The invention relates to the field of electronic cards, in particular in the field of aeronautics and space, and more precisely the mounting surface mount components on printed circuits.

BACKGROUND

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

Usually the CMS are soldered surface or reflow ( "reflow soldering" in English) or wave ( "solder wave" in English).

In the case of reflow soldering, the printed circuit bare is first screen-printed covering the conductive layers of the printed circuit (usually copper) with a solder paste using a printing screen (or stencil) so that only the locations for receiving the terminations of the components are covered by the solder paste. The solder paste comprises, in known manner, a metallic alloy slurry in a brazing flux. Then the termini of the components (SMD) are placed on the solder paste before undergoing a thermal reflow treatment, during which heat is recast alloy and evaporate the flux of solder to form solder joints from of the metal alloy present in the solder paste.

The reliability and service life of solder joints which attach the CMS to the printed circuit, depends on the vertical height (usually designated by the term "standoff") between the upper face of the copper brazing layer and the point bottom conductive terminations CMS once brazed. The standoff height corresponds to the alloy interface between the CMS and the copper layer. Indeed, in use, the CMS and the surface on which it is soldered expand differently, causing relative movements particularly in the surface plane (X, Y). Thus, over the standoff, the greater the brazed joint is flexible and therefore robust.

However, increasing the standoff is limited by conventional connecting means provided in production and by the variety of geometries of components Tin solder surface.

It was therefore proposed placing a wedge under the CMS to increase the standoff. However, the CMS may be damaged in the case where it expands Z (that is to say in a direction normal to the surface of the printed circuit on which is soldered the CMS), and come off prematurely circuit printed. Moreover, this solution is not applicable to all types of CMS, regardless of their size, weight and type of endings they wear.

It has also been proposed to increase the amount of cream solder applied on the layers of the PCB. For this, the size (width) of the apertures in the printing screen may for example be increased, so that during the reflow step, the height of the solder joint is increased by effect of coalescence: thickness the brazed joint is greater than the alloy of equivalent thickness obtained after reflow with smaller apertures in the printing screen, since the metal alloy can be spread beyond the layers of the printed circuit by effect wettability in its liquid phase (liquidus) during soldering. This method actually increases the standoff.

In addition, the height cream solder deposited on the printed circuit board for solder several CMS is substantially the same for each CMS, since it is deposited by screen printing using a screen. Of course, there are screens having a variable thickness to locally increase the height of the openings of the screen, and therefore the amount of solder paste

filed. However, these often involve varying thicknesses screen printing problems for adjacent CMS (cream solder height in adjacent holes unsatisfactory) and makes it difficult to optimize the implementation of CMS in the electronic card.

Finally, the deposition of the solder paste by screen printing limits the packing density of electronic components on the printed circuit and / or the type of component can be used, particularly in the case of fine pitch components. Indeed, the window size of the printing screen is limited by the following condition so that the screen can be removed from the mold without damaging the solder paste which has been filed as follows: the ratio between the surface of the window (in the plane of the screen, which is parallel to the plane (X, Y)) and the surface of the inner walls of the window (which extend perpendicularly to the plane of the screen) must be greater than or equal to 0.66. To meet such a report, it is necessary to reduce the thickness of the screen,

SUMMARY OF THE INVENTION

An object of the invention is therefore to propose a new method for securing a surface mounted component on a printed circuit which allows to increase the component life by increasing the associated standoff, that is also simple to perform and moderate cost whatever component packing density on the PCB and / or the component type (including the fine pitch components), without impacting the performance of assembling the electronic board.

