PCB layout

Abstract

The invention relates to a printed circuit board arrangement comprising a printed circuit board (1) with a top (11) and a bottom (12), and a heat sink (2) with which the printed circuit board (1) is thermally coupled at its bottom (12). It is provided that the printed circuit board (1) has at least one electrically insulating component (3) on its bottom (12), which comprises a ceramic material and projects from the bottom (12) of the printed circuit board (1), wherein the electrically insulating component (3) is thermally coupled to the heat sink (2).

Description

[0001] The invention relates to a printed circuit board arrangement according to the preamble of patent claim 1.

[0002] It is common practice to press printed circuit board-based power electronics assemblies against a heat sink for cooling. The electrical modules to be cooled, each equipped with a power semiconductor – also known as prepackage modules – are located on the underside of a printed circuit board and are pressed against a heat sink, for example, using a thermal interface material (TIM), thereby cooling them.

[0003] However, losses also occur in the circuit board, leading to an increase in temperature within the circuit board. The power loss, or rather the heat flow generated by the power loss, depends on the current and the copper cross-section of the circuit board available for current conduction. In addition to these ohmic losses, there are also thermal losses from components arranged on the top or bottom of the circuit board that dissipate heat into the circuit board. In particular, the aforementioned prepackage modules with power electronics components contribute to additional heating of the circuit board, as the circuit board cannot be coupled to the heat sink for cooling in the area of ​​the prepackage modules; instead, these represent an additional heat source during operation and dissipate part of their power loss into the circuit board.

[0004] It is therefore necessary to provide sufficient cooling for the circuit board to avoid exceeding the permissible circuit board temperature.

[0005] The invention is based on the object of providing a printed circuit board arrangement which reduces the risk of local overheating of the printed circuit board.

[0006] This object is achieved by a printed circuit board assembly having the features of claim 1. Embodiments of the invention are specified in the dependent claims.

[0007] The invention then considers a printed circuit board assembly comprising a printed circuit board with a top side and a bottom side and a heat sink to which the printed circuit board is thermally coupled at its bottom side.

[0008] It is provided that the printed circuit board has at least one electrically insulating component on its underside, which component comprises a ceramic material and protrudes from the underside of the printed circuit board, wherein the electrically insulating component is thermally coupled to the heat sink.

[0009] The inventive solution is based on the idea of ​​improving the cooling of a printed circuit board by using electrically insulating components comprising a ceramic material to provide electrically insulating paths between the printed circuit board and the heat sink, paths that simultaneously offer high thermal conductivity. These electrically insulating components are, in a sense, local vertical thermal connectors. The provision of thermally conductive paths to the heat sink is achieved by integrating a ceramic material into the electrically insulating components. This is because ceramic materials exhibit comparatively high thermal conductivity, especially compared to typical printed circuit board materials such as FR4.

[0010] The electrically insulating components with integrated ceramic material thus provide an improved thermal connection between a circuit board and a heat sink.

[0011] A further advantage associated with the invention is that electrical components connected to the circuit board and exhibiting power dissipation can also be cooled more effectively via the circuit board. This opens up an additional cooling path for such electrical components via the circuit board and the electrically insulating components into the heat sink.

[0012] This also applies, and in particular, to electrical prepackage modules with integrated power semiconductors, which are also located on the underside of the circuit board and are thermally coupled to the heat sink via a first cooling path. For such prepackage modules, a second cooling path is available, extending over the circuit board and the electrically insulating components into the heat sink.

[0013] Please note that the electrically insulating components are only arranged locally, i.e., in specific areas on the underside of the circuit board. The electrically insulating components can, in principle, have any cross-section, for example, a round, oval, or polygonal, or in particular a rectangular cross-section.

[0014] It is further pointed out that, in the context of the present invention, the side of the circuit board facing the heat sink is always referred to as the underside of the circuit board, regardless of the spatial orientation of the circuit board and the heat sink.

[0015] One embodiment of the invention provides for the electrically insulating component to be soldered to the underside of the circuit board, forming a solder layer. In principle, however, the thermal connection can also be achieved using other joining techniques, such as sintering.

