DIELECTRIC COMPOSITE MATERIAL AND POWER MODULE INCLUDING AN ENCAPSULATION LAYER FORMED OF SUCH MATERIAL

Abstract

COMPOSITE DIELECTRIC MATERIAL AND POWER MODULE COMPRISING AN ENCAPSULATION LAYER FORMED OF SUCH MATERIAL. The invention relates to a composite dielectric material comprising a benzocyclobutene (BCB) type thermosetting resin and an inorganic filler, the inorganic filler comprising zirconium tungstate particles. The invention also relates to a power electronic module (1) comprising a substrate (10), a power electronic component (11) disposed on the substrate (10), and at least one encapsulation layer (12) formed of the composite dielectric material and disposed on the substrate (10) and the power electronic component (11). Figure to be published with the abstract: Figure 1

Description

Title of the invention: COMPOSITE DIELECTRIC MATERIAL AND POWER MODULE COMPRISING AN ENCAPSULATION LAYER FORMED FROM SUCH A MATERIAL TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of dielectric materials which can be used as encapsulation material in power electronic modules.

[0002] The present invention relates more particularly to a composite dielectric material comprising a thermosetting resin and an inorganic filler. The invention also relates to an electronic module comprising a power electronic component and at least one encapsulation layer formed from the composite dielectric material. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Power electronic modules allow the implementation of basic functions such as switching, rectification, voltage division, etc.

[0004] A power module generally comprises one or more power electronic components soldered onto a ceramic substrate with metal tracks. The ceramic substrate is fixed onto a copper base plate which serves as a mechanical support and transfers the heat generated by the components to a cooling device, typically a heat sink. The electrical connections inside the power module, between the components and the metal tracks of the substrate, are partly provided by bonding wires. The components are enclosed in a housing filled with an encapsulating material.

[0005] The purpose of the encapsulation material is to ensure the dielectric strength of the power module and to protect the components from external aggressions (humidity, contamination, etc.). In addition, it reinforces the electrical insulation between the conductors and improves the resistance to partial discharges due in particular to defects in the metallizations of the ceramic substrate.

[0006] The encapsulation material generally used in power modules is a crosslinked silicone resin. Silicone resin offers the best performance in terms of flexibility and has a wide operating temperature range, from approximately -45°C to 200°C. Since this polymer material is flexible, it exerts little or no stress on the elements of the power module, such as the connecting wires. On the other hand, silicone resin has little resistance to shock and vibration.

[0007] Power electronic components can be transistors, thyristors and diodes. These components are formed from a semiconductor material. The Wide bandgap semiconductor materials, such as gallium nitride (GaN), gallium arsenide (GaAs), silicon carbide (SiC) and diamond, are tending to replace silicon, due to their better performance in terms of current density, operating frequency and voltage resistance.

[0008] Wide gap semiconductor-based components can also operate at temperatures much higher than those of silicon components, typically of the order of 250°C - 300°C. These high operating temperatures require the choice of an encapsulation material other than silicone resin, and more precisely a material having a glass transition temperature higher than the highest operating temperature (i.e. 250°C, or even 300°C).

[0009] The encapsulation material is then in the glassy state and has a relatively high Young's modulus (of the order of > 1 GPa, compared to > 1 kPa for silicone resin). However, a high Young's modulus can be the cause of significant thermomechanical stresses inside the housing which reduce the lifetime of the power module. Summary of the invention

[0010] There is therefore a need to provide a dielectric material suitable for power electronic modules operating at high temperatures (typically above 250°C).

[0011] According to a first aspect of the invention, this need is tended to be satisfied by providing a composite dielectric material comprising a thermosetting resin of the benzocyclobutene (BCB) type and an inorganic filler comprising zirconium tungstate particles (ZrW2O8).

[0012] The incorporation of zirconium tungstate particles into the BCB resin reduces its coefficient of thermal expansion, favoring its use as an encapsulation material in a power electronic module. Indeed, since the coefficient of thermal expansion of the encapsulation material is closer to those of the other elements of the power module, the thermomechanical stresses are reduced, which is beneficial for the life of the power module. In addition, the BCB resin loaded with zirconium tungstate particles is capable of withstanding temperatures ranging from -70 °C to 300 °C and of being crosslinked over several centimeters of thickness.

