Substrate for a power module and power module encompassing the substrate

Abstract

Substrate (10) for a power module, wherein the substrate (10) comprises a ceramic layer (12) and at least a first metal layer (14a) and a second metal layer (14b), wherein the ceramic layer (12) is arranged between the first metal layer (14a) and the second metal layer (14b), wherein in the first metal layer (14a) a side edge (16a) of the metal layer (14a) and an area (18) adjoining the side edge (16a) on a surface (20a) of the first metal layer (14a) facing away from the ceramic layer (12) is coated with a coating (22), and wherein in the second metal layer (14b) a side edge (16b) of the metal layer (14b) is coated with a coating (22) and a surface (20b) of the second metal layer (14b) facing away from the ceramic layer (12) is free of the coating (22).

Description

Technical field

[0001] The invention relates to a substrate for a power module.

[0002] Furthermore, the invention relates to a power module comprising the above substrate. background

[0003] Metal-ceramic substrates, and in particular copper-ceramic substrates, are increasingly being used as base substrates or printed circuit boards in power modules designed for higher operating voltages, for example, operating voltages of 600 V and higher.

[0004] In the typical case, such substrates have a ceramic layer which is provided with a metallization on both surface sides, whereby the metallization on the top side can be structured, for example by using an etching technique, in such a way that this metallization then forms the conductor tracks, contact surfaces and / or similar required for the circuit.

[0005] Another essential component of metal-ceramic substrates is the insulating layer formed by the ceramic layer. Due to their comparatively high insulating strength, ceramic insulating layers have proven particularly advantageous in power electronics.

[0006] A prerequisite for providing a metal-ceramic substrate is a permanent bond between the metal layer and the ceramic layer. Besides a so-called direct bonding method, i.e., a DCB (direct copper bonding) or DAB (direct aluminum bonding) method, it is known from the prior art to bond the metal layer to the ceramic layer using a brazing material (AMB method; active metal brazing).

[0007] Due to the different coefficients of thermal expansion of the metal in the metal layer and the ceramic in the ceramic layer, as well as due to different thicknesses and / or structures of the layers, thermomechanical stresses regularly arise, which can even lead to bending of the metal-ceramic substrate. Furthermore, corrosion resistance can be problematic, especially at higher moisture levels in the substrate. Description

[0008] Starting from the prior art, the present invention aims to provide a substrate for power modules that has high corrosion resistance, a reduced tendency to bend, reduced thermomechanical stress and / or high thermal resistance, especially under repeated temperature changes.

[0009] This problem is solved by a substrate for a power module according to claim 1 and a power module according to claim 15. Further advantages and properties will become apparent from the dependent claims, the description, and the accompanying figures.

[0010] According to the present invention, a substrate for a power module is provided, wherein the substrate comprises a ceramic layer and at least one first metal layer and a second metal layer, wherein the ceramic layer is arranged between the first and the second metal layer, wherein in the first metal layer a side edge of the metal layer and an area adjoining the side edge on a surface of the first metal layer facing away from the ceramic layer is coated with a coating, and wherein in the second metal layer a side edge is coated with a coating and a surface of the second metal layer facing away from the ceramic layer is free of the coating.

[0011] The side edge of the metal layer is, in particular, that edge of the metal layer which does not run parallel to the ceramic layer. Instead, the side edge of the metal layer preferably connects the surface of the metal layer, which preferably extends substantially parallel to the ceramic layer and faces away from the ceramic layer, with the surface of the metal layer, which preferably also extends substantially parallel to the ceramic layer and faces the ceramic layer. The side edge preferably runs approximately perpendicular to the ceramic layer and may also be chamfered. Furthermore, it is possible for the side edge to have a concave or convex profile.

[0012] One aspect of the invention is that the side edge of the first metal layer and the side edge of the second metal layer are each at least partially coated with the coating. Furthermore, the surface of the second metal layer facing away from the ceramic layer is free of the coating, whereas the surface of the first metal layer facing away from the ceramic layer is not completely free of the coating. Instead, an area on the surface of the first metal layer facing away from the ceramic layer is also coated with the coating, namely the area directly adjacent to the side edge. Preferably, the surface of the first metal layer is not completely coated, but only partially. In other words, preferably an area near the edge of the surface of the first metal layer is coated, and an area further away from the edge is uncoated.

