Method for manufacturing a printed circuit board comprising a substrate bearing a copper layer

Abstract

The invention relates to a method (100) for manufacturing a printed circuit board comprising a substrate bearing a copper layer and a heat sink, said method (100) comprising the following steps: - deposition (110) of an intermediate layer of thickness between 50 µm and 2 mm onto a surface of said heat sink by cold sputtering of copper particles; - hot assembly (160) of the substrate and the heat sink, such that the intermediate layer is located between the copper layer and the heat sink. Figure for the abstract: Fig. 2

Description

Title of the invention: Method for manufacturing a printed circuit board comprising a substrate bearing a copper layer. Technical field of the invention

[0001] The present invention relates generally to the manufacture of printed circuits.

[0002] It relates more specifically to a method for manufacturing a printed circuit board comprising a heat sink and a substrate bearing a copper layer. Prior art

[0003] A printed circuit board generally comprises a substrate, for example made of bakelite or ceramic, and copper traces located on this substrate to establish electrical connections between the electronic components fixed to the substrate. Some of these components, during their operation, heat up considerably, so the printed circuit board is equipped with a heat sink to dissipate the heat produced by this component.

[0004] Various solutions are known in this regard, one of which consists of fixing the heat sink on a copper layer carried by the substrate, at the level of the component that heats up, but on the other side of the substrate.

[0005] Typically, this solution consists of hot-assembling, for example by welding, the heat sink to the copper layer carried by the substrate.

[0006] When the heat sink is made of copper, this assembly is done without difficulty.

[0007] However, since the price of copper is high, it is preferable to use a heat sink made of aluminum, which will also be lighter.

[0008] However, the assembly will then be more complicated given the temperature difference inherent in welding aluminum and copper. Thus, this assembly can cause damage to the copper layer on the substrate or to the heat sink. Presentation of the invention

[0009] In order to remedy the aforementioned drawback of the prior art, the present invention proposes a manufacturing process as defined in the introduction, comprising the following steps: - deposition of an intermediate layer with a thickness between 50 pm and 2 mm on a surface of the heat sink by cold spraying of copper particles; - hot assembly of the substrate and the heat sink, such that the intermediate layer is between the copper layer and the heat sink.

[0010] Thus, according to the invention, a cold spray coating is applied to the face of the heat sink. This operation does not damage the heat sink since it is carried out at a low temperature (i.e., below 200°C). Furthermore, the resulting intermediate layer protects the heat sink during the hot assembly step.

[0011] Advantageously, the cold-sprayed material is the same as that carried by the substrate, namely copper, so that the hot assembly can be done at an ideal temperature, which also limits damage to the assembled elements.

[0012] Other advantageous and non-limiting features of the manufacturing process, taken individually or according to all technically possible combinations, are listed below.

[0013] In one embodiment, said copper particles are 99.95% pure by weight (of copper).

[0014] In one embodiment, the copper particles have a particle size between 10 pm and 60 pm.

[0015] In one embodiment, the heat sink having before the assembly step a coating which includes at least said intermediate layer, during the assembly step, the coating and the copper layer of the substrate have a temperature below 230 °C.

[0016] In one embodiment, the heat sink having, before the assembly step, a coating which includes at least said intermediate layer, said process includes, before the assembly step, a preliminary treatment step of an external face of the coating comprising at least one of the following treatments: - mechanical machining; - ultrasonic cleaning using an organic solution; - cleaning with an aqueous solution; - cleaning by air jet; - removal of oxidized particles by soaking in a solution containing an acid or by exposure to an acid; - degreasing by solvent or alkaline degreasing.

[0017] In one embodiment, the process comprises at least one additional step of depositing at least one additional layer onto the intermediate layer, said at least one additional layer and the intermediate layer forming at least part of a coating carried by the heat sink, said at least one additional layer comprising: - a tin layer with a thickness between 5.00 µm and 2.00 mm, and / or - a nickel layer with a thickness between 5.00 pm and 50.00 pm.

[0018] In this case, which said at least one other step of depositing the additional layer can be carried out by cold spraying of a mixture of particles.

[0019] In one embodiment, the process includes a machining step of an external surface of the copper layer carried by the substrate to provide a substrate face having at least one of the following characteristics: - an average roughness of less than 100 pm; - a flatness less than or equal to 20 pm.

