TECHNICAL FIELD

The present invention relates to a power semiconductor device comprising an encapsulant. Additionally, the present disclosure relates to a power semiconductor device comprising an encapsulant as a packaging or insulation material. The present disclosure further relates to use of the encapsulant as a packaging or insulation material in a power semiconductor device, such as an IGBT or a device including at least one IGBT. Furthermore, the present disclosure relates to a method of encapsulating electrical components in a power semiconductor device.

BACKGROUND ART

Power semiconductor device is a semiconductor device which may be used as a switch or converter/inverter in power electronics, for example as power converters/inverters for various applications, such as, industrial drives, variable speed drives, power supplies, power quality, renewable energies, solar energy, battery backup systems (UPS), electrical vehicles, wind turbines, traction converters, DC (direct current) transmission/distribution systems, FACTS (flexible AC transmission systems), multi-level inverters, frequency converters, and pulse- power applications, such as thyratron replacement. The power semiconductor applications may include an insulated-gate bipolar transistor (IGBT) and/or integrated gate-commutated thyristor (IGCT) devices. These devices normally have an encapsulated structure in which a solid-like insulation material can be used as an encapsulant.

Silicone gel is currently commonly used as an encapsulant in an IGBT module package, i.e. the IGBT device. Silicone gel has a relative high fire point, i.e. the lowest temperature at which the vapour of a silicone oil in the gel will continue to burn for at least 5 seconds after ignition by an open flame. When the open flame, i.e. source of the fire is removed, the gel is generally considered to be self-extinguishable. However, silicone gels comprise silicone oils which contain methypolysiloxanes, which can generate formaldehyde at temperatures over 149 °C. This is undesirable from an environmental point of view, since formaldehyde can be a skin and respiratory sensitizer, eye and throat irritant, and is believed to be a potential cancer hazard. Therefore, the operation temperature should be limited below the formaldehyde-generating temperature. This may be problematic in power semiconductor devices, especially in view that the operation temperatures are often above 150 °C and there is a desire to further increase the operation temperature up to/above 175 °C. Also, the price of a silicone product is much (by up to 10 times) higher than that of a mineral oil based product, a silicone oil takes more saturation water than a mineral oil at ambient condition, and the dielectric strength of the DOW silicone gels is about 22 kV/mm which is much lower than that of 40 kV/mm of a mineral oil.

Therefore, despite previous attempts, there remains a need for an improved encapsulant in power semiconductor applications.

SUMMARY OF THE INVENTION

The present disclosure identifies several shortcomings in the prior art encapsulants used in power semiconductor devices. There is a desire to provide a power semiconductor device comprising an encapsulant that is fire safe, environmentally friendly and has a lower material cost than commonly used silicone-based gels. Also, in some cases, there is a desire to increase the operational temperatures of the power semiconductor device. At the same time, it is desirable that the dielectric properties are maintained or even improved.

Thus, it is an objective of the present invention to provide a power semiconductor device that comprises an encapsulant as a packaging or insulation material, which encapsulant is fire safe, environmentally friendly and has a lower material cost than commonly used silicone-based gels.

It is a further objective to provide a power semiconductor device in which an increase of the operational temperature is possible.

Furthermore, it is an objective to provide a power semiconductor device having an

encapsulant with dielectric properties that are at least equal but preferably improved compared to the existing encapsulants. These objects are achieved by the present power semiconductor device as defined in the appended claims. The encapsulant used in the power semiconductor device is a dielectric gel and the gel comprises an electrical insulation oil in an amount of 50 % by weight or more, one or more thickeners and optionally an additive. The thickeners render the dielectric gel thermo- reversible, wherein the gel has a certain gelling temperature, which is referred to as so-called knee temperature. At a temperature higher than the knee temperature the gel is in a liquid like state, whereby the gel is easy to manufacture and mix. At a temperature below the knee temperature, the viscosity values of the gel are suitably high and thus the gel is in a solid-like gel state. When the gel is in its solid-like state, it has a good stability. Thus, power

semiconductor devices, in which the gel is used as an encapsulant, can be operated in a reliable & safe way.

The thickeners may comprise one or more tri-block copolymer thickeners and one or more di block copolymer thickeners. Such thickeners are suitable for use in power semiconductor devices.