[0016] In particular, it can be provided that the circuit board has a bottom metal layer and the electrically insulating component is soldered to the bottom metal layer. The bottom metal layer, like other metal layers of the circuit board, is, for example, a copper layer, although metal layers made of another material such as aluminum, silver, or tungsten, or of metal alloys, can also be used. Because the bottom layer of the circuit board is a metal layer and this is soldered to the electrically insulating component, the vertical heat dissipation into the electrically insulating component and from there into the heat sink is further improved.

[0017] One embodiment provides for the bottom metal layer to be structured into individual solder pads, with the electrically insulating component being soldered to the individual solder pads. These are, for example, rectangular in shape and arranged along a rectangle. Structuring the bottom metal layer into individual metal pads or solder pads has the advantage of reducing the overall soldering area, thus simplifying soldering. Furthermore, the reduced soldering area reduces the risk of local expansion and deflection of the circuit board due to the solder material. Gaps between the solder pads can be filled and insulated with an underfill material after soldering.

[0018] For effective vertical heat conduction from the circuit board into the electrically insulating components and from these into the heat sink, one embodiment of the invention provides that the circuit board has a plurality of vertically spaced metal layers (e.g. made of copper, aluminum, silver or tungsten) which comprise a bottom metal layer. Furthermore, the circuit board has thermal vias which thermally connect at least some of the metal layers to one another and extend down to the bottom metal layer. The vias are therefore plated through to the bottom layer of the circuit board. This ensures that power loss occurring in the individual metal layers is conducted via the thermal vias to the bottom metal layer and from there into the electrically insulating components. The vias can also be electrically conductive and can be made of copper or another metal, for example.

[0019] A further embodiment of the invention provides that the edge region of the electrically insulating component is filled with an electrically insulating material, such as an epoxy resin, using an underfill process and is electrically insulated. If the solder layer only partially fills the area between the top side of the electrically insulating components and the bottom side of the circuit board, the underfill material fills the remaining areas of this gap, thereby reinforcing the mechanical connection. In some cases, only an edge region is insulated, in which case the underfill process represents an edge bonding process.

[0020] One embodiment of the invention provides that the electrically insulating component has at least one electrically insulating ceramic layer as a ceramic material.

[0021] This can be additionally coated. The insulating ceramic layer exhibits high thermal conductivity due to the ceramic material.

[0022] In particular, the electrically insulating component can comprise an insulating ceramic layer, an upper metallization layer arranged on top of the ceramic layer, and / or a lower metallization layer arranged on the underside of the ceramic layer. The ceramic layer provides electrical insulation and, at the same time, effective thermal coupling between the circuit board and the heat sink. The metallization layers enable a connection to the circuit board and the heat sink.

[0023] Such ceramic materials are well known. One example is so-called DBC materials (DBC = "Direct Bonded Copper").

[0024] In exemplary embodiments, the ceramic material of the electrically insulating component has a thermal conductivity of at least 30 W / (m·K), in particular of at least 50 W / (m·K). The ceramic layer used consists, for example, of silicon nitride (Si3N4), which can achieve thermal conductivity values ​​of up to 90 W / (m·K). Other examples of suitable ceramic materials are aluminum oxide (Al2O3) and aluminum nitride (AIN). In principle, any ceramic material can be used.

[0025] A further embodiment of the invention provides that the underside of the electrically insulating component rests on the top side of the heat sink via a thermal interface material in order to optimize the thermal connection.

[0026] Furthermore, it can be provided that a plurality of electrical modules, each comprising a semiconductor component, in particular a power semiconductor, is arranged on the underside of the circuit board. Such electrical modules are directly coupled to the heat sink. However, the electrically insulating components provide an additional cooling path into the heat sink, which leads via the circuit board.

[0027] It can be provided that the electrically insulating components have the same vertical height as the electrical modules. This allows the electrical modules and the electrically insulating components to be coupled to the heat sink simultaneously without the need for additional spacers, for example, via a common thermal interface material.

[0028] The local arrangement of the electrically insulating components on the circuit board naturally depends on the locations or areas of the circuit board where high power dissipation or elevated temperatures occur. Localized cooling of hot spots can be achieved there.

[0029] One embodiment for the suitable placement of the electrically insulating components provides for the electrical modules to be arranged in at least two spaced-apart rows on the underside of the circuit board, with a row of local electrically insulating regions being arranged between the two rows of electrical modules, i.e., in the space between them. This allows each electrically insulating component to contribute to the cooling of two adjacent electrical modules.

