FR3042941A1 - FLEXIBLE PRINTED CIRCUIT BOARD - Google Patents

Abstract

A printed circuit board comprising a flexible sheet comprising electrical interconnection tracks, wherein the flexible sheet includes a laminated composite material including glass fibers; the flexible sheet has at least one location adapted to receive at least one electronic component so that this location corresponds to a stiffening element, the electronic component and the stiffening element corresponding thereto being respectively placed on opposite sides of the flexible sheet , and the printed circuit board is attached to at least one stiffening element adapted to fix the printed circuit board on a support.

Description

Flexible printed circuit board

Technical area

The present invention relates to a printed circuit board. More specifically, the present invention relates to a printed circuit board comprising a flexible sheet attached to stiffening elements disposed at the locations for receiving electronic components.

Prior state of the art

A printed circuit board, sometimes called PCB (acronym for Printed Circuit Board), is a support for electronic components, this support comprising conductive material tracks that electrically connect components and potentially external elements. à la carte.

When the environment of the printed circuit board requires it to be curved, the printed circuit board generally comprises a flexible sheet of polyimide on which are fixed the electronic components.

Such a card with a polyimide sheet has at least three problems.

First, polyimide is much more expensive than FR4 (Flame Retardant 4), which is another common material for printed circuit boards, FR4 being a fiber-reinforced epoxy resin composite material of glass, the whole being rolled.

Secondly, the electronic components are attached to the printed circuit board by means of connections which are particularly sensitive to side voltages. However, such voltages appear on these connections when a flexible printed circuit board deforms near an electronic component. This deformation can be involuntary, for example when the card is manipulated in the workshop, or voluntary to adapt the shape of the card to the desired geometry. Whether this deformation is voluntary or involuntary, it causes tension on the component connections and weakens them. As a result, flexible printed circuit boards have poor reliability because the connections of the electronic components have been weakened and broken easily.

Third, another problem of a printed circuit board with a polyimide sheet is that it is difficult to secure the polyimide sheet to a carrier, such as a housing incorporating the card. Indeed, the polyimide sheet is flexible, use as a point of attachment on the support gives rise to a flexible attachment, which is not necessarily desirable, especially when the electronic components are the LEDs (acronym for Light Emitting Diode in English: light emitting diode) for lighting a vehicle, for example traffic lights. Indeed, the front and rear lighting of a vehicle is regulated precisely, and it is not conceivable in this context that the light sources do not have a well-defined position. The electronic components themselves are not made to serve as a point of attachment on the support.

DISCLOSURE OF THE INVENTION The object of the invention is to solve these problems by proposing an inexpensive flexible printed circuit board, in which the electronic components and their connections are not subjected to voltages due to deformation of the card and easy to fix on a support. For this purpose, there is provided, according to the invention, a printed circuit board comprising a flexible sheet comprising electrical interconnection tracks, characterized: a. in that the flexible sheet includes a laminated composite material including glass fibers, b. in that the flexible sheet has at least one location adapted to receive at least one electronic component so that this location corresponds to a stiffening element, the electronic component and the corresponding stiffening element being placed respectively on opposite faces of the flexible sheet, and c. in that the printed circuit board comprises at least one stiffening element adapted to fix the printed circuit board on a support.

Such a printed circuit board is less expensive than the printed circuit board made of polyimide known from the prior art because the laminated composite material including glass fibers is typically less expensive than the polyimide.

The stiffening elements allow the flexible sheet to remain straight at the locations of the components and their connections. This lack of bending at the component level means that the connections of the components and the components themselves are not subjected to voltages which could damage them or even break them, and thereby reduce the reliability of the card assembly. printed circuit board and electronic components.

Such a printed circuit board is also easy to fix on a support or in a housing because it is possible to use for this purpose at least one of the stiffening elements.

According to various embodiments of the invention, which can be taken together or separately: the laminated composite material is FR4; the flexible sheet has a thickness of at most substantially 600 micrometers; - The contact surface between the stiffening element and the card is at least substantially equal to the surface on the map of the location adapted to receive at least one electronic component; - The stiffening element adapted to fix the printed circuit board on a support comprises a fixing means for fixing said stiffening element on the support; the fixing means comprises holes on the stiffening element; the fixing means comprises an adhesive; - The card is pierced with at least one hole containing a thermal conductive material for thermal contact between the electronic component and the stiffening element corresponding thereto; at least one electronic component is an electroluminescent diode; - the card is suitable for a lighting application; and the card is adapted to be used for lighting and / or signaling devices for motor vehicles.