The tri-block copolymer thickener may be selected from polystyrene-block-poly(ethylene- ethylene/propylene)-block-polystyrene (SEEPS), polystyrene-block-poly(ethylene/butylene)- block-polystyrene (SEBS), polystyrene-block-poly(ethylene/propylene)-block-polystyrene (SEPS), enhanced rubber segment (ERS) polymers, S-EB/S-S (styrene- ethylene/butylene/styrene-styrene), and mixtures thereof.

The di-block copolymer thickener may be polystyrene-block-poly(ethylene/propylene), SEP. By the use of these thickeners, the viscosity and knee-temperatures may be varied to be suitable for the aimed purpose.

The molecular weight of the di-block and tri-block copolymer thickeners may be from at least 10 kDa or from at least 1000 kDa. By varying the molecular weight, the knee temperature can be controlled. The higher the molecular weight of the thickeners, the higher the knee temperature will be. Suitably, the molecular weight of the di-block and tri-block copolymer thickeners may be from about 10 kDa to about 1000 kDa. The dielectric gel may comprise the thickener in an amount from 0.001 to 50 % by weight or from 0.1 to 10 % by weight. The values of gel viscosity are controllable by controlling the content of the thickeners.

The additive may comprise an antioxidant. The antioxidant may comprise or consist of an antioxidant selected from 2, 6-Di-tert-butyl-4-methylphenol (DBPC (C15H24O)) and Octadecyl 3- (3,5-di-tert-butyl-4-hydroxyphenyl)propionate (Irganox L 107 (C35H62O3)) . The gel may comprise the antioxidant in an amount of less than 10 % by weight. By using antioxidants, the oxidation stability of a gel system is further enhanced, and thus the gel system will have a long reliable service lifetime.

The dielectric gel may further comprise an additive selected from boron nitride, hexagonal boron nitride (h-BN), the nano structures of Fe ₃ C> ₄ , Fe2C>3, ZnO, AI2O3, SiC>2, CeC>2, T1O2, MgO, BaTiC>3, CaCu3TUOi2 (CCTO), and Bao.85Cao.15Zro.1Tio.9O3 (BCZT). Such inorganic additives may increase the thermal and dielectric performances of the gel. The dielectric gel may comprise the inorganic additive in an amount of less than 50 % by weight, whereby thermal and dielectric performances can be enhanced depending on the application, where the dielectric gel is used as an encapsulant.

The electrical insulation oil may be an iso-paraffinic oil or a hydrocarbon based oil having a flashpoint of 190 °C or more. The flashpoint may be measured according to any known method, for example according to IEC 60695, IEC 60707, UL94, ASTM D92, D93A, ISO 2719 or EN ISO 2592:2000. Thus, the working temperature of an application using the dielectric gel can be more than 150 °C.

The dielectric gel may further comprise a tackifier selected from hydrocarbon resins, rosin esters, or polyterpenes. The tackifier may be present in an amount of less than 50% by weight, preferably between 0.1 % and 10 % by weight. The tackifier may be used to improve tack and peel adhesion of the encapsulant composition.

The dielectric gel may have a gelling temperature of at least 60 °C, at least 100 °C, at least 120 °C, at least 150 °C, or at least 175 °C, whereby it is usable in many applications. The present invention also relates to use of the encapsulant as described above as a packaging or insulation material in a power semiconductor device, such as an IGBT or a device including at least one IGBT.

Additionally, the present invention relates to a method of encapsulating electrical components in a power semiconductor device comprising: providing an encapsulant by mixing an electrical insulation oil in an amount of 50 % by weight or more, one or more thickeners and optionally an additive at a temperature exceeding a knee temperature of the encapsulant, and injecting the encapsulant at a temperature above the knee temperature, in which the encapsulant is in a liquid-like state, inside a device body of the power semiconductor device comprising the electrical components, and cooling the power semiconductor device to a temperature below the knee temperature such that the encapsulant reaches a solid-like gel state.

Further objects, advantages and novel features of the present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a full understanding of the present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings.

Fig. 1 shows an example of an IGBT device from a partially cut side view comprising the present encapsulant;

Fig. 2 shows another example of an IGBT device from a partially cut side view comprising the present encapsulant;

Fig. 3 shows the dynamic viscosity of tested dielectric gel compositions as a function of temperature.

DETAILED DESCRIPTION

In this disclosure encapsulants in the form of dielectric gels are disclosed. The dielectric gels may be fire safe, environmentally friendly and have a lower material cost than commonly used silicone-based gels.

