Method for attaching an element 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), the method comprising the following steps: - filling (E2) the windows (8) of a first screen printing mesh with solder paste (9); - applying (E3) heat treatment to the printed circuit board (2) to convert the solder paste (9) into contact pads (11); - filling (E5) the windows (13) of a second screen printing mesh with solder paste; - placing (E6) the component (3a) on the solder paste (14); - applying (E7) heat treatment to the printed circuit board (2) to convert the solder paste (14) into solder joints (6a).

Description

Technical Field

[0001] This review covers the field of electronic boards, particularly in the aerospace or automotive sectors, where these electronic boards are intended for integration into portable and built-in devices, and more specifically, the attachment of electronic components to printed circuit boards. Background Technology

[0002] In a manner known per se, an electronic board may include surface mount components (SMCs), that is, electronic components, through-hole components, or printed circuit boards that are directly soldered onto the surface of the printed circuitry of the electronic board.

[0003] SMC is typically surface-mounted using reflow soldering or wave soldering.

[0004] In reflow soldering, the bare printed circuit board is first screen-printed using a screen (or stencil) to cover the conductive layer of the printed circuit (usually made of copper) with solder paste, so that the solder paste only covers the locations intended to receive the components. The solder paste comprises a metallic alloy suspended in flux, in a manner known per se. The terminals of the component (SMC) are then attached to the solder paste, followed by a thermal reflow soldering process. During thermal reflow, the heat remelts the alloy and evaporates the flux, thus forming solder joints based on the metallic alloy present in the solder paste.

[0005] However, the resulting solder joints are subjected to high stress. In particular, the portion of the solder joint located between the component and the solder pad is very thin. Therefore, the solder joint is exposed to very high stress, especially shear stress, and thus there is a risk of rapid cracking when the electronic board is exposed to harsh environments in terms of temperature and / or vibration. This significantly shortens the lifespan of the electronic board.

[0006] Furthermore, during use, the electronic board can be exposed to harsh temperature environments, resulting in relative displacement between the SMC and the printed circuit board (in the surface plane). These relative displacements are typically caused by differential expansion between the SMC and the printed circuit board due to temperature changes, as the SMC and the printed circuit board usually have different coefficients of thermal expansion in the plane. The effect of these relative displacements is to generate stresses that damage the solder joints, thereby limiting the lifespan and reliability of the solder joints.

[0007] To reduce these stresses, document FR3069128, submitted on behalf of the applicant, stipulates that the vertical height (commonly referred to as the "space distance") between the upper surface of the solder pad (which may be made of copper coated with nickel-gold or any other suitable material) and the lowest point of the conductive terminal of the SMC once soldered should be increased to increase the alloy height at the interface between the SMC and the solder pad. To this end, the document stipulates that an insulating layer is applied to the electronic card, creating cavities in the insulating layer, and the solder joints are placed within these cavities. The resulting solder joints are thus more flexible, therefore more robust, and contain a greater amount of alloy. However, this method involves adding an additional layer to the electronic board, which may be difficult to manufacture on some electronic boards.

[0008] Furthermore, the document WO 2019 / 120969 submitted on behalf of the applicant stipulates the following: the spacing between solder joints is determined based on the distance between the terminals of the SMC and the coefficients of thermal expansion of the SMC, the printed circuit board, and the solder joints. This solution effectively reduces stress in the solder joints and increases component lifespan. However, in some configurations, this solution may affect the component mounting density. Moreover, the implementation of this solution may prove complex because it involves considering the coefficients of thermal expansion of several different materials in different cells.

[0009] The following provisions are also made: solder joints can be strengthened by increasing the thickness of the screen, allowing filler material to overflow during screen printing to increase the amount of alloy, or adding alloy preforms. However, these methods are limited by area ratio (which prevents the methods from being applied to components with finer pitch), proximity to other components, and / or the presence of vias.

[0010] Finally, during the soldering process of attaching electronic components, solder paste microbeads may form as the paste spreads beneath the components. However, the presence of these microbeads is undesirable because they can compromise the quality of the connection or, over time, separate, leading to malfunctions.

[0011] The aerospace industry continues to seek improvements to electronic boards and enhance the reliability and lifespan of SMC solder joints. Summary of the Invention

[0012] Therefore, one object of the present invention is to provide an electronic board comprising a printed circuit board and surface mount elements having improved lifespan, reliability and strength.

[0013] Another object of the present invention is to provide an electronic board in which the robustness of the solder joints of all or some of the components is improved, and the electronic board can be selective, easy to manufacture and cost-effective, regardless of the mounting density of the components on the printed circuit board and / or the type of components, without affecting the assembly yield of the electronic board or the wiring of the components.