For this, the invention provides a method of attaching an electronic component on a printed circuit, said printed circuit comprising a connection face having at least one conductive layer and defining a

Z axis, said Z axis being normal to the connection face, the fixing method comprising the steps of:

- applying an insulating layer comprising an electrically insulating material on the circuit of the connection face, the insulating layer having a predetermined minimum thickness along the Z axis,

- forming a cavity in the insulating layer over the conductive layer so that at least a portion of the conductive layer is disclosed, the cavity having a predetermined minimum depth along the Z axis,

- filling the cavity with a metal alloy together with a brazing flux,

- placing the electronic component above the cavity,

- applying a heat treatment to the printed circuit on which is placed the component to transform the brazing flux accompanied metal alloy solder joint so as to fix the component to the printed circuit.

The minimum thickness of the insulating layer is such that the depth of the cavity is at least equal to 100 m.

Some preferred but not limiting features of the method described above are as follows:

- the electrically insulating material of the insulating layer has a first coefficient of thermal expansion along the axis Z, the metal alloy has a second coefficient of thermal expansion along the Z axis, and wherein the first coefficient of thermal expansion is more larger than the second coefficient of thermal expansion,

- the steps of applying the insulating layer and of forming the cavity are formed by photolithography on the surface,

- the cavity is made using at least one of the following techniques: laser drilling of the insulating layer, mechanical cutting of the insulating layer, chemical machining of the insulating layer.

- the insulating layer is attached and fixed on the connection face, and wherein the cavity is formed by cutting or drilling before or after attaching the insulating layer on the connecting face,

- the insulating layer is formed by a printed circuit,

- the method further comprises, prior to the step of filling the cavity, a step of metallization of the conductive pad 12,

- the cavity is filled by screen printing, with or without printing screen,

- the cavity has an area in a plane normal to the Z axis, said cavity being filled by screen printing with screen printing stencil, said printing screen having a window having a surface in the plane normal to the Z axis, the surface of the window being at least the surface of the cavity, and / or

- the thermal treatment comprises remelting the metal alloy.

BRIEF DESCRIPTION OF DRAWINGS

Other features, objects and advantages of the invention appear better on reading the detailed description that follows, and the accompanying drawings given as non-limiting examples and in which:

Figures 1a to 1g show steps of an exemplary embodiment of a mounting process according to the invention.

2 illustrates an alternative embodiment of recesses formed in an insulating layer coated on a conductive layer of a printed circuit.

3 illustrates an alternative embodiment of the cavity filling step.

Figure 4 is a flowchart illustrating an example of the fixing steps an electronic component on a printed circuit according to one embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

An electronic card 1 comprises a printed circuit board 10 comprising a connection face 14 having at least one conductive layer 12, and at least one surface mounted component (SMD 2), preferably several CMS 2 soldered on the face of 1 connection.

The connection face 14 is substantially planar and defines a plane (X, Y), normal to an axis Z.

According to the invention, the two CMS is determined in accordance with following steps:

- applying an insulating layer 20 (step S1) comprising electrically insulating material on the connecting face 14 of the printed circuit 10,

- form a cavity 22 (step S2) in the insulating layer 20 over the conductive layer 12 so that at least part of the conductive layer 12 is at least partially uncovered,

- filling the cavity 22 (step S3) with a metal alloy 4 accompanied by a brazing flux 5,

- placing the component 2 above the cavity 22 (step S4),

- applying a heat treatment (step S5) to the printed circuit 10 on which is placed the component to transform the metal alloy 4 accompanied by brazing flux 5 in the solder joint 6 so as to fix the component to the printed circuit 10.

On couche will notice that the insulation 20 présente une minimal online

E determined while the cavity 22 has a minimum depth p determined, wherein the thickness E p and depth dimensions are along the axis Z. The minimum thickness E of the insulating layer 20 is then determined so that the depth p of the cavity 22 is at least equal to 100 m.

Generally, the thickness E p and the depth may be substantially constant.

Thanks to the presence of the insulating layer 20, it is thus possible to obtain a higher standoff H than in the prior art insofar as the standoff H obtained is at least equal to the depth p of the cavity 22, after heat treatment. Furthermore, the p minimum depth of the cavity 22 in the insulating layer 20 increases the life of the solder joint 6 sufficiently.

