Method for Producing a Printed Circuit Board Having Thermal Through-Contacts, and Printed Circuit Board

Abstract

In a printed circuit board (1), thermal vias (19) are formed between the lower surface (A) and an upper surface (B) of the substrate plate (10) of the printed circuit board through the steps of: applying a respective solder resist mask (21, 31) to the lower surface (A) and the upper surface (B); applying solder to the lower surface (A) and reflow soldering the solder, wherein the solder penetrates into the boreholes (20) and forms convex menisci (26) protruding beyond the edge (22) of the respective boreholes on the lower surface (A); and creating regions (35) on the upper surface (B), which are freed from solder resist material, and which are intended for contacting at least one electronic component (17) on the upper surface and each of which comprise at least one of the thermal vias. Subsequently, the upper surface (B) can be provided with electrical components (17) on these regions (35). The first solder resist mask (21) has a respective region (23) that is free of solder resist on the lower surface around the edge of every borehole (20).

Claims

1. A method for producing thermal vias (19) in a printed circuit board, proceeding from a substrate plate (101) with a plurality of boreholes (11, 12, 13, 14) preformed therein, which are formed between an lower surface (A) and an upper surface (B) of the substrate plate (101) and are located at positions where thermal vias are to be produced respectively, the method comprising the following steps: applying a first and a second solder resist mask (21, 31) onto the lower surface (A) or respectively the upper surface (B), wherein the first solder resist mask (21) has, at the preformed boreholes (11-14), respective regions (23) free from solder resist around the edges (22) of each bore on the lower surface, wherein the second solder resist mask (31) extends to at least the edges (32) of the boreholes on the upper surface for at least a majority, preferably each, of the preformed boreholes (11-14) respectively; applying solder (16) onto the lower surface (A) and reflow soldering of the solder, wherein the solder penetrates into the bores (11-14) and forms on the lower surface (A) convex menisci (26) protruding beyond the edge (22) of the respective boreholes; and then clearing regions (35) on the upper surface (B), which regions are predetermined for the contacting of at least one electronic component (18) on the upper surface and respectively comprise at least one of the thermal vias, by removal of the second solder resist mask (31) at least in said regions.

2. The method according to claim 1, wherein in the first solder resist mask (21) the regions (23) free from solder resist are configured circular ring-shaped.

3. The method according to claim 2, wherein the solder-resist-free regions (23) of immediately adjacent boreholes touch one another, whereby between the free regions, areas (25) with solder resist are formed.

4. The method according to claim 1, wherein the second solder resist mask (31) in at least a part of said boreholes extends over the edge (32), forming a freestanding inwardly projecting ring (34) there, respectively.

5. The method according to claim 1, wherein the second solder resist mask (31) in at least a portion of said boreholes extends to the edge (32), wherein in these boreholes the edge (33) of the second solder resist mask is flush with the edge (32) of the borehole.

6. The method according to claim 1, wherein the substrate plate is held with the lower surface (A) oriented upwards during the step of the applying of solder.

7. The method according to claim 1, further comprising a subsequent additional step: equipping the upper surface (B) with at least one electronic component (18) on the cleared regions (35) for contacting.

8. The method according to claim 1, wherein the menisci (26) produced during the reflow soldering of the solder on the lower surface (A) form convex calottes respectively over the boreholes.

9. The method according to claim 1, further comprising an additional step, carried out in advance: lining of the preformed boreholes (11-14) with a metal.

10. The method according to claim 1, wherein during the reflow soldering of the solder, the bores (11-14) into which the solder penetrates are filled by the solder (16).

11. A printed circuit board (1) comprising: a substrate plate (10), and a plurality of thermal vias (19) formed in the substrate plate (10), which vias extend along boreholes (20), which are formed between a lower surface (A) and an upper surface (B) of the substrate plate (10), wherein: the thermal vias have, on the lower surface (A), convex menisci (26) protruding beyond the edge of the respective boreholes (20), and on the upper surface (B), regions (35) cleared of a solder resist mask (37) are provided, which regions (35) are equipped with solder material for the contacting of at least one electronic component (18) on the upper surface and are in solder connection with at least one of the thermal vias (19).

