FR3114214A1 - Electronic board comprising components buried in cavities - Google Patents

Abstract

The present invention relates to an electronic card (1) comprising:- a printed circuit (2) comprising: - at least one insulating layer (3); and - a first and a second conductive skin layer (4, 5) on either side of the insulating layer (3);- an electronic component (6) comprising a first termination (7) and a second termination (8 ); and- a through-cavity (12) formed in the insulating layer (3) and being open to the first and conductive skin layer (4) and the second conductive skin layer (5); the electronic component (6) being housed in the cavity (12), the first termination (7) being soldered to the first conductive skin layer (4) and the second termination (8) being soldered to the second conductive skin layer ( 5). Figure for the abstract: Fig. 1

Description

Electronic board comprising components buried in cavities

FIELD OF THE INVENTION

The invention relates to the field of electronic cards, in particular when these electronic cards are intended to be integrated into a portable and on-board device, for example in the field of aeronautics and space, and more precisely the attachment of components to circuits printed.

STATE OF THE ART

In a manner known per se, an electronic card can comprise surface-mounted components (CMS), that is to say electronic components soldered directly to the surface of the printed circuit of an electronic card, through-hole components, or even printed circuits.

Usually, SMDs are soldered on the surface either by reflow ("reflow soldering" in English) or by wave ("solder wave" in English).

In the case of reflow soldering, the bare circuit board is first screen printed by covering the conductive layers of the circuit board (usually copper) with a solder paste using a screen printing screen (or stencil) so that only the locations intended to receive the terminations of the components are covered by the solder paste. The solder paste comprises, in a manner known per se, a metal alloy in suspension in a soldering flux. Then the component terminations (CMS) are placed on the solder paste before undergoing a reflow heat treatment, during which the heat remelts the alloy and evaporates the solder flux so as to form solder joints from of the metal alloy present in the solder paste.

In order to increase the implantation density of the components of a printed circuit comprising surface-mounted components, it has already been proposed to reduce the size of the case of the electronic components, to change the interconnection technology, to stack electronic chips, bringing them closer or even burying them in the internal layers of the printed circuit, and making through vias in order to connect the two sides of the printed circuit.

Thus, document US 2014/158414 proposes to bury components in the printed circuit. More precisely, a cavity is produced by laser cutting in the printed circuit, after its manufacture, so as to reveal soldering areas at the bottom of the cavity. The component is then placed at the bottom of the cavity and soldered on the soldering pads, then the electronic board is screen-printed in order to embed the component in an insulating layer. This solution thus makes it possible to free up the skin layer of the electronic card. However, the insulating layer in which the component is embedded prevents replacement of the components in the event of their failure and complicates the development phase of the electronic card.

Document FR 3 069 127 in the name of the Applicant has also proposed a method for manufacturing an electronic card comprising a multilayer printed circuit having at least four conductive layers separated two by two by insulating layers, including:

• two conductive skin layers each fixed on an associated insulating layer,
• internal conductive layers, extending between the insulating layers and separated by a central insulating layer, at least one of the internal conductive layers being treated so as to form at least one soldering pad.

A cavity is formed in one of the conductive skin layers and in the associated insulating layer, opposite the soldering pad of the internal conductive layer, so that at least part of the soldering pad is exposed. This cavity is then filled with a metal alloy accompanied by a soldering flux, for example solder paste, then an electronic component is placed in front of the cavity. Finally, a reflow-type heat treatment is applied to the printed circuit on which the component is placed in order to transform the metal alloy accompanied by the soldering flux into a solder joint so as to fix the component to the printed circuit.

This process thus makes it possible to effectively increase the density of implantation and the mix of surface-mounted components by creating cavities making it possible to reduce the size of the electronic card, while allowing the replacement of defective SMDs.

In addition, it appears that the case size gains are part of the choices already used and that the stacking of chips is only possible for a small part of the electronic components and concerns rather consumer components which are not necessarily compatible. with more severe environments, especially in terms of vibrations, temperature ranges and temperature variations.

