Junction and method of joining

Abstract

A method of joining a first substrate and a second substrate, thereby forming a joint. A stack including a filling material and a plurality of holding media is provided between the first substrate and the second substrate. The stack is heated to melt the filling material, and the first and second substrates are wetted with the melted filling material. The melted filling material is solidified to form a joint.

Description

Technical Field

[0001] The present invention relates to joints, and more particularly to soldered or brazed joints and methods of joining.

Background Art

[0002] Soldering and brazing are methods of joining metal parts (referred to as "substrates") together using a low melting point filler material (e.g., solder or brazing filler metal).

[0003] In both soldering and brazing, the filler material is melted, and as a result, i) the filler material flows to fill the space between the solid substrates, and ii) the filler material wets the opposing surfaces of the solid substrates to be joined. Depending on the chemical nature of the materials, the solid substrates may dissolve in the liquid filler material to form an intermetallic layer. Typically, this intermetallic layer is brittle but secures the filler material to the substrates. After the remaining liquid filler material has solidified, a thermal, electrical, and / or mechanical connection is established between the substrates.

[0004] The main difference between soldering and brazing lies in the melting point of the filler material. In soldering, the filler material has a melting point below 450°C, while in brazing, the melting point of the filler material exceeds 450°C. Apart from this difference, soldering and brazing are mostly the same. In both soldering and brazing, the substrates remain below their melting points.

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a novel and useful joint and a method for manufacturing the same.

Means for Solving the Problems

[0006] According to a first aspect of the present invention, there is provided a method of joining a first substrate and a second substrate, thereby forming a joint. A stack including a filling material and a plurality of holding media is provided between the first substrate and the second substrate. The stack is heated to melt the filling material, and the first and second substrates are wetted with the melted filling material. The melted filling material is solidified to form a joint.

[0007] According to a second aspect, there is provided an apparatus comprising a first substrate, a second substrate, and a joint between the first substrate and the second substrate. The joint includes a plurality of holding media arranged to form a stack between the first substrate and the second substrate, and a filling material extending between the first substrate and the second substrate and penetrating the stack.

[0008] According to a third aspect, there is provided a plasma confinement vessel comprising the apparatus according to the second aspect, wherein each of the first and second substrates is a coil of a superconducting magnet.

[0009] Further embodiments are provided in claims 2 and later.

Brief Description of the Drawings

[0010] Here, some embodiments of the present invention will be described by way of example with reference to the accompanying drawings.

Figure 1A

Figure 1B

Figure 2A

Figure 2B

Figure 3

Figure 4

Figure 5

Figure 6

[0011] In practice, the thickness of a soldered or brazed joint has a lower limit and an upper limit for the following reasons. · For joints with a thin thickness (<5 μm), a brittle intermetallic layer occupies a significant proportion of the joint. Therefore, joints with a thin thickness (<5 μm) are too brittle to be used for load-bearing components. · For joints with a thick thickness (>250 μm), the molten filler material tends to flow away from the space between the substrates to be joined under the influence of gravity or other factors. This results in incomplete filling and void formation in the joint. Correspondingly, joints with a thick thickness are also mechanically weak.

[0012] If the joint needs to be manufactured to a specified tolerance of strength, electrical conductivity, and / or thermal conductivity, further thickness limitations may be imposed.

[0013] In the case of large (e.g., with a maximum dimension exceeding 30 cm) heavy substrates and / or substrates with complex shapes, it is difficult to establish joints having a maximum thickness of less than 250 μm and a minimum thickness exceeding 5 μm across the entire joint area. It is even more difficult to achieve stricter tolerances. Therefore, there is a need in the industry for improved joints that can be more easily fabricated to a specified thickness tolerance and / or methods for retaining solder within joints of proposed non-conventional dimensions (e.g., variable thickness, thickness exceeding 250 μm).

[0014] Accordingly, it is proposed to arrange a (filler) holding medium having a thickness exceeding 5 μm, configured to hold a filler of a predetermined volume, between substrates to be joined. The holding medium is configured such that the flow of the liquid filler away from the space to be joined is restricted. Advantageously, the thickness of the joint is not limited to the above upper limit (e.g., incomplete filling due to filler loss is avoided), and joints having a thickness exceeding 250 μm can be achieved. For example, using a single holding medium, a joint having a thickness exceeding 1 mm is possible. Using two or more holding media, even thicker joints are possible. Thicker joints generally have improved strength compared to thinner joints.

