Patent Publication Number: US-9410985-B2

Title: Printed circuit board and magnetic field or current sensor

Description:
RELATED APPLICATIONS 
     This application is the national stage entry under 35 USC 371 of international application PCT/EP2012/067453, filed on Sep. 6, 2012, which claims the benefit of the Sep. 7, 2011 priority date of French application 1157932, the contents of which are herein incorporated by reference. 
     FIELD OF INVENTION 
     The invention relates to a printed circuit, a magnetic field or current sensor incorporating the printed circuit, and a method for fabricating the printed circuit. 
     BACKGROUND 
     A printed circuit, also known by the acronym PCB (Printed Circuit Board), is a support that makes it possible to electrically link a set of electrical components. Such a printed circuit generally takes the form of a laminated plate. This printed circuit can be single-layer or multi-layer. A single-layer printed circuit comprises only a single metallization layer in which are printed conductive tracks that electrically connect the different electrical components together. A multilayer printed circuit comprises, on the other hand, a plurality of metallization layers. Such a printed circuit has at least two layers and, preferably, more than four or six layers. Hereinafter in this description, these multilayer printed circuits will be the focus of interest. 
     A metallization layer is one of the layers of the laminated plate forming the printed circuit in which are produced one or more conductive tracks that electrically connect the different electrical components together. This layer is planar and extends parallel to a plane of the laminated plate. Generally, the metallization layer is obtained by depositing a uniform layer of a conductive material, typically a metal such as copper, then etching this uniform layer to leave only the conductive tracks remaining. 
     The different metallization layers of the printed circuit are spaced apart mechanically from one another by insulating layers made of electrically insulating material. This insulating material exhibits a high dielectric strength, typically greater than 3 MV/m and, preferably, greater than 10 MV/m. For example, the electrically insulating material is produced from epoxy resin and/or glass fiber. The insulating layer generally takes the form of a rigid plate produced in a material that does not become viscous during its assembly with other layers. For example, the insulating layer can be produced from a thermosetting resin that has already undergone an irreversible thermosetting process. 
     The different layers of the multilayer printed circuit are assembled together, with no degree of freedom, using adhesive layers called “prepreg.” 
     A prepreg typically consists of a thermosetting resin impregnating a reinforcement, such as a fabric. Typically, the resin is an epoxy resin. During the fabrication of the printed circuit, the transformation of the thermosetting resin involves an irreversible polymerization that transforms the prepreg into a solid and rigid material that irreversibly bonds together the different layers of the printed circuit. Typically, each transformation occurs when the prepreg is heated to a high temperature and is compressed with a high pressure. Here, a high temperature is a temperature greater than 100° C. and, preferably, greater than 150° C. A high pressure is a pressure greater than 0.3 MPa and, typically, greater than 1 MPa. 
     The conductive tracks of the different metallization layers can be electrically connected via conductive bump contacts passing through the insulating layers. The conductive bump contacts are better known as “vias.” The vias generally extend at right angles to the plane of the layers. There are different ways of fabricating these vias. One of the most common ways is to produce a hole in the insulating layer or layers to pass through and then to cover the inner wall of these holes with a metal. These are called metalized holes. 
     A via does not necessarily pass through all the layers of the printed circuit. Thus, there are blind vias that emerge on a single outer face of the printed circuit. It is also possible to produce “buried” vias for example, using known technologies such as HDI (High Density of Integration). A buried via does not emerge on any of the outer faces of the printed circuit. For example, a buried via makes it possible to electrically connect conductive tracks produced in metallization layers embedded inside the printed circuit. 
     To obtain magnetic field or current sensors, it has already been proposed to fabricate them from a printed circuit notably comprising a magnetic core. A known magnetic field sensor produced from a printed circuit is described in Kubik, et al., “Magnetometer with pulse-excited miniature fluxgate sensor”, Journal of electrical engineering, vol. 57, No. 8/S, 2006, 80-83. 
     Such a magnetic field sensor is particularly accurate. On the other hand, this magnetic field sensor is very sensitive to the ambient conditions and, in particular, to temperature and mechanical stresses. For example, its accuracy varies as a function of the temperature, which degrades its performance. 
     SUMMARY 
     The invention aims to remedy this drawback by providing a printed circuit in which the printed circuit also comprises an anti-creep structure that is suitable for preventing the prepreg from flowing, by creep, during the assembly of the insulating and metallization layers, into the cavity until it comes into direct contact with the magnetic core over more than 20% of the surface of its top face. Such a printed circuit makes it possible to produce more accurate sensors. 
     In practice, it has been observed by the inventor that, in the absence of a creep suppressor, the prepreg of the magnetic field sensor disclosed in Kubik creeps during the assembly of the printed circuit and covers most of the top face of the magnetic core. After assembly, the association of the prepreg and of the magnetic core forms a composite with anisotropic thermal expansion. After thermosetting, the prepreg bonded onto the magnetic core forms a rigid material that has a thermal expansion coefficient that is different from that of the magnetic core. Consequently, in response to a change of temperature, mechanical stresses occur in the magnetic core. These mechanical stresses modify the magnetic properties of the core by magnetostriction. It is very difficult to fabricate soft magnetic materials that are strictly free of magnetostriction. The modification of the magnetic properties of the core degrades the accuracy of the sensor. Furthermore, the temperature calibration of the magnetic core is very difficult to perform because the modifications of the magnetic properties of the core that are generated in these conditions are nonlinear. 
     The above printed circuit, when equipped with a creep suppressor, prevents the formation of the composite with anisotropic thermal expansion over a substantial part of the surface of the core. This also makes it possible to conserve at least one degree of freedom on the magnetic core when it is subjected to the mechanical stresses. Thus, the problems associated with the thermal expansion of the core as described previously for the sensor described in Kubik are greatly reduced. This enhances the sensitivity of the sensor and makes its calibration easier. 