[0013] Thus, an asymmetrically coated substrate is provided with respect to the first and second metal layers. This offers advantages in terms of corrosion resistance, as the corrosion-prone edges in particular are coated. Furthermore, the overall stress within the substrate is reduced. As a result, the substrate exhibits high thermal resistance, especially under repeated temperature changes.

[0014] Furthermore, the substrate has the advantage that the manufacturing process simplifies and ensures that the side edge of the first metal layer is completely coated, and that no gaps occur in the coating of the side edge of the first metal layer due to the manufacturing process. The substrate also offers advantages in further processing, especially in high-temperature processes such as soldering.

[0015] Since the surface of the second metal layer is also free of coating, a further advantage is improved processability of the substrate for certain processes such as ultrasonic welding and / or soldering. In addition, the coating-free surface of the second metal layer enables optimal heat dissipation to a heat sink. Furthermore, the service life of solder joints that can be applied to the first and / or second metal layer, for example, for attaching a power semiconductor, is increased, especially under temperature fluctuations.

[0016] The substrate is therefore particularly suitable for power modules with power semiconductors designed for a voltage range of 3.3 kV or more.

[0017] The substrate can have a ceramic layer sandwiched between exactly two metal layers—that is, the first and second metal layers. Alternatively, it can have a ceramic layer sandwiched between more than two metal layers. For example, on the first side of the ceramic layer, where the first metal layer is located, there may be additional metal layers alongside the first metal layer. These additional metal layers are preferably arranged flat next to the first metal layer on this first side. A gap may exist between the first metal layer and the additional metal layers on this first side. It is also possible for the first metal layer to have recesses. In other words, the first metal layer does not necessarily have to cover the entire surface of the ceramic layer.According to a preferred embodiment of the invention, the substrate comprises a conductive track pattern on the side of the first metal layer. A conductive track pattern preferably means that the first metal layer does not completely cover the ceramic layer or that further metal layers are present alongside the first metal layer, so that areas exist where the ceramic layer is free of the first metal layer or of a further metal layer.

[0018] The described coating pattern, in which a side edge of the first metal layer and an area adjoining that side edge on the surface of the first metal layer facing away from the ceramic layer are coated, can also be implemented for the subsequent metal layers on the first side. Alternatively, the subsequent metal layers can be coated differently.

[0019] Analogous to the first metal layer, additional metal layers can be located on a second side of the ceramic layer where the second metal layer is situated. These additional metal layers are preferably arranged flat next to the second metal layer on the second side. A gap may exist between the second metal layer and the additional metal layers. The described coating pattern, in which a side edge of the second metal layer is at least partially coated and the surface of the second metal layer facing away from the ceramic layer is free of coating, can also be implemented for the additional metal layers on the second side. Alternatively, the additional metal layers on the second side can be coated in a different way. As a further alternative, the second side of the ceramic layer can consist solely of the second metal layer.

[0020] According to a preferred embodiment of the invention, the coating comprises nickel and preferably nickel and phosphorus. Alternatively or additionally, it is preferred that the coating comprises silver, palladium, and / or gold. These coatings have proven particularly suitable for providing a corrosion-resistant substrate. Preferably, the coating comprises nickel.

[0021] In this context, it is further preferred that the coating be an electroless nickel plating. In other words, the coating is preferably applied by electroless electroless nickel plating. Unlike electroplating, this differs, among other things, from the fact that no external electric current, such as from a rectifier, is used for deposition; instead, the electrons necessary for depositing the nickel ions are generated within the plating bath itself by means of a redox reaction. This allows for coatings with very precise contours. Preferably, the electroless nickel plating is a nickel-phosphorus plating, whereby properties such as the hardness or brittleness of the coating can be controlled by the phosphorus content. Preferably, the coating has a phosphorus content of less than 14% by mass and, more preferably, more than 3% by mass.

[0022] As an alternative to chemical coating, it is also possible to apply the coating using an electroplating process, and / or vapor deposition, and / or sputtering.