[0014] Another object of the present invention is to provide an electronic board in which the robustness of solder joints can be selectively improved without changing the commonly used assembly method.

[0015] Therefore, according to a first aspect, a method for attaching components to an electronic board is provided, the method comprising the following steps: - Place a first screen, including a first window, on the connection side of the printed circuit board including the solder area, such that the first window covers the solder area; - Use a first solder paste to fill the first window, the first solder paste comprising a metal alloy suspended in flux; - A first heat treatment is applied to the printed circuit board to convert the first solder paste into pads; - Place the second mesh, including the second window, on the connection side, such that the second window spans the pad; - Use a second solder paste to fill the second window, the second solder paste comprising a metal alloy suspended in flux; - Place the component on the second solder paste, with the component facing the solder pad; - A second heat treatment is applied to the printed circuit board to convert the second solder paste into solder joints.

[0016] The attachment method according to the first aspect is beneficial for reducing the applicant's environmental footprint. Specifically, the method enables increased manufacturing, production, and / or repair capabilities, as well as optimization of these capabilities, and thus a significant reduction in associated greenhouse gas emissions. This optimization also enables a reduction in raw material consumption. The method enables an extension of component lifespan, and thus a reduction in the number of replacements by new parts. Furthermore, the method enables a significant reduction in the amount of waste parts that may be difficult to recycle.

[0017] Certain preferred, but non-limiting, features of the attachment method according to the first aspect are as follows, and such features may be used individually or in combination: - The first and second solder pastes are the same; - The thickness of the second mesh is at least 75% equal to the thickness of the first mesh; - The thickness of the first and second meshes is selected from the following thicknesses: 60μm, 80μm, 100μm, 120μm, 150μm, 200μm; - The first and second heat treatments include reflow soldering. - The first solder paste and the second solder paste comprise at least 50% metal alloy, and the remainder of the first solder paste and the second solder paste comprises flux; - The attachment method further includes the following steps: processing the pads when the thickness of the pads is greater than or equal to the thickness of the second window; and / or - The second screen includes at least one additional window, and the printed circuit board includes at least one additional solder pad. The second screen is further positioned such that each additional window covers the corresponding additional solder pad. During the filling step, a second solder paste is also used to fill each additional window. The attachment method further includes placing an additional component on the second solder paste before applying a second heat treatment, such that the additional component faces the corresponding additional solder pad.

[0018] According to a second aspect, an electronic board is provided, which is obtained using the attachment method according to the first aspect, the electronic board comprising: - Printed circuit board, which includes solder pads and additional solder pads; - Components, including terminals connected to solder joints via solder pads, are electronic components; and - Additional components, including additional terminals connected to additional solder areas via additional solder joints, are also electronic components. The volume of the metal alloy in the solder joint is at least 20% larger than the volume of the metal alloy in the additional solder joint.

[0019] According to a third aspect, a method for repairing an electronic board is provided, the method comprising the following steps: - Provide electronic boards; - Remove worn-out electronic components; - Provide alternative electronic components; and - Using the attachment method according to the first aspect, the alternative electronic components are attached to the electronic board. Attached Figure Description

[0020] Figure 1 An example of a printed circuit board according to an embodiment is illustrated. The printed circuit board includes solder pads, and according to the steps of the attachment method, a first solder paste is applied to a portion of these solder pads using a first screen. Figure 2 The illustration schematically shows an embodiment. Figure 1 An example of a printed circuit board after heat treatment of the first solder paste according to the steps of the attachment method, and after applying the second solder paste to the solder areas of the printed circuit board using a second screen. Figure 3 The illustration schematically shows the positioning of electronic components and Figure 2 Electronic boards obtained after heat treatment of printed circuit boards; Figure 4 This is a flowchart illustrating the steps of the attachment method according to an embodiment; Figure 5 This is a flowchart illustrating the steps of a repair method according to an embodiment; and Figure 6 This is a flowchart illustrating the sub-steps used to determine the thickness of the wire mesh. Detailed Implementation

[0021] Electronic board 1 includes printed circuit board 2, which includes one or more conductive layers separated by one or more insulating layers, and components 3a and 3b are attached to these conductive layers.

[0022] Components can be electronic or mechanical. Throughout this text, the terms "component" 3a, 3b should be understood to mean any component intended to be assembled with other components to achieve one or more electronic functions (electronic, mechanical) and including one or more terminals 5 (electrical or mechanical) across its thickness. As a non-limiting example, electronic components 3a, 3b may include discrete electronic components 3a, 3b, such as surface mount components (SMCs) (typically resistors, capacitors, resistor networks, capacitor networks, etc.). Mechanical components may include gaskets, covers, etc.

