Method for Filing at least One Hole formed in a Printed Circuit Board, a Printed Circuit Board filled in such a Manner, and a Vehicle Comprising such a Printed Circuit Board

Abstract

Disclosed are a method for filling at least one hole formed in a printed circuit board, a printed circuit board filled in such a manner, and a vehicle having such a printed circuit board. The method of filling at least one hole formed in a printed circuit board comprises: introducing a paste comprising an electrically conductive metal powder and an electrolyte into the at least one hole of the printed circuit board, S1; and galvanic metallization of the printed circuit board so that elemental metal is deposited from the electrolyte in the at least one hole during the galvanic metallisation, S2.

Claims

1. A method for filling at least one hole formed in a printed circuit board, the method comprising: introducing a paste comprising an electrically conductive metal powder and an electrolyte into the at least one hole of the printed circuit board; and galvanic metallisation of the printed circuit board so that elemental metal is deposited from the electrolyte in the at least one hole during the galvanic metallisation.

2. The method according to claim 1, wherein prior to the introduction of the paste (S1), an inner peripheral wall of the at least one hole galvanically pre-metallised to form a sleeve-like metal layer and the paste is introduced into a space delimited by the sleeve-like metal layer.

3. The method according to claim 1, wherein the printed circuit board exposed to galvanic metallisation is provided with a protective layer on the printed circuit board bottom side and/or the printed circuit board top side which protects against deposition of metal during galvanic metallisation on a copper layer applied to the printed circuit board bottom side and/or the printed circuit board top side.

4. The method according to claim 1, wherein the metal powder comprises copper particles, silver particles and/or silver-plated copper particles.

5. The method according to claim 1, wherein an average particle diameter of the metal powder is greater than or equal to 10 μm.

6. The method according to claim 5, wherein the paste comprises 90 wt % or more of the metal powder.

7. The method according to claim 6, wherein the electrolyte comprises a solvent and cations dissolved in the solvent, the cations corresponding to a metal of the metal powder.

8. The method according to claim 7, wherein the electrolyte comprises CuCN, CuSO.sub.4, Cu[BF.sub.4].sub.2, Cu(SO.sub.3 NH.sub.2).sub.2, Cu[P.sub.2O.sub.7], AgCN, K[Ag(CN).sub.2] or a mixture of at least two components thereof dissolved in the solvent and/or wherein the cations originate from a salt dissolved in the solvent.

9. The method according to claim 1, wherein the electrolyte has a cation concentration greater than a cation concentration of an electrolyte in an electroplating bath of the galvanic metallisation.

10. The method according to claim 1, wherein the paste comprises 10 wt % or less of the electrolyte.

11. The method according to claim 1, wherein the paste further comprises an additive.

12. The method according to claim 1, wherein the paste is free of an epoxy resin phenolic resin, polyamide resin.

13. A printed circuit board manufactured according to claim 1.

14. A vehicle comprising a printed circuit board according to claim 13.

15. The method according to claim 1, wherein the paste comprises 90 wt % or more of the metal powder.

16. The method according to claim 15, wherein the electrolyte comprises a solvent and cations dissolved in the solvent, the cations corresponding to a metal of the metal powder.

17. The method according to claim 16, wherein the electrolyte comprises CuCN, CuSO.sub.4, Cu[BF.sub.4].sub.2, Cu(SO.sub.3 NH.sub.2).sub.2, Cu[P.sub.2O.sub.7], AgCN, K[Ag(CN).sub.2] or a mixture of at least two components thereof dissolved in the solvent and/or wherein the cations originate from a salt dissolved in the solvent.

18. The method according to claim 1, wherein the paste further comprises a viscosity stabiliser.

Description

[0052] Exemplary but non-limiting embodiments of the invention are explained in more detail below.

[0053] FIG. 1 shows a flowchart describing a method for filling at least one hole formed in a printed circuit board.

[0054] FIG. 2 shows a photograph of a cross-sectional view of a printed circuit board, the holes of which are filled with a solidified or galvanically converted/bonded, previously paste-like filling according to the present invention.

[0055] FIG. 3 (a) and FIG. 3 (b) each show a photograph of a cross-sectional view of a printed circuit board showing an applied optional protective layer.

[0056] FIG. 4 shows a photograph of a cross-sectional view of a multilayer printed circuit board in which a plurality of holes are formed, each extending in the thickness direction of the printed circuit board across multiple/all layers.

