Method for the direct integration of a power electronic substrate onto a heat sink

Abstract

A method for the direct integration of a power electronic substrate onto a heat sink is proposed, comprising the steps of providing several three-layer stacks, each consisting of a top layer of a metal plate, a middle layer of an electrically non-conductive resin plate, and a bottom layer representing a top surface of a heat sink, wherein the heat sink includes a rib structure provided on a bottom surface of the heat sink, the rib structure being arranged such that an outer edge of the bottom surface remains free of the rib structure;and providing a metal base plate comprising several openings, each of which is designed to receive one of the stacks, each opening having a groove along its outer circumference in which a sealing ring is inserted, each opening being sized such that the ribbed structure of a stack can be fully inserted into the opening. Furthermore, generating an arrangement of metal base plate and stacks is achieved by inserting the stacks into the openings of the metal base plate such that each outer edge of a stack overlaps at least partially with the metal base plate, so that the stack and the metal base plate are sealed by the sealing ring.

Description

[0001] The present invention relates to a manufacturing method which consequently provides more effective heat dissipation of power semiconductors.

[0002] In a power electronics application, the heat generated by the power semiconductors in the semiconductor chips (circuits) must be dissipated as quickly as possible to avoid damage to the semiconductors and their adjacent structures. Generally, the power semiconductors are physically mounted on a metallic heat sink, which has a heat-dissipating fin structure on the opposite side and is electrically grounded. Since the power semiconductors are electrically connected to the circuitry, which must also be electrically isolated from ground, several stacked layers are placed between the semiconductors and the heat sink. These layers should include a metal plate beneath the semiconductors and an insulating layer between the metal plate and the heat sink.

[0003] As is known from the prior art, a ceramic plate, typically made of aluminum oxide, is used as the insulating layer. Both sides of the ceramic plate are directly bonded to metal plates, usually made of copper. This sandwich-like three-layer stack is typically called DBC, which stands for "direct bonded copper." One of the metal plates of the DBC is bonded to the underside of the power semiconductors using a bonding material such as solder. The other metal plate is bonded to the heat sink (which is preferably also made of copper) using a thermal interface material (TIM).

[0004] The TIM can also be soldered, but a much softer material such as thermal grease is often preferred, as the soldering process for the heatsink and DBC presents many complications. One of the biggest problems is the large difference in the coefficient of thermal expansion (CTE) between the ceramic and the heatsink.

[0005] Cooling the solder from its solidification temperature results in high stress for both, sometimes leading to deformation. Therefore, the soldering process must be carefully designed to ensure that the stresses can be properly relieved and the resulting shape is as flat as possible after cooling. The use of thermal grease can mitigate the above-mentioned problems, as the grease application process can skip the heating process in the assembly step and easily relieve the stresses between the DBC and the heat sink when the DBC is heated by the semiconductors in the semiconductor chips arranged on the top copper plate. On the other hand, its thermal conductivity is generally much lower than that of solder, and the thickness of the grease layer is very prone to inhomogeneity both in the assembly step and during long-term operation with repeated heating and cooling.As a result, the lateral distribution of thermal resistance at the grease layer is very uneven, which means that the multiple semiconductors mounted on the DBC cannot operate at the same temperature due to the different heat dissipation environments. Since the electrical resistance of semiconductors is very temperature-sensitive, it is very important to keep the semiconductors operating at the same temperature to fully exploit their potential.

[0006] Recently, an epoxy-based resin sheet with ceramic particles as a filler was developed as a replacement for the ceramic sheet of the DBC. By recently improving the resin's thermal conductivity to nearly 1 / 3 that of alumina, equivalent thermal resistance to alumina was achieved by achieving a necessarily high breakdown voltage at only 1 / 3 the thickness of alumina. The CTE of the resin can be closely matched to that of copper, allowing the use of solder for the TIM without incurring various complications caused by the different CTE between the insulating layer and the copper when the heat sink is made of copper.

[0007] The use of the resin plate as a replacement for DBC's aluminum oxide already offers a mechanical advantage in the assembly process, but there is still potential for optimization in both the assembly process and the heat dissipation effect. Therefore, the object of the present invention is to provide a method for integrating a power electronics substrate onto a heat sink so that both the heat dissipation effect and the assembly process are optimized.