The dielectric gels may have at least equal or enhanced dielectric properties compared to the known dielectric gels, e.g. the silicone oil based gels. Such dielectric properties may include breakdown strength and permittivity. The present dielectric gels may also have a better healing effect, have less moisture uptake, and may be partial discharge (PD) free. For example, the thermal expansion coefficient of a silicone oil is higher than that of a mineral oil, such as by up to 40 % higher, which may affect the thermal properties of gels, a higher thermal expansion coefficient is an unfavourable feature because the volume increase will become larger for a gel at a higher temperature. The permittivity of the silicone oil can be over 27 % higher than that of the mineral oil. In combination with the moisture uptake issue, a silicone oil based product will generate more power losses than a mineral oil based product.

The encapsulant in the form of a dielectric gel of the present disclosure may be used alone as an encapsulant, i.e. without being carried by a carrier, e.g. a lapped impregnated solid insulation system. The dielectric gel of the present disclosure has an advantage in that it can be filled into corners and micro structures of complex structures, such as IGBT and IGCT product structures. Impregnated lapped solid system would not work for example in the IGBT and IGCT product, since there is difficulty in filling a gel impregnated solid insulation system into each corner and micro locations in an IGBT product.

The present dielectric gel differs from a silicone gel which may be cross-linked. The present encapsulant in the form of a dielectric gel is a thermo-reversible gel. An electrical insulation oil is converted into a gel by adding a thickener, which may be a block copolymer, into the insulation oil. By mixing the insulation oil and the thickener, a dielectric gel composition can be obtained in an easy, simple, and effective way.

The gel has a certain gelling temperature, which is also referred to as a knee temperature, in the viscosity of the gel. At a temperature higher than the knee temperature the gel is in a liquid-like state. The knee/gelling temperature may be gelling temperature of at least 60 °C, at least 100 °C, at least 120 °C, at least 150 °C, or at least 175 °C. The liquid-like state is

advantageous for the processing and healing effect. At a temperature below the knee temperature, the viscosity values of the gel are suitably high and thus the gel is in a solid-like gel state. In the solid-like state the gel features enhanced dielectric properties, mechanical properties and transportation benefit. The gels of the present disclosure are low cost and environmentally friendly. The gels do not contaminate to the air and the soil, especially when in the solid state. The gel may be in the solid-like state at desired working temperatures of the power semiconductor devices, such as the IGBT and IGCT. The gel may thus have an upper working temperature limit of up to about 200 °C, such as about 180 °C, about 160 °C, about 120 °C, about 100 °C, about 80 °C or about 60 °C, which may be the lowest of the upper working temperature limits. Thus, the upper working temperature limit may substantially correspond to the knee temperature of the encapsulant. Below the upper working

temperature limit, the viscosity may be more than 10 Pa-s and at a temperature above the upper working limit temperature the viscosity value may be considerably lower than 1 Pa-s.

After reaching the upper working temperature limit, there is a transition from high to low viscosity. The transition temperature is defined as the temperature at which the gel passes from the high viscosity regime to the transitionary viscosity regime.

The dielectric gel of the present invention comprises or consists of an electrical insulation oil as a base oil and the thickener. The electrical insulation oil may be a mineral oil. Alternatively, the electrical insulation oil may be an iso-paraffinic oil or a hydrocarbon based oil, or a mixture thereof. The iso-paraffinic oil or a hydrocarbon based oil may have a flashpoint of 190 °C or more, and may be measured according to IEC 60695, IEC 60707, UL94, ASTM D92, D93A, ISO 2719 or EN ISO 2592:2000. In this way, a fire-safe encapsulant can be provided and working temperatures of the application can be increased in a safe way. Such insulation oils are available in the market and examples of them are e.g. for iso-paraffinic oils trade name Shell Diala S4 ZX-I, for mineral oils trade names NS 100, which is a hydrotreated naphthenic oil and Nytro 10XN ^® by Nynas and for hydrocarbon based oils, trade name Beta fluid by DSI ventures Inc. By

hydrocarbon-based oil is meant that the base oil in these oils consists of 100 % hydrocarbon.

The amount of the electrical insulation oil in the gel composition is at least 50 % by weight, but less than 100 % by weight, since the composition also comprises the thickener. The amount may be thus from 50 % to 99.999 % by weight, preferably from 50 % to 90 % by weight.

It should be noted that the total amount of the electrical insulation oil (base oil), one or more thickeners and the optional additives sums up to 100 % by weight. This means that if a high amount of additives is used, the amount of the base oil is lowered correspondingly, but the amount of the base oil is always at least 50 % by weight.