[0015] The joints described herein can be formed by either soldering or brazing. As already mentioned, the main difference between soldering and brazing is the melting point of the filler. Accordingly, soldering and brazing should be treated as interchangeable methods unless otherwise specified.

[0016] FIG. 1A shows a column of liquid 104 held between two parallel plates 102 by capillary action. The weight of the column of liquid 104 acts downward, while the surface tension of the liquid acts upward due to the generation of a meniscus 106. The total height of the column of liquid 104 that can be held between the parallel plates is inversely proportional to i) the separation of the plates 102 and ii) the density of the liquid, and proportional to the surface tension of the liquid. This principle applies equally to other shapes, for example, when the parallel plates 102 are replaced by a cylindrical tube or mesh. If the weight of the liquid 104 is too large compared to the capillary force, the volume of the liquid 104 held between the plates 102 decreases and equalizes until it reaches equilibrium with the capillary force. As a result, the liquid 104 flows out of the space between the plates 102 and the liquid level in FIG. 1A drops.

[0017] Referring now to FIG. 1B, a further parallel plate 108 is disposed between the two parallel plates 102 shown in FIG. 1A. As the minimum separation between plates 102, 108 decreases, the height of the liquid 104 held between plates 102, 108 can increase. The further parallel plate 108 increases the capillary force holding the liquid 104 between the plates 102, thereby allowing a greater separation between the plates 102 before loss of liquid due to gravity as compared to the configuration of FIG. 1A.

[0018] More generally, a holding medium can hold a liquid filling by capillary action, provided that the holding medium is structured to define a plurality of internal and / or interfacial gaps with the substrate having a minimum dimension below a threshold. Referring to FIG. 1A, the further parallel plate 108 defines a gap 110 between each parallel plate 102 and the plate 108.

[0019] FIG. 2A shows a joint 200 between two substrates 202 including a holding medium 204 disposed between the substrates and a filling material (e.g., solder or brazing, referred to herein as "filler") disposed between the substrates and within the gaps of the holding medium.

[0020] Referring now to FIG. 2B, the holding medium 204 can include a plurality of interconnected or interlaced elements 206 (e.g., wires) forming an open network such as a mesh or wire mesh. The network defines a plurality of gaps 208 in which the liquid filling can be held by capillary action. The minimum dimension 210 of the gaps is below a threshold such that the liquid filling can be held by capillary action within the gaps 208 without loss of liquid due to gravity. In the illustrated embodiment, the network forms a honeycomb structure. As will be appreciated by those skilled in the art, other network structures, such as cubes, rectangles, triangles, are also possible.

[0021] In the case of a gap having a regular shape (e.g., square, hexagon, rectangle), the minimum dimension 210 is defined as the shortest straight-line distance between any point on one element 206 passing through the center point of the gap (i.e., the center of symmetry) and any point on another element 206. For example, the minimum dimension of an equilateral triangle is the distance between the midpoint of one side and the opposite angle. The minimum dimension of a square is the distance between the midpoints of opposite sides. In the case of a gap having an irregular shape, the minimum dimension is the shortest straight-line distance between any point on one element 206 passing through the centroid of the gap and any point on another element 206. In other words, in the case of a gap having a regular or irregular shape, the minimum dimension of the gap is such that the shortest distance between any point within the gap 208 and the holding medium (e.g., a point on one of the elements 206) is less than or equal to half of the above threshold value.

[0022] The network of the holding medium 204 shown in FIG. 2B is two-dimensional. That is, the honeycomb structure extends only on one side. In such an example, the interconnected elements 206 have a thickness sufficient to extend across the gap between the substrates 202. One of ordinary skill in the art will understand that, in practice, the thickness of the joint to be fabricated is predefined. One of ordinary skill in the art can, for example, adapt the thickness of the element 206 to match that thickness.

[0023] In another example, the network of the holding medium 204 (e.g., the honeycomb structure) may extend three-dimensionally (e.g., thereby forming a (hexagonal) prism structure) so as to have a thickness sufficient to extend across the gap between the substrates 202. Accordingly, the gap 208 is also three-dimensional.

