Method for assembling an electronic component in a printed circuit, method for manufacturing a multilayer printed circuit and printed circuit obtained by this method
The method of using a copper-tin solder paste for diffusion soldering addresses the challenge of solder reflow in assembling buried components, ensuring reliable connections and preventing short circuits in printed circuits.
Patent Information
- Application Number
- FR2022007676
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2022-07-26
- Publication Date
- 2025-06-20
- Estimated Expiration
- 2042-07-26
AI Technical Summary
Existing methods for assembling buried components in printed circuits face challenges such as solder reflow, which can lead to short circuits and compatibility issues with high-temperature soldering.
A method using a solder paste containing copper and tin for diffusion soldering, allowing for the assembly of buried components without reflowing the soldering material, thereby eliminating the risk of short circuits.
This method effectively connects buried components in printed circuits without the risk of reflow, ensuring reliable assembly and preventing short circuits, while being compatible with standard electronic components and printed circuits.
Smart Images
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Abstract
Description
Title of the invention: Method for assembling an electronic component in a printed circuit, method for manufacturing a multilayer printed circuit and printed circuit obtained by this method TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a method of assembling an electronic component in a printed circuit in which the electronic component is connected to the printed circuit by diffusion soldering. It also relates to a method of manufacturing a multilayer printed circuit avoiding problems related to reflows of the soldering material. The invention also relates to a printed circuit comprising buried components obtained by this method.
[0002] The invention finds applications in the field of manufacturing electronic cards and, in particular, in the field of manufacturing electronic cards intended for aeronautics. TECHNOLOGICAL BACKGROUND OF THE INVENTION
[0003] In aeronautics, many functions are performed using thermal or hydraulic devices. However, the desire to reduce greenhouse gas emissions has led to the replacement of these thermal and hydraulic functions with electrical or electronic functions. Power electronics applied to aeronautics has therefore tended to develop in recent years. The power electronics available on the market are, however, not entirely adapted to aeronautics. For use in aeronautics, it is indeed necessary for electronic cards to gain in efficiency as well as in mass or size. To this end, aeronautical manufacturers seek to increase the functions and power available within an electronic card, also called a printed circuit, by integrating a maximum number of components within a single printed circuit. Indeed, as shown in [Fig.l], a conventional printed circuit 10 comprises electronic components 14 connected on an outer face 12 (or external conductive layer) of the printed circuit or on both outer faces 12 of the printed circuit, each external conductive layer 12 resting on a dielectric layer 11. Techniques have been implemented to also integrate electronic components, called buried components, inside the printed circuit. Such buried components must then be connected on an internal conductive layer, housed between two dielectric layers.
[0004] One of the ways of integrating components into printed circuits is to bury the components in the substrates of the printed circuits, such as, for example, PCBs (or "Printed Circuit Boards" in English terms) of the organic type, in order to form highly integrated digital modules of the SIP (or "System in Package" in English terms) type or power modules. However, the burying of components in the substrates of printed circuits has limitations caused by the assembly of said components on the metal tracks, also called metal layers or conductive layers, of the printed circuits. Indeed, each of the methods currently used for burying components within a printed circuit has drawbacks linked to the assembly of the components on the internal metal layer, generally copper, of the printed circuit.
[0005] The most widely used method currently involves the manufacture of copper-filled laser vias. This method requires the use of copper-terminated components and chips. However, the variety of this type of component is still quite low, and these components are only available for very large production volumes;
[0006] An alternative method, still under study, for connecting components buried in a printed circuit consists of conductive bonding with silver-finished components. This method is still poorly documented and does not appear to offer sufficient reliability for aeronautical devices subject to high stress, particularly in vibrations.
