PRINTING SCREEN FOR USE IN A METHOD FOR THROUGH-PLATING A PRINTED CIRCUIT BOARD AND USE OF SUCH A PRINTING SCREEN IN SUCH A METHOD

Abstract

A printing screen for use through-plating a printed circuit board. The printing screen has at least one screen hole for filling a larger hole compared to a reference hole in a ceramic substrate. This printing screen has an area-reducing and area-dividing geometry that divides the screen hole into at least two hole sections.

Claims

1.-17. (canceled)

18. A printing screen used for through-plating a printed circuit board, comprising: at least one screen hole for filling a hole in a ceramic substrate which is larger than a reference hole, the at least one screen hole having an area-reducing and area-dividing geometry that divides the at least one screen hole into at least two hole sections.

19. The printing screen as claimed in claim 18, wherein the area-dividing geometry is a geometry that divides the at least one screen hole into three hole sections is used for the at least one screen hole.

20. The printing screen as claimed in claim 19, wherein three identical screen sections configured as lands are arranged offset from one another by 120 and are used for the geometry, and these lands meet in a center of the at least one screen hole, being formed onto one another at the center of the at least one screen hole.

21. The printing screen as claimed in claim 18, wherein the area-dividing geometry is a geometry that divides the at least one screen hole into four hole sections is used for the at least one screen hole.

22. The printing screen as claimed in claim 21, wherein four identical screen sections configured as lands are arranged offset from one another by 90 and are used for the geometry, and these lands meet in a center of the at least one screen hole, being formed onto one another at the center of the at least one screen hole.

23. The printing screen as claimed in claim 18, wherein the area-dividing geometry is a geometry that divides the at least one screen hole into five hole sections is used for the at least one screen hole.

24. The printing screen as claimed in claim 23, wherein one annular screen section and four identical screen sections configured as lands are arranged offset from one another by 90 and are used for the geometry, and these lands are formed onto the one annular screen section on an outer radius.

25. The printing screen as claimed in claim 18, wherein the area-dividing geometry is a geometry that divides the at least one screen hole into eight hole sections is used for the at least one screen hole.

26. The printing screen as claimed in claim 25, wherein eight identical screen sections configured as lands are arranged offset from one another by 45 and are used for the geometry, and these lands meet in a center of the at least one screen hole, being formed onto one another at the center of the at least one screen hole.

27. The printing screen as claimed in claim 18, wherein the area-dividing geometry is a geometry that divides the at least one screen hole into sixteen hole sections is used for the at least one screen hole.

28. The printing screen as claimed in claim 27, wherein one annular screen section and eight identical screen sections configured as lands are arranged offset from one another by 45 and are used for the geometry, and these lands meet in a center of the at least one screen hole, being formed onto one another at the center of the at least one screen hole and intersecting with the one annular screen section.

29. The printing screen as claimed in claim 18, comprising at least one row of holes having at least one screen hole with an area-reducing and area-dividing geometry.

30. The printing screen as claimed in claim 18, further comprising a respective area-reducing and area-dividing geometry for hole diameters fixed within a range from 100 to 450 m.

31. The printing screen as claimed in claim 18, wherein a hole diameter of 300 to 600 m is fixed for the at least one screen hole without taking account of the area-reducing and area-dividing geometry.

32. The printing screen as claimed in claim 18, wherein the printing screen has a screen thickness of 30 to 150 m.

33. A method of through-plating a printed circuit board with conductor tracks formed on two sides of a sintered ceramic substrate, comprising: providing a printing screen used for through-plating the printed circuit board, comprising: at least one screen hole for filling a hole in a ceramic substrate which is larger than a reference hole, the at least one screen hole having an area-reducing and area-dividing geometry that divides the at least one screen hole into at least two hole sections simultaneously filling multitude holes in the sintered ceramic substrate of different hole diameter under compression pressure with a metallic sintering paste in a single filling process manufacturing step; drying and firing the metallic sintering paste to fully sinter the metallic sintering paste due to the firing, wherein the metallic sintering paste in its fully sintered state enters into at least a cohesive bond with the ceramic substrate and fills the hole.