[0023] According to a preferred embodiment of the invention, the first and second metal layers are each connected to the ceramic layer via an intermediate layer, and the coating covering the side edge of the metal layer extends over the intermediate layer. It is further preferred that the intermediate layer comprises silver.

[0024] To bond the metal layer to the ceramic layer, the so-called DCB (direct copper bonding) process is known to be used. This process employs metal foils or sheets forming the metallization, which have at least on their surface facing the ceramic layer a fused layer or a coating of a chemical compound consisting of the metal and a reactive gas, preferably oxygen. During substrate production, the fused layer or coating forms a eutectic with a melting temperature below that of the metal in the metal layer. This allows the foil to be placed on the ceramic layer and all layers to be bonded together by heating. A disadvantage of this process is the potential for process-related defects between the metallization and the ceramic layer, which can negatively affect the partial discharge resistance of the substrate.

[0025] As an alternative to the DCB process, the substrate can be produced by an active soldering process, and particularly preferably by active brazing. In this embodiment of the invention, it is therefore preferred that the first metal layer and / or second metal layer is connected to the ceramic layer by means of active brazing. In active brazing, the metal layer is connected to the ceramic layer via a solder. Preferably, the solder forms the intermediate layer. More preferably, the solder comprises silver.

[0026] Regardless of the manufacturing process, it can occur due to the process that the interlayer between the metal and ceramic layers is slightly larger in area than the metal layer and therefore protrudes at the side edge of the metal layer. This protrusion of the interlayer also has the advantage of ensuring a complete bond between the ceramic and metal layers, avoiding voids and thus helping to reduce stress. Furthermore, the protrusion of the interlayer above the metal layer reduces local stresses, which can have a positive effect on the partial discharge resistance of the substrate. It can also reduce the risk of cracks forming in the ceramic layer, which can have a beneficial effect on both the service life and the partial discharge resistance.To increase the corrosion resistance of the substrate, it is particularly preferred that this protruding portion of the intermediate layer is also coated with the coating. In other words, the coating extends over the intermediate layer.

[0027] In this context, according to a further preferred embodiment of the invention, the planar extent of the first metal layer and / or the second metal layer is smaller than that of the ceramic layer, so that the ceramic layer protrudes and the coating covering the side edge of the metal layer extends to the surface of the protruding ceramic layer. In other words, the coating at the side edge of the metal layer extends down to the ceramic layer and preferably lies directly against the ceramic layer.

[0028] With regard to the side edge, a further preferred embodiment may provide that the side edge of the first metal layer and / or the second metal layer is chamfered, and / or concave and / or convex. This offers particular advantages for the application of the coating.

[0029] With regard to the material of the metal layers, a further preferred embodiment of the invention provides that the first and / or second metal layer comprises copper, aluminum, a copper alloy, and / or an aluminum alloy. It is further preferred that the first and / or second metal layer consists of copper, aluminum, a copper alloy, or an aluminum alloy. Particularly preferred is that the metal layer comprises copper or consists of copper.

[0030] With regard to the ceramic layer, a further preferred embodiment of the invention provides that the ceramic layer comprises aluminum oxide, aluminum nitride, and / or silicon nitride. More preferably, the ceramic layer consists of aluminum oxide, aluminum nitride, or silicon nitride. Particularly preferably, the ceramic layer comprises aluminum oxide and / or aluminum nitride and preferably consists of aluminum oxide or aluminum nitride.

[0031] Preferably, the ceramic layer is connected to the copper-encompassing or copper-containing metal layers via the silver-encompassing intermediate layer, wherein the coating is the electroless nickel coating.

[0032] As already mentioned, it is provided that the surface of the first metal layer facing away from the ceramic layer is not completely free of coating, but rather that the area directly adjoining the side edge is coated on the surface. In this context, according to a further preferred embodiment of the invention, it is provided that the area adjoining the side edge on the surface of the first metal layer has a perpendicular extension from the side edge of at least 0.05 mm and preferably at least 0.1 mm.

