Printed circuit board

A circuit board (1) has a top face (2) for positioning an electronic component and a bottom face (4) used as a support on a heat-dissipating base. A plurality of heat transfer holes (12) provide heat transfer from the top face (2) to the bottom face (4). The heat transfer holes (12) are unevenly or non-uniformly distributed on the top face (2) in such a way that the top face (2) is provided with several free sectors (14) which are free of heat transfer holes (12) in order to connect the electronic component to the circuit board (1). The free sectors (14) are configured as columns or lines. A plurality of heat transfer holes (12) are placed at least along the long sides of the free sectors (14). The circuit board has a low thermal resistance between the electronic component and the heat-dissipating base.

FIELD OF THE INVENTION

The invention relates to a printed circuit board with heat transfer holes or thermal vias extending through it.

BACKGROUND INFORMATION

A printed circuit board of this type is known from obvious prior use. Such printed circuit boards are used for SMD assembly with electronic components. These electronic components can be those, which in operation produce a substantial waste heat. In order to prevent destruction of the electronic components this waste heat must be efficiently dissipated. With this heat dissipation the printed circuit board as a rule is the component with the highest thermal resistance or heat (transfer) resistance, respectively. The thermal resistance of the prior known printed circuit board is still too high for sophisticated assembly tasks.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to further develop a printed circuit board of the initially mentioned type such that the thermal resistance of the printed circuit board between the electronic component and the heat dissipating base is reduced.

This object is achieved in accordance with the invention by a printed circuit board having the features disclosed and claimed herein. An inventive printed circuit board has

a top face for positioning an electronic component,

a bottom face to support on a heat-dissipating base,

a plurality of heat transfer holes, which create a heat transfer from the top face to the bottom face,

wherein the heat transfer holes are non-uniformly distributed on the top face in such way that the top face has several free sectors, which are free from heat transfer holes in order to connect the electronic component to the printed circuit board,

characterized in that the free sectors are each configured as a respective elongated free area having an elongated strip shape with two opposite longer sides extending parallel to an elongation direction and two opposite shorter sides extending transversely to the elongation direction, wherein the longer sides are longer than the shorter sides, and wherein a respective plurality of the heat transfer holes are positioned adjacent to each one of the longer sides and are spaced apart from one another in a respective row extending parallel to the elongation direction along each one of the longer sides of the elongated free areas, and wherein an in-row spacing distance between adjacent ones of the heat transfer holes in one of the rows is less than a transverse spacing distance between two of the rows that are respectively adjacent to the two opposite longer sides of one of the elongated free areas.

At the free sectors the electronic component is connected to the printed circuit board. As these free sectors are free of heat transfer holes for the purpose of enlargement of the connecting surface, at this place there is a more complicated heat transfer. It was recognized according to invention that the concept of the sectors being free of heat transfer holes can be maintained, if the free sectors are at least lengthwise surrounded by heat transfer holes. These heat transfer holes reduce the thermal resistance of the printed circuit board. By the arrangement according to invention it is ensured that in direct vicinity of the free sectors a sufficient number of heat transfer holes is available, which enable a heat passage, so that the thermal resistance of the printed circuit board as a whole is sufficiently low and thus uncritical also in the region of the free sectors. Heat transfer holes are assigned to each long side of the free sectors.

For a sufficient connection of the electronic component to the printed circuit board a certain minimum connecting surface must be provided on the top face of the printed circuit board. As was recognized in a preferred embodiment with at least three or preferably four free sectors, this minimum connecting surface can be distributed to a plurality of free sectors. The smaller the surface of the individual free sectors, the smaller the risk that a dangerous heat accumulation arises here.

A preferred arrangement in which at least two but preferably all of the free sectors are surrounded all around by heat transfer holes increases the heat dissipation within the region adjacent to free sectors.

A packing density of fifty heat transfer holes per 100 mm2of surface of the circuit board results in a thermal resistance of the printed circuit board being sufficiently low also for sophisticated assembly tasks.

The heat transfer holes are most efficient to the heat transfer at that place, where they are covered by the electronic component. Therefore, the heat transfer holes are preferably located exclusively where they will be covered at least partially by the mounted electronic component, which results in an advantageously low thermal resistance.

