FR2653595A1 - Electronic circuit inter-connection grid - Google Patents

Abstract

The present invention relates to an electronic circuit of the type comprising a substrate (1) supporting electrical conductors and/or electronic components in which at least one conductor is suitable for providing an electrical link between two points of the said circuit. The said conductor (4) consists of a series of segments shaped into a single electrically conducting part including at least one segment (5) for absorbing thermal expansions which are unconstrained with respect to the said substrate (1), between two fixed segments (3) for linking to the said substrate (1).

Description

 The present invention relates to a particularly hybrid electronic circuit.

 It applies more particularly to an electronic power circuit and / or requiring a low line resistance in conductors connecting electronic components which it comprises.

 Up to now, the connection lines between different electronic components on the same circuit have been made by tracks traced on a metallized substrate made of a tinning material.

 This technique, if it is suitable for low power electronic circuits, requires in the case of power circuits comprising components dissipating a large amount of energy by the Joule effect and in particular if high currents are supplied, that the metallized tracks of the substrate having a thickness of the order of 15 μm are covered with one or more additional metallization thicknesses.

 These additional thicknesses are generally produced either by screen printing or by transferring a metallic element over the entire surface of the tracks.

 In the first case, the increase in thickness is limited and it is often necessary to have recourse to an increase in the width of the tracks, which in particular affects the size of the circuit.

 In the second case, such a circuit involves risks of rupture or detachment of the metallic elements due to thermal expansions due to the temperature variations involved either during manufacture or even due to climatic constraints, or during operation due of the powers conveyed by the tracks.

 The problems of expansion in manufacturing can come from bending of the metallic elements due to the difference in coefficient of expansion between the material of the substrate, generally alumina, and the tinable material constituting the metallic element generally of copper. It has already been proposed to avoid this bending to provide the substrate with a copper expansion base with a thickness of approximately 5 times the thickness of the substrate. But if this limits the risks of bending, the four thicknesses thus produced (base, alumina, track, metallic element) tend to separate from each other in the event of thermal shock.

 These bending problems are all the more present as the circuit is intended for a higher power due to the corresponding increase in the thickness of the tracks.

 The aim of the present invention is to solve the above problems by proposing an electronic circuit which can withstand thermal stresses which are linked to the manufacture, to the climatic conditions or to the powers conveyed in the circuit without harming either or the line resistance of the conductors, nor to the congestion of the circuit.

 According to its main characteristic, the present invention relates to an electronic circuit of the type comprising a substrate supporting electrical conductors and / or electronic components in which at least one conductor is capable of ensuring an electrical connection between two points of said circuit, characterized in that said conductor consists of a series of sections formed in a single electrically conductive part comprising at least one section for absorbing free thermal expansion relative to said substrate, between two fixed sections for connection to said substrate.

 It is therefore provided that, instead of overloading the metallized tracks of the substrate by tinning or even adding brazed conductors on the metallized tracks over their entire length in order to increase the thickness, that a conductor between two points of the circuit is bonded by welding or brazing discontinuously on metallized portions of the substrate.

 The fixed sections provide, in addition to the capacity for bonding to the substrate, a capacity for separating the free sections from the surface of the substrate by the thickness of the metallized portion and the thickness of solder, and the free sections provide a capacity for expansion. and are intended to take up the thermal forces of the assembly.

 The fixed sections also help to convey at least part of the heat energy to the substrate where it diffuses.

 In a particularly advantageous manner, the fixed sections are dimensioned so that the shear stress undergone by the weld is less than the elastic limit of the latter, and in that the free sections are dimensioned so that the stresses which they undergo, in particular the stresses of expansion, are lower than its elastic limit.

 The ratio between the length of the free sections and the length of the fixed sections is for example between 1/1 and 4/1.

 The lengths of the fixed sections can for example be chosen between 3 and 20 mm.

 It is particularly advantageous to provide the dimensioning as a function of the temperature extremes to which the circuit must be subjected, the free sections having to be capable of absorbing the damping force which corresponds to the extreme of lower temperature.

 According to a particularly advantageous characteristic of the invention, the circuit comprising a plurality of similar conductors suitable for ensuring an electrical connection between different points of the circuit, is characterized in that said conductors each comprise at least one free section between two fixed sections, and in that they constitute a single-piece interconnection grid in which at least one of said conductors comprises at least one free section between two interconnection points of the grid.

