PRINTED CIRCUIT BOARD AND METHOD OF MANUFACTURING THE SAME

Abstract

A circuit board includes a plurality of circuit board layers arranged one on top of the other, wherein through-holes are integrated in the circuit board, wherein the through-holes are configured to receive metal screws that screw the circuit board to a heat sink. Insulating sleeves made of an insulating material are integrated into the circuit board, wherein the through-holes are formed in the circuit board in the region of the insulating sleeves.

Claims

1. A circuit board comprising: a plurality of circuit board layers arranged one on top of another; through-holes configured to receive metal screws that screw the circuit board to a heat sink being integrated into the circuit board; and insulating sleeves integrated into the circuit board, wherein the insulating sleeves comprise an insulating material, and wherein the through-holes are formed in the circuit board in a region of the insulating sleeves.

2. The circuit board of claim 1, wherein the insulating sleeves have a mechanical compressive strength greater than a mechanical compressive strength of a non-conductive material of the circuit board.

3. The circuit board of claim 1, wherein the insulation material is a plastics material, a glass fiber reinforced plastics material, or a ceramic.

4. The circuit board of claim 1, wherein the insulation material has a comparative tracking index (CTI) value of a creepage current resistance of at least 175.

5. The circuit board of claim 1, wherein the insulating sleeves are embedded in a milled-in cavity of the circuit board.

6. The circuit board of claim 1, wherein an intermediate layer made of plastics material is formed between an insulating sleeve of the insulating sleeves and the circuit board, and wherein the intermediate layer forms an outer sleeve that surrounds the insulating sleeve.

7. The circuit board of claim 6, wherein the intermediate layer comprises an epoxide resin or silicone.

8. The circuit board of claim 1, wherein an insulating sleeve of the insulating sleeves is covered by an uppermost circuit board layer and/or a lowermost circuit board layer of the circuit board.

9. The circuit board of claim 1, further comprising: an additional metal surface is formed on an upper side of the circuit board and/or an underside of the circuit board above or below an insulating sleeve of the insulating sleeves.

10. The circuit board of claim 9, wherein the insulating sleeve is covered by an uppermost circuit board layer and/or a lowermost circuit board layer of the circuit board, and wherein the additional metal surface is arranged on the uppermost circuit board layer and/or the lowermost circuit board layer and extends into microbores of the corresponding circuit board layer.

11. The circuit board of claim 1, wherein at least some of the metal layers present in the circuit board extend up to an outer rim of an insulating sleeve of the insulating sleeves, and wherein the insulating sleeve is dimensioned such that, in a case of a metal screw of the metal screws screwed into a through-hole of the through-holes, a sufficient creepage distance between the corresponding metal layer and the metal screw is realized by the insulating sleeve.

12. The circuit board of claim 1, wherein each insulating sleeve of the insulating sleeves is cylindrical or cuboid.

13. A method for producing a circuit board, the method comprising: providing a circuit board having a plurality of circuit board layers arranged one on top of another; milling a cavity into the circuit board, the cavity involving at least some of the circuit board layers of the circuit board; inserting an insulating sleeve made of an insulating material into the cavity; filling a free volume in the cavity that extends between the inserted insulating sleeve and a rim of the cavity with a plastics material when the free volume is present after the insulating sleeve has been inserted; and drilling a through-hole through the insulating sleeve.

14. The method of claim 13, wherein the cavity ends above a lowermost circuit board layer of the circuit board.

15. The method of claim 13, further comprising: providing a non-conductive circuit board layer on an upper side of the circuit board that covers the insulating sleeve at a top of the circuit board, after the inserting of the insulating sleeve and the filling of the free volume in the cavity.

16. The method of claim 13, further comprising: forming an additional metal surface on an upper side of the circuit board and/or an underside of the circuit board above or below the insulating sleeve.

17. The method of claim 13, wherein the filling of the free volume comprises filling the free volume with epoxide resin in a pressing process.

18. The method of claim 13, further comprising: attaching the circuit board to a heat sink via screwing a metal screw through the through-hole of the insulating sleeve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The disclosure is explained in more detail below with reference to the figures of the drawing based on a plurality of embodiments. In the drawings:

[0032] FIG. 1 depicts a schematic sectional view of an embodiment of a circuit board into which an insulating sleeve made of an insulating material is integrated, a through-hole being formed in the region of the insulating sleeve.

