Electrical assembly comprising a stiffening element

Abstract

The invention relates to an electrical assembly (1) comprising: - at least one electronic component (2), - a flexible printed circuit board (3), configured to receive, on its top face (4), the at least one electronic component (2), and comprising a flexible substrate (6) bearing connection conductor tracks connected to the electronic components (2), - at least one stiffening element (10) comprising: • a copper top layer (30), • an insulating intermediate layer (31), • at least one metallized or non-metallized via (33) passing through the copper top layer (30) and the insulating intermediate layer (31), characterized in that the stiffening element (10) is bonded to the bottom face (5) of the printed circuit board (3).

Description

Electrical assembly including a stiffening element. Scope of the invention

[0001] The field of invention is that of electrical assemblies. State of the art

[0002] A printed circuit board, sometimes called a PCB (acronym for Printed Circuit Board), is a support for electronic components, this support including traces of conductive material that electrically connect the components and potentially elements external to the board.

[0003] These electronic components generate heat that must be dissipated or they will be damaged and the electronic board will be disrupted.

[0004] When the environment of the printed circuit board requires it to be curved, the printed circuit board generally includes a flexible polyimide-based substrate onto which the electronic components are fixed.

[0005] These electronic components are attached to the printed circuit board using connections that are particularly sensitive to lateral stresses. Such stresses appear on these connections when a flexible printed circuit board deforms near an electronic component, weakening the connections. Consequently, flexible printed circuit boards have low reliability because the connections to the electronic components are weakened and break easily.

[0006] Another problem with a printed circuit board using a polyimide substrate is the difficulty of attaching the polyimide substrate to a support, such as an enclosure housing the board. Because the polyimide substrate is flexible, using it as an attachment point on the support results in a flexible connection, which is not necessarily desirable.

[0007] To solve these problems, we know of the use of stiffening elements placed under the flexible printed circuit board, and used to locally stiffen the board at the electrical connections.

[0008] We know of stiffening elements made of aluminum. However, this material is prone to expansion when exposed to the heat generated by electronic components, and this expansion weakens the connection between the stiffening element and the printed circuit board.

[0009] Stiffening elements made of plastic, or based on an epoxy formulation more commonly known as FR4 (Flame Retardant 4), are also known. FR4 is another common material for printed circuit boards, being a composite material of epoxy resin reinforced with glass fibers, all laminated together. However, these materials do not have sufficient thermal conductivity to dissipate the heat emitted by the electronic components, which risks damaging the printed circuit board.

[0010] The invention aims to solve these problems by proposing an electrical assembly comprising a flexible printed circuit board, in which the stiffening elements are present in the area requiring thermal and mechanical optimization of the assembly.

[0011] To this end, the invention proposes an electrical assembly comprising: at least one electronic component, a flexible printed circuit board, configured to receive on its upper face at least one electronic component, and comprising a flexible substrate carrying conductive connection tracks connected to the electronic components, at least one stiffening element comprising: an upper copper layer, an insulating intermediate layer, at least one via, metallized or not, passing through the upper copper layer and the insulating intermediate layer, characterized in that the stiffening element is glued to the lower face of the printed circuit board.

[0012] The electrical assembly may include one or more of these additional features, taken separately or in combination:

[0013] The stiffening element is separate from the printed circuit board.

[0014] The stiffening element is electrically isolated from the printed circuit board and electronic components.

[0015] At least one of the vias is a thermal via. This allows for increased heat dissipation.

[0016] The stiffening element comprises a lower copper layer, placed below the upper copper layer and the intermediate insulating layer, and at least one via passes through the upper copper layer, the intermediate insulating layer and the lower copper layer.

[0017] The printed circuit board has a top face and a bottom face, the top face having conductive traces for connecting electronic components, and the bottom face being glued to the stiffening element.

[0018] At least one via passing through the top copper layer is metallized.

[0019] At least one via of the stiffening element is a thermal via, specifically a thermal via, which may be filled or copper-plated on an inner face of the via.

[0020] In particular, all the vias of the stiffening element are copper thermal vias.

[0021] Advantageously, the use of thermal via improves the conductivity between the upper copper layer and the lower copper layer, and improves heat dissipation by the stiffening element.

[0022] In particular, the use of thermal vias makes it possible to achieve the required heat dissipation performance in a compact space.

[0023] Thermal vias have a diameter between 0.1mm and 3mm, specifically between 0.3mm and 0.7mm, specifically equal to 0.5mm.

