Method for producing a metal-ceramic substrate and heating furnace

Abstract

The invention relates to a method for producing a metal-ceramic substrate and a heating furnace suitable for carrying out the method. With the method, a metal-ceramic substrate having increased thermal and current conductivity can be obtained. The method comprises the following steps: a) providing a stack comprising a1) a ceramic body, a2) a metal foil, and a3) a solder material in contact with the ceramic body and the metal foil, the solder material comprising: (i) a metal having a melting point of at least 700°C, (ii) a metal having a melting point of less than 700°C, and (iii) an active metal; and b) heating the stack through a heating zone for heating.

Description

[0001] The present invention relates to a method for producing metal-ceramic substrates and a heating furnace suitable for carrying out the method.

[0002] Metal-ceramic substrates play a crucial role in power electronics. They are key components in the construction of electronic parts, ensuring the rapid dissipation of large amounts of heat during operation. Metal-ceramic substrates typically consist of a ceramic layer and a metal layer bonded to the ceramic layer.

[0003] Several methods for attaching metal layers to ceramic layers are known from the prior art. In the so-called direct copper bonding (DCB) method, a copper foil surface has a copper compound (typically copper oxide) with a melting point lower than copper, resulting from the reaction of copper with a reactive gas (usually oxygen). When the copper foil treated in this way is applied to a ceramic body and the composite is heated, the copper compound melts and wets the surface of the ceramic body, thereby forming a stable, integral bond between the copper foil and the ceramic body. This method is described, for example, in US 3744120 A or DE 2319854 C2.

[0004] Despite its obvious advantages, the DCB method has two main drawbacks. First, it must be carried out at relatively high temperatures (specifically slightly below the melting point of copper). Second, it can only be used for oxide-based ceramics such as alumina or surface-oxidized aluminum nitride. Therefore, an alternative method is needed to produce metal-ceramic substrates under less stringent conditions. In one alternative method, a metal foil can be attached to a ceramic body at temperatures ranging from approximately 650°C to 1000°C, using a specific solder containing a metal (typically silver) with a melting point of at least 700°C and an active metal. The active metal acts to react with the ceramic material, allowing it to bond to the remaining solder to form a reaction layer, while the metal with a melting point of at least 700°C is used to attach this reaction layer to the metal foil. For example, JP4812985 B2 proposes using a solder containing 50% to 89% by weight of silver, along with copper, bismuth, and the active metal, to attach copper foil to a ceramic body. Using this method, the copper foil can be reliably bonded to the ceramic body. To avoid problems associated with silver migration, it may be advantageous to use a silver-free solder to attach the metal foil to the ceramic body. These solders are based, for example, on high-melting-point metals (particularly copper), low-melting-point metals (such as bismuth, indium, or tin), and active metals (such as titanium). This technique is proposed, for example, in DE 102017114893 A1. This technique, in principle, can create a new, independent class of connections because the base material of the solder used is formed from another metal (copper instead of silver), which leads to changes in material properties and creates adaptations to other solder compositions and improved bonding conditions. Therefore, the metal-ceramic substrate produced in this way has, in addition to the metal and ceramic layers, a bonding layer containing an active metal located between the metal and ceramic layers.

[0005] Due to increasing demand in the power electronics field, there is a need to further improve the thermal conductivity and current conductivity of metal-ceramic substrates produced using solder materials containing metals with a melting point of at least 700°C, metals with a melting point of less than 700°C, and active metals.

[0006] Existing methods for increasing the thermal and current conductivity of metal-ceramic substrates focus on altering the composition of the bonding layer between the metal and ceramic layers. However, this may be advantageous when the composition of the bonding layer remains constant for various reasons. Therefore, for example, a given composition of the bonding layer could ideally meet technical requirements beyond thermal and current conductivity, be easy to manufacture, or even be more cost-effective. Thus, it is advantageous to improve the thermal and current conductivity of a metal-ceramic substrate with a given composition of the bonding layer through appropriate methodological measures.

[0007] Therefore, the object of the present invention is to provide a method by which a metal-ceramic substrate with increased thermal conductivity and current conductivity can be obtained using a solder material comprising a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, and an active metal.

[0008] This objective is achieved by the method of claim 1. Therefore, the present invention provides a method for producing a metal-ceramic substrate, the method comprising the following steps:

[0009] a) Provide a stack, the stack containing

[0010] a1) Ceramic body,

[0011] a2) Metal foil, and

[0012] a3) A solder material in contact with the ceramic body and the metal foil, the solder material comprising:

[0013] (i) Metals with a melting point of at least 700°C

[0014] (ii) Metals with a melting point less than 700°C, and

[0015] (iii) Active metals, and

[0016] b) Heating the stack, which passes through a heating zone for heating.

[0017] The present invention also relates to a heating furnace suitable for carrying out the method.

[0018] In the method according to the invention, a stack is first provided, the stack comprising a ceramic body, a metal foil, and a solder material in contact with the ceramic body and the metal foil.

