Arrangement including a substrate for power components and a heat sink, and a method for manufacturing the arrangement

[0002] A conventional arrangement is described in German Patent Application No. 1 95 28 62. In this publication, as the substrate a circuit board is indicated, which on its upper side is provided with an electronic circuit, which includes at least one power component that generates waste heat. Beneath the power component, the circuit board is provided with via holes, which divert the heat generated by the power component to the lower side of the circuit board. Between lower side of the circuit board and a control unit housing functioning as a heat sink, a thermally conductive filler is arranged. During operation, the heat produced by the power components is diverted via the via holes to the lower side of the circuit board and from there is delivered via the thermally conductive filler to the housing, which functions as a heat sink. In this context, it is disadvantageous that printed circuit traces that conduct potential on the lower side of the circuit board can come into contact with the heat sink during assembly of the circuit board in the control unit. A short-circuit that is caused in this way can damage or destroy the sensitive electronic components on the circuit board.

[0003] In addition, German Patent Application No. 1 97 23 409 describes an arrangement having a substrate and a heat sink. On the upper side of the circuit board, a power component is deposited on a large-surface printed circuit trace, which is connected by via holes to a large-surface printed circuit trace on the lower side of the circuit board. On the lower side of the circuit board, a metal layer is deposited under the large-surface printed circuit trace arranged there and over an insulation layer, which in turn is deposited through a solder stop mask onto a control unit housing part that is provided as a heat sink. In this arrangement, although an electrical contact between the printed circuit traces and a heat sink is prevented by the insulation layer, it is disadvantageous that the insulation layer and the further metal layer complicate the direct heat transfer to the heat sink, increase the space requirements of the arrangement, and also make manufacturing more expensive.

[0004] The arrangement according to the present invention avoids the disadvantages arising in the conventional arrangements. As a result of spacer elements placed on the side of the substrate opposite the power components and a thermally conductive filler introduced between the substrate and the heat sink, an effective heat coupling of the substrate to the heat sink is advantageously achieved, on the one hand, and an undesirable electrical contact between the printed circuit traces carrying potential and located on the side of this substrate and the heat sink is dependably avoided, on the other hand. In addition, a particularly space-saving arrangement can be realized. Additional layers, making manufacturing more expensive, such as an additional insulating layer or a further metal layer deposited onto the insulation layer, are not necessary, so that the costs in this regard can be saved.

[0005] It is also advantageous if the spacer elements are composed of conductor surface elements on the lower side of the substrate, which are coated using a preselected quantity of solder, since for this purpose, in particular for substrates that are fitted with components on both sides, no additional manufacturing step is required. The conductor surface elements can be manufactured together with the connecting surfaces of the electronic components provided on the lower side, and can be coated with solder.

[0006] A solder resist deposited on the side of the substrate opposite the power components prevents solder from mistakenly reaching locations that are not provided therefore during the application of the solder.

[0007] If the power component and the heat sink have the same electric potential, it is advantageous to integratedly manufacture the conductor surface elements directly in the second large-surface printed circuit trace, since in this way the heat transfer is improved.

[0008] In addition, it is advantageous if the thermally conductive filler provided between the substrate and the heat sink is a thermally conductive adhesive or a thermally conductive adhesive foil, by which the substrate can also be mechanically secured to the heat sink.

[0009] The spacer elements resting on the heat sink can advantageously also be used as a ground connection of the substrate to the heat sink and for improving the EMV behavior (electromagnetic compatibility).

[0010] In addition, the present invention relates to a method for manufacturing an arrangement composed of a substrate and a heat sink. In particular, in the case of substrates having components on both sides, no additional manufacturing steps are necessary for carrying out the method according to the present invention. The conductor surface elements can be manufactured together with the printed circuit traces provided on the second side. The deposition of solder necessary for the manufacture of the spacer elements can be carried out together with the soldering of the connecting surfaces for components, which makes the method particularly economical, since scarcely any additional costs arise for manufacturing the spacer elements.

[0011] It is advantageous to imprint the solder onto the conductor surface elements in a solder paste imprinting station, since this technology is particularly well suited for depositing a defined quantity of solder and can be controlled very well. In a subsequent reflow soldering step, the solder is melted, the spacer elements being formed at a height defined by the quantity of solder deposited. The reflow solder step can advantageously take place with the reflow soldering of the SMD components provided on the substrate.

