Elastomeric material filled with diamond and boron nitride for encapsulating electronic components

Abstract

One aspect of the invention relates to an elastomeric material (4) for encapsulating an electronic component (2) on a printed circuit board (1), the elastomeric material containing a silicone-based elastomer, diamond powder and boron nitride powder, and the elastomer being preferably crosslinkable at room temperature and based on polydimethylsiloxane. This elastomeric material may further contain a reactive silicone oil, such as hydroxy-terminated polydimethylsiloxane oil, serving as a plasticiser. This elastomeric material is suitable for aeronautics and has improved thermal conductivity. Another aspect of the invention relates to a printed circuit board including at least one electronic component encapsulated in such an elastomeric material.

Description

DIAMOND AND BORON NITRIDE-LOADED ELASTOMERIC MATERIAL FOR ENCAPSULATION OF ELECTRONIC COMPONENTS TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to an elastomeric material suitable for encapsulating electronic components on a printed circuit board, as well as to a method for preparing such an elastomeric material.

[0002] The present invention also relates to a printed circuit board, also called an electronic board, on which there are electronic components encapsulated in such an elastomeric material, as well as a method of manufacturing such a board.

[0003] The invention relates to on-board electronics in general, but applies more particularly to the aeronautical and space fields where specific constraints are exerted. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0004] In electronics, it is known to coat electronic components on printed circuit boards in a protective material to protect them from moisture and the external environment. This technique is called encapsulation or "potting" in English.

[0005] To achieve this coating, it is important to choose a material that is both waterproof to protect the components, electrically insulating to avoid causing short circuits, easy to install without damaging the components and which is also a good thermal conductor.

[0006] Indeed, when they operate, active electronic components (especially semiconductors) generate a very significant amount of heat, which must be dissipated. In addition, although they are passive devices, resistors or inductors (chokes) can also heat up enormously. For these reasons, the encapsulation material must not retain heat around the encapsulated components, at the risk of damaging them, but must instead be able to dissipate it very quickly.

[0007] In addition, when these electronic cards are intended for aeronautics, additional constraints are added and weigh on the encapsulation material.

[0008] Since the mass of on-board equipment is essential to limit energy consumption, the density of the encapsulation material must be low so as not to add too much weight to the electronic card.

[0009] Furthermore, depending on their position in aircraft (in the wings or near the engines), electronic equipment is subject to very varied temperature conditions and must be able to operate over a wide temperature range from -55°C to +125°C. The encapsulation material must therefore be able to withstand and fulfill its role over this entire temperature range.

[0010] Furthermore, when the board is used in high temperature conditions, it is important that the encapsulation material does not, by expanding, exert too much pressure on the electronic components that are soldered to the board and their connections, in order to avoid damaging them. This material must be able to flow between the components without tearing them off when it expands.

[0011] Additionally, in an on-board application, the encapsulation material may come into contact with many fluids used in aeronautics, including oil, fuel, or skydrol™-type liquids. The encapsulation material must be chemically resistant to these fluids to protect the encapsulated components.

[0012] In addition, it is hoped that this encapsulation material will allow an electronic component to be repaired without the need to replace the entire electronic board. To achieve this, it must be easily removable locally and replaced once the component has been changed.

[0013] There are two types of materials commonly used on the market to encapsulate electronic components on a printed circuit board: resins and elastomers.

[0014] By nature, resins are very hard and very inelastic materials, which is problematic at high temperatures because they are then likely to cause irreversible damage to the coated electronic components and their connections.

[0015] Furthermore, a resin does not allow any disassembly or repair. Indeed, it is not possible to remove a portion of it, or even to remove it entirely without breaking it and tearing off the components it coats because it forms a block with these elements. In addition, if we add fresh resin on an older layer of resin, in the context of a repair for example, we only obtain a simple bonding of the two layers without real intermolecular chemical cohesion between them. Subsequently, for example in the event of vibrations, cracking can occur between the two layers.

[0016] For all these reasons, resins are not suitable for the intended applications and we will limit ourselves to elastomers in the context of the present invention.