[0007] Another method consists of connecting the components buried in the printed circuit by a soldering method identical to that used to connect the components on the external faces of the printed circuit. This method makes it possible to use standard components, easy to find on the market and to connect them by a known soldering technique. These standard components, called COTS (for "Commercial Off-The-Shelf", in English terms), are assembled on the internal metal circuit by conventional soft soldering, at a melting temperature close to 220°C, with a known tin-based soldering material. In its most currently used form in electronics, the soldering material, also called solder cord, is formed from an alloy of tin, silver and copper (Sn96.5Ag3.0Cu0.5) and known under the name "SAC305".This method, however, causes the solder bead of the buried component to reflow when soldering the surface components with the same type of alloy, which leads to risks of short circuit inside the printed circuit. Indeed, this method consists of assembling the buried components on an internal metal layer of the printed circuit by soldering using a SAC305 solder bead. This first soldering involves bringing the buried components and the internal metal layer to the temperature. melting temperature of the solder bead, generally a temperature of around 245°C or 260°C depending on the type of component to be soldered. Above a dielectric layer covering the buried components and the internal metal layer, extends an external metal layer on which external electronic components are assembled. These external electronic components are connected by the same soldering process as the buried components. This soldering of the external components involves bringing said external components and the external metal layer to the same temperature as for the first soldering. Given the stacking of the layers, the second soldering also involves bringing the buried components and the internal metal layer back to this same melting temperature of the solder bead. There is therefore reflow of the solder material in the internal layer.This reflow of the internal solder material, combined with the possibility of delaminations within the printed circuit board, can cause short circuits. This is because reflow causes the solder material to become liquid again and can expand into the spaces between the metal layer and the dielectric layer, creating an unwanted connection with the metal layer and / or another internal electronic component.
[0008] To avoid this reflow, it has been considered to use, for the connection of buried components, soldering materials based on alloys with a higher melting point than SAC305. However, most electronic boxes are not designed or validated for this type of assembly. Indeed, it is necessary that the electronic components used are qualified to withstand these higher temperatures. However, the components found on the market, such as so-called "lead-free compliant" components, known as RoHS, are qualified for assembly processes at 245°C or 260°C (according to JEDEC standards) and are therefore not compatible with high-temperature soldering.
[0009] There is therefore a real need for a new method of assembling buried components, avoiding the remelting of the soldering material with the consequences that follow. Summary of the invention
[0010] To address the above-mentioned problems of reflowing the soldering material used during the assembly of buried electronic components, the applicant proposes a method for assembling a buried component using a solder paste containing essentially copper and tin and allowing diffusion soldering. The applicant also proposes a method for manufacturing a multilayer printed circuit, also called PCB (Printed Circuit Board, in English terms) in which the buried component(s) are connected by diffusion soldering using this solder paste.
[0011] "Brazing" is a permanent joining process that establishes a bond metallic between two metal parts, without melting the edges of the metal parts and, in particular, between an electronic component and a metal track of a printed circuit. In the invention, the brazing considered is brazing with the addition of a metal-based brazing material, in which the brazing material is brought to its melting temperature (lower than that of the metals to be joined) to become liquid and thus wet, by capillarity, the parts to be joined.
[0012] According to a first aspect, the invention relates to a method of assembling an electronic component on a conductive layer of a printed circuit, comprising the following operations: • depositing a solder paste on the conductive layer, said solder paste comprising tin, copper balls and a soldering flux, • positioning the electronic component on the solder paste, then • diffusion soldering of said electronic component.
[0013] This method makes it possible to connect an internal electronic component in a printed circuit without the risk of reflowing the soldering material when assembling the components in external layers.
[0014] A second aspect of the invention relates to a method of manufacturing a multilayer printed circuit, comprising the following steps: a. assembly of at least a first electronic component on an internal conductive layer of a printed circuit, b. depositing a dielectric layer on the first electronic component and the internal conductive layer, and c. assembly of at least one second electronic component on an external conductive layer of the printed circuit, step a) of assembling the first electronic component being in accordance with the assembly method as defined above.
[0015] This method makes it possible to manufacture a multi-layer printed circuit with internal electronic components without the risk of short circuits, in the internal layers, caused by the reflow of the soldering material. Indeed, the fact of using two distinct soldering materials, one of which has a reflow temperature significantly higher than its initial melting temperature, makes it possible to create one or more internal layers without the risk of reflow of the soldering material used in these internal layers.