[0023] The printed circuit board 2 includes at least one electrically insulating layer having a connection side to which a conductive layer is attached, the connection side being processed to form a solder pad 4. Typically, the printed circuit board 2 can be a single-layer type (also referred to as single-layer) including only a single conductive layer, can be a double-layer type (also referred to as double-sided) including conductive layers on both sides of the insulating layer, or can be multi-layered and including at least four conductive layers. Each insulating layer may include at least one of the following materials: glass fiber, epoxy resin, polyimide, polyester, polymer, and Teflon. For example, the insulating layer includes epoxy resin and glass fiber. Meanwhile, the conductive layer may be made of copper (or a copper-based alloy).

[0024] The terminals 5 of electronic components 3a and 3b are connected to the solder area 4 of printed circuit board 2 via solder points 6a and 6b.

[0025] To increase the lifespan of the electronic board 1, the following provision is made: electronic components 3a are attached by selective double screen printing, thereby increasing the effective cross-section of the solder joints 6a of all or part of the electronic components exposed to high stress. In this way, the electronic board 1 may include a first electronic component 3a, which includes a terminal 5 used with the stronger solder joint 6a, and, where applicable, the electronic board 1 may include a second electronic component 3b, which includes a terminal 5 used with a solder joint 6b, which may be a conventional solder joint. Because the solder joint 6a is stronger, it deforms less. Furthermore, the time to cracking is increased, which improves the lifespan of the electronic board 1. In addition, the increased height of the solder joint 6a allows for a reduction in the shear angle for the same expansion difference. Therefore, the stress in the solder joint 6a is reduced and the lifespan of the solder joint 6a is increased.

[0026] For this purpose, screen printing is performed on the solder area 4 intended to receive the first electronic component 3a, and then screen printing is performed on the solder area 4 intended to receive the first component 3a and the second component 3b.

[0027] More specifically, during step E1, a first screen 7 is placed on the connection side 2a of the printed circuit board 2. The first screen 7 includes a first thickness e1 and includes at least a first through-hole 8. Preferably, the first screen 7 includes as many through-holes as the solder pads 4 included in the printed circuit board 2, the solder pads 4 being designed to connect to a terminal 5 of one of the first elements 3a.

[0028] The first wire mesh 7 is positioned such that each first window 8 covers the corresponding solder area 4 (see...). Figure 1 In one embodiment, each first window 8 is globally centered on the solder pad 4. Furthermore, the surface of each first window 8 covers at least 50% of the surface of the solder pad 4, preferably at least 75%, typically between 95% and 120%, and preferably at least 100%. For example, the surface of the first window 8 is substantially equal to the surface of the corresponding solder pad 4 to optimize the amount of solder paste 9 applied to the solder pad 4. In particular, when the surface of the first window is at least equal to the surface of the solder pad 4, the wettability of the solder paste on the solder pad is optimized. Furthermore, this allows for further improvement in the reproducibility of the height of the pad 11 obtained after reflow soldering. Finally, the reduced thickness of the first screen 7 for depositing the same volume of solder paste 9 increases the area ratio and thus reduces the variability of the deposit.

[0029] During step E2, for example, scraper 10 is used (see...) Figure 1 This forces solder paste 9 into each first window 8 to fill the first window 8. Solder paste 9 comprises a metal alloy suspended in flux.

[0030] Attachment method P allows the use of any type of solder paste 9, particularly lead-free solder paste, thus enabling compliance with current standards, especially the EU RoHS Directive 2002 / 95 / CE – which prohibits the use of lead, hexavalent chromium, mercury, cadmium, polybrominated biphenyls (PBBBs), and decabromodiphenyl ethers (DBDs). For example, the metal alloys of solder paste 9 may include one of the most commonly used compounds: tin / lead 63 / 37 or 10 / 90 or 90 / 10 for products exempt from RoHS, tin / lead / silver 62 / 36 / 2, or tin / silver alloy 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 applicable to all types of permitted substances (including bismuth, antimony, etc.). The flux depends on the type of metal alloy suspended in the solder paste 9 and the assembly method used, or not, with or without cleaning.

[0031] The flux in solder paste 9 typically includes resins (usually natural, modified, or synthetic resins), activators, and additives to optimize screen printing and reflow soldering. The flux serves to ensure stripping from solder pad 4 (using the activators contained in the flux), to protect solder pad 4 during the temperature rise steps of heat treatment, and to act as a surfactant to promote wetting of the metal alloy. For example, the flux may include rosin.