These processes have therefore already made it possible to increase the density of component implantation in an electronic board. However, the aeronautics and space industry is constantly seeking to improve electronic boards. Moreover, in certain fields, the cost associated with the proposed solutions remains too high and cannot therefore be envisaged because the cost price of the electronic card obtained would be too high.

An object of the invention is to remedy the drawbacks of the prior art.

Thus, an object of the invention is to propose an industrializable solution making it possible to improve the density of implantation of electronic components on an electronic card and to reduce their manufacturing cost.

For this purpose, according to a first aspect of the invention, an electronic card is proposed comprising:
- a printed circuit comprising:
- at least one insulating layer; And
- a first and a second conductive skin layer on either side of the insulating layer;
- an electronic component comprising a first termination and a second termination; And
- a through-cavity formed in the insulating layer and being open to the first and conductive skin layer and the second conductive skin layer.

The electronic component is housed in the cavity. Furthermore, the first termination is soldered to the first conductive skin layer and the second termination is soldered to the second conductive skin layer.

Certain preferred but non-limiting characteristics of the electronic card according to the first aspect are the following, taken individually or in combination:

• the electronic component comprises a larger surface face and is placed in the cavity so that the larger surface face extends parallel to the first conductive layer of skin in the cavity;
• the printed circuit has a thickness, the electronic component has a distance between the first termination and the second termination and the distance is between 0.9 and 1.1 times the thickness of the printed circuit;
• the first termination is further soldered to the second conductive skin layer and the second termination is further soldered to the first conductive skin layer;
• the electronic component further comprises an additional electronic component soldered to the first conductive skin layer, an additional termination of the additional electronic component being connected to the first termination of the electronic component via a solder joint;
• the additional electronic component at least partially covers the cavity;
• the additional electronic component is placed on the first conductive skin layer without overlapping with the cavity; and or
• the additional termination of the additional electronic component is soldered to the termination of the electronic component.

According to a second aspect, the invention proposes a method for manufacturing an electronic card comprising the following steps:
S1: provide a printed circuit including:
- an insulating layer having two opposite faces; And
- a first and a second conductive skin layer, each conductive skin layer being fixed on a corresponding face of the insulating layer;
S2: forming a cavity in the insulating layer, the cavity being open to the first conductive skin layer and the second conductive skin layer;
S3: placing an electronic component in the cavity, the electronic component comprising a first termination and a second termination; And
S4: solder the first termination of the electronic component to the first conductive skin layer and the second termination to the second conductive skin layer.

Certain preferred but non-limiting characteristics of the manufacturing method according to the second aspect are the following, taken individually or in combination:

• the method further comprises, prior to step S3, a step of laying the printed circuit on a support, step S4 comprising the following sub-steps:

- Placing the printed circuit on a support, the first conductive skin layer facing the support;

- application of a solder paste to the second conductive skin layer and reflow heat treatment so as to form a first solder joint between the second termination and the second conductive skin layer;

- reversal of the printed circuit comprising the soldered electronic component; And

- application of a solder paste to the first conductive skin layer and reflow heat treatment so as to form a second solder joint between the first termination and the first conductive skin layer;

• the electronic component includes a larger area face and wherein, in step S3, the electronic component is placed in the cavity such that the larger area face extends parallel to the first conductive skin layer in the cavity;
• during step S4, the first termination is further soldered to the second conductive skin layer and the second termination is further soldered to the first conductive skin layer; and or
• the method further comprises a step of soldering an additional electronic component to the first conductive skin layer by connecting an additional termination of the additional electronic component to the first termination of the electronic component via a solder joint .