The thickener in the dielectric gel composition may comprise or consist of styrenic block copolymers having a function as thickeners. The thickeners may suitably comprise or consist of one or more tri-block copolymer thickeners and/or one or more di-block copolymer thickeners. The molecular weight of the di-block and tri-block copolymer thickeners may be at least about 10 kDa, or at least about 1000 kDa. Suitably, the molecular weight of the di-block and tri-block copolymer thickeners may be from about 10 kDa to about 1000 kDa.

The tri-block copolymer thickener may comprise, essentially consist of, or consist of polystyrene-b-poly(ethylene-ethylene/propylene)-b-polystyren e, for example as sold under the name Septon SEEPS 4099. Alternatives to SEEPS 4099 include, but are not limited to SEEPS 4077, SEEPS 4055, SEEPS 4044, SEEPS 4033, SEBS 8006 (polystyrene-b- poly(ethylene/butylene)-b-polystyrene), SEBS 8004, SEBS 8007, SEBS 8076, SEPS 2006

(polystyrene-b-poly(ethylene/propylene)-b-polystyrene), SEPS 2104, SEPS 2005, Kraton A1535 (S-EB/S-S polymer) or A1536, Kraton G1633 (SEBS polymer) or G1651, and G1641 (ERS polymer) or G1640.

The di-block copolymer thickener may comprise, essentially consist of, or consist of polystyrene-b-poly(ethylene/propylene), for example as sold under the name Septon SEP 1020. Alternatives to SEP 1020 include, but are not limited to Septon SEP 1001 and Kraton G1701 and G1702 (SEP).

The dielectric gel compositions may comprise from 0.001 to 50 %, and preferably from 0.1 % to 10 % by weight the thickener. For example, the dielectric gel may comprise from about 0.1 % to about 10 % by weight tri-block copolymer thickener, such as from about 0.5 % to about 8 % by weight tri-block copolymer thickener, from about 1 % to about 5 % by weight tri-block copolymer thickener or from about 2 % to about 5 % by weight tri-block copolymer thickener.

The gel can be obtained with the mixture of the base oil and the additives at a suitable temperature, which is higher than the knee temperature, such as between 60 °C and 200 °C. The knee temperature can be controlled by using different types of dielectric insulation oils, alternatively by using thickeners with different molecular weights. The higher the molecular weight of the thickeners, the higher the knee temperature will be. The values of gel viscosity are also controllable for example by controlling the content of the thickeners.

The gel of the present disclosure may comprise, essentially consist of, or consist of a mineral oil, one or more tri-block copolymer thickeners and/or one or more di-block copolymer thickeners. The oil may for example be a hydrotreated naphthenic mineral oil, such as NS100 or Nytro 10XN, although alternative mineral oils with similar characteristics may be chosen.

Alternatively, the gel of the present disclosure may comprise, essentially consist of, or consist of an iso-paraffinic oil, one or more tri-block copolymer thickeners and/or one or more di block copolymer thickeners. By using iso-paraffinic oil as a base oil, the knee temperature may be higher and thus a working temperature of the desired power semiconductor application. The oil may for example be Shell Diala S4 ZX-I, although alternative iso-paraffinic oils with similar characteristics may be chosen.

Alternatively, the gel of the present disclosure may comprise, essentially consist of, or consist of a hydrocarbon based oil, one or more tri-block copolymer thickeners and/or one or more di-block copolymer thickeners. By using the hydrocarbon based oil as a base oil, the knee temperature may be higher and thus a working temperature of the desired power

semiconductor application. The oil may for example be Beta fluid by DSI ventures Inc., although alternative hydrocarbon-based oils with similar characteristics may be chosen.

The gel compositions may alternatively or additionally comprise from about 0.1 % to about 10 % by weight di-block copolymer thickener, such as from about 0.5 % to about 8 % by weight di-block copolymer thickener, from about 1 % to about 5 % by weight di-block copolymer thickener, from about 2 % to about 5 % by weight di-block copolymer thickener.

The dielectric gel of the present invention may optionally comprise an additive. The additive may comprise an antioxidant. Examples of antioxidants include, 2, 6-Di-tert-butyl-4- methylphenol, i.e. DBPC (C15H24O) or Octadecyl 3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate, e.g. Irganox L 107 ^® (C35H62O3) . The amount of the antioxidant may be up to 10 % by weight, preferably up to 1.0 % by weight.

