Method for attaching a component to an electronic board, electronic board and repair method

Abstract

The present invention relates to a method for attaching (P) a component (3a) to an electronic board (1), comprising the following steps: - filling (E2) the window (8) of a first screen-printing screen with solder paste (9); - applying (E3) a heat treatment to the printed circuit board (2) in order to transform the solder paste (9) into a contact pad (11); - filling (E5) the window (13) of a second screen-printing screen with solder paste; - placing (E6) a component (3a) on the solder paste (14); - applying (E7) a heat treatment to the printed circuit board (2) so as to transform the solder paste (14) into a solder joint (6a).

Description

[0001] DESCRIPTION

[0002] TITLE: METHOD FOR ATTACHING A COMPONENT TO AN ELECTRONIC BOARD, ELECTRONIC BOARD AND REPAIR METHOD

[0003] TECHNICAL FIELD

[0004] This presentation concerns the field of electronic cards, in particular when these electronic cards are intended to be integrated into a portable and embedded device, for example in the field of aeronautics, space or even automobiles, and more precisely the fixing of electronic components to printed circuits.

[0005] STATE OF THE ART

[0006] In a manner known per se, an electronic card may comprise surface-mounted components (SMCs), i.e. electronic components soldered directly to the surface of the printed circuit of an electronic card, through-hole components, or even printed circuits.

[0007] Usually, SMDs are surface soldered either by reflow soldering or by solder wave.

[0008] In the case of reflow soldering, the bare PCB is first screen-printed by covering the conductive layers of the PCB (usually copper) with solder paste using a screen printing screen (or stencil) so that only the locations intended to receive the component terminations are covered by the solder paste. The solder paste comprises, in a manner known per se, a metal alloy suspended 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 soldering flux so as to form solder joints from the metal alloy present in the solder paste.

[0009] However, the resulting joints are subject to high stress. In particular, the part of the joints located between the component and the soldering area is very thin. The joints are therefore subject to very high stresses, particularly in shear, and therefore risk cracking quickly when the electronic board is subjected to harsh temperatures and / or vibrations. This therefore significantly reduces the lifespan of the electronic board.

[0010] Furthermore, in use, electronic cards can be subjected to harsh temperature environments creating relative displacements (in the plane of the surface) between the SMD and the printed circuit. These relative displacements are generally due to differential expansion between the SMDs and the printed circuit resulting from a temperature variation, the SMDs and the printed circuit generally having different coefficients of thermal expansion in the plane. However, these relative displacements have the effect of generating stresses which damage the solder joints, thus limiting their lifespan and reliability.

[0011] In order to reduce these constraints, document FR3069128 in the name of the Applicant proposes increasing the vertical height (usually referred to by the English term "standoff") between the upper face of the soldering pad (which can be made of copper, nickel-gold coated copper or any other suitable material) and the low point of the conductive terminations of the CMS once soldered, in order to increase the alloy height at the interface between the CMS and the soldering pads. To this end, this document proposes applying an insulating layer to the electronic board, creating a cavity there and placing the solder joint there. The solder joints thus obtained are then more flexible and therefore more robust and include a greater quantity of alloy. However, this process involves adding an additional layer to the electronic board, which can be difficult to achieve in certain electronic boards.

[0012] It has also been proposed in document WO 2019 / 120969, also in the name of the Applicant, to size the spacing between the soldering pads as a function of the spacing between the SMD terminations and the thermal expansion coefficients of the SMD, the printed circuit and the joints. This solution effectively reduces stresses in the joints and increases the lifetime of the components. However, in certain configurations, it can have an impact on the implantation density of the components. Its implementation can also be complex since it involves taking into account the thermal expansion coefficient of several different materials in the different packages.

[0013] It has also been proposed to strengthen the solder joint by increasing the thickness of the screen, by extending the solder pads during screen printing to increase the amount of alloy, or by adding an alloy preform. However, these methods are limited by the demolding ratio (which prevents its application to fine-pitch components), the proximity of other components, and / or the presence of vias.

[0014] Finally, when attaching electronic components by soldering, microbeads of solder paste can appear due to the spread of the solder paste under the component. These microbeads are, however, undesirable as they can disrupt the quality of connections or detach over time, thus causing failures. The aerospace industry remains in constant research to improve electronic boards and increase the reliability and lifespan of SMD solder joints.

[0015] STATEMENT OF THE INVENTION

[0016] An objective of the invention is therefore to propose an electronic card comprising a printed circuit and surface-mounted components whose lifespan, reliability and robustness are improved.