[0020] In one embodiment, the heat sink having, before the assembly step, a coating which includes at least said intermediate layer, said method further includes a step of adding a solder flux between the coating and the copper layer;

[0021] In one embodiment, the hot assembly being carried out by welding, the welding of the substrate to the heat sink is obtained by depositing a weld element between a coating carried by the heat sink before the assembly step and comprising said intermediate layer and an outer face of the copper layer carried by the substrate, said weld element having, after the welding step, a thickness of between 75 pm and 250 pm.

[0022] In one embodiment, the hot assembly being carried out by sintering, the sintering of the substrate to the heat sink is obtained by deposition of a sintering element between a coating, carried by the heat sink before the assembly step and comprising said intermediate layer, and an outer face of the copper layer carried by the substrate, said sintering element having, after the sintering step, a thickness of between 50pm and 400pm.

[0023] Of course, the various features, variants, and embodiments of the invention can be combined in various ways, provided they are not incompatible or mutually exclusive. Detailed description of the invention

[0024] The following description with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0025] On the attached drawings:

[0026] [Fig. 1] is a schematic profile representation of a printed circuit board manufactured according to a process according to the invention.

[0027] [Fig.2] is a schematic representation of the process according to the invention.

[0028] In this disclosure, each step can be performed over an entire surface considered or on one or more parts of the surface considered. This depends in particular on the printed circuit board to be manufactured. For the sake of brevity, in the In the examples described, the steps are carried out over the entire area considered unless otherwise specified.

[0029] Fig. 1 thus illustrates an example of the realization of a printed circuit board 1.

[0030] This printed circuit board 1 comprises a substrate 20, at least one electronic component (here a chip 30) located on one side of the substrate 20, and a heat sink 10 located on the other side of the substrate, at the level of the chip 30.

[0031] Before describing how this printed circuit board will be manufactured, we will describe the printed circuit board 1 in the assembled state.

[0032] The heat sink 10 could be made of different materials, provided they are good thermal conductors. Here, it is made of aluminum (and more precisely, aluminum alloy). It typically comprises a base from which cooling fins extend.

[0033] The substrate 20 is made of an electrically non-conductive material. This substrate 20 is in the form of a flat plate, with two main faces 24, 25.

[0034] In this example, the substrate has on one of its main faces 25, the one facing the heat sink 10, a flat layer of copper 21.

[0035] Preferably, this "substrate and copper layer" assembly is of the active brazing type, also known by the English name "Active Metal Braze" (AMB). Thus, the substrate 20 is formed of a ceramic plate 20, for example made of SiC or Si3N4, onto which the copper layer 21 is welded.

[0036] In a preferred mode, the substrate 20 also carries on its other main face 24 another layer 22 on which the chip 30 to be cooled is connected.

[0037] The printed circuit board 1 illustrated in [Fig.1] includes a coating 40 positioned sandwiched between the heat sink 10 and the copper layer 21. The coating 40 includes at least one copper layer 41 (also called the "intermediate layer") obtained by cold spraying directly onto one face of the base of the heat sink 10.

[0038] In a first embodiment, the coating 40 could consist of only the copper layer 41, in which case the latter will be in direct contact with the copper layer 21 carried by the substrate 20.

[0039] Optionally, the coating 40 may include at least one additional layer positioned on the copper layer 41 projected onto the heat sink 10.

[0040] In the illustrated example, the coating 40 includes a first additional layer 42 of nickel positioned on the copper layer 41 fixed to the heat sink.

[0041] The coating 40 also comprises a second additional layer 43 in tin, here positioned between the first additional layer 42 in nickel and the copper layer 21 carried by the substrate 20.

[0042] Here, it should be noted that we are talking about a layer of copper, tin and nickel. In practice, these layers are more precisely formed in alloys comprising respectively a content of more than 50% of copper, tin and nickel.

[0043] An embodiment of a method 100 according to the invention, allowing the printed circuit illustrated in [Fig.1] to be assembled, will now be described with the aid of [Fig.2].

[0044] In the remainder of this description, the term "external surface of the coating 44" will be used to describe the surface of the coating that is exposed to the air and located on the side opposite the heat sink 10. Since this coating 40 may comprise several layers successively superimposed on one another, the external surface of a layer may be exposed to the air at one time and then subsequently covered (in which case it will no longer be referred to as external). Similarly, the expression "external surface 26 of the copper layer 21" will be used to designate the surface of the copper layer 21 carried by the substrate 20, which is exposed to the air.