[0032] For example, solder paste 9 comprises 50% by volume a metal alloy and 50% by volume flux.

[0033] In a manner known per se, the scraper 10 may include a metal plate applied to the first screen 7 at an angle between 45° and 60° to better push the solder paste 9 into the first window 8.

[0034] Once the first window 8 has been filled with solder paste 9, the first screen 7 can be removed.

[0035] During step E3, a first heat treatment is applied to the printed circuit board 2 on which solder paste 9 has been applied to convert the solder paste 9 into pads 11 (see...). Figure 2 ).

[0036] Specifically, the heat treatment may include reflow soldering of the metal alloy present in the solder paste 9. In a manner known per se, the reflow heat treatment sequentially includes a temperature rise step, a so-called preheating step (soaking or pre-flowing and preparing to clean the solder terminals 5 of the printed circuit board 2 and components) for drying the flux, a so-called reflow step (in which the metal alloy melts), and a rapid cooling step to ambient temperature. Further details regarding the reflow soldering heat treatment can be found in document WO2019 / 012136 filed on behalf of the applicant.

[0037] It should be noted that the flux gradually evaporates during the temperature rise (and preheating / preflow) steps, leaving only the metal alloy on solder pad 4. The resulting solder pad 11 no longer contains any material other than the metal alloy.

[0038] During step E4, a second screen 12 is placed on the connection side 2a of the printed circuit board 2. The second screen 12 includes a second thickness e2 and includes at least a second through-window 13. Preferably, the second screen 12 includes as many second through-windows 13 as the solder pads 4 included in the printed circuit board 2, the solder pads 4 being designed to connect to terminals 5 of the first electronic component 3a and the second electronic component 3b. Therefore, when the electronic board 1 to be manufactured includes the first electronic component 3a and the second electronic component 3b, the second screen 12 includes more windows 13 than the first screen 7.

[0039] The second wire mesh 12 is positioned such that each second window 13 covers the corresponding solder area 4 (see...). Figure 2 The placement of the second screen 12 on the connecting side 2a and the size of the second window 13 relative to the solder pad 4 conform to the same standards as the placement of the first screen 7 and the size of the first window 8. Preferably, the surface of the second window 13 is at least equal to the surface of the solder pad 4 to completely cover the solder pad 11, thereby enabling the second screen 12 to be flattened against the printed circuit board 2. Specifically, the raised areas of the second screen 12 can easily cause alloy beads from the solder paste to pass under the screen, contributing to the occurrence of short circuits or the formation of microbeads.

[0040] Preferably, the second thickness e2 is greater than or equal to the thickness of the pad 11 obtained in step E3 to ensure that the electronic component 3a intended to be placed on the pad 11 remains in place and does not squeeze the solder paste 14. Advantageously, the flux of the solder paste 14, in addition to its role during heat treatment, also enables the electronic component 3 to be held in place and cleans the terminals to ensure soldering during preheating. Therefore, it is preferred that the electronic component 3 contacts the solder paste 14, and not just the pad 11.

[0041] Therefore, a second mesh 12 is selected such that the second thickness e2 is greater than the thickness of the pad 11 obtained during step E3. Preferably, e2 is greater than or equal to the sum of the thickness of the pad 11 and the diameter of the alloy bead with the largest size.

[0042] When the second thickness e2 of the second screen 12 is too small compared to the thickness of the pad 11, method P may further include the step of processing the pad 11 (E8) to reduce the thickness of the pad 11 until the thickness of the pad 11 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 the alloy bead. In particular, this processing can be achieved by leveling. It should be noted that embodiments of this form allow for an increase in the volume of the metal alloy in the pad 11 obtained in step e3 (because a higher pad 11 can be formed during the first screen printing (steps E2 to E5), thus filling a larger area of ​​the second window 13), and thus increasing the volume of the metal alloy in the cured solder joint (see step E6 described below).

[0043] Therefore, some of the second windows 13 of the second screen 12 are placed on the printed circuit board 2 to cover the pads 11 obtained in step E3, while other second windows 13 are placed on solder areas 4 that do not have any pads 11 (these solder areas 4 were covered in step E2 by the portion of the first screen 7 that does not have any windows 8, so as not to accept solder paste 14 - see...). Figure 1 ).

[0044] During step E5, for example, scraper 10 is used (see Figure 2 This forces solder paste 14 into each second window 13 to fill the second window 13. Solder paste 14 comprises a metal alloy suspended in flux.