[0024] In another example, the retention medium 204 may include a plurality of "two-dimensional" mesh layers stacked on top of each other (e.g., the honeycomb structure shown in FIG. 2B). When the layers are stacked together, they have a thickness sufficient to extend across the gap between the substrates 202. One of ordinary skill in the art will understand how to vary the thickness of these layers and the number of layers to fill the gap between the substrates 202. As an example, each layer may include a mesh formed from interconnected or interlaced wires, and the thickness of the layer depends on the thickness of the wires. When the wires are interlaced (e.g., a woven mesh), the thickness of the layer can be approximately twice the thickness of the wires. The thickness of each retention medium can be in the range of 25 μm to 200 μm, or more preferably in the range of 50 μm to 150 μm.

[0025] The gaps 208 of each directly adjacent layer can be aligned with each other ("AA" stacking) or offset ("AB" or "ABC" stacking). In an AB stack, the gaps 208 of each alternating layer are aligned with each other. In an ABC stack, the gaps 208 of the layers (e.g., layer "A") in the retention medium 204 that are immediately separated by two layers (e.g., layers "BC") are aligned with each other. For example, the gap of the first layer in the retention medium 204 is aligned with the fourth layer, the gap of the second layer is aligned with the fifth layer, and the gap of the third layer is aligned with the sixth layer. In both AB and ABC stacks, the gaps 208 of each directly adjacent layer are offset.

[0026] The numerical value of the threshold depends on the number of parameters including the following and their interactions. · The density of the liquid filler (the greater the density of the filler, the smaller the threshold because the weight increases, while the magnitude of the capillary force does not change); and · The difference between the surface energy of the liquid filler - retention medium interface and the surface energy of the liquid filler - air (or vacuum) interface (the greater the difference, the greater the threshold can be, because this results in a greater capillary force). The interfacial surface energy controllably depends on the following. ○ The degree of impurities / contaminants (e.g., oxides, or Ar / N adsorption layers) on the liquid filler and / or substrate formed by chemisorption or adsorption of the gas present in the surroundings. To some extent, the degree of impurities / contaminants can be controlled by using a controlled atmosphere (e.g., Ar, N, air atmosphere) or by treating the substrate to be joined with the flux to remove surface oxides or other contaminants. ○ Temperature; and ○ Pressure.

[0027] In certain examples, the threshold is 225 μm or less, more preferably 200 μm or less, even more preferably 150 μm, 125 μm or 100 μm or less, such that the minimum dimension of one or any one of the gaps 210 within the mesh is less than 225 μm, more preferably less than 200 μm, even more preferably 150 μm, 125 μm or 100 μm. This ensures that the capillary action is large enough to hold the filler in the holding medium 204 or, at least in part, to restrict the flow of the filler from the holding medium 204. The minimum size of the gaps in the holding medium 204 depends on the ability of the liquid filler to penetrate or soak into the holding medium and can reduce the trapped gas / vacuum voids. Thus, the size of the gaps can be in the range of 10 μm to 200 μm, or more preferably 50 μm to 150 μm.

[0028] Note that the joint 200 including the holding medium 204 may have a lower electrical / thermal conductivity compared to an equivalent joint without the holding medium 204, as the holding medium 204 can represent a barrier to heat conduction or electrical conduction. In this regard, when such characteristics of the joint are important, the minimum dimension of the gap 208 is preferably a value close to the threshold to avoid an excessive barrier within the joint and to increase the proportion of the filler and the number of minimized holding medium layers.

[0029] Referring back to FIG. 2A, the joint 200 is formed by inserting a holding medium 204 pre-impregnated with a (solid) filler material between two substrates 202, the holding medium 204 extending between the substrates 202 when inserted; and heating the holding medium 204 to melt the filler material, enabling the liquid filler material to wet (preferably react chemically with) each substrate 202. After a predetermined time period (e.g., a time period sufficient for an intermetallic layer to develop between the filler material and each substrate 202), heating of the holding medium is stopped / reduced and the filler material solidifies, thereby establishing the joint 200 between the substrates 202. Such a pre-impregnated holding medium is referred to as a "foil".