[0017] Elastomers are generally more flexible and elastic than resins and are therefore more suitable for desired on-board applications.

[0018] Many elastomer-based encapsulation products have been proposed in the prior art. It is also known to add fillers to increase their thermal conductivity. These fillers are generally metal oxides or nitrides, such as aluminum oxide, zinc oxide or aluminum nitride.

[0019] However, to be effective, these fillers must be added in large quantities, which considerably increases the density of the assembly and makes these materials unusable for embedded applications.

[0020] It is therefore desirable to find an encapsulation material for electronic components on printed circuit boards, which is more satisfactory and better suited to on-board electronics applications, particularly for aeronautics and space. SUMMARY OF THE INVENTION

[0021] The invention aims to meet this unmet need.

[0022] For this, a first aspect of the invention teaches an elastomeric material for encapsulating an electronic component on a printed circuit board, which contains a silicone-based elastomer and which further contains diamond powder.

[0023] Silicone elastomers advantageously have good elasticity and low hardness. They retain their properties over a wide temperature range, including the -55°C to 125°C range found in aeronautics.

[0024] In addition, they are not electrically conductive and are inert to various fluids used in aeronautics. They are particularly resistant to oil, fuel and Skydrol™.

[0025] Additionally, they do not chemically adhere to virtually any surface, allowing them to be removed from the electronic devices they encapsulate without risk of damaging them, making repairs possible.

[0026] Advantageously, the silicone-based elastomer is preferably an elastomer that can be crosslinked at room temperature. This means that its crosslinking is likely to occur at room temperature (i.e. 20 to 25°C at atmospheric pressure) without the need to heat it. Of course, even if the crosslinking of such an elastomer is capable of taking place at room temperature, it can also be carried out at a higher temperature with a higher speed. In addition, an elastomer whose crosslinking is likely to occur at a temperature below room temperature is also considered to be an elastomer that can be crosslinked at room temperature, since its crosslinking can also occur at room temperature.

[0027] Choosing an elastomer that can be crosslinked at room temperature makes it easier to install on the electronic board without risking damaging the electronic components present, which may be fragile.

[0028] Indeed, some silicone elastomers must be crosslinked by a hot vulcanization reaction of the radical type, for example, initiated by a peroxide. This reaction only occurs at high temperatures, above 150°C, which are not compatible with certain fragile electronic components and could damage them. Such silicone elastomers will not be used if the printed circuit board contains fragile electronic components that are not compatible with their high crosslinking temperature.

[0029] Room temperature crosslinkable silicone elastomers are products obtained from a two-component mixture, comprising oligomers on the one hand and a crosslinking agent on the other, often with a catalyst in one of the two, which begins to crosslink when the two components are brought into contact. This reaction occurs slowly at room temperature (it lasts for example up to a day) and can be accelerated by increasing the temperature.

[0030] This type of elastomer can easily be used for encapsulation, with the two components of the elastomer being mixed just before being deposited on the electronic board and its components. Since crosslinking occurs slowly, the mixture is sufficiently fluid to flow between the components or be applied on and between them, and thus perfectly coat them. Once the electronic board is satisfactorily covered, it is sufficient to wait or accelerate the crosslinking process by placing the assembly in an oven set at a temperature below 125°C (for example at 100 or 110°C for one hour).

[0031] In other embodiments, the chosen silicone-based elastomer will only be crosslinked from a temperature above room temperature, preferably between 25 and 125°C, for example 30, 40 or 50°C, which remains below the deterioration temperature of the electronic components used.

[0032] Advantageously, the silicone-based elastomer may be a polydimethylsiloxane-based elastomer, for example the elastomer marketed under the name TUFgel 331™.

[0033] A polydimethylsiloxane-based elastomer is an elastomer that contains predominantly polydimethylsiloxane (also known as PDMS) and preferably at least 80% or at least 90% polydimethylsiloxane. It can also consist solely of polydimethylsiloxane.