[0008] To achieve this object, the present invention proposes a method for directly integrating a power electronic substrate onto a heat sink, comprising the steps of providing a plurality of three-layer stacks, each of which is made of an upper layer of a metal plate, a middle layer of an electrically non-conductive resin plate, and a lower layer constituting an upper side of a heat sink, wherein the heat sink comprises a fin structure provided on a lower side of the heat sink, wherein the fin structure is arranged such that an outer edge of the lower side remains free of the fin structure; and providing a metal base plate comprising a plurality of openings, each of which is designed to receive one of the stacks, wherein each opening has a groove along its outer circumference into which a sealing ring is inserted, wherein each opening has a size such thatthat the rib structure of a stack can be fully inserted into the opening. Furthermore, the assembly of metal base plate and stacks is created by inserting the stacks into the openings of the metal base plate such that each outer edge of a stack at least partially overlaps the metal base plate, so that the stack and the metal base plate are sealed by the sealing ring. Furthermore, a single vacuum film web is provided on top of the assembly, the vacuum film having a size at least equal to the size of the metal base plate, and the area beneath the vacuum film is evacuated. Finally, the assembly is subjected to an autoclaving process.

[0009] In one embodiment, the layers of each stack are pre-fixed to each other.

[0010] In one embodiment, the vacuum film web has a vacuum sealing adhesive on edges of the metal base plate and is fixed to the metal base plate.

[0011] In one embodiment, a venting fabric is provided for filling gaps under the vacuum film web, and a plurality of air tubes are provided in the vacuum film web for evacuating the gaps.

[0012] In one embodiment, the rib structure is made of pinfins.

[0013] In one embodiment, the metal plate is structured into metal plate islands at the time of creating the stack.

[0014] In one embodiment, the semiconductor chips are arranged on the metal plate islands after the autoclaving process.

[0015] In one embodiment, the metal plate is made of copper. In one embodiment, the metal plate surface made of copper is covered with another metal.

[0016] Furthermore, a power electronic device, in particular an inverter, is provided which is manufactured according to the described method.

[0017] For a more complete understanding of the invention, the invention will now be described in detail with reference to the accompanying drawings. The detailed description illustrates and describes what are considered to be preferred embodiments of the invention. It should, of course, be evident that various modifications and changes in form or detail may readily be made without departing from the spirit of the invention. It is therefore intended that the invention be limited neither to the precise form nor detail shown and described herein, nor to anything less than the entirety of the invention herein disclosed and hereinafter claimed. Furthermore, the features described in the specification, drawings, and claims which disclose the invention, considered alone or in combination, may be essential to the invention.In particular, any reference signs in the claims shall not be construed as limiting the scope of the invention.

[0018] The attached drawings show: Fig. 1 shows a DBC arranged on a heat sink according to the prior art; Fig. 2 shows a three-layer stack to be processed according to an embodiment of the invention; Fig. 3 shows a stack ready for evacuation according to an embodiment of the invention; Fig. 4 shows a stack ready for processing by an autoclaving process according to an embodiment of the invention; Fig. 5 a metal plate with multiple openings according to an embodiment of the invention; Fig. 6 is an enlarged view of two openings and their grooves according to an embodiment of the invention; Fig. 7 a metal plate with two stacks placed so that their rib structure is inserted into the opening of the metal plate before evacuation, according to an embodiment of the invention; Fig. 8 a flowchart of the method according to an embodiment of the invention.

[0019] While the present disclosure is subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will be described in detail herein. The invention is not intended to be limited to the particular forms disclosed, but on the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.

[0020] In the following, embodiments are described with reference to metal plates made of copper, as this is the most commonly used material. Of course, other metals can be used instead of copper where appropriate. Furthermore, the surface of a copper plate can be covered with another metal such as silver or nickel. Furthermore, the invention is not limited to the use of DBC as a substrate. Where appropriate, other substrates comprising at least a top metal plate, an insulating layer, and a heat sink can also be used.

[0021] In Fig. Figure 1 schematically shows a DBC with an upper copper plate 1 and a lower copper plate 3 sandwiching a ceramic layer or an epoxy-based resin layer 2, as is known in the art. The upper copper plate 1 is processed, for example, by etching, to create copper plate islands 11 for supporting semiconductor chips 4, such as high-side and low-side switches. A heat sink 5 with a fin structure 51 on its underside is connected to the underside of the lower copper plate 3 via a thermally conductive material 6.

[0022] As mentioned above, using Resin Plate 2 as a replacement for DBC's aluminum oxide already offers a mechanical advantage in the assembly process. However, there is still potential for optimization.