[0030] The foil can be formed by pre-impregnating the holding medium 204 with the filler material, immersing / dipping the holding medium in a pool of the liquid filler material such that the filler material penetrates the holding medium. As described above, the holding medium 204 can include a plurality of mesh layers stacked on top of each other. The stack may be formed after solidification of the filler material in this pre-impregnation process. The advantage of pre-impregnating the holding medium is that the process can be more easily controlled and optimized compared to in-situ impregnation (see below). Thus, a high filling ratio of the holding medium with the filler material (e.g., >95%) is possible and the degree of gas / vacuum trapped within the holding medium can be minimized. Further, a "fixed" intermetallic layer can be formed before the substrates 202 are joined to each other, which means that there are fewer surfaces that need to be wetted with the filler material during joining. Ultimately, this reduces the risk of gas being trapped or voids being formed in the joint.

[0031] However, when the holding medium 204 is pre-impregnated with the filler, the compatibility of the holding medium decreases. The planar holding medium may not be suitable for joining substrates on non-identical planes, i.e., substrates at an oblique angle to each other. However, referring to FIG. 3, the joint 300 between the substrates 202 on non-identical planes may be formed using a stack of foils 204a, 204b, 204c of different lengths to correspond to the variable-thickness gap such that the resulting stack substantially extends across the gap between the substrates. Each foil 204a, 204b, 204c is inserted between the substrates 202 and can be heated sequentially or simultaneously (as described above) with the other holding media 204a, 204b, 204c. By compressing the stack between the substrates when the stack is heated, the compatibility of the stack with the substrates on non-identical planes can be improved.

[0032] Alternatively, the holding medium may be shaped such that after impregnation, it can be inserted between two substrates on non-identical planes to fill the space to be joined. Those skilled in the art will understand that the desired shape of the holding medium is set by the shape and relative positioning of the substrates and will actually vary. Referring back to FIG. 3, the desired shape of the foil is a wedge having a thickness that varies along its length. However, in another example, the desired shape may be annular and of a constant thickness.

[0033] Referring now to FIG. 4, a joint 400 is shown being fabricated that includes one or more layers of a filling material 402 (e.g., a sheet or preform of the filler having a thickness below the threshold described above) and one or more layers of a holding medium 404 disposed between two substrates 202. The holding medium layer 404 is not pre-impregnated with the filler and is thus flexible. Similarly, the filler layer 402 in sheet form is also flexible. Such a configuration is well-suited for joining substrates on non-identical planes because the layers 402 and the holding medium 404 are malleable enough to conform to the variable-thickness gap that will be filled between the substrates 202. Nevertheless, a pre-impregnated holding medium may be used as part or all of the holding medium layer 404.

[0034] Each of one or more holding media 404 within the joint assembly may be arranged, for example in an alternating arrangement, directly adjacent to one or more of the filler layers 402. Alternatively, as in the example shown in FIG. 4, the filler layer 402 may be provided directly on one or both of the two substrates 202 with a stack of holding media 404 (and / or holding media foil) therebetween.

[0035] The joint 400 can be produced by forming a stack of one or more filler layers 402 and one or more holding media layers 404; inserting the stack between the substrates 202 such that the stack extends between the substrates 202; heating the one or more filler layers 402 and the holding media layers 404 until the filler layer 402 melts and for a period of time long enough for the liquid filler to i) penetrate the holding media layer 404 by capillary action and ii) chemically wet the holding media layer 404 and each substrate 202 to enable the formation of a fixed intermetallic layer. Thereafter, the remaining liquid filler is solidified to establish the joint.

[0036] Alternatively, the joint 400 may be produced by forming a stack of one or more foils, each foil comprising a holding media layer 404 impregnated with a filling material; inserting the stack between the substrates 202; heating the stack of foils such that the filling material melts and wets each substrate. As already mentioned, the holding media may include gaps sized small enough such that the molten filling material is at least partially retained within the gaps by surface tension. Thus, the joint can be assembled in any configuration without substantially losing the molten filling material. Thereafter, the filling material is solidified to establish the joint.

[0037] Depending on the size and shape of the gap between the substrates 202, the above-described combinations of techniques can also be used. For example, the filler layer 402 may be provided on one or both of the substrates having a stack of the holding medium 404 therebetween. Some or all of the holding medium 404 may be a pre-impregnated foil to increase the amount of filler material within the gap and help ensure that the filler material bridges the gap to establish a joint.

[0038] Preferably, although not essential, the joint 400 is formed under vacuum or partial vacuum conditions. The joint in FIG. 2 may be formed under vacuum or partial vacuum. Under such conditions, the gas present within the holding medium 404 or between the layers 402, 404 can be removed to avoid or reduce the entrapment of gas in the joint after solidification of the filler. The pre-impregnated holding medium is advantageous in that such entrapped gas is not normally present initially prior to joining.