[0013] In addition to the characteristics which have just been mentioned in the preceding paragraphs, the composite dielectric material according to the first aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: • the composite dielectric material has a particle charge rate of zirconium tungstate less than 50% by weight, preferably between 1% and 49% by weight; • the composite dielectric material is arranged in a layer with a thickness greater than 100 pm; • zirconium tungstate particles are nanoparticles or microparticles; • the zirconium tungstate particles are functionalized using a silane coupling agent, such as 3-glycidyloxypropyl trimethoxysilane; and • the zirconium tungstate particles are coated with a layer of polymer material, for example polypyrrole.

[0014] A second aspect of the invention relates to an electronic power module comprising: • a substrate: • a power electronic component arranged on the substrate; and • at least one encapsulation layer formed from a composite dielectric material and arranged on the substrate and the power electronic component; the composite dielectric material comprising a crosslinked benzocyclobutene (BCB) resin and an inorganic filler, the inorganic filler comprising zirconium tungstate particles.

[0015] In one embodiment, the electronic module comprises: • a first encapsulation layer formed from the composite dielectric material and arranged on the substrate and the power electronic component, the benzocyclobutene (BCB) type resin in the first encapsulation layer having a first crosslinking rate; • a second encapsulation layer formed from the composite dielectric material and disposed on the first encapsulation layer, the benzocyclobutene (BCB) type resin in the second encapsulation layer having a second crosslinking rate greater than the first crosslinking rate.

[0016] In addition to the characteristics which have just been mentioned in the preceding paragraphs, the electronic module according to the second aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: • benzocyclobutene (BCB) type resin has a crosslinking rate of between 20% and 100%; • the substrate and the power electronic component are completely covered by the composite dielectric material; • the electronic module further comprises a housing containing the substrate and the power electronic component, the housing being filled with the composite dielectric material; • the power electronic component is based on wide-gap semiconductor material, for example based on silicon carbide, gallium nitride, aluminum nitride or diamond; • the power electronic component is an active component, for example a transistor, a diode or a thyristor. BRIEF DESCRIPTION OF THE FIGURES

[0017] Other characteristics and advantages of the invention will emerge clearly from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which: • [Fig.l] schematically represents an electronic power module according to a first embodiment of the invention; • [Fig.2] shows the Fourier transform infrared (FTIR) absorption spectra of a BCB resin for different curing conditions; • [Fig.3] represents the crosslinking rate of the BCB resin as a function of temperature and time; and • [Fig.4] schematically represents an electronic power module according to a second embodiment of the invention.

[0018] For clarity, identical or similar elements are identified by identical reference signs throughout the figures. DETAILED DESCRIPTION

[0019] [Fig.l] is a schematic sectional view of an electronic power module 1 according to a first embodiment.

[0020] The power module 1 comprises a substrate 10 and at least one power electronic component 11 arranged on the substrate 10. Preferably, the substrate 10 is metallized, that is to say that it comprises metal tracks 110 on the surface, and the electronic component 11 is electrically connected to one or more metal tracks 110 of the substrate 10. For example, the electronic component 11 is welded or soldered onto a metal track 110 of the substrate 10.

[0021] The substrate 10 is for example a ceramic substrate metallized by copper (substrate called DCB for “Direct Copper Bonding” in English) or aluminum (DAB, for “Direct Aluminum Bonding”), a brazed ceramic substrate (substrate called AMB, for “Active Metal Brazing”) or an insulated metal substrate (SMI).

[0022] The electronic component 11 may comprise several electrical contact surfaces, also called metallizations (because they are formed of a metal), for example a first contact surface on its lower face and a second contact surface on its upper face. The first contact surface is electrically connected to a first metal track 110 of the substrate 10 (by soldering or welding) and the second contact surface is electrically connected to a second metal track 110, for example by means of a connecting wire 13 (also called bonding wire).