34. The method as claimed in claim 33, wherein all holes in the ceramic substrate that are to be filled are filled simultaneously in a single filling process manufacturing step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The invention will be discussed in detail in the following text with reference to the illustrations in the figures. Further advantageous refinements of the invention emerge from the dependent claims and the description below of preferred embodiments. The figures show:

[0051] FIG. 1 is a schematic illustration of a metallization of a substrate hole according to the prior art;

[0052] FIG. 2 is a schematic illustration of an inventive metallization of a substrate hole;

[0053] FIG. 3 is a schematic illustration of a compression pressure filling device;

[0054] FIG. 4 is a further schematic illustration of a metallization of a substrate hole; and

[0055] FIG. 5 are screen hole geometries used for filling of substrate holes.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0056] FIG. 1 illustrates a substrate 2 as part of a printed circuit board 1. The substrate 2, which may have been produced from a sintered ceramic, for example an alumina ceramic, has a hole 3 and has been printed on a first side with a first electrically conductive layer 4 or thick layer 4 and on a second side, opposite the first side, with a second electrically conductive layer 5 or thick layer 5. The hole 3 here is of conical design for production-related reasons. The two thick layers 4, 5 extend partly into the hole 3 and overlap in so doing. Such a coating of the hole 3 constitutes a through-plating of the substrate 2, by which through-plating conductor tracks 4, 5 formed on the two sides of the substrate 2 are connected to one another.

[0057] Such a coating of the hole 3 is achieved in that the two thick layers 4, 5 are successively partly sucked into the hole 3 from the respective opposite side of the substrate 2 by a negative pressure. In this example, the thick layer 4 has first been drawn in and then fully sintered in a furnace. Thereafter, the thick layer 5 has been sucked in and fully sintered in the furnace.

[0058] Weak points with very small layer thicknesses may be formed here, for instance a weak point 6 at the lower of the two hole edges. Such a weak point 6, which may have a layer thickness of about 1 to 2 m, can even lead to a failure of the through-plating under a high current load. If the hole 3 is also closed, for instance by a further printed layer or by further printed layers, or in that, for example, a glass compound is incorporated or introduced into the hole 3, because for instance one of the two substrate sides is to be hermetically closed off, such a filling of the hole 3 can lead to an excessive change in the resistance and hence also to an excessive change in the electrical behavior of the through-plating, and this change as such may be unacceptable.

[0059] FIG. 2 illustrates the proposed improvement whereby the hole 3 in the substrate 2 is completely filled with a metal-containing sintering paste 7 or conducting paste, preferably a silver-palladium paste. The sintering paste 7 is at least cohesively bonded to the substrate 2. In addition to this, the sintering paste 7 may also be bonded to the substrate 2 in a form-fitting manner, although this is not illustrated in FIG. 2. This depends on whether the filling of the hole with the sintering paste 7 forms an excess of material or a material plug that engages behind the respective substrate side or the respective hole edge. In addition, the substrate 2 is printed in the region of the completely filled hole 3 on each side with an electrically conductive thick layer 4, 5.

[0060] The sintering paste 7 that completely fills the hole 3 here is a pasty mixture including, for example, at least silver, palladium, a glass, a resin and a thinner. As it passes through a sintering furnace, this sintering paste 7 is solidified and consolidated to form a physically solid and electrically conductive structure. The sintering paste 7 contains a palladium content of preferably 10% to 15%. The sintering paste 7 here may be lead-containing or lead-free depending on the application.

[0061] An advantage of such a metallization of the hole 3 is that sufficient electrically conductive material is present at every point in the hole in order to assure failsafe through-plating of the substrate 2.

[0062] In addition, the region X around the hole 3 required for metallization according to FIG. 2 is smaller by comparison with the region X according to FIG. 1. Therefore, the proposed mode of metallization also leads to saving of space. The region X may be about 600 to 900 m, and the region X about 300 m or less. As a result, the region X is thus at most half the size of the region X.

[0063] The substrates illustrated in FIGS. 1 and 2 each have a thickness of about 0.63 mm. Furthermore, the holes 3 illustrated in FIGS. 1 and 2 each have a conical shape. Such a conical shape arises for production-related reasons when drilling the holes by a laser. The upper hole diameter here may be about 0.1 to 0.3 mm.

[0064] FIG. 3 illustrates an arrangement 30 of a substrate matrix 20 in a compression pressure filling device 10. Such a substrate matrix 20 consequently gives rise to a multitude of substrates 2 (cf. FIG. 2). For example, the substrate matrix 20 may comprise a total of 16 substrates 2, for instance in a 28 arrangement, i.e. in an arrangement having two rows each of 8 substrates 2. The compression pressure filling device 10 here enables at least simultaneous filling of the holes 3, arranged in a row, in the substrate matrix 20 with the sintering paste 7. This row of holes extends notionally in vertical direction with respect to the plane of the drawing of FIG. 3.