[0039] Referring again to FIG. 2B, the total volume of the liquid filler that can be held within the holding media 204, 404 (their volume) is determined by the total gap volume. This depends on the structure (e.g., hexagonal, cubic, etc.), the gap size 208, etc. Since void formation can negatively affect the quality of the joint, the total volume of the filler layer 402 is preferably equal to or greater than the total volume of the holding medium 404 that holds the filler. This ensures that there is sufficient (or excess) filler material to fill the holding media 204, 404.

[0040] Furthermore, when multiple holding media 404 are present, the total volume of the filler layer 402 directly adjacent to each holding medium 404 is greater than or equal to the volume of the holding medium 404. Preferably, since the solid-solid interface between the filler layers may trap gas within the joint 400, two filler layers 402 within the stack configuration are not directly adjacent to each other.

[0041] Since the minimum dimension of the gaps within the holding media 204, 404 is below the above-described threshold value, the thickness and / or number of the filler layers 402 can be selected so as to ensure that a sufficient volume of filler exists in each holding media 404. That being said, the thickness of the filler layer 402 should remain low enough so that the layer 402 remains flexible enough to be formed.

[0042] The holding media layer 404 may also have different lengths, for example as shown in FIG. 3, so that the above-described stack can fill the space between non-coplanar substrates before bonding.

[0043] In the foregoing example, the layers of the holding media 404 are shown as planar, but this is not essential. For example, the holding media 404 can be shaped to fill the space between two non-coplanar substrates before bonding. Then, an appropriate number and thickness of filler layers 402 can be provided adjacent to the shaped holding media 404 within the stack.

[0044] It should be noted that melting results in an increase in volume, while solidification results in a decrease in volume. The volume of the filler layer 402 described above relates to the volume of the solid filler. For example, when the total volume of the (solid) filler layer 402 substantially matches the total volume of the holding media 404, the volume of the liquid generated by melting the filler layer 402 will be greater than the volume of the holding media 404. However, shrinkage occurs during solidification, which ideally results in the uptake of at least a portion of the remaining liquid filler and a pressure drop that effectively fills the remaining voids within the holding media 404. Excess liquid filler wets the opposing outer surfaces of the substrate 202 and the holding media 404 and solidifies to fill the space therebetween, or wets the opposing outer surfaces of adjacent holding media 404 and solidifies to fill the space therebetween.

[0045] As already described, the retaining medium 404 acts as a barrier to heat conduction and / or electrical conduction, which may adversely affect the resulting thermal and electrical conductivities of the joint. FIG. 5 shows a joint 500 formed between two substrates 202 that includes a plurality of retaining media 504 and filler layers 502 (e.g., sheets of the filler material described above). Prior to joining, the layers 504, 502 are arranged orthogonally (rather than parallel as in FIG. 4) to the substrates 202 to be joined. In this configuration, the joint 500 includes high-conductivity channels (excess filler layers 502) where there are no thermal / electrical barriers. Thus, the thermal and electrical conductivities of the joint 500 are improved compared to the joint 400 shown in FIG. 4. In these examples, the total volume of the filler layer 502 is greater than the total volume of the retaining medium 504, thereby enabling the manifestation of "high-conductivity" channels. Similar to the joint 400, the joint 500 is formed by preparing a stack of one or more filler layers 502 and retaining medium layers 504; inserting the stack between the substrates 202 such that the stack extends between the substrates; heating the one or more filler layers 502 and retaining medium layers 504 until the filler layer 502 melts and the liquid filler has sufficient time to i) penetrate the retaining medium layer 504 by capillary action and ii) chemically wet the retaining medium layer 504 and each substrate 202 to enable the formation of an immobilizing intermetallic layer. Thereafter, the remaining liquid filler is solidified to establish the joint 500.