 This constitutes an interconnection grid intended to be welded in a single operation by its fixed sections on metallized portions of the substrate, which considerably simplifies the construction of the circuit and which moreover reduces the line resistance due to the absence of solder to connect the conductors.

 Depending on the powers to be conveyed by the conductors and the other thermal stresses which they must bear during their lifetime, the size of the conductors is dimensioned in thickness, in width and the length of the free sections and of the fixed sections.

 In addition, since we are no longer limited in section in particular because the expansion is absorbed by the free sections, we can provide the section of the conductors according to the currents to flow in the conductors and we can thus achieve a circuit accepting higher currents, without risk of breaking the conductors under the effect of the current.

According to other advantageous characteristics of the circuit according to the invention
the length of a free section formed between two fixed sections is such that it represents an elongation of the course relative to a straight line which would connect the two fixed sections
a free section formed between two fixed sections follows a path in a parallel plane or plane of the substrate, said parallel plane comprising said two fixed sections
a free section formed between two fixed sections follows a course in a plane perpendicular or oblique to the plane of the substrate, said perpendicular or oblique plane comprising said two fixed sections
- and a free section, presents an accordion line.

 The shape of the layout of the free sections can thus be chosen according to the requirements of the circuit, for example its size.

 If the circuit must be as flat as possible, it will be advantageous to choose a route of the free sections in a plane parallel to that of the substrate, so as to achieve the increases in length of the free sections parallel to the circuit.

 If the circuit must be as small as possible on the surface, it will be advantageous to choose a layout of the free sections in a plane perpendicular to that of the substrate so as to achieve the increases in length of the sections in the height of the circuit.

 A compromise between these two solutions can be found by choosing for the layout of the free sections, a plane oblique to that of the substrate.

 The shape of the layout of the free sections (rounded, at right angles, accordion, etc.) will be chosen according to the manufacturing requirements and the desired expansion damping.

 According to a particularly advantageous embodiment of the device according to the invention, the section of the sections of the conductors is parallelepipedic and preferably rectangular.

 This also allows, in a particularly advantageous manner that electronic components, in particular semi-conductors, whether they are encapsulated or not, are soldered on a fixed section, the upper surface of the fixed section constituting support surface for said component at least. partially.

 The materials used for the realization of the circuit can be all the materials commonly used in this field, for example alumina for the substrate, copper, silver, silver-palladium or gold for the metallization of the substrate, tin, lead or silver for the solder and copper or silver for the conductors.

 The conductor or the conductor grid can be made from a copper sheet, by insulating a photoresist and then applying a strip of chemical etching to the strip so that only the corresponding non-exposed parts remain of the strip. to the desired route for the driver or for the grid. This method is particularly suitable for making free sections in a plane parallel to the plane of the substrate.

 It is also possible to make the conductor or the grid by cutting or stamping a sheet using a tool suitable for reproducing the desired shape and layout. In this case, the production is simplified if the free sections are produced in a plane perpendicular to the plane of the substrate by deformation of the straight part obtained by stamping, insofar as this then avoids having to use a reproducing tool. in a plane parallel to the plane of the sheet of the shapes, for example rounded, desired for the free sections.

 The thicknesses which can be chosen for example for the production of a circuit according to the invention are from 0.4 to 1 mm for the substrate, from 10 to 50 μm for the metallization, from 20 to 100 μm for the soldering or soldering, 0.1 to 1 mm for conductors.

 The section of a free section may advantageously be less than the section of neighboring fixed sections, which allows the traction exerted by the free section on the fixed sections to be better absorbed by the latter.

 In addition, if the free section is produced in a plane parallel to the plane of the substrate, this leads to providing fixed sections that are wider than the free section, which gives for example, if the fixed sections are twice as wide as the free section, a support almost equivalent to that which would have fixed sections twice as long, but considerably reducing the shear stresses and therefore the risks of breaking or detachment of the fixed sections. In addition, this gives better support to the components possibly attached to the fixed sections.