[0045] At the beginning of the process, it will be assumed that the substrate 20 already carries the copper layer 21 and optionally the other copper layer 22 comprising the chip 30.

[0046] Advantageously, the process 100 initially includes, and optionally includes, a mechanical machining step 101 of a flat surface 11 of the heat sink 10.

[0047] The process 100 then includes a step 110 of depositing the copper layer 41 onto the flat surface 11 of the heat sink 10 (here, onto the entire flat surface 11 of the heat sink). Of course, in one embodiment and depending on the printed circuit board to be manufactured, the deposition step 110 can be carried out on one or more parts of the flat surface 11. The deposition step 110 is carried out by cold spraying of copper-containing particles, also known as "Cold Spray," to a thickness of between 50.00 µm and 2.00 mm.

[0048] Cold spraying refers to a material deposition process carried out by a cold spray deposition device in which particles, preferably micrometer-sized in the solid state, are accelerated to speeds of up to 1200 m / s to deposit a coating onto a surface by an impaction process in which the particles are deformed. The powder particles are accelerated to high speeds by upstream injection of a high-speed gas stream developed by a convergent-divergent de Laval nozzle included in the cold spray deposition device.

[0049] Thus, the deposition step 110 is carried out by a cold spray deposition device described above in which the copper particles are accelerated and projected onto the flat surface of the heat sink 10.

[0050] Preferably, the particles used correspond to a single-component projected copper powder.

[0051] In a preferred configuration, the copper particles are 99.95% pure by weight and contain less than 0.05% dioxygen by weight (i.e., 500 ppm dioxygen). This characteristic makes it possible to obtain an intermediate layer with a very high copper content (i.e., almost pure), which improves its thermal performance and provides very good mechanical properties.

[0052] The copper particles are preferably spherical and, in this example, have a particle size between 10.00 pm and 60.00 pm. Preferably, the copper particles have a similar size, for example, 38.00 pm ± 15 pm. These particles can, for example, be obtained by gas atomization.

[0053] For example, cold spraying is carried out by a known cold spraying device as described above which uses nitrogen gas heated to a temperature between 400°C and 850°C and a pressure between 35 bar and 60 bar as the cold spraying treatment gas. In a particular embodiment, the copper particle feed rate is between 100 g / min and 450 g / min and the spray distance, defined between the flat surface of the heat sink and the tip of the convergent-divergent Laval nozzle of the cold spraying device, is between 5 mm and 45 mm.

[0054] Optionally, the process 100 illustrated in [Fig.1] includes, after the deposition step 110, a preliminary treatment step 120 of an external face of the coating 40 (at this stage, the coating consists of only one layer, the sprayed copper layer).

[0055] The preliminary treatment step 120 may include a mechanical machining step 121 of the external surface of the copper layer 41 to obtain a desired average surface roughness and / or average flatness. In practice, in this step, the external surface is mechanically machined to have an average roughness of less than 100 µm and / or a flatness of less than or equal to 20 µm. This mechanical machining step may be carried out by grinding or milling the surface of the coating 40.

[0056] After this machining step, the preliminary treatment step 120 includes a cleaning step 122 of the machined surface. This cleaning step 122 can be carried out using an aqueous solution. In practice, the external surface of the copper layer 4L is rinsed with water. In one embodiment, the cleaning step can be ultrasonic cleaning using one or more commercially available mildly acidic or alkaline solutions for ultrasonic cleaning of copper (Cu) and its alloys, or one or more organic solvents such as acetone, isopropyl alcohol, or terpenes to remove organic contaminants. In another embodiment, the step of Cleaning can be carried out by air jet, for example with flat nozzles or commercially available circular orifices for air jets between 0.1 and 10 MPa and with air flow rates between 1 and 500 Nl / min. Such a cleaning step 122 makes it possible to remove the copper particles that were machined in the mechanical machining step 121.

[0057] After the cleaning step 122, or directly after the deposition step if no machining is performed, the preliminary treatment step 120 may include a step 123 for removing oxidized particles by soaking in a solution containing an acid. Typically, only the outer surface of the copper layer 41 is soaked in an acidic solution for a period of time whose duration depends on the acid used and / or the copper layer deposited on the heat sink in the deposition step 110. The duration of this period may be from approximately 5.00 seconds to 5.00 minutes.

