for a motor vehicle characterized in that it comprises a pressure sensor (1) as defined according to any one of claims 1 to 6.
8. Method for manufacturing a pressure sensor (1) comprising a connector (2), a printed circuit (3), a metal body (4) and a pressure-sensitive element (5) comprising a substrate (6), the method comprising: - a step of depositing a dielectric layer (8) on an external face (7) of the substrate (6), - a step of depositing a first metallic layer on the dielectric layer (8), - a step of depositing a second metal layer on the first metal layer, a step of etching on the first metal layer to form four resistors (9, 10, 11, 12) on the dielectric layer (8) arranged to form four electrical poles (13, 14, 15, 16), a conductive zone (17, 18, 19, 20) being formed on a portion of each electrical pole (13, 14, 15, 16) from the second metal layer, and - a step of connection between the conductive zone (17, 18, 19, 20) and a connection zone (21, 22, 23, 24) formed on the printed circuit (3) by a connection element (25, 26, 27, 28), characterized in that it comprises: - an operation of grounding the pressure-sensitive element (5) comprising a step of forming at least one grounding zone (30, 31) on the external face (7) of the substrate (6) and a step of electrical connection between the grounding zone (30, 31) and the printed circuit (3) by a connecting means (32, 33).
9. Manufacturing method according to claim 8, characterized in that during the step of depositing the dielectric layer (8) on the external face (7) of the substrate (6), at least one spared portion (38, 39) is formed on the external face (7) of the substrate (6) by masking a portion of the dielectric layer (8).
10. Manufacturing method according to claim 8, characterized in that the external face (7) of the substrate (6) is entirely covered by the dielectric layer (8) during the step of depositing the dielectric layer (8) on the external face (7) of the substrate (6), a laser ablation operation then being applied to at least a portion of the dielectric layer (8) to form at least one spared portion (40, 41) on the external face (7) of the substrate (6).
11. Manufacturing method according to any one of claims 9 or 10, characterized in that the spared portion (38, 39, 40, 41) forms the grounding zone (30, 31), the method comprising an electrical connection step during which the connection means (32, 33) connects the spared portion (38, 39, 40, 41) to a connection zone (36, 37) formed on the printed circuit (3).
12. Manufacturing method according to any one of claims 9 or 10, characterized in that the first metal layer covers the spared portion (38, 39, 40, 41) during the step of depositing the first metal layer, the grounding zone (30, 31) being formed by a portion of the first metal layer covering the spared portion (38, 39, 40, 41) and at the same time as the conductive zones (17, 18, 19, 20).
13. Manufacturing method according to any one of claims 9 or 10, characterized in that a portion of the first metal layer covers the spared portion (38, 39, 40, 41) during the step of depositing the first metal layer, a second metal layer being deposited on the portion of the first metal layer covering the spared portion (38, 39, 40, 41) to form a metal conduction zone (56, 57), the metal conduction zone (56, 57) forming the grounding zone (30, 31).