An easily achievable electronic circuit is thus obtained which can be used for power applications and / or in difficult climatic conditions without harming its service life. In addition, the free sections considerably reduce the line resistance and the inductive effect of the conductors, which in particular makes it possible to obtain switching operations with very rapid transition fronts compared to known embodiments during operation of the circuit.
It is also advantageous to choose the dimensions of the conductors, in particular their cross-section, as a function of the power of the circuit and in particular as a function of the current which must flow therein. Electronic circuits can easily be produced for powers ranging from 5 W to 1000 W at 12 V.

 From the design of the electronic circuit according to the invention follows a particularly advantageous possibility of producing from the same substrate a circuit comprising both a control stage whose interconnections between the various components are made directly by metallized tracks of the substrate and a power stage, the interconnections of which between the various components are made by means of a conductor grid according to the invention connected punctually to the substrate and kept spaced from the latter by means of its fixed sections.

 It is thus possible to considerably reduce the size of such a type of circuit and the usual layout constraints of a circuit which prohibits crossings between two tracks not having to be in electrical connection are eliminated, insofar as the power stage is removed from the substrate and nty is connected only occasionally.

According to other advantageous characteristics of the circuit which is the subject of the invention
- this comprises, near the periphery of the substrate, at least one free connection section connected by a first end to a fixed section and projecting from the periphery of the substrate by a second end, said free connection section constituting a clean connector to connect the circuit with the outside
- And the substrate is covered with a cover bonded to its periphery and having openings for passage of the free sections of connection with the outside, the inside cavity of the cover being filled with a circuit protection product, for example silicone.

A particular embodiment of the invention will now be described in more detail which will make it better understand the essential characteristics and the advantages, it being understood however that this embodiment is chosen by way of example and that it is not at all limiting. Its description is illustrated by the accompanying drawings, in which
- Figure 1 shows schematically and in section an electronic circuit according to the invention
- Figure 2 shows a perspective view of the circuit of Figure 1
- Figure 3 schematically shows an exploded perspective view of an electronic circuit according to the invention;
- Figures 4a and 4b schematically show, respectively in elevation and in section along line AA 'of the elevation, a first series of embodiments of a free section of a circuit according to the invention
- Figures Sa and 5b schematically show, respectively in elevation and in section along line BB 'of the elevation, a second series of embodiments of a free section of a circuit according to the invention.

 On all these representations given as examples, the scales are not respected for reasons of clarity.

 The electronic circuit shown in section in FIG. 1 and in perspective in FIG. 2 comprises a substrate 1 made of an electrically insulating material such as alumina and with a thickness of the order of 0.200 μm, for example 635 μm.

 The substrate 1 is metallized into portions of its surface intended to receive by welding fixed sections 3 of a conductor 4. The conductor 4 comprises each time between two fixed sections 3 a free section 5.5 ′ not welded to the substrate and intended in particular to absorb the expansion forces to which the conductor 4 is subjected under the effect of temperature variations due to the climatic conditions of operation, to the manufacturing processes and to the energy dissipated by the Joule effect by the circuit during its operation.

 The metallized portions 6 of the substrate 4 are made of a tinning material such as for example copper, and have a thickness of the order of 15 μm. The solders 2 or solders are produced for example from tin, lead or silver and their thickness is of the order of 50 μm. The thicknesses of the metallized portions 6 and of the welds 2 make it possible, in cooperation with the fixed sections 3 of the conductor 4, to separate the free sections 5.5 ′ from the surface of the substrate, which also makes it possible to increase the capacity of expansion of the conductor 4 and reduce the capacitive effects during the operation of the circuit.

 The conductor 4, for example made of copper, is dimensioned as a function of the power that it must convey during operation and as a function of the extreme temperatures that it must withstand. To this end, the length of each fixed section 3 is chosen so that the shear stress undergone by the solder 2 which binds it to the substrate via a metallized portion 6 is less than its elastic limit and the length of each free section 5.5 ′ is chosen so that the stresses which it undergoes, in particular the stresses of expansion, are less than its elastic limit.

 One can for example provide a length of 1 to 8 mm for the fixed sections and a length of 1 to 20 mm for the free sections.

 The free sections 5.5 ′ are shaped so that their layout has a length greater than the length that would have the layout of a straight line directly connecting the two neighboring fixed sections 3.

