ELECTRONIC ASSEMBLY AND PRESSURE MEASURING DEVICE WITH IMPROVED RESISTANCE

Description

FIELD OF INVENTION

[0001] The present invention relates to the field of electronic assemblies and more particularly to the field of electromechanical fluid pressure sensors for aeronautical applications. BACKGROUND OF THE INVENTION

[0002] An electronic assembly typically consists of a substrate onto which an electronic component is attached using a sintered or brazed joint. The assembly provides a mechanical and electrical connection between the electronic component and the substrate. During operation, the assembly is subjected to thermal cycles, and the differential expansion of the substrate and the component can cause stress on the joint and the electronic component, potentially affecting its measurement accuracy and durability.

[0003] The use of a flexible conductive adhesive joint allows for mechanical decoupling of the substrate and the component, reducing the effect of differential expansion on sensor accuracy and long-term performance. However, while the mechanical performance is good, migration of the conductive metallic flakes in the adhesive joint and a significant increase in its electrical resistance are observed over time.

[0004] Thus, existing electronic assemblies do not guarantee a durable mechanical and electrical connection between the component and the substrate while establishing a level of thermomechanical decoupling suitable for the size and sensitivity of the component and the measurement devices.

[0005] Documents US2006 / 0196408 and US2007 / 0013014 describe examples of such electronic assemblies. Document EP1667508 describes a method for assembling components. SUBJECT OF THE INVENTION

[0006] The invention aims to improve the fatigue resistance of an electronic assembly to thermomechanical stresses. SUMMARY OF THE INVENTION

[0007] To this end, a device is provided comprising an electronic assembly with an electronic component mounted on a first substrate and a body defining a cavity. One end of the body is in fluidic contact with a fluid, the electronic component extends within the cavity, and the first substrate includes a portion in contact with a cavity wall. Advantageously, the coefficient of thermal expansion of the first substrate material is lower than that of the electronic component, and the electronic component is assembled using a heat-assisted brazing method on the first substrate. Such a device allows for the attenuation of vibrations and thermomechanical stresses transmitted from the surrounding environment to the electronic component. Securing the assembly to the device is also simplified by this design.

[0008] The device of the invention is particularly suitable for pressure measurement when the electronic component is a pressure sensor.

[0009] Vibrations and thermomechanical stresses are even more strongly attenuated when the substrate is a flexible substrate.

[0010] The extent of the movements of the first substrate is effectively limited when the portion in contact with a wall of the cavity is a curved portion.

[0011] The attenuation of vibrations and thermomechanical stresses transmitted from the surrounding environment to the electronic component is improved when the first substrate exerts an elastic force on the cavity wall.

[0012] Pressure transients and microparticles are effectively filtered when the first substrate includes orifices designed to allow a fraction of the fluid whose pressure is to be measured to pass through.

[0013] The construction of the device is facilitated when the first substrate is connected to a second substrate which carries a communication unit and a communication port.

[0014] Advantageously, the second substrate is a rigid substrate or a flexible substrate.

[0015] Advantageously still, the second substrate extends partly into the cavity. The filtration of the fluid to be measured is improved when the device includes a grid extending in front of the first end of the cavity.

[0016] The construction of the device is also economical when the cavity is defined at least partially by a metal jacket, preferably made of stainless steel.

[0017] The construction of the device is economical when the body is made of thermoplastic material.

[0018] The invention also relates to a method for assembling an electronic component, comprising the steps of, during a preparatory phase, selecting an electronic component and selecting a first substrate on which the electronic component is to be assembled such that the coefficient of thermal expansion of the substrate material is lower than that of the electronic component. The method also includes, during an assembly phase, the step of joining the electronic component and the substrate using a heat-assisted assembly method such as brazing.

[0019] The difference in coefficient of thermal expansion between the component and the first substrate causes, after assembly, a differential contraction of the component and the first substrate, which pre-stresses the first substrate. When the assembly is subjected to temperature cycles, the maximum stress on the bond points between the substrate and the component is reduced by the pre-stress value, thus improving the fatigue resistance of the assembly of the invention.

[0020] The robustness of the assembly is improved when the preparatory phase includes the additional step of selecting a first substrate and a component whose materials both have a glass transition temperature or a melting temperature higher than a service temperature of the electronic assembly.