DESCRIPTION OF FIGURES

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings in which:

FIG. 1 schematically illustrates an example of an electronic card according to a first embodiment, on which an example of an electronic component positioned horizontally in a cavity has been illustrated;

FIG. 2 schematically illustrates the screen printing of the second conductive skin layer and of the second termination of an example of an electronic component, placed on a support with a view to obtaining an example of an electronic card according to a second embodiment , the electronic component being positioned vertically in the cavity;

FIG. 3 is a flowchart of steps of a method of manufacturing an electronic card according to one embodiment of the invention.

In all the figures, similar elements bear identical references.

DETAILED DESCRIPTION OF THE INVENTION

An electronic card 1 comprises a printed circuit 2 comprising conductive layers separated by insulating layers on which electronic components are fixed.

In general, a printed circuit 2 can be of the monolayer type (also called single layer) and only include a single conductive layer, double layer (also called double face) and include a conductive layer on either side of an insulating layer or multilayer and include at least four conductive layers.

The insulating layers may comprise, in a manner known per se, an epoxy resin and glass fibers. The conductive layers can be made of copper (or a copper-based alloy).

By electronic component, we will understand in what follows any element intended to be assembled with others in order to perform one or more electronic functions and comprising one or more terminations on the thickness of the component, for example a first termination 7 and a second termination 8. By way of non-limiting example, an electronic component may include a discrete electronic component such as a surface mount component (SMT) (typically a resistor, capacitor, capacitor, resistor network, abilities, etc).

An electronic card 1 comprises a printed circuit 2 comprising:

- A printed circuit 2 comprising at least one insulating layer 3 and a first and a second conductive skin layer 4, 5 on either side of the insulating layer 3;
- an electronic component 6 comprising a first termination 7 and a second termination 8; And
- a through-cavity 12 formed in the insulating layer 3 and being open to the first conductive skin layer 4 and the second conductive skin layer 5.

The printed circuit 2 can be monolayer, bilayer or multilayer.

The electronic component 6 is housed in the cavity 12. In addition, its first termination 7 is soldered to the first conductive skin layer and its second termination 8 is soldered to the second conductive skin layer.

Thus, instead of placing the electronic component 6 on one of the conductive skin layers 4, 5, the invention proposes using the thickness of the printed circuit 2 to house the component, and thus reduce the density of implantation electronic components on the surface, on the electronic card 1. Furthermore, by placing the electronic component 6 in a through-cavity (and not a blind cavity having a bottom) each termination 7, 8 of the electronic component 6 can be connected at the same time to the first conductive skin layer 4 and to the second conductive skin layer 5, which makes it possible to connect said layers 4, 5 without requiring the formation of a crossing via or metallizing the cavity 12, which are costly steps. A printed circuit 2 is thus obtained at low cost and with low layout density.

The electronic component 6 has a face 11 of larger surface, or larger face 11. The orientation of the electronic component 6 in the cavity 12 depends on the thickness e ₁ (distance between the first and the second conductive layer of skin 4 , 5) of the printed circuit 2 and the dimensions of the electronic component 6. Generally, the printed circuits 2 being thin e ₁ , the electronic component 6 is placed in the cavity 12 so that its largest face 11 extends parallel to the first conductive skin layer 4. By placing the second electronic component 6 in a cavity 12 in a horizontal position (since its largest face 11 is parallel to the first conductive skin layer 4), it is thus possible to take advantage of the space usually not used under the electronic component 6 and thus gain in place and density of implantation. The first and the second termination 7, 8 preferably extend over the entire thickness e ₂ (dimension of the electronic component 6 along an axis normal to the largest face) of the second electronic component 6, thus allowing their connection to the two layers skin conductors 4, 5.

As a variant, for electronic components of short length L and/or printed circuits of significant thickness e ₁ , the electronic component 6 is placed in the cavity 12 so that its largest face 11 extends perpendicularly to the first layer skin conductor 4. The first and the second termination 7, 8 preferably extend over the entire length (dimension of the electronic component 6 along an axis parallel to the largest face) of the second electronic component 6, thus allowing their connection to the two conductive skin layers 4, 5.