[0046] FIG. 6 shows another fabricated joint 600 between two substrates 202, including a holding medium having an elongated core element 604 from which a plurality of lateral elements 606 extend outwardly with respect to one of the substrates 202, and a plurality of filler layers 602 disposed between the lateral elements 606. The spacing between adjacent lateral elements 606 is below the above-described threshold. In fact, the spacing between the lateral elements 606, the central element 604, and each substrate 202 defines a gap having a minimum dimension below the above-described threshold. In some examples, a plurality of filler layers 602 are disposed within each defined gap. As described above, the total volume of the filler layers 602 is such that the holding medium 604, 606 can be filled with filler without leaving voids. That is, the volume of the filler in the filler layers 602 is greater than or equal to the capacity of the holding medium. The lateral elements 606 are, in this example, similar to the bristles of a paintbrush. Similar to joints 400, 500, joint 600 is formed by arranging one or more filler layers 602 between the lateral elements 606 of the holding medium to form a stack; inserting the resulting stack between the substrates 202 such that the stack extends between the substrates; heating one or more filler layers 602 and the holding medium until the filler layers 602 melt and the liquid filler has a long enough period of time to i) penetrate the holding medium 604, 606 by capillary action and ii) chemically wet the holding medium layers 604, 606 and each substrate 202 to enable the formation of a fixed intermetallic layer. Thereafter, the remaining liquid filler is solidified to establish joint 600.

[0047] Suitable fluxes known to those skilled in the art can be used, for example, to remove or reduce surface oxides present on the surfaces of the substrates and / or the holding medium to be joined, if applicable, prior to impregnation.

[0048] The holding medium is composed of copper, brass, stainless steel, nickel, gold, silver, their alloys, or surface-treated carbon or glass fibers and can be coated with any of the above.

[0049] The filling material may be composed of lead-tin, Sn, In, and other solders known to those skilled in the art (e.g., lead-free, high-temperature solder, low-temperature or ultra-low-temperature solder). Alternatively, the filling material may be silver, copper, copper-zinc (brass), copper-tin (bronze), gold, gold-silver alloy, or other known brazing known to those skilled in the art.

[0050] The material selection of the holding medium 204 and the filling material, although not necessarily so, preferably ensures that the (solid) filling material can wet and form an intermetallic layer with the holding medium so that the filling material is effectively fixed in the holding medium when the joint is formed. The melting point of the holding medium 204 is higher than the melting point of the filling material, and heating of the joint to be produced melts the filling material rather than the holding medium.

[0051] The base material may be composed of nickel or nickel alloy, copper or copper alloy, brass or brass-containing alloy, or stainless steel or stainless steel-containing alloy.

[0052] In a specific example, each base material to be joined is a magnet (e.g., a pancake coil). In another example, at least one of the base materials is a current lead terminal (e.g., within a cryostat).

[0053] In yet another specific example, each base material to be joined is a coil of a superconducting magnet. For example, a limb of a toroidal magnetic field coil within a plasma confinement vessel. More specifically, each base material to be joined is a terminal including a superconducting tape (e.g., HTS ReBCO) or encapsulated / laminated winding housed within copper or other suitable metallic material. The plasma confinement vessel can be a tokamak, preferably a spherical tokamak. Although not necessarily so, preferably, the aspect ratio of the spherical tokamak is 2.5 or less. The aspect ratio is the ratio of the major radius to the minor radius of the toroidal plasma confinement region of the tokamak. Alternatively, the plasma containment vessel may be a stellarator or other plasma confinement system. Each limb may be several meters in length.

[0054] The present invention is summarized in the following numbered clauses.

[0055] Clause 1: An apparatus comprising a first substrate, a second substrate, and a joint between the first substrate and the second substrate, the joint being disposed between the first substrate and the second substrate and being at least one holding medium defining at least one gap having a minimum dimension below a threshold value, whereby the shortest distance between any point within one of the at least one gap and the holding medium is below half of the threshold value, the at least one holding medium, and a filling material disposed between the first substrate and the second substrate and within the at least one gap, the threshold value being 225 μm or less.

[0056] Clause 2: The apparatus according to clause 1, wherein the threshold value is 200 μm or less, more preferably 150 μm or less, even more preferably 125 μm or 100 μm or less.

[0057] Clause 3: The apparatus according to any one of clauses 1 to 2, wherein the at least one holding medium includes a plurality of interconnected elements defining the at least one gap.

[0058] Clause 4: The apparatus according to any one of the preceding clauses, wherein the at least the holding medium is a single holding medium sized and shaped to fill the space between the first substrate and the second substrate.

[0059] Clause 5: The apparatus according to clause 4, wherein the holding medium has a variable thickness.

[0060] Clause 6: The apparatus according to any one of clauses 1 to 3, wherein the joint includes a plurality of holding media.