Furthermore, the order described above for the steps S3 and S4 is not limiting. Typically, the CMS 2 can be placed above the cavity 22 before the latter is filled, especially when the component is soldered to the wave.

In one embodiment, the thermal expansion coefficient ( "coefficient of thermal expansion" in English) along the axis Z of the material of the insulating layer 20 is larger than the coefficient of thermal expansion along the axis Z of the metal alloy 4 so that, after the heat treatment S5, after the solidification of the metallic alloy 4, the insulating layer 20 undergoes a greater shrinkage than the solder joint 6 (see Fig. 1g). CMS 2 is no longer supported by the solder joint 6 and a gap e is created between the underside of the CMS 2 and the free face of the insulating layer 20, thereby removing the risk of contact between these two parts despite any differential expansion in Z CMS 2 and the insulating layer 20. Furthermore,

1 electronic maps of the temperature of use it will be appreciated that is necessarily lower than the alloy of the melting temperature

metal 4, so that the CMS 2 remains at all times at a distance from the insulating layer 20.

The thickness E of the insulating layer 20 is selected to be large enough to ensure a significant standoff, even when the coefficient of thermal expansion along the axis Z of the insulating material layer 20 is not greater than of the metal alloy 4, without penalizing the overall size and weight of the electronic board 1.

For example, the insulating layer 20 may have a thickness S defined such that the depth p (along the Z axis dimension) of the cavity 22, which corresponds to the distance between the face unveiled the conductive layer 12 and the outer face the insulating layer 20, is at least equal to 100 m. Since a conductive layer 12 generally has a thickness of about 17.5 m to 35 m and can then be recharged during a metallization step, which further increases its thickness of about 20 mm, we obtain a thickness E of the insulating layer at least equal to a hundred micrometers. This thickness E can reach more than 300 mm according to the desired duration of life (over the layer will be thick, the better the durability).

The insulating layer 20 may cover all or part of the connection face 14 of the circuit board 10. In one embodiment, the insulating layer 20 covers the entire face of connection 14.

The filling step S3 can be carried out for example by filling the cavity with a solder paste comprising a metal alloy 3 4 suspended in a brazing flux 5. In step S4, the CMS 2 is then placed on the solder paste 3, above the insulating layer 20. Alternatively, the step S3 may be performed by an input through a molten alloy bath, in which case the CMS 2 is placed on a point glue (step S4) before the passage of the printed circuit in the molten alloy bath.

In the following, the invention will be described in the case where the circuit 10 is single layer (thus comprising a single face of

connection 14, a conductive layer 12 and insulating layer 16 can be epoxy and glass fibers) and two cavities 22 are formed. Moreover, filling the cavity S3 is performed by placing a solder paste comprising a metal alloy 3 4 suspended in a brazing flux 5. This is however not limited to the number of cavities 22 made on the printed circuit board 10 depending on the number and the type of CMS to be fixed on its connection face 14 in order to realize the electronic board 1. in addition, both sides of the printed circuit 10 could form the connection face 14, the electronic card 1 may CMS comprise on each of said faces.

Optionally, the S method may further comprise, before the filling step S3 of the cavity 22, a metallization step S6 of the conductive pad 12 to facilitate the brazing step S5.

The S5 heat treatment may in particular comprise a reflow

( "Reflow soldering" in English) of the metal 4 present in the cream solder alloy 3. For this purpose, in a first step so-called temperature rise ( "ramp up" in English), the temperature is gradually increased. This step rise in temperature may, in known manner, be carried out following a slope between 1 ° C / s and 4 ° C / s to 100 ° C to 150 ° C (maximum 7 ° C / s , maximum temperature slope advocated by SMD components before brazing).