[0033] FIG. 2a-2c schematically depicts an example of a method for producing a circuit board according to FIG. 1.

[0034] FIG. 3 depicts an example of a modification of the circuit board of FIG. 1, wherein an additional metal surface is formed on the upper side and/or the underside of the circuit board.

[0035] FIG. 4 depicts an example of a circuit board according to the prior art, the circuit board being screwed to a metallic heat sink by metal screws.

[0036] FIG. 5 depicts an example of a circuit board according to FIG. 4, the through-hole in the circuit board being reinforced by an annular copper layer on the surface of the circuit board and a through-plating made of copper (copper via).

[0037] FIG. 6 schematically depicts the representation of required distances between metal layers of the circuit board when the through-hole is provided with additional copper layers according to FIG. 5.

[0038] FIG. 7 depicts an example of a flowchart of a method for producing a circuit board.

DETAILED DESCRIPTION

[0039] For better understanding of the background of the present disclosure, a circuit board arrangement according to the prior art is first described with reference to FIG. 4.

[0040] FIG. 4 shows a circuit board arrangement that includes a circuit board 1 and a metallic heat sink 4. The circuit board 1 includes a plurality of circuit board layers 10 arranged one on top of the other. An uppermost circuit board layer 11 forms an upper side of the circuit board 1 and a lowermost circuit board layer 12 forms an underside of the circuit board 1.

[0041] The circuit board 1 is screwed to the heat sink 4 by metal screws 2. The metal screws 2 are inserted into through-holes 15 in the circuit board and extend into a corresponding opening 45 of the heat sink, which is provided, for example, with an internal thread (not shown). The metal screws 2 have a screw head 2 and a screw shank 22, the screw head 2 resting against the upper side of the circuit board 1 via a metallic washer 3 and the screw shank 22 being screwed into the opening 45 of the heat sink 4 through the through-hole 15 of the circuit board 1.

[0042] By the screws 2, the underside of the circuit board 1 is pressed against the surface 40 of the heat sink 4 in order to provide a good thermal transition.

[0043] The heat sink 4 includes a metal such as aluminum or an aluminum alloy and is thus electrically conductive. It may be on the ground potential or alternatively a different potential. This means that the metal screw 2 coming into contact with the heat sink 4 is also subjected to the ground potential or the other potential of the heat sink 4.

[0044] The individual circuit board layers 10 of the circuit board 1 are formed, for example, by prepreg layers, (e.g., glass fiber mats impregnated with epoxide), and copper layers, which are connected to each other and structured in a manner known per se by lamination and etching processes. The circuit board 1 may also contain printed circuit board cores (PCB cores). Within the meaning of the present disclosure, no distinction is made between prepreg layers and printed circuit board cores. The contours of the circuit board 1 are realized with milling and drilling processes. In accordance with the standard EN IEC 60664-1 for the design of clearances and creepage distances, a solid insulation is assumed between individual prepreg layers (in the vertical direction of FIG. 6), so that no clearances and creepage distances have to be observed between prepreg layers. However, the risk of failures exists along a prepreg layer (horizontal plane of FIG. 6), (e.g., by delamination of the prepreg layers), so that a solid insulation between two potentials within a layer cannot be assumed. Accordingly, creepage distances, which indicate the shortest permitted distance along the surface between two conductive parts, are to be taken into account within a prepreg layer.

[0045] Because the screws 2 are subjected to the ground potential (or a different potential of the heat sink 4), creepage distances are also provided within the prepreg layers 10. Such a creepage distance 72 is shown in FIG. 6. A further creepage distance 71 relates to the distance between a copper layer and the washer 3 on the upper side of the circuit board 1. The need for providing creepage distance 72 in the individual prepreg layers leads to unused rim regions around the through-holes 15. As a result, the copper cross-section is reduced at each fastening point, and the electrical performance of the circuit board 1 is impaired.