[0061] Clause 7: The apparatus according to clause 6, wherein each of the plurality of holding media extends between the first substrate and the second substrate.

[0062] Clause 8: The apparatus according to clause 6, wherein the plurality of holding media are of different lengths.

[0063] Clause 9: The apparatus according to any one of Clauses 1 to 3, wherein the holding medium includes an elongated core element from which a plurality of lateral elements extend outwardly with respect to the first or second base material, and the spacing between the lateral elements defines at least one gap.

[0064] Clause 10: The apparatus according to any of the preceding clauses, wherein each base material is a limb of a toroidal magnetic field coil within a plasma confinement vessel.

[0065] Clause 11: The apparatus according to any of the preceding clauses, wherein the holding medium is composed of copper or an alloy thereof, brass or an alloy thereof, stainless steel or an alloy thereof, nickel or an alloy thereof, gold or an alloy thereof, or silver or an alloy thereof.

[0066] Clause 12: The apparatus according to any one of Clauses 1 to 10, wherein the holding medium is composed of carbon fibers or glass fibers coated with copper or an alloy thereof, brass or an alloy thereof, stainless steel or an alloy thereof, nickel or an alloy thereof, gold or an alloy thereof, or silver or an alloy thereof.

[0067] Clause 13: The apparatus according to any of the preceding clauses, wherein the filling material is solder.

[0068] Clause 14: The apparatus according to any of the preceding clauses, wherein the filling material is a brazing material containing silver, copper, brass, bronze, gold, or a gold-silver alloy.

[0069] Clause 15: A plasma confinement vessel comprising the apparatus according to any one of Clauses 1 to 14, wherein each of the first and second base materials is a coil of a superconducting magnet.

[0070] Clause 16: The plasma confinement vessel is a tokamak, preferably a spherical tokamak, more preferably a spherical tokamak having an aspect ratio of 2.5 or less, where the aspect ratio is defined as the ratio of the major radius to the minor radius of the toroidal plasma confinement region of the tokamak, and each of the first and second substrates is a toroidal magnetic field coil, the plasma confinement vessel according to Clause 15.

[0071] Clause 17: The plasma confinement vessel according to Clause 15, wherein the plasma confinement vessel is a stellarator.

[0072] Clause 18: A method of joining a first substrate and a second substrate to thereby form a joint, the method comprising the steps of inserting a holding medium pre-impregnated with a filling material between the first substrate and the second substrate, the holding medium extending between the substrates and defining at least one gap having a minimum dimension below a threshold of 225 μm, whereby the shortest distance between any point within one of the at least one gaps and the holding medium is below half of the threshold; heating the filling material until it melts such that the molten filling material chemically wets the substrates; and solidifying the filling material to form the joint, whereby during the step of melting the filling material, the molten filling material is at least partially retained within the holding medium.

[0073] Clause 19: A method of joining a first substrate and a second substrate, thereby forming a joint, the method comprising the steps of: arranging one or more filler material layers and one or more holding medium layers to form a stack, each of the one or more holding medium layers defining at least one gap having a minimum dimension below a threshold of 225 μm, whereby the shortest distance between any point within one of the at least one gap and the holding medium is below half of the threshold; inserting the stack between the first substrate and the second substrate, the stack extending between the first substrate and the second substrate; heating the one or more filler material layers until melted such that the melted filler material chemically wets the holding medium layer and the first and second substrates, and the melted filler material penetrates the holding medium layer by capillary action; and solidifying the filler material to form the joint.

[0074] Clause 20: The method according to clause 19, wherein the number and / or thickness of the filler material layers in the stack are such that the volume of the filler material is at least as large as the total volume of at least one gap defined by the one or more holding medium layers.

[0075] Clause 21: The method according to any one of clauses 19 to 20, wherein each of the one or more holding medium layers is arranged directly adjacent to one or more filler material layers in the stack.

[0076] Clause 22: The method according to any one of clauses 18 to 21, wherein the joint is formed under vacuum.

[0077] As described above, the present invention has been described with respect to preferred embodiments, but it should be understood that these embodiments are merely illustrative and the claims are not limited to those embodiments. Features from different examples may be combined as appropriate to form other examples.