     In some embodiments, the printed circuit comprises at least one coil wound around the magnetic core, turns of this coil being formed by conductive tracks produced in metallization layers situated above and below the magnetic core and a bump contact passing through the insulating layers electrically connecting these conductive tracks. 
     In other embodiments, the creep suppressor comprises a cap arranged between the prepreg and a top face of the magnetic core facing toward the metallization layer, this cap covering all the top face of the magnetic core over more than 80% of its length. 
     In yet other embodiments, the insulating layer comprises two shoulders hollowed out at the intersection between the cavity and the top face of the insulating layer, the depth p of each shoulder in the vertical direction being strictly less than the difference h−e, where h is the depth, from the top face, in the vertical direction, of the cavity and e is the vertical thickness of the core, and the cap is a plate with the edges thereof resting on the shoulders; 
     In those embodiments that use a cap as a creep suppressor, the cap is produced from a soft material, i.e. with a Young&#39;s modulus less than 0.1 GPa, poured into a gap between the magnetic core and the cavity and covering more than 80% of the top face of the magnetic core. 
     A variety of creep suppressors can be used in alternative embodiments. For example, the creep suppressor, in some embodiments, comprises openings produced in the prepreg facing the cavity, these openings being wider than the mouth of the cavity in the top face of the insulating layer to prevent the prepreg from flowing as far as the magnetic core during assembly. In other embodiments, the creep suppressor includes a thermosetting resin incorporated in the prepreg with a viscosity greater than or equal to 10,000 pascal-seconds at the glass transition temperature during its temperature rise. 
     A printed circuit as described above offers numerous advantages. 
     One such advantage is that producing the turns of the coil using conductive tracks formed in the metallization layers makes it possible to industrialize the fabrication of the coil. 
     Another advantage is that a creep-suppressing cap makes it possible to prevent the prepreg from flowing as far as the magnetic core in a relatively simple way. 
     Yet another advantage, in some embodiments, is that having a shoulder to receive the cap makes it possible to prevent the cap from mechanically stressing the magnetic core and also more effectively prevents the prepreg from reaching the magnetic core. 
     Another advantage in some embodiments arises from the use of a soft material to form the cap poured over the magnetic core. This makes it possible to hold the magnetic core in place while avoiding the formation of a composite with anisotropic thermal expansion with the prepreg. 
     Another subject of the invention is a magnetic field or current sensor comprising a magnetic core and the above printed circuit, the magnetic core of the sensor being formed by the magnetic core of the printed circuit. 
     Another subject of the invention is a method for fabricating the above printed circuit. Such a method includes the acts of hollowing out a cavity in an insulating layer, this cavity emerging on a top face of this layer, the insertion inside this cavity of a magnetic core, longer than it is wide, fabricated independently of the insulating layer, this magnetic core having transversal dimensions that are smaller than the corresponding transversal dimensions of the cavity so as to form a gap of at least 5 micrometers between, on the one hand, vertical walls and a roof of this cavity and, on the other hand, facing vertical and top faces of the magnetic core, the deposition of a metallization layer above the insulating layer, this metallization layer at least partly covering the cavity, the assembly of these layers by bonding using a prepreg directly deposited on the insulating layer and below the metallization layer, the prevention, using anti-creep means, of the flow, by creep during the assembly of the insulating and metallization layers, of the prepreg into the cavity until it comes into direct contact with the magnetic core over more than 20% of the surface of its top face. 
     In some practices, the fabrication method can include, before the assembly of the insulating and metallization layers, the insertion into the cavity of a temporary block in place of the magnetic core, and, after assembly, the replacement of this temporary block by the magnetic core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description, given solely as a nonlimiting example and with reference to the drawings in which: 
         FIG. 1  is a schematic illustration, in cross section and in perspective, of a current sensor; 
         FIG. 2  is a schematic illustration, in cross section and in perspective, of a printed circuit used to produce the sensor of  FIG. 1 ; 
         FIG. 2A  is a schematic illustration of the enlargement of a detail of  FIG. 2 ; 
         FIG. 3  is a schematic illustration, in cross section and in an exploded view, of a first embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 4  is a schematic illustration in cross section of the printed circuit of  FIG. 2 ; 
         FIG. 5  is a flow diagram of a method for fabricating the printed circuit of  FIG. 2 ; 
         FIG. 6  is a schematic illustration, in cross section and in an exploded view, of a second embodiment of the printed circuit of  FIG. 2 ; 
         FIGS. 7 to 9  are schematic illustrations in cross section of a third embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 10  is a schematic illustration, in cross section and in an exploded view, of a fourth embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 11  is a schematic illustration, in cross section and in an exploded view, of a fifth embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 12  is a schematic illustration in cross section of a sixth embodiment of the printed circuit of  FIG. 2 ; 
         FIGS. 13 and 14  are schematic illustrations, in cross section and in exploded views, respectively, of a seventh and of an eighth embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 15  is a schematic illustration, in cross section and in an exploded view, of a ninth embodiment of the printed circuit of  FIG. 2 ; 
         FIG. 16  is a schematic illustration in perspective of a prepreg used in the embodiment of  FIG. 15 ; 
         FIG. 17  is a schematic illustration, in cross section and in perspective, of another embodiment of the printed circuit of the sensor of  FIG. 1 ; 
         FIG. 18  is a flow diagram of a method for fabricating the printed circuit of  FIG. 17 ; 
         FIGS. 19 to 21  are schematic illustrations of different fabrication steps in the method of  FIG. 18 , and 
         FIG. 22  is a schematic illustration in perspective of a triaxial magnetic field sensor. 
     
    
    
     In these figures, the same references are used to designate the same elements. 
