METHOD FOR MANUFACTURED A STACK OF PRINTED CIRCUIT BOARDS AND STACK OF PRINTED CIRCUIT BOARDS

Abstract

METHOD FOR MANUFACTURING A STACK OF PRINTED CIRCUIT BOARDS AND STACK OF PRINTED CIRCUIT BOARDS The invention relates to a method for manufacturing a stack of printed circuit boards (1) comprising at least: a first step, in which at least three pads (101) are positioned on a first printed circuit board (10), in particular a double-sided board; a second step, in which a layer of adhesive (4) is deposited on the pads (101); a third step, in which a core (2) is positioned on the pads (101); and a fourth step, in which the layer of adhesive (4) is polymerized. Figure to be published with the abbreviation: Figure 4

Description

Title of the invention: METHOD FOR MANUFACTURING A STACK OF PRINTED CIRCUITS AND STACK OF PRINTED CIRCUITS TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of printed circuits, more particularly the field of stacks of printed circuits.

[0002] The present invention relates to a method for manufacturing a stack of printed circuits comprising a toroidal magnetic core between two printed circuits and allowing precise and stable placement of the magnetic core during the various stages of the manufacturing process. The invention also relates to a stack of printed circuits obtained by such a manufacturing method. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Transformers are extremely classic electrical components. They meet various needs of electronic systems. For transmission of high-frequency electrical signals, for transport of electrical energy, for transmission of digital data and for certain current sensors, toroidal magnetic cores of small or very small dimensions, of the order of a few millimeters in diameter for example, are used.

[0004] According to conventional manufacturing, electrical windings are made of enameled copper wires wound around the toroidal core. For the majority of uses, such a technique is satisfactory, despite dispersions in electrical characteristics linked to the more or less precise placement of the electrical windings around the toroidal core.

[0005] Reproducibility of electromagnetic performance is fundamental, particularly for digital bus transformers, because the rejection of disturbances arriving in common mode, particularly “BCI”, acronym for “Bulk Current Injection” in English, lightning or any electromagnetic disturbances likely to pollute the useful signals, or even, in certain cases, leading to destruction of the electronic interfaces, is all the more effective as the characteristics of the transformer are precise and above all those expected.

[0006] On the other hand, the parasitic parameters intrinsic to current measuring sensors of the current transformer type, such as leakage inductance and parasitic capacitances, need to be known and reproduced with little dispersion.

[0007] However, manual or mechanized wire winding does not lead to sufficiently constant electrical characteristics to ensure the required performance.

[0008] To ensure good reproducibility, the positioning of the different elements must be precise and reliable during a manufacturing process of a stack of printed circuits.

[0009] Furthermore, a positioning of the toroidal magnetic core is critical and must remain stable during all stages of the manufacturing process. It is, in fact, necessary to know very well a lateral positioning of the core within the printed circuit with respect to the metallized holes, also called vias, so that these do not approach too close to the inner cylinder formed by the central hole of the toroidal magnetic core.

[0010] The manufacturing process must also not stress the core too much, which could produce micro-cracks within it and render it unusable.

[0011] Operational thermal stresses must also not generate mechanical forces resulting from differences in the coefficient of thermal expansion, also known as "CTE" for "Coefficient of Thermal Expansion" in English, which could lead to breakage of the magnetic cores.

[0012] In addition, deposition of the resin layer and pressing of the assembly may cause displacement of the core. Summary of the invention

[0013] The invention offers a solution to the problems mentioned above, by allowing precise and stable positioning, during all stages of the manufacturing process, without generating too much stress on the core.

[0014] To this end, a first aspect of the invention relates to a method for manufacturing a stack of printed circuits comprising at least: • a first step, during which at least three pads are positioned on a first printed circuit, in particular double-sided; • a second step, during which a layer of glue is applied to the pads; • a third step, during which a toroidal magnetic core, in particular a toroidal magnetic core, is positioned on the pads; and • a fourth step, during which the layer of adhesive is polymerized.