[0012] It is easy to apply a thermally conductive adhesive or a thermally conductive adhesive foil first onto the heat sink and subsequently to place the substrate onto the heat sink coated with the adhesive or the adhesive foil such that the spacer elements are impressed into the adhesive, they being able to contact the heat sink via the solder layer.

[0013] FIG. 1 shows a cross-section of a conventional arrangement having an insulation layer and an additional copper plate.

[0014] FIG. 2 shows a cross-section of a first exemplary embodiment of an arrangement according to the present invention.

[0015] FIG. 3 shows a cross-section of a second exemplary embodiment of the arrangement according to the present invention.

[0016] FIG. 1 shows a conventional arrangement for diverting the waste heat produced in a control unit by a power component. On upward-facing first side 8, a circuit board 2 is provided with a first large-surface printed circuit trace 10 and, on the downward-facing second side 9, with a second large-surface printed circuit trace 11. The first large-surface printed circuit trace 10 and a second large-surface printed circuit trace 11 are well connected with each other in a thermally conductive manner, by numerous via holes 4 extending across the circuit board. A power component 3, for example, an SMD component, is applied onto first large-surface printed circuit trace 10, the component being conductively connected via connections 14 to further printed circuit traces 12, insulated from large-surface printed circuit trace 10, on upper side 8 of circuit board 2. For simplicity's sake, only one printed circuit trace 12 is depicted. On lower side 9, further potential-carrying printed circuit traces 13 are located, which are arranged so as to be insulated from second large-surface printed circuit trace 11. In addition, on lower side 9, provision is made for further undepicted SMD components, which are soldered on the lower side to undepicted connecting surfaces. Furthermore, a copper plate 6, which is used as a heat sink, is deposited onto second large-surface printed circuit trace 11 and printed circuit traces 13 via an electrically insulating, thermally conductive insulation layer 5. A solder resist 15, in turn, is applied onto copper plate 6, the resist preventing a deposition of the solder on the copper during the soldering of the connecting surfaces for the SMD components. Underneath solder resist 15, provision is made for a thermally conductive adhesive 7, which is applied onto a base part 1 control unit housing, the base part being provided as a heat sink. It is also known to press the copper plate directly onto the heat sink using a screw-type mounting means. In any case, the heat produced by power component 3 is dissipated via the via holes onto large-surface printed circuit trace 11 and via insulation layer 5 onto copper plate 6. From there, the heat is transferred to heat sink 1 by solder resist 15 either directly or via thermally conductive adhesive 7. If circuit board 2 were applied to the heat sink without insulation layer 5 and copper sink 6, a short-circuit between potential-carrying printed circuit traces 13 could lead to damaging the components. However, in the arrangement shown in FIG. 1, it is disadvantageous, that in order to apply insulation layer 5 and additional copper plate 6, two further, separate manufacturing steps are required.