In all cases, the electronic component 6 is positioned and oriented in the cavity 12 so as not to protrude from the first conductive skin layer 4 nor from the second conductive skin layer 5. In other words, the horizontal/vertical orientation of the large face 11 of the electronic component 6 is chosen so that the electronic component 6 does not protrude from the cavity 12. Thus, if the length L of the largest face 11 of the electronic component 6 is greater than l thickness e ₁ of the printed circuit 2 (within 1 mm), the electronic component 6 is positioned horizontally in the cavity 12, that is to say that its largest face 11 is parallel to the first conductive skin layer 4 Preferably, either the length L of the largest face of the electronic component 6 is substantially equal (to within 10%) to the thickness e ₁ of the printed circuit 2  in which case the electronic component 6 is placed vertically in the printed circuit 2  is the thickness e ₂ of the electronic component 6 is substantially equal (to within 10%) to the thickness e ₁ of the printed circuit 2  in which case the electronic component 6 is placed horizontally in the printed circuit 2.

It will be understood that, to solder the electronic component 6 to the first conductive skin layer 4 and to the second conductive skin layer 5, the first termination 7 and the second termination 8 of the second electronic component 6 preferably extend laterally with respect to to the electronic component 6, that is to say on faces which are substantially normal with respect to its largest face 11. Preferably, both the first termination 7 and the second termination 8 are lateral.

In one embodiment, the cavity 12 opens onto a soldering pad formed in the first and the second conductive skin layer 4, 5. These soldering pads therefore each form a flange at the exit from the cavity 12, that is that is to say that they at least partially surround the cavity 12.

Optionally, the cavity 12 can be partially metallized.

If necessary, one or more additional electronic components can be soldered to the soldering pad of the first and/or second conductive skin layer 4, 5, and/or to one of the terminations 7, 8 of the electronic component 6. Thus, the additional electronic component(s) can share a soldering area with the electronic component 6, which makes it possible to reduce the length of the connection tracks between the electronic component 6 and the additional electronic component(s) and improves electrical performance . For this, the additional electronic components can be placed on the first and/or the second conductive skin layer 4, 5 so as to at least partially overlap the cavity 12, or alternatively without overlapping it. Reference may be made to document FR1901926 filed on February 25, 2019 in the name of the Applicant for more details on the soldering of an electronic component 6 on a cavity, with or without overlap.

The additional termination 10 (whose role is played by the additional soldering pad, in the case of an additional printed circuit 2) of the additional electronic component 9 is connected to the first termination 7 of the electronic component 6 via a solder joint. The soldering of the additional electronic component 9 can be carried out via the conductive skin layer 4, 5 or directly on the termination 7, 8 of the electronic component 6.

By way of non-limiting example, the additional electronic component 9 may comprise a discrete electronic component 6 such as a surface-mounted component (CMS) and comprising an additional termination 10 (typically a resistor, a capacitor, a BGA (English acronym Ball Grid Array for ball matrix), etc.) or an additional printed circuit 2 comprising an additional soldering area 10 on which are fixed one or more discrete electronic components.

This embodiment variant is in particular advantageous in the case where the additional electronic component 9 comprises a BGA (English acronym for Ball Grid Array for matrix of balls). Indeed, the soldering of a BGA on one of the terminations 7, 8 of the electronic component 6 and/or on the conductive skin layer 4, 5 also makes it possible to reduce the length of the connection tracks between the BGA and the electronic component 6 which, in addition to improving the electrical performance, facilitates routing between the BGA and the electronic component 6, since the electronic component 6 can be placed in the cavity 12 in the printed circuit 2, instead of being placed on the second conductive skin layer 5.