12. The printed circuit board according to claim 11, wherein on the lower surface (A) the menisci (26) are circular, and preferably the menisci (26) of immediately adjacent boreholes touch one another, wherein between these menisci respectively areas (25) with solder resist are present, which preferably have the shape of quadrilaterals or triangles delimited by concave curve segments.

13. The printed circuit board according to claim 12, wherein the menisci (26) are formed by solder material (16), which at the same time fills the boreholes (20).

14. The method of claim 9, wherein the metal has a high electrical conductivity.

15. The method of claim 14, wherein the metal is copper.

16. The method of claim 9, wherein the lining is by a galvanic method.

17. The method of claim 3, wherein the areas (25) with solder resist have the shape of quadrilaterals or triangles delimited by concave curve segments.

Description

[0034] The invention including further advantages is explained below based on exemplary embodiments, which are illustrated in the drawings. The drawings show in schematic form by means of a sequence of respective sectional views of the substrate plate:

[0035] FIG. 1 shows a substrate plate of a first embodiment of the invention, wherein the substrate plate is temporarily turned around, so that its lower surface is turned upwards;

[0036] FIG. 2 shows the substrate plate with applied solder resist masks, and specifically in a sectional view in partial image (a) and in a top view onto the lower surface of the substrate plate in partial image (b);

[0037] FIG. 3 illustrates the applying and reflow soldering of solder;

[0038] FIG. 4 shows the state achieved by the reflow soldering with filled boreholes in the substrate plate;

[0039] FIG. 5 illustrates the following step of clearing of regions on the upper surface, wherein previously the substrate plate is turned around;

[0040] FIG. 6 illustrates the additional step of equipping the substrate plate with electronic components;

[0041] FIG. 7 shows the thus obtained printed circuit board; and

[0042] FIG. 8 shows a substrate plate with solder resist masks according to a second embodiment of the invention.

[0043] It shall be understood that the embodiments which are described here serve merely for illustration and are not to be interpreted as being restrictive for the invention; rather, all configurations which the person skilled in the art can find based on the description, are within the scope of protection of the invention, wherein the scope of protection is defined by the claims.

[0044] In the following figures, the same reference numbers are used for identical or comparable elements for the purpose of simpler explanation and illustration. The reference numbers used in the claims are intended further only to facilitate the readability of the claims and the understanding of the invention and are in no way impeding in nature with regard to the scope of protection of the invention.

[0045] In FIG. 1 a substrate plate 101 is shown, which in the first example embodiment serves as the starting point for the production method. The sectional views shown in FIG. 1 and in the subsequent figures correspond to a section through a row of boreholes, respectively along a section plane which corresponds to the section plane 2-2 in FIG. 2(a), wherein respectively the same portion of the substrate plate is shown in the successive stages of the production method.

[0046] The substrate plate 101 includes a base plate 10, for example a single- or multi-layered FR4 plate; FR4 plates are well known as base plates for printed circuit boards. The two surfaces of the substrate plate correspond to an lower surface A and an upper surface B. It is pointed out that in the illustrations of FIGS. 1 to 4 the substrate plate is oriented in a turned around manner, i.e. the lower surface A is turned upwards and the upper surface B is turned downwards. As explained in the introduction, the upper surface B is the side on which, with equipping of the substrate plate on one side, the electronic components are applied (cf. FIG. 6). Depending on the application layout of the printed circuit board which is to be produced, provision can also be made that for various regions of the substrate plate the roles of the lower surface and upper surface are exchanged, for example if components in certain regions are also mounted on the opposite side.

[0047] In the base plate 10 of the substrate plate 101 a plurality of boreholes are formed, of which four boreholes 11, 12, 13, 14 are shown in a representative manner in FIG. 1, and which have been produced by means of a suitable known method. Generally, the boreholes are situated on the substrate plate in a two-dimensional arrangement which has been established in advance. The positions in the two-dimensional arrangement are selected according to application and requirements at the locations where respectively vias (thermal vias) are to be produced; the actual positions of the vias or respectively boreholes, however, are not of further relevance for the invention. The boreholes 11-14 (and likewise the bores 211-214 of FIG. 2(b)) are provided respectively with their own reference numbers, but are realized in a similar way for the purposes of the present disclosure.