     DETAILED DESCRIPTION 
     Hereinafter in this description, the features and functions that are well known to a person skilled in the art are not described in detail. 
       FIG. 1  represents a current sensor  2  suitable for measuring the intensity of the current circulating in a conductor wire  3 . 
     The sensor  2  comprises a superposition  4  of a plurality of printed circuits in which is formed a hole  5  passing through this superposition  4  from side to side in a vertical direction Z. 
     The wire  3  is received in the hole  5 . To this end, the transversal dimensions of the hole  5  say in a horizontal plane, are typically greater by at least 100 μm than the outer diameter of the wire  3 . 
     Here, a stack, in the direction Z, of two printed circuits  6  and  7 , forms the superposition  4 . These printed circuits are fastened one on top of the other with no degree of freedom. The outline of these circuits is represented by broken lines to allow the various elements of which they are composed to show. 
     The circuit  6  comprises a stack, in the direction Z, of a plurality of metallization layers mechanically separated from one another by insulating layers. The different layers of this circuit  6  extend horizontally parallel to orthogonal directions X and Y. 
     The circuit  6  comprises a magnetic core that surrounds the hole  5 . This core is a magnetic ring  10  that extends essentially in a horizontal plane. 
     To increase the sensitivity of the sensor  2 , the magnetic ring  10  is made of a magnetic material exhibiting a static relative permeability, that is to say for a zero frequency, greater than 1,000 and, preferably greater than 10,000. For this, the magnetic ring  10  is made of a magnetic material naturally exhibiting a strong relative permeability. For example, the magnetic ring  10  can be made of a ferromagnetic material, such as a mu-metal or a magnetic material, including those known by the marketing name of VITROVAC® 6025. 
     Preferably, the center of the ring  10  is centered on the hole  5 . 
     An excitation coil  12 , which is intended to be passed through by an excitation current i ex  at a frequency f ex , is wound around the ring  10 . This coil  12  generates, inside the ring  10 , an excitation magnetic field B ex  that periodically saturates this ring. 
     Preferably, the coil  12  extends over the entire length of the ring  10 . The “length of the ring  10 ” here designates the perimeter of the magnetic ring in the horizontal plane. Each turn of this coil passes through the interior of the ring. To simplify  FIG. 1 , only one turn of the coil  12  is represented. 
     The printed circuit  6  also comprises a measurement coil  13  intended to measure the intensity of the magnetic field inside the ring  10 . This coil  13  is wound around the excitation coil  12 . To simplify  FIG. 1 , only two turns of this coil  13  are represented. Each turn passes through the center of the ring  10 . This coil  13  extends over the entire length of the magnetic ring  10 . Hereinafter in this description, the measurement of this coil  13  is given by the following relationship: M 1 =B i +B ex , in which:
         M 1  is the measurement of the coil  13 ,   B i  is the magnetic field generated by the current i that circulates in the wire  3 , and   B ex  is the excitation magnetic field.       

     To write this formula, it is assumed that the sign of the magnetic field is positive when this magnetic field rotates inside the ring  10  in the counterclockwise direction. 
     The use of a magnetic ring makes it possible to automatically compensate for the external magnetic field influence, which is uniform over the entire surface of the printed circuit  6 . As used herein, “compensate” designates the action of eliminating or of greatly reducing the contribution of a magnetic field in measuring the intensity of the current i. For example, this makes it possible to automatically compensate for the influence of the Earth&#39;s magnetic field. 
     The printed circuit  7  is the symmetrical counterpart of the printed circuit  6  relative to a horizontal plane, except that the excitation coil is here wound in the reverse direction. The magnetic ring, the excitation coil and the measurement coil of the printed circuit  7  are respectively given the references  15 ,  16  and  17 . The coil  16  is powered by the same excitation current i ex  as the excitation coil  12 . In these conditions, the measurement of the coil  17  is given by the following relationship: M 2 =B i −B ex , in which M 2  is the measurement of the coil  17 . 
     Here, the coils  13  and  17  are connected in series to automatically compensate for the contribution of the excitation magnetic field B ex . As used herein, “compensate” designates the fact of best eliminating the influence or the contribution of this magnetic field B ex  in the final result, that is to say, here, in the measurement of the current. With the sign conventions adopted here, the coils  13  and  17  are connected in such a way as to add together the measurements M 1  and M 2 . 
     The sensor  2  also comprises an electronic processing unit  18  connected to the superposition  4  via wired links. This unit  18  processes the measurements obtained from the coils  13  and  17  to obtain a measurement of the intensity of the current i that circulates in the wire  3 . The unit  18  also includes a current source  19  suitable for generating the excitation current i ex  that circulates in the coils  12  and  16 . 
       FIG. 2  represents, in more detail, the printed circuit  6  used to produce the sensor  2 .  FIG. 2  represents a cross section of the printed circuit  6  to show the details that are embedded inside this printed circuit. 
     The printed circuit  6  is a multilayer printed circuit. It is therefore formed by a stack  20 , in the vertical direction Z, of a plurality of metallization layers separated by insulating layers. In  FIG. 2 , only two metallization layers  22  and  24  separated by one insulating layer  26  are represented. This stack  20  is described in more detail in relation to the subsequent figures. 
     A cavity  28  is hollowed out in the insulating layer  26  to receive the magnetic ring  10 . The cavity  28  and its details are more visible on the enlarged portion of the cross section of the printed circuit represented in  FIG. 2A . Here, the cavity  28  is annular. It is situated between the two metallization layers  22  and  24 . This cavity admits a vertical axis of symmetry  30 . The axis  30  is also an axis of symmetry for the hole  5 . This cavity has a rectangular transversal section. A transversal section is the section contained in a vertical plane containing the axis  30 . More specifically, the cavity  28  has an outer vertical wall  32 , an inner vertical wall  33 , a flat roof  34  and a flat bottom  36 . The vertical walls  32  and  33  are circular. The transversal section of the cavity is constant over its entire length. The length of the cavity is its perimeter, for example internal, in a horizontal plane. 
