Potting compound, method for electrically insulating an electrical or electronic component using the potting compound, electrically insulated component manufactured using such a method and the potting compound

Abstract

Potting compound containing: - 5.0 - 30.0 wt.% reactive particles, comprising a mixture of a) i) magnesium oxide particles with a particle size of no more than 5.0 µm and / or porous magnesium oxide particle agglomerates and ii) crystalline magnesium hydroxide particles, wherein the weight ratio between the magnesium oxide particles and / or porous magnesium oxide particle agglomerates and the magnesium hydroxide particles is in the range of 95:5 to 70:30, as first particles, and b) silicon dioxide particles with a particle size of no more than 0.5 µm and / or silica particles with a particle size of no more than 0.5 µm as second particles, - 45.0 - 90.0 wt.% filler particles with a particle size of more than 1 µm and / or filler fibers, - 5.0 - 20.0 wt.% water.

Description

The present invention relates to a potting compound. It further relates to a method for electrically insulating an electrical or electronic component using the potting compound, an electrically insulated component produced by the method, and a use of the potting compound. State of the art For packaging and protection, as well as for the effective dissipation of waste heat from electrical and electronic components, organically bound potting compounds are used as standard, which are mixed with ceramic fillers to increase thermal conductivity. DE 10 2018 214 641 B4 describes a process for producing a magnesium oxide-based casting compound which, through infiltration with polysiloxane resins, forms a moisture-insensitive composite with an adapted coefficient of thermal expansion. Disclosure of the invention A potting compound is proposed which contains as components at least 5.0 to 30.0 wt.%, preferably 5.0 to 20.0 wt.%, particularly preferably 5.0 to 15.0 wt.%, reactive particles, 45.0 to 90.0 wt.% filler particles and / or filler fibers, and 5.0 to 20.0 wt.% water. These and all subsequent wt.% values ​​refer to 100 wt.% of the total potting compound. The reactive particles contain magnesium oxide particles with a maximum particle size of 5.0 µm and crystalline, in particular non-porous, magnesium hydroxide oxide particles, in particular with a maximum particle size of 5.0 µm, and / or porous magnesium oxide particle agglomerates, in particular with a maximum particle size of 75 µm. These particles are referred to as the first particles. Furthermore, the reactive particles contain silicon dioxide particles, in particular amorphous silicon dioxide particles, with a maximum particle size of 0.5 µm and / or silica particles with a maximum particle size of 0.5 µm. These particles are referred to as the second particles. The reactive particles enable the potting compound to harden through a reaction of the reactive particles with water, forming a thermally stable magnesium silicate hydrate phase. The weight ratio between the first particles and the second particles is preferably in the range of 99:1 to 40:60. More preferably, this weight ratio is between 90:10 and 70:30. The weight ratio between the magnesium oxide particles and / or porous magnesium oxide particle agglomerates on the one hand and the magnesium hydroxide particles on the other hand is in the range of 95:5 to 70:30. Preferably, the weight ratio is between 90:10 and 85:15. It has been found that the processing properties of a potting compound produced according to the method of DE 10 2018 214 641 B4 are significantly influenced by the aging state of the magnesium oxide used in its production. A fresh batch of magnesium oxide exhibits excessive reactivity, rapidly increasing viscosity when mixed with the other components, and significant shrinkage during curing. This results in high shrinkage of the potting compound. Only a crystalline magnesium hydroxide layer, formed by atmospheric humidity on the surface of the magnesium oxide particles after two years of storage at room temperature, reduces the potting compound's reactivity at room temperature. However, this layer cannot be artificially produced on the surface of fresh magnesium oxide within a few hours using a simple process. The presence of magnesium hydroxide crystal nuclei plays a crucial role in the microstructure development of the hardening potting compound. Needle-like growth is assumed, with these crystal needles becoming entangled and hindering volume shrinkage. It is hypothesized that, without these crystal nuclei, an amorphous, gel-like magnesium hydroxide precipitate initially forms from the magnesium ions dissolved by the magnesium oxide particle surfaces. This precipitate only crystallizes towards the end of the curing process, causing volume shrinkage because the entire potting compound matrix is ​​affected. The present method is based on the finding