[0023] The electronic component 11 is advantageously an electronic component based on a wide-gap semiconductor material, i.e. a semiconductor material having a bandgap width (EG) much greater than that of silicon, typically greater than 3 eV at a temperature of 300 K (the bandgap width of silicon being 1.1 eV at 300 K). Such an electronic component is capable of reaching a high operating temperature, typically greater than 250°C. The electronic component 11 is preferably an active component, such as a transistor, a diode or a thyristor. Examples of wide-gap semiconductor materials include silicon carbide, gallium nitride, aluminum nitride or diamond.

[0024] In [Fig. 1], a single power electronic component 11 is shown. The power module 1 may, however, comprise several power electronic components 11 (advantageously based on wide-gap semiconductor material) arranged on the substrate 10. These electronic components 11 may be electrically connected to each other (for example by the metal tracks 110 of the substrate 10 and / or by connecting wires 13) or electrically isolated from each other.

[0025] The power module 1 further comprises at least one encapsulation layer 12 arranged on the substrate 10 and the electronic component(s) 11. The encapsulation layer 12 is formed from a composite dielectric material. It advantageously has a thickness greater than 100 μm, preferably greater than 1 mm, for example equal to 30 mm. It ensures the dielectric strength of the power module 1 and protects the electronic component 11 from external aggressions (humidity, contamination, etc.). In addition, it reinforces the electrical insulation between the various conductive elements of the power module 1 (metal tracks 110, connecting wires 13, electrical contact surfaces, etc.) and improves the resistance to partial discharges.

[0026] Preferably, the substrate 10 and the electronic component 11 are entirely covered by the composite dielectric material of the encapsulation layer 12. In other words, the composite dielectric material completely covers the upper face and the sides of the substrate 10 and the electronic component 11. The protection of the substrate 10 and the electronic component 11 is then maximum.

[0027] The power module 1 may also comprise a housing 14 containing the substrate 10 and the power electronic component(s) 11. The housing 14 is filled with the composite dielectric material. Advantageously, the composite dielectric material occupies all the available interior space of the housing 14. The housing 14 is preferably sealed, that is to say it is impermeable to liquids and gases (air, water, etc.).

[0028] The housing 14 preferably comprises a support 141 called a sole, on which the substrate 10 rests, and a cover (or lid) 142 attached to the support 141. The support 141 is preferably metallic, for example made of copper or a copper alloy. The substrate 10 is preferably brazed or welded to the support 141. The cover 142 is fixed to the support 141, for example by means of screws (not visible in the section plane of [Fig.l]).

[0029] The power module 1 may further comprise a cooling device 15, fixed to the support 141 on the side opposite the substrate 10, for example by soldering or welding. The support 141 transfers the heat generated by the power electronic component(s) 11 to the cooling device 15. The cooling device 15 is for example a heat sink.

[0030] Finally, the power module 1 may comprise connection terminals (or connectors) 16, fixed to the metal tracks 110 of the substrate 10 and which extend outside the housing 14.

[0031] The composite dielectric material of the encapsulation layer 12, also called encapsulation material, was obtained by crosslinking a thermosetting resin of the benzocyclobutene (BCB) type into which an inorganic filler in the form of particles was incorporated. Benzocyclobutene (BCB) is a polycyclic aromatic hydrocarbon of formula C8H8, having a benzene ring joined to a cyclobutane ring. A thermosetting resin of the benzocyclobutene type, more simply called BCB resin hereinafter, denotes a prepolymer containing benzocyclobutene.

[0032] The composite dielectric material of the encapsulation layer 12 therefore comprises a crosslinked (or “thermoset”) BCB resin and an inorganic filler in the form of particles, the crosslinked BCB resin forming an organic matrix which coats the particles.

[0033] The crosslinking rate of the BCB resin is advantageously between 20% and 100%. Such a crosslinking rate makes it possible to cover different types of modules. For example, for a module comprising numerous connecting wires, it is preferable to use a more weakly crosslinked resin so as not to generate too many stresses (but to the detriment of the dielectric properties).