[0065] What is specifically apparent is the substrate matrix 20 disposed on a carrier 25. The substrate matrix 20 here is preferably framed by a reinforcing frame 22 and positioned with respect to the carrier 25 in such a way that the holes 3 of the individual substrates 2 are aligned with channels 26 of the carrier 25 that are arranged at right angles to one another. These channels 26 are additionally arranged at least essentially at right angles to the substrate matrix 20. The channels 26 serve firstly to accommodate slight material excesses that form on the underside of the substrate matrix 20 in the filling of the individual holes 3, and secondly to ventilate the individual holes 3.

[0066] As an alternative to these channels 26, the ventilation as such could also be ensured by a porous stone that may be disposed on the carrier 25. Such a porous stone could be in the form, for example, of a rectangular slab and be disposed in a corresponding receptacle of the carrier 25, such that the carrier 25 laterally frames the stone. The face of the stone facing the substrate matrix 20 appropriately concludes flush with the face of the carrier 25 facing the substrate matrix 20 in order to assure correspondingly two-dimensional contact with the substrate matrix 20.

[0067] The positionally accurate alignment of the substrate matrix 20 may be assured here, for example, via at least one corresponding stop (not shown) formed for instance on the carrier 25 and can be abutted, for example, by the reinforcing frame 22. The carrier 25 further comprises a multitude of vertical suction channels 28 via which the substrate matrix 20 is drawn and hence fixed against the carrier 25 by a negative pressure. The formation of the channels 26 results in formation of individual squares on the side of the carrier 25 facing the substrate matrix 20. Each of these squares is assigned at least one suction channel 28, for example with a central arrangement of the suction channel 28 (cf. FIG. 3).

[0068] In the case of the aforementioned porous stonenot shown herethe suction channels 28 may be arranged, for example, on two opposite sides of the stone in the carrier 25, such that they correspondingly flank the stone, in order to bring about the suction of the substrate matrix 20 along two edges of the stone.

[0069] Between the substrate matrix 20 and the carrier 25 is appropriately disposed a flexible, air-permeable layer 24, for instance in the form of a paper layer, which catches the sintering paste 7.

[0070] Atop the substrate matrix 20 appropriately lies a screen 18 having a plurality of holes 19 aligned with the holes 3 that have to be filled. The thickness of the screen is, for example, about 0.03 mm. It may alternatively be up to about 0.1 mm Indicated above the screen 18 is a print squeegee 14, by which the row of holes in the substrate matrix 20 is completely filled with the sintering paste 7. This print squeegee 14 comprises a collecting chamber 16 and an adjoining, smaller chamber 17 that can cover said row of holes in the substrate matrix 20.

[0071] The filling of the substrate matrix 20 here proceeds as follows:

[0072] By a plunger of elongate design, in the form of a sword 12 which is movable in the collecting chamber 16, the sintering paste 7 present in the chamber 16 is forced in vertical direction Y into the holes 3 of the row of holes via the chamber 17 and the screen 18. A compression pressure of about 1 to 4 bar is applied here. In this example, a compression pressure of about 3 bar is applied. The sintering paste 7 is introduced here into the holes 3 by metering in such a way that, on the underside of the substrate matrix 20, there is formation of only very minor excesses of material or material plugs that extend into the channel 26 and in so doing locally bend the paper layer 24 without tearing or damaging it. The individual plugs here form a material excess with respect to the underside of the substrate matrix 20 of about 2 to 5 m.

[0073] The print squeegee 14 moves from row of holes to row of holes in horizontal direction X to successively fill the individual rows of holes with the sintering paste 7. Both the screen 18 over which the print squeegee 14 sweeps and the paper layer 24 serve to prevent smearing of the substrate matrix 20.

[0074] It is also possible for a slight material excess with respect to the upper side of the substrate matrix 20 to form, with the result that the fillings of the individual holes 3 substantially have the form of a rivet.

[0075] Subsequent to the above-described filling operation, the substrate matrix 20 runs through a drying furnace and a sintering furnace. The fillings of the individual holes 3 are solidified and consolidated here to form a physically solid and electrically conductive structure.