[0019] Therefore, the solder material is preferably located between the ceramic body and the metal foil in the stack. According to a preferred embodiment, the stack includes a ceramic body, a (first) metal foil, a (first) solder material in contact with the ceramic body and the first metal foil, a second metal foil, and a second solder material in contact with the ceramic body and the second metal foil. According to this embodiment, the (first) solder material is preferably located between the ceramic body and the (first) metal foil, and the second solder material is preferably located between the ceramic body and the second metal foil. Furthermore, according to this embodiment, the first solder material preferably corresponds to the second solder material.

[0020] Therefore, the ceramic body preferably has a first surface and a second surface. The metal foil preferably has a first surface. The second metal foil (if present) preferably has a first surface. According to a preferred embodiment, the (first) solder material is thus located between the first surface of the ceramic body and the first surface of the (first) metal foil in the stack. According to another preferred embodiment, the stack includes a second solder material in contact with the second surface of the ceramic body and the first surface of the second metal foil. According to this embodiment, the (first) solder material is preferably located between the first surface of the ceramic body and the first surface of the (first) metal foil in the stack, and the second solder material is preferably located between the second surface of the ceramic body and the first surface of the second metal foil. According to another preferred embodiment, no additional layer is located between the ceramic body and the (first) metal foil except for the solder material according to the invention. According to yet another embodiment, no additional layer is located between the ceramic body and the second metal foil (if present) except for the solder material according to the invention.

[0021] The ceramic body is preferably an insulating ceramic. According to one preferred embodiment, the ceramic is selected from the group consisting of oxide ceramics, nitride ceramics, and carbide ceramics. According to another preferred embodiment, the ceramic is selected from the group consisting of metal oxide ceramics, silicon oxide ceramics, metal nitride ceramics, silicon nitride ceramics, boron nitride ceramics, and boron carbide ceramics. According to a particularly preferred embodiment, the ceramic is selected from the group consisting of aluminum nitride ceramics, silicon nitride ceramics, and alumina ceramics (such as zirconia-toughened alumina (ZTA) ceramics). The ceramic body preferably has a thickness in the range of 0.05 mm to 10 mm, more preferably in the range of 0.1 mm to 5 mm, and particularly preferably in the range of 0.15 mm to 3 mm.

[0022] The metal of the foil is preferably selected from the group consisting of copper, aluminum, and molybdenum. According to a particularly preferred embodiment, the metal of the foil is selected from the group consisting of copper and molybdenum. According to a very particularly preferred embodiment, the metal of the foil is copper. The metal foil preferably has a thickness in the range of 0.01 mm to 10 mm, more preferably in the range of 0.03 mm to 5 mm, and particularly preferably in the range of 0.05 mm to 3 mm.

[0023] The solder material comprises (i) a metal with a melting point of at least 700°C, (ii) a metal with a melting point of less than 700°C, and (iii) an active metal.

[0024] According to a preferred embodiment, the solder material contains at least one metal component, namely (i) a metal with a melting point of at least 700°C, (ii) a metal with a melting point of less than 700°C, and (iii) an active metal. Therefore, the solder material preferably comprises at least one metal component containing (i) a metal with a melting point of at least 700°C, (ii) a metal with a melting point of less than 700°C, and (iii) an active metal. For example, it may be preferred that the solder material comprises: a metal component (i) containing a metal with a melting point of at least 700°C, a metal component (ii) containing a metal with a melting point of less than 700°C, and a metal component (iii) containing an active metal. Furthermore, it may be preferred that the solder material comprises: a metal component (i) containing members of the group consisting of (i) metals with a melting point of at least 700°C, (ii) metals with a melting point of less than 700°C, and (iii) active metals; and a metal component (ii) containing members of the group consisting of (i) metals with a melting point of at least 700°C, (ii) metals with a melting point of less than 700°C, and (iii) active metals, not included in metal component (i). The term "metal component" is not further limited. In addition to metals and metal alloys, it also includes metal compounds such as intermetallic phases and other compounds such as metal hydrides. According to a preferred embodiment, the metal component is therefore selected from the group consisting of metals, metal alloys, and metal compounds.

[0025] The solder material comprises (i) a metal with a melting point of at least 700°C. The metal with a melting point of at least 700°C preferably has a melting point of at least 850°C, and particularly preferably at least 1000°C. According to a preferred embodiment, the metal with a melting point of at least 700°C is selected from the group consisting of copper, nickel, tungsten, and molybdenum. According to a particularly preferred embodiment, the metal with a melting point of at least 700°C is copper. According to another preferred embodiment, the solder material comprises a metal component (i) containing a metal with a melting point of at least 700°C. According to a particularly preferred embodiment, the solder material comprises a metal component (i) containing copper. According to another preferred embodiment, the metal component (i) is copper.