The additive may alternatively or additionally comprise an inorganic additive, for example, boron nitride (BN), hexagonal boron nitride (h-BN), nano structures of Fe3C>4, Fe2C>3, ZnO, AI2O3, S1O2, Ce0 ₂ , T1O2, MgO, BaTi0 ₃ , CaCu ₃ TUOi ₂ (CCTO), a nd Bao.85Cao.15Zro.1Tio.9O3 (BCZT).

The inorganic additives are used to increase the thermal and dielectric performances of the gel. The amount of the inorganic additive may be up to 50 % by weight, preferably less than 30 % by weight.

Further, the additive may alternatively or additionally comprise a tackifier. The tackifiers may be included in the formulation to improve tack and peel adhesion. The tackifiers may be for example hydrocarbon resins, rosin esters, or polyterpenes. The tackifier may be included in an amount between 0.001 % and 50 % by weight, preferably between 0.1 % and 10 % by weight.

The present invention also relates to a method of encapsulating electrical components in a power semiconductor device. The method comprises providing an encapsulant by mixing an electrical insulation oil in an amount of 50 % by weight or more, one or more thickeners and optionally an additive at a temperature exceeding a knee temperature of the encapsulant.

Thus, the encapsulant will be in a liquid-like state and may have a viscosity of less than 1 Pa-s, whereby the mixture can be easily processed and mixed. The method comprises injecting the encapsulant at a temperature above the knee temperature, in which the encapsulant is in a liquid-like state, inside a device body of the power semiconductor device comprising the electrical components. Since the encapsulant is in a liquid like state, it can effectively fill all structures and microstructures inside the device body, which is an advantage. The method additionally comprises cooling the power semiconductor device to a temperature below the knee temperature such that the encapsulant reaches a solid-like gel state. In the solid-like state, the viscosity may be more than 10 Pa-s. The solid-like state provides stable operating conditions for the semiconductor device.

Table 1 below summarises the contents of the encapsulants in the form of a dielectric gel according to the present disclosure. Table 1. Examples of materials used in the proposed dielectric gels for the application in IGBT & IGCT as encapsulant.

There are several advantages obtained by the invented dielectric gels described above, and as further evidenced by the appended examples. Higher dielectric performance can be obtained, such as, having higher electrical breakdown strength. Higher electrical breakdown strength may lead to increased reliability or a compact design of an IGBT or IGCT device, in which the dielectric gel of the present invention is used as encapsulant.

Additionally, lower manufacturing cost may be obtained since the material costs of mineral oil based gels are much (by up to 10 times) lower than that of a silicone gel product.

Further, there is less moisture uptake compared to silicone oil based encapsulants. The water saturation at ambient condition is typically less than 55 ppm, while for a silicone oil it has much higher moisture uptake with a typical value of 220 ppm at ambient condition.

More environmentally friendly performance is also obtained, since the invented gels do not release any cancer related hazardous chemicals, and have no contamination to the air and the soil. Additionally, the invented gels are easily recycled and have better bio-degradable feature than a silicone gel.

The present dielectric gels can be partial discharge (PD) free due to the thermo-reversible nature, which has a better healing effect as well.

Furthermore, if a fire resistant oil, e.g. the Beta fluid by DSI ventures Inc. is selected, the invented gel has a feature of high fire point, such as higher than B00 °C. Therefore, the gel thus obtains a fire safe formulation and is less flammable, which is about the same as a silicone product.

Reference is now made to the appended Fig. 1 and Fig. 2, in which two examples of a power semiconductor device are shown. The device in both drawings illustrates examples of an IGBT, in which the encapsulant described above is used. The encapsulant is used as a packaging or insulation material. Generally, IGBT devices may be used in for example transition systems, transmission and distribution systems, renewable energy systems, e.g. wind and solar energy systems, industrial drives, and frequency converters.

The IGBT device shown in Fig. 1 comprises a device body 10 comprising a base plate 11 and a housing 13 comprising an upper wall 15 and a side wall 17, which surround and house electrical components, which are generally depicted by a reference sign 50, inside the device body 10. The electrical components may comprise for example semi-conducting chips, power connecting means, etc., which are obvious for the skilled person. Two power connectors PC extend through an upper wall 15 of the device body housing 13. The electrical components 50 inside the device body 10 are assembled on a support plate 40, which may be a ceramic plate, which is arranged in connection with the base plate 11, which, may be made of for example aluminum silicon carbide. The base plate 11 is attached to the side wall or side walls of the device body housing 13. The space 20 within the device body is filled with the encapsulant 30 of the present disclosure.