An electronic card 1 in accordance with the invention can for example be obtained in accordance with the following steps:
S1: provide a printed circuit 2 comprising:
- an insulating layer 3 having two opposite faces; And
- A first and a second conductive skin layer 4, 5, each conductive skin layer being fixed on a corresponding face of the insulating layer 3;
S2: forming a through cavity 12 in the insulating layer 3, the cavity 12 being open to the first conductive skin layer 4 and the second conductive skin layer 5;
S3: placing an electronic component 6 in the cavity 12, the electronic component 6 comprising a first termination 7 and a second termination 8; And
S4: solder the first termination 7 of the electronic component 6 to the first conductive skin layer 4 and the second termination 8 to the second conductive skin layer.

During step S2, cavity 12 can be formed in insulating layer 3 by any appropriate means. For example, the cavity 12 can be made by laser drilling, by milling or by punching the insulating layer 3.

If necessary, the cavity 12 can then be partially metallized, according to the desired embodiment.

Steps S3 and S4 can be performed according to the following substeps:

• placement of the printed circuit 2 on a carrier 19, the first conductive skin layer 4 facing the carrier 19;
• placement of the electronic component 6 in the cavity 12 (step S3), by placing it on the carrier 19 (horizontally or vertically, depending on the dimensions L, e ₂ of the electronic component 6 and the thickness e ₁ of the printed circuit 2);
• application of a solder paste on the second conductive layer of skin 5 and on the second termination 8 of the electronic component 6. Optionally, the electronic component 6 can comprise an additional second termination 8, in which case the solder paste is also applied to the second additional termination 8. The solder paste comprises, in a manner known per se, a metal alloy suspended in a soldering flux;
• reflow heat treatment of the electronic card 1 comprising the printed circuit 2 and the electronic component 6 so as to form a first solder joint 14 between the second termination 8 (and optionally the additional second termination 8) of the electronic component 6 and the flange at the soldering area of the second conductive skin layer 5;
• reversal of the printed circuit 2 comprising the electronic component 6 soldered. The electronic component 6 being brazed, the carrier 19 is then no longer necessary;
• application of a solder paste to the first termination 7 of the electronic component 6 and at the level of the soldering area of the first conductive layer of skin 4. Optionally, the electronic component 6 can comprise an additional second termination 8, in which case the solder paste is also applied to the second additional termination 8;
• reflow heat treatment of the electronic card 1 comprising the printed circuit 2 and the electronic component 6, so as to form a second solder joint 14 between the first termination 7 of the electronic component 6 (and optionally the first additional termination 7) and the soldering area of the first conductive skin layer 4.

By reflow heat treatment, we will here understand a heat treatment during which the heat remelts the alloy of the solder paste and evaporates the solder flux so as to form solder joints from the metal alloy present in solder paste. Reference may be made in particular to document FR 3 069 128 in the name of the Applicant for more details on reflow soldering.

Of course, the soldering of the first termination 7 of the electronic component 6 can be carried out before the soldering of its second termination 8 on the second conductive skin layer 5.

Where appropriate, when the electronic card 1 further comprises at least one additional electronic component 9, this or these components are placed on the corresponding second conductive layer of skin 4, 5 after application of a solder paste on this layer and previously to its heat treatment.

Solder paste can be applied by any means, such as:

• screen printing;
• the local soldering of a solder paste by local addition of alloy and temperature, thanks to a soldering iron by a repair machine. For this, the first termination 7 is soldered to the first conductive skin layer 4, then the electronic card is turned over before soldering the second termination 8 to the second conductive skin layer 5;
• wave soldering. For this, the brazing is carried out in accordance with the following sub-steps: placing a dot of glue on the edge of the cavity between the terminations 7, 8 (or a dot of glue on the two edges between the terminations 7, 8) ; placing the second component 8 in the cavity; polymerization of the glue; placing the first component(s) 9 on the second conductive skin layer 5; passage to the wave of the second conductive skin layer 5 of the first conductive skin layer 4.