[0045] The solder paste 14 applied in step E5 can be the same as the solder paste 9 applied in step E2. This configuration ensures that the second solder paste 14 fuses before the pad 11, while ensuring optimal mixing of the metal alloy forming the pad 11 and the metal alloy of the solder paste 14, thereby ensuring better adhesion. As a variation, solder pastes 9 and 14 can be different.

[0046] Once the second window 13 has been filled with solder paste 14, the second screen 12 can be removed. Then, the second circuit 2 includes: - The first deposit 15a of solder paste 14 located on pad 11 obtained in step E3; and - A second deposit 15b of solder paste 14 located on solder area 4 where there are no solder pads 11.

[0047] When the second thickness e2 of the second mesh 12 is constant, the thickness of the assembly formed by the pad 11 and the associated first deposit 14a is the same as the thickness of the second deposit 15b of the solder paste 14.

[0048] During step E6, the first electronic component 3a and the second electronic component 3b are placed on the printed circuit board 2 such that each terminal 5 of the electronic components 3a and 3b is located on the corresponding deposits 15a and 15b of the solder paste 14. More specifically, the terminal 5 of the first electronic component 3a is placed on the first deposit 15a of the solder paste 14, and the terminal 5 of the second electronic component 3b is placed on the second deposit 15b of the solder paste 14. When the second thickness e2 of the second screen 12 is greater than or equal to the thickness of the pad (in particular, equal to the sum of the thickness of the pad 11 and the diameter of the metal alloy beads in the solder paste 14), the first electronic component 3a is supported by the pad 11, which prevents the solder paste 14 from being squeezed (and thus causing the solder paste 14 to spread).

[0049] During step E7, a second heat treatment is applied to the printed circuit board 2 to which solder paste 14 has been applied, in order to convert the solder paste 14 into solder joints 6a, 6b. This heat treatment may be the same as that described in step E3. Where applicable, the temperature, gradient, and duration of the different steps of heat treatment E3 may be adapted to the composition of the solder paste 14 used in step E5.

[0050] The resulting electronic board 1 includes solidified solder joints 6a formed from the solder pads 11 obtained in step E3 and conventional solder joints 6b obtained from the second deposit 15b of solder paste 14 in step E7 (see...). Figure 3 In step E7, additional solder joints are formed on pad 11. Therefore, the cured solder joint 6a comprises a larger volume of alloy compared to the conventional solder joint 6b, which increases the effective cross-section of the cured solder joint 6a. Specifically, steps E2 and E3 allow the second window 13 of the second mesh 12 to be partially filled using pad 11, which consists only of the metal alloy, and then steps E5 and E7 fill the remaining space in the second window 13 with solder paste 14. This allows the initial function of the flux (holding the electronic component 3a during heat treatment and cleaning the parts to be assembled) to be preserved while increasing the final volume of the metal alloy in the cured solder joint 6a. Furthermore, it should be noted that during the heat treatment in step E7, when the reflow temperature of the metal alloy of solder paste 9 is less than or equal to the reflow temperature of the metal alloy of solder paste 14, the pad 11 fuses with the metal alloy in solder paste 14 to form only a single identical solder joint 6a. On the other hand, when the reflow temperature of the metal alloy of solder paste 9 is greater than that of the metal alloy of solder paste 14 (this can be the case where solder paste 9 and 14 are not at the same time), the pad 11 will not fuse with the metal alloy in solder paste 14.

[0051] It should be noted that since the cured solder joint 6a does not contain a large volume of metal alloy, the strength of the cured solder joint 6a is increased, which can improve the lifespan of the electronic board 1. In addition, the cured solder joint 6a is easier to visually inspect compared to the conventional solder joint 6b, which has a smaller volume.

[0052] The first thickness e1 of the first wire mesh 7 and the second thickness e2 of the second wire mesh 12 are selected based on the volume of the metal alloy desired by the corresponding electronic components 3a and 3b.

[0053] The applicant noted that the presence of pads 11 on the electronic board during step E7 reduces or even eliminates the formation of microbeads. This is because these microbeads are typically generated during the reflow step by the diffusion of solder paste beneath the electronic components. However, when the electronic board includes pads 11, the space left between the components and the printed circuit board 2 during reflow draws the solder paste 14 toward the pads 11, thus eliminating the risk of microbead formation. Furthermore, selecting a second screen 12 with a thickness e2 (the size of the alloy beads) slightly greater than the thickness of the pads 11 prevents the solder paste 14 from being squeezed and thus diffused, further reducing the risk of microbead formation. Finally, applying the second screen 12 against the surface of the printed circuit board 2 also reduces microbead formation, thereby reducing the risk of solder paste 14 diffusing beneath the electronic components.