[0026] Solder material (ii) comprises a metal with a melting point less than 700°C. The metal with a melting point less than 700°C preferably has a melting point less than 600°C, and particularly preferably less than 550°C. According to a preferred embodiment, the metal with a melting point less than 700°C is selected from the group consisting of tin, bismuth, indium, gallium, zinc, antimony, and magnesium. According to a particularly preferred embodiment, the metal with a melting point less than 700°C is tin. According to another preferred embodiment, the solder material comprises a metal component (ii) containing a metal with a melting point less than 700°C. According to a particularly preferred embodiment, the metal component (ii) is an alloy of a metal with a melting point less than 700°C and another metal. The other metal may be selected, for example, from the group consisting of metals with melting points less than 700°C, metals with melting points of at least 700°C, and active metals. According to another preferred embodiment, the metal component (ii) containing a metal with a melting point less than 700°C is selected from the group consisting of tin, bismuth, indium, gallium, zinc, antimony, magnesium, tin-copper alloys, tin-bismuth alloys, tin-antimony alloys, tin-zinc-bismuth alloys, and indium-tin alloys. According to yet another particularly preferred embodiment, the metal component (ii) containing a metal with a melting point less than 700°C is selected from the group consisting of tin, tin-copper alloys, tin-bismuth alloys, tin-antimony alloys, tin-zinc-bismuth alloys, and indium-tin alloys.

[0027] The solder material contains an active metal. The active metal is preferably a metal that forms a bond between the solder and ceramic formed by a chemical reaction of the solder material components. According to a preferred embodiment, the active metal is selected from the group consisting of hafnium, titanium, zirconium, niobium, tantalum, vanadium, and cerium. According to a more preferred embodiment, the active metal is selected from the group consisting of hafnium, titanium, zirconium, niobium, and cerium. According to a particularly preferred embodiment, the active metal is selected from the group consisting of hafnium, titanium, and zirconium. According to a very particularly preferred embodiment, the active metal is titanium. According to another preferred embodiment, the solder material contains a metal component (iii) containing an active metal. According to a particularly preferred embodiment, the metal component (iii) is an active metal alloy or an active metal compound, particularly preferably an active metal hydride. The metal component (iii) is preferably selected from the group consisting of titanium hydride, titanium-zirconium-copper alloy, zirconium hydride, and hafnium hydride. According to a particularly preferred embodiment, the metal component (iii) is selected from the group consisting of hafnium hydride, titanium hydride, and zirconium hydride. According to a very particularly preferred embodiment, the metal component (iii) is titanium hydride.

[0028] According to a preferred embodiment, the proportion of metal with a melting point of at least 700°C is 50% to 90% by weight, more preferably 55% to 90% by weight, particularly preferably 65% ​​to 90% by weight, and very particularly preferably 70% to 90% by weight, relative to the total metal weight of the solder material. According to another preferred embodiment, the proportion of metal with a melting point less than 700°C is 5% to 45% by weight, more preferably 5% to 40% by weight, particularly preferably 5% to 30% by weight, and very particularly preferably 5% to 25% by weight, relative to the total metal weight of the solder material. According to yet another preferred embodiment, the proportion of active metal is 1% to 20% by weight, more preferably 1% to 15% by weight, particularly preferably 1% to 12% by weight, and very particularly preferably 1% to 10% by weight, relative to the total metal weight of the solder material.

[0029] The solder material is preferably silver-free or contains only a small amount of silver. Therefore, the proportion of silver relative to the total metal weight of the solder material is preferably less than 3.0% by weight, particularly preferably less than 1.0% by weight, and very particularly preferably less than 0.2% by weight. The absence of silver or the presence of only a small amount of silver means that silver migration at the edges of the bonding layer in the finished metal-ceramic substrate can be avoided or reduced. Surprisingly, it has been found that the method according to the invention can also improve the current conductivity and thermal conductivity of metal-ceramic substrates with such reduced silver content. This is surprising because, since silver is replaced as a solder base, such metal-ceramic substrates effectively represent a separate category of bonding with other material properties, which sometimes requires adaptation to other solder compositions and modified bonding conditions.

[0030] According to another preferred embodiment, the solder material contains a small amount of silicon or no silicon at all. Therefore, relative to the total weight of all metals and semi-metals in the solder material, the proportion of silicon is preferably less than 3.0% by weight, particularly preferably less than 1.0% by weight, and very particularly preferably less than 0.5% by weight.

[0031] The solder material is in contact with both the ceramic body and the metal foil. Therefore, the solder material is preferably located between the ceramic body and the metal foil. For example, the solder material can be provided on the ceramic body, and then the metal foil can be applied onto the solder material. The solder material is preferably at least one material selected from the group consisting of a paste, foil, and deposit containing a metal with a melting point of at least 700°C, a metal with a melting point less than 700°C, and an active metal. Therefore, the solder material can also be formed from two or more materials with different compositions. For example, the first material, preferably in direct contact with the ceramic body, may contain a metal component (iii) containing an active metal, and the second material, preferably disposed between the first material and the metal foil, may contain a metal component (i) containing a metal with a melting point of at least 700°C and a metal component (ii) containing a metal with a melting point less than 700°C.