[0034] In addition to a glass transition temperature TG greater than 300°C, BCB resins have (in the crosslinked state) interesting properties for the encapsulation of power modules (low dielectric constant, low moisture absorption, very good thermal stability and excellent chemical resistance). They are usually used for microelectronic applications in the form of thin layers (thickness less than 100 μm) obtained by spin-coating.

[0035] However, BCB resins have a coefficient of thermal expansion greater than 40 ppm / °C, which makes them initially incompatible with the application of encapsulating power modules (due to the thermomechanical constraints generated by a difference in coefficient of thermal expansion that is too great with the other elements of the power module 1).

[0036] In order to obtain an encapsulation material having a coefficient of thermal expansion closer to that of the other elements of the power module 1, an inorganic filler comprising zirconium tungstate particles of chemical formula ZrW2O8 is added to the BCB resin. In other words, the BCB resin is "doped" with zirconium tungstate. Zirconium tungstate is a ceramic material characterized by isotropic negative thermal expansion behavior over a wide temperature range. It improves the dimensional stability of the BCB resin by considerably reducing its coefficient of thermal expansion.

[0037] The composite dielectric material advantageously has a ZrW2O8 particle loading rate of less than 50% by weight, preferably between 1% and 49% by weight. In other words, the ZrW2O8 particles represent less than 50% of the weight of the composite dielectric material. Such a loading rate makes it possible to obtain a good compromise between the viscosity (before crosslinking), the mechanical properties (after crosslinking) and the cost of the composite dielectric material. Too high a viscosity would complicate the process of encapsulating the power module.

[0038] For example, the coefficient of thermal expansion is equal to approximately 28 ppm / °C when the filler rate is equal to 20% and to approximately 15 ppm / °C when the filler rate is equal to 45%. For comparison, the coefficient of thermal expansion of the BCB resin alone (zero filler rate) is equal to approximately 45 ppm / °C.

[0039] The zirconium tungstate particles may be microparticles or nanoparticles. A microparticle is a particle whose characteristic size (typically the diameter in the case of a spherical particle) is between 0.1 pm and 100 pm. A nanoparticle is a particle whose characteristic size is between 1 nm and 100 nm.

[0040] For example, Fisher Scientific markets zirconium tungstate in microparticulate form. Cubic ZrW2O8 nanoparticles can be synthesized by the hydrothermal method described in the article [“Carbon Fiber-Reinforced Cyanate Ester / Nano-ZrW2O8 Composites with Tailored Thermal Expansion”, P. Badrinarayanan et al., ACS Appl. Mater. Interfaces, vol. 4, no. 2, pp. 510-517,2012],

[0041] ZrW2O8 nanoparticles can be functionalized using a silane coupling agent, such as 3-glycidyloxypropyl trimethoxysilane, to improve the compatibility of the particles with the organic matrix. This method involves replacing the hydroxyl groups on the surface of the particles with more hydrophobic groups of the O-Si-R type.

[0042] Alternatively, the ZrW2O8 particles may be coated with a layer of polymeric material, for example polypyrrole, in order to improve the dispersion of the particles in the organic matrix. The layer of polymeric material on the surface of the ZrW2O8 particles may be obtained by plasma polymerization (for example of a pyrrole-type monomer).

[0043] The BCB resin used to form the composite dielectric material of the encapsulation layer 12 is advantageously a resin based on 1,3-divinyl-1,1,3,3-tetramethyldisiloxane-bisbenzocyclobutene (DVS-èA-BCB), for example that marketed by the company Dupont under the registered trademark CYCLOTENE™.

[0044] The structure of the DVS-èA-BCB monomer is recalled below:

[0045] [Chem.l]

[0046] CYCLOTENE™ resin, like other commercially available BCB resins, is crosslinked under the action of heat. This thermal crosslinking follows a two-step process, specific to benzocyclobutene (BCB) chemistry: • the four-membered BCB ring opens thermally from 160 °C to produce o-quinodimethane (o-QDM), a highly reactive chemical species also known as o-xylylene; and • o-quinodimethane undergoes a Diels-Alder reaction with a dienophile to form a tetrahydronaphthalene group.