[0017] Another objective of the invention is to propose an electronic card in which the robustness of the joints of all or part of the components is improved, which can be selective, simple to produce and of moderate cost whatever the density of implantation of the components on the printed circuit and / or the type of component, without impacting the assembly yield of the electronic card or the routing of the components.

[0018] Yet another objective of the invention is to provide an electronic card in which the robustness of the joints can be selectively improved, without modifying the assembly method usually used.

[0019] For this purpose, according to a first aspect, a method of fixing a component on an electronic card is proposed, comprising the following steps:

[0020] - placing a first screen printing screen comprising a first window on a connection face of a printed circuit comprising a soldering pad so that the first window covers the soldering pad;

[0021] - filling the first window with a first soldering cream comprising a metal alloy suspended in a soldering flux;

[0022] - apply a first heat treatment to the printed circuit so as to transform the first soldering cream into a pad;

[0023] - placing a second screen printing screen comprising a second window on the connection face, so that the second window overlaps the pad;

[0024] - filling the second window with a second solder paste comprising a metal alloy suspended in a soldering flux;

[0025] - place a component on the second solder paste opposite the soldering pad;

[0026] - apply a second heat treatment to the printed circuit board so as to transform the second soldering cream into a solder joint.

[0027] The fastening method according to the first aspect is advantageous for reducing the Applicant's environmental footprint. Indeed, the method makes it possible to increase and optimize manufacturing, production and / or repair capacity and, consequently, to significantly reduce the associated greenhouse gas emissions. This optimization also makes it possible to reduce the consumption of raw materials. The method makes it possible to extend the service life of the components and, consequently, to reduce the number of replacements with new parts. The method also makes it possible to significantly reduce the number of discarded parts that may be difficult to recycle.

[0028] Some preferred but non-limiting features of the attachment method according to the first aspect are the following, taken individually or in combination:

[0029] - the first and second soldering cream are identical;

[0030] - a thickness of the second screen printing screen is at least equal to 75% of a thickness of the first screen printing screen;

[0031] - a thickness of the first and second screen printing screens are chosen from the following thicknesses: 60 pm, 80 pm, 100 pm, 120 pm, 150 pm, 200 pm;

[0032] - the first and second heat treatments include a remelting treatment;

[0033] - the first and second soldering creams comprise at least 50% metal alloy, the remainder being made up of the soldering flux;

[0034] - the fixing method further comprises a step of machining the stud when a thickness of the stud is greater than or equal to a thickness of the second window; and / or

[0035] - the second screen printing screen comprises at least one additional window and the printed circuit comprises at least one additional solder pad, the second screen printing screen being further positioned such that each additional window covers a corresponding additional solder pad, each additional window also being filled with the second solder paste during the filling step, and the attachment method further comprising placing an additional component on the second solder paste opposite a corresponding additional solder pad before applying the second heat treatment.

[0036] According to a second aspect, there is provided an electronic card obtained in accordance with a fixing method according to the first aspect and comprising:

[0037] - a printed circuit board comprising a soldering pad and an additional soldering pad;

[0038] - a component comprising a termination connected to the solder pad by a solder joint, the component being an electronic component; and

[0039] - an additional component comprising an additional termination connected to the additional solder pad by an additional solder joint, the additional component also being an electronic component; a volume of metal alloy in the solder joint being at least 20% greater than a volume of metal alloy in the additional solder joint.

[0040] According to a third aspect, there is provided a method of repairing an electronic board comprising the steps of: providing an electronic board; removing a used electronic component; providing a replacement electronic component; and attaching the replacement electronic component to the electronic board in accordance with the attachment method according to the first aspect.

[0041] DESCRIPTION OF FIGURES

[0042] Figure 1 schematically illustrates an example of a printed circuit comprising solder pads and the application of a first solder paste using a first screen printing screen on a portion of these solder pads in accordance with a step of an attachment method in accordance with an embodiment;

[0043] Figure 2 schematically illustrates the example printed circuit of Figure 1 after heat treatment of the first solder paste and the application of a second solder paste using a second screen printing screen on the solder pads of the printed circuit in accordance with a step of an attachment method according to one embodiment;

[0044] Figure 3 schematically illustrates the electronic card obtained after positioning electronic components and heat treatment of the printed circuit of Figure 2;

[0045] Figure 4 is a flowchart illustrating steps of a fastening method according to one embodiment;

[0046] Figure 5 is a flowchart illustrating steps of a repair method according to one embodiment; and

[0047] Figure 6 is a flowchart illustrating substeps for determining the thickness of screen printing screens.

[0048] DETAILED DESCRIPTION OF THE INVENTION An electronic card 1 comprises a printed circuit 2 comprising one or more conductive layers separated by one or more insulating layers, on which components 3a, 3b are fixed.