[0032] Solder material can be a paste. The paste preferably comprises (a) at least one metallic component containing a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, and an active metal, and (b) an organic medium.

[0033] The organic medium is preferably an organic medium commonly used in the relevant technical field. Preferably, the organic medium comprises an organic binder, an organic dispersion medium, or a mixture thereof.

[0034] Preferably, the organic binder is removed from the solder material during the heating process. The organic binder is preferably thermoplastic or thermosetting. Examples of organic binders include cellulose derivatives (such as ethyl cellulose, butyl cellulose, and cellulose acetate), polyethers (such as polyoxymethylene), and acrylic resins (such as polymethyl methacrylate and polybutylene methacrylate).

[0035] The organic dispersion medium is preferably an organic compound that imparts a suitable viscosity to the paste and is discharged during the drying or heating process of the paste. The organic dispersion medium can be selected from, for example, aliphatic alcohols, terpene alcohols, alicyclic alcohols, aromatic cyclic carboxylic acid esters, aliphatic esters, carbitol, and aliphatic polyols. Examples of organic dispersion media include octanol, decanol, terpineol (e.g., dihydroterpineol), cyclohexanol, dibutyl phthalate, carbitol, ethyl carbitol, ethylene glycol, butanediol, and glycerol.

[0036] In addition, the paste may contain conventional additives. Examples of such additives include inorganic binders (such as glass frit), stabilizers, surfactants, dispersants, rheology modifiers, wetting agents, defoamers, fillers, and hardeners.

[0037] According to a preferred embodiment, the proportion of at least one metallic component, comprising a metal with a melting point of at least 700°C, a metal with a melting point less than 700°C, and an active metal, is 20% to 95% by weight, more preferably 30% to 95% by weight, and particularly preferably 75% to 95% by weight, relative to the total weight of the paste. According to another preferred embodiment, the proportion of the organic medium is 5% to 80% by weight, more preferably 5% to 70% by weight, and particularly preferably 5% to 25% by weight, relative to the total weight of the paste.

[0038] According to another preferred embodiment, the ratio of the total weight of (a) at least one metal component comprising a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, and an active metal, to the weight of (b) the organic medium is at least 5:1, particularly preferably at least 7:1, and very particularly preferably at least 8:1. According to another preferred embodiment, the ratio of the total weight of (a) at least one metal component comprising a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, and an active metal, to the weight of (b) the organic medium is in the range of 1:1 to 20:1, particularly preferably in the range of 2:1 to 20:1, and very particularly preferably in the range of 5:1 to 15:1.

[0039] To provide a stacking effect, the paste is preferably applied to the surface of the ceramic body. The paste can be applied, for example, by a dispersion method or a printing method. Suitable printing methods include, for example, screen printing, inkjet printing, and offset printing. Preferably, the paste is applied to the surface of the ceramic body by screen printing.

[0040] After applying the paste, it can be pre-dried if necessary. Pre-drying can be carried out at room temperature or high temperature. The pre-drying conditions can vary depending on the organic medium contained in the paste. The pre-drying temperature can be, for example, in the range of 50°C to 180°C, and preferably in the range of 80°C to 150°C. Pre-drying typically takes from 2 minutes to 2 hours, preferably from 5 minutes to 1 hour.

[0041] The surface of the metal foil can then be applied to the paste, and pre-dried if necessary, to obtain a stack.

[0042] Solder material can also be foil.

[0043] The foil comprises (i) a metal with a melting point of at least 700°C, (ii) a metal with a melting point of less than 700°C, and (iii) an active metal. Additionally, the foil may contain other components, such as a suitable binder.

[0044] The foil can be obtained, for example, by homogenizing at least one metal component containing a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, an active metal, and optional additional components, and heating them to a temperature below the melting temperature of the metal with a melting point of at least 700°C, the metal with a melting point of less than 700°C, and the active metal, but sufficient to form a bond between the metals. This temperature can be, for example, at least 200°C.

[0045] Alternatively, the foil can be obtained, for example, by mixing at least one metal component comprising a metal with a melting point of at least 700°C, a metal with a melting point of less than 700°C, an active metal, and a binder, and forming and heating the mixture to form a green compact. During the heating process, the binder can solidify and form a matrix in which the metal is distributed.

[0046] For example, to provide a stack, a foil can be placed on ceramic. The surface of a metal foil can then be applied to the foil located on the ceramic to obtain the stack.

[0047] According to another embodiment, the solder material can be a deposit. The solder deposit can be produced, for example, by electroplating or chemical vapor deposition. Preferably, the solder material deposit is produced on a ceramic body. A metal foil can then be applied to the solder material deposited on the ceramic to obtain a stack.

[0048] After the stack is provided, it is heated, and the stack passes through a heating zone for heating.