[0023] Thus, a new construction is provided in which the resin plate 2 is directly bonded to the heat sink 5, providing a three-layer stack 100 made of the upper copper plate 1, resin plate 2, and heat sink 5 with a fin structure 51. In this construction structure, TIM and the other copper plates 3 in the stack of the DBC and the heat sink 5 are omitted, as shown in Fig. 1, which further reduces the thermal resistance to dissipate heat from the power semiconductor chips 4. Direct bonding of resin to the heat sink 5 even enables the integration of a 6-in-1 power module circuit and heat sink, while each copper plate island 11 remains electrically isolated. This structure reduces subsequent process steps and lowers the production cost of semiconductor power modules.

[0024] However, there is a difficulty in integrating this three-layer stack 100 made of the upper copper plate 1, which is preferably already processed to include copper plate islands 11 for accommodating the semiconductor chips 4 (after the autoclaving process), the resin plate 2, and the heat sink 5, which has a heat-dissipating fin structure 51 on its underside. This is because almost all types of available resin plates require a curing process at a relatively high temperature of around 200 degrees Celsius using a mechanical press whose pressure is typically greater than 1 MPa and uniform over the entire resin plate 2, including the area where the copper plate islands 11 are not present (i.e., the gap between them).

[0025] In a conventional manufacturing method, such copper islands 11 are formed by chemical etching after curing using a single copper plate 1 with a size almost identical to the resin plate 2. In this way, the resin plate 2 should be properly cured with uniform pressure, at least from the top side, where the flat surface of the copper plate 1 contacts the upper-side step of the press device. However, the lower-side step of the press device has difficulty pressing evenly. The step should not be flat, but should somehow fit the underside of the heat-dissipating fin structure 51.However, applying uniform pressure from the bottom side is not easy due to the non-flat surface of the bottom side of the heat sink 5 and tends to affect the long-term reliability of the product due to insufficient bonding between the resin plate 2 and the copper plate 1.

[0026] Instead of a mechanical press, a so-called autoclaving process can solve the problem described above. In such a process, the physically stacked materials (copper plate 1, resin plate 2, and heat sink 5) are placed in an airtight chamber and pressed by high-pressure air of approximately 1 MPa fed into the chamber. Since the pressure is uniform in all directions, the pressing force is evenly transmitted from the bottom of the heat sink to the resin plate 2. It is also possible to press already structured copper islands 11, since the high-pressure air evenly presses the resin at the gap between the copper plate islands 11 with the copper islands 11.

[0027] To apply an autoclaving process, the stack 100 of the heat sink 5, the resin plate 2, and the copper plate 1 is covered with a vacuum film 9, typically made of multi-layer, hard-to-tear nylon. The edge of the vacuum film 9 is sealed to the back of the heat sink 5 outside the heat-dissipating fin structure 51 with a vacuum sealing adhesive 92. Furthermore, a venting fabric 93, which is a felt-like nonwoven fabric typically made of polyester, can be filled between the vacuum film 9 and the stack 100 to assist in evacuating the gaps beneath the vacuum film 9, particularly the slot / gap between the heat sink 5 and the resin plate 2, and between the resin plate 2 and the copper plates 1.Before supplying high-pressure air, the space between the vacuum film 9 and the stack 100 must be sufficiently evacuated so that the volume of the venting fabric 93 is shrunk as thin as possible. The (optional) venting fabric 93 assists in evacuating the slots / gaps on and beneath the resin plate 2. To this end, air is sucked out of the gaps beneath the vacuum film 9 through one or more air tubes 91 attached to the vacuum film 9 (the provision of an air tube 91 in the vacuum film 9 is standard in autoclaving processes). Subsequently, the entire chamber is heated to the curing temperature of the resin of the resin plate 2 while maintaining the evacuation, and high-pressure air is supplied for the required curing duration. The stack 100 ready for evacuation is shown in . Fig. 3. The stack 100 ready for processing by an autoclaving process, ie for evacuation, is shown in Fig. 4 shown.

[0028] The biggest disadvantage of this process is its cost. The chamber itself is relatively expensive, as is the high-pressure air supply system. This type of problem must be overcome through large-scale production. Nevertheless, evacuating the stack 100 times requires additional labor and time.

[0029] Preparing a large chamber and increasing the number of stacks 100 to be processed simultaneously therefore contributes to reducing the average processing costs for the stack 100 of the heat sink 5. This invention provides a solution to at least the above-mentioned problem.

[0030] According to the invention, a method for the direct integration of a power electronic substrate, ie the copper plate 1 structured into copper plate islands 11 for supporting the semiconductor chips 4, on the heat sink 5 is provided and presented below.