[0054] The thickness e1 of the first wire mesh 7 and the thickness e2 of the second wire mesh 12 can be determined to optimize the volume of the metal alloy in the obtained solidified solder joint 6a.

[0055] Therefore, during the first sub-step (S1), the thickness e2 of the second wire mesh 12 is determined. This determination is made to obtain the optimal (i.e., maximum) volume of the metal alloy, while taking into account the characteristics of the electronic board. The greater the thickness e2 of the second wire mesh, the larger the volume of the deposited alloy can be. However, for some components (especially those with finer pitch), it is impossible to use a wire mesh with excessive thickness due to the area ratio. Specifically, the ratio of the window's surface surface to the inner wall surface of the window must be greater than or equal to 0.66 to allow for the removal of the stencil from the solder paste. Therefore, the thickness e2 of the second wire mesh is selected to be as high as possible to optimize the amount of solder paste 14 deposited in step E5, while ensuring compliance with the area ratio of all electronic components on the electronic board.

[0056] During the second sub-step (S3), the thickness e1 of the first screen 7 is determined. For this purpose, the thickness of the pad 11 to be obtained is first estimated (S2). This is an estimate because the thickness of the obtained pad 11 will vary depending on the surface of the solder area where the pad 11 is formed and the surface of the windows of the screens 7 and 12. In an exemplary embodiment, the thickness of the pad 11 obtained in step E3 is preferably slightly smaller than the thickness e2 of the second screen 12, so that the electronic component 3a can be placed on the solder paste 14 covering the tip of the pad 11 during step E6. For example, the thickness of the pad 11 may be equal to the difference between the diameter of the metal alloy beads of the solder paste 14 and the thickness e2. Since the beads can have slightly different diameters, the diameter chosen to estimate the thickness of the pad 11 corresponds to the maximum diameter of the beads. For example, the thickness of the pad 11 may be equal to the difference between the thickness e2 of the second screen 12 and 35 μm.

[0057] Then, the thickness e1 of the first screen 7 is selected to be substantially equal to 90% (close to the 2% mark) of the estimated thickness of the pad 11. Specifically, the applicant notes that when the thickness e1 of the first screen 7 is substantially equal to 90% of the estimated thickness of the pad 11 and the surface of the window 8 is at least equal to the surface of the solder area, the actual thickness of the pad 11 obtained according to step E3 is necessarily slightly less than the thickness e2 of the second screen 12, and this is true even for pads with small surfaces (here, considering the small surface area of ​​the pad, this is a limiting factor). Therefore, selecting a thickness e1 equal to 90% of the estimated thickness of the pad 11, combined with the window surface 8 being at least equal to the surface of the solder area, ensures that the thickness e2 of the second screen is greater than the thickness of the obtained pad 11 regardless of the shape of the solder area. Thus, the pad 11 is high enough to support the electronic component 3a, even when the pad has a small surface area. Furthermore, since the thickness e1 of the first screen 7 is determined based on the optimal thickness e2 of the second screen 12, the volume of the metal alloy in the solder joint 6a is optimized.

[0058] For example, the first thickness e1 and the second thickness e2 can be selected from the following thicknesses: 50μm, 60μm, 80μm, 100μm, 120μm, 150μm, and 200μm. It should be noted that the first thickness e1 and the second thickness e2 can be different.

[0059] When the first thickness e1 and the second thickness e2 are equal and constant across the entire surface of the wire meshes 7 and 12, and when the surface of the first window 8 and the surface of the second window 13 are equal, the volume of the metal alloy in the solidified solder joint 6a is equal to 150% of the volume of the metal alloy in the conventional solder joint 6b.

[0060] Furthermore, the volume of the alloy in the cured solder joint 6a obtained according to the attachment method P using a first screen 7 and a second screen 12 with constant thicknesses e1 and e2 equal to 100 μm is equal to 125% of the volume of the metal alloy obtained according to the conventional attachment method P (single screen printing step) using a screen with a thickness equal to 120 μm. Similarly, the volume of the alloy in the cured solder joint 6a obtained according to the attachment method P using a first screen 7 and a second screen 12 with constant thicknesses e1 and e2 equal to 100 μm is equal to the volume of the metal alloy obtained according to the conventional attachment method P (single screen printing step) using a screen with a thickness equal to 150 μm.