[0024] At least one thermal via is a hole passing through the top copper layer, the intermediate insulating layer and the bottom copper layer.

[0025] Advantageously, the holes through the top layer of copper prevent the formation of air bubbles when gluing the stiffening element to the underside of the printed circuit board.

[0026] According to another embodiment of the invention, the stiffening element comprises at least one solid via.

[0027] Advantageously, the use of fully metallized vias improves heat dissipation through the stiffening element.

[0028] At least one via is conical in shape, and is formed by drilling with a laser beam.

[0029] At least one via is cylindrical in shape, and is formed by mechanical drilling with a drill bit.

[0030] The electrical assembly includes an adhesive layer placed between the printed circuit board and the stiffening element and arranged to stick the printed circuit board and the stiffening element together.

[0031] Advantageously, the use of an adhesive layer makes it possible to secure the stiffening element to the printed circuit board without modifying the structure or the manufacturing process of the printed circuit board.

[0032] At least one electronic component is placed on the top surface of the printed circuit board, and electrically connected to at least one conductive trace of the printed circuit board.

[0033] At least one electronic component includes at least one connection pad, in particular a plurality of connection pads arranged to be electrically connected to conductive traces of the printed circuit board.

[0034] At least one electronic component is substantially rectangular in shape, and has a characteristic dimension between 5mm and 15mm, in particular between 7mm and 12mm, preferably on the order of 10mm.

[0035] At least one electronic component has a central copper area, called a thermal pad, arranged to promote the transfer of heat dissipated by the electronic component to the stiffening element.

[0036] The electrical assembly comprises a plurality of electronic components, including at least three electronic components, including four, five or six electronic components.

[0037] The electrical assembly includes at least one thermal pad placed between an electronic component and the printed circuit board, and arranged to conduct the heat dissipated by the electronic component to the printed circuit board.

[0038] In particular, the electrical assembly includes as many thermal pads as there are electronic components with a thermal base.

[0039] Each thermal pad extends under the thermal base of an electronic component.

[0040] The stiffening element is electrically isolated from the printed circuit board and electronic components, notably by the adhesive layer.

[0041] The stiffening element has a thickness between 0.5 mm and 2 mm, specifically between 0.8 mm and 1.8 mm, for example 1.6 mm

[0042] The electrical assembly includes a support, and the stiffening element is fixed to the support.

[0043] Advantageously, the stiffening element thus allows the printed circuit board to be fixed to the support.

[0044] The intermediate insulating layer is made of insulating material, a molecular base epoxy composite material containing glass fibers, in particular FR4 (acronym for Flame Retardant 4 in English).

[0045] The intermediate insulating layer has a coefficient of thermal expansion (CTE) between 10 and 20 ppm / K, specifically between 15 and 18 ppm / K.

[0046] Advantageously, the insulating intermediate layer expands very little when exposed to the heat generated by the electronic components. In particular, the insulating intermediate layer expands similarly to the printed circuit board, which helps maintain a reliable mechanical connection during the operation of the electronic components.

[0047] Advantageously, the use of a stiffening element comprising two layers of copper, an intermediate layer of insulating material and at least one thermal via makes it possible to combine the mechanical properties of the insulating material, in particular its low thermal expansion, with the thermal dissipation properties of the copper layers and the thermal vias.

[0048] The printed circuit board is flexible and can form a bending radius of less than 10 mm, specifically between 1 mm and 10 mm.

[0049] The printed circuit board has a thickness between 100 and 600 micrometers, specifically between 150 and 500 micrometers, specifically 150 and 400 micrometers.

[0050] Advantageously, the thinness of the printed circuit board allows it to be flexible.

[0051] The flexible substrate of the printed circuit board is made of insulating material, for example polyimide.

[0052] The flexible substrate is designed to be curved.

[0053] Advantageously, the use of polyimide ensures the reliability of the printed circuit board and its compatibility with electronic soldering processes in a reflow oven.

[0054] The printed circuit board has a first layer of copper, placed on the flexible substrate.

[0055] The first copper layer of the printed circuit board forms the conductive traces for connecting the electronic components.

[0056] The printed circuit board has a top protective layer extending over the first copper layer, and arranged to protect the first copper layer from corrosion.

[0057] According to a first embodiment of the invention, the printed circuit board is a double-sided board, and the printed circuit board includes a second layer of copper placed under the flexible substrate, and which may optionally form other conductive tracks.