[0031] One of the requirements of the invention is a metal base plate 7 (step S1). This metal base plate 7 is typically made of (stainless) aluminum alloy with a typical thickness of 10 mm. The metal base plate 7 has several openings 71 for accommodating the same number of heat-dissipating heat sinks 5 (as part of a stack 100) as in Fig. 5. Therefore, the width and depth of the metal base plate 7 are large enough to accommodate several stacks 100. Therefore, its width and depth can be, for example, approximately 1 m x 1 m, which is not too small given the purpose described later and not too large given the weight of the total loads thereon. Each opening 71 has a groove 72 around it, as shown in Fig. 6. The purpose of the groove 72 is to accommodate a sealing ring 8, such as an O-ring, whose length corresponds to the circumference of the groove 72 and whose thickness is greater than the depth of the groove. Such a sealing ring is typically made of FFKM (perfluoroelastomer) or FKM (fluororubber material).

[0032] In addition, a three-layer stack 100 is produced by stacking the heat sink 5, the resin plate 2 and the copper plates 1 on top of each other (step S1).

[0033] To prevent the layers 5, 2, 1 from moving relative to each other before they are finally fixed together by the autoclaving process, they are pre-fixed with an adhesive. Depending on where the adhesive is placed, it must be thermally conductive (if it is placed near the location where the semiconductor chips 4 will later be arranged) or less thermally conductive (e.g., if it is placed at edges or corners of the layer that are as far away as possible from the semiconductor chips 4).

[0034] Furthermore, an assembly is created by placing each stack 100 on the metal base plate 7 (step S2) such that the rib structure 51, which may be pinfins or another protruding structure that can dissipate heat, is inserted into the opening 71 so that it protrudes from the underside of the metal base plate 7, as shown in Fig. 7 can be seen. In Fig. Figure 7 also shows that the heat sink 5, i.e., its plate, has the fin structure 51 only on a portion of its lower side. The fin structure 51 is provided such that an outer edge 52 on the lower side of the plate remains empty, see, for example, Fig. 2. At this outer edge 52 (or at least a portion thereof), the heat sink 5 overlaps with the metal base plate 7 after both have been joined. In particular, the groove 72 and the sealing ring 8 are designed to seal the area surrounding the fin structure 51 to prevent air from escaping when the sealing ring 8 is pressed by both the heat sink 5 and the metal base plate 7. In particular, if the heat sink 5 is intended for water cooling, the same size of the opening 71 and the groove 72 can be used for a cooling jacket.

[0035] The provision of the metal base plate 7 and the creation of the three-layer stack 100 can be carried out simultaneously, since two different processes are required, or in successive steps.

[0036] Furthermore, a vacuum film 9 is required for the use of the autoclaving process. According to the invention, a web of vacuum film 9 is used (step S3) that is at least the size (depth and width) of the metal base plate 7. This means that only one web is required for all stacks 100 instead of one web per stack 100. This is only possible by introducing the metal base plate 7. The web of vacuum film 9 covers the metal base plate 7, on which the stacks 100 with the heat sinks 5 are located, which are sealed against the opening 72 by the sealing rings 8. In addition, to optimize evacuation, the vent fabric 93 can be filled under the web of vacuum film 9. Furthermore, the web of vacuum film 9 must have one or more air tubes 91 for evacuation and vacuum sealing adhesive 92 at its edge (connected to the metal base plate 7) to be able to generate a vacuum.By providing a metal base plate 7 with only one sheet of vacuum film 9, the number of vacuum systems for evacuating on and under the resin plate 2 can be significantly reduced. This leads to a reduction in process costs and process steps when a large number of stacks 100 of heat sink 5 / resin plate 2 / copper plates 1 are loaded simultaneously into an autoclaving chamber to apply the autoclaving process.

[0037] After evacuation, the assembly is placed in an autoclaving chamber to undergo an autoclaving process (step S4). Using the autoclaving process, the three layers of copper plate 1, resin plate 2, and heat sink 5 are pressed together at a specific temperature so that the resin plate 2 acts as an adhesive. After the autoclaving process is completed, each stack can be removed from the opening 71, and the semiconductor chips 4 can be arranged on the copper plate islands 11. The thus-finished device can be used, for example, as part of an inverter for automotive applications, depending on the function of the semiconductor chips 4.

[0038] By applying the described autoclaving process, a further advantage can be achieved. Since the copper plate 1, which is already divided into several copper plate islands 11, and the resin plate 2 and heat sink 5 are pressed together in the process, a copper plate 1 thicker than 0.8 mm can be used, and the copper plate islands 11 can have a border closer to the vertical than before.