[0076] In the sintering furnace, the substrate matrix 20 runs through a temperature profile with temperatures of up to 850 C. The fillings of the individual holes 3 here undergo both a reduction and an oxidation and in so doing enter into at least a cohesive bond with the ceramic substrate 2. In the fully sintered state, these fillings completely fill the respective holes.

[0077] The substrate matrix 20 is finally divided into the individual substrates 2, for instance using corresponding intended breakage sites in the substrate matrix 20 formed between the individual substrates 2.

[0078] FIG. 4 illustrates a substrate 2 according to the illustration in FIG. 2, but with an illustrated glass layer 8 that covers the electrically conductive thick layer 4. The printed circuit board 1 comprising the substrate 2 (according to FIG. 2 and FIG. 4) forms part of what is called a magnetic passive position sensor, used in a fuel tank of a motor vehicle to detect the fuel fill level. Sensors of this type are also referred to as MAPPS (MAgnetic Passive Position Sensor). A sensor of this type contains a printed circuit board, which, on one side of a substrate, is equipped with conductor tracks and a contact spring structure, wherein the contact spring structure is contacted with the conductor tracks by a magnet depending on the fuel fill level of the tank. However, the use of such a substrate 2 is not fundamentally reduced to such sensors.

[0079] This glass layer 8 contributes to hermetic encapsulation of the opposite side of the substrate 2 equipped with the conductor tracks and the contact spring structure from an aggressive fuel, and hence to protecting it from contamination and corrosion.

[0080] In a further configuration, the substrate matrix 20 has a multitude of substrates 2, each having a multitude of holes 3 of different diameter that are to be through-plated. According to the application, such a substrate matrix 20 may be of different size and have, for example, a multitude of substrates 2 or else just two substrates 2, depending on what is called the useful size of the individual substrates 2. Such a substrate matrix 20 may have, for example, up to about 800 holes 3 of different diameters that are to be through-plated, distributed over the individual substrates 2. The individual diameters of the substrate holes 3, 3a, 3b, 3c, 3d, 3e here are, for example, in the range from about 100 to 450 m.

[0081] FIG. 5 illustrates different diameters of substrate holes 3, 3a, 3b, 3c, 3d, 3e that may be formed in the individual substrates 2 and are to be filled with the sintering paste 7. Filling of the individual holes 3, 3a, 3b, 3c, 3d, 3e, owing to the different diameters, in principle requires different print parameter sets that would have to be applied in relation to the above-described compression pressure filling method. Such a print parameter set here includes a squeegee fill pressure, also called compression pressure, a squeegee offset pressure, and a squeegee speed. For simplification and for avoidance of repetition, reference is made at this point to the definition of these parameters introduced by way of introduction.

[0082] Accordingly, processing of the substrate matrix 20 comprising a multitude of substrates 2 with a multitude of holes 3, 3a, 3b, 3c, 3d, 3e in principle requires multiple compression pressure filling operations and hence a multistage compression pressure filling method.

[0083] Print parameters used for different substrate hole diameters are cited by way of example for illustration below. The compression pressure is about 3 bar.

TABLE-US-00001 Substrate hole Squeegee offset Squeegee diameter [m] pressure [N] speed [mm/s] 100 130 20 150 110 35 200 100 60 300 100 60

[0084] The first two diameters of 100 m and 150 m each relate to a signal wire for transmission of a current of, for example, about 10 A. The other two diameters of 200 m and 300 m each relate to a power supply wire for transmission of a current of, for example, about 65 to 110 A, for instance in conjunction with a power supply of a motor driver.

[0085] What is apparent here is that the squeegee offset pressure is at its highest at the smallest diameter of 100 m, and that the squeegee speed, by contrast, is at its lowest.

[0086] For simplification of the compression pressure filling method, what is now proposed is the use of a single print parameter set that is ideally fixed on the basis of the smallest hole 3 to be filled with the smallest hole diameter.

[0087] The other screen holes 19a, 19b, 19c, 19d, 19e that are larger than the smallest screen hole are provided with an area-reducing and area-dividing geometry 32 that divides the assigned screen hole 19a, 19b, 19c, 19d, 19e into multiple hole sections in order to fill the respectively assigned substrate holes 3a, 3b, 3c, 3d, 3e with the print parameters fixed for the smallest substrate hole 3 (see the table above) with the sintering paste 7.