     The magnetic ring  10  is housed inside the cavity  28 . The axis of revolution of the ring  10  is merged with the axis  30 . The transversal section of the magnetic ring  10  is also rectangular. This transversal section is also constant over the entire length of the ring  10 . 
     In order to increase the static relative permeability, the thickness  e  in the direction Z of the core is as small as possible. For example, the core has a thickness that is typically less than 250 μm and preferably less than 125 μm or 25 μm. Its width  l  in a radial direction is typically between 0.5 mm and 10 mm. For example, here, the width  l  is between 1 and 2 mm. 
     The magnetic ring  10  has an inner face that rests on the bottom  36  of the cavity  28 , for example only under the action of the force of gravity. The ring  10  also has a top face  40  facing the roof  34  of the cavity  28  and outer and inner vertical faces respectively facing the vertical walls  32  and  33 . 
     The dimensions of the transversal section of the ring  10  are less than the corresponding dimensions of the transversal section of the cavity  28 . Thus, its width  l  and its thickness  e  are less, respectively, than the width and the thickness of the cavity  28 . This makes it possible to separate the ring  10  from the vertical walls  32  and  33  by a lateral gap  i  and to separate the top face  40  of the ring  10  from the roof  34  by a gap  j . The gaps  i  and  j  are greater than 5 μm and, preferably greater than 100 μm. Thus, in these conditions, the cavity  28  exerts no mechanical stress on the ring  10 . The fact that the ring  10  is not mechanically stressed increases the accuracy of the sensor  2  because that limits the modifications of its magnetic properties. 
     The circuit  6  also comprises the coils  12  and  13 . To simplify the representation, only the turns of the coil  12  are represented. For example, these turns are evenly arranged at regular intervals over the entire length of the ring  10 . 
     Each turn of the coil  12  is formed by: a conductive track  44  produced in the metallization layer  22 , and a conductive track  46  produced in the metallization layer  24 . 
     Each turn also comprises a vertical via  48  electrically linking the ends of the tracks  44  and  46  of one and the same turn. The other end of the conductive tracks is also connected by another vertical via to the end of a conductive track of an adjacent turn. 
       FIG. 3  represents a first embodiment of the printed circuit  6  in an exploded view. In this embodiment, the stack  20  is produced by stacking, from bottom to top: a metalized prepreg  50 , an insulating layer  52 , and a metalized prepreg  54 . 
     The metalized prepreg  50  comprises a prepreg  56  and the metallization layer  24  fixed onto the bottom face of the prepreg  56 . 
     The metalized prepreg  54  comprises a prepreg  60  and the metallization layer  22  fixed onto a top face of the prepreg  60 . 
     Here, the prepregs  56  and  60  are prepregs qualified as “standard.” Standard prepregs have a low viscosity during the assembly of the printed circuit. “Low viscosity” denotes a viscosity less than 5,000 pascal-seconds and, preferably, less than 1,000 pascal-seconds at the glass transition temperature when the temperature of the prepreg is high according to the specifications of the manufacturer of this prepreg. 
     For example, the prepregs  56  and  60  can be produced in one of the following materials: FR-2 (phenolic paper cotton), FR-3 (epoxy resin, paper and cotton), FR-4 (woven glass fiber and epoxy resin), FR-5 (woven glass fiber and epoxy resin), FR-6 (coating of glass and polyester), G-10 (woven glass fiber and epoxy resin), CEM-1 (paper cotton and epoxy resin), CEM-2 (paper cotton and epoxy resin), CEM-3 (woven glass fiber and epoxy resin), CEM-4 (woven glass fiber and epoxy resin), CEM-5 (woven glass fiber and polyester). Here, these prepregs  56  and  60  are prepregs from the family of products under the reference 33N (for example, the prepreg of reference 33N2355) sold under the mark ARLON®. 
     The layer  52  is an insulating layer such as that described in the introduction to this patent application. For example, this layer  52  conforms to the FR-4 standard. It has a bottom face  64  facing the prepreg  56  and a top face  66  turned toward the prepreg  60 . The prepregs  56  and  60  entirely cover, respectively, the faces  64  and  66 . They are also directly deposited on these faces  64  and  66  and are therefore in direct contact without the intermediary of other layers. 
     The cavity  28  is hollowed out from the face  66  inside the layer  52 . This cavity  28  therefore has a mouth that emerges in the face  66 . This mouth forms two shoulders  70  and  72 , respectively, at the top of the vertical walls  32  and  33  of the cavity  28 . The depth of these shoulders  70  and  72  is given by the following relationship: p=h−e−j, in which:
         p is the depth of the shoulders  70 ,  72  measured in the Z direction,   h is the depth of the cavity  28 , measured in the Z direction.   e is the thickness of the ring  10 , and   j is the vertical gap between the ring  10  and the roof  34  of the cavity  28 .       

     The depths p and h are measured in the direction Z. 
     The printed circuit  6  also comprises an anti-creep cap  76  specifically for preventing the flow of the prepreg  60 , by creep during the assembly, on the magnetic ring  10 . To this end the cap  76  here takes the form of a circular ring the axis of revolution of which is merged with the axis  30 . The inner diameter and the outer diameter of this cap  76  are chosen in such a way that they rest, respectively, on the shoulders  72  and  70 . Thus, this cap  76  does not bear on the magnetic ring  10  when the printed circuit is assembled and therefore does not exert any mechanical stress on this magnetic ring  10 . 