BRIEF DESCRIPTION OF THE FIGURES Other characteristics, details and advantages of the invention will emerge from the description given below, in a nonlimiting manner and with reference to the appended drawings.

Figure 1 illustrates a side view of a printed circuit board diagram according to an embodiment of the invention.

Figure 2 illustrates a side view of a printed circuit board schematic mounted in a housing according to another embodiment of the invention.

Figure 3 illustrates a side view of a printed circuit board diagram according to yet another embodiment of the invention.

Embodiments of the invention

The present invention is described with particular embodiments and references to figures but the invention is not limited by them. The drawings or figures described are only schematic and are not limiting. In drawings, some elements are not scaled.

In the figures where an element is represented several times, the number of times an element is present is not necessarily close to the number of times that element is present in embodiments of the invention.

In the figures, identical or similar elements may bear the same references.

Figure 1 illustrates a side view of a printed circuit board according to an embodiment of the present invention. A printed circuit board 101 is a carrier for an electronic component 102 which is connected to the printed circuit board by connections 103. A stiffener element 104 is disposed on the face of the printed circuit board 101 opposite to that of the electronic component. 102, at the right of the latter, so as to correspond to the electronic component 102. The stiffening element 104 is fixed to a support 105.

The printed circuit board 101 comprises a FR4 plate and electrical interconnection tracks made of conductive material. It preferably comprises a single layer of FR4 with a layer of conductive material per face. Such a printed circuit board 101 is thinner than the usual printed circuit boards, which allows it to be flexible. In one embodiment of the invention, the printed circuit board 101 has a thickness of about 150 to 600 microns. In one embodiment of the invention, the printed circuit board 101 comprises a layer of about 100 micrometers of FR4 laminated with a layer of about 35 microns of copper on one of its faces and a layer of about 35 microns copper on the other of its faces. A layer of OSP (acronym for Organic Soldering Preservative) or layers of nickel and gold may be present on copper to prevent oxidation of copper. The printed circuit board 101 may possibly comprise more than two layers of conductive material and several FR4 layers as long as the printed circuit board is flexible.

The printed circuit board 101 is curvatable in the zones 106, 107 where there is no stiffening element 104 and is constrained to remain straight by the stiffening element 104 at the location 108 of the electronic component 102 and connections 103. The printed circuit board 101 may comprise holes or thermal via, a thermal via in a printed circuit board being a hole which pierces the circuit board between its two faces and which provides a passage for the heat generated. on one side of the printed circuit board can be evacuated to the other side of the printed circuit board.

The electronic component 102 may be an LED (acronym for Light Emitting Diode).

The connections 103 have an electrical function by connecting the electronic component 102 to tracks of conductive material on the printed circuit board 101 and a mechanical function by mechanically fixing the electronic component 102 to the printed circuit board 101. In an embodiment of the In the invention, the connections 103 are made by wire wiring, as illustrated in FIG. 1. In another embodiment of the invention, not shown, the connections 103 are solder balls, especially if the electronic component is a chip. intended to be mounted on the surface of the printed circuit board. The stiffening element 104 is stiffer than the printed circuit board 101. It makes it possible to avoid the strong deformations of the printed circuit board 101 at the location of the electronic component 102 and the connections 103. Indeed, such deformations would weaken connections 103 and risk breaking them. It could also happen that the electronic component 102 is broken, especially if it is a ceramic capacitor. In one embodiment of the invention, the stiffening element 104 has a surface at least substantially equal to the surface of the electronic component 102 and locations provided for the connections 103 on the printed circuit board 101, which forces the flexible card to stay straight over the entire area on which voltages would damage the electronic component 102 and / or its connections 103.

In one embodiment of the invention, the stiffening element 104 is made of plastic or metal. Advantageously, the stiffening element 104 does not comprise a conductive track, which makes it particularly suitable for fixing the printed circuit board 101 on the support 105. In one embodiment of the invention, the stiffening element 104 is pierced with holes. to be fixed on the printed circuit board 101 and / or the support 105. In one embodiment of the invention, the stiffening element 104 is fixed on the printed circuit board 101 and / or the support 105 by an adhesive material. In one embodiment of the invention, the stiffener element 104 is a radiator capable of diffusing the heat received from the electronic component 102 by one or more holes in the printed circuit board 101 comprising a thermal conductor.