[0047] This two-step process can be illustrated as follows:

[0048] [Chem.2] c-QOM tetrahydronaphtheiene

[0049] In the case of a resin based on DVS-Z?zs-BCB, the dienophile and the BCB are present within the same molecule (the DVS-Z?zs-BCB monomer) thus allowing the crosslinking of the system:

[0050] [Chem.3]

[0051] The crosslinking mechanism of DVS-Z?zs-BCB-based resins and a synthesis technique for the DVS-Z?zs-BCB monomer have been described in detail in [“Benzocyclobutene-based polymers for microelectronic applications.”, Ying-Hung So et al., Polymers for Microelectronics and Nanoelectronics, Chapter 21, pp. 279-293, 2004].

[0052] [Fig.2] shows the evolution of the crosslinking reaction of the CYCLOTENE™ resin, here monitored by Fourier transform infrared spectroscopy (FTIR). We observe the growth of the absorption band at 1500 cm 1 associated with a vibration of the tetrahydronaphthalene groups formed during the reaction and the concomitant decrease of the absorption band at 1475 cm 1 associated with the deformation vibrations of the 4-membered BCB ring.

[0053] Crosslinking of BCB resin is advantageously carried out in the absence of oxygen (< 100 ppm) because the polymer is sensitive to oxidation, particularly at high temperatures. Oxidation occurs mainly at the tetrahydronaphthalene groups formed during the Diels-Alder reaction. The oxidation reaction is clearly visible in [Fig. 2] (dotted curve: annealing at 250 °C in air for 2h), which shows the disappearance of the absorption bands associated with the tetrahydronaphthalene groups, this disappearance being accompanied by the appearance of absorption bands linked to the carbonyl-type groups after 1650 cm1.

[0054] The crosslinking of the BCB resin of the composite dielectric material is therefore advantageously carried out in a chamber under an oxygen-free atmosphere, preferably under an inert atmosphere (for example under N2), or under vacuum.

[0055] Crosslinking can be accomplished by following the following temperature profile (also called firing profile). The heating and cooling rates are set at ± 10°C / min. The temperature is first increased from 25°C to 150°C, then maintained for 15 min at 150°C, then increased again to a so-called firing temperature T of between 190°C and 250°C, then maintained at this firing temperature T for a time t which is a function of the desired crosslinking rate. Finally, the temperature T is decreased. The inert atmosphere is preferably maintained at least until the temperature reaches 150°C during cooling.

[0056] [Fig.3] is a chart giving the crosslinking rate of the BCB resin as a function of the cooking temperature T and the duration t (duration of the plateau at temperature T).

[0057] The BCB resin loaded with ZrW2O8 particles is particularly advantageous for the encapsulation of power modules, because it is capable of withstanding temperatures of the order of 300°C continuously (unlike the silicone resins of the prior art) and of being able to be crosslinked in a layer with a thickness greater than 100 μm (unlike the BCB resins devoid of ZrW2O8 particles), and even in a layer several centimeters thick. Furthermore, its coefficient of thermal expansion is variable depending on the ZrW2O8 particle loading rate and its hardness is variable depending on the crosslinking rate. It therefore offers great flexibility of use.

[0058] In the first embodiment shown in [Fig. 1], the power module 1 comprises a single encapsulation layer 12 formed from the composite dielectric material (crosslinked BCB resin loaded with ZrW2O8 particles). This encapsulation layer 12 preferably occupies the entire available interior volume of the housing 14.

[0059] In the second embodiment shown in [Fig. 4], the power module 1 comprises a stack of several encapsulation layers 12a-12c formed from the composite dielectric material. The different encapsulation layers 12a-12c of the stack have different crosslinking rates, and more particularly increasing rates moving away from the substrate 10 (from bottom to top in the figure).

[0060] Thus, the power module 1 comprises (at least): • a first encapsulation layer 12a arranged on the substrate 10 and the electronic component 11 and having a first crosslinking rate; and • a second encapsulation layer 12b arranged on the first encapsulation layer 12a and having a second crosslinking rate greater than the first crosslinking rate.