 The shape given to the free sections 5.5 ′ in this example is an arc of a circle in a plane perpendicular to the plane of the substrate 1.

 Depending on the constraints to be observed, it can be provided, as in the case of the free section 5 ′, that the section of a free section is less than the section of the two neighboring fixed sections.

 The section of the fixed sections is preferably rectangular so as to ensure good contact of their lower face with the welds 2 and to allow their upper face to serve as a support surface for electronic components 7, for example semiconductors, encapsulated or not and that the latter can be fixed by welding 8 on the upper surface of the fixed sections 3.

We can for example provide that the conductor has a thickness of about 400 t-Lm ~
The conductor 4 has at one end a free section of connection 9 with the outside of the circuit.

This free section 9 is generally of a length greater than the other free sections 5.5 ′ of the circuit in order to extend laterally from the periphery of the substrate 1.

These free sections are shown rectilinear but could also have other shapes.

 For reasons of clarity we have spoken above of a conductor but in practice we prefer to make an interconnection grid comprising several conductors and made in one piece.

Once the circuit has been completed, it can be covered with a cover 10 bearing on the periphery of the substrate 1 and linked to the latter in a sealed manner by bonding 11. The internal cavity formed by the cover 10 and the substrate 1 is preferably filled with a circuit protection product 12 such as silicone
FIG. 3 represents an electronic circuit according to the invention in exploded view so as to better understand the manufacture of such a circuit.

 The elements are designated by the same references as in the previous figures.

 The substrate 1 of the circuit includes, as before, metallized portions 6. These are intended to constitute electrical contact surfaces by means of welds 2 for fixed sections 3 of a grid of conductors 13.

 The fact of making an interconnection grid 13 intended to be attached in one piece to the substrate 1 considerably simplifies the manufacture of the circuit.

 This grid comprises, in addition to the fixed sections 3, free sections 14, 14 ′ electrically connecting two neighboring fixed sections 3 and free sections 9 for connection to the outside of the circuit.

 In this example, the free sections have the shape of an arc of a circle in a plane parallel to the plane of the substrate, which facilitates the machining of the grid without affecting the efficiency of the free sections. The free section 14 ′ shows an embodiment similar to that of the free section 5 ′ in FIG. 1, in which a free section has a smaller section than that of the two neighboring fixed sections.

 Finally, electronic components 7 are added by welding 8 to the upper surface of the fixed sections 3 provided for this purpose.

 Depending on the purpose of the circuit and therefore of the components it comprises, provision may be made for the pins of a component to be welded to other fixed sections than that serving as a support for the component.

 It can also be provided that only the pins of a component are welded to fixed sections and that this component is kept in the air by its pins.

 In addition and in particular in the case of components mounted on the surface, provision may be made for the tabs of a component to be welded on different fixed sections provided at positions allowing the component to be held by its tabs.

 This FIG. 3 clearly shows that in addition to a power stage constituted by the interconnection grid 13, it is possible to provide a control stage produced in the conventional manner directly on the substrate, without thereby increasing the size of the circuit.

 Figures 4a to 5b show different embodiments of the free sections of a circuit according to the invention.

 These forms can be chosen in particular as a function of the technical requirements for making the circuit.

 Figures 4a and 4b show three examples of shapes of free sections in a plane parallel to the substrate 1, Figure 4b being a section of Figure 4a along the line AA '.

 Between fixed sections 3, 3 ′, 3 "linked to the substrate by means of metallized surface portions 6 and of solder 2, it is possible to provide free sections in an arc of a circle 14, in a point 15 or in an accordion 16. This last embodiment making it possible to considerably increase the expansion capacity of the free section 16, while facilitating the connection between two non-aligned fixed sections 3 ′ and 3 ″.

 Figures Sa and 5b show three examples of free section shapes (circular arc 5, point 17 and accordion 18) having shapes similar to those shown in Figures 4a and 4b above but produced in a plane perpendicular to the plane of the substrate , Figure 5b being a section of Figure Sa along the line B-B '.

 These embodiments of the free sections 5, 17, 18, in a plane perpendicular to the plane of the substrate have the advantage of reducing the size of the circuit in the plane of the substrate.