[0016] In addition to the characteristics which have just been mentioned in the preceding paragraph, the manufacturing method according to one aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: • step c) of assembling the second electronic component comprises the operations of depositing a solder bead on an external conductive layer, positioning the second electronic component on the solder bead, and soft soldering the second electronic component. • the solder cord comprises a tin-based alloy, distinct from the solder paste used for the first component. • the solder paste comprises a melting temperature and a reflow temperature distinct from each other, the reflow temperature being substantially higher than the melting temperature, and the solder bead comprises a single melting temperature. • the reflow temperature of the solder paste is at least 100°C higher than the melting temperature of said solder paste. • step a) of assembling the first electronic component and step c) of assembling the second electronic component each comprise an operation of heating the solder bead and the solder paste to a maximum temperature of 260°C. • it comprises a plurality of steps a) of assembling the first electronic component and steps b) of depositing a dielectric layer, carried out successively one after the other before step c) of assembling the second electronic component, an internal layer of the multilayer printed circuit being formed after each set of a step a) and a step b).
[0017] A third aspect of the invention relates to a multilayer printed circuit comprising at least a first and a second electronic component connected, respectively, to an internal conductive layer and an external conductive layer, said internal and external conductive layers being separated from each other by a dielectric layer, characterized in that it is obtained by the manufacturing method as defined above. BRIEF DESCRIPTION OF THE FIGURES
[0018] Other advantages and characteristics of the invention will appear on reading the following description, illustrated by the figures in which:
[0019] [Fig.l], already described, represents a schematic sectional view of a conventional multilayer printed circuit;
[0020] [Fig. 2] represents two examples of solder paste used, according to the invention, for the assembly of components buried on the internal conductive layer; and
[0021] [Fig. 3] represents, according to schematic sectional views, the different operations and stages of the method of manufacturing a printed circuit according to the invention. DETAILED DESCRIPTION
[0022] An exemplary embodiment of a method for assembling a buried component and an exemplary embodiment of a method for manufacturing a multilayer printed circuit incorporating this assembly method are described in detail below, with reference to the appended drawings. These examples illustrate the characteristics and advantages of the invention. It is however recalled that the invention is not limited to these examples.
[0023] In the figures, identical elements are identified by identical references. For reasons of readability of the figures, the size scales between elements represented are not respected.
[0024] An example of a method 100 for assembling an internal electronic component, also called first electronic component or buried component, on a conductive layer of a printed circuit, is shown in [Fig. 3]. This assembly method 100 comprises an operation 110 for producing the conductive layer 23 on which it is intended to connect the buried component 25. This conductive layer 23, also called metal track, is a track etched in an electrically conductive material such as copper for example. This conductive layer 23, initially deposited on a dielectric layer 21, is etched by photolithography or by any other etching technique known in the field of printed circuits.
[0025] The assembly method 100 then comprises an operation 120 of depositing the solder paste 26 at the location where the buried component 25 will be positioned. An operation 130 then consists of positioning the buried component 25 above the solder paste 26. These operations 110 to 130 together constitute an operation known as positioning the buried component.
[0026] The assembly method 100 then comprises an operation 140 of diffusion soldering the buried component 25. This soldering operation 140 consists of bringing the solder paste 26 to a soldering temperature making said solder paste fluid. The soldering temperature, more simply called the melting temperature, is a temperature at least equal to the melting temperature of the solder paste. The temperature of the solder paste 26 can be brought to a temperature by installing the partially formed assembly of the printed circuit - that is to say the assembly comprising (at this stage of the method) the conductive layer 23 resting on the dielectric layer 21, the buried component 25 and the solder paste 26 - in a heating device, such as an oven, in which a heat prevails substantially higher than the melting temperature of the solder paste.Under the influence of the melting temperature and time above the melting point, the solder paste 26 transforms into internal solder joints allowing electrical connection with the internal conductive layer.