[0030] It can be provided that the electrical modules of a row each form a logic switch of a three-phase inverter with three phases U, V, and W. Another embodiment for the suitable placement of the electrically insulating components provides that at least one electrically insulating component is arranged adjacent to a contact point of the circuit board for tapping the current of such a phase.

[0031] Another embodiment for the suitable placement of the electrically insulating components provides for a plurality of electrically insulating components to be arranged in regions on the underside of the circuit board, in which capacitors of an inverter are arranged opposite one another on the top side of the circuit board. These capacitors are, for example, DC-link capacitors. Heat generated by the DC-link capacitors on the top side of the circuit board can be directly dissipated via the electrically insulating components on the underside of the circuit board.

[0032] The electrical modules may also comprise a ceramic material. One embodiment provides that the electrical modules each comprise: - a ceramic circuit carrier having an insulating ceramic layer and an upper metallization layer arranged on top of the ceramic layer, - an electrical component arranged on top of the upper metallization layer and electrically connected thereto, - a top side of the electrical module arranged on the underside of the printed circuit board, and - a bottom side of the electrical module, which is thermally coupled to the heat sink, for example via a thermal interface material.

[0033] The electrical component of the electrical module is, for example, a power MOSFET or an IGBT component. The ceramic circuit carrier serves to electrically insulate the electrical component from the heat sink and simultaneously provides thermal connection to the heat sink. The ceramic circuit carrier, together with the semiconductor component and a casing, e.g., made of potting material, forms the electrical module, which can be connected to the printed circuit board or a carrier board via contacts formed on its surface. Such an electrical module is also referred to as a prepackage module.

[0034] The invention is explained in more detail below with reference to the figures of the drawing using several exemplary embodiments. They show: Fig. 1 shows an embodiment of a printed circuit board assembly comprising a printed circuit board, a heat sink, and an electrically insulating component with a ceramic layer that thermally couples the printed circuit board to the heat sink; Fig. 2 a printed circuit board assembly comprising a plurality of electrically insulating components according to the Fig. 1 and comprises a plurality of electrical modules; Fig. 3 a view of the underside of an embodiment of a printed circuit board arrangement according to the Fig. 2, wherein the circuit board forms an inverter; Fig. 4 a bottom metal layer of a printed circuit board according to the Fig. 1, wherein the bottom metal layer is structured and forms a plurality of individual solder pads; and Fig. 5 shows an example of a printed circuit board arrangement not relating to the invention.

[0035] For a better understanding of the background of the invention, an example of a printed circuit board arrangement not relating to the invention will first be described with reference to Fig. 5 explained.

[0036] The circuit board arrangement of the Fig. 5 comprises a multilayer printed circuit board 1 having a top side 11 and a bottom side 12. The printed circuit board 1 comprises a plurality of layers, which are copper layers 113', 115', 117' and insulating layers 112', 114', 116' made of a printed circuit board material such as FR4. The copper layers 113', 115', 117' are connected to one another by vertically running copper vias 118'. An electrical component 6 is schematically shown on the top side 11 of the printed circuit board, which has a power loss and dissipates a heat flow into the printed circuit board 1.

[0037] The printed circuit board 1 is pressed onto a heat sink 2 on its underside 12 via a thermal interface material 8. The thermal interface material 8 ensures a defined thermal transition and consists, for example, of thermal pastes or thin so-called gap pads.

[0038] The heat sink 2 is shown only schematically and is made of an electrically conductive material. It is therefore important to provide effective electrical insulation of the circuit board 1 from the heat sink 2. For this purpose, the bottommost non-conductive layer 112 of circuit board material is thicker than the other non-conductive layers, or several non-conductive layers are formed one above the other on the underside of the circuit board 1.

[0039] However, printed circuit board material such as FR4 has a relatively low thermal conductivity of less than 1 W / (m·K), which means that the layer structure according to the Fig. 5, a relatively high thermal resistance results. Vias 118' serve to improve the thermal resistance. However, these can only improve the thermal resistance within circuit board 1. The at least one electrical insulation layer 112' remains as a thermal bottleneck.