[0049] Preferably, the stack is heated to obtain a metal-ceramic substrate. According to a preferred embodiment, heating is performed to obtain the metal-ceramic substrate, thereby forming an integral bond between the ceramic body and the metal foil via a solder material. The integral bond is preferably formed such that an active metal becomes part of the connection with the ceramic body, and metals with a melting point of at least 700°C, metals with melting points less than 700°C, and the metal foil are connected to form an alloy. During subsequent solidification, the integral bond is then formed between the ceramic body and the metal foil via a solder material.

[0050] Therefore, conditions enabling the formation of an integral bond between the ceramic and metal foil via solder material are preferably dominant in the heating zone. The temperature and atmosphere present in the heating zone are preferably adjustable. The heating zone preferably has an inlet and an outlet. When passing through the heating zone, the stack preferably enters the heating zone through the inlet and exits through the outlet. Preferably, the inlet is different from the outlet.

[0051] According to a preferred embodiment, the stack and the heating zone are arranged such that the position of the stack can be changed relative to the position of the heating zone to allow the stack to pass through the heating zone. Preferably, the distance between the stack and the heating zone decreases before passing through the heating zone, reaches a minimum during passing through the heating zone, and increases after passing through the heating zone. According to a preferred embodiment, this is done by relative movement of the stack and the heating zone, wherein the stack and the heating zone initially perform relative movement toward each other, and perform relative movement away from each other after passing through. For this purpose, the stack can be arranged in a stationary manner and the heating zone can be arranged to be movable, or the stack and the heating zone can be arranged in a stationary manner.

[0052] As the stack passes through the heating zone, it experiences a temperature rise. Therefore, the stack is kept at a certain distance from the heating zone as it passes through, which ensures the temperature input required to form a monolithic bond between the ceramic body and the metal foil.

[0053] Surprisingly, it has been found that metal-ceramic substrates with improved thermal conductivity and current conductivity can be obtained when the stack passes through the heating zone used for heating.

[0054] The desire to avoid being bound by theory is likely due to the fact that targeted control of energy input can be achieved by passing the stack through the heating zone. If the stack passes through the heating zone, the temperature within the zone and the speed at which the stack passes through can be ideally matched to the stack's structure and dimensions, ensuring that only the energy input required to form an integral bond between the ceramic body and the metal foil occurs. This prevents excessive energy input, which typically leads to increased diffusion of metals with melting points at least 700°C and those with melting points less than 700°C into the metal foil, potentially resulting in reduced electrical and thermal conductivity in the finished metal-ceramic substrate. Furthermore, uniform distribution of temperature and gases (e.g., inert gases) within the heating zone is ensured during passage. Therefore, when producing multiple metal-ceramic substrates using the method according to the invention, the resulting metal-ceramic substrates exhibit less variation in quality compared to conventional methods (e.g., using a batch heating furnace). In this respect, heating the stack by passing through the heating zone has proven superior to fixed heating (e.g., in a batch heating furnace).

[0055] According to a particularly preferred embodiment, the stack is heated in a heating furnace, preferably in a continuous heating furnace.

[0056] The heating furnace preferably has a heating zone and a carrier system. The stack is preferably arranged on the carrier system. Preferably, the heating zone and carrier system are designed such that the position of the stack can be changed relative to the position of the heating zone to allow heating of the stack as it passes through the heating zone. Preferably, the heating zone and carrier system are therefore designed such that the distance between the stack and the heating zone can decrease until it reaches a minimum during passage through the heating zone, and can increase after passing through the heating zone. According to a preferred embodiment, the heating zone and carrier system are therefore designed for relative movement, such that the stack and the heating zone initially perform relative movement toward each other, and perform relative movement away from each other after passing through.

[0057] The heating furnace is preferably a continuous heating furnace. Therefore, according to a preferred embodiment, a stack is heated in a continuous heating furnace, wherein the stack passes through a heating zone of the continuous heating furnace during the heating process. The continuous heating furnace preferably has at least one heating zone and, for example, a rotary conveyor chain, conveyor roller system, or sliding system as a carrier system on which the workpiece can be conveyed through the heating zone. In the conveying direction, there may be additional zones in the continuous heating furnace before and after the heating zone. For example, it may be advantageous for a cooling zone to be located after the heating zone in the continuous heating furnace. Additionally, it may be advantageous for a gas inlet and a gas outlet (through which gases (e.g., inert gases, such as nitrogen) can be supplied to these zones) to be located in the heating zone and any other zones that may be present. Such continuous heating furnaces are well known in the prior art (see, for example, DE 4008979 C1 and EP 0085914 A2).

[0058] In a preferred embodiment of the method according to the invention, the stack is first applied to a carrier. For example, the carrier may be made of silicon carbide. The silicon carbide carrier may be provided with additional supports, such as graphite foil.

[0059] The stacks, preferably arranged on a carrier, are then placed on a carrier system (e.g., a conveyor belt). The conveyor belt can be, for example, a conveyor chain, a conveyor roller system, or a sliding system of a continuous heating furnace.