In a second step said preheating ( "preheat" in English) operates the flow drying operation ( "soak" gold "preflow" in English) and cleaning preparation soldering terminals of the printed circuit and components, the temperature is increased gradually to about 170 ° C and maintained for at least half a minute to several minutes (depending on the flow used) to allow the evaporation of the volatile parts of the brazing flux 5 and homogeneity of temperature all components before reflow phase. The insulating layer 20 expands.

During a third step said reflow ( "reflow" in English), the temperature is again increased to reach a critical temperature, generally greater than 20 to 50 ° C above the melting temperature of alloy of the metal alloy used 4.

When the temperature passes through the melting point of the metallic alloy 4 contained in the solder paste 3 (or for example about 180 ° C when the metal alloy 4 comprises a tin / lead 63/37, of the order 217 ° C in the case of a metallic alloy 4 tin / silver / copper 95.6 / 3.0 / 0.5 and of the order of 221 ° C in the case of a metallic alloy 4 tin / silver 96.5 / 3.5), the metal alloy 4 background.

During this step, the insulating layer 20 continues to expand, further lifting the CMS 2 with respect to the connection face 14. It will be noted that at any time, the metal alloy that is liquid 4 remains in contact with the terminations 2 CMS and the conductive layer 12 under the effect of the wettability.

Preferably, the increase in temperature during the reflow step is fast and in any case quicker than the rise of temperature step in order to avoid that the CMS 2 does not suffer long elevated temperatures. The printed circuit 10 may also be kept at the critical temperature above the melting temperature of the alloy for a time which may be between twenty and ninety seconds, depending on the thermal mass of the components to be assembled. This time above the alloy melting temperature allows the creation of intermetallic that will maintain the components between the metallic alloy 4 and the parts to be assembled.

During a fourth step, the printed circuit 10 is rapidly cooled to room temperature. When the temperature rises through the melting point and becomes lower than the melting temperature of the metallic alloy 4, it solidifies, thereby forming the solder joint 6. The layer 20 then guarantees a minimum height H of the component. When the coefficient of thermal expansion in Z of the insulating layer 20 is larger than that of the metallic alloy 4, the insulating layer 20 undergoes a shrinkage which is greater than that of the metallic alloy 4, thus creating space e between the underside of the CMS 2 and the free face of the insulating layer 20. Alternatively, when the coefficient of thermal expansion in Z of the insulating material of the insulating layer 20 n '

Note that the brazing flux 5 gradually evaporates during the temperature rise step (and preheating / drying), leaving only the metallic alloy 4 on the conductive layer 12. The solder joint 6 does not include so that the metal alloy 4.

The thermal reflow processing is well known in the technical field of soldering SMD 2, so it is not necessary to detail the further here. Moreover, temperatures, slopes and durations of steps S5 heat treatment are given by way of example and obviously depend on the solder paste 3 used. One skilled in the art will therefore adapt without difficulty depending on the type of metal alloy 4 and 5 used brazing flux.

The insulating layer 20 may comprise any electrically insulating material. Where appropriate, the material constituting the insulating layer 20 may be thermally conductive. Moreover, according to the thermal environment and vibration and to further increase the life of the printed circuit 10, the material of the insulating layer 20 can, as we have seen above, be selected to present a coefficient of thermal expansion in Z greater than that of the metallic alloy 4.

Typically, the insulating layer 20 may include at least one of the following materials: glass fiber, epoxy resin, polyimide, polyester, polymer, Teflon.

As can be seen, the method of the invention S allows using any type of solder paste 3, and in particular unleaded solder pastes, allowing comply with current standards and in particular the European RoHS Directive n 2002/95 / EC - banishment of lead, hexavalent chromium, mercury, cadmium, and Polybrominatedbiphenyls

decabromodiphenyl ethers. For example, the metal alloy 4 of the cream solder 3 may comprise one of the following compositions, which are most commonly used: tin / lead 63/37 or 10/90 or 90/10 tin / lead / silver 62/36/2, for products exempt from RoHS or tin alloy / silver 96.5 / 3.5., tin / silver / Copper 96.5 / 3.8 / 0.7 or 96.6 / 3.0 / 0.5 or 98.5 / 1 .0 / 0.5. This technique is suitable for all type of alloy (comprising bismuth, antimony, etc.)