     The thickness of the cap  76  is less than or equal to the depth p of the shoulders  70  and  72 . For example, the thickness of the cap  76  is less than the depth by at most 10%. Thus, after assembly, the cap does not form a protuberance beyond the top face  66 . 
     The cap  76  is produced in a rigid material. As used herein, a rigid material is one with a Young&#39;s modulus that is greater than 2 GPa and preferably greater than 10 or 100 GPa. In this description, the values of the Young&#39;s modulus are given at a temperature of 20° C. For example, the cap  76  can be produced in FR-4. For example, the cap  76  can be produced by cutting from a rigid plate. The cap  76  is produced in a non-magnetic material. As used herein, a non-magnetic material is one with a static relative permeability that is equal to 1. 
       FIG. 4  represents the same printed circuit as  FIG. 3  but in an assembled position. Furthermore, in  FIG. 4 , the vertical vias  48  that link the tracks  44  to the tracks  46  are represented. Once assembled, the superposition of the layer  52  and of the prepregs  56  and  60  forms the insulating layer  26 . 
     A method for fabricating the printed circuit  6  will now be described with reference to the method of  FIG. 5 . 
     In a step  80 , the layer  52  is machined to form the cavity  28 . 
     In parallel, in step  82 , the magnetic ring  10  is fabricated independently of the fabrication of the layer  52 . For example, the magnetic ring  10  is fabricated by cutting from a sheet of magnetic material. 
     In parallel with the step  82 , in step  84 , the cap  76  is also fabricated independently of the fabrication of the ring  10  and of the layer  52 . For example, the cap  76  is fabricated by cutting from a panel of a rigid non-magnetic material. 
     Then, in step  86 , the magnetic ring  10  is inserted into the cavity  28 . 
     In step  88 , the cap  76  is deposited on the shoulders  70  and  72  of the cavity  28 . 
     In step  90 , the metallization layers are assembled. To this end, in this embodiment, the metalized prepregs  50  and  54  are arranged, respectively, facing the faces  64  and  66  of the insulating layer  52 . Then, the prepregs are heated to a temperature greater than or equal to 150° C. and, at the same time, a vertical pressure greater than 1 MPa is applied to the assembly of the layers to provoke the thermosetting of the prepregs  56  and  60 . In this step, the prepregs  56  and  60  are transformed by polymerization into a rigid material that definitively bonds the metallization layers to the faces  64  and  66  of the insulating layer  52 . 
     In step  90 , the prepreg  60  creeps. However, the cap  76  prevents the prepreg from flowing to the magnetic ring  10  to form a composite with an anisotropic thermal expansion. 
     In step  92 , the conductive tracks  44  and  46  are produced in the metallization layers, respectively,  22  and  24 . For example, these tracks are produced by etching the metallization layers. 
     Then, in step  94 , the vias are produced to electrically connect the different conductive tracks and thus form the coil  12 . For example, the vias are produced by drilling holes and then metalizing the inner walls of these holes. 
     The steps  90  to  94  can be reiterated to add extra metallization layers in the stack. 
       FIG. 6  represents another embodiment of the printed circuit  6  in which the shoulders  70  and  72  are omitted. In these conditions, the cap  76  is replaced by a cap  100  whose dimensions are adjusted for the latter to be able to be housed inside the cavity  28 . In this embodiment, the cap  100  bears mechanically on the ring  10  but does not form a composite with an anisotropic thermal expansion with the ring. 
       FIG. 7  represents another embodiment of the printed circuit  6  in an exploded view. This embodiment is identical to that described with reference to  FIG. 3 , except that the insulating layer  52  is replaced by an insulating layer  102  and the cap  76  is replaced by a cap  104 . 
     The insulating layer  102  is identical to the insulating layer  52  except that the central core of the layer  52  around which the cavity  28  is wound is replaced by a bore that passes vertically right through the insulating layer  102 . 
     The cap  104  is identical to the cap  76  except that the central core that has been eliminated from the insulating layer  102  is now incorporated in the cap  104 . The cap  104  therefore takes the form of a disk whose central core is arranged to be introduced in the bore of the insulating layer  102 . 
       FIG. 8  represents the embodiment of  FIG. 7  in the assembled position. 
       FIG. 9  represents the printed circuit of  FIG. 8  after new metalized prepregs, respectively  110  and  112 , have been stacked on the top and bottom outer faces of this printed circuit. This therefore makes it possible to add two metallization layers. Here, these additional metallization layers are used to form conductive tracks  114  and  116  linked by vertical vias  119  to form, for example, the turns of the measurement coil  13 . 
       FIG. 10  represents another embodiment of the printed circuit that is identical to the embodiment of  FIG. 7  except that the shoulders of the cavity  28  are omitted. In this figure, the insulating layer and the cap bear, respectively, the references  120  and  122 . 
       FIG. 11  represents another embodiment of the printed circuit  6  identical to that described with respect to  FIG. 3  except that it does not use any metalized prepreg but instead uses metalized insulating layers. More specifically, the metalized prepreg  50  and the insulating layer  52  are replaced by a metalized insulating layer  124 . The metalized insulating layer  124  comprises an insulating layer  126  on the bottom face of which is fixed the metallization layer  24 . The prepreg  54  is replaced by a metalized insulating layer  130  formed from the metallization layer  22  and from an insulating layer  132 . The metalized insulating layers  130  and  124  are assembled with one another using a prepreg  134  covering all of the top face of the insulating layer  126 . 
     The fabrication of this embodiment of the printed circuit  6  is identical to that described with respect to  FIG. 5  except that, in the assembly step  90 , the prepreg  134  is introduced between the insulating layers  124  and  130  to produce the bonding of these metalized insulating layers. 