that the crystalline magnesium hydroxide layer present on aged magnesium oxide can be replaced by adding crystalline magnesium hydroxide powder to the formulation with fresh magnesium oxide. This allows for a very low shrinkage of the potting compound (0.1%–0.3%) even when using fresh magnesium oxide, making it suitable for potting electronic components. Porous magnesium oxide particle agglomerates can be produced by calcination. Non-porous magnesium oxide particles can be produced by a melting process. The particle size of all particles can be determined by measuring grain size using a laser granulometer and by measuring primary particle size using scanning electron microscopy (SEM). Filler particles are defined as particles with a particle size, particularly a primary particle size, greater than 1 µm. Materials suitable as filler particles include magnesium oxide, aluminum oxide, silicon, aluminosilicates, silicon carbide, and boron nitride. Other suitable materials include kyanite, rutile, quartz, and fluorspar. Further materials suitable for use as filler particles include inorganic silicates, metal oxides, mixed oxides, spinels, nitrides, carbides, and borides. In addition, surface-oxidized silicon powder and surface-oxidized metal powders, such as copper or aluminum powder, can also be used as filler particles. Carbon, particularly in the form of diamond, can be used as a filler particle. Aluminium oxide, kyanite, quartz, fluorspar, aluminosilicates, surface oxidized silicon powder, surface oxidized silicon carbide powder and mixed oxides (especially ferrites) are particularly preferred. The filler fibers can be, in particular, carbon fibers or carbon nanotubes. Other organic or inorganic fibers, such as aramid fibers or glass fibers, are also suitable as filler fibers. The filler particles and fibers do not react with the water in the potting compound during curing, or at most only superficially. If non-porous magnesium oxide particles with a particle size greater than 5.0 µm are used as fillers, the curing process is interrupted before further hydroxide formation leads to the swelling of the already hardened compound. Therefore, the use of such magnesium oxide particles as fillers is preferably avoided. The high proportion of filler particles that can be contained in the potting compound makes it possible to achieve very high thermal conductivities. Depending on the ratio between reactive particles, filler particles, and water, the potting compound can have a thixotropic to pasty consistency, which impairs its flowability. It is therefore preferred that the potting compound also contains 0.5 to 5.0 wt.% of at least one superplasticizer. Suitable superplasticizers are, in particular, polycarboxylate ethers (PCE), polycondensates, and wetting agents, with polycarboxylate ethers being preferred. Furthermore, the superplasticizer preferably contains carboxyl groups, and especially preferably a proportion of carboxyl groups corresponding to an acid number of at least 20 mg KOH per gram of the superplasticizer. The acidic nature of such superplasticizers reduces the reactivity of fresh magnesium oxide. Furthermore, the potting compound preferably contains at least one defoamer to facilitate its processing. The defoamer is particularly polymer-based. Furthermore, it is preferred that the potting compound also contains 1.0 to 15.0 wt.% of at least one crosslinkable synthetic resin. This synthetic resin is particularly preferably a polysiloxane. If a pore space forms during the drying of the potting compound, its pore channels can be sealed by the synthetic resin, and its pore walls can be coated by the synthetic resin, thus rendering them hydrophobic. The potting compound can be used for the electrical insulation of a component, particularly an electrical or electronic component. Here, "component" also includes parts of assemblies. Specifically, the electronic component could be a circuit with high-performance semiconductors, such as wide-bandgap semiconductors (WBG) on a direct-bonded copper (DBC) substrate. Furthermore, the component could be a passive component, such as an inductor or a transformer coil in a metallic enclosure, for example, in a pot transformer, a heat sink with recesses, or frame modules. Battery cells can also be electrically insulated using the potting compound. In addition to electrical insulation, improved heat dissipation is also achieved. In a process for electrically insulating a component, the component is first encased in the potting compound. The encased component is then heat-treated in a water-saturated atmosphere at a temperature between 50°C and 95°C. The minimum temperature is necessary to initiate a reaction between the reactive particles and the water in the potting compound. The maximum temperature ensures an accelerated setting reaction, while the water-saturated atmosphere acts as a barrier against evaporation. The heat treatment