[0049] The component may be electronic or mechanical. By component 3a, 3b, we will understand in the following any element intended to be assembled with others in order to perform one or more functions (electronic, mechanical) and comprising one or more terminations 5 (electrical or mechanical) on the thickness of the component. By way of non-limiting example, an electronic component 3a, 3b may comprise a discrete electronic component 3a, 3b such as a surface-mounted component (SMD) (typically a resistor, a capacitor, a resistor network, a capacitor network, etc.). A mechanical component may comprise a washer, a cover, etc.

[0050] The printed circuit 2 comprises at least one electrically insulating layer having a connection face on which is fixed a conductive layer treated so as to form soldering pads 4. In general, the printed circuit 2 may be of the single-layer type (also called single-layer) and comprise only a single conductive layer, double-layer (also called double-sided) and comprise a conductive layer on either side of an insulating layer, or multi-layer and comprise at least four conductive layers. Each insulating layer may comprise at least one of the following materials: glass fibers, epoxy resin, polyimide, polyester, polymer, Teflon. For example, the insulating layer(s) comprise an epoxy resin and glass fibers. The conductive layers may be made of copper (or a copper-based alloy).

[0051] The terminations 5 of the electronic component 3a, 3b are connected to the soldering pads 4 of the printed circuit 2 via solder joints 6a, 6b.

[0052] In order to increase the lifetime of the electronic card 1, it is proposed to increase the resistant section of the solder joint 6a of all or part of the electronic components 3a subjected to high stresses by fixing the electronic components 3a by double selective screen printing. In this way, the electronic card 1 can comprise first electronic components 3a comprising terminations 5 whose solder joint 6a is more resistant and, where appropriate, second electronic components 3b comprising terminations 5 whose solder joint 6b can be conventional. The solder joint 6a being more resistant, it deforms less. Furthermore, the cracking time is increased, which improves the lifetime of the electronic card 1. Increasing the height of the solder joint 6a also makes it possible to reduce the shear angle for the same difference in expansion.The stress is therefore reduced in the joint 6a and its lifespan increased.

[0053] To do this, screen printing of the soldering pads 4 intended to receive the first electronic components 3a is carried out, followed by screen printing of the soldering pads 4 intended to receive the first and second components 3a, 3b.

[0054] More precisely, during a step E1, a first screen printing screen 7 is placed on the connection face 2a of the printed circuit 2. The first screen printing screen 7 comprises a first thickness e1 and at least one first through window 8. Preferably, the first screen 7 comprises as many through windows as the printed circuit 2 comprises soldering pads 4 intended to be connected to a termination 5 of one of the first components 3a.

[0055] The first screen 7 is positioned such that each first window 8 covers a corresponding soldering pad 4 (see FIG. 1). In one embodiment, each first window 8 is generally centered on the soldering pad 4. Furthermore, the surface of each first window 8 covers at least 50% of the surface of the soldering pad 4, preferably at least 75% of the surface of the soldering pad 4, typically between 95% and 120% of the surface of the soldering pad 4, preferably at least 100% of the surface of the soldering pad. For example, the surface area of ​​the first window 8 is substantially equal to the surface area of ​​the corresponding soldering pad 4 in order to optimize the quantity of soldering cream 9 applied to the soldering pad 4. In particular, when the surface area of ​​the first windows 8 is at least equal to the surface area of ​​the soldering pad 4, the wettability of the soldering cream on the soldering pads is optimized.This also improves the reproducibility of the height of the pad 11 obtained after reflow. Finally, for the same volume of solder paste 9 deposited, the thickness of the first screen 7 is reduced, which increases the demolding ratio and therefore reduces the variability of the deposits.

[0056] During a step E2, solder paste 9, comprising a metal alloy suspended in a soldering flux, is forced into each first window 8 to fill the first windows 8, for example using a scraper 10 (see figure 1).

[0057] The P fixing process allows the use of any type of solder paste 9, and in particular lead-free solder pastes, thus allowing compliance with current standards and in particular the European RoHS directive n°2002 / 95 / CE - banning of Lead, Hexavalent Chromium, Mercury, Cadmium, Polybrominated biphenyls and decabromodiphenyl ethers. For example, the metal alloy of solder paste 9 may comprise one of the following compositions, which are the most commonly used: tin / lead 63 / 37 or 10 / 90 or 90 / 10, tin / lead / Silver 62 / 36 / 2, for products exempted from the Rohs directive or tin / silver alloys 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 types of alloy (including bismuth, antimony, etc.). The soldering flux depends on the type of metal alloy suspended in the solder paste 9 and the assembly process with or without cleaning.