[0060] According to a preferred embodiment, the stack passes through a heating zone on a carrier system. The carrier system is preferably driven, for example, by means of rollers.

[0061] During the heating process, the stack is heated to a peak temperature. The peak temperature is not further limited, but is preferably less than or equal to the melting point of a metal with a melting point of at least 700°C and lower than the melting point of the metal in the metal foil. According to a preferred embodiment, the peak temperature is at least 10°C lower than the melting point of the metal in the metal foil, particularly preferably at least 50°C lower. According to another preferred embodiment, the peak temperature is at least 700°C. The peak temperature is preferably in the range of 700°C to 1100°C, particularly preferably in the range of 750°C to 1050°C, and very particularly preferably in the range of 800°C to 1000°C. As used herein, the peak temperature refers to the temperature measured at the stack using a thermocouple. The peak temperature is the maximum temperature measured at the stack. To prevent adverse effects due to excessive fluidity of the molten metal, such as excessive shrinkage or seepage of the molten metal, those skilled in the art will seek to avoid excessively high peak temperatures.

[0062] During the heating process, the stack experiences a temperature input for the duration of the heating. The heating duration herein preferably refers to the period during which the stack is exposed to a temperature of at least 200°C during the heating process. The heating duration is not further limited, as long as it is sufficient to ensure the wetting of the surfaces to be joined and their availability for overall bonding. According to a preferred embodiment, the heating duration is at least 2 minutes, particularly preferably at least 10 minutes. According to another preferred embodiment, the heating duration does not exceed 5 hours, particularly preferably not more than 2 hours, and very particularly preferably not more than 90 minutes. The heating duration is preferably in the range of 2 minutes to 5 hours, particularly preferably in the range of 2 minutes to 2 hours, and very particularly preferably in the range of 10 minutes to 90 minutes.

[0063] During the heating process, the stack experiences a temperature input for a duration of high-temperature heating. The high-temperature heating duration herein preferably refers to the time period during which the stack is exposed to a temperature corresponding to at least -250°C of the peak temperature during heating. Therefore, for an exemplary peak temperature of 900°C, the high-temperature heating duration corresponds to the time period during which the stack is exposed to a temperature of at least 650°C during heating. According to a preferred embodiment, the high-temperature heating duration does not exceed 60 minutes, more preferably not more than 50 minutes, particularly preferably not more than 45 minutes, and very particularly preferably not more than 40 minutes. The high-temperature heating duration is preferably in the range of 2 minutes to 60 minutes, more preferably in the range of 3 minutes to 50 minutes, particularly preferably in the range of 5 minutes to 45 minutes, and very particularly preferably in the range of 10 minutes to 40 minutes.

[0064] During the heating process, the stack experiences a temperature input during the peak temperature heating duration. The peak temperature heating duration herein preferably refers to the time period during which the stack is exposed to a temperature corresponding to at least -50°C of the peak temperature during the heating process. Therefore, for an exemplary peak temperature of 900°C, the peak temperature heating duration corresponds to the time period during which the stack is exposed to a temperature of at least 850°C during the heating process. According to a preferred embodiment, the peak temperature heating duration does not exceed 30 minutes, more preferably not more than 25 minutes, particularly preferably not more than 20 minutes, and very particularly preferably not more than 15 minutes. The peak temperature heating duration is preferably in the range of 1 minute to 30 minutes, more preferably in the range of 1 minute to 25 minutes, particularly preferably in the range of 2 minutes to 20 minutes, and very particularly preferably in the range of 3 minutes to 15 minutes.

[0065] According to another particularly preferred embodiment, the high-temperature heating duration is in the range of 10 to 40 minutes, and the peak temperature heating duration is in the range of 3 to 15 minutes.

[0066] Surprisingly, shorter high-temperature heating duration and shorter peak temperature heating duration have been found to have a favorable effect on the current conductivity and thermal conductivity of the finished metal-ceramic substrate.

[0067] According to another preferred embodiment, the ratio of peak temperature heating duration (in minutes) to heating duration (in minutes) does not exceed 1:2. The ratio of peak temperature heating duration (in minutes) to heating duration (in minutes) is preferably in the range of 1:2 to 1:15, more preferably in the range of 1:2 to 1:10, particularly preferably in the range of 1:2 to 1:7, and very particularly preferably in the range of 1:3 to 1:6. Surprisingly, it has been found that when the ratio of peak temperature heating duration (in minutes) to heating duration (in minutes) is within the specified range, the thermal conductivity and current conductivity of the metal-ceramic substrate can be further improved.

[0068] When the stack is heated, temperature input occurs during the heating duration. The heating duration here preferably refers to the time required for the stack to reach its peak temperature from a starting temperature of 100°C. According to a preferred embodiment, the heating duration does not exceed 60 minutes, particularly preferably not more than 45 minutes, and very particularly preferably not more than 30 minutes. The heating duration is preferably in the range of 1 minute to 60 minutes, more preferably in the range of 5 minutes to 45 minutes, and particularly preferably in the range of 10 minutes to 30 minutes.