[0034] Advantageously, this type of elastomer is highly repairable, because in addition to being able to be perfectly removed from the printed circuit board without damaging the electronic components, it can be replaced locally once the repair has been carried out, guaranteeing real cohesion of the new part of elastomer with the older parts. Indeed, when polydimethylsiloxane is deposited during crosslinking on a layer of polydimethylsiloxane that is already crosslinked, a real fusion of the two layers is obtained by creating intermolecular chemical bonds.

[0035] TUFgel 331™, formerly known as Q-gel 331™, is a polydimethylsiloxane-based elastomer marketed by CHT (formerly ACC Silicones LTD). This elastomer was selected because it has low hardness and good elasticity. In addition, it advantageously contains a flame retardant that prevents flames from spreading in the event of a fire. However, other silicone elastomers can be used as needed.

[0036] Within the encapsulating elastomer material, the silicone elastomer is a matrix that serves to bind the filler particles improving thermal conductivity.

[0037] Indeed, the elastomeric encapsulating material of the invention contains diamond powder to improve its thermal conductivity.

[0038] Diamond has exceptional thermal conductivity (in the order of 1000 to 2000 W rrr 1 K' 1), while being a perfect electrical insulator. In addition, due to its loosely packed crystalline structure, it is advantageously less dense compared to other minerals.

[0039] Diamond is thus more advantageous than the fillers conventionally used in the prior art, because in addition to being a better thermal conductor than metal oxides or nitrides, it is much less dense. It thus makes it possible to obtain an elastomeric encapsulation material that is both thermally efficient and lighter, which is crucial in the aeronautics field.

[0040] In addition, diamond advantageously has a very low coefficient of expansion (of the order of 1 ppm / K) which is favorable for use in encapsulation in an application such as those targeted where the equipment must endure high temperatures.

[0041] Furthermore, it is surprisingly noted that the elastomer encapsulation material obtained by mixing diamond powder in a silicone elastomer has a micro-spongy structure highlighted in Figure 2. This structure is probably due to the lack of regularity of the diamond grains which have varied angular shapes as can be seen in Figures 3 and 4, which is very different for example from the spherical ball structure of aluminum oxide conventionally used in encapsulation materials.

[0042] This micro-spongy structure is particularly advantageous because it significantly reduces the pressure exerted by the encapsulation material on the electronic components when it expands at high temperatures. This greatly reduces the risk of damage.

[0043] The invention thus makes it possible to satisfactorily achieve the sealing and thermal transfer of on-board electronic cards, by encapsulation using an elastomer material which exerts only limited pressure on the electronic components at high temperature.

[0044] Furthermore, it is also surprisingly observed that the diamond grains are not distributed uniformly in the elastomer that binds them, but rather tend to organize themselves in curved lines where they are placed close to each other by a percolation phenomenon. The diamond grains thus form conduction paths allowing faster heat transmission. The appearance of such conduction paths has been highlighted in Figure 5.

[0045] Advantageously, the elastomer material may contain a volume percentage of diamond powder of between 10% and 55%, preferably of between 25 and 35% or between 45 and 55%, and even more preferably be substantially equal to 30% or 54%.

[0046] In this patent application, the volume percentage of an element will be defined as being the percentage of volume occupied by this element relative to the sum of the volumes of the elements to be mixed to produce the encapsulating elastomer material, this sum not including the possible volume of reactive oil which may be added to the elastomer.

[0047] Due to the very low density of diamond, a very highly loaded elastomer material can be produced while maintaining a low overall density, which is crucial for an embedded application.

[0048] Advantageously, the diamond powder may have a particle size of between 6 and 60 μm in diameter, preferably between 12 and 50 μm, even more preferably between 40 and 50 μm.

[0049] The particle size chosen for the diamond powder has little influence on thermal conductivity, but allows the viscosity of the mixture to be modified during crosslinking in order to facilitate its placement on the electronic card.

[0050] Advantageously, the elastomer material further contains boron nitride powder, preferably hexagonal.

[0051] The combination of a different type of filler, boron nitride powder, with diamond powder surprisingly allows the thermal conductivity of the encapsulation material to be considerably increased, as demonstrated by the graph in Figure 6.