[0058] The soldering flux of the solder paste 9 generally comprises a resin (typically a natural, modified or synthetic resin), activating agents and additives to optimize screen printing and reflow. The role of the soldering flux is to ensure the stripping of the solder pads 4 (using the activators contained in the soldering flux), to ensure their protection during the temperature increase steps during the heat treatment and to play a surfactant role to promote the wetting of the metal alloy. For example, the soldering flux may comprise rosin.

[0059] Solder paste 9 comprises, for example, 50% by volume of metal alloy and 50% by volume of soldering flux.

[0060] In a manner known per se, the scraper 10 may comprise a metal sheet which is applied to the first screen 7 at an angle which may be between 45° and 60° in order to better push the soldering cream 9 into the first windows 8.

[0061] Once the first windows 8 are filled with solder paste 9, the first screen 7 can be removed.

[0062] During a step E3, a first heat treatment is applied to the printed circuit 2 on which the solder paste 9 has been applied so as to transform the solder paste 9 into pads 11 (see figure 2).

[0063] The heat treatment may in particular comprise reflow soldering of the metal alloy present in the solder paste 9. In a manner known per se, a reflow heat treatment successively comprises a temperature ramp-up step, a so-called preheat step for drying the fluxes (soak or preflow) and preparing for cleaning the solder terminations 5 of the printed circuit 2 and components, a so-called reflow step where the metal alloy melts and a rapid cooling step to room temperature. Reference may be made to document WO2019 / 012136 in the name of the Applicant for further details on the reflow heat treatment.

[0064] It should be noted that the brazing flux gradually evaporates during the temperature rise (and preheating / drying) step, thus leaving only the metal alloy on the brazing pads 4. The pads 11 obtained therefore only comprise the metal alloy.

[0065] During a step E4, a second screen printing screen 12 is placed on the connection face 2a of the printed circuit 2. The second screen printing screen 12 comprises a second thickness e2 and at least one second through window 13. Preferably, the second screen 12 comprises as many second through windows 13 as the printed circuit 2 comprises soldering pads 4 intended to be connected to a termination 5 of the first and second electronic components 3a, 3b. When the electronic card

[0066] I to be manufactured comprises first and second electronic components 3a, 3b, the second screen 12 therefore comprises more windows 13 than the first screen 7.

[0067] The second screen 12 is placed so that each second window 13 covers a corresponding soldering pad 4 (see figure 2). The placement of the second screen 12 on the connection face 2a and the dimensioning of the second windows 13 relative to the soldering pads 4 comply with the same criteria as the placement of the first screen 7 and the dimensioning of the first windows 8. Preferably, the surface area of ​​the second windows 13 is at least equal to the surface area of ​​the soldering pad 4 in order to completely cover the pad 11 in order to be able to press the second screen 12 onto the printed circuit 2. Raising the second screen 12 is in fact likely to cause the alloy balls of the soldering paste to pass under the screen and promote the occurrence of short circuits or the formation of microbeads.

[0068] Preferably, the second thickness e2 is greater than or equal to the thickness of the pads 11 obtained in step E3 in order to ensure that the electronic component 3a intended to be placed on the pad 11 remains in place and does not crush the solder paste 14. Advantageously, the soldering flux of the solder paste 14, in addition to its role during the heat treatment, makes it possible to keep the electronic component 3 in position and to clean the termination to ensure soldering during preheating. It is therefore preferable for the electronic component 3 to be in contact with the solder cream 14 and not only with the pad 11.

[0069] For this purpose, either the second screen 12 is chosen so that the second thickness e2 is greater than the thickness of the pad 11 obtained in step E3. Preferably, e2 is greater than or equal to the sum of the thickness of the pad 11 and the diameter of an alloy ball of maximum size.

[0070] When the second thickness e2 of the second screen 12 is too small compared to the thickness of the pad 11, the method P can further comprise a step of machining the pad

[0071] II (E8) in order to reduce its thickness until it becomes less than the second thickness e2, preferably less than or equal to the difference between the thickness e2 of the second screen and the diameter of an alloy ball. The machining can in particular be carried out by leveling. It will be noted that this embodiment makes it possible to increase the volume of metal alloy in the pad 11 obtained in step E3 (since it is possible to produce a higher pad 11 during the first screen printing (steps E2 to E5) which will therefore fill the second window 13 more) and therefore the volume of metal alloy in the consolidated solder joint 6a (see step E6 described below).

[0072] Thus, certain second windows 13 of the second screen 12 are placed on the printed circuit 2 so as to cover the pads 11 obtained in step E3 while the other second windows 13 are placed on soldering pads 4 without pads 11 (these soldering pads 4 having been covered by a portion of the first screen 7 without windows 8 in step E2 so as not to receive soldering cream 14 - see figure 1).