[0061] The first crosslinking rate is for example between 20% and 49%. The second crosslinking rate is for example between 50% and 74%.

[0062] As illustrated in [Fig. 4], the power module 1 may also comprise a third encapsulation layer 12c arranged on the second encapsulation layer 12b and having a third crosslinking rate greater than the second crosslinking rate. The third crosslinking rate is for example between 75% and 100%.

[0063] The stack of encapsulation layers preferably occupies the entire available interior volume of the housing 14.

[0064] The first encapsulation layer 12a of the stack, in contact with the substrate 10 and the electronic component 11, is weakly crosslinked (20% - 49%) in order to limit the mechanical stresses on the electronic component 11 and possibly the connecting wires 13, for example by using a short cooking time t (see [Fig. 3]). Thus, cyclobutene and vinyl functions remain present to react with the second encapsulation layer 12b, which is more crosslinked (and so on, if applicable). The last encapsulation layer of the stack (the furthest from the substrate 10, i.e. the second encapsulation layer 12b in the case of a two-layer stack or the third encapsulation layer 12c in the case of the three-layer stack illustrated by [Fig.4]) is highly crosslinked (75% - 100%), in order to provide a certain hardness which protects the power 1 module against external forces.

Claims

Claims

1. A composite dielectric material comprising a benzocyclobutene (BCB) type thermosetting resin and an inorganic filler, characterized in that the inorganic filler comprises zirconium tung state particles.

2. Composite dielectric material according to claim 1, having a zirconium tungstate particle loading rate of less than 50% by weight, preferably between 1% and 49% by weight.

3. Composite dielectric material according to one of claims 1 and 2, arranged in a layer with a thickness greater than 100 pm.

4. A composite dielectric material according to any one of claims 1 to 3, wherein the benzocyclobutene (BCB) type thermosetting resin is a 1,3-divinyl-1,1,3,3-tetramethyldisiloxane-bisbenzocyclobutene (DVS-bis-BCB) based resin.

5. A composite dielectric material according to any one of claims 1 to 4, wherein the zirconium tungstate particles are functionalized using a silane coupling agent, such as 3-glycidyloxypropyl trimethoxysilane.

6. A composite dielectric material according to any one of claims 1 to 4, wherein the zirconium tungstate particles are coated with a layer of polymeric material, for example polypyrrole.

7. A power electronic module (1) comprising: - a substrate (10); - a power electronic component (11) arranged on the substrate (10); and - at least one encapsulation layer (12) formed of a composite dielectric material and arranged on the substrate (10) and the power electronic component (11); the composite dielectric material comprising a crosslinked benzocyclobutene (BCB) type resin and an inorganic filler, the inorganic filler comprising zirconium tungstate particles.

8. The electronic module (1) according to claim 7, wherein the benzocyclobutene (BCB) type resin has a crosslinking rate between 20% and 100%.

9. Electronic module (1) according to one of claims 7 and 8, comprising: - a first encapsulation layer (12a) formed from the composite dielectric material and arranged on the substrate (10) and the power electronic component (11), the benzocyclobutene (BCB) type resin in the first encapsulation layer (12a) having a first crosslinking rate; - a second encapsulation layer (12b) formed from the composite dielectric material and arranged on the first encapsulation layer (12a), the benzocyclobutene (BCB) type resin in the second encapsulation layer (12b) having a second crosslinking rate higher than the first crosslinking rate.

10. Electronic module (1) according to any one of claims 7 to 9, wherein the substrate (10) and the power electronic component (11) are entirely covered by the composite dielectric material.

11. Electronic module (1) according to any one of claims 7 to 10, further comprising a housing (14) enclosing the substrate (10) and the power electronic component (11), the housing (14) being filled with the composite dielectric material.

12. Electronic module (1) according to any one of claims 7 to 11, in which the power electronic component (11) is based on wide gap semiconductor material, for example based on silicon carbide, gallium nitride, aluminum nitride or diamond.

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