[0048] The bores 11-14 are preferably, but not necessarily, provided with a metallic lining (coating) 15. The metallic material of the coating 15 preferably has a high electrical conductivity. Preferred materials are e.g. copper, aluminium or carbon coating. This lining is produced for example immediately after the introducing of the boreholes into the base plate 10 by a suitable method known per se, e.g. a galvanic method. The lining 15 is illustrated with an exaggerated thickness in the drawings, for the sake of clarity.

[0049] Solder resist masks 21, 31 are now applied onto the substrate plate 101 on either sides. This produces the substrate plate 102, shown in FIG. 2, with a first solder resist mask 21 on the lower surface A of the substrate plate 102 and with a second solder resist mask 31 on the upper surface B of the substrate plate 102, as can be seen in the sectional view of FIG. 2(a). The height of the solder resist masks 21, 31 is illustrated in an exaggerated manner in the figures, for the purpose of clarity and in many embodiments is distinctly less than as shown. In order to enable a flat filling of the vias in the later stage of the method, cleared regions are provided in the solder resist masks 21, 31, so that the solder resist masks respectively exclude at least areas which correspond to the openings of the boreholes 11-14 on the upper surface or respectively lower surface.

[0050] FIG. 2(b) shows a detail of the first solder resist mask 21 in a top view of the substrate plate 102. Each boreholein FIG. 2(b) beyond the boreholes 11-14 bores 211, 212, 213, 214 of a second row, arranged in parallel, are also illustratedopens on the lower surface into an opening, the edge of which can be seen in FIG. 2(b) in each case as a circle: smaller circles 22. Around each of the edges 22 a solder resist clearance 23 is provided, so that a free region remains up to an outer edge, shown in each case as a larger circle 24. Preferably, the clearance regions 23, in so far as possible and compatible with the electronic structure which is provided, are set so closely against one another that the outer edges 24 of immediately adjacent boreholes 11, 12, 212, 211 or respectively 13, 14, 214, 211 touch one another. Between such immediately adjacent boreholes, respectively a non-wettable area 25 of the solder resist mask remains; in the example embodiment which is shown, this area 25 has the shape of a rhombus or diamond shape. These areas 25 are provided in order, in the later step, to delimit the amount of the solder received in the cleared regions and in order to better define the surface of the solder. In general, the shape of such an area, which is bordered by several clearance regions, corresponds to a concave polygon, i.e. a polygon-like figure, which is formed from concave curve segments, in particular circle segments, mostly a concave quadrilateral or concave triangle. With such a concave polygon, therefore, the mentioned concave curves or respectively circle segments occur instead of straight-lined edges. Between clearance regions, which are provided for vias, between which no electrical contact must come about in the electronic layout, a bridge remains, however, with a minimum width which is determined in advance. The value of the minimum width depends respectively on layout specifications; a typical value is e.g. 100 m, but can also be 50 m or less. Generally, a value is selected which ensures an electrical separation between the produced vias. In these cases, the concave polygons run out at the points, which lie between such electrical separated vias, into the mentioned bridges and are therefore connected with one another if applicable by these bridges. Clearances are also provided on the upper surfacesolder resist mask 31at the sites of the openings of the boreholes, corresponding to the edges 32. Here, however, the clearance in the solder resist mask 31 on the upper surface B is preferably realized as small as possible; this is to prevent solder material situated in the bores from projecting over the surface of the upper surface after the first reflow soldering of the solder (cf. FIG. 4). The solder resist mask 31 therefore preferably reaches at least up to the edges 32 of the boreholes on the upper surface B. In the embodiment which is shown, the clearance is such that the edge 33 of the second solder resist mask 31 is flush with the edge 32 of the bore. Here, therefore, with each borehole, the edge 33 of the solder resist mask 31 corresponds to the borehole edge 32. Alternatively, in a variant embodiment which is not illustrated, a narrow clearance region can also be taken out around the edge 32, so that the edge of the solder resist mask reaches just up to the edge 32; the remaining distance to the edge 32 is small, so that the meniscus occurring over the opening of the borehole does not project over the surface of the solder resist mask 31 on the upper surface.

[0051] Examples of dimensions in the solder resist mask 21 are 0.7 mm diameter of the clearance regions 23 with a diameter of the openings 22, 32 of 0.35 mm.