The support 105 is preferably more rigid than the printed circuit board 101. The support 105 may be made of plastic or aluminum. The carrier 105 may be a housing of the printed circuit board 101. The carrier 105 may be attached to the stiffener element 104 by adhesive or screws through holes in the stiffener element 104 and / or the carrier 105.

Fig. 2 illustrates a side view of a printed circuit board according to another embodiment of the present invention. A printed circuit board 201 is a carrier for electronic components 202, 212, 222 which are connected to the printed circuit board by respective connections 203, 213, 223. Stiffening elements 204, 224 are disposed on the face of the printed circuit board. printed circuit board 201 opposed to that of the electronic components 202, 212, 222 so as, as previously, to correspond to the electronic components 202, 212, 222. The stiffening elements 204, 224 are attached to a support 251 which is part of a housing 250. A closure glass 252 is also part of the housing 250 and closes the housing 250 when attached to the support 251.

The printed circuit board 201 is curved at the location 210 where there is no stiffening element 204 224 and is stiffened by the stiffening elements 204, 224 at the locations 205, 215, 225 of the electronic components 202, 212, 222. The stiffening element 204 corresponds to two electronic components 202, 212 while the stiffening element 224 corresponds to a single electronic component 222.

The stiffening elements 204, 224 are glued or fastened for example by screws to the support 251. The support 251 may have a curvature, as illustrated in FIG. 2, which it imposes on the printed circuit board 101 via the succession of the elements stiffener 204, 224. The card 101 then has a curvature in certain areas corresponding to the areas of the card where there is no electronic component 102.

This system makes it possible to put a printed circuit board in a curved environment because the card curves in places where there are no stiffening elements and remains straight at the points of the stiffening elements, thus avoiding tensions on the connections 203, 213, 223.

The support 251 is fitted with the closure glass 252 to form the housing 250. In one embodiment of the invention, the closure glass 252 is made of a transparent material.

In one embodiment of the invention which is not shown in the figures, the card 101 is curved in several directions.

Figure 3 illustrates a side view of a printed circuit board according to yet another embodiment of the present invention. A printed circuit board 501 is pierced with holes 510, 520. Electronic components 512, 522 are arranged on the holes 510, 520.

The electronic component 512 is disposed on a hole 510 and is connected to the printed circuit board 501 by connections 513. The hole 510 is filled with a material 514 intended to allow the heat generated by the component to evacuate to through the card 501 to heat dissipation means. This material 514 must be a good thermal conductor and electrically insulating. This may be an adhesive or a thermal paste, for example a thermal paste based on silicone and / or containing ceramic particles. This material 514 is in contact on one side of the card 501 with the component 512 and on the other side of the card 501 with a layer of a material 515 similar to the material 514, thermal paste for example. The thermal paste layer 515 is in contact with the heat dissipation means, a radiator 516 for example. The thermal paste layer 515 could be absent at the hole 510, which would allow direct thermal contact between the heat conducting element 514 and the radiator 516.

Similarly, the electronic component 522 is disposed on a hole 520 and is connected to the printed circuit board 501 by connections 523. The hole 520 is filled with a thermal paste 524 which is in contact with one side of the printed circuit board 501 with the electronic component 522 and the other side of the printed circuit board 501 with a thermal paste layer 525. The thermal paste layer 525 is in contact with a heat sink 526. The thermal paste layer 525 could be absent at the hole 520, which would allow direct thermal contact between the thermal conducting element 524 and the radiator 526.

The electronic components 512, 522 may be so-called "power" electronic components that generate a large amount of heat. If this heat is not removed, the temperature inside the electronic component increases which degrades the electronic characteristics of the component and potentially can lead to its destruction. In the case of an LED, a heating of the diode leads to a decrease in its efficiency. The embodiment of the invention of FIG. 3 allows a thermal contact via the hole 510 (respectively 520) and passing through the materials 514, 515 (respectively 524, 525) between the electronic component 512 (respectively 522) and the element stiffener 516 (respectively 526) corresponding thereto, which allows a good evacuation of the heat generated by the electronic component and limits its heating.

The printed circuit board 501 has a curved area 550.