[0069] A non-oxidizing atmosphere is preferably present in the heating zone. The non-oxidizing atmosphere is preferably an inert gas atmosphere. Preferably, a nitrogen, helium, or argon atmosphere is present in the heating zone. According to a particularly preferred embodiment, a nitrogen atmosphere is present in the heating zone. Preferably, the proportion of reactive gases, particularly oxygen, in the non-oxidizing atmosphere is less than 1000 ppm, more preferably less than 500 ppm, and particularly preferably less than 40 ppm.

[0070] During heated stacking, an integral bond is formed between the ceramic body and the metal foil via solder material to obtain a metal-ceramic substrate. If desired, the metal-ceramic substrate can undergo further processing steps. For example, the exposed surface of the metal foil, preferably the metal foil of the metal-ceramic substrate, can be polished. Preferably, the surface of the metal foil of the metal-ceramic substrate is physically or chemically polished. Furthermore, the metal-ceramic substrate can be constructed. For example, the metal-ceramic substrate can be provided with conductive traces. The conductive traces are preferably formed by etching.

[0071] The metal-ceramic substrate produced according to the present invention can be used particularly for electronic applications, especially in the field of power electronics.

[0072] According to a preferred embodiment, the present invention also relates to a heating furnace as described above with respect to the method. The heating furnace preferably has...

[0073] (1) Heating zone,

[0074] (2) Carrier system, and

[0075] (3) Stacking, the stacking being arranged on the carrier system and comprising

[0076] a1) Ceramic body,

[0077] a2) Metal foil, and

[0078] a3) A solder material in contact with the ceramic body and the metal foil, the solder material comprising:

[0079] (i) Metals with a melting point of at least 700°C

[0080] (ii) Metals with a melting point less than 700°C, and

[0081] (iii) Active metals,

[0082] The heating zone and the carrier system are designed such that the position of the stack can be changed relative to the position of the heating zone, so as to allow the stack to be heated as it passes through the heating zone.

[0083] According to a particularly preferred embodiment, the heating furnace is a continuous heating furnace. The carrier system is preferably designed as a rotary conveyor chain, a conveyor roller system, or a sliding system. According to a preferred embodiment, the stack is arranged on the conveyor chain and can move along the conveyor chain through the heating zone.

[0084] The heating furnace is particularly suitable for implementing the method according to the invention.

[0085] Exemplary Implementation :

[0086] In the embodiments, metal-ceramic substrates are produced under various conditions. In each case, a stack comprising a ceramic body, a metal foil, and a solder material in contact with the ceramic body and the metal foil is provided, and then heated. The solder material is a standard paste containing copper, tin, and titanium as metals. Current and thermal conductivity are then qualitatively evaluated. Similar results can be obtained with other material combinations.

[0087] Example 1 :

[0088] To produce the metal-ceramic substrate, 31.67 wt% SnCu0.7 powder, 7.24 wt% titanium hydride, and 9.50 wt% Texanol-containing organic mediator were first mixed in a vertical mixer at 35 Hz for 20 minutes. Then, 51.59 wt% copper powder was added incrementally. The resulting mixture was stirred at high speed until a homogeneous paste was obtained.

[0089] Using the paste produced in this way, the opposing surfaces of the ceramic body are bonded to both sides of the copper foil. For this purpose, a ceramic body (obtained from Toshiba Materials) with the same front and back characteristics and dimensions of 177.8mm × 139.7mm × 0.32mm is used in each case. The paste is screen-printed onto the back of this ceramic body with dimensions of 137 × 175mm using a 165-mesh sieve. 2 The paste was pre-dried at 125°C for 15 minutes in the designated area. The thickness of the pre-dried paste was 35μm ± 5μm. The resulting arrangement was then flipped over, and the paste was similarly printed onto the front side of the ceramic body and pre-dried. Copper foil made of oxygen-free, highly conductive copper with a purity of 99.99% and dimensions of 174mm × 137mm × 0.3mm was then applied to both sides of the ceramic body with the paste on both sides to obtain a stack with the following structure: copper foil - pre-dried paste - ceramic - pre-dried paste - copper foil.

[0090] The stack is then heated in a continuous heating furnace. For this, a silicon carbide plate with graphite foil is first placed on the conveyor belt of the continuous heating furnace. The stack is placed on the graphite foil, then covered with another graphite foil and weighted with another silicon carbide plate (weight = 600g). The structure is then conveyed through the heating zone of the continuous heating furnace. The peak temperature (measured at the stack using a type K thermocouple manufactured by Temperatur Messelemente Hettstedt GmbH) is 935°C. The resulting metal-ceramic substrate is then cooled to room temperature to obtain a metal-ceramic substrate comprising ceramic layers connected to copper layers on both sides via interconnecting layers.

[0091] Example 2 :

[0092] Example 2 was performed similarly to Example 1, but the peak temperature (measured at the stack using a K-type thermocouple manufactured by Temperatur MesselementeHettstedt GmbH) was 910°C.