Claims

1. A method for joining a first substrate and a second substrate, thereby forming a joint, The steps include providing a stack between the first substrate and the second substrate, which includes a filler material and a plurality of holding media, The steps include heating the stack to melt the filler material and wetting the first and second substrates with the molten filler material, The steps include solidifying the molten filling material to form the joint and Methods that include...

2. The method according to claim 1, wherein each holding medium is an open structure comprising a plurality of interconnected or interlaced elements.

3. The method according to claim 2, wherein the open structure includes a mesh.

4. The method according to claim 1, wherein each retaining medium defines a plurality of gaps, the size of which the gaps are such that molten filler material can penetrate the retaining medium by capillary action and / or the molten filler material is held in the gaps by surface tension.

5. The method according to claim 4, wherein the minimum dimension of the gap in each holding medium does not exceed a threshold, and as a result, the shortest distance between any point in one of the gaps and the holding medium is less than or equal to half of the threshold, and the threshold is 225 μm or less.

6. The method according to claim 4, wherein the size of the gap in each holding medium is in the range of 10 to 200 μm.

7. The step of providing one or more further stacks between the first substrate and the second substrate, each containing a filler material and a plurality of holding media. It further includes, The method according to claim 1, wherein the number of holding media arranged in the stack is changed.

8. The method according to claim 1, wherein the first and second substrates are at an oblique angle to each other, and the number of holding media in the stack varies across the first and second substrates so as to change the height of the stack.

9. The method according to claim 1, wherein the length of the holding medium in the stack changes monotonically between the first substrate and the second substrate.

10. The step of providing the stack and / or one or more further stacks, The step of arranging multiple foils to form a stack, each foil containing a retaining medium impregnated with a filler material. The method according to claim 7, including the method described in claim 7.

11. The step of providing the stack and / or one or more further stacks, Steps to form a stack by arranging at least one filler material layer and multiple holding media. The method according to claim 7, including the method described in claim 7.

12. The method according to claim 1, wherein the thickness of each holding medium is in the range of 25 to 200 μm.

13. The step of compressing the stack or each stack, which includes the filling material and the plurality of holding media, between the first substrate and the second substrate. The method according to claim 1, further comprising:

14. An apparatus comprising a first substrate, a second substrate, and a joint between the first substrate and the second substrate, wherein the joint is A plurality of holding media arranged to form a stack between the first substrate and the second substrate, A filling material extending between the first substrate and the second substrate and penetrating the stack A device including a device.

15. The apparatus according to claim 14, wherein each holding medium is an open structure comprising a plurality of interconnected or interlaced elements.

16. The apparatus according to claim 15, wherein each holding medium includes a mesh.

17. One or more further stacks, each containing a plurality of holding media, between the first substrate and the second substrate. It further includes, The filling material extends through each of the stacks, The number of holding media placed in the stack changes. The apparatus according to claim 14.

18. The apparatus according to claim 14, wherein the first and second substrates are at an oblique angle to each other, and the number of holding media in the stack varies across the first and second substrates so as to change the height of the stack.

19. The apparatus according to claim 14, wherein the length of the holding medium in the stack changes monotonically between the first substrate and the second substrate.

20. The apparatus according to claim 14, wherein the minimum dimension of any of the multiple gaps within each holding medium does not exceed a threshold, and as a result, the shortest distance between any point within one of the gaps and the holding medium is less than or equal to half of the threshold, and the threshold is 225 μm or less.

21. The apparatus according to claim 14, wherein the thickness of each holding medium is in the range of 25 to 200 μm.

22. The apparatus according to claim 14, wherein the holding medium is made of copper or an alloy thereof, brass or an alloy thereof, stainless steel or an alloy thereof, nickel or an alloy thereof, gold or an alloy thereof, or silver or an alloy thereof.

23. The apparatus according to claim 14, wherein each substrate is a limb of a toroidal magnetic field coil in a plasma confinement vessel.

24. The apparatus according to claim 14, wherein the filling material includes silver, copper, brass, bronze, gold, or a gold-silver alloy.

25. A plasma confinement vessel comprising the apparatus according to any one of claims 14 to 24, wherein each of the first and second substrates is a coil of a superconducting magnet.

26. The plasma confinement vessel according to claim 25, wherein the plasma confinement vessel is a tokamak, and each of the first and second substrates is a toroidal magnetic field coil.

27. The plasma confinement vessel according to claim 25, wherein the plasma confinement vessel is a stellarator.

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