[0052] Boron nitride, with the chemical formula BN, is an electrical insulator with good thermal and chemical stability. It has an acceptable density and high thermal conductivity (although significantly lower than that of diamond), particularly in its hexagonal conformation.

[0053] Advantageously, the elastomer material may contain a volume percentage of boron nitride powder less than or equal to 15% and preferably substantially equal to 10%.

[0054] By thus limiting the quantity of boron nitride used, the increase in the density of the resulting elastomeric encapsulation material remains low and compatible with an on-board application, while leading to a notable improvement in thermal conductivity.

[0055] Advantageously, boron nitride powder has a smaller particle size than diamond powder. This allows the boron nitride grains to fit into the gaps between the diamond grains. By filling these gaps, the boron nitride grains improve the continuity of the heat conduction paths, thereby improving the thermal conductivity of the material.

[0056] Advantageously, the elastomer material may also contain a plasticizer of the reactive silicone oil type, preferably a hydroxy-terminated poly-dimethylsiloxane oil and even more preferably the oil marketed under the name HSR750™ by the company AB chimie (CAS No.: 70131-67-8).

[0057] The addition of this oil helps to reduce the viscosity of the mixture during crosslinking and to increase the elasticity of the final encapsulation material after crosslinking. In fact, the siloxane monomers and oligomers contained in this oil increase the chain length of the elastomer and thus reduce its crosslinking rate. The material obtained is therefore more flexible and less viscous during crosslinking.

[0058] Advantageously, the elastomer material may contain a mass percentage of reactive silicone oil less than or equal to 10%, preferably between 2.5% and 10%, and even more preferably between 5% and 7% relative to the mass of the elastomer.

[0059] In this patent application, the mass percentage of reactive silicone oil will be defined in relation to the mass of the elastomer only and not in relation to the total mass of the mixture. Indeed, its action specifically targets the elastomer.

[0060] With such a percentage, the mechanical properties of the elastomer material are significantly improved, without reducing its thermal conductivity too significantly. In addition, the risk of the oil bleeding out of the elastomer material is avoided.

[0061] Furthermore, when the mass percentage of reactive silicone oil is between 5% and 7%, the pressure exerted by the resulting elastomeric material when the temperature increases is considerably limited.

[0062] A second aspect of the invention relates to a printed circuit board comprising at least one electronic component encapsulated in an elastomeric material as described above.

[0063] A third aspect of the invention relates to a method for preparing an elastomeric material as described above, in which: a first component and a second component of a two-component silicone-based elastomer are provided, to be mixed to initiate crosslinking of the silicone-based elastomer; diamond powder and boron nitride powder of a particle size smaller than that of the diamond powder; a first mixture is carried out by mixing the first component with the diamond powder; a second mixture is carried out by mixing the second component with the boron nitride powder; makes a final mix by mixing the first mix with the second mix.

[0064] The production of the elastomer material according to this process advantageously makes it possible to obtain a more homogeneous elastomer material than if all the reagents are mixed together directly, especially when the quantities of diamond powder and boron nitride are large.

[0065] Diamond powder and boron nitride powder can be mixed with either of the elastomer components without affecting the final result. For example, diamond powder can be mixed with the component containing the oligomers and boron nitride powder with the component containing the crosslinking agent, as this allows for better visualization of the homogeneity of the mixture. However, the reverse can also be done.

[0066] When the elastomer material to be prepared must also contain a reactive silicone oil type plasticizer, the process can then advantageously contain the following steps: a reactive silicone oil type plasticizer is additionally provided, and the first mixture is produced by mixing the first component with the diamond powder and the reactive silicone oil type plasticizer.

[0067] A fourth aspect of the invention relates to a method of manufacturing a printed circuit board as described above, in which: a printed circuit board comprising at least one electronic component is provided; an elastomeric material is prepared according to the method described above; the final mixture is deposited on the printed circuit board so as to completely cover the electronic component, before the crosslinking of the silicone-based elastomer is completed. the crosslinking of the silicone-based elastomer is allowed to complete.

