Method for soldering an electronic component to a coated base and electronic component for an inverter

Abstract

The invention relates to a method for soldering an electronic component (2) to a coated base (1) which is made of a metal and is at least partially coated with a coating (1a) made of nickel or a nickel-based alloy (1a), comprising the steps of: a) chemically cleaning the coating (1a) by removing an oxide layer using a cleaning gas; b) soldering the electronic component (2) on the cleaned coating (1a) using a solder, wherein a soldering temperature is selected in such a way that the coating (1a) remains in a solid state and the solder forms a connection (3) to the cleaned coating (1a).

Description

[0001] Description

[0002] Method for soldering an electronic component to a coated base and electronic component for an inverter

[0003] The invention relates to a method for soldering an electronic component to a coated base and to an electronic component for an inverter.

[0004] Soldering methods are known from the prior art for connecting electronic components, such as a substrate on which a power electronic element is provided, to a coated base, which may be, for example, a coated heat sink for thermal dissipation of heat generated by the power electronic module. Particularly in high-performance electronics for inverters, reliable, efficient heat dissipation or a tight thermal connection, and thus a durable solder connection with a low void rate between the heat sink and the substrate with the power electronic module, is essential.

[0005] In this context, copper is typically used as the material for the heat sink, which has high thermal conductivity and is bonded to the substrate using a brazing process. However, the copper heat sink must be protected, particularly against chemical attack from the coolants used, such as glycol mixed with water. This is conventionally achieved by partially nickel-plating the surface of the heat sink, with the solder joints to be soldered being exempt from the coating.

[0006] The production of partially coated copper coolers requires additional process steps such as using masks or masking with stencils, and is complex and expensive. The use of the brazing process, which bonds the copper to the substrate at the solder joints at relatively high soldering temperatures using a silver solder, also negatively impacts costs.

[0007] From a cost perspective, the production of fully coated copper heat sinks is therefore preferred. However, soldering on nickel-plated surfaces traditionally requires liquid or pasty fluxes, since to achieve a durable solder joint, an oxide layer that impairs the soldering process—which typically forms on a nickel-based coating under atmospheric influence—must be removed, at least from the sections of the nickel-plated heat sink to be soldered.

[0008] However, the fluxes used are often chemically aggressive and can damage electronic components through corrosion. Therefore, flux residues must be carefully removed after soldering, as they can otherwise severely impair the functionality of the power electronic module. Failure to clean sufficiently can lead to subsequent failure of the power electronic module.

[0009] The use of fluxes and the cleaning processes before and after soldering are also complex and expensive and significantly increase production costs.

[0010] The invention is therefore based on the object of providing teachings concerning favorable solder joints with low void rates between electronic components and surfaces susceptible to oxide layers. At the very least, the object of the invention is to provide an alternative.

[0011] This object is achieved by a method for soldering an electronic component with a coated base according to claim 1 and an electronic component for an inverter according to claim 6.

[0012] Preferred embodiments are the subject of the dependent patent claims. The invention was conceived against the background of process development for developing a cost-optimized, flux-free soldering process for connecting a heat sink to a substrate and a power electronics module located thereon to form an electronic component for an inverter.

[0013] The inventive method for soldering an electronic component essentially uses a gas or gas mixture instead of a flux for chemically cleaning a coating of a base or for removing an oxide layer on the coating of the base by chemically reducing the surface of the coating. The method provides for the heat sink and substrate to be joined at relatively low temperatures compared to the brazing process for copper, which allow the heat sink coating to remain in a solid state and thus protect it, creating a high-quality, defect-free solder joint with a high production yield.

[0014] The method can be used to obtain the electronic component according to the invention for an inverter, which in particular has a high durability because the solder joint is of high quality and electronic components located thereon are not corroded by flux residues.

[0015] The electronic component for an inverter manufactured in this way is used particularly in high-performance electronics for inverters for e-mobility.