[0058] Preferably, the acid used is an inorganic acid or a mixture of inorganic acids.

[0059] Typically, the acid solution used in this step 123 may comprise: - dilute hydrochloric acid; - sulfuric acid; or a mixture of the aforementioned acids.

[0060] After the use of the acid, the removal step 123 includes a cleaning step 124 of the external surface of the copper layer 41 soaked in the acid with an aqueous solution. Here, the surface of the coating is, for example, rinsed with deionized water in order to remove the inorganic acid residues present on the surface of the coating 40.

[0061] The process 100 may also include a mechanical machining step 130 of the surface of the copper layer 21 supported by the substrate 20 in order to obtain a satisfactory average surface roughness and / or average flatness. In practice, in this mechanical machining step 130, the external surface is mechanically machined to have an average roughness of less than 100 µm and / or a flatness of less than or equal to 20 µm. Such a mechanical machining step 130 makes it possible to obtain on the surface of the copper layer 21 a roughness and / or flatness similar to that of the external surface of the copper layer 4L. This improves their adhesion properties. As before, this mechanical machining step can be carried out by grinding or milling the surface of the copper layer 21.

[0062] When the external surface of the copper layer 21 has been mechanically machined, the machining step 130 preferably includes a cleaning step 131 of the machined surface. This cleaning step 131 can be carried out using an aqueous solution. In practice, the surface of the copper layer 21 is rinsed with water. In a Alternatively, cleaning step 131 can be ultrasonic cleaning using one or more commercially available mildly acidic or alkaline solutions for ultrasonic cleaning of copper (Cu) and its alloys, or one or more organic solvents such as acetone, isopropyl alcohol, or terpenes to remove organic contaminants. In another alternative, the cleaning step can be performed by air jet cleaning.

[0063] Optionally, the process may include another deposition step 140 during which at least one additional layer 42, 43 is deposited on the copper layer 41. In the following, the coating 40 will then be considered to include not only the copper layer 41, but also the additional layer(s) which cover it.

[0064] Here, in the example presented, this further deposition step 140 comprises a first deposition operation 141 of a nickel layer 42 onto the copper layer 41 deposited in the deposition step 110. By nickel layer, we mean a layer with a high nickel content, namely a layer made of at least 99.95% nickel by weight, the remainder of the composition consisting of residual elements present in concentrations typically on the order of tens of ppm. These residual elements may include aluminum (Al), carbon (C), chromium (Cr), iron (Fe), manganese (Mn), or titanium (Ti).

[0065] Here, the nickel layer 42 is directly deposited on the surface of the copper layer 41 attached to the heat sink. Typically, the deposited nickel layer 42 has a thickness greater than or equal to 5.00 pm, here between 5.00 pm and 50.00 pm.

[0066] This step is particularly relevant when the subsequent steps of process 100 (described below) are not carried out directly after the deposition step 110. Typically, this can occur when the heat sink coated with the copper layer (of the coating 40) is stored for a certain period of time. Typically, this period of time can be one or two years.

[0067] Indeed, this nickel 42 layer prevents the excessive formation of brittle Cu3Sn or Cu6Sn5 type intermetallic compounds, which would have a negative impact during the assembly of the heat sink to the substrate described below.

[0068] In practice, this nickel layer 42 can be deposited by electrolysis or, preferably, by cold spraying of a particle mixture. In the case of cold spraying of a particle mixture, this first deposition operation 141 is carried out in a similar manner to the deposition step 110 described above, except that in this case, the particle mixture essentially comprises nickel.

[0069] Preferably, the particles used in this first deposition operation 141 can comprise nickel present at 99.95% by weight in the mixture of particles.

[0070] In a preferred mode, the particle mixture comprises 99.95% by weight of nickel and 0.05% of residual elements as described above. This characteristic makes it possible to obtain a layer within the coating 40 with a high nickel content, which improves the properties described above.

[0071] This other deposition step 140 herein further includes a second deposition operation 142 of a tin layer 43. By tin layer 43 is meant a layer with a high tin content, namely a layer made of at least 99.95% by weight of tin.

[0072] Here, the tin layer 43 is deposited on the previously deposited nickel layer 42. Of course, in the case where the other deposition step 140 does not include the first deposition operation 141 described above, the tin layer 43 is in this case directly deposited on the copper layer 41 fixed to the heat sink 10.

[0073] The tin layer 43 has a thickness between 5.00 µm and 2.00 mm.