[0021] Fatigue resistance is further improved when the preparatory phase includes the additional step of selecting a first substrate and an electronic component, as well as an assembly technique that results in a post-assembly stress at the junction points between the substrate and the component of less than 0.1 N / mm².

[0022] Reliable and economical electronic assembly is possible when the heat-assisted assembly method is brazing or sintering or silver sintering or includes the making of connections by gold beads.

[0023] The invention also relates to an electronic assembly obtained by the above process.

[0024] The assembly can be easily integrated into a compact device when the first substrate selected is a flexible substrate.

[0025] Bonding operations between the substrate and the component are facilitated when the first substrate is a textured substrate.

[0026] Fatigue resistance is improved when the first substrate is a liquid crystal polymer-based substrate.

[0027] The assembly process is facilitated when the assembly includes a component assembled with the chip reversed.

[0028] The compactness of the assembly is improved when the assembly includes at least one component with a stack of chips.

[0029] The assembly is effectively protected when it is at least partially covered with a layer of parylene.

[0030] Advantageously, the substrate has a thickness of less than one hundred microns and / or a face of the substrate which includes a portion covered with a layer of metallization less than fifteen microns thick.

[0031] Other features and advantages of the invention will become apparent from the following description of non-limiting embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Reference will be made to the attached figures, among which: there figure 1 is a schematic cross-sectional representation of a first step in the production of an assembly according to the invention; the figure 2 is a schematic cross-sectional representation of a second stage in the assembly of the figure 1 ; there figure 3 is a schematic cross-sectional representation of a third stage in the assembly of the figure 1 ; there figure 4 is a schematic cross-sectional representation of a fourth step in the assembly of the figure 1 ; there figure 5 is a schematic cross-sectional representation of a pressure measuring device according to a first embodiment of the invention; the figure 6 is a schematic cross-sectional representation of a pressure measuring device according to a second embodiment of the invention; the figure 7 is a schematic cross-sectional representation of a pressure measuring device according to a third embodiment of the invention; the figure 8 is a schematic cross-sectional representation of a pressure measuring device according to a fourth embodiment of the invention; the figure 9 is a schematic cross-sectional representation of a pressure measuring device according to a fifth embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] With reference to the figure 1 An electronic assembly according to the invention, and generally designated 1, is made by brazing an electronic component 2 onto a first substrate 3. The assembly is intended to operate at a maximum service temperature of around two hundred degrees Celsius.

[0034] Here, component 2 is a piezoresistive microelectromechanical pressure measurement system made of inorganic material such as Al2O3 ceramic. Such component 2 has a coefficient of thermal expansion approximately equal to 7.10 -6< K -1< and a melting temperature greater than 2000 degrees Celsius. Component 2 is provided with a first electrical connector 2.1 coated with a first tin solder ball 2.10 and a second electrical connector 2.2 coated with a second tin solder ball 2.20.

[0035] Here, the first substrate 3 chosen is a flexible, textured silicon substrate eighty microns thick. The first substrate 3 has a coefficient of thermal expansion approximately equal to 4.10⁻⁶ K⁻¹ and a melting point of 1414 degrees Celsius.

[0036] For the purposes of this application, a substrate is said to be flexible if it can be elastically bent at more than forty-five degrees.

[0037] The first substrate 3 is provided with a third electrical connector 3.1 and a fourth electrical connector 3.2 respectively connected to a first conductive track 4.1 and a second conductive track 4.2 obtained by a selective metallization of the first face 3.3 of the first substrate 3 over a thickness of ten microns.

[0038] Component 2 is placed on the first substrate 3 so that the first connector 2.1 and the second connector 2.2 of component 2 face respectively the third connector 3.1 and the fourth connector 3.2 ( figure 2 ). The first ball 2.10 rests substantially on the third connector 3.1 and the second ball 2.20 rests substantially on the fourth connector 3.2. This assembly technique is also called "flip chip assembly".

[0039] Assembly 1 is then heated (here by induction) and the first connector 2.1 and the third connector 3.1 are then joined using a first tin solder joint 5.1. The second connector 2.2 and the fourth connector 3.1 are joined using a second tin solder joint 5.2 ( figure 3 ).