[0033] Furthermore, it is preferably provided in this context that the area adjoining the side edge on the surface of the first metal layer has a perpendicular extension from the side edge of up to 2 mm. By extension, we mean in particular the length of the area on the surface measured from the side edge towards the center of the surface.

[0034] It is possible, and preferably preferred, that the surface of the first metal layer has the coated area around the entire perimeter near the edge. In other words, according to a preferred embodiment of the invention, the coated area on the surface of the first metal layer extends around the entire perimeter, and preferably completely, along the side edge. Alternatively, it is possible that the surface of the first metal layer has areas at the edge where no coating is present on the surface.

[0035] It is further preferably provided that the coating is applied by means of a mask on the side edge of the first metal layer, on the side edge of the second metal layer, and on the area adjacent to the side edge on the surface of the first metal layer. The mask, or a resist applied by means of the mask, prevents in particular the area on the surface of the first metal layer adjacent to the edge area from being coated. Furthermore, the mask or resist prevents in particular the surface of the second metal layer from being coated. Alternatively, a full coating followed by an etching process can also be used to apply the coating.

[0036] It is further preferably provided that the coating layer thickness is greater than or equal to 1 µm. It is further preferably provided that the coating layer thickness is less than or equal to 15 µm. Such coating layer thicknesses can be achieved particularly well with electroless nickel plating.

[0037] Alternatively, particularly when using coatings that do not include nickel, the coating thickness can be greater than or equal to 50 nm. This is especially advantageous in conjunction with gold or silver.

[0038] As previously mentioned, the substrate may include a conductor track pattern on the side of the first metal layer. In this context, it is further preferably provided that all side edges defined by the conductor track pattern are coated with the coating. Alternatively, it may be provided that only some side edges are coated. It is further preferably provided that all areas on the surface adjoining the side edges, extending up to 2 mm perpendicularly from the side edge, are coated with the coating. Alternatively, it may be provided that only those conductor track sections connected to a power semiconductor have a coating in the area near the side edges on the surface. It is further preferably provided that conductor track patterns with a planar extent of up to 4 mm in one dimension are completely coated with the coating in this dimension.Furthermore, it may be provided that local uncoated windows are present within coated areas. Likewise, local coatings may be present within larger uncoated areas. Additionally, it is possible that contact windows for power semiconductor connections, for example for mounting chips or terminals, are located in fully coated areas.

[0039] According to a further preferred embodiment of the invention, it is more preferably provided that the substrate on the side of the second metal layer is free of a conductive track pattern and / or that the second metal layer is continuous across its entire surface. In other words, the second metal layer preferably has no areas where the ceramic layer is not covered by the second metal layer, except for the edge region where the ceramic layer protrudes. In this way, the second metal layer enables particularly good heat transfer. Alternatively, the substrate can also include a conductive track pattern on the side of the second metal layer.

[0040] As already mentioned, the invention also relates to a power module comprising the substrate mentioned above. In this context, the problem is also solved by a power module comprising a substrate and one or more power semiconductors, wherein the substrate comprises a ceramic layer and at least one first metal layer and a second metal layer, wherein the ceramic layer is arranged between the first and second metal layers, wherein, in the case of the first metal layer, a side edge of the metal layer and an area adjoining the side edge on a surface of the first metal layer facing away from the ceramic layer are coated with a coating, wherein, in the case of the second metal layer, a side edge is coated with a coating and a surface of the second metal layer facing away from the ceramic layer is free of the coating, and wherein the one or more power semiconductors are applied to the first metal layer.

[0041] The power module is particularly well-suited for voltages of 3.3 kV or higher.

[0042] The invention is explained in more detail below with reference to the accompanying drawings and a preferred embodiment.

[0043] In the drawings shows: Fig. 1 a schematic top view of a substrate according to a preferred embodiment of the invention, and Fig. 2 a schematic cross-sectional view of the substrate made of Fig. 1 at the location marked AA. Detailed description of the exemplary embodiment

[0044] Fig. Figure 1 shows a top view of a substrate 10 for a power module according to a preferred embodiment of the invention, while Fig. 2 a cross-sectional view of the substrate 10 of the Fig. 1 at the point marked AA.