A preferred feature, whereby the shape of the area of the circuit board provided with heat transfer holes is matched to the shape of the mounting surface of the electronic component, leads to an efficient, in particular compact heat transfer arrangement.

Grid dimensions of the heat transfer holes spaced apart at most 1.5 mm, preferably at most 1.25 mm, and especially preferably 1.0 mm proved to be particularly advantageous for the creation of a low thermal resistance.

A simple cubic packing of the heat transfer holes permits the accommodation of a high number of heat transfer holes with given grid dimension and given surface. Compared to less close packings this can again reduce the thermal resistance.

A solder layer may be provided on the circuit board, preferably only in the free sector areas, which prevents solder from penetrating into the heat transfer holes during soldering of the electronic component. The advantageous low thermal resistance of the heat transfer holes is then maintained. A limitation of the surface expansion of the solder layer in such a manner that it is available only within the free sectors, is preferably effected by use of a layer of solder masking paint surrounding the solder layer and if necessary individual sectors thereof. By the limitation of the solder layer to the free sectors a defined soldering result is obtained. Short-circuits on the bottom face of the printed circuit board, e.g. towards a cooling body, are avoided. As far as solder masking paint is used, preferably the free sectors are free of soldering masking paint, so that solder can be applied there.

Solder layer dimensions preferably of 1 mm×10 mm within a free sector area represent a good compromise between sufficient size of the connecting surface for safe connection of the electronic component to the printed circuit board on the one hand and lowest possible surface for reduction of the thermal resistance on the other hand. Besides, the limitation of size of the free sectors leads to the fact that during soldering cavities in the solder layer are avoided.

Heat transfer holes preferably having a width of 0.3 mm to 0.5 mm can be packed particularly closely.

Heat transfer holes having a heat-conducting coating, preferably of metal about 30 mm thick, comprise an advantageously low thermal resistance.

Heat transfer holes that are preferably free, i.e. unfilled, can be manufactured at low costs.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1shows schematically an exemplary structure, in which a printed circuit board1, a so-called pad, is used. This schematic structure usually does not have anything in common with the actual field of application of the printed circuit board1and is used for determining the thermal resistance Rththrough the printed circuit board1. In fact, the printed circuit board1in the form as represented in the drawing and described hereinafter is merely a cutout of a real printed circuit board with a plurality of electronic components and associated conductor paths. Therefore, if in this description a printed circuit board1is addressed, a cutout of a real printed circuit board is actually meant. An electronic component3lies on a top face2of the printed circuit board1. This is e.g. a field-effect transistor FET for use in particular in the automotive field, e.g. with ignition systems. Due to the support on the printed circuit board1the electronic component3is installed in SMD (Surface-Mounted-Device) style. A bottom face4of the printed circuit board1rests upon a heat dissipating base, a copper block5. Instead of a copper block5also an aluminum block can be used. Generally, aluminum can be used instead of copper in case of a variant of the shown embodiments.

The vertical cut according toFIG. 5through the structure according toFIG. 1makes clear the different layers, of which the printed circuit board1and the electronic component3are composed, and, moreover, shows the connecting layers between the components of the structure according toFIG. 1. The highest layer shown inFIG. 5is a silicon layer6, which represents the actual electronic component. This silicon layer6is connected to an underlying copper layer7, the so-called spreader, for the purpose of heat dissipation. The copper layer7is connected via a solder layer8in sectors to the printed circuit board1. Between the individual sectors of the solder layer8a solder masking paint can be applied onto the printed circuit board1. The solder masking paint guarantees a defined expansion of the individual sectors of the solder layer8, wherein ideally each individual sector of the solder layer8is completely surrounded by solder masking paint, which represents in this way a limitation of the solder layer8in all directions parallel to the level of the printed circuit board1. The printed circuit board1in turn represents a layer structure from a plurality of copper layers9and prepreg layers10. Two of the copper layers9limit the printed circuit board1on top and bottom, so that both along the top face and along the bottom face a good superficial thermal conduction is available over the entire surface of the printed circuit board1. Via a further solder layer11the printed circuit board1is soldered onto the copper block5. Instead of the bottom solder layer11shown inFIG. 5also a thermal conductive adhesive can be used. Such a thermal conductive adhesive can fulfill in additional the function of an electrical insulator.