[0027] The solder paste 26 according to the invention is a solder paste generally used in TLPS (Transient Liquid Phase Sintering) technology for soldering standard surface-mounted components (or CMS), with a tin finish, with a furnace profile close to that used with a conventional soldering material (of the SAC305 type described below). This solder paste 26 is a substance consisting of a mixture of tin (Sn) and copper (Cu) balls. In a first variant, shown in drawing A of [Fig.2], the solder paste is in the form of copper balls 31 and tin balls 32 mixed in a soldering flux 33. In a second variant, shown in drawing B of [Fig.2], the solder paste 26 is in the form of copper balls 34 covered with a thin layer of tin 35 and mixed with a flux 33.Whatever the variant (drawing A or drawing B), the tin melts during soldering, i.e. under the effect of the melting temperature, and a tin / copper interdiffusion occurs to form a bronze (CuSn) whose melting point is much higher than 400°C. The melting temperature of the solder paste 26, i.e. the temperature to which the partially formed assembly of the printed circuit is brought during operation 140, is of the order of 250°C. After cooling, the buried component 25 is assembled to the conductive layer 23. The bronze formed by the tin / copper interdiffusion has a melting point much higher than 400°C and therefore much higher than the melting temperature. This melting point of the bronze corresponds to a so-called “reflow” temperature, i.e. the second melting temperature of the solder paste.This reflow temperature being significantly higher than the melting temperature of the solder paste 26, there is no risk of reflow of said solder paste when soldering the external electronic components 14, or surface components, as explained below.
[0028] This type of diffusion soldering using a solder paste such as that described above makes it possible to assemble components with a tin finish at soldering temperatures similar to those used with the alloy known as "SAC305", with the advantage that the solder paste no longer remelts during subsequent assemblies, which eliminates the risk of having short circuits when assembling surface-mounted components with the SAC305 alloy.
[0029] The manufacturing method 200 of a multilayer printed circuit according to the invention is shown functionally in [Fig. 3]. This manufacturing method 200 comprises all the operations of the assembly method 100 described previously. It further comprises, after the soldering operation 140, a step 250 of depositing at least one dielectric layer 21 on the buried component 25 and the internal conductive layer 23. This step 250 comprises, in the example of [Fig. 3], the deposition of an upper dielectric layer 21a, deposited above the internal conductive layer 23, and the deposition of a lower dielectric layer 21b, deposited below said internal conductive layer 23. These dielectric layers 21 are layers formed in a dielectric material and deposited according to any conventional technique in the field of printed circuits to form a dielectric layer. The dielectric layer 21, also called pre-impregnated, can be formed, for example, of a structuring fabric and a dielectric resin, the structuring fabric being able to be in particular a glass fabric and the dielectric resin an epoxy resin.
[0030] Step 250 also includes an operation of etching an external conductive layer 22, or metal track, on the surface of the printed circuit. This external conductive layer 22 is produced in the same way as the internal conductive layer 23.
[0031] The manufacturing method 200 then comprises a step of assembling the second electronic component 24, or surface component, on the external conductive layer 22. This step of assembling the surface component 24 comprises an operation 260 of depositing the solder bead 27 at the location where the surface component 24 is to be positioned. The surface component 24 is then positioned on the solder bead 27 before carrying out the soft soldering operation 270 using a tin-based soldering material 27. This solder bead 27 is a conventional soldering material, as usually used in the field of printed circuits. This solder bead may, for example, be formed essentially of tin, such as the alloy SAC305.
[0032] The soldering operation 270 consists of bringing the entire printed circuit to the soldering temperature adapted to the material of the solder bead 27 to make said material fluid. The soldering temperature of the operation 270 is a temperature at least equal to the melting temperature of the solder bead 27. The heating of the solder bead 27 can be carried out, as in step 140 of the assembly method 100, by installing the entire printed circuit (i.e. all the layers comprising the components, the soldering materials, the conductive layers and the dielectric layers) in a heating device, such as an oven, in which there is a heat substantially equal to the melting temperature of the solder bead 27. For a solder bead of the SAC305 type, the melting temperature of the soldering operation 270 is 260°C maximum.Thus, the melting temperature of the soldering operation 270 of the surface component is approximately the same as the melting temperature of the soldering operation 140 of the buried component. The soldering operation 270 allows the solder bead 27 to melt and form external solder joints (connecting the surface component 24 to the external conductive layer) without reflowing the internal solder joints (connecting the buried component to the internal conductive layer) - that is, without the internal solder joints becoming fluid again - because the melting point of said solder joints . internals became higher (400°C), after the first fusion, than that of the solder bead 27.
[0033] The manufacturing method 200 is therefore implemented with a soldering temperature below 260°C; it is therefore perfectly compatible with standard electronic components and standard printed circuits. It also offers the advantage of not causing reflow of the internal soldered joint, which allows components to be buried inside the printed circuit using a simple soldering method, without the risk of generating short circuits.