[0068] Advantageously, after depositing the final mixture on the printed circuit board, said printed circuit board can be placed in an oven at a temperature below 125°C in which the crosslinking of the silicone-based elastomer is allowed to complete. The crosslinking process is thus accelerated.

[0069] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0070] The figures are presented for information purposes only and in no way limit the invention.

[0071] [Fig. 1] is a schematic sectional view of a printed circuit board with an electronic component encapsulated in an elastomeric material according to the invention.

[0072] [Fig. 2] is a photograph taken with a scanning electron microscope (magnification x 341) of a section of an example of elastomeric material according to the invention, highlighting its spongy structure.

[0073] [Fig. 3] and [Fig. 4] are photographs taken with a scanning electron microscope (magnification x 500 for Figure 3 and x 300 for Figure 4) of powdered diamond with a grain diameter ranging respectively from 8 to 12 pm for Figure 3 and from 40 to 60 pm for Figure 4.

[0074] [Fig. 5] is a photograph taken with a scanning electron microscope (magnification x 300) of the upper face of a block of an elastomeric material according to the invention, highlighting the formation of heat conduction lines.

[0075] [Fig. 6] is a graph representing the thermal conductivity of an elastomeric encapsulation material as a function of the percentage of diamond powder it contains, with the first curve being a material containing only diamond and the second curve being the same material but containing an additional 10% boron nitride.

[0076] [Fig. 7] is a graphical representation of three curves illustrating the variation, at three different temperatures, of the volume pressure coefficient P of the elastomeric material as a function of the quantity of reactive silicone oil it contains.

[0077] [Fig. 8] is a graphical representation, at three different temperatures, of the pressure exerted by the elastomeric material as a function of its reactive silicone oil content.

[0078] [Fig. 9] a graphical representation, at three different temperatures, of the pressure exerted by the elastomeric material as a function of its diamond powder content. DETAILED DESCRIPTION

[0079] In Figure 1, there is shown schematically an example of a printed circuit board 1, also called a printed circuit or electronic board or PCB in English, on which an electronic component 2 is mounted.

[0080] Although a single electronic component 2 has been shown for reasons of simplification, it is obvious that the printed circuit board can contain many others of various kinds.

[0081] A layer 3 of elastomer material 4 covers the upper face 5 of the printed circuit board 1 and the upper 6 and lateral 7 faces of the electronic component 2 which is thus completely encapsulated, that is to say coated, by the elastomer material 4 according to the classic encapsulation technique ("potting"). This layer 3 can be relatively thick and for example of the order of a few millimeters to several centimeters.

[0082] Advantageously, the elastomer material 4 is an elastomer material according to the invention which has improved thermal conductivity, which makes it possible to evacuate to the outside a large part of the heat produced by the electronic component 2 during its operation. This heat flow is symbolized by the arrow 8.

[0083] In the photograph of Figure 2, the microscopic structure of the elastomer material 4 seen in section was observed using a scanning electron microscope.

[0084] It can be seen that the diamond grains 9 are coated and bonded to each other by the elastomer 10. However, the elastomer does not completely or uniformly fill the volume occupied by the elastomer material 4. Holes or empty spaces 11 remain between the grains 9 and the elastomer 10, thus giving the assembly a spongy internal structure at the microscopic level, probably due to the geometric irregularity of the diamond grains 9 (clearly visible in Figures 3 and 4). This micro-spongy internal structure allows the material 4 to easily deform and compress, which makes it more elastic and allows it to expand and flow between electronic components 2 without damaging them.

[0085] However, material 4 remains waterproof despite everything because the elastomer is closed-cell, the outer faces of material 4 remaining smooth as can be seen in the photograph in figure 5.

[0086] In the photograph of Figure 5, it can be seen that the diamond grains 9 are not uniformly distributed in the elastomer 10, but are rather arranged next to each other to form a network of curved paths. These paths, some of which have been shown in Figure 5 by the dotted lines 12 for greater visibility, allow heat to be transferred directly from grain to grain. They thus constitute thermal conduction paths which allow faster and more efficient heat transfer.