[0015] The first printed circuit comprises at least one positioning target constituting an optical positioning reference. The term “positioning target” is understood to mean at least one optical reference serving as a position reference for an assembly. The positioning target serves as a positioning reference for the other elements of the stack of printed circuits, in particular the pads.

[0016] The first printed circuit can be etched to make the pads that serve as support for the layer of glue and the core. The pads allow the core to be raised without using too much glue, which would be detrimental to positioning accuracy. The pads could be made of another material deposited on the printed circuit, without departing from the scope of the present invention.

[0017] The polymerization of the glue allows the core to be fixed for all subsequent stages of the manufacturing process. The number of pads is preferably 3, as this guarantees the absence of hyperstaticity.

[0018] In addition, the manufacturing method may comprise at least: • a fifth step, during which a layer of resin is placed around and in the center of the core; • a sixth step, during which a second printed circuit, in particular single-sided or double-sided, is positioned on the resin layer; and • a seventh step, during which a stack of printed circuits comprising at least the core, the resin layer, the first printed circuit and the second printed circuit is compressed.

[0019] The pads leave a space under the core, between it and the first printed circuit, to allow the resin, during its liquid phase, to flow freely around the core, and particularly under the core.

[0020] Advantageously, the resin layer comprises a fabric pre-impregnated with resin. The fabric is structuring and is, for example, made of glass fibers.

[0021] In addition, the fifth step may comprise at least one sub-step, during which an intermediate printed circuit is arranged on the resin layer. In particular, the intermediate printed circuit is arranged between two resin layers which may each comprise a pre-impregnated fabric. It is thus possible to superimpose several intermediate printed circuits interposed with resin layers.

[0022] The pads may be made of copper. In particular, the printed circuits comprise one or more layers of copper on one or both sides. In particular, the pads are made by etching the printed circuits. The pads are therefore made of copper material.

[0023] In particular, the pads have a height of between 5 pm and 210 pm, in particular between 35 pm and 105 pm, more specifically substantially 70 pm. Such heights correspond to the standard thicknesses of the copper layer of a printed circuit.

[0024] In addition, the pads may be cylindrical in shape with a diameter greater than or equal to 200 μm. Such a diameter provides a sufficient surface area to deposit at least one drop of glue without it protruding from the pad.

[0025] Preferably, the dimensions of the pads are less than the thickness of the core.

[0026] The adhesive layer is, in particular, electrically insulating. Such a characteristic is particularly interesting when the pads are made of copper.

[0027]

[0028]

[0029]

[0030]

[0031]

[0032]

[0033] Specifically, the glue layer has a thickness of between 5 pm and 50 pm, in particular between 10 pm and 40 pm. The second printed circuit board may include at least one vent to allow excess air and / or resin to escape from the assembly during pressing. The resin is then polymerized. The vents may be provided for any stack of printed circuits consisting of a superposition of a first printed circuit, a second printed circuit and a layer of resin. In particular, the PCB stack may include • at least one intermediate layer, such as a pre-impregnated fabric and / or an intermediate printed circuit), • components, • a core located between the first printed circuit and the second printed circuit. Another aspect of the invention relates to a stack of printed circuits comprising at least: • a core, • at least one layer of resin, • a first printed circuit, notably double-sided, and • a second printed circuit board, in particular single-sided or double-sided • the stack of printed circuits being obtained by the manufacturing process with at least one of the preceding characteristics. BRIEF DESCRIPTION OF THE FIGURES The invention and its various applications will be better understood upon reading the following description of embodiments given as examples and in no way limiting the invention and upon examining the accompanying figures, in which: • [Fig.l] is a perspective view of an example of a first printed circuit at a first stage of the manufacturing method according to the invention; • [Fig.2] is a perspective view of the first printed circuit of [Fig.l] at a second step of the manufacturing process according to the invention; • [Fig.3] is a perspective view of the first printed circuit board of [Fig.2] at a third step of the manufacturing process according to the invention; • [Fig.4] is a section of the first printed circuit of [Fig.3]; • [Fig.5] illustrates the steps of the manufacturing process according to the invention according to a first embodiment; • [Fig.6] illustrates the steps of the manufacturing process according to the invention according to a second embodiment; • [Fig.7] is a schematic view showing a flow of a resin under the magnetic core; • [Fig.8] is a sectional view of a pressing step of the stack of printed circuits of the first embodiment; • [Fig.9] is a sectional view of a pressing step of the stack of printed circuits of the second embodiment; • [Fig. 10] illustrates the drilling and metallization steps according to the first embodiment; and • [Fig.l 1] is a section of the stack of printed circuits after a step of etching the tracks according to the first embodiment. DETAILED DESCRIPTION