preferably lasts between 30 minutes and 10 hours. Following the heat treatment, the encased component is dried at a temperature preferably no more than 40°C, and particularly preferably in the range of 30°C to 40°C. If drying is carried out at a pressure of less than 100 kPa, it can also be performed at a temperature of less than 30°C.This type of drying prevents undesirable progressive magnesium hydroxide formation, which would lead to an expansion of the hardened potting compound. Drying leads to pore formation in the hardened potting compound. To prevent subsequent re-absorption of water into these pores, it is preferred to seal them with a synthetic resin. If the potting compound itself already contains a synthetic resin, it is preferred that the potted component undergoes further heat treatment at a temperature in the range of 150°C to 200°C after drying. During this process, the synthetic resin melts, and the molten material is drawn into the finest pores with submicrometer diameters by capillary action through the pore space left behind by residual water in the binder matrix. In this way, the open-pored structure closes into a largely dense structure or at least coats the capillary surface with a hydrophobic layer. This prevents capillary transport of polar media such as water or aqueous solutions in the resulting end product. If the crosslinkable synthetic resin is a polysiloxane, it reacts under the specified temperature in the presence of the basic hydroxides of the cured potting compound by crosslinking to form a thermosetting plastic that is thermally stable up to a continuous temperature of 350°C. Sealing the pore space in the hardened potting compound typically leads to a reduction in the water absorption of the potting compound when stored underwater, at least by a factor of 10. Using this method, an electrically insulated component can be produced, which is insulated in particular with a specific resistance of more than 108Ω x cm. Brief description of the drawings Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Fig. 1 shows a flowchart of exemplary embodiments of the method according to the invention. Fig. 2 shows a schematic sectional view of an electrically insulated component according to an exemplary embodiment of the invention. Exemplary embodiments of the invention The sequence of a comparative example of a method according to DE 10 2018 214 641 B4 and an embodiment of the method according to the invention is shown in Fig. 1. Here, a potting compound VB1 (comparative example) or B1 (example according to the invention) is first provided, the composition of which is given in Table 1: Table 1 Table 1 Al2O310-2007840,0052,4840,0052,48 Al2O33-30106,007,866,007,86 Al2O31-207718,3714,0018,37 MgO0-51,08,409,947,408,76 MgOH0-92,01,801,39 SiO20-10,11,601,161,501,09 PCE10,800,810,450,46 PCE20,650.47 H2O25,006,1624,005,83 Siloxane 4,203,224,203,22 In Table 1, d indicates the particle size distribution of the reactive particles and the filler particles, and d50 indicates their mean particle size. A fresh, unaged batch of MgO was used. In addition to the 100% by weight of the potting compound, 0.10 wt.% of a polymer-based defoamer was added to each potting compound. In step 11, an electronic component, which in this exemplary embodiment is a WBG semiconductor on a DBC substrate, is encapsulated with the respective potting compound. In step 12, a first heat treatment takes place in a water-saturated atmosphere in a hardening oven. The treatment temperature is 80°C and the treatment duration is 2 hours. In step 13, the heat-treated, encapsulated component is dried. Drying takes place at a temperature of 80°C. For this purpose, the encapsulated components are each placed in a convection oven. In a second heat treatment step 14, heat treatment takes place in a convection oven at a temperature of 150°C. During this process, the polysiloxane contained in the hardened casting compound is melted, drawn to the walls of the pores by capillary forces, and hardens there by cross-linking. The cured potting compounds treated in this way exhibit a thermal conductivity of 5–10 W / mK. However, while potting compounds produced according to the comparative example show a shrinkage of 0.8–1.0%, those produced according to the inventive example show a shrinkage of only 0.1–0.3%. The shrinkage was determined by heating the potting compound from room temperature to 150°C for 10 hours and then cooling it back to room temperature. The shrinkage corresponds to the irreversible change in length of the potting compound after cooling, relative to the length of the casting mold. The process ends with the production of an electrically insulated component 20, as shown in Fig. 2. This component consists of the electronic component 21 in the form of the WBG semiconductor on a substrate 22 in the form of the DBC substrate. The component 21 is surrounded by the cured potting compound 30.

