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

In an arrangement composed of a substrate and a heat sink, the substrate is provided on a first side with at least one power component arranged on a first large-surface printed circuit trace and, on a second side opposite the power component, with a second large-surface printed circuit trace, which is connected in a thermally conductive manner to the first printed circuit trace via via holes. The substrate is mounted at the second side onto the heat sink in a thermally conductive manner, in order to achieve a good heat coupling of the substrate to the heat sink and the same time to avoid an undesirable electrical contact between the potential-carrying printed circuit traces and the heat sink. The substrate having spacer elements arranged on the second side is placed onto the heat sink and to hold it at a defined distance from the heat sink, the gap formed by the distance between the substrate and the heat sink being filled with a thermally conductive filler.

FIELD OF THE INVENTION
 BACKGROUND INFORMATION
 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.
 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.
 SUMMARY OF THE INVENTION
 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.
 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.
 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.
 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.
 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.
 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).
 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.
 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.
 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.

DETAILED DESCRIPTION
 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.
 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.
 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.
 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.
 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.