[0074] It is preferably used in the case of hot welding or brazing because it protects the copper layer 41 attached to the heat sink 10. It also allows good wettability.

[0075] This tin layer 43 can be deposited by dipping or electroplating. Advantageously, however, the second deposition operation is carried out by cold spraying of a particle mixture. In the case of cold spraying of a particle mixture, this second deposition operation 142 is carried out similarly to the deposition step 110 described above, except that in this case, the particle mixture comprises tin and optionally lead.

[0076] Preferably, the particle mixture used in the second deposition step 142 may include tin particles. It may also optionally include lead particles. In particular, in the present disclosure, the particle mixture used in this step comprises between 50% and 99.99% by weight of tin (i.e., tin particles), and optionally between 35% and 50% by weight of lead (i.e., lead particles).

[0077] In a preferred configuration, the particle mixture comprises 99.95% by weight of tin and 0.05% of residual elements as described above. This characteristic makes it possible to obtain a high-tin intermediate layer, which improves the properties described above. In an alternative configuration, the particle mixture may comprise at least 50% by weight of tin and optionally 40% of lead.

[0078] If the tin layer 43 is deposited by cold spraying, this second deposition operation 142 can be carried out immediately after the deposition step 110 of the copper layer 41 on the heat sink 10. In this case, it is not necessary to carry out the removal steps 123, which improves the ease of implementation of the process 100 since no acid-based step is performed. This second Deposition operation 142 is therefore commonly used in the case of mass production of the printed circuit board since process 100 is free from constraints related to the handling of acid solutions on an industrial scale.

[0079] Optionally, the process 100 also includes another preliminary step 150 comprising a cleaning step 151 of the external surface 26 of the copper layer 21 and of the external surface 44 of the coating 40. Such a step is similar to the cleaning step 122 described above. It facilitates the assembly of the heat sink 10 to the substrate 20, in particular because it provides good wettability for welding or good sintering properties. This cleaning step 151 may include, for example, rinsing with a dry air jet to remove any dust or debris. This cleaning step may also be followed by a removal step 152 of oxidized particles by soaking in a solution containing an acid. This removal step 152 is carried out in a similar manner to the removal step 123 described above.

[0080] This other preliminary step 150 may also include a degreasing step 153 using an alkaline solvent on the external surface 44 of the coating 40 and on the external surface 26 of the copper layer 21 supporting the substrate 20 to remove substances present on the surfaces to be joined. Typically, such a step makes it possible to remove any trace of grease, oil, or organic substance present on the surfaces to be joined. By way of example, this step 153 may be carried out by vapor condensation of solvents such as trichloroethylene, or by immersion in one or more liquid solvents such as isopropyl alcohol or alkaline detergent solutions. The surfaces must then be rinsed or cleaned thoroughly with water to remove any trace of previously used solutions.

[0081] The process 100 then includes a hot-bonding step 160 of the substrate 20 and the heat sink 10. This step is carried out by placing the two elements one on top of the other such that the coating 40 is located between the heat sink 10 and the copper layer 21 carried by the substrate 20. Thus, if no additional layer has been deposited (if the deposition step 140 has not been implemented), then the intermediate layer 41 deposited in the deposition step 110 is hot-bonded to the surface of the substrate 20 carrying the copper layer 21. Conversely, if at least one additional layer 42, 43 has been deposited in the deposition step 140 (for example, the tin layer 43 and / or the nickel layer 42 described above), the copper layer 21 carried by the substrate 20 is hot-bonded to the additional layer of the coating 40. deposited last, here typically on the tin layer 43 deposited in the second deposition operation 142.

[0082] It will then be said that the substrate 20 and the heat sink 10 are assembled by bringing them together (directly or by adding a material between them for this assembly).

[0083] In this disclosure, the hot joining step 160 is preferably carried out by welding or sintering. For this purpose, brazing will be considered a type of welding.

[0084] We will now describe the operations carried out during hot welding assembly.

[0085] Welding

[0086] This section describes the steps that can be carried out in the hot joining step 160 using a welding operation. A welding operation 170 and a preliminary step 1700 to the welding operation will then be described.

[0087] In the case where the assembly step 160 is carried out by welding, the preliminary step 1700 to the welding operation 170 may include a step of adding a welding flux 1710 between the two external faces 44, 26 of the coating 40 and the copper layer 21.