[0073] During a step E5, solder paste 14, comprising a metal alloy suspended in a soldering flux, is forced into each second window 13 to fill the second windows 13, for example using a scraper 10 (see figure 2).

[0074] The solder paste 14 applied in step E5 may be identical to the solder cream 9 applied in step E2. This configuration ensures that the second solder cream 14 melts before the pad 11, while ensuring optimal mixing of the metal alloy forming the pad 11 with that of the solder cream 14 and therefore better adhesion. Alternatively, the solder creams 9, 14 may be different.

[0075] Once the second windows 13 are filled with solder paste 14, the second screen 12 can be removed. The printed circuit 2 then comprises:

[0076] - first piles 15a of solder paste 14 positioned on the pads 11 obtained in step E3; and

[0077] - second piles 15b of solder cream 14 positioned on soldering pads 4 without a pad 11.

[0078] When the second thickness e2 of the second screen 12 is constant, the thickness of the assemblies formed by the pads 11 and the associated first piles 14a and the thickness of the second piles 15b of solder paste 14 is identical.

[0079] During a step E6, first and second electronic components 3a, 3b are placed on the printed circuit 2 so that each termination 5 of the electronic components 3a, 3b is located on a corresponding pile 15a, 15b of solder paste 14. More precisely, the terminations 5 of the first electronic components 3a are placed on the first piles 15a of solder paste 14 and the terminations 5 of the second electronic components 3b are placed on the second piles 15b of solder paste 14. When the second thickness e2 of the second screen 12 is greater than or equal to the thickness of the pad (and in particular equal to the sum of the thickness of the pad 11 and the diameter of the metal alloy balls in the solder paste 14), the first electronic components 3a are supported by the pads 11, which avoids crushing (and therefore spreading) the solder paste 14.

[0080] During a step E7, a second heat treatment is applied to the printed circuit 2 on which the solder paste 14 has been applied so as to transform the solder cream 14 into solder joints 6a, 6b. The heat treatment may be identical to the heat treatment described in step E3. Where appropriate, the temperatures, slopes and durations of the different steps of the heat treatment E3 are adapted to the composition of the solder cream 14 used in step E5.

[0081] The electronic card 1 thus obtained comprises both consolidated solder joints 6a, formed by the pads 11 obtained in step E3 on which additional solder joints have been made in step E7, and conventional solder joints 6b obtained in step E7 from the second piles 15b of solder paste 14 (see figure 3). The consolidated solder joints 6a therefore comprise a larger volume of alloy than the conventional solder joints 6b, which improves their resistant section.Indeed, steps E2 and E3 make it possible to partially fill the second windows 13 of the second screen 12 with a pad 11 comprising only metal alloy, then steps E5 and E7 complete the remaining space in the second windows 13 with solder paste 14, which makes it possible to preserve the initial functions of the soldering flux (cleaning of the parts to be assembled and holding the electronic component 3a during installation and heat treatment) while increasing the final volume of metal alloy in the consolidated solder joint 6a. It will also be noted that, during the heat treatment of step E7, when the reflow temperature of the metal alloy of the solder paste 9 is less than or equal to the reflow temperature of the metal alloy of the solder cream 14, the pad 11 fuses with the metal alloy in the solder cream 14 to form a single solder joint 6a.On the other hand, when the reflow temperature of the metal alloy of the solder paste 9 is higher than the reflow temperature of the metal alloy of the solder paste 14 (which may be the case when the solder pastes 9, 14 are different), the pad 11 does not fuse with the metal alloy in the solder paste 14.

[0082] It will be noted that, since the consolidated solder joints 6a comprise a larger volume of metal alloy, their strength is increased, which makes it possible to improve the service life of the electronic card 1. Their visual inspection is further facilitated in comparison with the inspection of conventional solder joints 6b which are of smaller volume.

[0083] The first thickness e1 of the first screen 7 and the second thickness e2 of the second screen 12 are chosen as a function of the volume of metal alloy sought for the corresponding electronic component 3a, 3b.

[0084] The Applicant has noticed that the presence of the pads 11 on the electronic card during step E7 makes it possible to reduce, or even eliminate, the formation of microbeads. Indeed, these microbeads generally result from the spreading of solder paste under the electronic component during the reflow step. However, when the electronic card comprises pads 11, the solder paste 14 is attracted to the pads 11 thanks to the space left between the component and the printed circuit 2 during reflow and therefore no longer risks forming microbeads. The choice of a second screen 12 having a thickness e2 slightly greater (the size of an alloy ball) than the thickness of the pad

[0085] 11 further makes it possible to avoid crushing, and therefore spreading, the solder paste 14 and, consequently, further reduces the risks of microbead formation. Finally, the reduction in the formation of microbeads is also made possible by the application of the second screen 12 against the surface of the printed circuit 2, thus reducing the risks of spreading the solder paste 14 under the electronic component.