       FIG. 12  represents another embodiment of the printed circuit  6  identical to that described with reference to  FIG. 6  except that, in this embodiment, the cap  76  is replaced by a cap  140 . The cap  140  is produced in a soft material. As used herein, a soft material is one with a Young&#39;s modulus less than 0.1 GPa. 
     In this embodiment, the insertion of the cap on the magnetic ring  10  consists in pouring the soft material into the cavity  28  on the ring  10 . When it is poured, the viscosity of this material is low enough for the latter to entirely fill the gaps i and j without exerting any mechanical stress on the ring  10 . Typically, the material used to produce the cap  140  is a non-thermosetting flexible resin. The chosen resin is also able to withstand the printed circuit fabrication temperatures. One example of such a resin is silicone. The rigidity of this material  140  is sufficiently low for it to avoid exerting any mechanical stresses on the magnetic ring  10  and for it to avoid forming, with the latter, a composite with anisotropic thermal expansion that would be likely to deform the ring  10  in response to temperature variations. Once the cap  140  has been poured into the cavity  28 , the rest of the fabrication steps are identical to those described with reference to  FIG. 5 . 
       FIG. 13  represents an embodiment of the printed circuit  6  that is identical to that of  FIG. 6  except that the cap  100  is omitted. In this embodiment, the metalized prepreg  54  is replaced by a metalized prepreg  150 . The metalized prepreg  150  is identical to the prepreg  54  except that the prepreg  60  is replaced by a prepreg  152 . The prepreg  152  exhibits a viscosity at the glass transition temperature at least ten times and, preferably, twenty times greater than the viscosity of the prepreg  60  when measured in the same conditions. For example, the prepreg  152  comprises only a thermosetting resin with a viscosity greater than 10,000 pascal-seconds and, preferably, greater than 20,000 pascal-seconds at the glass transition temperature when its temperature is raised in accordance with the specifications of the manufacturer. For example, the temperature is raised by 5° C./minute. The viscosity of a prepreg can also be measured in “mils” or in millimeters. For this, a hole of 1 inch diameter (0.03 meters) is hollowed out in the prepreg. Then, insulating layers are assembled with this prepreg in the same conditions as those that are applied in the step  90 . During the assembly, the prepreg creeps and reduces the diameter of the hole. 
     The difference between the diameter of the hole before assembly and the average diameter of the hole after assembly constitutes a measurement of the viscosity of the prepreg. This difference is expressed in “mil” (2.54*10 −5  meter) or in millimeters. The viscosity of the prepreg  152  measured according to this method is typically less than 3.5 mm and, preferably, less than 2 mm or 1.5 mm. For example, the prepreg  152  is one from the family of reference 37N sold by the company ARLON®. For example, it is the one bearing the reference 37N0 666. 
     Such a prepreg is known by the term “no-flow prepreg.” The prepreg  152  forms a creep suppressor. In practice, given the very high viscosity of this prepreg even during assembly, the latter cannot flow as far as the magnetic ring  10  even in the absence of a cap. 
     The fabrication of this embodiment is identical to that described with reference to  FIG. 5  except that the fabrication of the cap and the deposition of this cap can be omitted. 
       FIG. 14  represents an embodiment of the printed circuit  6  identical to that described with reference to  FIG. 11 , except that the prepreg  134  is replaced by a prepreg  160 . The prepreg  160  is a no-flow prepreg, for example identical to the prepreg  152 . In these conditions, the cap  76  can be omitted since the high viscosity of this prepreg  160  during the assembly prevents it from flowing as far as the ring  10 . 
     The embodiment of  FIG. 15  is identical to the embodiment of  FIG. 11  except that the cap  76  and the shoulders are omitted and that the prepreg  134  is replaced by a prepreg  170 . The prepreg  170  is represented in more detail in  FIG. 16 . The prepreg  170  is identical to the prepreg  134  except that it includes openings produced in such a way as to be located facing the mouth of the cavity  28  during assembly. For example, here, it includes four openings  172  to  175 , each in the form of a portion of a ring, and centered on the axis of revolution  30 . 
     The transversal width of each of these openings is equal to the width of the mouth of the cavity  28  in the top face  66 , to which is added an offset margin. The offset margin is such that the prepreg  170  does not extend as far as the lips of the mouth of the cavity  28  during assembly. Thus, before assembly, the edges of the openings  172  to  175  are offset, in a radial direction, relative to the lips of the mouth of the cavity  28  by a distance R. The distance R is determined, for example by trial and error, so that, during assembly, the prepreg  170  cannot flow as far as the magnetic ring  10 . The distance R is, for example, greater than or equal to 100 micrometers and, preferably, greater than or equal to 200 micrometers or 300 micrometers or 1 mm. Thus, these openings  172  to  175  form a creep suppressor. 
     The prepreg  170  also comprises bridges  178  to  181  distributed at regular intervals around the axis  30  in order to mechanically link a central core  184  of the prepreg  170  to the periphery  186  of the prepreg  170 . The width of these bridges  178  to  181  is chosen such that their aggregate surface area is less than 20% and, preferably, less than 10% or 5% of the surface area of the top face of the ring  10 . 
     In these conditions, even if these bridges are, during assembly, made to creep as far as the magnetic ring  10 , the covered surface area of the ring  10  is small enough not to create mechanical stresses likely to significantly disrupt the operation of the sensor  2 . 
       FIG. 17  represents a printed circuit  178  that can be used in place of the circuit  6 . This circuit  178  is identical to the printed circuit  6  except that the magnetic ring  10  is replaced by two magnetic bars  180  and  182 . It therefore comprises a creep-suppressor as described previously. These magnetic bars are parallelepipeds that are arranged parallel to one another in the Y direction. To clarify  FIG. 17 , the bars are shown protruding beyond the circuit  178  but, in reality, they are entirely housed inside this circuit. For example, the thickness and the width of these bars  180  and  182  are identical to those of the ring  10 . The length of the bars  180  and  182  in the Y direction is typically between 5 and 60 mm. An excitation coil and a measurement coil are wound around each of these bars  180 ,  182 . These coils are produced as described for the printed circuit  6 . The top and bottom conductive tracks of the excitation coils bear the references, respectively,  184  and  186  in  FIG. 17 . The measurement coils and the processing unit  18  are not represented to simplify the illustration. 