[0088] The soldering flux can typically be in paste or liquid form and be applied by immersion. If it is not in paste form, this soldering flux can be applied by any technique known to those skilled in the art. This soldering flux can be a non-corrosive organic flux based on rosin.

[0089] In the case of welding, it is preferable that the external surfaces 26, 44 being joined be free from oxidation. For this purpose, it is understood that the oxidized particle removal steps 123, 152 described above can be carried out just before the welding operation. In combination or as an alternative, these removal steps 123, 152 can be carried out by exposing the external surfaces 26, 44 to formic acid, for example, in the furnace that will subsequently be used in the hot joining step 160.

[0090] The welding of the substrate 20 to the heat sink 10 is achieved by deposition of a welding element (step 1720) applied between the outer face 44 of the coating 40 carried by the heat sink 10 and the outer face 26 of the copper layer 21 carried by the substrate.

[0091] In practice, the welding element is applied to one of the two external surfaces, i.e. either to the external surface 44 of the coating or to the external surface 26 of the copper layer 21 carried by the substrate 20. The welding element can be applied to the entire external surface or to one or more parts of the external surface.

[0092] In this disclosure, the solder element comprises at least one of the following materials: tin, lead, silver, or is a mixture of these materials. Typically, the solder element may be a eutectic alloy of the 60Sn-40Pb type or a 62.5Sn-36.1Pb-1.4Ag eutectic alloy.

[0093] The applied weld element has, after the welding step, a thickness between 75.00 µm and 250.00 µm.

[0094] Typically, the deposited weld element can be in paste form. In this case, the two surfaces to be joined are spaced apart to allow the weld element to be applied to one of the surfaces to be joined.

[0095] In this case, the weld element can be applied in various ways, for example by stencil printing, screen printing, or a bulk dispensing technique, etc. In this case, after welding, the weld element can have a thickness of between 80.00 µm and 250.00 µm.

[0096] After application of the welding element in paste form, the surfaces to be joined are then brought together and made in contact (i.e. cold assembly) before the welding operation.

[0097] Alternatively, the weld element can be a film. In this case, the film can typically be pre-cut to have dimensions similar to the external surfaces 26, 44 to be joined. The film is then placed between the two external surfaces 26, 44, and the heat sink 10 is cold-assembled with the substrate 20 (i.e., the elements are brought together) before the welding operation. In this variant, the weld element can have a thickness of between 75.00 µm and 250.00 µm after the welding step.

[0098] In another embodiment, the hot assembly step 160 includes, prior to the welding operation, a cold assembly of the substrate 20 and the heat sink 10, leaving sufficient space between these elements (i.e., the two external surfaces 26, 44 to be joined) to allow the insertion of the weld element by capillary action. In practice, this can be achieved by an automated process such as vapor / condensation reflow soldering performed during the welding operation. In this case, the weld element can have a thickness of between 75 µm and 150 µm after the welding operation.

[0099] It is thus understood that the hot assembly step 160 includes, prior to the welding operation 170, a cold assembly step 1730 of the heat sink and the substrate. In practice, the substrate 20 and the heat sink 10 are brought together so that they form a single unit or element. The weld element may already be positioned in contact with one of the surfaces to be joined during this cold assembly step or may be inserted afterward, particularly during the welding operation 170. Indeed, as explained above, the implementation of this cold assembly step 1730 depends on the weld element used.

[0100] Then a preheating step 1740 is planned for the external surface 44 of the coating (covered or not by the welding element) and for the external surface 26 of the copper layer 21.

[0101] The welding element is a consumable having a liquid state temperature below 450°C. Thus, in practice, the surfaces to be cold-assembled are heated, for example in a furnace, or by laser technology (typically based on infrared radiation), or by vapor phase reflow or other means, to a welding temperature and for an appropriate time period to ensure good wettability.

[0102] This preheating step 1740 is set so that the surfaces to be joined by welding reach a temperature between 100°C and 120°C, preferably at a rate of less than 4°C / s.

[0103] When the surfaces to be joined reach this temperature, the preheating step 1740 continues so that the surfaces to be joined maintain a constant temperature over time. In other words, the surfaces to be joined have a stable temperature, meaning that it does not vary by more than 5°C for less than one minute. In practice, the surfaces to be joined have a temperature between 120°C and 170°C. This temperature can be selected according to the welding flux. The surfaces to be joined remain in a solid state.