[0034] The use of two separate soldering materials, for the internal components and the external components, can also allow for better cleaning of the printed circuits. Indeed, the solder paste 26 has the advantage of not forming a meniscus (unlike a conventional tin solder) under the component, which creates, under the component, a larger space, conducive to good circulation of liquids during cleaning. The use of two separate soldering materials, for the internal components and the external components, can also allow for better adhesion of the epoxy resin on the internal soldered joints during the operations of depositing the dielectric layers. Indeed, since the joint does not fuse, it is granular, which allows for better adhesion of the resin.
[0035] The assembly method 100 and the manufacturing method 200 according to the invention have been described for a buried component 25 and a surface component 24. Those skilled in the art will understand that several buried components can be assembled in the same way as the buried components 25 and that several surface components can be assembled in the same way as the surface component 24. Those skilled in the art will also understand that several internal layers, each comprising an internal conductive layer 23 and one or more internal components 25, can be buried inside the printed circuit, each internal layer being separated from the next internal layer by a dielectric layer 21. Indeed, since the internal components of each internal layer are soldered using the solder paste 26, there is no risk of reflow of the internal soldered joints regardless of the number of soldering operations carried out on the printed circuit.
[0036] The assembly method 100 according to the invention thus allows the production of internal layers with connection of components buried in printed circuits, such as organic electronic cards, SIP modules (for System In Package, in English terms), power modules or PCB packaging, without risk of remelting the soldering of the internal components during the final assembly in an oven.
[0037] Although described through a number of examples, variants and embodiments, the method of assembling a buried component and the method of manufacturing a multilayer printed circuit according to the invention comprise various variations, modifications and improvements which will be obvious to those skilled in the art, it being understood that these variations, modifications and improvements are part of the scope of the invention.
Claims
Claims
1. A method of manufacturing (200) a multilayer printed circuit, comprising the following steps: a. assembly of at least one first electronic component (25) on an internal conductive layer (23) of a printed circuit, said assembly comprising the following operations:
1. depositing (120) a solder paste (26) on the conductive layer (23), said solder paste comprising tin, copper balls and a soldering flux, 2. positioning (130) of the electronic component (25) on the solder paste, then 3. soldering (140) by diffusion of said electronic component. b. depositing (250) a dielectric layer (21) on the first electronic component (25) and the internal conductive layer (23), and c. assembly (260, 270) of at least one second electronic component (24) on an external conductive layer (22) of the printed circuit, said assembly (260, 270) of the second electronic component (24) comprising the following operations: i. depositing (260) a solder bead (27) on an external conductive layer (22), ii. positioning the second electronic component (24) on the solder cord (27), and iii. soft soldering (270) of the second electronic component (24) characterized in that the solder bead (27) comprises a tin-based alloy, distinct from the solder paste (26).
2. Method according to claim 1, characterized in that: - the solder paste (26) comprises a melting temperature and a reflow temperature distinct from each other, the reflow temperature being substantially higher than the melting temperature, and - the weld bead (27) has a single melting temperature.
3. Method according to claim 2, characterized in that the reflow temperature of the solder paste (26) is at least 100°C higher than the melting temperature of said solder paste.
4. Method according to any one of claims 1 to 3, characterized in that step a) of assembling the first electronic component (25) and step c) of assembling the second electronic component (24) each comprise an operation of heating the solder bead (27) and the solder paste (26) to a maximum temperature of 260°C.
5. Method according to any one of claims 1 to 4, characterized in that it comprises a plurality of steps a) of assembling the first electronic component (25) and steps b) of depositing a dielectric layer (21), carried out successively one after the other before step c) of assembling the second electronic component (24), an internal layer of the multilayer printed circuit being formed after each set of a step a) and a step b).
6. Multilayer printed circuit comprising at least a first and a second electronic component (25, 24) connected, respectively, to an internal conductive layer (23) and an external conductive layer (22), said internal and external conductive layers being separated from each other by a dielectric layer (21), characterized in that it is obtained by the manufacturing method according to any one of claims 1 to 5.