The metallized portions 6 shown in FIGS. 4a to 5b have a surface greater than the lower surfaces of the fixed sections 3, 3 ′, 3 ". This can for example make it possible to ensure connection to a component of a control stage
Of course, the invention is in no way limited by the features which have been specified in the preceding examples or by the details of the particular modes of implementation chosen to illustrate the invention.

All kinds of variations can be made to the operating conditions as well as to the nature and proportions of the constituents and reagents without departing from the scope of the invention.

Claims (16)

 1. Electronic circuit of the type comprising a substrate (1) supporting electrical conductors and / or electronic components in which at least one conductor is capable of ensuring an electrical connection between two points of said circuit, characterized in that said conductor (4) consists of a series of sections formed in a single electrically conductive part comprising at least one section (5) for absorbing free thermal expansion relative to said substrate (1), between two fixed sections (3) connecting to said substrate ( 1).  2. Electronic circuit according to claim 1, characterized in that each fixed section is linked to said substrate (1) by welding (2) over its entire length on a metal portion of said substrate (1).  3. Electronic circuit according to claim 2, characterized in that the fixed sections (3) are dimensioned so that the shear stress undergone by the weld (2) is less than the elastic limit of the latter and in that the sections free (5.5 ′) are dimensioned so that the stresses they undergo, in particular the stresses of expansion, are less than its elastic limit.  4. Electronic circuit according to claim 3, characterized in that the ratio between the length of the free sections <5.5 '; 14.14 ') and the length of the fixed sections (3) is between 1/1 and 1/4.  5. Electronic circuit comprising a plurality of similar conductors suitable for ensuring an electrical connection between different points of the circuit, characterized in that said conductors each comprise at least one free section between two fixed sections in accordance with claims 1 to 4, and in that 'They constitute an interconnection grid (13) in one piece in which at least one of said conductors comprises at least one free section (14,14') between two interconnection points of the grid (13).  6. Electronic circuit according to claim 5, characterized in that each interconnection point corresponds to a point on the circuit and in that said interconnection points are produced in the form of fixed sections (3).  7. Electronic circuit according to any one of claims 1 to 6, characterized in that the length of a free section (5.5 '; 14.14') formed between two fixed sections (3) is such that it represents an elongation of course with respect to a straight line which would connect the two fixed sections (3).  8. Electronic circuit according to claim 7, characterized in that a free section (14,15,16) formed between two fixed sections (3,3 ', 3 ") follows a path in a parallel plane or plane of the substrate ( 1), said parallel plane comprising the two fixed sections (3).  9. Electronic circuit according to claim 7 or 7, characterized in that a free section (5,17,18) formed between two fixed sections (3) follows a path in a plane perpendicular or oblique to the plane of the substrate (1) , said perpendicular or oblique plane comprising the two fixed sections (3).  10. Electronic circuit according to any one of claims 1 to 9, characterized in that a free section (16,18), has an accordion pattern.  11. Electronic circuit according to any one of claims 1 to 10, characterized in that the section of the sections of the conductors is parallelepipedal and preferably rectangular.  12. Electronic circuit according to claim 11, characterized in that the section of a free section is smaller than the section of the fixed sections which it connects.  13. Electronic circuit according to any one of claims 1 to 12, characterized in that it comprises at least one electronic component (7), in particular semiconductor, encapsulated or not, soldered on a fixed section (3).  14. Electronic circuit according to any one of claims 1 to 13, characterized in that the substrate (1) comprises metallized portions (6) intended to receive fixed sections (3) of an interconnection grid (13) of power components (7) and metallized tracks of interconnection of control components placed directly on the substrate.  15. Electronic circuit according to any one of claims 1 to 14, characterized in that it comprises, near the periphery of the substrate (1), at least one free connection section (9) connected by a first end to a fixed section (3) and projecting from the periphery of the substrate (1) by a second end, said free connection section (9) constituting a connector suitable for connecting the circuit with the outside.  16. Electronic circuit according to any one of claims 1 to 15, characterized in that it comprises a cover (10) bonded to the substrate (1) on its periphery and having orifices for passage of the connector connector (9) to the exterior, the interior cavity of said cover (10) being filled with a circuit protection product (12).