[0016] According to the invention, a method for soldering an electronic component to a coated base formed from a metal and at least partially coated with a coating of nickel or a nickel-based alloy comprises the steps of: a) chemically cleaning the coating by removing an oxide layer using a cleaning gas; b) soldering the electronic component to the cleaned coating using a solder, wherein a soldering temperature is selected such that the coating remains in a solid state and the solder forms a bond with the cleaned coating.

[0017] The soldering process can be classified as a soft soldering process, whereby the solder forms a material bond with the surfaces to be joined at characteristic soft soldering temperatures below 450 °C. The surfaces are bonded during the soldering step by forming a surface alloy with the solder or by melting the solder and diffusing the solder into the surfaces to be soldered.

[0018] The soldering process, which comprises a chemical cleaning step and a soldering step, is intended to connect the electronic component to the coated base, or in particular to the coating of the base. The electronic component is preferably a power electronic module, such as one or more power semiconductor switches, e.g., one or more Si, SiC, or GaN semiconductor switches, on a substrate. The substrate is, for example, a direct bond copper (DBC) substrate or an active metal brazed (AMB) substrate. However, the substrate can also be provided with another electronic module or without a module.

[0019] The coated base is preferably a coated molded body made of a thermally conductive metal with a coating of nickel or a nickel-based alloy, which serves to protect against mechanical and chemical stress and conventionally forms the oxide layer on a surface. The oxide layer is an approximately one nanometer thick layer of a metal-oxygen compound, which is formed particularly on base metals exposed to an oxygen-containing atmosphere.

[0020] The shaped body can in particular be made of copper, aluminum or steel, wherein the coating is preferably a metallic surface formed by electroplated nickel or nickel-based alloys such as nickel-titanium, nickel-tin or electroless nickel-phosphorus. The oxide layer of the nickel-based coating is reactively removed in the chemical cleaning step using the cleaning gas, preferably an acidic gas mixture, by chemically reducing the surface. For this purpose, the electronic component and the base are introduced, for example, into a hermetically sealed furnace or a vacuum furnace. In this furnace, as described further below, the atmosphere in the furnace or the composition of the gas mixture used for chemical cleaning can be adjusted in order to clean the sections that are intended to be soldered for the soldering step and to prevent the solder orTo enable wetting of the sections to be soldered during the soldering step.

[0021] Gas pressure and temperature can also be adjusted, with particular attention being paid to setting the parameters in such a way as not to damage the electronic component and the base.

[0022] In addition, the temperature used in the chemical cleaning step is preferably significantly below the melting temperature of the solder used to prevent the solder from melting before the soldering step. Preferably, the base coating can be chemically cleaned or chemically reduced at a temperature between 180°C and 230°C, in particular between 180°C and 200°C.

[0023] In the step of soldering the coating of the base and the electronic component, the base and component are preferably locked in a predefined spatial relationship to each other.

[0024] This is preferably a defined distance that significantly influences the thickness of the solder layer between the sections to be soldered. The distance or layer thickness is adjusted to ensure a good service life of the connection, taking into account the thermomechanical stresses that occur during operation, while not excessively increasing the thermal resistance between the electronic component and the base. The solder is preferably a preform or a rolled metal plate made of a solder alloy, which is arranged on the sections to be soldered between the component and the base, particularly preferably on the base.

[0025] The solder alloy of the solder is also preferably a flux-free tin-based solder that is suitable for soft soldering but has a melting temperature that is significantly higher than the set temperature in the chemical reduction step and lower than the melting temperature of the base coating.

[0026] The internal temperature in the furnace, or the temperature at the sections to be soldered with the solder, is adjusted so that the solder melts and wets the sections to be soldered, forming a bond with the coating, which is in a solid state and is attached to the electronic component. This is preferably a diffusion soldering process.

[0027] The method according to the invention enables a fast and simple soldering process for robust solder joints on coatings based on nickel or nickel-based alloys without complex cleaning processes for removing residues of liquid solder.