[0104] After that, the actual welding operation 170 begins.

[0105] To achieve this, the solder element is heated to a temperature 15°C to 30°C above its melting point. Typically, if the solder element is a eutectic solder element with the formula 63Sn-37Pb, its melting point is 183°C. Therefore, this solder element must be heated to a temperature of at least 198°C, preferably between 198°C and 213°C. For a lead-free solder element, such as SnAgCu (corresponding to a lead-free SAC alloy), the melting point of such an alloy is around 220°C. Consequently, it must be heated to approximately 235°C.

[0106] In the case of soldering, the hot assembly step 160 then includes a cooling step 171 after the soldering operation 170. Typically, heating of the assembled elements is stopped and their temperature is reduced, preferably at a rate of less than or equal to 4°C / s to avoid thermal shocks damaging the printed circuit board materials.

[0107] The total time of the hot assembly step 160 is generally between 3 and 6 minutes.

[0108] If a soldering flux has been used, depending on the flux actually used, the complete assembled component must then be cleaned using one or more commercial chemical solvents under conditions known to those skilled in the art. This step helps prevent corrosion or short circuits during operation of the printed circuit board.

[0109] Sintering

[0110] The operations carried out during the hot assembly 160 can now be described by a sintering operation 180. This sintering operation 180 is preceded by a preliminary step 1800.

[0111] This preliminary step 1800 includes a step 1810 of adding a solder flux between the outer face 44 of the coating 40 and the outer face 26 of the copper layer 21.

[0112] In this case, the assembled surfaces must be free from oxidation and particles. For this purpose, it is understood that the steps 123, 152 for removing oxidized particles described above can be carried out.

[0113] As described previously, this soldering flux can typically be in paste form and be applied by immersion.

[0114] Alternatively or additionally, preliminary step 1800 includes at least one of the following steps: - a plasma activation step 1820 of the external surface of the coating 40; - an activation step 1830 of the outer face of the copper layer 21. Activation makes it possible to obtain a surface free of inorganic and organic contaminants.

[0115] In this disclosure, the substrate is sintered at the heat sink by depositing a sintering element (deposition step 1840) between the outer surface 44 of the coating 40 and the outer surface 26 of the copper layer 21. After the sintering operation 180, the sintering element has a thickness between 50.00 pm and 400.00 pm. This sintering element can be deposited over the entire outer surface considered or over one or more portions thereof.

[0116] Typically, the sintering element can be in the form of a paste, for example a paste containing copper nanoparticles, which can be deposited on one of the external faces 26, 44 to be joined. In this case, in the deposition step 1840, the two faces to be joined are spaced apart to allow the application of the sintering element to one of the faces to be joined.

[0117] After the sintering element has been deposited, this paste is then dried (step 1850), for example by heating in an oven under a protective atmosphere. The faces to be joined can then be brought into contact and joined by a cold assembly step 1860 before the actual sintering operation 180.

[0118] In one embodiment, the sintering element is in the form of a film. In this case, the film can typically be cut by preforming to have dimensions similar to the contact surfaces to be assembled. In this case, the film is deposited between the coating face and the copper layer carried by the substrate, and the heat sink is cold-assembled with the substrate (i.e., the elements are brought together) before the operation sintering 180.

[0119] It is thus understood that the hot assembly step 160 includes, prior to the sintering operation 180, a cold assembly step 1860 of the heat sink and the substrate. In practice, the substrate and the heat sink are brought together so that they form a single unit or element.

[0120] The process then includes the operations necessary for sintering.

[0121] In this case, the hot assembly step includes a preheating step 1870 of the sintering element.

[0122] For this purpose, after the cold assembly 1860, the joined faces are preheated, for example in a furnace, preferably under a protective atmosphere, or by infrared technology or by thermal conduction. Typically, this preheating step 1870 is set so that the surfaces to be joined by sintering reach a temperature between 130°C and 170°C, preferably at a rate of 5°C / s or less.

[0123] Next, the sintering operation 180 can begin.

[0124] Here, the parts to be assembled are preferably transferred under a protective atmosphere into a furnace in which the sintering operation will be carried out. The parts are then heated until they reach a sintering temperature. Typically, the surfaces to be assembled have a temperature between 120°C and 170°C.

[0125] In a preferred embodiment, when the sintering temperature is reached, the sintering operation 180 includes a step of applying a pressure force on the free surface of the substrate 20 (chip side) and / or on a free face of the heat sink 10.