[0086] The thickness e1, e2 of the first and second screens 7, 12 can be determined in order to optimize the volume of metal alloy in the consolidated brazing joints 6a obtained.

[0087] For this, during a first sub-step (S1), the thickness e2 of the second screen

[0088] 12 is determined. This determination is carried out in order, on the one hand, to obtain an optimal (i.e. maximum) volume of metal alloy, while taking into account the specificities of the electronic card. The greater the thickness e2 of the second screen, the greater the volume of alloy deposited can be. However, for certain components (particularly with fine pitch), it is not possible to use a screen of excessive thickness due to the demolding ratio. In particular, the ratio between the surface area of ​​the window and the surface area of ​​the internal walls of the window must be greater than or equal to 0.66 to allow demolding. Thus, the thickness e2 of the second screen is chosen to be as high as possible, in order to optimize the quantity of solder paste 14 deposited in step E5, while ensuring compliance with the demolding ratio for all the electronic components of the electronic card.

[0089] During a second sub-step (S3), the thickness e1 of the first screen 7 is determined. For this, a thickness of pad 11 to be obtained is first estimated (S2). This is referred to as an estimation, since the thickness of the pad 11 obtained will vary as a function of the surface area of ​​the soldering pad on which the pad 11 is formed, but also of the surface area of ​​the windows of the screens 7, 12. In an exemplary embodiment, the thickness of the pads 11 obtained in step E3 is preferably slightly less than the thickness e2 of the second screen 12 in order to be able to place the electronic component 3a on the solder paste 14 covering the top of the pad 11 during step E6. The thickness of the pad 11 can for example be equal to the difference between the thickness e2 and the diameter of the balls of the metal alloy of the solder paste 14. Since the balls can have slightly different diameters, the diameter chosen to estimate the thickness of the pad 11 corresponds to the maximum diameter of the balls.For example, the thickness of the pad 11 can be equal to the difference between the thickness e2 of the second screen 12 and 35 pm.

[0090] The thickness e1 of the first screen 7 is then chosen so as to be substantially equal (to within 2%) to 90% of the thickness of the pad 11 thus estimated. Indeed, the Applicant noticed that, when the thickness e1 of the first screen 7 was substantially equal to 90% of the thickness of the pad 11 thus estimated and the surface area of ​​the window 8 was at least equal to the surface area of ​​the soldering pad, the thickness of the pad 11 actually obtained following step E3 was necessarily slightly less than the thickness e2 of the second screen 12, even for the soldering pads of small surface area (which are the limiting factor here given their small surface area). The choice of a thickness e1 equal to 90% of the estimated thickness of the pad 11, combined with a window surface 8 at least equal to the surface of the soldering area, thus makes it possible to guarantee that, whatever the soldering area, the thickness e2 of the second screen is greater than the thickness of the pad 11 obtained.The pads 11 are therefore high enough to support the electronic components 3a, even when the soldering area is small. Furthermore, since the thickness e1 of the first screen 7 is determined from the optimal thickness e2 of the second screen 12, the volume of metal alloy in the consolidated solder joint 6a is optimized.

[0091] For example, the first and second thicknesses e1, e2 may be chosen from the following thicknesses: 50 pm, 60 pm, 80 pm, 100 pm, 120 pm, 150 pm, 200 pm. It will be noted that the first thickness e1 and the second thickness e2 may be different.

[0092] When the first thickness e1 and the second thickness e2 are equal and constant over the entire surface of the screens 7, 12 and the surface of the first and second windows 8, 13 are equal, the volume of metal alloy in the consolidated solder joints 6a is equal to 150% of the volume of metal alloy in the conventional solder joints 6b.

[0093] Furthermore, the volume of alloy in the consolidated solder joints 6a obtained in accordance with the attachment method P with a first and a second screen 7, 12 of constant thickness e1, e2 equal to 100 μm is equal to 125% of the volume of metal alloy obtained with a conventional attachment method P (a single screen printing step) with a screen of thickness equal to 120 μm. Similarly, the volume of alloy in the consolidated solder joints 6a obtained in accordance with the attachment method P with a first and a second screen 7, 12 of constant thickness e1, e2 equal to 100 μm is equal to the volume of metal alloy obtained with a conventional attachment method P (a single screen printing step) with a screen of thickness equal to 150 μm.