[0028] In addition, if the base is made of a material not suitable for soldering, such as aluminum or steel, it can be indirectly connected to an electronic component by soldering the coating of the base.

[0029] In the process, the cleaning gas in the chemical cleaning step is preferably a gas mixture comprising nitrogen enriched with formic acid, and a cleaning temperature of between 180°C and 230°C, in particular between 180°C and 200°C, is provided in the chemical cleaning step. The cleaning gas in the chemical cleaning step is preferably a gas mixture comprising nitrogen enriched with formic acid, wherein the concentration of formic acid in the nitrogen is preferably 4-10%, in particular at least 5%. However, other concentrations are also suitable, which are selected taking into account the base to be soldered and the electronic component. The nitrogen can be blown into a bubbler via a lance for enrichment with formic acid and fed into the furnace through an outlet opening of the bubbler.

[0030] Such a gas mixture is particularly suitable if nickel or nickel-phosphorus is chosen for the coating.

[0031] Alternatively, other gas mixtures may also be suitable, for example, nitrogen can be enriched with forming gas and / or hydrogen. Furthermore, other gases can also be added to the nitrogen-formic acid gas mixture according to the invention.

[0032] The chemical cleaning step is preferably carried out at a cleaning temperature suitable for chemical activation of the cleaning gas, preferably between 180 °C and 200 °C, but can also be carried out in a two-stage process, for example, after an intermediate activation step at 120 °C. The cleaning temperature can be individually selected depending on the gas mixture used.

[0033] The gas pressure of the gas mixture in the furnace is preferably between 900 mbar and 1000 bar and is also chosen appropriately to enable chemical reduction of the base coating.

[0034] Furthermore, the solder is preferably a flux-free solder, which in particular comprises SnSb5, and in the soldering step the soldering temperature is between 230 °C and 300 °C.

[0035] After removing the oxide layer in the chemical cleaning step, the flux-free solder, which is preferably tin-based and preferably has a melting point between 220°C and 290°C, can be melted in the soldering step at a suitably adjusted soldering temperature between 230°C and 300°C and wet the sections to be soldered. The solder can diffuse into the sections to be soldered on the base or electronic component in a diffusion soldering process and establish the connection between the sections. SnSb5, which has a melting point between 230 and 240°C, is particularly preferred and is melted at a soldering temperature of preferably 280°C.

[0036] Also preferably, in the method, the base is a fully coated heat sink for the electronic component, which is formed at least from copper, aluminum or steel, and wherein the electronic component has a functional coating of silver, palladium or a nickel alloy in sections, which represents a section of the electronic component to be soldered.

[0037] Especially if the base is a heat sink made of copper, the coating is necessary to protect the heat sink from chemical and mechanical stress caused, for example, by the coolant.

[0038] For cost and manufacturing reasons, one advantage of soldering directly onto the heat sink plating is that inexpensive fully nickel-plated coolers can be used instead of expensive partially coated coolers with deliberately provided uncoated or exposed sections to be soldered, which are, however, difficult to maintain and significantly increase production complexity and costs.

[0039] Furthermore, a complete coating of nickel or a nickel-based alloy enables metals that are conventionally unsuitable for soldering, such as aluminum or steel, which may also be suitable for heat dissipation, to be soldered indirectly via the coating in the process. Particularly preferably, the heat sink can therefore alternatively be formed from coated aluminum or steel, which is more favorable than copper. In addition, the electronic component can have a functional coating on a section of the electronic component, in particular on the section of the substrate of the electronic component to be soldered. The functional coating can be a silver or palladium coating that oxidizes little or not at all, or a nickel-based coating comparable to the base coating, which is cleaned of an oxide layer in the chemical reduction step.

[0040] This enables particularly good solder joints with low void rates to be obtained between the functional coating or the electronic component and the coating or the base.