[0093] Comparative example :

[0094] Similar to Example 1 in the comparative example, but the stack was heated in a batch heating furnace instead of a continuous heating furnace. For this purpose, the stack was placed in a batch heating furnace and heated. The peak temperature (measured at the stack using a type K thermocouple manufactured by Temperatur MesselementeHettstedt GmbH) was 910°C.

[0095] The thermal conductivity and current conductivity of the metal-ceramic substrates obtained in the embodiments are then evaluated as follows:

[0096] Example 1 high Example 2 high Comparative example Low

[0097] It has been found that the conductivity of metal-ceramic substrates can be significantly improved when using the method according to the invention. This improvement, not to be bound by theory, is likely due to the fact that, compared to conventional methods, the method according to the invention achieves a precise target energy input by only temporarily passing through the heating zone. This target energy input is high enough on the one hand to ensure integral bonding, but on the other hand to prevent the metal from penetrating deeply from the solder material into the metal foil. Therefore, it can be shown that, in the method according to the invention, the depth to which metals with melting points less than 700°C (tin in the examples) penetrate into the metal foil is less than in conventional methods. Depth distribution analysis of the metal-ceramic substrates obtained in Examples 1 and 2 shows that the diffusion of tin into the copper foil is reduced by more than 30% compared to the metal-ceramic substrates of the comparative examples. Therefore, in Examples 1 and 2, the conductivity of the finished metal-ceramic substrates is less compromised.

Claims

1. A method for producing a metal-ceramic substrate, the method comprising the following steps: a) Provide a stack, the stack containing a1) Ceramic body, a2) Metal foil, and a3) Solder material in contact with the ceramic body and the metal foil, wherein the solder material is a homogeneous paste comprising the following components: (i) Metals with a melting point of at least 700°C (ii) Metals with a melting point less than 700°C, and (iii) Active metals, and b) Heating the stack, the stack passing through a heating zone for heating. The proportion of the metal with a melting point of at least 700°C relative to the total metal weight of the solder material is 50% to 90% by weight. The proportion of the metal with a melting point less than 700°C relative to the total metal weight of the solder material is 5% to 45% by weight. The proportion of the active metal relative to the total metal weight of the solder material is from 1% to 20% by weight. The proportion of silver relative to the total metal weight of the solder material is less than 3.0% by weight, and Heating the stack includes forming an integral bond between the ceramic body and the metal foil via the solder material.

2. The method according to claim 1, characterized in that, The ceramic body is selected from the group consisting of aluminum nitride ceramics, silicon nitride ceramics, and alumina ceramics.

3. The method according to claim 1 or 2, characterized in that, The metal in the metal foil is copper.

4. The method according to claim 1 or 2, characterized in that, The metal with a melting point of at least 700°C is copper.

5. The method according to claim 1 or 2, characterized in that, The metals with a melting point less than 700°C are selected from the group consisting of tin, bismuth, indium, gallium, zinc, antimony, and magnesium.

6. The method according to claim 1 or 2, characterized in that, The active metal is selected from the group consisting of hafnium, titanium, zirconium, niobium, tantalum, vanadium, and cerium.

7. The method according to claim 1 or 2, characterized in that, A non-oxidizing atmosphere exists in the heating zone.

8. The method according to claim 1 or 2, characterized in that, A nitrogen atmosphere is present in the heating zone.

9. The method according to claim 1 or 2, characterized in that, The peak temperature heating duration shall not exceed 30 minutes, wherein the peak temperature heating duration refers to the duration during which the stack is exposed to a temperature corresponding to at least -50°C of the peak temperature.

10. The method according to claim 9, characterized in that, The heating duration shall not exceed 60 minutes, wherein the heating duration represents the time required for the stack to reach the peak temperature from a starting temperature of 100°C.

11. A heating furnace, said heating furnace having at least the following features: (1) Heating zone, (2) Carrier system, and (3) Stacking, the stacking being arranged on the carrier system and comprising a1) Ceramic body, a2) Metal foil, and a3) Solder material in contact with the ceramic body and the metal foil, wherein the solder material is a homogeneous paste comprising the following components: (i) Metals with a melting point of at least 700°C (ii) Metals with a melting point less than 700°C, and (iii) Active metals, The proportion of the metal with a melting point of at least 700°C relative to the total metal weight of the solder material is 50% to 90% by weight. The proportion of the metal with a melting point less than 700°C relative to the total metal weight of the solder material is 5% to 45% by weight. The proportion of the active metal relative to the total metal weight of the solder material is from 1% to 20% by weight. The proportion of silver relative to the total metal weight of the solder material is less than 3.0% by weight. The heating zone and the carrier system are designed to allow the position of the stack to be changed relative to the position of the heating zone, so as to allow heating of the stack as it passes through the heating zone. Heating the stack includes forming an integral bond between the ceramic body and the metal foil via the solder material.