[0058] The printed circuit board has a lower protective layer extending under the second copper layer, and arranged to protect the second copper layer from corrosion.

[0059] The printed circuit board has circuit board holes passing through the printed circuit board between the top and bottom faces.

[0060] The printed circuit board holes are copper-plated.

[0061] The holes in the printed circuit board provide a passage for heat generated on one side of the printed circuit board to escape to the other side of the printed circuit board, and form thermal vias in the printed circuit board.

[0062] Printed circuit board holes can also form electrical vias of the printed circuit board, and electrically connect the conductive traces on the top and bottom faces of the printed circuit board.

[0063] According to another embodiment of the invention, the printed circuit board is a single-sided board, and does not have a second copper layer.

[0064] The invention also relates to a lighting device, in particular a signaling device, comprising an electrical assembly as described.

[0065] Method for manufacturing an electrical assembly comprising: at least one electronic component, a flexible printed circuit board, at least one stiffening element, the manufacturing process comprising the following steps: a step of manufacturing the flexible printed circuit board, a step of manufacturing the stiffening element, a step of gluing the stiffening element to the underside of the printed circuit board, a step of welding or brazing the electronic components to the upper side of the printed circuit board.

[0066] The printed circuit board manufacturing stage includes a lamination step.

[0067] The manufacturing step of the stiffening element includes: a step of drilling the vias of the stiffening element possibly, a step of filling the vias of the filling element possibly, a step of copper plating the vias of the stiffening element, forming copper thermal vias.

[0068] Advantageously, the step of bonding the stiffening element to the underside of the printed circuit board allows the printed circuit board and the stiffening element to be manufactured separately, without requiring any modification to the printed circuit board manufacturing process. Thus, the stiffening element is attached to the printed circuit board. It is therefore understood that the stiffening element is separate from the printed circuit board.

[0069] The drilling of the vias of the stiffening element is carried out by laser drilling.

[0070] Alternatively, the step of drilling the vias of the stiffening element is carried out by mechanical drilling with a drill bit.

[0071] The soldering or brazing step of the electronic components on the top side of the printed circuit board is carried out after the step of gluing the stiffening element on the bottom side of the printed circuit board.

[0072] Advantageously, the presence of the stiffening element makes it easier to solder or braze the electronic components onto the top face of the printed circuit board. List of figures

[0073] Other features, details and advantages of the invention will become clearer upon reading the following description on the one hand, and several illustrative and non-limiting examples of embodiments given with reference to the attached schematic drawings on the other hand, in which:

[0074] Laest is a representation of an electrical assembly according to the invention, comprising a single-sided printed circuit board.

[0075] Laest representation of an electrical assembly according to the invention, comprising a double-sided printed circuit board.

[0076] The features, variations, and different embodiments of the invention may be combined in various ways, provided they are not incompatible or mutually exclusive. In particular, variations of the invention may include only a selection of features, described hereafter in isolation from the other features described, if this selection of features is sufficient to confer a technical advantage and / or to differentiate the invention from prior art. Detailed description

[0077] We have represented on the electrical assembly 1 according to the invention, comprising a single-sided printed circuit board.

[0078] The electrical assembly 1 comprises two electronic components 2, and a single-sided flexible printed circuit board 3, configured to receive on its upper face 4 the electronic components 2.

[0079] The printed circuit board includes a flexible substrate 6 carrying conductive connection tracks, not shown here, and connected to electronic components 2.

[0080] The printed circuit board 3 has a first layer of copper 40, placed on the flexible substrate 6 and forming the conductive tracks for connecting the electronic components 2.

[0081] The printed circuit board 3 has a top protective layer 41 extending over the first copper layer 40, and arranged to protect the first copper layer against corrosion.

[0082] The electronic components 2 are substantially rectangular in shape, and have a characteristic dimension between 5mm and 15mm, in particular between 7mm and 12mm, preferably around 10mm.

[0083] Each one includes a plurality of connection pads 11 arranged to be electrically connected to conductive tracks of the printed circuit board 3.

[0084] The flexible substrate 6 is configured to be curved.

[0085] The electrical assembly 1 includes a stiffening element 10 glued to the lower face 5 of the printed circuit board.

[0086] The printed circuit board 3 has an upper face 4 and a lower face 5, the upper face bearing the conductive traces for connecting the electronic component, and the lower face 5 being glued onto the stiffening element 10.