Claims (13)

Electronic card (1) comprising:
- a printed circuit (2) comprising:
- at least one insulating layer (3); And
- a first and a second conductive skin layer (4, 5) on either side of the insulating layer (3);
- an electronic component (6) comprising a first termination (7) and a second termination (8); And
- a through-cavity (12) formed in the insulating layer (3) and being open to the first and conductive skin layer (4) and the second conductive skin layer (5);
the electronic card being characterized in that the electronic component (6) is housed in the cavity (12) and in that the first termination (7) is soldered to the first conductive skin layer (4) and the second termination (8 ) is soldered to the second conductive skin layer (5). Electronic card according to claim 1, in which the electronic component (6) comprises a face (11) of greater surface and is placed in the cavity (12) so that the face (11) of greater surface extends parallel to the first conductive skin layer (4) in the cavity (12). Electronic card (1) according to one of Claims 1 or 2, in which the printed circuit (2) has a thickness (e ₁ ), the electronic component (6) has a distance (L, e ₂ ) between the first termination (7) and the second termination (8) and the distance (L, e ₂ ) is between 0.9 and 1.1 times the thickness (e ₁ ) of the printed circuit (2). Electronic card (1) according to one of Claims 1 to 3, in which the first termination (7) is additionally soldered to the second conductive skin layer (5) and the second termination (8) is additionally soldered to the first conductive layer of skin (4). Electronic card (1) according to one of Claims 1 to 4, further comprising an additional electronic component (9) soldered to the first conductive skin layer (4), an additional termination (10) of the additional electronic component (8) being connected to the first termination (7) of the electronic component (6) via a solder joint. Electronic card (1) according to Claim 5, in which the additional electronic component (9) at least partially covers the cavity (12). Electronic card (1) according to claim 5, in which the additional electronic component (9) is placed on the first conductive skin layer (4) without overlapping with the cavity (12). Electronic card (1) according to one of Claims 5 to 7, in which the additional termination (10) of the additional electronic component (8) is soldered to the termination (9) of the electronic component (6). Method for manufacturing an electronic card (1) comprising the following steps:
S1: provide a printed circuit (2) comprising:
- an insulating layer (3) having two opposite faces; And
- a first and a second conductive skin layer (4, 5), each conductive skin layer being fixed on a corresponding face of the insulating layer (3);
S2: forming a cavity (12) in the insulating layer (3), the cavity (12) being open to the first conductive skin layer (4) and the second conductive skin layer (5);
S3: placing an electronic component (6) in the cavity (12), the electronic component (6) comprising a first termination (7) and a second termination (8); And
S4: soldering the first termination (7) of the electronic component (6) to the first conductive skin layer (4) and the second termination (8) to the second conductive skin layer (5). Method according to claim 9, further comprising, prior to step S3, a step of laying the printed circuit (2) on a support, step S4 comprising the following sub-steps:
- placing the printed circuit (2) on a support (19), the first conductive skin layer (4) facing the support (19);
- application of a solder paste to the second conductive skin layer (5) and reflow heat treatment so as to form a first solder joint (14) between the second termination (8) and the second conductive skin layer ( 5);
- reversal of the printed circuit (2) comprising the soldered electronic component; And
- application of a solder paste to the first conductive skin layer (4) and reflow heat treatment so as to form a second solder joint (15) between the first termination (7) and the first conductive skin layer ( 4).
Method according to one of Claims 9 or 10, in which the electronic component (6) comprises a face (11) of larger surface and in which, during step S3, the electronic component (6) is placed in the cavity (12) so that the larger surface face (11) extends parallel to the first conductive skin layer (4) in the cavity (12). Method according to one of Claims 9 to 11, in which, during step S4, the first termination (7) is additionally soldered to the second conductive skin layer (5) and the second termination (8) is further soldered to the first conductive skin layer (4). Method according to one of Claims 9 to 12, further comprising a step of soldering an additional electronic component (9) onto the first conductive skin layer (4) by connecting an additional termination (10) of the additional electronic component ( 9) to the first termination (7) of the electronic component (6) via a solder joint.