[0040] During the soldering operations and under the effect of the heat supplied (around 180 degrees Celsius), the first substrate 3, whose material has a coefficient of thermal expansion lower than that of the material of component 2, will expand, just like component 2. Thus, the value ∂3 of the thermal expansion of the first substrate 3 is less than the value ∂2 of the thermal expansion of the material of component 2. The soldering of the first connector 2.1 to the third connector 3.1 as well as the soldering of the second connector 2.2 to the fourth connector 3.1 takes place when component 2 and the first substrate 3 are expanded.

[0041] During the cooling of assembly 1, the greater shrinkage of component 2 with respect to the first substrate 3 creates a prestressing of the first portion 6 of first substrate 3 located between the first braze 5.1 and the second braze 5.2 ( figure 4 ).

[0042] The stresses after assembly in the first brazing 5.1 and the second brazing 5.2 are substantially less than 0.1N / MM 2< .

[0043] A five-micron layer of parylene is then deposited on the assembly thus obtained.

[0044] In operation, when assembly 1 is subjected to temperature variations, the amplitude of the thermomechanical stress cycle undergone by the first braze 5.1 and the second braze 5.2 is reduced by the value of the prestress of portion 6. The fatigue resistance of the first braze 5.1 and the second braze 5.2 is therefore improved.

[0045] We then obtain an assembly 1 in which the first substrate 3 has a coefficient of thermal expansion lower than that of the component 2 to which it is connected.

[0046] With reference to the figure 5 assembly 1 is integrated into a pressure measuring device 10.

[0047] The device 10 comprises a polyamide body 11 in the form of a right cylinder with longitudinal axis Oy. The body 11 delimits a cavity 12 defined by a stainless steel sleeve 13. The sleeve 13 is in the form of a right cylinder with longitudinal axis Oy and has a series 13.1 of lugs projecting radially from the outer surface 13.2 of the sleeve 13. A first end 12.1 of the cavity 12 opens into a first end 11.1 of the body 11 to the outside of the cavity 12 to be in fluidic contact with an external fluid 14 whose pressure is to be measured. A grid 15 extends in front of the first end 12.1 of the cavity 12. The second end 11.2 of the body 11 is closed by a transverse wall 16. The wall 16 is crossed by a first intermediate portion 17.1 of a printed circuit 17 made of epoxy resin, a second portion 17.2 of which extends into the cavity 12 and a third portion 17.3 protrudes from the outside of the body 11.

[0048] The printed circuit board 17 includes a microprocessor 18 mounted on a lower face 17.21 of the second portion 17.2 of the printed circuit board 17, and a communication module 19 mounted on the upper face 17.12 of the first portion 17.1 of the printed circuit board 17. As can be seen in figure 5 The communication module 19 is embedded in the body 11. Assembly 1 is connected to the upper face 17.22 of the second portion 17.2 of the printed circuit board 17, and a first portion 7.1 of the first substrate 3 extends from the second portion 17.2 of the printed circuit board 17 to the conduit 13 such that a first end 7.11 of the portion 7.1 of the first substrate 3 comes into contact with the inner surface 13.3 of the sleeve 13. Depending on its length and / or its inclination with respect to the longitudinal axis Oy, the first end 7.11 of the portion 7.1 of the first substrate 3 exerts a greater or lesser elastic force on the sleeve 12.

[0049] Conductive tracks not shown carried by the printed circuit 17 connect the assembly 1, the microprocessor 18 and the communication unit 19 to a first output port 20. A connector 21 shown in dotted lines allows the device 10 to be connected to a processing unit not shown.

[0050] The flexibility of the first substrate 3 and its particular assembly makes it possible to improve the performance of the device 10 with regard to resistance to vibrations and thermomechanical stresses.

[0051] Elements identical or analogous to those previously described shall bear a numerical reference identical to those in the following description of the second, third, fourth and fifth embodiments.

[0052] With reference to the figure 6 , and according to a second embodiment of the invention, the first portion 7.1 of the first substrate 3 which is in contact with the jacket 12 is curved and exerts an elastic force.

[0053] With reference to the figure 7 and according to a third embodiment of the invention, the first substrate 3 comprises a first curved portion 7.1 which is in contact with the inner surface 13.3 of the sleeve 13 and a second portion 7.2 extending substantially transversely into the cavity 12. The first substrate 3 also comprises a third curved portion 7.3 which is in contact with the inner surface 13.3 of the sleeve 13. Advantageously, the second portion 7.2 comprises ten orifices 8 which are intended to allow the fluid 14 to pass through.