[0087] The electronic components 2 have a central copper area, called a thermal pad 20, arranged to promote the transfer of heat dissipated by the electronic component 2 to the stiffening element 10.

[0088] The printed circuit board is flexible and can form a bending radius of less than 10 mm, specifically between 1 mm and 10 mm.

[0089] Printed circuit board 3 has a thickness between 100 and 600 micrometers, specifically between 150 and 500 micrometers, specifically 150 and 400 micrometers.

[0090] Advantageously, the thinness of the printed circuit board 3 allows it to be flexible.

[0091] The flexible substrate 6 of the printed circuit board is made of an insulating material, for example, polyimide. Advantageously, the use of polyimide ensures the reliability of the printed circuit board 3 and its compatibility with electronic soldering processes using a reflow oven.

[0092] According to an embodiment not shown, the electrical assembly 1 comprises a single electronic component 2, or alternatively three or more electronic components, in particular at least four electronic components, in particular five or six electronic components.

[0093] The electrical assembly 1 includes two thermal pads 21, each placed between an electronic component 2 and the printed circuit board 3, and arranged to conduct the heat dissipated by the electronic component 2 to the printed circuit board.

[0094] The electrical assembly 1 has as many thermal pads 21 as there are electronic components 2, and each thermal pad 21 extends under the thermal base 20 of an electronic component 2.

[0095] The electrical assembly 1 includes an adhesive layer 25 positioned between the printed circuit board 3 and the stiffening element 10, arranged to bond the printed circuit board and the stiffening element together. It is therefore understood that the stiffening element 10 is separate from the printed circuit board 3.

[0096] Advantageously, the use of an adhesive layer 25 makes it possible to secure the stiffening element 10 to the printed circuit board 3 without modifying the structure or the manufacturing process of the printed circuit board 3.

[0097] The stiffening element 10 is electrically isolated from the printed circuit board 3 and the electronic components 2, in particular by this adhesive layer 25.

[0098] The stiffening element 10 comprises: an upper copper layer 30, an insulating intermediate layer 31, a lower copper layer 32, and at least one via 33 passing through the upper copper layer 30 and the insulating intermediate layer 31, and the lower copper layer. 32

[0099] The vias 33 passing through the upper layer of copper 30 are metallized, and are thermal vias, in particular thermal vias that can be filled or copper-lined.

[0100] According to an embodiment of the invention not shown, at least one via 33 of the stiffening element is not metallized.

[0101] Advantageously, the use of thermal vias 33 improves the conductivity between the upper copper layer and the lower copper layer, and improves heat dissipation by the stiffening element.

[0102] In particular, the use of thermal vias makes it possible to achieve the required heat dissipation performance in a compact space.

[0103] Thermal vias have a diameter between 0.1mm and 3mm, specifically between 0.3mm and 0.7mm, specifically equal to 0.5mm.

[0104] In this embodiment, at least one thermal via is a hole 35 passing through the upper copper layer 30, the intermediate insulating layer 31 and the lower copper layer 32.

[0105] Advantageously, the holes 35 through the upper layer 30 of copper prevent the formation of air bubbles when gluing the stiffening element 10 onto the lower face 5 of the printed circuit board 3.

[0106] According to another embodiment of the invention not shown, the stiffening element 10 comprises at least one solid via.

[0107] Advantageously, the use of solid metallized vias improves heat dissipation through the stiffening element 10.

[0108] The vias 33 are cylindrical in shape, and are formed by mechanical drilling with a drill bit.

[0109] According to another embodiment of the invention not shown, at least one via 33 is conical in shape, and is formed by drilling with a laser beam.

[0110] The intermediate insulating layer 31 is made of insulating material, of epoxy molecular base composite material containing glass fibers, in particular FR4 (acronym for Flame Retardant 4 in English: flame retardant 4).

[0111] Advantageously, the insulating intermediate layer 31 expands very little when exposed to the heat emitted by the electronic components. In particular, the insulating intermediate layer expands similarly to the printed circuit board, which helps maintain a reliable mechanical connection during the operation of the electronic components 2.

[0112] The stiffening element 10 has a thickness between 0.5 mm and 2 mm, specifically between 0.8 mm and 1.8 mm, for example 1.6 mm

[0113] The electrical assembly includes a support 50, and the stiffening element 10 is fixed to the support

[0114] Advantageously, the stiffening element 10 thus allows the printed circuit board 3 to be fixed onto the support 50.