[0041] The process therefore describes a general process for soldering metallic components that are functionalized by the aforementioned metallic or, in particular, nickel-plated coatings.

[0042] Furthermore, the steps of the method are preferably carried out in at least one hermetically sealed furnace, in particular a vacuum furnace, wherein in the furnace, before the chemical cleaning step, the atmosphere in the furnace is cleaned by at least one flooding with nitrogen gas such that the oxygen concentration in the furnace is preferably below 5 ppm, and the furnace is evacuated before the brazing step.

[0043] During the cleaning step, the atmosphere in the furnace is purified, preferably before using the cleaning gas, by flooding the furnace with nitrogen gas at least once. The aim is to achieve as little oxygen as possible, preferably an oxygen concentration of less than 5 ppm.

[0044] This reduces the likelihood of the oxide layer forming again before the soldering step, allowing for better solder joints to be obtained.

[0045] Before the soldering step, the furnace is preferably evacuated so that the soldering step can be carried out in the furnace at low gas pressure or vacuum to minimize gas inclusions or cavities in the joint. According to the invention, an electronic component for an inverter is obtainable by the method according to the invention.

[0046] According to the invention, in particular a connection or solder connection obtainable by the method between the coated base, which as described above is at least partially coated with a coating of nickel or a nickel-based alloy, and the electronic component, whereby an electronic component for an inverter is obtained, offers the advantages and effects described above.

[0047] Preferably, the electronic component comprises a power electronic module on a substrate, and the connection comprises a diffusion solder connection made of a solder between the substrate and the base, in particular between the functional coating of the substrate of the electronic component and the coating of the base.

[0048] The base with the electronic component soldered onto it is preferably the heat sink made of copper, which is galvanically or chemically nickel-plated, whereby the electronic component is the power electronic module on the substrate or a DBC or AMB substrate for use in an inverter for e-mobility.

[0049] However, the coated base according to the invention with the electronic component soldered thereon, the connection or soldered connection of which is obtainable in particular by the described method, offers further fields of application, such as in other electronic devices, e.g. in shipping, the railway industry, aviation, power generation or even for other mechanical / structural applications.

[0050] A preferred embodiment of the invention is explained below by way of example with reference to the figure. It shows:

[0051] Figure 1 is a schematic representation of an electronic component according to the invention for an inverter with a coated base and an electronic component in a side view, which are connected by a connection obtained by the method according to the invention.

[0052] Fig. 1 is a schematic representation of an electronic component according to the invention for an inverter 10 with a coated base 1 and an electronic component 2 in a side view, which are connected by a connection 3. The electronic component comprises a substrate 2a with two power electronic modules 2b. There is a connection 3 or a solder connection between a surface of the base 1 coated with a coating 1a and a surface with a functional coating 2c of the substrate 2a, wherein in this exemplary embodiment the coating 1a is made in particular of nickel or nickel-phosphorus and the functional coating 2c is made of silver.

[0053] The electronic component 10 illustrated according to the invention is intended for use in an inverter, particularly for e-mobility, and was obtained by the method according to the invention. The electronic component 2 and the base 1 were first introduced at a predefined distance into a hermetically sealed furnace or a vacuum furnace (not shown).

[0054] Then, the chemical cleaning step and the soldering step were carried out using the method according to the invention to obtain the connection 3 between the electronic component 2 and the base 1 and between the functional coating 2c of the substrate 2a and the coating 1a of the base, respectively.

[0055] The base 1 is a heat sink made of copper, aluminum, or steel and is galvanically coated with a nickel or nickel-phosphorus coating 1a. Said metals are preferably suitable for protecting the base against mechanical and chemical influences, whereby they can be chemically cleaned using the gas or gas mixture according to the invention, or their oxide layer can be removed by chemical reduction using the gas or gas mixture.