Claims

Potting compound containing: - 5.0 - 30.0 wt.% reactive particles, comprising a mixture of a) i) magnesium oxide particles with a particle size of no more than 5.0 µm and / or porous magnesium oxide particle agglomerates and ii) crystalline magnesium hydroxide particles, wherein the weight ratio between the magnesium oxide particles and / or porous magnesium oxide particle agglomerates and the magnesium hydroxide particles is in the range of 95:5 to 70:30, as first particles, and b) silicon dioxide particles with a particle size of no more than 0.5 µm and / or silica particles with a particle size of no more than 0.5 µm as second particles, - 45.0 - 90.0 wt.% filler particles with a particle size of more than 1 µm and / or filler fibers, - 5.0 - 20.0 wt.% water. Casting compound according to claim 1, characterized in that the weight ratio between the first particles and the second particles is in the range of 99:1 to 40:

60. Potting compound according to claim 1 or 2, characterized in that the magnesium oxide particles are pore-free with a maximum size of 5µm. A potting compound according to one of the preceding claims, characterized in that the filler particles are selected from the group consisting of aluminium oxide, kyanite, quartz, fluorspar, aluminosilicates, surface oxidized silicon powder, surface oxidized silicon carbide powder, mixed oxides and mixtures thereof. A potting compound according to one of the preceding claims, characterized in that it comprises aluminium oxide particles as filler particles, wherein the proportion of silicon oxide particles and / or silica particles in the reactive particles is at least 8 wt.% based on 100 wt.% of the reactive particles. A potting compound according to one of the preceding claims, characterized in that it further contains 0.5 - 5.0 wt.% of at least one flow agent. Potting compound according to claim 6, characterized in that at least one flow agent contains carboxyl groups. A potting compound according to one of the preceding claims, characterized in that it further contains 1.0 - 15.0 wt.% of at least one crosslinkable synthetic resin. Potting compound according to claim 8, characterized in that the synthetic resin is a polysiloxane. Use of a potting compound according to one of claims 1 to 9 for electrically insulating a component (20) or an electronic component (21) of a component (20). A method for electrically insulating a component (21) comprising the following steps: - potting (11) the component (21) with a potting compound according to one of claims 1 to 9, - heat-treating (12) the potted component (21) at a temperature in the range of 50°C to 95°C in a water-saturated atmosphere, and - drying (13) the potted component (21) to obtain an electrically insulated component (20). Method according to claim 11, characterized in that the potting compound is a potting compound according to one of claims 1 to 7 and the potted component (21) is subjected to pressure infiltration (14) with a crosslinkable synthetic resin or with a solution of a solid synthetic resin in an organic solvent after drying. Method according to claim 11, characterized in that the potting compound is a potting compound according to claim 8 or 9 and the potted component (21) is subjected to a further heat treatment (15) at a temperature in the range of 150°C to 200°C after drying. Electrically insulated component (20) manufactured by a method according to one of claims 11 to 13. Electrically insulated component (20) with a hardened potting compound (30) according to one of claims 1 to 9, in particular for electrically insulating the component (20) or an electronic component (21) of the component (20).

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