[0057] FIG. 5 shows a photograph of a cross-sectional view of a printed circuit board showing a solder mounted on a solder pad that is electrically connected to an electronic component.

[0058] In the following description, reference is made to the accompanying figures which form part thereof and in which specific embodiments in which the invention may be practised are shown by way of illustration.

[0059] It is understood that other embodiments may be used and structural or logical changes may be made without departing from the scope of protection of the present invention. It is understood that the features of the embodiments described herein may be combined with each other, unless specifically stated otherwise. The following description is therefore not to be construed in a limiting sense, and the scope of protection of the present invention is defined by the appended claims.

[0060] FIG. 1 shows a flowchart describing a method for filling at least one hole formed in a printed circuit board. Referring now to FIG. 1, a method for filling at least one hole formed in a printed circuit board according to one embodiment of the invention is described.

[0061] First, for example, a paste comprising an electrically conductive metal powder and an electrolyte may be introduced into the at least one hole of the printed circuit board, step S1.

[0062] The printed circuit board can be, for example, a standard single-layer printed circuit board with two electrically conductive outer layers (for example, copper layers) or a multilayer printed circuit board. In this regard, FIG. 4 shows, by way of example, a photograph of a cross-sectional view of a multilayer printed circuit board in which several holes (in this case through-holes) are formed, each of which extends in the thickness direction of the printed circuit board across several/all layers.

[0063] The electrically conductive metal powder in the paste may comprise, for example, copper particles (in another embodiment, silver particles or silver-plated copper particles may be used, for example). An average particle diameter of the copper metal powder may be, for example, greater than or equal to 10 μm and may be, for example, in the range of 30 μm to 50 μm. The paste may comprise, for example, 90 wt % or more of the metal powder, for example 90 wt % to 99 wt %.

[0064] For example, the electrolyte may comprise a suitable solvent and cations dissolved in the solvent, in this case copper cations. For example, the electrolyte may contain CuCN or CuSO.sub.4 dissolved in the solvent. Further, the electrolyte may be, for example, an aqueous solution. The cations may, for example, be derived from a salt dissolved in the solvent. The paste may comprise, for example, 10 wt % or less of the electrolyte, for example, 1 wt % to 10 wt %. The paste may further comprise, for example, an additive. The additive may be, for example, a viscosity stabiliser.

[0065] The paste may be free of an epoxy resin, for example, free of epoxy resin, phenolic resin, polyamide resin and polyamide-imide resin, for example, free of any resin binder.

[0066] Further with reference to FIG. 1, after the paste is introduced into the at least one hole of the printed circuit board, S1, for example, av galvanic metallisation of the printed circuit board, S2, may be performed. Thereby, for example, during the galvanic metallisation, S2, elemental metal may be deposited from the electrolyte in the at least one hole.

[0067] Thus, for example, the deposited elemental metal can bond with the particles of the metal powder and altogether form a cohesive (for example, one-piece), solid, thermally and electrically conductive metal filling 2 in the at least one hole of the printed circuit board. In this respect, FIG. 2 shows by way of example a solidified/galvanically bonded metal filling 2 according to one embodiment of the present invention.

[0068] The paste electrolyte may, for example, comprise a cation concentration that is greater than a cation concentration of an electroplating bath or its electrolyte in which the galvanic metallisation is carried out (for example, by immersing the PCB in it), in order to specifically promote metallisation in the hole.

[0069] According to an exemplary aspect of the present invention, prior to the introduction of the paste, S1, for example, an inner peripheral wall of the at least one hole may optionally be pre-metallised by electroplating to form a sleeve-like metal layer 3, S3. The paste may then be introduced, for example, into a space delimited by the sleeve-like metal layer 3, S1. An example of such a sleeve-like metal layer 3 is shown in FIG. 2. The sleeve-like metal layer 3 may thus be an electrolytically deposited metal sleeve of, for example, the same metal as that of the paste metal powder and the paste cations.

[0070] The formation of the sleeve-like metal layer 3 may, for example, allow the current density to grow in a controlled/defined manner over the axial extent of the hole in the direction R (cf. FIG. 2) from the sleeve-like metal layer 3 on the inner circumferential wall of the at least one hole towards a centre of the at least one hole during the galvanic metallisation, S2. For example, the filling may first be connected to the sleeve-like metal layer 3. Fillings produced in this way may, for example, have particularly good properties, such as electrical conductivity. However, the method according to the invention or the metallisation/solidification of the filling can also be carried out without galvanic pre-metallisation, S3.