[0115] Advantageously, the presence of the stiffening element 10 makes it easier to solder or braze the electronic components onto the upper face 4 of the printed circuit board 3.

[0116] The use of a stiffening element 10 comprising two layers of copper 30, 32, an intermediate layer 31 of insulating material and at least one thermal via 33 makes it possible to combine the mechanical properties of the insulating material, in particular its low thermal expansion, with the thermal dissipation properties of the copper layers and the thermal vias.

[0117] We have represented on the electrical assembly according to a second embodiment of the invention in which the printed circuit board 3 is double-sided.

[0118] Printed circuit board 3 has a second copper layer 42 placed under the flexible substrate which may optionally form other conductive tracks.

[0119] The printed circuit board 3 has a lower protective layer 43 extending under the second copper layer 42 and arranged to protect the second copper layer 42 against corrosion.

[0120] The printed circuit board 3 has 60 printed circuit board holes passing through the printed circuit board between the top face 4 and the bottom face 5.

[0121] The 60 printed circuit board holes are copper-plated, and provide a passage for heat generated on one side of the printed circuit board to escape to the other side of the printed circuit board, forming thermal vias of the printed circuit board.

[0122] The 60 printed circuit board holes can also form electrical vias of the printed circuit board, and electrically connect the conductive tracks on the top face 4 and the bottom face 5 of the printed circuit board.

Claims

Electrical assembly (1) comprising: at least one electronic component (2), a flexible printed circuit board (3), configured to receive on its upper face (4) at least one electronic component (2), and comprising a flexible substrate (6) carrying conductive connection tracks connected to the electronic components (2), at least one stiffening element (10) comprising: an upper layer (30) of copper, an insulating intermediate layer (31), at least one via (33), metallized or not, passing through the upper layer (30) of copper and the insulating intermediate layer (31), characterized in that the stiffening element (10) is glued to the lower face (5) of the printed circuit board (3). Electrical assembly (1) according to claim 1, wherein the stiffening element (10) is separate from the printed circuit board (3). Electrical assembly (1) according to claim 1 or 2, wherein the stiffening element (10) is electrically isolated from the printed circuit board (3) and the electronic components. Electrical assembly (1) according to any one of claims 1 to 3, wherein at least one via (33) is a thermal via. Electrical assembly (1) according to any one of claims 1 to 4, wherein the stiffening element (10) comprises a lower copper layer (32), placed under the upper copper layer (30) and the intermediate insulating layer (31), and at least one via (33) passes through the upper copper layer (30), the intermediate insulating layer (31) and the lower copper layer (32). Electrical assembly (1) according to any one of the preceding claims, comprising an adhesive layer (25) placed between the printed circuit board (3) and the stiffening element (10) and arranged to stick the printed circuit board (3) and the stiffening element (10) together. Electrical assembly (1) according to any one of the preceding claims, comprising at least one thermal pad (21) placed between an electronic component (2) and the printed circuit board (3), and arranged to conduct the heat dissipated by the electronic component (2) to the printed circuit board (3). Electrical assembly (1) according to any one of the preceding claims, comprising a support (50), and in which the stiffening element (10) is fixed to the support (50). Electrical assembly (1) according to any one of the preceding claims, wherein the printed circuit board (3) is flexible and can form a radius of curvature less than 10 mm, in particular between 1 mm and 10 mm Electrical assembly (1) according to any one of the preceding claims, wherein the printed circuit board (3) comprises: a first copper layer (40), placed on the flexible substrate (6) and forming the conductive tracks for connecting the electronic components (2), a top protective layer (41) extending over the first copper layer (40), and arranged to protect the first copper layer (40) against corrosion. Electrical assembly (1) according to any one of the preceding claims, wherein the printed circuit board (3) is a double-sided board, and comprises a second copper layer (42) placed under the flexible substrate (6), and which may optionally form other conductive tracks. Electrical assembly (1) according to claim 11, wherein the printed circuit board (3) has a lower protective layer (43) extending under the second copper layer (42), and arranged to protect the second copper layer (42) against corrosion. Electrical assembly (1) according to any one of the preceding claims, wherein the printed circuit board (3) is a single-sided board, and does not have a second copper layer (42). Electrical assembly (1) according to any one of the preceding claims, wherein the printed circuit board (3) has printed circuit board holes (60) passing through the printed circuit board (3) between the upper face (4) and the lower face (5). A lighting device, in particular a signaling device, comprising an electrical assembly (1) according to one of the preceding claims.

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