     The operation of a current sensor produced using the printed circuit  178  is similar to that of the sensor  2 . However, the use of bars rather than a ring makes it possible to measure a current of greater intensity while retaining a reduced footprint. 
       FIG. 18  represents a method for fabricating the printed circuit  178 . 
     This fabrication method is identical to that described with reference to  FIG. 5  except that the magnetic ring  10  is replaced by two magnetic bars and a temporary block is used during assembly. 
     More specifically, step  84  is replaced by step  190 , which includes fabrication of two temporary blocks  191  ( FIG. 19 ) that have dimensions greater than or equal to those of the magnetic bars  180  and  182  and strictly less than the dimension of the cavity  28  in which these bars are received. 
     Step  86  is replaced by step  192  during which the temporary blocks are inserted into each of the cavities in place of the magnetic bars  180 ,  182 . 
     Then, steps  90 ,  92  and  94  are carried out. These steps  90 ,  92  and  94  can be reiterated to stack more than two metallization layers. 
     On completion of step  94 , in step  196  the temporary blocks  191  are replaced by the magnetic cores  180  and  182 . For this, an end of the printed circuit  178  is cut to open the ends of the cavities in which the temporary blocks  191  are housed. The state represented in  FIG. 19  is then obtained. Then, the temporary blocks  191  are removed through these opened ends. The state represented in  FIG. 20  is then obtained. Then, the magnetic cores  180  and  182  are inserted into their respective cavities via these same ends ( FIG. 21 ). 
       FIGS. 19 to 21  have been represented in the particular case in which the creep-suppressors are produced as described with reference to  FIG. 16 . These creep-suppressors are therefore, here, an open work prepreg  198  in which openings are formed facing the cavity to prevent the flow of this prepreg to the magnetic core. 
       FIG. 22  represents a magnetic field sensor  200 . The sensor  200  is one that is sufficiently accurate so that the root mean square of the power spectral density of the noise (“RMS PSD”) is less than 1 nT/√{square root over (Hz)} or 100 pT/√{square root over (Hz)} and, in certain configurations, less than 10 pT √{square root over (Hz)}. 
     Here, this sensor  200  is a sensor familiarly known as a “fluxgate” sensor. The operation of such a sensor is well known from, for example, U.S. Pat. No. 7,372,261, the contents of which are herein incorporated by reference, and therefore need not be described further. 
     Hereinbelow, only the elements of this sensor that are necessary for understanding of the invention are described. 
     Here, the sensor is used to measure a continuous or static magnetic field. For example, this continuous magnetic field corresponds to the Earth&#39;s magnetic field. 
     The sensor  200  is capable of measuring the orientation, and, optionally, the intensity of an external magnetic field T. More specifically, the sensor  2  measures the components T X , T Y  and T Z  corresponding, respectively, to the projections of the magnetic field T in three non-collinear directions X, Y and Z. In the embodiments described herein, the directions X, Y and Z are mutually orthogonal, with the Z direction being the vertical direction and the X and Y directions defining a horizontal plane. 
     The sensor  200  comprises a multilayer printed circuit  204 . Typically, the printed circuit  204  comprises more than five metallization layers and, preferably, more than ten metallization layers. In the embodiments described herein, the printed circuit  204  comprises ten metallization layers. The metallization layers are stacked one on top of the other in the direction Z. The topmost metallization layer in the direction Z corresponds to the top face while the bottommost metallization layer corresponds to the bottom face of the printed circuit. 
     The printed circuit  204  comprises a magnetic ring  206  housed in a horizontal annular cavity. The ring  206  is situated between the top and bottom faces of the printed circuit. This printed circuit  204  is fabricated as described previously for the printed circuit  6 . It therefore comprises, notably, one of the anti-creep structures previously described. Thus, only the main differences between this circuit  204  and the circuit  6  are underlined here. 
     The ring  206  comprises two bars  208  and  209  parallel to the X direction and two bars  211  and  212  parallel to the Y direction. The ends of these bars are linked to one another by a corner made of magnetic material to form the ring  206 . 
     Four excitation coils  214  to  217  are produced in the printed circuit  204  to saturate the magnetic ring  206 . An excitation current i 0H  at an excitation frequency f H  passes through these excitation coils. Typically, the excitation frequency is greater than 300 Hz and preferably greater than 10 kHz. Each of the coils  214  to  217  is wound respectively around the bars  211 ,  208 ,  212  and  209 . 
     The coils  214  to  217  are connected in series to one another so as to generate an excitation magnetic field B exH  of the same direction when they are passed through by the current i OH . Each coil  214  to  217  is formed by conductive tracks produced in two metallization layers of the printed circuit situated, respectively, above and below the magnetic ring  206 . The ends of these conductive tracks are connected together by vertical vias to form the turns of the coils  214  to  217 . 
     The printed circuit  204  also comprises four measurement coils  220  to  223  that are wound, respectively, around the bars  211 ,  208 ,  212 ,  209  to measure the magnetic field inside these bars. The measurements of the magnetic field produced by each of these coils  220  to  223  are denoted, respectively, M 1 , M 2 , M 3  and M 4 . These measurements are given by the following relationships:
 
 M   1   =T   Y   −B   exH ,
 
 M   2   =T   X   +B   exH ,
 
 M   3   =T   Y   +B   exH , and
 
 M   4   =T   X   −B   exH .