[0039] Conventionally, the resin plate 2 is mechanically pressed into a sandwich with copper plates 1, 3 stacked on both sides. The upper copper plate 1 is later divided into several copper plate islands 11 because the gap between the copper plate islands 11 and the area under the copper plate islands 11 must be pressed. This is only possible if the upper copper plate 1 is not processed before the stack is pressed together. Isolation is usually achieved by chemical etching using a patterned etching resist. Due to the side effect of chemical etching, the sidewall of the copper plate islands 11 tapers by 45 to 60 degrees, and the edge shape of the upper surface becomes less clear as the copper plate 1 becomes thicker.

[0040] Furthermore, the minimum distance between the copper plate islands 11 should be 1 mm to avoid electrical discharge when a high voltage is applied between the copper plate islands 11. On the other hand, it is desirable to utilize the surface area as much as possible. Therefore, a top copper plate 1 thicker than 0.8 mm is not realistic from a product control perspective. On the other hand, thermal simulation shows that a top plate with a thickness of 2 mm to 3 mm more effectively dissipates the heat generated by the power semiconductor chips 4 mounted on the top copper plate 1. When the autoclaving process is used as the resin integration process, the copper plate islands 11 can be patterned before stacking with the resin plate 2 and the heat sink 5, because the high air pressure presses even the gap between the copper plate islands 11 as well as the area under the copper plate islands 11.The isolation process is not limited to chemical etching of a pattern, and the problem of tapering can be almost avoided.

[0041] According to the invention, resin integration by the autoclave leads to more effective heat dissipation from the power semiconductor chips 4. Reference numbers 100 three-layer stacks 1 upper copper plate 11 Copperplate Islands 2 resin plate 3 lower copper plate 4 semiconductor chips 5 heat sinks 51 Rib structure 52 outer edge 6 Thermal interface material (TIM) 7 Metal base plate 71 Opening 72 grooves 8 Sealing ring 9 Vacuum film 91 Air pipe 92 Vacuum sealing adhesive 93 Ventilation fabric

Claims

[1] Method for the direct integration of a power electronic substrate on a heat sink (5), comprising the following steps: - S1: Providing a plurality of three-layer stacks (100), each of which is made of an upper layer of a metal plate (1), a middle layer of an electrically non-conductive resin plate (2), and a lower layer constituting an upper surface of a heat sink (5), wherein the heat sink (5) comprises a fin structure (51) provided on a lower surface of the heat sink (5), wherein the fin structure (51) is arranged such that an outer edge (52) of the lower surface remains free of the fin structure (51); and providing a metal base plate (7) comprising a plurality of openings (71), each of which is designed to receive one of the stacks (100), wherein each opening (71) has a groove (72) along its outer circumference into which a sealing ring (8) is inserted, wherein each opening (71) is sized such that the fin structure (51) of a stack (100) can be fully inserted into the opening (71), - S2: Creating an arrangement of metal base plate (7) and stacks (100) by inserting the stacks (100) into the openings (71) of the metal base plate (7) such that each outer edge (52) of a stack (100) at least partially overlaps the metal base plate (7), so that the stack (100) and the metal base plate (7) are sealed by the sealing ring (8), - S3: providing a single sheet of vacuum film (9) on top of the assembly, the vacuum film (9) having a size at least the size of the metal base plate (7), and evacuating the area under the vacuum film (9); - S4: Applying an autoclaving process to the assembly. [2] The method of claim 1, wherein in step S1 the layers of each stack (100) are pre-fixed to each other. [3] Method according to claim 1 or claim 2, wherein the web of vacuum film (9) has a vacuum sealing adhesive (92) on the edges of the metal base plate (7) and is fixed to the metal base plate (7) in step S3. [4] Method according to one of the preceding claims, wherein a ventilation fabric (93) is provided for filling the gaps under the web of vacuum film (9) and a plurality of air pipes (91) are provided in the web of vacuum film (9) for evacuating the gaps. [5] Method according to one of the preceding claims, wherein the rib structure (51) is made of pinfins. [6] Method according to one of the preceding claims, wherein the metal plate (1) is structured into metal plate islands (11) at the time of production of the stack (100). [7] A method according to any one of the preceding claims, wherein semiconductor chips are arranged on the metal plate islands after the autoclaving process. [8] Method according to one of the preceding claims, wherein the metal plate (1) is made of copper. [9] A method according to claim 8, wherein the surface of the metal plate (1) made of copper is covered with another metal. [10] Power electronic device, in particular inverter, manufactured by a method according to one of the preceding claims.

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