In a manner known per se, the brazing flux 5 depends on the metal alloy type 4 suspended in the cream solder 3 and the assembly process with or without cleaning. The brazing flux 5 generally comprises a resin (typically a natural resin, modified or synthetic), activating agents and additives for optimizing screen printing and reflow. The role of the brazing flux 5 is to ensure the stripping of the conductive layers 12 (using activators), to protect them during the temperature rise of steps and play a role surfactant to promote wetting of metal alloy 4.

E.g., brazing flux 5 may comprise rosin.

The insulating layer 20 may be applied by any means on the connection face 14.

In a first embodiment, the insulating layer 20 can be applied by photolithography on the surface. In this case, the cavities 22 may also be formed by photolithography on the surface (see Figure 2).

For this purpose, in a first step, a photosensitive resin for forming the insulating layer 20 is applied on the connection face 14 in the form of a film. The resin may be a negative resin (ultraviolet radiation causing polymerization of the exposed areas thereby imparting to these areas a particular outfit to the developing solvent while the portions unexposed selectively disappear in this solvent) or positive (the ultraviolet radiation causes a break macromolecules, resulting in increased solubility of the exposed areas in the developing solvent). The resin may in particular comprise an epoxy resin.

During a second step, a mask is applied on the resin film. The mask comprises transparent areas and opaque areas to form the cavities 22 and the insulating layer 20.

During a third step, the resin film is exposed to light radiation: in the case of a positive photoresist (typically epoxy resin), the portions of the film present in the transparent areas will then respond to this light radiation and solubilize while the parties present in the opaque areas will be protected. Conversely, in the case of a negative resist, the cavities 22 are formed under the opaque areas of the film.

In all cases, the solubilized portions are then removed with a developing solvent, thereby obtaining the insulating layer 20 in which are formed the recesses 22.

These steps are known per se, they will not be detailed further here.

In a second embodiment, the insulating layer 20 may be attached and fixed on the connection face 14, for example by lamination or gluing with an adhesive layer (see figures 1 and 1b). The adhesive layer may comprise any type of adhesive conventionally used in the field of printed circuit board material to join the layers together, typically an epoxy adhesive.

The cavities 22 can then be preformed in the insulating layer 20 before it is placed on the connection face 14, or after its attachment (as in FIG 1 c, for example).

For example, the cavities 22 may be formed by cutting the insulating layer 20. The cutting may be performed mechanically (using a mechanical milling type cutting tool or mechanical drilling or laser drilling before laying on the face 14 ). In the case of laying on the face 14 before the realization of cavities, the cutting may be achieved by chemistry on dry film or liquid photoimageable or mechanical milling or mechanical drilling or laser drilling. The realization by laser drilling may be realizable only from the time the surface (in the plane (X, Y)) to the bottom of the cavity 22 is smaller than the conductive layer 12 made of copper so that the layer copper extends beyond the bottom of the cavity 22 to allow the CO2 laser action stop towards copper.

When the cavities 22 are formed by laser drilling of the insulating layer 20, the laser may be of the laser type gas (carbon dioxide). The parameters selected for the laser may then be similar to those commonly used for the realization of laser vias.

The technique of laser drilling the insulating layer 20 makes it possible to form cavities 22 with high accuracy. In particular, it is possible to position the cavities 22 with high accuracy with respect to the conductive layer 12 and realize the cavities with low dimensional tolerances. Typically, the dimensional tolerance is of the order of

25 microns (pour des cavités de Ι ΟΟμηι minimum fine) lorsque les cavités

22 are made by laser drilling, as opposed to about 100 microns (for a minimum of 300 m cavities side) when they are produced by cutting (mechanical or chemical).