[0052] In an advantageous variant embodiment, which is shown in FIG. 8 in a section view of a substrate plate 103, the solder resist mask on the upper surface B can be modified to the effect that the solder resist mask 38 projects over the edges 32 of the boreholes into the respective openings. The solder resist mask 38 therefore forms, in the case of these boreholes, respectively a freestanding inwardly projecting ring 34. This ring 34 prevents an undesired flowing away of the solder, which could lead to undesired voids in the via. Such protruding rings 34 can be provided in all boreholes or in a portion of the boreholes. Otherwise, the substrate plate 103 of FIG. 8 corresponds to the substrate plate 102 of FIG. 2.

[0053] The vias are advantageously not closed with solder resist, because this prevents air pockets from occurring in the via and impairing an efficient filling. The solder resist mask 31, 38 therefore prevents solder from being able to exit via the openings, and at the same time through the remaining openings (according to the edges 33 or respectively 34) itself enables an outgassing of the flux. This provides for a forming of the vias without undesired voids (cavities).

[0054] With reference to FIG. 3, then in the next step, namely a first reflow soldering pass for example by means of the known SMT method, solder 16 is applied onto the lower surface A. In FIG. 3 the solder 16 is illustrated symbolically by a rectangle with broken hatching. Preferably, the substrate plate is held with the lower surface A oriented upwards at least during this step.

[0055] During the reflow soldering, the solder 16 penetrates into the boreholes 11, 12, 13, 14, wherein it advances up to the edges of the solder resist masks 21, 31.

[0056] The thus obtained state of the substrate plate 104 is illustrated in FIG. 4. The solder advantageously advances on the lower surface A up to the edges 24 of the clearance regions 23 of the solder resist mask 21, so that largely filled boreholes 30 are obtained. The solder 16 forms menisci 26 here on the regions 23, which project over the lower surface edges 22 of the boreholes 30. These menisci 26 preferable receive, through a suitable choice of solder material and quantity, a shape such that they form several small hills in the form of convex calottes. This produces an enlarged surface of the menisci, which improves the radiation or respectively dissipation of heat. The menisci 26 correspond in their arrangement naturally to the arrangement of the clearance regions 23 (see FIG. 2(b)), so that they preferably touch one another, whereas between the menisci 26 the areas 25, described above, remain with solder resist.

[0057] If applicable, according to a variant of the method, during the first reflow soldering pass of FIGS. 3 and 4 at the same time components (not shown) on the lower surface A can be equipped.

[0058] On the upper surface B, on the other hand, such menisci are possible in this stage, but are not necessary, with regard to the later solder pass for the upper surface, as described further below.

[0059] With reference to FIG. 5, a step of the clearing of regions 35 on the upper surface B then takes place. For this, it is generally expedient that the substrate plate is previously turned, so that from now on the upper surface B is turned upwards. The regions 35 are established in advance and correspond to the regions on which, in the subsequent step, electronic components are contacted. At least some of the regions 35 also comprise here the sites of one or more of the previously produced filled boreholes 30.

[0060] This step of clearing serves for the regions 35 to be made free from solder resist. For this, the parts of the solder resist mask 31, which are situated in the regions 35, are removed by means of suitable methods C, for example with a chemical or plasma-chemical etching method of known type, by means of lithographic methods, or with the use of a marking laser C. Outside these regions 35 expediently the solder resist of the thus processed solder resist mask 37 remains.

[0061] Then, as illustrated in FIG. 6, in the next step a second reflow soldering pass takes place on the upper surface B, for example by means of the known SMT method. Here, the upper surface is again printed with solder material 36 in a manner known per se and is equipped with components; in FIG. 6 a component 17 for generally any desired number of components is shown in a representative manner. The solder 36 therefore penetrates during the reflow soldering into the cleared regions 35 and connects on the other side with the contact surfaces 18 of the components 17. Hereby, a direct connection of the contact surfaces 18 with the filled bores 20 is produced.

[0062] FIG. 7 shows the thus obtained printed circuit board 1. At the sites of the boreholes 20 a number of vias 19 are now formed, which are filled with solder material, which extends, free of interruption, to the contact surfaces 18 of the components 17 on the upper surface B (and, if applicable, the lower surface A; not shown), and thus provides for a good thermal and, if applicable, electrical contacting of the components.

[0063] Thus, the method according to the invention makes it possible that vias 19 have a position directly under components 17 and can nevertheless be filled in a targeted manner, without, in so doing, having to accept an impairment of the paste printing in the SMT process or a tipping of the components 17.