[0061] Based on the required amount of metal alloy for electronic components 3a and 3b, select the type of solder joint (cured solder joint 6a obtained through double screen printing in steps E2 to E7, or conventional solder joint 6b obtained in steps E5 to E7) and the volume of deposited solder paste 9 and 14: - Electronic components 3b that do not require increased strength (subject to lower mechanical stress during use) can be attached to solder areas 4 by a single screen printing process according to steps E5 to E7 (only); - Electronic components 3a requiring average strength (subject to average mechanical stress during use), such as BGAs, can be attached to solder pads 4 using a first mesh 7 and a second mesh 12 according to steps E2 to E7 (see E8), wherein the first mesh 7 has an average aperture (e.g., a first thickness e1 of approximately 100 μm or 120 μm, and a surface area of ​​approximately 75% of the surface area of ​​solder pad 4), and the second mesh 12 has a small aperture (e.g., a second thickness e2 of approximately 80 μm or 100 μm, and a surface area of ​​approximately 100% of the surface area of ​​solder pad 4); and - Electronic components 3a requiring high mechanical strength (subject to high mechanical or thermomechanical stress during use), such as R1206 resistors, can be attached to solder pads 4 using a first mesh 7 and a second mesh 12 according to steps E2 to E7 (or E8). The first mesh 7 has a thicker aperture (e.g., a first thickness e1 of approximately 150 μm or 200 μm, with a surface area approximately 95% or 100% of the surface area of ​​solder pad 4), and the second mesh 12 has a thinner aperture (e.g., a second thickness e2 of approximately 100 μm, with a surface area approximately 95% or 100% of the surface area of ​​solder pad 4). It should be noted that the use of openings with a smaller second thickness e2 in steps E4 and E5, compared to the pads 11 obtained in step E3, restricts the diffusion of solder paste 14.

[0062] The attachment method P can be applied to any electronic components 3a, 3b (capacitors, BGA (ball grid array) resistors, LGA (contact grid array), QFN or DFN (quadrature flat no lead or dual in-line flat no lead), etc.), regardless of the spacing of the terminals 5 of these electronic components 3a, 3b (0.5mm, etc.), and the attachment method P can be implemented without reducing the mounting density on the electronic board 1: this is because the position of the solder area 4 is not affected by the attachment method P.

[0063] Furthermore, the volume of the metal alloy in the cured solder joint 6a is no longer limited by the dimensions of the window 8 of the screen 7 and the window 13 of the screen 12. In particular, recalling that the dimensions of the window or screen are limited by the ratio of the window's surface area to its inner wall surface, this ratio must be greater than or equal to 0.66 to allow the stencil to be removed from the solder paste. Until now, to tightly mount electronic components together on a printed circuit board, either the screen thickness needed to be reduced, which involved reducing the volume of the solder paste, or the surface area of ​​the window needed to be increased, which prevented the placement of finely pitched components. Therefore, attaching P electronic components 3a and 3b by selective double screen printing according to steps E1 to E7 (and, where applicable, according to E8) overcomes this difficulty while increasing the volume of the metal alloy in the cured solder joint 6a, and simultaneously allowing a reduction in the surface area 13 of the second screen 12, thereby mounting finely pitched components 3a and / or increasing the density of electronic components 3a and 3b on the printed circuit board 2, without altering the wiring of the solder area 4.

[0064] In addition, repairs can be made to electronic board 1 (see R). Figure 5 The attachment method P is implemented in the circuit. Specifically, electronic board 1 can be repaired according to the following steps: - Remove the worn electronic component S2 from electronic board 1; - Provide alternative electronic components 3a, S3; and - The alternative electronic component 3a is attached to the electronic board 1 according to the above attachment method (P).

[0065] In particular, when it is necessary to use a solidified solder joint 6a (i.e., with a large volume) to attach electronic component 3a, the alternative electronic component 3a can be soldered onto the electronic board 1 by sequentially applying the above steps E1 to E7 (and, where applicable, E8).

[0066] Where applicable, the repair method R further includes the following operations: after step S2 and before attaching the electronic component according to the attachment method P, preparing the solder pads to standardize the amount of alloy and clean the remaining flux used in the aforementioned method.

Claims

1. A method (P) for attaching a component (3a) to an electronic board (1), comprising the following steps: - A first screen (7) for screen printing, including a first window (8), is placed (E1) on the connection side (2a) of the printed circuit board (2) including the solder area (4), such that the first window covers the solder area (4), and the surface of the first window (8) is at least equal to the surface of the solder area (4). - Use a first solder paste (9) to fill (E2) the first window (8), the first solder paste (9) comprising a metal alloy suspended in flux; - Apply (E3) first heat treatment to the printed circuit board (2) to convert the first solder paste (9) into pads (11); - Place (E4) a second wire mesh (12), the second wire mesh (12) including a second window (13) in contact with the connection side (2a) such that the second window (13) spans the pad (11) and the surface of the second window (13) is at least equal to the surface of the solder area (4); - Use a second solder paste (14) to fill (E5) the second window (13), the second solder paste (14) comprising a metal alloy suspended in flux; - Place (E6) component (3a) on the second solder paste (14) with component (3a) facing the solder area (4); - Apply a second heat treatment (E7) to the printed circuit board (2) to convert the second solder paste (14) into solder joints (6a).