The laser drilling allows increased density

CMS 2 on the connection face 14, since the size of the cavities 22 may be reduced (the dimensional tolerance being smaller).

When the insulating layer 20 comprises a printed circuit, it may be attached and fixed to the connection face 14 by brazing or gluing.

In one embodiment, the cavities 22 are then made in the printed circuit of the insulating layer.

The printed circuit forming the insulating layer 20 can then extend over all or part of the connection face 14. Alternatively, it may extend only locally, as CMS.

This embodiment is particularly advantageous in the case where the CMS comprises tabs gull wings, the assembly tends to break.

The insulating layer 20 may be attached to the connection face 14 after completion of the underlying layers of the printed circuit 10. In this embodiment, the printed circuit 10 is provided in finished form (the case of Figures 1 to appended ).

Alternatively (not shown in the figures), when the printed circuit comprises at least four conductive layers 12, the insulating layer 20 may be reported when stacking the printed circuit 10 on the connection face 14 of the printed circuit 10 when the stack before it is rolled. Indeed, a printed circuit board 10 is generally achieved by stacking and compression temperature in accordance with the following steps:

- provide a double-sided PCB (that is to say comprising an insulating layer 16 and two conductive layers 12 on both sides of the insulating layer),

- report and fix an additional insulating layer on each conductive layer, the additional insulating layers may for example comprise epoxy and glass fibers,

- report and fix an outer conductive layer on each of the additional insulating layers,

- etching one of the outer conductive layers to form the tracks on one connection face 14,

- report and fix an insulating layer 20 on the outer conducting layer forming the connection face 14, and

- compressing the assembly thus formed.

In this case, the insulating layer 20 is thus fixed before the compression step of the assembly (ie when stacking), by gluing or lamination on the side of connection 14.

Note that regardless of the embodiment, the printed circuit 10 may be of the monolayer type (also called single-layer: the printed circuit comprises a single conductive layer 12), double layer (also called double-sided: a conductive layer 12 on either side of an insulating layer 16) or multilayer (at least four conductive layers 12).

In the embodiments shown in the figures, the printed circuit board has for example four conductive layers 12. The connection face 14 of the printed circuit includes a conductive layer 12, on which are formed two cavities 22 in order to fix a CMS . The circuit 10 also includes a through-via 18. This is however not limited to, the PCB 10 may include more or fewer conductive layers 12 as we have just seen, a larger number of cavities on its connection face 14 and a different number (possibly zero) of via through or non-through.

The cavities 22 may be filled in (step S3) by any suitable means.

The cavities 22 may be filled according to any one of the following methods contained herein are not limited to:

- by inkjet printing ( "jetting" in English) of the metallic alloy 4 accompanied by a brazing flux 5,

- by passage through a turbulent wave metal alloy 4 melt together with a brazing flux 5.

- by dipping or passing it through a metal alloy bath 4 reflow front screen and laying the CMS.

- the passage of the card into a bath of metallic alloy 4 after placement of CMS above the cavities 22 on at least one glue point when soldering wave. The use of a turbulent wave is advantageous in that the position of the copper layers 12 in the bottom of the cavities 22.

Alternatively, the cavities 22 may be filled by screen printing a solder paste 3 comprising the metal alloy 4 accompanied by a brazing flux 5, with or without display 30.

For example, the cavities 22 may be filled in using, in known manner, a screen 30 (or stencil). For this purpose, in a first step, a screen printing screen 30 in which have been formed two windows 34 is positioned on the insulating layer 20. The display 30 is positioned such that the windows 34 are located opposite the cavities 22 to complete.

Of course, the screen 30 may comprise a different number of windows 34 if a different number of recesses 22 must be filled in the insulating layer 20.