[0056] In the chemical cleaning step, an oxide layer was removed at least on the coating 1 a using a gas mixture comprising nitrogen enriched with formic acid, preferably at 180 °C, wherein in the soldering step, a preform solder made of SnSb5 was melted preferably at 280 °C in order to connect the points to be soldered to one another by means of the connection 3.

[0057] The power electronic modules 2b represent semiconductor or power chips, such as silicon chips, silicon carbide chips or

[0058] Gallium nitride chips, which in Fig. 1 have been soldered, sintered, or glued onto an upper surface of the substrate 3 for the power electronic modules 2b. These are electronically connected on the substrate 2a in a suitable manner using a wire connection made of copper or aluminum.

[0059] The substrate 2a is a DCB or AMB substrate which, in Fig. 1, has a functional coating 2c on the lower soldered side, which may be, for example, silver or palladium, but may also be a comparable layer to the coating of the base.

[0060] The connection 3 exists between the substrate 2a or the functional coating 2c and that of the base 1 or the coating 1a, which is formed by a flux-free tin-based solder.

[0061] The module shown in Fig. 1 can be a power supply module for e-mobility with a power electronics input, a silicon carbide frame module and a silicon carbide mold module.

[0062] In this embodiment, the base 1 or a fully coated cooler was connected to the electronic component 2 or substrate 2a using the fluxless soldering process. A high-quality, defect-free solder joint with a high production yield was obtained by combining a solder preform process with a tin-based solder, the use of a gas mixture comprising nitrogen enriched with formic acid for the fluxless soldering, and appropriately selected coatings suitable as the sections to be soldered.

[0063] List of reference symbols

[0064] Base 1

[0065] Coating 1a

[0066] Electronic component 2

[0067] Substrate 2a

[0068] Power electronics module 2b

[0069] Functional coating 2c

[0070] Connection 3

[0071] Electronic component for an inverter 10

Claims

Patent claims 1. A method for soldering an electronic component (2) to a coated base (1) made of a metal and at least partially coated with a coating (1a) made of nickel or a nickel-based alloy (1a), comprising the steps of: a) chemically cleaning the coating (1a) by removing an oxide layer using a cleaning gas; b) soldering the electronic component (2) to the cleaned coating (1a) using a solder, wherein a soldering temperature is selected such that the coating (1a) remains in a solid state and the solder forms a connection (3) with the cleaned coating (1a).

2. The method according to claim 1, wherein the cleaning gas in the chemical cleaning step is a gas mixture comprising nitrogen enriched with formic acid, and a cleaning temperature between 180 °C and 230 °C, in particular between 180 °C and 200 °C, is provided in the chemical cleaning step.

3. The method according to claim 1 or 2, wherein the solder is a flux-free tin-based solder, which in particular comprises SnSb5, and in the soldering step the soldering temperature is between 230 °C and 300 °C.

4. Method according to one of claims 1 to 3, wherein the base (1) is a fully coated heat sink for the electronic component (2), which is formed at least from copper, aluminum or steel, and wherein the electronic component (2) has in sections a functional coating (2c) made of silver, palladium, nickel or a nickel-based alloy, which represents a section of the electronic component (2) to be soldered.

5. A method according to claims 1 to 4, wherein the steps of the method are carried out in at least one hermetically sealed furnace, in particular a vacuum furnace be carried out, wherein in the furnace, before the chemical cleaning step, the atmosphere in the furnace is cleaned by at least one flooding with nitrogen gas such that the oxygen concentration in the furnace is preferably below 5 ppm, and the furnace is evacuated before the brazing step.

6. Electronic component for an inverter (10) obtainable by a method according to one of claims 1 to 5.

7. Electronic component (10) according to claim 6, wherein the electronic component (2) comprises a power electronic module (2b) on a substrate (2a), and the connection (3) comprises a diffusion solder connection made of a solder between the substrate (2a) and the base (1), in particular between the functional coating (2c) of the substrate (2a) of the electronic component (2) and the coating (1a) of the base (1).

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