2. The attachment method (P) according to claim 1, wherein, The first solder paste and the second solder paste (9, 14) are the same.

3. The attachment method (P) according to any one of claims 1 and 2, wherein, The thickness (e2) of the second wire mesh (12) is greater than the thickness of the pad (11).

4. The attachment method (P) according to any one of claims 1 to 3, wherein, The attachment method further includes the following sub-steps: - Determine the thickness (e2) of the second wire mesh (12) according to the parameters of the electronic board. - Determine the maximum diameter of the metal alloy beads present in the second solder paste (14); and - Thus, the thickness (e1) of the first wire mesh (7) is derived, and the first thickness (e1) is equal to 90% of the difference between the thickness (e2) of the second wire mesh (12) and the maximum diameter determined therefrom.

5. The attachment method (P) according to any one of claims 1 to 4, wherein, The thickness (e2) of the second screen (12) is greater than the thickness of the pad (11) such that when the component (3a) is placed on the second solder paste (14), the component (3a) is placed (E6) on the pad (11).

6. The attachment method (P) according to claim 5, wherein, The thickness (e2) of the second wire mesh (12) is substantially equal to the sum of the thickness of the pad (11) and the maximum diameter of the metal alloy beads present in the second solder paste (14).

7. The attachment method (P) according to claim 5, wherein, The thickness (e2) of the second mesh (12) is substantially equal to the sum of the thickness of the pad (11) and 35 micrometers.

8. The attachment method (P) according to any one of claims 1 to 7, wherein, The thickness (e2) of the second wire mesh (12) is at least 75% equal to the thickness (e1) of the first wire mesh (7).

9. The attachment method (P) according to any one of claims 1 to 8, wherein, The thicknesses (e1, e2) of the first wire mesh and the second wire mesh (12) are selected from the following thicknesses: 50μm, 60μm, 80μm, 100μm, 120μm, 150μm, 200μm.

10. The attachment method (P) according to any one of claims 1 to 9, wherein, The first heat treatment and the second heat treatment include reflow soldering.

11. The attachment method (P) according to any one of claims 1 to 10, wherein, The first solder paste and the second solder paste (9, 14) comprise at least 50% metal alloy, and the remainder of the first solder paste and the second solder paste comprises the flux.

12. The attachment method (P) according to any one of claims 1 to 11, wherein, The attachment method further includes the following steps: when the thickness of the pad (11) is greater than the thickness (e2) of the second wire mesh (12), preferably when the thickness of the pad (11) is greater than the difference between the thickness (e2) of the second wire mesh (12) and the maximum diameter of the metal alloy beads present in the second solder paste (14) or greater than the difference between the thickness (e2) of the second wire mesh (12) and 35 micrometers, the pad (11) is processed (E8).

13. The attachment method (P) according to any one of claims 1 to 12, wherein, The second screen (12) includes at least one additional window (13), and the printed circuit board (2) includes at least one additional solder area (4). The second wire mesh (12) is further placed (E4) such that each additional window (13) covers the corresponding additional solder area (4). During the filling step (E5), the second solder paste (14) is also used to fill each additional window (13), and The attachment method (P) further includes: placing the attachment element (3b) on the second solder paste (14) before applying (E7) the second heat treatment, such that the attachment element (3b) faces the corresponding attachment solder area (4).

14. An electronic board (1), obtained using the attachment method (P) according to claim 13, the electronic board comprising: - Printed circuit board (2), the printed circuit board includes solder pads (4) and additional solder pads (4); - Component (3a), the component including a terminal (5) connected to the solder area (4) via a solder joint (6a), the component being an electronic component; and - An additional component (3b), which includes an additional terminal (5) connected to the additional solder area (4) via an additional solder joint (6b), the additional component (3b) also being an electronic component. The volume of the metal alloy in the solder joint (6a) is at least 20% larger than the volume of the metal alloy in the additional solder joint (6b).

15. A method for repairing (R) electronic board (1), comprising the following steps: - Provide electronic board (1) (R1); - Remove worn electronic components (R2); - Provide alternative (R3) electronic components (3a); as well as - The alternative electronic component (3a) is attached to the electronic board (1) using the attachment method (P) according to any one of claims 1 to 12.

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