[0017] FIG. 2 shows a first exemplary embodiment of an arrangement according to the present invention for dissipating the waste heat of a power component, the arrangement being preferably installed in an electronic control unit of a motor vehicle. At least one power component, e.g., a power semiconductor, is applied onto a large-surface printed circuit trace 10 on upper side 8 of a substrate 2, which can be a circuit board, a hybrid, or another substrate furnished with an electronic circuit. In the exemplary embodiment depicted here, substrate 2 is a circuit board having components on both sides. As can be seen further in FIG. 2, a connection 14 of power component 3 is electrically connected to a connecting printed circuit trace 12 of upper side 8 of circuit board 2, the connecting printed circuit trace being insulated from printed circuit trace 10. On lower side 9 of circuit board 2, a second large-surface printed circuit trace 11 is applied, which is connected in a thermally conductive manner to first printed circuit trace 10 on upper side 8 by via holes 4. In addition, still other undepicted printed circuit traces, associated with the circuit, as well as a number of connecting surfaces for SMD components are provided on lower side 9. As is depicted in FIG. 2, on lower side 9 of circuit board 2, provision is further made for conductor surface elements 17, which can be manufactured together with the remaining printed circuit traces and connection surfaces and can be made from the same material on the lower side of the circuit board. This can take place using the customary known technologies. In the exemplary embodiment depicted in FIG. 2, a voltage is transmitted to second large-surface printed circuit trace 11 from power component 3 by via holes 4. Conductor surface elements 17 are therefore arranged on the substrate insulated from printed circuit trace 11. Furthermore, a solder resist 15 is applied in the known manner onto lower side 9, provision being made for recesses in the solder resist at the locations of conductor surface elements 17 and at the undepicted connection surfaces for SMD components. In the manufacture of the arrangement depicted in FIG. 2, circuit board 2 is turned with lower side 9 facing upwards, and, in a solder paste imprinting station, is imprinted using solder paste. In this context, solder is applied onto conductor surface elements 17 and onto connection surfaces for SMD components. Solder resist 15 prevents solder from reaching other circuit parts in the process. After the imprinting of the solder, circuit board 2 is conveyed to an assembly machine, which impresses the SMD components into the solder paste applied on the connection surfaces on upwards-facing lower side 9 of the circuit board. Subsequently, the circuit board passes through a reflow solder station, in which the solder is melted. In this context, it is advantageous if the SMD components are soldered to the connecting surfaces and, at the same time, the spacer elements 17, 18 are formed. This takes place by liquefying solder 18 that is impressed onto conductor surface elements 17, in the reflow oven, and, as a result of the surface tension of the solder on surface pieces 17, solder humps or solder caps of a defined size are formed, whose shape is a function only of the size of the conductor surface elements 17 and of the quantity of solder that is impressed. In particular, using the described method, it is possible to configure all the spacer elements having one precisely defined uniform height. In this context, it is advantageous that the solder paste imprinting step and the reflow soldering step must in any case be carried out for circuit boards having components on two sides, so that no additional manufacturing step is required for producing the spacer elements. In place of the manufacturing process described above, it is also possible to moisten the conductor surface elements, e.g., in a wave solder bath using liquefied solder. It is advantageous that the spacer elements be manufactured at a defined height. After the manufacture of spacer elements 17, 18, a thermally conductive adhesive 7 is applied onto a heat sink 1 using a dispensing device. In a further exemplary embodiment, provision is made for employing a thermally conductive foil, having adhesive on both sides, in place of the adhesive. Functioning as heat sink is a housing part of the control unit, for example, the housing base. Circuit board 2 is then placed onto the adhesive with its lower side 9 turned toward heat sink 1 and, in this manner, is pressed in the direction of the heat sink so that spacer elements 17, 18 penetrate into adhesive 7. In this context, they can contact heat sink 1 and thus obtain a minimal spacing. In this context, a trough-shaped recess, not depicted in FIG. 2, in the housing base of the control unit receives the components arranged on the lower side of the circuit board. As a result of spacer elements 17, 18, a defined gap is formed between the lower side of the circuit board and the heat sink, the gap, as shown in FIG. 2, being completely filled with adhesive 7. Because the spacer elements can be manufactured having a defined small height, the gap can be set very small without the heat sink contacting the lower side of the circuit board, which improves the heat removal to the heat sink.

[0018] The arrangement shown in FIG. 2 can, also be manufactured, for example, in that circuit board 2 having spacer elements 17, 18 is first placed onto the heat sink, only subsequently an adhesive capable of capillary flow being introduced into the gap between the circuit board lower side and the heat sink.

[0019] Spacer elements 17, 18 can advantageously be used also for the EMV protection (electromagnetic compatibility) of the arrangement. Since the spacer elements are composed of an electrically conductive material, an electrical contact to the heat sink is generated by them, i.e., spacer elements and heat sink have the same potential. If at least some conductor surface elements 17 are connected to printed circuit traces associated with the circuit, then, via the spacer elements, a short and thus low-radiation ground connection can be realized.

[0020] A second exemplary embodiment is depicted in FIG. 3. The same numbers denotes the same parts. The arrangement depicted in FIG. 3 differs from the arrangement depicted in FIG. 2 in that power component 3 and heat sink have the same potential. Therefore, the conductor surface elements 17 can advantageously be directly integrated into second large-surface printed circuit trace 11 of substrate 2. Solder resist mask 15, applied on second side 9 of substrate 2, has recesses which define conductor surface elements 17. Solder caps 18, as described above, are formed on these conductor surface elements. Subsequently, the substrate is placed onto heat sink 1 or adhesive 7. Since power component 3, printed circuit trace 11, and heat sink 1 have the same potential, no short-circuit is generated by spacer elements 18. In comparison to the first embodiment shown in FIG. 2, the heat transfer by the arrangement of solder caps 18 on second large-surface printed circuit trace 11 can be improved.