The printed circuit board1comprises a plurality of heat transfer holes12, which create a heat transfer from the top face2to the bottom face4, i.e. with an installed printed circuit board1between the electronic component3and the copper block5. In the embodiment according toFIG. 2the heat transfer holes12are free, thus unfilled. Each heat transfer hole12carries in the interior a heat conducting coating not shown in detail. This is a copper coating with a layer thickness from 30 to 35 μm. The heat transfer holes12are embodied as through-holes running perpendicular to a main level13(comp.FIG. 5) of the printed circuit board1. In the embodiment according toFIG. 2the heat transfer holes12have a width of 0.3 mm.

The printed circuit board1is rectangular with external dimensions of 9 mm (in x-direction, i.e. in the extension direction being horizontal inFIG. 4) times 10 mm (in y-direction, i.e. in the extension direction being vertical inFIG. 4).

In total, in the embodiment according toFIG. 2there is a number of 58 heat transfer holes12. These are irregularly i.e. non-uniformly distributed on the top face2of the printed circuit board1in such a manner that the top face2comprises several free sectors14, in the embodiment according toFIG. 2altogether four free sectors14. These free sectors14are free from heat transfer holes12in order to connect the electronic component3to the printed circuit board1, so that in the region of the free sectors14there are no heat transfer holes12. Due to the solder masking paint the solder layer8can be applied only in the region of the free sectors14on the top face2for connection to the electronic component3. In the embodiment according toFIG. 4the free sectors14are arranged as four columns parallel to the longer side of the rectangular printed circuit board1(y-direction). The free sectors14lie within a rectangular connecting region15, while in case of an installed electronic component3this rests against the top face2of the printed circuit board1. The four free sectors14are rectangular with dimensions of 0.9 mm in x-direction and 8 mm in y-direction. The heat transfer holes12are arranged on the printed circuit board1in such a manner that with the applied electronic component3they are covered at least partially or geometrically, respectively, by it.

The side dimensions of the printed circuit board1or of the pads, respectively, are such that a surface region16of the top face2of the printed circuit board1, which is provided with heat transfer holes12, is adapted geometrically to geometry of the connecting region15of the electronic component3, which rests against the top face2of the printed circuit board1.

The rectangular connecting region15has the dimensions 8 mm×9 mm, the adaptation of the surface region16provided with heat transfer holes12to the dimensions of the connecting region15being such that the surface region16projects beyond all four sides of the connecting region15by 0.5 mm each.

In the embodiment according toFIG. 2apart from the free sectors14the heat transfer holes12are arranged in a regular grid without offset with a grid width of 1.0 mm.

Adjacent to each one of the long sides17running in y-direction of the free sectors14, respectively eight heat transfer holes12are arranged. Adjacent to each one of the small sides18running in x-direction of the free sectors14, again respectively one heat transfer hole12is arranged. Besides, adjacent to each corner of the free sectors14another heat transfer hole12is arranged, so that each of the four free sectors14is surrounded all around by a total of twenty-two heat transfer holes12.

FIGS. 6 to 20show further arrangement variants of heat transfer holes12on printed circuit boards. The printed circuit boards as well as the arrangements of the heat transfer holes12in the variants according toFIG. 6 to 20are only described, where they differ from that, what was discussed already for the embodiment according toFIGS. 1 to 5.

The embodiment according toFIG. 6comprises rectangular free sectors14with an extension of 1.2 mm in x-direction and 8.2 mm in y-direction. By the enlarged free sectors14and the enlarged solder surfaces resulting therefrom an improvement of the adhesion of the electronic component3at the top face2of the printed circuit board1is effected.

In the embodiment according toFIG. 7the free sectors14have dimensions of 0.9 mm in x-direction and 8.2 mm in y-direction. Also through this compared to the free sectors14according toFIG. 4an improvement of the adhesion of the electronic component3at the printed circuit board1is made possible. Compared to the variant according toFIG. 6the long sides17are distanced slightly further from the heat transfer holes12adjacent to them, so that the risk of an unwanted entrance of solder into the heat transfer holes12is reduced.