[0087] In order to complete the description of the invention, several examples of elastomeric encapsulation materials are detailed below.

[0088] In these examples, all the materials were prepared by independently mixing one component of the two-component elastomer (the one containing the oligomers) with the diamond powder and the reactive oil when the material contains them and the second component of the elastomer (the one containing the crosslinking agent) with the boron nitride powder, then bringing together and mixing the two initial mixtures to allow the crosslinking of the elastomer. The elastomeric materials obtained were all homogeneous. EXAMPLES

[0089] Example 1:

[0090] Several formulas have been produced using the silicone elastomer marketed by DOW CORNING under the trade name Sylgard 184™.

[0091] Composition of these formulas in volume percentage: Sylgard 184™: 78% diamond powder: 22%

[0092] Different particle sizes were used for the diamond powder. Powders with particle diameters in the following ranges were tested: 8 to 12 pm, 12 to 22 pm, 22 to 36 pm and 40 to 50 pm.

[0093] The thermal conductivity of the elastomeric materials obtained after crosslinking was measured and compared to that of Sylgard 184™ alone.

[0094] The results are grouped in the table below.

[0095] In the examples, the average thermal conductivity value shown corresponds to an average value calculated from the values ​​measured on five 2.5 mm thick samples.

[0096] [Table 1]

[0097] It is found that the addition of diamond powder significantly increases the thermal conductivity of Sylgard 184™. In contrast, the particle size of the diamond powder has little influence on the thermal conductivity.

[0098] It was also observed that the viscosity of the mixture decreases when the size of the diamond particles increases.

[0099] Example 2:

[0100] Several formulas were created using the silicone elastomer marketed by MOMENTIVE under the trade name XE14-B7892™.

[0101] Composition of these formulas in volume percentage: XE14-B7892™: 72% diamond powder: 28%

[0102] Diamond powders with particle diameters in the following ranges, respectively: 12 to 22 pm, 22 to 36 pm and 40 to 50 pm, were tested.

[0103] The thermal conductivity of the elastomeric materials obtained after crosslinking was measured and compared to that of XE14-B7892™ alone.

[0104] The results are grouped in the following table.

[0105] [Table 2]

[0106] It is observed that the addition of diamond powder considerably increases the thermal conductivity of XE14-B7892™, which is almost doubled.

[0107] As previously, we observe that the grain size of the diamond powder has little influence on the thermal conductivity, but varies the viscosity which decreases when the size of the diamond grains increases.

[0108] Example 3:

[0109] Several formulas were created using the polydimethylsiloxane-based elastomer TUFgel 331™ marketed by CHT and diamond powder. Their composition is shown in the table below.

[0110] This time, the volume proportion of diamond powder to that of the elastomer was varied.

[0111] Diamond powders with particle size in the range of 12-22 pm and 22-36 pm were used.

[0112] The thermal conductivity of the elastomeric materials obtained after crosslinking was measured and compared to that of TUFgel 331™ alone.

[0113] The results are grouped in the following table.

[0114] [Table 3]

[0115] It is observed that the addition of diamond powder significantly increases the thermal conductivity of TUFgel 331™ and the higher the proportion of diamond, the greater the thermal conductivity.

[0116] Example 4:

[0117] Several formulations were made using TUFgel 331™ as the elastomer, diamond powder, and boron nitride powder. Their composition is shown in the table below.

[0118] The grains of the powders used had a diameter between 22 and 36 pm for diamond and approximately 5 pm for boron nitride.

[0119] The thermal conductivity of the elastomeric materials obtained after crosslinking was measured. The results are grouped in the following table.

[0120] [Table 4]

[0121] It is found that the addition of boron nitride powder to diamond powder significantly increases the thermal conductivity of the resulting elastomeric material.

[0122] To better illustrate this effect, the results of Example 3 and part of the results of Example 4 corresponding to the addition of 10% boron nitride are plotted in parallel on the graph in Figure 6.