[0034] Unless otherwise specified, the same element appearing in different figures presents a unique reference.

[0035] Throughout this description, the terms "top" or "bottom" refer to the top of the figures and the terms "bottom" or "upper" refer to the bottom of the figures.

[0036] A stack of printed circuits 1 obtained by the manufacturing method according to the invention comprises: • a first printed circuit 10, • a second printed circuit 11, • a layer of resin 3 placed between the first printed circuit 10 and the second printed circuit 11, and • a core 2, such as a toroidal magnetic core 2.

[0037] Other elements can also be interposed between the first printed circuit 10 and the second printed circuit 11.

[0038] The stack of printed circuits 1 may be, for example, that of an electrical transformer, a magnetic flux valve sensor, a coupling transformer for digital buses, in particular Ethernet, an electromagnetic field filtering component, a radiofrequency circuit or a very high frequency switching low power conversion circuit.

[0039] The stack of printed circuits 1 illustrated in figures 8 to 10 comprises the first printed circuit 10 and the second printed circuit 11 between which the resin layer 3 and the core 2 are arranged.

[0040] The first printed circuit 10 comprises two positioning marks 100 arranged axially on either side of the first printed circuit 10 along an axis X perpendicular to a surface of the first printed circuit. The positioning marks 100 may be produced by etching the first printed circuit 10, which will then be double-sided.

[0041] Pads 101 are made on an upper surface 102 of the first printed circuit 10. The pads 101 are covered with a layer of glue 4 on which the core 2 is placed, as can be seen in [Fig.4].

[0042] The method of manufacturing the stack of printed circuits 1 will be described for a transformer. The principle remains however the same for a magnetic flux valve sensor, a coupling transformer for digital buses, an electromagnetic field filtering component, a radiofrequency circuit or a very high frequency switching low power conversion circuit.

[0043] [Fig. 1] is a perspective view of an example of a first printed circuit 10 at a first stage of the manufacturing method according to the invention.

[0044] During the first step of the manufacturing method illustrated in [Fig. 1], the pads 101 are precisely made on the first printed circuit 10 relative to an upper positioning target 100a, located inside the stack of printed circuits 1. A lower positioning target 100b (not visible in [Fig.l] but referenced in [Fig.4]), located outside the stack of printed circuits 1, will make it possible to locate the position of the core 2 when the stack of printed circuits 1 is complete. The lower positioning target 100b is located, for example, to simplify the location, on a lower face of the first printed circuit 10 opposite the upper positioning target 100a located on the opposite face.

[0045] The pads 101 are preferably produced by etching a layer of copper of the first printed circuit 10.

[0046] A height h0 of the pads 101 corresponds to a height of the copper layer of the first printed circuit 10, in particular a height of between 5 pm and 210 pm, in particular a height of between 35 pm and 105 pm, more specifically a height of substantially 70 pm. The height h0 allows the resin 3 to flow under the core 2 without reducing the magnetic coupling, which would induce a reduction in the qualities of the transformer. Indeed, the smaller this height, the more difficult it will be for the resin to flow; the greater this height, the more the magnetic coupling is reduced.