[0040] The Fig. Figure 1 shows an embodiment of a printed circuit board assembly. The printed circuit board assembly comprises a multilayer printed circuit board 1 having a top side 11 and a bottom side 12. Furthermore, the printed circuit board assembly includes a heat sink 2 and an electrically insulating component 3, wherein the electrically insulating component 3 thermally couples the bottom side 12 of the printed circuit board 1 to the heat sink 2 in certain regions or locally.

[0041] The circuit board 1 comprises a plurality of layers, which are copper layers 111, 113, 115, 117 and insulating layers 112, 114, 116 made of a circuit board material such as FR4. The copper layers 111, 113, 115, 117 are connected to one another by vertically extending copper vias 118. An electrical component 6 is schematically shown on the top side 11 of the circuit board 1. This component has a power dissipation and dissipates a heat flow into the circuit board 1.

[0042] The lowest layer of the circuit board 1 is the copper layer 111, i.e. the copper layer 111 forms the underside 12 of the circuit board 1 in the area in which it extends.

[0043] The electrically insulating component 3 comprises an insulating ceramic layer 31. The ceramic layer is, for example, a layer of silicon nitride (Si3N4), which has thermal conductivity values ​​of up to 90 W / (m·K). An upper metallization layer 32 is formed on the top side of the ceramic layer 31, and a lower metallization layer 33 is formed on the underside of the ceramic layer 31. The metallization layers 32, 33 consist, for example, of copper, silver, or aluminum. Such layered components are also referred to as DBC materials (DBC = "Direct Bonded Copper").

[0044] The electrically insulating component 3 is soldered to the underside 12 of the circuit board 1, forming a solder layer 5. The solder connection is made between the upper metallization layer 32 and the lowest copper layer 111 of the circuit board 1, ensuring a good thermal connection between the copper layer 111 and the component 3. The thermal vias 118 are plated through to the lowest copper layer 111, so that heat generated by power loss in the individual copper layers 113, 115, 117 can be conducted via the vias 118 into the lowest copper layer 111 and thus effectively removed from the circuit board 1.

[0045] The ceramic layer 31 provides the required electrical insulation to the heat sink 2. The lower metallization layer 32 ensures a good thermal transition to the heat sink 2. The electrically insulating component 3 is thermally connected to the heat sink 2 via a thermal interface material 8 (for example, a thermal paste or a gap pad).

[0046] The electrically insulating component 3 thus serves to provide electrical insulation to the circuit board 1 and, at the same time, to provide thermal connection to the heat sink 2. It represents a vertical thermal connection element between the circuit board 1 and the heat sink 2.

[0047] It should be noted that the lower metallization layer 33 is merely optional. Alternatively, the ceramic layer 31 is directly coupled to the heat sink 2 via the thermal interface material 8. In a further alternative, the thermal interface material 8 is also not present, so that the ceramic layer 31 is directly connected to the heat sink 2.

[0048] Furthermore, the circuit board arrangement of the Fig. 1, the edge region of the electrically insulating component 3 is insulated by an electrically insulating material 4. This is applied, for example, via an underfill process or edge bonding. The electrically insulating material 4 extends along the edge of the electrically insulating component 3 from the top side of the ceramic layer 31 to the bottom side 12 of the circuit board.

[0049] The heat sink 2 is shown schematically. It is made of a conductive material, for example, an aluminum alloy, and may have cooling surfaces (not shown). The heat sink 2 can be actively cooled (by a fan or liquid cooling) or passively cooled.

[0050] The electrically insulating component 3 has in the embodiment of the Fig. 1 has a rectangular or square cross-section, although this is only an example.

[0051] The Fig. 2 shows a sectional view of an embodiment of a printed circuit board arrangement with a printed circuit board 1 and a heat sink 2, wherein the printed circuit board 1 has on its underside a plurality of electrically insulating components 3 according to the Fig. 1 and, on the other hand, electrical modules 7. With regard to the structure of the electrically insulating components 3, reference is made to the explanations for Fig. 1.

[0052] The electrical modules 7 each comprise an electrical component (not shown separately), in particular a power semiconductor. They are connected, for example, via surface mounting to corresponding contact pads on the underside 12 of the circuit board 1. The current flow to and from the electrical modules 7 is realized via corresponding, schematically illustrated copper layers 118, 119 of the circuit board 1.