In the embodiment according toFIG. 8the surface region16of the printed circuit board1is widened in x-direction by 1 mm, so that a square printed circuit board1with dimensions 10 mm×10 mm is effected. Otherwise the embodiment is unchanged regarding the relative arrangement of the printed circuit board1to the electronic component3. InFIG. 8on the right a further column19with a total of ten heat transfer holes12results, which is not covered by the electronic component3. Thus, the number of heat transfer holes in the embodiment according toFIG. 8is increased to a total of 68. By means of the ten additional heat transfer holes12the heat transfer through the printed circuit board1is improved.

In the embodiment according toFIG. 9the dimensions of the solder layer8coincides with those of the connecting region15. Consequently, at this place solder is present also outside of the free sectors14. In the embodiment according toFIG. 9altogether three free sectors14are provided. Apart from the free sectors14in the embodiment according toFIG. 9the heat transfer holes12are provided in a regular, offset-free, i.e. cubic-face-centered grid with a grid width of 1.25 mm. In this form of embodiment five heat transfer holes12each are adjacent to the long sides of the free sectors14. In this form of embodiment one heat transfer hole12each is adjacent to the small sides of the free sectors14. Also to the four corners of the free sectors14one heat transfer hole12each is adjacent in this embodiment, so that all three free sectors14in this embodiment are surrounded by a total of sixteen transfer holes12. Towards the outside a squarely rotating external path with nine heat transfer holes each per side is attached to the heat transfer holes12adjacent to the free sectors of this embodiment. Therefore, in the embodiment according toFIG. 9there is a total of 66 transfer holes12. The surface region16, which is provided with heat transfer holes12, has dimensions of 11.5 mm×11.5 mm in case of the printed circuit board1. With an electronic component3with connecting region dimensions of 8 mm×9 mm a projection of the surface region16beyond the connecting region15in x-direction of 1.75 mm and in y-direction of 1, 25 mm results. The path rotating outside of heat transfer holes12is not covered by the electronic component3. All other heat transfer holes12are covered by the electronic component3. The enlargement of the accessible solder surface leads to an improvement of the adhesion of the electronic component3at the top face2of the printed circuit board1. The external path of heat transfer holes12at least partially compensates the decrease of the heat transfer by the increase of the grid dimension of the heat transfer holes12compared to the embodiments according toFIGS. 1 to 8.

The embodiment according toFIG. 10corresponds to that according toFIG. 9with the difference that in the embodiment according toFIG. 10the solder layer8for connecting the electronic component3to the printed circuit board1is freely accessible only within the free sectors14on the top face2.

In the embodiment according toFIG. 10there are 66 heat transfer holes12. The reduction of the freely accessible solder layer8to the free sections14prevents that solder penetrates into the heat transfer holes12during soldering.

The embodiment according toFIG. 11resembles that according toFIG. 10, so that the embodiment according toFIG. 11is described only, where it differs from that according toFIG. 10. In the embodiment according toFIG. 11the free sectors14along their long sides17are extended in such a manner that with an equal grid dimension now seven heat transfer holes12are adjacent to the long sides17. Consequently, in contrast to the embodiment according toFIG. 10six heat transfer holes are omitted at the expense of an appropriate enlargement of the free sectors14. Thus, in the embodiment according toFIG. 11there are 60 heat transfer holes12. The surface of the free sectors14enlarged in comparison to the embodiment according toFIG. 10improves adhesion of the electronic component3at the printed circuit board1.

The embodiment according to Fig. resembles12that according toFIG. 11, so that the embodiment according toFIG. 1is described in the following only, where it differs from that according toFIG. 11. Between two adjacent free sectors14in the embodiment according toFIG. 12the middle each of the seven heat transfer holes12along the long sides17of the free sectors14is omitted. At the place of this omission an additional free surface section in form of a line results in the case of the representation according toFIG. 12. Also at this place the solder layer8is freely accessible, so that compared to the embodiment according toFIG. 11adhesion of the electronic component3at the printed circuit board1is improved again. The embodiment according toFIG. 12has a total of 58 heat transfer holes, as in comparison toFIG. 11two heat transfer holes12are omitted.