[0047] The number of pads 101 is greater than or equal to three to guarantee balance of the core 2. Preferably, the number of pads 101 is equal to three in order to avoid any hyperstatic stress, to facilitate the finishing of the resin and to limit any possible magnetic disturbances.

[0048] The pads 101 are preferably cylindrical in shape and comprise a circular surface to allow uniform distribution of the adhesive layer 4. The pads 101 may have a diameter d greater than or equal to 200 μm, in order to allow the adhesive layer 4 to be deposited. Furthermore, the diameter d of the pads 101 will be, preferably less than a thickness e of the core 2.

[0049] [Fig.2] is a perspective view of the first printed circuit 10 of [Fig.l] at a second step of the manufacturing process according to the invention.

[0050] Subsequently, according to the second step of the manufacturing method illustrated [Fig.2], the layer of glue 4 is deposited on each of the pads 101. The layer of glue 4 has a height hl of between 5 pm and 50 pm, in particular a height hl of between 10 pm and 40 pm, depending on the viscosity of the glue. The layer of glue 4 will be chosen with an expansion coefficient, a shrinkage coefficient and a stiffness coefficient as close as possible to the surrounding materials. The surface of the pads 101 is chosen so that it is sufficient so that the layer of glue 4 does not overflow.

[0051] [Fig. 3] is a perspective view of the first printed circuit 10 of [Fig. 2] at a third step of the manufacturing process according to the invention.

[0052] Subsequently, according to the third step of the manufacturing method illustrated [Fig.3], the core 2 is placed on the layer of glue 4 of each pad 101.

[0053] The adhesive layer 4 can then be polymerized during a fourth step of the manufacturing process, during which the core 2 is fixed, so as to guarantee the position of the core 2 in all subsequent steps of the manufacturing process according to the invention.

[0054] [Fig.4] is a section of the first printed circuit 10 of [Fig.3], at the end of the third step of the manufacturing process according to the invention.

[0055] More particularly, as can be seen in [Fig.4], the core 2 has an outer diameter D0 and an inner diameter DI. Thus, the difference between the outer diameter D0 and the inner diameter DI defines the thickness e of the core 2.

[0056] [Fig.5] illustrates the steps of the manufacturing method according to the invention according to a first embodiment.

[0057] For this purpose, according to the first embodiment illustrated [Fig.5], consequently, during a fifth step of the manufacturing method, a layer of resin 3 is arranged, in particular in a pre-impregnated fabric 3a, on the first printed circuit 10.

[0058] The pre-impregnated fabric 3a can be composed of: • an external layer 30 comprising a hole 33 having a diameter D3 greater than the external diameter D0 of the core 2, and • a cylindrical portion 31 having a diameter D2 less than the internal diameter DI of the core 2.

[0059] Thus, at the end of the fifth step of the manufacturing method according to the invention, the pre-impregnated fabric 3a is arranged on the first printed circuit 10, so that: • the core 2 is surrounded externally by the external layer of the pre-impregnated fabric 3a, and • the cylindrical portion 31 of the pre-impregnated fabric 3a is arranged inside the core 2.

[0060] Subsequently, during a sixth step of the manufacturing process, the second printed circuit 11 is arranged on the pre-impregnated fabric 3a and the toroidal magnetic core 2.

[0061] Subsequently, during a seventh step of the manufacturing method according to the invention, a pressure P is then exerted on the first printed circuit 10 and the second printed circuit 11, as can be seen in [Fig.8], showing a sectional view of the seventh step of the manufacturing method according to the invention, as a step of pressing the stack of printed circuits.

[0062] When the pressure P is applied, the resin of the resin layer 3, in particular of the pre-impregnated fabric 3a, will flow around and under the core 2, as represented in [Fig.7] by solid arrows.

[0063] In order to facilitate evacuation of the air A present inside the stack of printed circuits 1, at least one vent 110 may be provided in the thickness of the second printed circuit 11, as can be seen in [Fig.8]. In particular, several vents 110 are provided.