[0053] The electrical modules 7 and the electrically insulating components 3 have the same vertical height, so that they can be pressed together onto the heat sink 2 via a thermal interface material 8 to ensure effective heat conduction into the heat sink 2. Accordingly, the electrical modules 7 form a first thermal conduction path that extends from the electrical modules 7 via the thermal interface material 8 directly into the heat sink 2.

[0054] In addition, a second heat conduction path for cooling the electrical components 7 is opened via the electrically insulating components 3. This can be seen from the schematically illustrated heat conduction paths A, B of the Fig. 2. The second heat conduction paths A, B extend from the electrical module 7 upwards into the circuit board 1 and from there via the electrically insulating components 3 into the heat sink. This also allows heat dissipated upwards from the electrical modules 7 into the circuit board 1 to be dissipated into the heat sink 2. This prevents the circuit board temperature from exceeding an upper limit due to the heat dissipation of the electrical modules 7.

[0055] In the illustrated embodiment, the electrical modules 7 are arranged in rows, but not necessarily so, with each row forming a switch of a three-phase inverter with phases U, V, and W. Between each row of electrical modules 7, there is a row of electrically insulating components 3.

[0056] This is further from the Fig. 3, which shows a view of the underside 12 of an embodiment of a printed circuit board 1 according to the Fig. 2. The circuit board 1 and the electrical components arranged thereon form a three-phase inverter 9 in the illustrated embodiment.

[0057] Six rows 71-76 of electrical modules 7 are provided, each forming a logic switch of a three-phase inverter, with two rows or logic switches 71, 72 and 73, 74 as well as 75, 76 forming a half-bridge. The electrical modules 7 comprise power semiconductors such as MOSFETs or IGBT components as electrical components. Two logic switches or the half-bridge formed from them form a phase U, V, W of the three-phase inverter 9.

[0058] Electrical contacts on the circuit board 1 for the currents of phases U, V, W are provided by contact points 103, 104, 105.

[0059] The three-phase inverter 9 further comprises DC link capacitors arranged in an area 102 on the top side of the circuit board 1.

[0060] For selective cooling of the circuit board 1, a plurality of electrically insulating components 3 are arranged according to the Fig. 1 arranged on the underside of the circuit board 1.

[0061] On the one hand, according to the Fig. 2, a row 35 of electrically insulating components 3 is provided, which extends between two rows 75, 76 of the electrical modules 7. Corresponding rows of electrically insulating components 3 can also be arranged between the further rows of the electrical modules 7. In this case, an electrically insulating component 3 provides an additional heat conduction path (according to the heat conduction paths A, B of the Fig. 2) for two adjacent electrical modules 7.

[0062] Secondly, electrically insulating components are provided at other suitable locations. Thus, electrically insulating components 3a are arranged in areas on the underside 12 of the printed circuit board 1, in which the aforementioned DC link capacitors are arranged opposite one another on the top side. Furthermore, electrically insulating components 3b, 3c are arranged adjacent to the contact points 103, 104 for the phases U, V in order to protect the copper layers in the printed circuit board 1 that carry the corresponding current (corresponding to the copper layers 118, 119 of the Fig. 2). The temperature in these copper layers increases towards contact points 103, 104, 105, so that additional cooling of the circuit board in the area of ​​contact points 103, 104, 105 is sensible or necessary to prevent exceeding the permissible circuit board temperature.

[0063] The electrically insulating components 3b, 3c are in the Fig. 3 is shown only as an example and in different sizes. Another electrically insulating component can be assigned to contact point 105.

[0064] The Fig. 4 shows a modification of the embodiment of the Fig. 1 with regard to the design of the lowest metal layer 111. This is Fig. 4 is not formed over a large area, but is structured into individual rectangular solder pads 1110. These are each separately connected via a solder layer to the upper metallization layer 32 of the component 3 of the Fig. 1 soldered. The reduction in the size of the solder pads 1110 simplifies soldering. Furthermore, the risk of sagging is reduced, as any local expansion is confined to smaller areas. Gaps between the solder pads 1110 can be filled with an underfill material after soldering.

[0065] It is understood that the invention is not limited to the embodiments described above, and various modifications and improvements may be made without departing from the concepts described herein. It is further understood that any of the described features may be used separately or in combination with any other features, provided they are not mutually exclusive. The disclosure extends to and includes all combinations and subcombinations of one or more features described herein. Where ranges are defined, these include all values ​​within these ranges, as well as all subranges that fall within a range.