On the basis of a comparison with the embodiment according toFIG. 4the embodiment according toFIG. 13is described, where it differs from the embodiment toFIG. 4. The four free sectors14comprise lengthwise seven heat transfer holes12, the outer two holes of these seven heat transfer holes12being located so far outside that they protrude slightly beyond the long sides17. One heat transfer hole12each is adjacent to the narrow sides18, so that each of the four free sectors14is surrounded all around by a total of sixteen heat transfer holes12. Further heat transfer holes12are arranged around the three free sectors14according to the type of a simple cubic packing with a grid width of 1.25 mm within the surface region16. Apart from the heat transfer holes12lacking in the region of the free sectors14there is a total of eleven columns of heat transfer holes12, with columns with eight heat transfer holes12, which are present also at the edge side, alternating with columns of nine heat transfer holes12being offset hereunto. The surface region16of the printed circuit board1, which is provided with heat transfer holes12, has dimensions of 12.5 mm in x-direction and of 11.5 mm in y-direction. All in all there are 69 heat transfer holes12with the variant according toFIG. 13. Due to the simple cubic packing there is a relatively high number of heat transfer holes12around the three free sectors14. Therefore, in this region this amounts to a good heat transfer.

The embodiment according toFIG. 14is described in the following only, where it differs from that according toFIG. 11. In the embodiment according toFIG. 14the heat transfer holes12adjacent to the long sides17and the narrow sides18, what concerns the long sides17and the narrow sides18facing each other, are arranged in the same way as the heat transfer holes12in the embodiment according toFIG. 11. At the two outer long sides17of the outside free sectors14two simply cubically packed columns20,21of heat transfer holes12are attached. The two interior columns20, directly adjacent to the long sides17, have eight heat transfer holes12each. The two outer columns21have nine heat transfer holes12each. The grid dimension is 1.25 mm. Therefore, in the embodiment according toFIG. 14there is a total of 58 heat transfer holes12. In the region of the long sides17of the two outer free sectors14there is a good heat passage due to the simple cubic packing of the heat transfer holes12available there. Compared to the cubic-face-centered packing with a given surface and grid dimension the simple cubic packing allows for a higher number of heat transfer holes12, which can be accommodated.

The embodiment according toFIG. 15is described only, where it differs from that according toFIG. 13. In the embodiment according toFIG. 15the outer left and the outer right column of heat transfer holes are omitted. There is a total of 53 heat transfer holes12in a simple cubic packing. The surface region16provided with heat transfer holes12is reduced to 10.5 mm in x-direction and to 11.5 mm in y-direction.

The embodiment according toFIG. 16has two free sectors14, which inFIG. 16are arranged in columns, i.e. along the y-direction. The solder layer8is accessible only in the region of the free sectors14on the top face2of the printed circuit board1. Eight heat transfer holes12each are assigned adjacent to the long sides17of the free sectors14. Between the two free sectors14as well as on the respective opposite side of the free sectors14there are two columns each of heat transfer holes12in cubic-face-centered manner with a grid dimension of 1.25 mm. All in all the embodiment according toFIG. 16thus comprises six columns of each eight heat transfer holes12, thus a total of 48 heat transfer holes12. The surface region16, which is provided with heat transfer holes12, has dimensions of 10.25 mm×10.25 mm.

The embodiment according toFIG. 17is described in the following only, where it differs from that according toFIG. 4. In the embodiment according toFIG. 17not eight, but seven heat transfer holes12are adjacent to the long sides of the free sectors14. Therefore, all in all the embodiment according toFIG. 17has five heat transfer holes12less than the embodiment according toFIG. 4, so that there is a total of 53 heat transfer holes12. The decrease of the number of heat transfer holes12along the long sides17is accompanied by an appropriate shortening of the long sides17, so that the surface region16, which is provided with heat transfer holes12, has dimensions of 9 mm×9 mm.

The embodiment according toFIG. 18is described only, where it differs from that according toFIG. 17. In the embodiment according toFIG. 18the two outer free sectors14are extended along their long sides17up to the edge of the surface region16in such a manner that the heat transfer holes12adjacent to the small sides18are not available there in contrast to the embodiment according toFIG. 17. Thus, all in all the embodiment according toFIG. 18has four heat transfer holes12less than the embodiment according toFIG. 17, so that this results in a total of 49 heat transfer holes12. Due to the outer two free sectors14enlarged through this adhesion of the electronic component3at the printed circuit board1is improved.