In Fig. 2 another example of application in IGBT is shown. The IGBT of the shown type may be used for example in HVDC (high voltage direct current) and FACTS (Flexible AC Transmission Systems). The IGBT comprises a device body 110 comprising a base plate 111 and a housing 113 comprising an upper wall 115 and an inner side wall 118, also referred to as a polymeric sub- module frame, which is connected to the base plate 111. The upper wall 115 may be of copper. The device body 110 further comprises an outer side wall 117, also referred to as a module outer frame, which can be fibreglass reinforced polymeric material. The outer side wall 117 surrounds the inner side wall 118 and the inner and outer side walls 118, 117 together with the upper wall 115 and the base plate 111 house electrical components, which are generally depicted by a reference sign 150, inside the device body 110. The electrical components comprise semi conducting chips 151, 152, and 153, spring washers 154, 155 and 156 and current bypasses 154a, 155a, 156a in connection with the respective spring washer. Further electrical components are obvious for the skilled person. The semi-conducting chips 151, 152, and 153 inside the device body 110 are assembled on the base plate 111. The base plate 111 is attached to the inner side wall 118 (or side walls 118) of the device body housing 113. The spaces 121, 122, 123 and 124 between the spring washers and/or between the inner side walls 118 and the spring washers within the device body 110 are filled with the encapsulant of the present disclosure.

The examples of Fig. 1 and 2 are only to be considered as examples of the present power semiconductor device structures, and are not exhaustive for the applications in which the encapsulant can be used.

Example 1

Gel compositions

The following dielectric gel compositions were prepared: Mineral oil based gel 1, "Test B002": Nytro 10XN + 1 wt. % SEPTON SEEPS 4099;

Mineral oil based gel 2, "Test 371": NS 100 + 1.0 wt.% SEPTON SEEPS 4099;

Iso-paraffinic oil based gel 1, "Test B007": Shell Diala S4 ZX-I + 1.0 wt.% SEEPS 4099; Iso-paraffinic oil based gel 2, "Test B011": Shell Diala S4 ZX-I + 2.0 wt.% SEEPS 4099;

Beta fluid based gel 1, "Test B012": Beta fluid by DSI Ventures Inc. + 1.5 wt. % SEEPS 4099; Beta fluid based gel 2, "Test B008": Beta fluid by DSI Ventures Inc. + 3.0 wt.% SEEPS 4099. "S4" is a reference, i.e. iso-paraffinic oil Shell Diala S4 ZX-I "Beta fluid" is a reference, i.e. Beta fluid by DSI Ventures Inc.

The dynamic viscosity of the dielectric gel compositions were tested as a function of temperature. Viscosity may be measured according to the methods of ISO 3219, ISO 3014, ASTM D445 or IEC 61868, e.g. using a rotational viscometer with defined shear rate.

In the present tests the dynamic viscosity was measured by using a KinexusPro+ rheometer equipped with a Peltier temperature control. The viscosity measurement was done with a concentric cylinder measuring system (C25), which comprises a rotating bob (inner cylinder) located in a fixed cup (outer cylinder). The gap size was set to 1 mm, and the analysis was performed in oscillation mode under the below conditions:

Geometry: Cup (C25 cylinder splined finish)

/ Bob (C25 cylinder with splined finish)

Gap distance: 1 mm

- Temperature sweep: from 160 °C to -10 °C at -1 °C/min

Oscillation mode at frequency of 0.1 Hz and strain rate of 10 %

The results are shown in Figure 3. It can be seen that all six gel compositions demonstrated a transition from a high viscosity regime to a low viscosity regime a temperature higher than 60 °C. In fact, the iso-paraffinic oil based gels and the beta fluid based gels have a transition, i.e. a knee temperature, from a high viscosity regime to a low viscosity regime at a temperature higher than 160 °C, whereby these oils are preferable if high working temperatures are required for the power semiconductor applications. All six gels exhibited excellent stability.

Example 2

Table 2 shows a comparison between the properties of the invented gels for which the values are an average for the invented gels listed above and those of silicone gels developed by DOW.

Table 2. A comparison between the invented dielectric gels and DOW silicone gels (Dowsil™ EG- 3896 ^a & 3810 ^b )

From the Table 2 above it can be concluded that the dielectric properties may be improved by the present encapsulant, while the encapsulant is environmentally friendly and more economic than silicone gels based on silicone oils.