[0046] FIG. 5 shows a variant of the circuit board of FIG. 4 according to the prior art, in which a through-plating (via) 51 made of copper is formed in the through-hole 15 of the circuit board 1 in order to reinforce the through-hole 15, which represents the screw hole for the metal screw 2. In order to secure the connection and adhesive strength of the through-plating 51 in the through-hole 15, further copper structures are provided, namely on the one hand copper layers 52 on the upper side or underside of the circuit board, and copper channels 53, which extend perpendicularly in the circuit board 1 at a distance from the through-hole 50. Furthermore, an additional annular copper layer 54 may be provided, for example between the washer 3 and the screw head 21 (or vice versa).

[0047] FIG. 6 illustrates what such additional copper structures 51-53 mean for providing a through-plating for the creepage distances and thus the minimum distances of the copper layers of the circuit board from the through-hole 15. For example, creepage distances 73, 74 are to be maintained on the upper side and in the inner layers between the copper structures 53 and the copper layers 10, 11. The non-usable region on the upper side of the circuit board 1 around the through-hole 15 is characterized here as d1. It may be in the range of 12 mm to 20 mm.

[0048] FIG. 1 shows a first embodiment of a circuit board 1. The heat sink 4 shown in FIG. 4 is no longer shown and may be designed in accordance with FIG. 4 or in another way.

[0049] The circuit board 1 includes a plurality of circuit board layers 10 arranged one on top of the other. An uppermost circuit board layer 11 forms an upper side of the circuit board 1 and a lowermost circuit board layer 12 forms an underside of the circuit board 1. For forming the circuit board layers 10, reference is made to the description of FIG. 4. The circuit board layers 10 are thus formed, for example, by prepreg layers and copper layers. One or more of the circuit board layers 10-12 may also be formed jointly by a prepreg layer and one or more copper layers. For example, in FIG. 1, the uppermost layer is formed by a prepreg layer 11a (for example from FP-4) and a copper layer 11b that is also formed thereon.

[0050] The circuit board 1 has a through-hole 15 that, together with further corresponding through-holes 15, is used to fix the circuit board 1 to a heat sink by metal screws. An insulating sleeve 6 is integrated into the circuit board 1 in the region of the through-hole 15. In the embodiment shown, the insulating sleeve 6 extends between the uppermost layer 11a and the lowermost layer 12, thus being covered by them. Alternatively, the insulating sleeve 6 may extend up to the corresponding surface and thus also through the uppermost layer 11a and the lowermost layer 12.

[0051] The through-hole 15 extends through the insulating sleeve 6. It is thus formed as a bore through the uppermost circuit board layer 11a, the insulating sleeve 6, and the lowermost circuit board layer 12. If, according to the aforementioned alternative embodiment, the insulating sleeve 6 extends to the corresponding surface of the circuit board 1, the through-hole 15 is formed exclusively in the insulating sleeve 6.

[0052] The insulating sleeve 6 includes an insulating material that may be formed by any non-conductor. It may be formed by a glass fiber reinforced plastics material or a ceramic. Examples of suitable insulating materials are aluminum oxide (Al.sub.2O.sub.3), zirconium (IV)-oxide (ZrO.sub.2), silicon nitride (Si.sub.3N.sub.4), polyamides filled with glass fibers (e.g., PA GF30), and polycarbonates filled with glass fibers (e.g., PC GF30).

[0053] The material of the insulating sleeve 6 is harder and has a higher mechanical compressive strength than the non-conductive material of the circuit board 1, so that a mechanical stability for the through-hole 15 is provided by the insulating sleeve 6, which allows the formation of through-platings and additional copper layers on the surface of the circuit board according to FIGS. 5 and 6 for reinforcing the through-hole 15 to be avoided. The non-usable region on the upper side of the circuit board 1 around the through-hole 15 is thereby reduced to a diameter d2, which is smaller than the diameter d1 of FIG. 6 and may be in the range of 6 mm to 10 mm.

[0054] Furthermore, it may be provided that the insulating material has a CTI value of the creepage current resistance of at least 175, in the range of 175 to 600, or in the range of 400 to 600. The insulating material thus has a high electric strength.

[0055] Furthermore, the insulating material may also have a high thermo-mechanical strength in order to absorb the pressing forces that occur when a screw head introduces forces into the circuit board 1 and presses it against a heat sink.

[0056] The insulating material furthermore may have a coefficient of thermal expansion that corresponds to the coefficient of thermal expansion of the non-conductive material of the circuit board (e.g., of the prepreg layers) or does not deviate from this substantially (for example, at most 20% relative to the coefficient of thermal expansion of the prepreg layers). This provides that there is no occurrence of stresses or gaps between the insulating sleeve and the circuit board material when there are temperature changes.