The embodiment according toFIG. 19is described in the following only, where it differs from that according toFIG. 4. Similar as in the embodiment according toFIG. 18also in the embodiment according toFIG. 19the heat transfer holes12adjacent the small sides of the two outer free sectors14are omitted and these two outer free sectors14are drawn up to the edge of the connecting region15. In comparison to the embodiment according toFIG. 4thus in the embodiment according toFIG. 19four heat transfer holes are lacking, so that there is a total of 54 heat transfer holes12.

The embodiment according toFIG. 20is described only, where it differs from that according toFIG. 19. In the embodiment according toFIG. 20also the heat transfer holes12adjacent to the small sides of the two interior free sectors14are omitted and these two interior free sectors14are drawn up to the edge of the connecting region15. Thus, there are five columns of heat transfer holes12of each ten heat transfer holes12, a free surface section14each being between two adjacent of these columns.

The diagram according toFIG. 21makes clear the influence of different parameters onto the heat transfer through the printed circuit board1. The thermal resistance Rthis applied dependent on the number of heat transfer holes12. The squares indicate the thermal resistance for arrangements with the grid width 1.0 mm.

The square at the number of 49 corresponds to the arrangement according toFIG. 18. Here, there is a thermal resistance of approximately 1.73 K/W. This value results from a model calculation, in which apart from geometry of the arrangement also heat conductivities, heat capacities and the densities of the different materials involved are taken into account.

The square at the number of 50 corresponds to the arrangement according toFIG. 20. There, in comparison to the arrangement according toFIG. 18, the thermal resistance is reduced to approximately 1.66 K/W. Apart from the enlargement of the number of heat transfer holes12this is also caused by the fact that the surface region16in the embodiment according toFIG. 20is enlarged in relation to the embodiment according toFIG. 18. Thus, a larger heat transfer surface is created, what reduces the thermal resistance.

The square at the number of 53 corresponds to the arrangement according toFIG. 17. In this arrangement there is a thermal resistance of approximately 1.69 K/W. In comparison to the arrangement according toFIG. 20here, two effects compete against each other: On the one hand in the arrangement according toFIG. 17the surface region16is smaller than in the arrangement according toFIG. 20, what increases the thermal resistance. On the other hand the arrangement according toFIG. 17has three heat transfer holes12more, what reduces the thermal resistance.

The square at the number of 54 stands for the arrangement according toFIG. 19. Here, there is a thermal resistance of approximately 1.59 K/W. In comparison to the arrangement according toFIG. 20there are four additional heat transfer holes12in this case, so that this results in an accordingly low thermal resistance.

The square at the number of 58 corresponds to the arrangement according toFIG. 4or6, respectively. Here, in comparison to the arrangement according toFIG. 19there are four further heat transfer holes12, so that the thermal resistance continues to be reduced to approximately 1.54 K/W.

The lozenge at the number of 53 corresponds to the arrangement according toFIG. 15. Here, there is a larger grid dimension of the heat-transfer holes-12, so that this results in a larger thermal resistance of approximately 1.81 K/W compared to the arrangement according toFIG. 17with the same number of heat transfer holes12.

The lozenge for the number of 58 corresponds to the arrangement according toFIG. 14. Here, there are two columns of heat transfer holes12, i.e. the two columns21, which are relatively far away from the connecting region15, in which the electronic component3rests upon the printed circuit board1. This explains the relatively high thermal resistance of approximately 1.87 K/W in the embodiment according toFIG. 14.

The lozenge for the number of 60 corresponds to the arrangement according toFIG. 11. Due to the high number of heat transfer holes12this results in a reduction of the thermal resistance to approximately 1.8 K/W.

While considering the parameterstotal area of the accessible solder layer8, which should be as large as possible for optimizing the adhesion of the electronic component3at the printed circuit board1,size of the surface region16, which for realizing a compact as possible printed circuit board1should not be too large,number of heat transfer holes12, which for realizing a good heat transfer should be large, but for reasons of manufacturing costs and possibilities should be not too large,
the embodiment according toFIG. 4turns out to be particularly suitable according to the current state.