The dimensions of the windows 34 are substantially equal to the dimensions of the cavities 22 associated fill order to optimize the filling of the cavities 22. Preferably, the dimensional accuracy to achieve the windows 34 is of the order of thirty microns. The windows 34 may be slightly larger than the cavities 22 to ensure proper filling of the latter and increase the amount of solder paste 3 deposited (Figure 2).

In a second step illustrated in Figure 1d, the cream solder 3 is deposited on the screen 30 and then forced into the windows 34 and into the cavities 22 using a squeegee 32. In a known manner per se, the scraper 32 may comprise a metal sheet, which is inclined at an angle which may be between 45 ° and 60 ° to better drive the solder paste 3 in the cavities 22.

During a third step, the screen 30 may be removed from the mold, so as to allow the solder paste into the cavities 22 (the thickness of the solder paste 3 deposited being greater than the thickness E of the layer insulator 20, due to the presence of the screen).

Alternatively, the cavities 22 may be filled by screen printing without the use of screen 30. Indeed, the squeegee 32 can be applied directly on the insulating layer 20, which then serves as a screen 30 (see Figure 3). When the squeegee 32 reaches the recesses 22, it then forces the solder paste 3 in the cavities 22 similar to what is usually done with a screen 30. In order to avoid the presence of solder paste on top of the layer 20, it is preferable to use a squeegee polymer. The few remaining alloy beads may be washed after the step S5 of the heat treatment. After filling of the cavities 22, the insulating layer 20 is not, however, removed from the mold so that

This embodiment 30 without display and reduces the manufacturing costs of the electronic board 1 to the extent that it is no longer necessary to perform a screen 30 of screen printing and use of equipment requiring high accuracy (more screen positioning face to face with the circuit board 10). The filling step S4 of the cavities 22 is further facilitated since it is no longer necessary to accurately position a screen 30 on the insulating layer 20.

The dimensions of the cavities 22 are chosen so that each cavity 22 reveals at least part of the conductive layer 12 opposite. For example, each cavity 22 may be dimensioned to cover and extend beyond the conductive layer 12. The definition of the cavities depends on the dimensions of the SMD components to be assembled. For example for 0603 case size cavities 22 can be of the order of 0.5 mm * 1 mm; for case size 1206: 1 mm * 2 mm; for case size 2010: 1 .5 mm *4.5 mm. For such cases not for the width of the cavity 22 may be for example in the order of 0.3mm. The bottom surface of the cavity 22 is larger than the surface that reveals the conductive layer 12. This is however not restrictive, the cavities 22 may be dimensioned so as not to extend beyond the conductive layer 12 .

Over the insulating layer 20 is thicker, the depth p of the cavities 22 and the greater the thickness of solder paste 3 inserted into the cavities 22 may be important. Indeed, in the case where the cavities 22 are filled with solder paste 3 by screen printing with a screen, the window size of the display 30 screen printing is no longer limited by the ability to demold the screen 30 after filling cavities 22, since the insulating layer 20 remains on the connecting face 14 and is no longer removed from the mold. Therefore, the height of cream solder 3 inserted into the cavities 22 is equal to the sum of the depth p of the cavity and the thickness E2 of the screen 30, the thickness E2 of the screen 30 can be low when the thickness E of the insulating layer 20 is large.

Specifically, as we have seen, the size of a window 34 of a 30 screen printing screen is limited by the ratio of the area of ​​the window 34 (in the plane of the screen 30, which is parallel to the plane (X, Y)) and the surface of the inner walls of the window 34 (which extend perpendicularly to the plane of the screen 30), which must be greater than or equal to 0.66. Until now it was necessary to reduce the thickness E2 of the screen 30, which meant a decrease in the amount of solder paste 3 and thus the standoff CMS 2 is to increase the area of window 34, which prevented to implant CMS two fine pitch or limited CMS 2 implantable density on the printed circuit 10.