[0054] With reference to the figure 8 , and according to a fourth embodiment of the invention, the device 10 is devoid of a second substrate 17 and the first substrate 3 extends through the wall 16. The first substrate 3 then carries the assembly 1 as well as the microprocessor 18, the module 19 and the output port 20.

[0055] With reference to the figure 9 and according to a fifth embodiment of the invention, the first substrate 3 is shaped to form a spiral with longitudinal axis Oy. The first substrate 3 is connected to the second substrate 17 and the communication port 20 is, according to this fifth embodiment, embedded in the body 11. A pin 22 extends from the communication port 20 through the body 11 to protrude from the body 11 and receive the connector 21 at its end. The spiral shape of the first substrate 3 allows the first portion 7.1 of the first substrate 3 and the third portion 7.3 of the first substrate 3, which are diametrically opposed, to exert an elastic force on the sleeve 13 in the manner of a spiral spring.

[0056] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0057] Especially, Although the electronic assembly is obtained using a process comprising a particular selection of the first substrate / component pair and assembly by heat application, the device 10 can integrate an electronic assembly obtained using a process other than that of the invention such as for example an assembly of a component on a flexible substrate obtained by bonding, or an assembly of a component on a substrate in which the substrate has a coefficient of thermal expansion greater than that of the electronic component; although here the maximum service temperature of the assembly is on the order of two hundred degrees centigrade, the invention also applies to other values ​​of maximum service temperature such as for example service temperatures between zero and two hundred degrees centigrade, negative temperatures or greater than two hundred degrees centigrade;Although here the component is a piezoresistive microelectromechanical pressure measurement system, the invention also applies to other types of components such as, for example, microsystems or nanosystems that may be capacitive or inductive and whose function is to measure pressure, temperature, or to perform a logic, calculation, or communication function; although here the component includes two connectors, the invention also applies to other types of components such as, for example, components comprising a single connector or more than two; although here the substrate has a thickness of eighty microns, the invention also applies to other types of substrate such as, for example, a substrate with a thickness between one and eighty microns, or a thickness greater than eighty microns, preferably less than one hundred microns;Although here the conductive tracks are obtained by selective metallization of a first face of the first substrate to a thickness of ten microns, the invention also applies to other means of electrical connection which may include, for example, conductive wires and / or internal conductive tracks and / or conductive tracks printed or obtained by selective insolation of the substrate, the thickness of the tracks being less than ten microns or greater than eleven microns, ideally less than fifteen microns and the tracks being able to extend over both faces of the substrate; although here the processing unit is a microprocessor, the invention also applies to other processing means such as, for example, a microcontroller, an FPGA or an ASIC type integrated unit (from the English "Application Specific Integrated Unit");Although here the assembly is covered with a five-micron layer of parylene, the invention also applies to other types of protective layers such as, for example, a parylene layer less than five microns or greater than six microns, preferably less than ten microns, a silicone, acrylic or polyurethane layer; although here the device includes a stainless steel sleeve that defines a cavity of the device, the invention also applies to other means of defining a cavity such as, for example, a sleeve partially defining the cavity with the body or a cavity entirely defined by the body of the device and without a sleeve;Although here the liner is made of stainless steel, the invention also applies to liners made of other materials such as, for example, a liner made of non-ferrous metal such as copper or bronze, or a liner made of synthetic material (polyamide for example) or of fiberglass or carbon; although here the body of the device is made of polyamide, the invention also applies to other types of thermoplastic materials such as, for example, low or high density polyethylene and PVC, or thermosetting materials such as formaldehyde or polyester; although here the cavity of the device is in the form of a straight cylinder, the invention also applies to other types of cavities such as, for example, cavities of spherical, conical or any other shape;Although here the second substrate is made of epoxy resin, the invention also applies to other types of second substrate such as, for example, a flexible substrate made of liquid crystal polymers or a rigid substrate made of polyimide; although here the communication module is carried by the upper face of the second substrate while being embedded in the body of the device, the invention also applies to other implantations of the communication module such as, for example, a communication module carried by an underside of the second substrate, a communication module extending totally or partially into the cavity, a module partially embedded in the body, a communication module located outside the body;Although here the pressure measurements are transmitted to an external communication circuit via a wired connector, the invention also applies to other means of communication such as wireless communication such as Bluetooth, Wi-Fi, or radio; although here the component is a simple component, the invention also applies to a component comprising a stack of chips; although here the first substrate comprises ten ports to allow the passage of the fluid whose pressure is to be measured, the invention also applies to a first substrate comprising a different number of ports, such as between one and nine ports or more than ten; well; qu'ici The components and substrate have been selected with regard to their melting temperatures; ideally, the glass transition temperature will be used when the materials are inorganic materials.