[0123] Example 5:

[0124] Three formulas were made by mixing the same quantities (in volume percentage) of TUFgel 331™ as elastomer, diamond powder and boron nitride powder, as well as a variable quantity of reactive silicone oil marketed by the company AB chimie under the name HSR750™.

[0125] The grains of the powders used had a diameter of approximately 40 pm for diamond and approximately 30 pm for boron nitride.

[0126] The amount of reactive silicone oil is expressed as a mass percentage relative to the mass of TUFgel 331™.

[0127] The composition of these three formulas is shown in the following table.

[0128] [Table ^

[0129] For each of these three materials, the volume pressure coefficient P (in bar / cm 3 / K), corresponding to the pressure exerted per unit volume of the material and per Kelvin, at three different temperatures (80°C, 100°C, 125°C).

[0130] The results obtained were graphically represented in Figure 7.

[0131] It is found that the addition of reactive silicone oil reduces the pressure exerted by an encapsulating elastomer material at a given temperature. Furthermore, for the same formula, the increase in pressure exerted at 125°C compared to that exerted at 80°C is much less significant when the elastomer material contains 7.5% reactive oil than when it contains 2.5%.

[0132] The pressure exerted by the elastomeric encapsulation material on the electronic components at high temperature is therefore reduced.

[0133] Example 6:

[0134] Four formulas were made by mixing the same quantities (in volume percentage) of TUFgel 331™ as elastomer, diamond powder and boron nitride powder, as well as a variable quantity of reactive silicone oil marketed by the company AB chimie under the name HSR750™.

[0135] The grains of the powders used had a diameter between 40 and 50 pm for diamond and around 10 pm (average value) for boron nitride.

[0136] The amount of reactive silicone oil is expressed as a mass percentage relative to the mass of TUFgel 331™.

[0137] The composition of these four formulas is shown in the following table.

[0138] [Table 6]

[0139] For each of these four materials, the pressure (in Newtons) exerted on a piston by a sample of parallelepiped material with dimensions equal to 50x75x8 mm, placed in a container resting against the faces of the sample in order to maintain its dimensions except for that in contact with the piston, was measured at three different temperatures (85°C, 100°C, 125°C).

[0140] The results obtained were graphically represented in Figure 8.

[0141] It is found that the addition of reactive silicone oil significantly reduces the pressure exerted by the elastomer material, regardless of the temperature tested. This advantageous effect is particularly noticeable when the quantity of reactive oil is between 4 and 8%, and even more so between 5 and 7% where the pressure exerted is almost three times less than with 2.5% reactive oil.

[0142] The pressure exerted by the elastomeric encapsulation material on the electronic components at high temperature is therefore reduced.

[0143] Example 7:

[0144] Five formulations were made by mixing a varying proportion of diamond powder, with an equal amount of boron nitride powder and TUFgel 331™ as an elastomer in a proportion matched to that of the diamond powder to complete the mixture to 100%.

[0145] The grains of the powders used had a diameter between 40 and 50 pm for diamond and around 10 pm (average value) for boron nitride.

[0146] HSR750™ reactive silicone oil, in constant proportion to the amount of elastomer, was added to the mixture. It is expressed as a mass percentage relative to the mass of TUFgel 331™.

[0147] The composition of these five formulas is shown in the following table.

[0148] [Table 7]

[0149] The pressure exerted by each of these materials at three different temperatures (85°C, 100°C, 125°C) was measured under the same experimental conditions as in Example 6.

[0150] The results obtained were graphically represented in Figure 9.

[0151] It is observed that the pressure exerted by the elastomeric material decreases significantly between 25 and 35% and between 45 and 55%, and even more significantly beyond 50%, whatever the temperature tested. Remarkably, a point convergence of the three curves to a particularly low value (almost ten times less than with 40% diamond) is advantageously observed around 54% diamond powder.

[0152] Example 8:

[0153] A preferred example of an elastomeric encapsulating material was obtained by mixing 50% (volume) of diamond powder, 10% (volume) of boron nitride powder, 40% (volume) of TUFgel 331™ and 10% (mass relative to the mass of TUFgel 331™) of HSR750™ oil.