Claims

[1] Printed circuit board assembly comprising: - a printed circuit board (1) having a top side (11) and a bottom side (12), - a heat sink (2) with which the circuit board (1) is thermally coupled on its underside (12), characterized by that the printed circuit board (1) has on its underside (12) at least one electrically insulating component (3) which comprises a ceramic material and protrudes from the underside (12) of the printed circuit board (1), wherein the electrically insulating component (3) is thermally coupled to the heat sink (2). [2] Printed circuit board arrangement according to claim 1, characterized by that the electrically insulating component (3) is soldered to the underside (12) of the printed circuit board (1). [3] Printed circuit board arrangement according to claim 1 or 2, characterized by that the printed circuit board (1) has a bottom metal layer (111) and the electrically insulating component (3) is soldered onto the bottom metal layer (111). [4] Printed circuit board arrangement according to claim 3, characterized by that the lowermost metal layer (111) is structured into individual solder pads (1110), wherein the electrically insulating component (3) is soldered to the individual solder pads (1110). [5] Printed circuit board arrangement according to claim 3 or 4, characterized by in that the circuit board (1) has a plurality of vertically spaced metal layers (111, 113, 115, 117), wherein the plurality of metal layers (111, 113, 115, 117) comprises the lowermost metal layer (111), and wherein the circuit board (1) has thermal vias (118) which thermally connect at least some of the metal layers (111, 113, 115, 117) to one another and extend to the lowermost metal layer (111). [6] Printed circuit board arrangement according to one of the preceding claims, characterized by that the edge region of the electrically insulating component (3) is filled with an electrically insulating material (4) via an underfill process. [7] Printed circuit board arrangement according to one of the preceding claims, characterized by that the electrically insulating component (3) has at least one insulating ceramic layer (31) as a ceramic material. [8] Printed circuit board arrangement according to one of the preceding claims, characterized by that the electrically insulating component (3) has: - an insulating ceramic layer (31) - an upper metallization layer (32) arranged on top of the ceramic layer (31), and / or - a lower metallization layer (33) arranged on the underside of the ceramic layer (31). [9] Printed circuit board arrangement according to one of the preceding claims, characterized by that the ceramic material of the electrically insulating component (3) has a thermal conductivity of at least 30 W / (m·K), in particular of at least 50 W / (m·K). [10] Printed circuit board arrangement according to one of the preceding claims, characterized bythat the underside of the electrically insulating component (3) rests on the top side of the heat sink (2) via a thermal interface material (8). [11] Printed circuit board arrangement according to one of the preceding claims, characterized by that on the underside (12) of the printed circuit board (1) there is further arranged a plurality of electrical modules (7), each comprising a semiconductor component, in particular a power semiconductor. [12] Printed circuit board arrangement according to claim 11, characterized by that the at least one electrically insulating component (3) has the same vertical height as the electrical modules (7). [13] Printed circuit board arrangement according to claim 11 or 12, characterized bythat the electrical modules (7) are arranged in at least two rows (71-76) spaced apart from one another on the underside (12) of the printed circuit board (1), wherein the at least one electrically insulating component (3) comprises electrically insulating components (3) arranged in a row (35), and wherein the row (35) of electrically insulating components (3) is arranged between two rows (75, 76) of electrical modules (7). [14] Printed circuit board arrangement according to claim 13, characterized by that the electrical modules (7) of a row (71-76) each form a switch of a three-phase inverter (9) with three phases (U, V, W). [15] Printed circuit board arrangement according to claim 14, characterized by that the at least one electrically insulating component (3) is arranged adjacent to a contact point (103, 104) of the printed circuit board (1) which serves to tap the current of a phase (U, V, W). [16] Printed circuit board arrangement according to one of the preceding claims, characterized by in that the at least one electrically insulating component (3) comprises a plurality of electrically insulating components (3a) which are arranged in regions on the underside (12) of the printed circuit board (1) in which capacitors of an inverter are arranged opposite one another on the upper side (11) of the printed circuit board (12). [17] Printed circuit board arrangement according to one of the preceding claims, characterized by that the electrically insulating component (3) has silicon nitride (Si3N4), aluminum oxide (Al2O3) or aluminum nitride (AIN) as ceramic material.

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