The application of the insulating layer 20 of the connection face 14 permits and so quite advantageous to lift this limitation since it becomes possible to drastically increase the height of cream solder 3 applied to the circuit 10 printed, without changing the thickness of the screen 30 of screen printing. It suffices to increase as much as necessary the thickness E of the insulating layer 20 and to achieve a window 34 having a surface adapted to the surface of the cavity 22 associated, its thickness being dictated by the above relation so it remains above 0.66.

Thus, the invention overcomes the difficulties release the screen 30 screen printing, when such a screen 30 is used and makes possible the implementation of CMS 2 fine pitch and / or a high density of CMS 2 on the circuit board 1. Typically, it may be envisaged to use a screen 30 having a thickness of about 50 m to 100 m with local thickness can be 100 m to 300 mm in case of need.

We of course understand that the removal of the screen 30 screen printing, made possible by the presence of the insulating layer 20, also allows to implement SMT fine pitch on the circuit board and / or increase the density on CMS the connection face 14, because this method of filling the cavities 22 does not require any demolding a screen 30.

WE CLAIMS

1. A method of attaching (S) of an electronic component (2) on a printed circuit (10), said printed circuit (10) comprises a connection face (14) having at least one conductive layer (12) and defining an axis Z , said Z axis being normal to the connecting face (14), the method of attachment (S) comprising the steps of:

- applying an insulating layer (20) (S1) comprising electrically insulating material on the connecting face (14) of the printed circuit (10), the insulating layer (20) having a minimum thickness (E) determined along the Z axis ,

- forming a cavity (22) in the insulating layer (20) over the conductive layer (12) (S2) so that at least a portion of the conductive layer (12) is disclosed, the cavity (22) having a depth (p) determined minimum along the Z axis,

- filling the cavity (22) with a metal alloy (4) together with a brazing flux (5), (S3)

- placing the electronic component (2) above the cavity (22) (S4), - applying a heat treatment (S5) to the printed circuit (10) on which is placed the component to transform the metal alloy (4 ) together with the brazing flux (5) brazed joint (6) so as to fix the component to the printed circuit (10),

the fixing method characterized in that the minimum thickness (E) of the insulating layer (20) is such that the depth (p) of the cavity (22) is at least equal to 100 m, and in that the electrically insulating material of the insulating layer (20) has a first coefficient of thermal expansion along the axis Z, the metallic alloy (4) has a second coefficient of thermal expansion along the Z axis, and wherein the first coefficient thermal expansion is greater than the second coefficient of thermal expansion.

2. Attachment Method (S) according to claim 1, wherein the steps of applying the insulating layer (20) and forming the cavity (22) are formed by photolithography on the surface.

3. fixing method (S) according to claim 1, wherein the cavity (22) is carried out using at least one of the following techniques:

- laser drilling of the insulating layer (20),

- mechanical cutting of the insulating layer (20),

- chemical cutting of the insulating layer (20).

4. A method of securing (S) according to claim 3, wherein the insulating layer (20) is attached and fixed on the connection face (14), and wherein the cavity (22) is formed by cutting or drilling before or after attaching the insulating layer (20) on the connecting face (14).

5. A method of manufacturing (S) according to one of claims 3 or 4, wherein the insulating layer (20) is formed by a printed circuit.

6. A method of manufacturing (S) according to one of claims 1 to 5, further comprising, prior to the cavity filling step

(22), a step (S6) of metallization of the conductive pad (12).

7. A method of attaching (S) according to one of claims 1 to 5, wherein the cavity (22) is filled by screen printing, with or without a screen (30) of screen printing.

8. fixing method (S) according to one of claims 1 to 7, wherein the cavity (22) has a surface in a plane normal to the Z axis, said cavity being filled by screen printing with a screen (30) screen printing, said screen (30) for screen printing having a window (34) having a surface in the plane normal to the Z axis, the surface of the window (34) being at least the surface of the cavity (22).

9. fastening method (S) according to claim 8, wherein the heat treatment (S5) comprises a reflow of the metal alloy (4).