[0126] The temperature exerted depends on the duration of the sintering operation and the pressure applied to the assembly during this operation. The sintering time and the pressure are interdependent. Typically, the sintering operation can reach temperatures between 180°C and 250°C and pressures between 5 MPa and 30 MPa.

[0127] The hot assembly step then includes a cooling step 181 after the sintering operation 180. Typically, heating of the assembled elements is stopped, and their temperature is reduced, preferably at a rate less than or equal to 5°C / s to avoid thermal shocks damaging the printed circuit board materials.

Claims

Claims

1. Method (100) for manufacturing a printed circuit (1) comprising a substrate (20) carrying a copper layer (21), and a heat sink (10), said method (100) comprising the following steps: - deposition (110) of an intermediate layer (41) of thickness between 50 μm and 2 mm on a surface (11) of said heat sink (10) by cold spraying of copper particles; - hot assembly (160) of the substrate (20) and the heat sink (10), such that the intermediate layer (41) is between the copper layer and the heat sink (10).

2. The method (100) of claim 1, wherein the copper particles are 99.95% pure by weight.

3. Method (100) according to claim 1 or 2, wherein the copper particles have a particle size between 10 pm and 60 pm.

4. Method (100) according to any one of claims 1 to 3, wherein, the heat sink (10) carrying before the assembly step (160) a coating (40) which comprises at least said intermediate layer (41), during the assembly step (160), the coating (40) and the copper layer of the substrate (20) have temperatures lower than 230 °C.

5. Method (100) according to any one of claims 1 to 4, wherein, the heat sink (10) carrying before the assembly step (160) a coating (40) which comprises at least said intermediate layer (41), said method (100) comprises, before the assembly step (160), a preliminary treatment step (120, 150) of an external face (44) of the coating (40) comprising at least one of the following treatments: - mechanical machining (121); - cleaning (122, 151) by ultrasound using an organic solution; - cleaning (122, 124, 151) with an aqueous solution; - cleaning (122, 151) by air jet; - removal (123, 152) of oxidized particles by soaking in a solution comprising an acid or by exposure to an acid; - degreasing (153) by solvent or alkaline degreasing.

6. Method (100) according to any one of claims 1 to 5, wherein said method (100) comprises at least one further step of depositing (140) at least one additional layer (42, 43) on the inter-layer intermediate layer (41), said at least one additional layer (42, 43) and the intermediate layer (41) forming at least part of a coating (40) carried by the heat sink (10), said at least one additional layer (42, 43) comprising: - a tin layer (43) having a thickness of between 5.00 pm and 2.00 mm, and / or - a nickel layer (42) having a thickness of between 5.00 pm and 50.00 pm.

7. Method (100) according to claim 6, wherein said at least one other step of depositing (140) the additional layer is carried out by cold spraying of a mixture of particles.

8. Method (100) according to any one of claims 1 to 7, wherein said method (100) comprises a step of machining (130) an external surface (26) of the copper layer (21) carried by the substrate (20) to provide a face (26) having at least one of the following characteristics: - an average roughness of less than 100 pm; - a flatness of less than or equal to 20 pm.

9. Method (100) according to any one of claims 1 to 8, wherein, the heat sink (10) carrying before the assembly step (160) a coating (40) which comprises at least said intermediate layer (41), said method further comprises a step of adding a solder flux between the coating (40) and the copper layer (21).

10. Method (100) according to any one of claims 1 to 9, wherein, the hot assembly (160) being carried out by welding (170), the welding (170) of the substrate (20) to the heat sink (10) is obtained by deposition (1720) of a welding element between a coating (40) carried by the heat sink (10) before the assembly step (160) and comprising said intermediate layer (41) and an outer face (26) of the copper layer carried by the substrate (20), said welding element having, after the welding step, a thickness of between 75 μm and 250 μm.

11. Method (100) according to any one of claims 1 to 9, in which, the hot assembly (160) being carried out by sintering (180), the sintering (180) of the substrate (20) to the heat sink (10) is obtained by deposition (1840) of a sintering element between a coating (40), carried by the heat sink (10) before the assembly step (160) and comprising said intermediate layer (41), and an outer face (26) of the copper layer carried by the substrate (20), said sintering element having, after the sintering step (180), a thickness of between 50 pm and 400 pm.

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