[0094] The choice of the type of solder joint (consolidated 6a by double screen printing in accordance with steps E2 to E7 or conventional 6b in accordance with steps E5 to E7) and the volume of solder paste 9, 1 deposited is made according to the requirements in terms of quantity of metal alloy of the electronic components 3a, 3b:

[0095] - an electronic component 3b not requiring increased resistance (low mechanical stress in use) may be fixed on a soldering pad 4 by simple screen printing in accordance with steps E5 to E7 (only);

[0096] - an electronic component 3a requiring average resistance (average mechanical stress in use), such as a BGÀ, may be fixed on a soldering pad 4 in accordance with steps E2 to E7 (or even E8), with a first screen printing screen 7 having an average opening (for example a first thickness e1 of the order of 100 pm or 120 pm and a surface area of ​​the order of 75% of the surface area of ​​the soldering pad 4) and a second screen 12 having a small opening (for example a second thickness e2 of the order of 80 pm or 100 pm and a surface area of ​​the order of 100% of the surface area of ​​the soldering pad 4); and

[0097] - an electronic component 3a requiring significant mechanical strength (high mechanical or thermomechanical stress in use), such as a resistor R1206, may be fixed on a soldering pad 4 in accordance with steps E2 to E7 (or even E8) with a first screen printing screen 7 having a thick opening (for example a first thickness e1 of the order of 150 pm or 200 pm and a surface area of ​​the order of 95% or 100% of the surface area of ​​the soldering pad 4) and a second screen 12 having a finer opening (for example a second thickness e2 of the order of 100 pm and a surface area of ​​the order of 95% or 100% of the surface area of ​​the soldering pad 4). It will be noted that the use of an opening having a second, smaller thickness e2 in steps E4 and E5 limits the spreading of the solder paste 14 relative to the pad 11 obtained in step E3.

[0098] The P fixing process can be applied to any electronic component 3a, 3b (capacitor, resistor, BGÀ (Ball Grid Array), LGA (Land Grid Array), QFN or DFN (Quad Flat No-leads or Dual Fiat No-leads, for flat integrated circuit package without pin), etc.), whatever the pitch of the terminations 5 of these electronic components 3a, 3b (0.5 mm, etc.) and without reducing the implantation density of the electronic board 1: the position of the soldering pads 4 is in fact not impacted by the P fixing process.

[0099] Furthermore, the volume of metal alloy in the consolidated solder joint 6a is no longer limited by the size of the window 8, 13 of the screen printing screens 7, 12. It is recalled that the size of a window of a screen printing screen is limited by the ratio between the surface area of ​​the window and the surface area of ​​the internal walls of the window, which must be greater than or equal to 0.66 to allow the solder paste to be demolded. Until now, to bring electronic components closer together on a printed circuit, it was either necessary to reduce the thickness of the screen, which involved a reduction in the volume of solder paste, or to increase the surface area of ​​the window, which prevented the installation of fine-pitch components.The fixing P of the electronic components 3a, 3b by double selective screen printing in accordance with steps E1 to E7 (and where appropriate E8) therefore makes it possible to overcome this difficulty by increasing the volume of metal alloy in the consolidated solder joint 6a while making it possible to reduce the surface area of ​​the second windows 13 of the second screen 12 and therefore to implant components 3a with fine pitches and / or to increase the density of electronic components 3a, 3b on the printed circuit 2 without modifying the routing of the solder pads 4.

[0100] The fixing method P can further be implemented in the repair R of an electronic card 1 (see figure 5). In particular, an electronic card 1 can be repaired in accordance with the following steps:

[0101] - remove a used electronic component S2 from the electronic card 1;

[0102] - provide a replacement electronic component 3a S3; and

[0103] - fix the replacement electronic component 3a on the electronic card 1 in accordance with the fixing method (P) described above.

[0104] In particular, when the electronic component 3a must be fixed using a consolidated solder joint 6a, i.e. of larger volume, the replacement electronic component 3a can be soldered onto the electronic card 1 by successively applying the steps E1 to E7 (and where appropriate E8) described above.

[0105] Where appropriate, the repair method R further comprises an operation of preparing the areas after step S2, before the step of fixing the electronic component in accordance with the fixing method P, in order to standardize the quantity of alloy and to clean the remains of flux used in the previous methods.