[0154] Another preferred example of elastomeric encapsulating material was obtained by mixing 54% (volume) of diamond powder, 2% (volume) of boron nitride powder, 44% (volume) of TUFgel 331™ and 5% (mass relative to the mass of TUFgel 331™) of HSR750™ oil.

[0155] These elastomeric materials advantageously have a thermal conductivity greater than 2 W rrr 1 K' 1 , a hardness less than 35 SHORE A, a volume pressure (P) less than 1.6 mbar / cm 3 / K at 100°C, a density less than 2.3 (two point three), a resistivity greater than 10 11Qm, a continuous use temperature range of -55°C to 204°C and contains a flame retardant.

Claims

CLAIMS

1. Elastomeric material (4) for encapsulating an electronic component (2) on a printed circuit board (1) containing a silicone-based elastomer, characterized in that it further contains diamond powder and boron nitride powder having a particle size smaller than that of the diamond powder.

2. Elastomeric material (4) according to claim 1 characterized in that the silicone-based elastomer is an elastomer which is crosslinkable at room temperature.

3. Elastomeric material (4) according to one of the preceding claims, characterized in that the silicone-based elastomer is a polydimethylsiloxane-based elastomer, preferably the elastomer marketed under the name TUFgel 331 ™.

4. Elastomeric material (4) according to one of the preceding claims, characterized in that it contains a volume percentage of diamond powder of between 10% and 55%, preferably of between 25 and 35% or between 45 and 55%, and even more preferably equal to 30% or 54%.

5. Elastomeric material (4) according to one of the preceding claims, characterized in that the diamond powder has a particle size of between 6 and 60 μm in diameter, preferably between 12 and 50 μm, even more preferably between 40 and 50 μm.

6. Elastomeric material (4) according to one of the preceding claims, characterized in that the boron nitride is hexagonal.

7. Elastomeric material (4) according to one of the preceding claims, characterized in that it contains a volume percentage of boron nitride powder less than or equal to 15% and preferably equal to 10%.

8. Elastomeric material (4) according to one of the preceding claims, characterized in that it further contains a plasticizer of reactive silicone oil type, preferably a polydimethylsiloxane-hydroxy terminated oil and even more preferably the oil marketed under the name HSR750™.

9. Elastomeric material (4) according to claim 8 characterized in that it contains a mass percentage of reactive silicone oil less than or equal to 10%, preferably between 2.5% and 10%, and even more preferably between 5% and 7% relative to the mass of the elastomer.

10. Printed circuit board (1) comprising at least one electronic component (2) encapsulated in an elastomeric material (4) according to one of the preceding claims.

11. Process for preparing an elastomeric material according to one of claims 1 to 9, characterized in that: - provides a first component and a second component of a two-component silicone-based elastomer, to be mixed to initiate crosslinking of the silicone-based elastomer; - provides diamond powder and boron nitride powder with a particle size smaller than that of diamond powder; - makes a first mixture by mixing the first component with the diamond powder; - makes a second mixture by mixing the second component with the boron nitride powder; - makes a final mix by mixing the first mix with the second mix.

12. A method according to claim 11 for preparing an elastomeric material according to claim 8 or 9, characterized in that a reactive silicone oil type plasticizer is additionally provided, and in that the first mixture is produced by mixing the first component with the diamond powder and the reactive silicone oil type plasticizer.

13. Method for manufacturing a printed circuit board (1) according to claim 10 characterized in that: a printed circuit board (1) is provided comprising at least one electronic component (2); - prepares an elastomeric material according to the method of claim 11 or 12; - deposits the final mixture on the printed circuit board so as to completely cover the electronic component (2), before the crosslinking of the silicone-based elastomer is complete; - allows the crosslinking of the silicone elastomer to complete.

14. Method according to claim 13 characterized in that after having deposited the final mixture on the printed circuit board, said printed circuit board is placed in an oven at a temperature below 125°C in which the crosslinking of the silicone-based elastomer is allowed to complete.

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