Claims

1. A device (10) having both an electronic assembly (1) comprising an electronic component (2) assembled on a first substrate (3), and also a body (11) defining a cavity (12), the electronic component (2) extending in the cavity, wherein the coefficient of thermal expansion of the material of the first substrate (3) is less than that of the electronic component (2), characterized in that a first end (12.1) of the cavity is in fluid flow communication with a fluid (14), the first substrate (3) includes a portion (7.1, 7.3) in contact with a wall of the cavity (12), and the electronic component (2) is assembled on the first substrate (3) by a brazing type assembly method involving the application of heat.

2. A device (10) according to claim 1, wherein the electronic component (2) is a pressure sensor.

3. A device (10) according to claim 1 or claim 2, wherein the first substrate (3) is a flexible substrate.

4. A device (10) according to any preceding claim, wherein the portion (7.1, 7.3) in contact with a wall of the cavity (12) is a curved portion.

5. A device (10) according to any preceding claim, wherein the first substrate (3) exerts a resilient force on the wall of the cavity (12).

6. A device (10) according to any preceding claim, wherein the first substrate (3) includes orifices (8) for passing a fraction of the fluid (14).

7. A device (10) according to any preceding claim, wherein the first substrate (3) is connected to a second substrate (17) that carries a communication unit (19) and a communication port (20).

8. A device (10) according to any preceding claim, wherein the second substrate (17) is a rigid substrate or a flexible substrate.

9. A device (10) according to claim 7 or claim 8, wherein the second substrate (17) extends in part into the cavity (12).

10. A device (10) according to any preceding claim, including a grid (15) extending in front of the first end (12.1) of the cavity (12).

11. A device (10) according to any preceding claim, wherein the cavity is defined, at least in part, by a metal liner (12).

12. A device (10) according to claim 11, wherein the liner (12) is made of stainless steel.

13. A device (10) according to any preceding claim, wherein the body (11) is made of thermoplastic material.

14. A device (10) according to claim 1, wherein the first substrate (3) is a textured substrate.

15. A device (10) according to claim 1 or 14, wherein the first substrate (3) is a substrate based on liquid crystal polymers.

16. A device (10) according to any one of claims 1, 14 and 15, including at least one flip chip assembled component.

17. A device (10) according to any one of claims 1 and 14 to 16, including at least one component comprising a stack chip.

18. A device (10) according to any one of claims 1 and 14 to 17, wherein the electronic assembly (1) is covered, at least in part, by a layer of Parylene.

19. A device (10) according to any one of claims 1 and 14 to 18, wherein the first substrate (3) has a thickness that is less than 100 µm.

20. A device (10) according to any one of claims 1 and 14 to 19, wherein at least one face of the first substrate (3) includes a portion covered in a layer of metallization of thickness less than 15 µm.

21. A method of making an electronic assembly (1) of a device (10) according to any preceding claim, the method comprising the following steps: during a preparatory stage: · selecting an electronic component (2); and · selecting a first substrate (3) on which the electronic component (2) is to be assembled, the first substrate (3) being selected in such a manner that the coefficient of thermal expansion of the material of the first substrate (3) is less than that of the electronic component (2); and the method being characterized in that, during an assembly stage, the method comprises the step of assembling together the electronic component (2) and the first substrate (3) by a brazing type assembly method involving the application of heat.

22. A method according to claim 21, wherein the preparatory stage includes the additional step of selecting an electronic component (2) and a first substrate (3), both made of materials having a glass transition temperature and / or a melting temperature that is higher than a service temperature of the electronic assembly (1).

23. A method of making an electronic assembly (1) according to claim 21 or claim 22, wherein the preparatory stage includes the additional step of selecting an electronic component (2) and a first substrate (3) and also an assembly technique that leads, after assembly, to stress of less than 0.1 N / mm2 in the junction points (5.1; 5.2) between the electronic component (2) and the first substrate (3).