Claims

CLAIMS 1. Method for fixing (P) a component (3a) on an electronic card (1) comprising the following steps: - placing (E1) a first screen printing screen (7) comprising a first window (8) on a connection face (2a) of a printed circuit (2) comprising a soldering area (4) so ​​that the first window covers the soldering area (4), a surface of the first window (8) being at least equal to a surface of the soldering area (4); - filling (E2) the first window (8) with a first soldering cream (9) comprising a metal alloy suspended in a soldering flux; - applying (E3) a first heat treatment to the printed circuit (2) so as to transform the first soldering cream (9) into a pad (11); - placing (E4) a second screen printing screen (12) comprising a second window (13) in contact with the connection face (2a), so that the second window (13) overlaps the pad (11), a surface of the second window (13) being at least equal to a surface of the soldering area (4); - filling (E5) the second window (13) with a second soldering cream (14) comprising a metal alloy suspended in a soldering flux; - placing (E6) a component (3a) on the second soldering cream (14) opposite the soldering pad (4); - apply (E7) a second heat treatment to the printed circuit (2) so as to transform the second soldering cream (14) into a solder joint (6a).

2. Fixing method (P) according to claim 1, in which the first and second soldering creams (9, 14) are identical.

3. Fixing method (P) according to one of claims 1 and 2, in which a thickness (e2) of the second screen printing screen (12) is greater than a thickness of the pad (11).

4. Fixing method (P) according to one of claims 1 to 3, further comprising the following sub-steps: - determine a thickness (e2) of the second screen printing screen (12) as a function of parameters of the electronic card; - determining a maximum diameter of metal alloy balls present in the second solder paste (14); and - deducing therefrom a thickness (e1) of the first screen printing screen (7), the first thickness (e1) being equal to 90% of a difference between the thickness (e2) of the second screen printing screen (12) and the maximum diameter thus determined.

5. Fixing method (P) according to one of claims 1 to 4, in which a thickness (e2) of the second screen printing screen (12) is greater than a thickness of the pad (11), so that the component (3a) is placed (E6) on the pad (11) when it is placed on the second solder paste (1).

6. Fixing method (P) according to claim 5, in which the thickness (e2) of the second screen printing screen (12) is substantially equal to a sum of the thickness of the pad (11) and a maximum diameter of metal alloy balls present in the second solder paste (14).

7. Fixing method (P) according to claim 5, in which the thickness (e2) of the second screen printing screen (12) is substantially equal to a sum of the thickness of the pad (11) and 35 micrometers.

8. Fixing method (P) according to one of claims 1 to 7, in which a thickness (e2) of the second screen printing screen (12) is at least equal to 75% of a thickness (e1) of the first screen printing screen (7).

9. Fixing method (P) according to one of claims 1 to 8, in which a thickness (e1, e2) of the first and second screen printing screens (12) are chosen from the following thicknesses: 50 pm, 60 pm, 80 pm, 100 pm, 120 pm, 150 pm, 200 pm.

10. Fixing method (P) according to one of claims 1 to 9, in which the first and second heat treatment comprise a reflow treatment.

11. Fixing method (P) according to one of claims 1 to 10, in which the first and second soldering cream (9, 14) comprise at least 50% of metal alloy, the remainder being constituted by the soldering flux.

12. Fixing method (P) according to one of claims 1 to 11, further comprising a step of machining (E8) the pad (11) when a thickness of the pad (11) is greater than a thickness (e2) of the second screen printing screen (12), preferably when the thickness of the pad (11) is greater than a difference between the thickness (e2) of the second screen printing screen (12) and a maximum diameter of metal alloy balls present in the second solder paste (14) or greater than a difference between the thickness (e2) of the second screen printing screen (12) and 35 micrometers.

13. Fixing method (P) according to one of claims 1 to 12, in which the second screen printing screen (12) comprises at least one additional window (13) and the printed circuit (2) comprises at least one additional soldering pad (4), the second screen printing screen (12) being further placed (E4) so ​​that each additional window (13) covers an additional soldering pad (4) corresponding, each additional window (13) also being filled with the second solder paste (14) during the filling step (E5), and the fixing method (P) further comprising placing an additional component (3b) on the second solder paste (14) opposite a corresponding additional solder pad (4) before the application (E7) of the second heat treatment.

14. Electronic card (1) obtained in accordance with a fixing method (P) according to claim 13 and comprising: - a printed circuit (2) comprising a soldering pad (4) and an additional soldering pad (4); - a component (3a) comprising a termination (5) connected to the soldering pad (4) by a solder joint (6a), the component being an electronic component; and - an additional component (3b) comprising an additional termination (5) connected to the additional solder pad (4) by an additional solder joint (6b), the additional component (3b) also being an electronic component; a volume of metal alloy in the solder joint (6a) being at least 20% greater than a volume of metal alloy in the additional solder joint (6b).

15. Method for repairing (R) an electronic card (1) comprising the following steps: - provide an electronic card (1) (R1); - remove a used electronic component (R2); - provide a replacement electronic component (3a) (R3); and - fixing the replacement electronic component (3a) on the electronic card (1) in accordance with the fixing method (P) according to one of claims 1 to 12.

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