[0088] Columns Ia) and Ib) of FIG. 5 show the substrate holes 3, 3a, 3b, 3c, 3d, 3e to be filled and the assigned screen holes 19, 19a, 19b, 19c, 19d, 19e, without said area-reducing and area-dividing geometry 32. Columns IIa) and IIb), by contrast, illustrate individual configurations of area-reducing and area-dividing geometries 32, specifically in relation to the screen holes 19a, 19b, 19c, 19d, 19e.

[0089] In a first configuration, a geometry 32 that divides the screen hole 19b into three hole sections is used for the assigned screen hole 19b. Specifically, three identical screen sections 34 in the form of lands arranged offset from one another by 120 are used here for the geometry 32, and these meet in the center of the screen hole 19b, being formed onto one another at the center of the screen hole 19b.

[0090] In a second configuration, a geometry 32 that divides the screen hole 19a into four hole sections is used for the assigned screen hole 19a. Specifically, four identical screen sections 34 in the form of lands arranged offset from one another by 90 are used here for the geometry 32, and these meet in the center of the screen hole 19a, being formed onto one another at the center of the screen hole 19a.

[0091] In a third configuration, a geometry 32 that divides the screen hole 19c into five hole sections is used for the assigned screen hole 19c. Specifically, one annular screen section 36 and four identical screen sections 34 in the form of lands arranged offset from one another by 90 are used for the geometry 32, and these are formed onto the annular screen section 36 on the outer radius.

[0092] In a fourth configuration, a geometry 32 that divides the screen hole 19e into eight hole sections is used for the assigned screen hole 19e. Specifically, eight identical screen sections 34 in the form of lands arranged offset from one another by 45 are used here for the geometry 32, and these meet in the center of the screen hole 19e, being formed onto one another at the center of the screen hole 19e.

[0093] In a fifth configuration, a geometry 32 that divides the screen hole 19d into sixteen hole sections is used for the assigned screen hole 19d. Specifically, one annular screen section 36 and eight identical screen sections 36 in the form of lands arranged offset from one another by 45 are used here for the geometry 32, and these meet in the center of the screen hole 19d, being formed onto one another at the center of the screen hole 19d. The screen sections 36 here intersect the annular screen section 34.

[0094] In one embodiment, the screen 18 illustrated in FIG. 3which may have been manufactured, for example, from a steel in a laser cutting method and may have a screen thickness of 30 to 150 mcomprises at least one of the above-described screen holes 19a, 19b, 19c, 19d, 19e. Furthermore, the screen 18 may, additionally or alternatively to the above-described screen holes 19a, 19b, 19c, 19d, 19e, comprise further screen holes having different area-reducing and area-dividing geometries.

[0095] The sole print parameter set may be fixed here for hole diameters of, for example, about 100 to 450 m. For the assigned screen hole 19, 19a, 19b, 19c, 19d, 19e, it is possible here, without taking account of the area-reducing and area-dividing geometry 32, to fix a hole diameter of, for example, about 300 to 600 m.

[0096] The use of a printing screen of the type described above simplifies a filling manufacturing operation in which a multitude of substrate holes having at least two hole diameters are to be filled, in that all holes to be filled are filled simultaneously with a sintering paste, ideally in a single filling manufacturing operation using merely a single print parameter set. The print parameter set used is ideally based on the substrate hole having the smallest diameter. Thus, in the ideal case, there will be just one pass through a drying and sintering furnace for drying and firing of the sintering paste introduced into the individual substrate holes. But it is at least possible to reduce the number of thermal treatment operations to a minimum.

[0097] The use of such a printing screen is thus the best possible way of counteracting the formation of what are called depletion zones especially in the sintering paste owing, for example, to migration of metal or glasson account of the aforementioned Kirkendall effectand hence the formation of holes in the material. Such depletion zones are promoted specifically by multiple passes through a sintering furnace.

[0098] For the sake of completeness, it should be mentioned by way of illustration at this point that it has been possible to date for such printed circuit boards to pass through a sintering furnace for thermal treatment up to about fifteen times. The proposed printing screen thus advantageously contributes to reducing this number of passes through the sintering furnace to a minimum.

[0099] Although exemplary embodiments have been elucidated in the above description, it should be noted that numerous modifications are possible. Furthermore, it should be noted that the exemplary embodiments are merely examples which are not intended to limit the scope of protection, the applications and the structure in any way. Instead, the above description gives the person skilled in the art a guideline for the implementation of at least one exemplary embodiment, wherein various changes may be made, especially with regard to the function and arrangement of the component parts described, without departing from the scope of protection as apparent from the claims and combinations of features equivalent thereto.

[0100] Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.