 
     These relationships are given with the following convention: the excitation magnetic field B ex  rotates in the counterclockwise direction and the components T X  and T Y  are directed in the same direction as the X and Y directions respectively. 
     The coils  220 - 223  are wound around respective excitation coils  214 - 217 . The coils  220 - 223  are formed by conductive tracks produced in metallization layers situated above and below those used to produce the conductive tracks of the excitation coils. 
     In this embodiment, four compensation coils  226 - 229  are also wound, respectively, around the measurement coils  220 - 223 . These coils  226 - 229  make it possible to cancel the magnetic field in the respective bars  211 ,  208 ,  212 ,  209 . In these conditions, the measurement of the components T X  and T Y  is deduced from the intensity of the compensation current i cH  that circulates in these coils  226 - 229 . 
     The coils  226 - 229  are formed by conductive tracks produced in metallization layers situated above and below those used to form the conductive tracks of the measurement coils. 
     Because it is a null magnetic field that is being worked with, there is reduced magnetic coupling between the measurements produced in the X and Y directions that could otherwise be provoked by the use of measurement coils that are wound around the same magnetic ring  206 . 
     The printed circuit  204  also comprises vertical coils for measuring the component T Z . As used herein, a “vertical coil” denotes a coil that is wound and extends along a vertical axis. 
     More specifically, two holes  230 ,  232  passing right through the printed circuit along, respective vertical axes  234 ,  236  are hollowed out in this printed circuit. These holes  230 ,  232  are intended to each receive a respective branch of a vertical magnetic ring. To simplify illustration, this vertical magnetic ring is omitted from  FIG. 22 . 
     Around these holes  230 ,  232 , the printed circuit comprises two excitation coils  238 ,  239  specifically for generating an excitation magnetic field B exv  capable of saturating the vertical magnetic ring when an excitation current i exv  of frequency f exv  passes through the coils  238 ,  239 . In some embodiments, the current i exv  and the frequency f exv  are equal, respectively, to the current i exH  and to the frequency f exH . 
     A measurement vertical coil  240  surrounds the two holes  230 ,  232 . This coil  240  is intended to measure the magnetic field in the vertical magnetic ring. 
     Finally, a compensation vertical coil  242  is also produced in the printed circuit  240 . This compensation vertical coil  242  surrounds the holes  230 ,  232 . As previously, the function of the compensation vertical coil  242  is to cancel the magnetic field in the vertical magnetic ring when a compensation current i cv  passes through it. 
     The coils  238 ,  239 ,  240 ,  242  are formed by conductive tracks produced in metallization layers of the printed circuit  204 . These conductive tracks are connected to one another via vertical vias to form the turns of each of these coils. 
     Finally, the sensor  200  comprises an electronic processing unit  250  suitable for controlling the power supply for the excitation and compensation coils and for processing the signals from the measurement coils to obtain a measurement of the magnetic field T. Typically, the measurement of each component of the magnetic field T is obtained from the amplitude of a harmonic of the excitation frequency in the magnetic field as measured by the measurement coils. 
     Many other embodiments are possible. For example, the insulating layer can be produced differently. In some examples, the insulating layer conforms to the standard such as FR-5, G10 or G11. 
     The vias can also be produced by metal rivets. 
     The conductive tracks can be produced by technologies other than the etching of a metallization layer. For example, the conductive tracks can equally be deposited on an insulating layer. With this latter technology, the metallization layer is the layer in which the conductive tracks are deposited. 
     In the embodiment of  FIG. 7 , the bore may not be a through bore. 
     The different embodiments of the creep-suppressors described here can be combined with one another. 
     The use of a temporary block can be applied regardless of the creep-suppressors used. 
     The method of  FIG. 18  has been described in the particular case where the step  196  is situated after the step  94 . However, the step  196  can also be carried out after one of the steps  90 ,  92  or  94 . 
     One and the same coil can fulfil both measurement coil and excitation coil functions. This is made possible by the fact that the measurement is made on harmonics of the excitation frequency. In this case, the coils  12  and  13  are, for example, replaced by one and the same coil. The same coil can also fulfill the measurement and compensation coil functions. 
     The turns of the excitation and measurement coils can be interleaved as described with reference to FIG. 6 of U.S. Pat. No. 7,372,261. 
     As a variant, the magnetic rings and the measurement and excitation coils are produced in the same printed circuit. In this case, it is not necessary to superimpose two printed circuits produced independently of one another. 
     Finally, in a simplified embodiment, the magnetic ring  15  and the coils  16  and  17  are omitted. In this case, the processing unit  18  is programmed to compensate for the excitation magnetic field present in the measurement delivered by the coil  13 . 
     The ring  10  is configured to pick up a maximum of magnetic flux generated by the conductor wire. For example, if the wire  3  is replaced by a conductive bar of a rectangular section, it is preferable to have the ring  10  be a rectangular ring. 
     The vertical magnetic ring housed in the holes  230 ,  232  can be produced using two “U”-shaped magnetic parts assembled head-to-tail or by winding a wire or a magnetic strip in the holes. In this case, preferably, the strip or the wire forms one or more turns each passing through the holes. In an alternate embodiment, two vertical and parallel magnetic bars can also replace the magnetic ring. 
     In the embodiment of  FIG. 22 , it is possible to superimpose several multilayer printed circuits in order to increase the length/width ratio of the magnetic rings, as well as the number of turns of the sensor. 
     In another variant, the wire  3  is formed by a rigid portion securely attached to the printed circuit. Typically, this rigid portion of the wire  3  is a vertical electrical track produced in the printed circuit. For example, this vertical electrical track is produced by metallization of the hole  5 . The flexible parts of the wire  3  are then connected to this electrical track to perform the measurement of current intensity.