The printed circuit board 501 preferably has a thickness between 150 and 600 micrometer, which allows it to be flexible. The printed circuit board 501 is more preferably between 150 and 500 microns thick, which allows it to be more flexible. The printed circuit board 501 even more preferably has a thickness between 150 and 400 micrometer, which allows it to be even more flexible.

In the embodiment of the invention illustrated in Figure 3, the areas of the printed circuit board 501 where the electronic components 512, 522 are located are not curved. They are kept un-curved by radiators 516, 526 which act as stiffeners.

Having holes 510, 520 filled with a heat conductive material opening on radiators 516, 526 allows the heat generated by the electrical components 512, 522 to be discharged through the holes 510, 520 to the radiators 516. , 526 where it is discharged to the ambient air away from the electrical components 512, 522 and over a large area.

In one embodiment of the invention, the total surface area of the hole or holes 510, 520 corresponding to an electronic component 512, 522 is sufficient to allow sufficient thermal discharge to the radiators 516, 526 so that the electronic components 512, 522 remain in a temperature range where their characteristics are not deteriorated by thermal effects.

In one embodiment of the invention, the total cross-sectional area of the hole or holes 510, 520 corresponding to an electronic component 512, 522 is greater than or substantially equal to the surface of the component that faces the printed circuit board and the card printed circuit board 501 has a thickness of at most about 600 micrometers.

In such a thin circuit board, the distance that the heat must travel between the electronic component and the heat sink is less than in conventional printed circuit boards. In addition, since the total cross-sectional area of the hole (s) is substantially equal to or greater than the area of the electronic component corresponding to the hole (s), the total heat exchange area between the electronic component and the heat sink is larger than for multiple narrow thermal vias.

The printed circuit board of the present invention can be used in lighting and / or signaling applications, particularly for motor vehicles. A particular application is the elevated central brake light, or raised brake light, also referred to as HMSL or CHMSL in English. Another application is the daytime running lights, also referred to by the acronym DRL. In one embodiment of the invention particularly suitable for the raised brake light, the card comprises a single layer of FR4, the electronic components are a plurality of LEDs, for example 6, a single electronic component is present per stiffening element and the components electronic devices are substantially aligned. The card according to the present invention allows the assembly comprising the card and the LEDs to follow the curvature of the rear windshield or the bodywork of a vehicle.

In one embodiment of the invention, some electronic components are on the top of the board and have their stiffening element on the underside of the board while other electronic components are on the underside of the board and have their stiffening element on the board. above the map. The attachment to the support is made by one or more stiffening elements, placed on one or two sides.

It is understood that the present invention is in no way limited to the embodiments described above and that many modifications can be made without departing from the scope of the appended claims.

Claims (11)

A printed circuit board comprising a flexible sheet comprising electrical interconnection tracks, characterized by: a. in that the flexible sheet includes a laminated composite material including glass fibers, b. in that the flexible sheet has at least one location adapted to receive at least one electronic component so that this location corresponds to a stiffening element, the electronic component and the corresponding stiffening element being placed respectively on opposite faces of the flexible sheet, and c. in that the printed circuit board comprises at least one stiffening element adapted to fix the printed circuit board on a support. The printed circuit board of claim 1, wherein the laminated composite material is FR4. The printed circuit board according to one of claims 1 or 2, wherein the flexible sheet has a thickness of at most substantially 600 microns. A printed circuit board according to any one of the preceding claims, wherein the contact area between the stiffening element and the card is at least substantially equal to the area on the card of the slot adapted to receive at least one electronic component. A printed circuit board according to any one of the preceding claims, wherein the stiffening element adapted to secure the printed circuit board to a support comprises a fixing means for fixing said stiffening element to the support. The printed circuit board of claim 5, wherein the securing means comprises holes on the stiffening member. 7. Printed circuit board according to one of claims 5 or 6, wherein the fixing means comprises an adhesive. 8. Printed circuit board according to any one of the preceding claims, pierced with at least one hole containing a thermal conductive material for thermal contact between the electronic component and the stiffening element corresponding thereto. A printed circuit board according to any one of the preceding claims wherein at least one electronic component is a light emitting diode. 10. Printed circuit board according to any one of the preceding claims adapted for lighting application. 11. Printed circuit board according to claim 10 adapted to be used for lighting and / or signaling devices for motor vehicles.