[0045] As in the Fig. As can be seen from Figures 1 and in particular 2, the substrate 10 comprises a ceramic layer 12, a first metal layer 14a and a second metal layer 14b, wherein the ceramic layer 12 is arranged between the first metal layer 14a and the second metal layer 14b.

[0046] A side edge 16a of the first metal layer 14a and an area 18 adjoining the side edge 16a on a surface 20a of the first metal layer 14a facing away from the ceramic layer 12 is coated with a coating 22.

[0047] In the second metal layer 14b, a side edge 16b is also coated with the coating 22. However, in the second metal layer 14b, a surface 20b of the second metal layer 14b facing away from the ceramic layer 12 is free of the coating 22.

[0048] Furthermore, in the top view in Fig. It is clearly evident that the substrate 10 comprises a conductive track pattern on the side of the first metal layer 14a. Therefore, in addition to the first metal layer 14a, further metal layers are arranged, so that in a top view the ceramic layer 12 is visible next to the metal layer 14a. The further metal layers can be coated with the coating 22 analogously to the metal layer 14a as described above. Alternatively or additionally, they can be coated differently. It is evident that the surface 20a of the further metal layers, particularly in the area of ​​the centrally arranged conductive track pattern, is completely coated with the coating 22.

[0049] In this embodiment, the ceramic layer 12 is made of aluminum nitride, and the metal layers 14a and 14b are made of copper. The coating 22, in this embodiment, is an electroless nickel coating 22 with a phosphorus content between 3 wt% and 14 wt%. The coating 22 has a thickness of 5 µm.

[0050] As in Fig. As can be seen in Figure 2, the side edges 16a, 16b of the first metal layer 14a and the second metal layer 14b are chamfered. Furthermore, the first metal layer 14a and the second metal layer 14b are each connected to the ceramic layer 12 via an intermediate layer 24. In this case, the intermediate layer 22 comprises silver and, during the production of the substrate 10, acted as a solder, enabling the joining of the metal layers 14a, 14b to the ceramic layer 12 by means of active brazing. The surface area of ​​the first metal layer 14a and the second metal layer 14b is smaller than that of the intermediate layer 24, so that the intermediate layer 24 protrudes slightly beyond the metal layers 14a, 14b. In particular, the coating 22 covering the side edges 16a, 16b of the metal layers 14a, 14b extends over the intermediate layer 24.

[0051] Also in Fig. 2 It is evident that the surface area of ​​the first metal layer 14a and the second metal layer 14b is smaller than that of the ceramic layer 12, so that the ceramic layer 12 protrudes. In particular, the coating 22 covering the side edge 16a, 16b of the metal layer 14a, 14b extends to the surface of the protruding ceramic layer 12.

[0052] As particularly in Fig. As can be seen in Figure 1, which shows the top view, the area 18 extends around the side edge 16a on the surface 20a of the first metal layer 14a. In the present embodiment, there are therefore no areas at the edge where there is no coating 22 on the surface 20a. As can be seen, the surface 20a of the first metal layer 14a is not completely coated, but only partially, with the area 18 near the edge of the surface 20a being coated and an area further from the edge being uncoated.

[0053] In the present embodiment, the area 18 adjoining the side edge 16a on the surface 20a of the first metal layer 14a has a dimension 26 perpendicular to the side edge 16a of 0.1 mm to 0.4 mm. By dimension 26, we mean in particular the extent of the area 18 on the surface 14a measured from the side edge 16a towards a center of the surface 14a. Fig. 1 evident that the extent 26 of area 18 at a point in the Fig. 1. The upper area is less than the lateral area of ​​the substrate. 10.