[0064] The vent 110 may also be provided in the first printed circuit 10, without departing from the scope of the present invention.

[0065] The vent 110 can be provided for any other type of stack of printed circuits comprising external printed circuits separated by a resin regardless of the other possible elements arranged between the printed and compressed circuits.

[0066] Once the stack of printed circuits 1 is compressed, the resin 3 is polymerized.

[0067] [Fig. 10] illustrates the drilling and metallization steps according to the first embodiment.

[0068] Subsequently, thanks to the lower positioning target 100b located outside the stack of printed circuits 1, holes 5 are made. The holes 5 consist of through holes.

[0069] In particular, the stack of printed circuits 1 comprises at least: • a first series 51 of holes 5 arranged at a distance 10 from the lower positioning sight 100b and outside the core 2, • a second series 52 of holes 5 arranged at a distance 11 to 1' 1, in particular greater than the distance 10 of the first series 51 of holes 5, from the lower positioning sight 100b and inside the core 2, and • a third series 53 of holes 5 arranged at a distance 12, in particular greater than a distance 1' 1 from the second series 52 of holes 5, from the lower positioning sight 100b and outside the toroidal magnetic core 2.

[0070] The holes 5 make it possible in particular to create vertical tracks making it possible to form a winding around the core 2.

[0071] In particular, once all the holes 5, in particular the first series 51, the second series 52 and the third series 53 of holes 5, have been made, they are metallized with copper. The metallization of the holes 5 consists of depositing a layer of copper 60 on the walls of the holes 5 and / or filling the holes 5 with copper, so as to form vias 6.

[0072] The vias 6 make it possible to connect horizontal tracks of the first printed circuit 10 and of the second printed circuit IL.

[0073] It is possible to use a conductive metal other than copper, such as aluminum or silver, without departing from the scope of the present invention.

[0074] As a result, it is possible to etch tracks 7 intended to transmit control signals to connect all or part of the vias 6 to each other and / or to control devices.

[0075] Subsequently, an insulating layer 8 is deposited in order to cover the tracks 7 and fill residual spaces between the tracks 7.

[0076] [Fig. 11] is a section of the stack of printed circuits 1 after the step of etching the tracks according to the first embodiment.

[0077] [Fig.6] illustrates the steps of the manufacturing method according to the invention according to a second embodiment.

[0078] More particularly, the second embodiment illustrated in [Fig.6] differs from the first embodiment in that the stack of printed circuits 1 comprises at least one intermediate printed circuit 12 between the first printed circuit 10 and the second printed circuit 11.

[0079] According to the second embodiment, a layer of resin 3 in the form of pre-impregnated fabric 32 is placed, on the one hand, between the first printed circuit 10 and the intermediate printed circuit 12 and, on the other hand, between the intermediate printed circuit 12 and the second printed circuit 11.

[0080] According to the second embodiment, the fifth step of the method presented in [Fig.6] comprises several sub-steps comprising at least: • a first sub-step, during which a first layer of resin 3, in particular in the form of a first pre-impregnated fabric 32, is placed on the first printed circuit 10 • a second sub-step, during which the intermediate printed circuit 12 is placed on the first layer of resin 3, and • a third sub-step, during which a second layer of resin 3, in particular in the form of a second pre-impregnated fabric 32, is placed on the intermediate printed circuit 12.

[0081] At the end of the first sub-step, the second sub-step and the third sub-step step of the manufacturing method according to the invention, the first pre-impregnated fabric 32 and the second pre-impregnated fabric 32 are arranged on the first printed circuit 10, so that: • the core 2 is surrounded externally by the external layer of the first pre-impregnated fabric 32 and the second pre-impregnated fabric 32, • the cylindrical portion 31 of the pre-impregnated fabric 32 is arranged inside the core 2, and • the intermediate printed circuit 12 is arranged around the core 2.