[0057] It is pointed out that the insulating sleeve 6 in FIG. 1 is dimensioned such that the copper inner layers 10 (e.g., the copper layers of the inner circuit board layers) may project as far as the insulating sleeve 6. The metal layers of the circuit board may thus be brought as close as possible to the through-hole 15 while maintaining the required creepage distances.

[0058] FIGS. 2a to 2e show an example of a method for producing a circuit board according to FIG. 1. According to FIG. 2a, a cavity 7 is formed in a circuit board 1. This is milled, for example, from above into the circuit board 1. In the illustrated embodiment, the cavity 7 ends above the lowermost circuit board layer 12, so that the layer forms a receiving floor for an insulating sleeve to be inserted. Alternatively, the cavity 7 extends completely through the circuit board 1, in which case a temporary base for supporting the insulating sleeve is to be provided below the circuit board 1.

[0059] According to FIG. 2b, the insulating sleeve 6 is then inserted into the cavity 7. In this case, it may be provided that a free volume 71 extends between the outer surface of the insulating sleeve 6 and the rim of the cavity 7. Whether this is the case or not depends of course on the dimensions of the insulating sleeve 6 to be inserted. If the insulating sleeve 6 is inserted into the cavity 7 with a press fit, for example, the free volume 71 is omitted.

[0060] If a free volume 71 is present corresponding to FIG. 2b, it is filled with a plastics material 60 according to FIG. 2c. For this purpose, for example, an epoxide resin 60 is pressed into the free volume 71. The plastics material 60 may also have a greater hardness and mechanical compressive strength than the non-conductive material of the circuit board 1. The intermediate layer 60 thus forms an outer sleeve for the insulating sleeve 6. It may be regarded as a part of an insulating sleeve 6 that includes two successive layers in the radial direction.

[0061] According to FIG. 2d, an upper circuit board layer (prepreg layer) 11a is then applied to the circuit board 1 and covers the insulating sleeve 6 at the top. Furthermore, additional copper layers 11b may be formed on the upper side of the circuit board 1. Finally, according to FIG. 2e, the through-hole 15 is introduced that extends through the insulating sleeve 6.

[0062] FIG. 7 in summary shows a flowchart of such a method for producing a circuit board arrangement. According to act 701, a circuit board is first provided that has a plurality of circuit board layers arranged one on top of the other. According to act 702, a cavity is milled into the circuit board, the cavity involving at least some of the circuit board layers of the circuit board. According to act 703, an insulating sleeve made of an insulating material is then inserted into the cavity.

[0063] If present, a free volume in the cavity that extends between the inserted insulating sleeve and the rim of the cavity is filled with a plastics material, for example, an epoxide resin, in act 704. Finally, a through-hole is drilled through the insulating sleeve in act 705.

[0064] FIG. 3 shows a modification of the circuit board of FIG. 1, in which an additional copper surface 81, 82 is applied to the uppermost or lowermost circuit board layer 11a, 12 in addition above and below the insulating sleeve 6. In this case, the additional copper surface 81, 82 extends into microbores 810, 820 of the corresponding circuit board 11a, 12. By the additional copper surfaces 81, 82 and the microbores 810, 820, an additional mechanical rigidity of the uppermost circuit board layer 11a below the screw head (see screw head 21 of FIG. 4) and the lowermost circuit board layer 12 adjacent to a heat sink is provided.

[0065] It is pointed out that the circuit board 1 is shown in the figures only schematically and by way of example. For example, a larger number of circuit board layers may be provided. The circuit board 1 also has a plurality of through-holes 15 for receiving screws. Furthermore, active components (not shown) may be arranged on and/or integrated into the circuit board 1.

[0066] The disclosure is not limited to the embodiments described above, and various modifications and improvements may be made without departing from the concepts described herein. Any of the described features may be used separately or in combination with any other features, provided that they are not mutually exclusive. The disclosure extends to all combinations and sub-combinations of one or more features described herein and includes them. If ranges are defined, they include all values within these ranges and all sub-ranges that fall within a range.

[0067] It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend on only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.

[0068] While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.