[0054] The described embodiment is merely an example that can be modified and / or supplemented in various ways within the scope of the claims. Each feature described for a particular embodiment can be used independently or in combination with other features in any other embodiment. Each feature described for an embodiment of a particular category can also be used accordingly in an embodiment of a different category. Reference symbol list 10 substrate 12 ceramic layers 14a first metal layer 14b second metal layer 16a Side edge of the first metal layer 16b Side edge of the second metal layer 18 Area on the surface of the first metal layer 20a Surface of the first metal layer 20b Surface of the second metal layer 22 Coating 24 Intermediate layer 26 Expansion

Claims

[1] Substrate (10) for a power module, wherein the substrate (10) comprises a ceramic layer (12) and at least a first metal layer (14a) and a second metal layer (14b), wherein the ceramic layer (12) is arranged between the first metal layer (14a) and the second metal layer (14b), wherein in the first metal layer (14a) a side edge (16a) of the metal layer (14a) and a region (18) adjoining the side edge (16a) on a surface (20a) of the first metal layer (14a) facing away from the ceramic layer (12) are coated with a coating (22), and wherein in the second metal layer (14b) a side edge (16b) of the metal layer (14b) is coated with a coating (22) and a surface (20b) of the second metal layer (14b) facing away from the ceramic layer (12) is free of the coating (22). [2] Substrate (10) according to claim 1, wherein the coating (22) comprises nickel and preferably comprises nickel and phosphorus, and / or wherein the coating (22) comprises silver, palladium and / or gold. [3] Substrate (10) according to claim 1 or 2, wherein the coating (22) is an electroless nickel coating. [4] Substrate (10) according to one of the preceding claims, wherein the first metal layer (14a) and the second metal layer (14b) are each connected to the ceramic layer (12) via an intermediate layer (24) and the coating (22) coating the side edge (16a, 16b) of the metal layer (14a, 14b) extends over the intermediate layer (24). [5] Substrate (10) according to the preceding claim, wherein the intermediate layer (24) comprises silver. [6] Substrate (10) according to one of the preceding claims, wherein the first metal layer (14a) and / or second metal layer (14b) is connected to the ceramic layer (12) by means of active brazing. [7] Substrate (10) according to one of the preceding claims, wherein the areal extent of the first metal layer (14a) and / or the second metal layer (14b) is smaller than that of the ceramic layer (12), so that the ceramic layer (12) protrudes and wherein the coating (22) coating the side edge (16a, 16b) of the metal layer (14a, 14b) extends to the surface of the protruding ceramic layer (12). [8] Substrate (10) according to one of the preceding claims, wherein the side edge (16a, 16b) of the first metal layer (14a) and / or the second metal layer (14b) is bevelled and / or has a concave and / or convex profile. [9] Substrate (10) according to one of the preceding claims, wherein the first metal layer (14a) and / or second metal layer (14b) comprises copper, aluminum, a copper alloy, and / or an aluminum alloy and preferably consists of copper, aluminum, a copper alloy, or an aluminum alloy. [10] Substrate (10) according to one of the preceding claims, wherein the ceramic layer (12) comprises aluminum oxide, aluminum nitride, and / or silicon nitride, preferably consists of aluminum oxide, aluminum nitride, or silicon nitride, and particularly preferably consists of aluminum nitride. [11] Substrate (10) according to one of the preceding claims, wherein the region (18) adjoining the side edge (16a) on the surface (20a) of the first metal layer (14a) comprises an extension (26) perpendicular to the side edge (16a) of at least 0.05 mm and preferably of at least 0.1 mm. [12] Substrate (10) according to one of the preceding claims, wherein the region (18) adjoining the side edge (16a) on the surface (20a) of the first metal layer (14a) comprises an extension (26) perpendicular to the side edge (16a) of up to 2 mm. [13] Substrate (10) according to one of the preceding claims, wherein the region (18) on the surface (20a) of the first metal layer (14a) extends circumferentially and preferably completely circumferentially along the side edge (16a). [14] Substrate (10) according to one of the preceding claims, wherein the first metal layer (14a) comprises a conductor track pattern, and / or wherein the second metal layer (14b) is free of a conductor track pattern and / or wherein the second metal layer (14b) is continuous over its entire surface. [15] Substrate (10) according to one of the preceding claims, wherein a layer thickness of the coating (22) is greater than or equal to 1 µm and / or wherein a layer thickness of the coating (22) is less than or equal to 15 µm. [16] Power module comprising a substrate (10) according to one of the preceding substrate claims, and one or more power semiconductors, wherein the one power semiconductor or the plurality of power semiconductors are applied to the first metal layer (14a).

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