[0082] If it is desired that several intermediate printed circuits 12 be arranged, another intermediate printed circuit 12 is arranged on the previous pre-impregnated fabric 32 then another pre-impregnated fabric 32 arranged on this other intermediate printed circuit 12. Such an operation is repeated until the desired number of intermediate printed circuits 12 is reached, ending with a last pre-impregnated fabric 32 before laying the second printed circuit 11.

[0083] Subsequently, during a seventh step of the manufacturing method according to an example of the invention, a pressure P is then exerted on the first printed circuit 10, the intermediate printed circuit 12 and the second printed circuit 11, as can be seen in [Fig.9], showing a sectional view of the seventh step of the manufacturing method according to the invention, as a step of pressing the stack of printed circuits.

[0084] When the pressure P is applied, the resin of the resin layer 3, in particular of the first pre-impregnated fabric 32 and of the second pre-impregnated fabric 32, will flow around and under the core 2, as represented in [Fig.7] by solid arrows.

[0085] In order to facilitate evacuation of the air A present inside the stack of printed circuits 1, vents 110 may be provided in the thickness of the intermediate printed circuit 12 and / or the second printed circuit 11, in a manner similar to the first embodiment.

[0086] The vents 110 may also be provided in the first printed circuit 10, without departing from the scope of the present invention.

[0087] The vents 110 may be provided for any other type of stack of printed circuits comprising external printed circuits separated by a resin regardless of the other possible elements arranged between the printed and compressed circuits.

[0088] Once the stack of printed circuits 1 is compressed, the resin 3 is polymerized.

[0089] In a similar manner to the first embodiment, the intermediate printed circuits 12 are drilled to fit around the core 2.

[0090] The subsequent steps according to the second embodiment are the same as for the first embodiment.

[0091] Alternatively, according to another example of stacking 1, components 9 may also be buried in resin 3, as illustrated [Fig.9].

Claims

Claims

1. Method for manufacturing a stack of printed circuits (1) comprising at least: - a first step, during which at least three pads (101) are positioned on a first printed circuit (10), in particular double-sided; - a second step, during which a layer of glue (4) is deposited on the pads (101); - a third step, during which a core (2) is positioned on the pads (101); and - a fourth step, during which the layer of glue (4) is polymerized.

2. Manufacturing method according to claim 1, characterized in that it comprises at least: - a fifth step, during which a resin layer (3) is arranged around and in the center of the core (2); - a sixth step, during which a second printed circuit (11), in particular single-sided or double-sided, is positioned on the resin layer (3); and - a seventh step, during which a stack of printed circuits (1) comprising at least the core (2), the resin layer (3), the first printed circuit (10) and the second printed circuit (11) is compressed.

3. Manufacturing method according to the preceding claim, characterized in that the resin layer (3) comprises a pre-impregnated fabric (3a, 32).

4. Manufacturing method according to one of claims 2 or 3, characterized in that the fifth step comprises at least one sub-step, during which an intermediate printed circuit (12) is arranged on the resin layer (3).

5. Manufacturing method according to one of the preceding claims, characterized in that the pads (101) are made of copper.

6. Manufacturing method according to one of the preceding claims, characterized in that the pads (101) have a height of between 5 pm and 210 pm, in particular between 35 pm and 105 pm, more specifically of

7.

8.

9.

10.

11. approximately 70 pm. Manufacturing method according to one of the preceding claims, characterized in that the pads (101) are cylindrical in shape having a diameter (d) greater than or equal to 200 μm. Manufacturing method according to one of the preceding claims, characterized in that the adhesive layer (4) is electrically insulating. Manufacturing method according to one of the preceding claims, characterized in that the adhesive layer (4) has a thickness of between 5 μm and 50 μm, in particular between 10 μm and 40 μm. Manufacturing method according to one of the preceding claims, characterized in that the second printed circuit (11) comprises at least one vent (110). Stack of printed circuits (1) comprising at least: - a core (2), - at least one layer of resin (3), - a first printed circuit (10), and - a second printed circuit (11), - the stack of printed circuits (1) being obtained by the manufacturing method according to any one of the preceding claims.

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