[0071] According to another exemplary aspect of the present invention, the printed circuit board subjected to galvanic metallisation may be provided with a protective layer 4, S4, for example on the PCB bottom side and/or the PCB top side. In this regard, FIG. 3 (a) and FIG. 3 (b) each show a photograph of a cross-sectional view of a printed circuit board showing an applied optional protective layer 4. The protective layer 4 may, for example, protect against deposition of metal during the galvanic metallisation S4 on a copper layer 1 (e.g., thick copper layer) applied to the PCB bottom side and/or PCB top side. For example, a conventionally used dry film can serve as a protective layer 4.

[0072] As shown in FIG. 1, the application of the protective layer, S4, may be performed, for example, before the paste is introduced into the printed circuit board, S1. Alternatively, the application of the protective layer, S4, may be carried out, for example, after the introduction of the paste, S1, into the printed circuit board. The sequence of steps S3 and S4 shown, if both are carried out, can also be reversed.

[0073] A printed circuit board made by the method for filling at least one hole formed in a printed circuit board according to the present invention may be used, for example, in a vehicle. A vehicle may be, for example, a land vehicle, aircraft or watercraft.

[0074] FIG. 5 shows a photograph of a cross-sectional view of a printed circuit board according to the invention, showing a solder 6 attached to a solder pad 5, which is electrically connected to an electronic component 7. The solder pad 5 may be, for example, a patterned portion of the copper layer 1 on the PCB surface, including an area disposed over the hole and (re)sealed by the galvanic metallisation. Such an arrangement may be of great relevance, for example, in high density interconnect circuits (for example, HDI circuits, for example, “high density interconnect” circuits), which may be used, for example, in the field of high current applications with large and heavy electronic components 7. Especially in vehicles, an adhesion (e.g., peel strength) of the solder pad may be highly stressed due to the constant vibration loads, for example. For this reason, for example, a high peel strength of the solder pad 5 at the filling of the hole or on the hole filling surfaces in vehicles can be of great importance.

Trial Examples

[0075] In accordance with the present invention, initial tests have been carried out.

[0076] First, approx. 95 wt % copper powder (manufacturer's data: electrolytically produced, dendritic 99.70% copper powder, oxygen: max. 0.3% (during production); bulk density: 1.8 g/cm.sup.3; 38 μm, 400 mesh, particle size distribution IS04497>106 μm: 0%, particle size distribution IS04497>63 μm: max. 5.0%, particle size distribution IS04497>45 μm: max. 10.0%; Werth metal) and approx. 5 wt % of an acidic CuSO.sub.4 electrolyte solution (Glanzkupferelektrolyt sauer from MARAWE GmbH & Co. KG, Art. No. 01-10-01000) were mixed in a beaker. The electrolyte solution was added gradually, drop by drop towards the end, and the mixture was stirred with a copper spatula for a longer time until the copper particles were well wetted with the electrolyte solution and a paste had formed. Using a doctor blade, the paste was then introduced into the through holes of a printed circuit board.

[0077] The PCB was then subjected to electrolysis. For this, the PCB was placed in an electroplating bath. The electrolyte solution of the electroplating bath was the same electrolyte solution that was used to mix with the copper particles and to make the paste. Furthermore, the printed circuit board was connected at its thick copper layer 1 as cathode. A copper plate, which was also in the electroplating bath, served as the anode.

[0078] These procedures/experimental set-up were/was repeated for the various experiments described below.

[0079] Different voltages were then applied in different experiments. The series of experiments was started with 3 V in the first experiment and the voltage was reduced to 1 V in the last experiment. The best results were observed for a voltage of 1 V in the present experimental setup. However, it must be expected that the voltage must be adjusted depending on the thickness of the PCB, the diameter of the holes and other influencing factors.

[0080] Furthermore, the duration of the electrolysis was varied in different experiments. In particular, different electrolysis durations from 30 to 60 minutes were investigated. It was found that in the present experimental set-up, a solid copper filling could be observed already from an electrolysis duration of 30 minutes. The circuit board produced could be ground down and showed a hole well and evenly filled with copper when viewed under the microscope.

[0081] Furthermore, tests were carried out in which the paste was introduced into galvanically pre-metallised holes (where the inner circumference of a hole had a sleeve-shaped metal layer 3) or metal-free holes (where the inner circumference of a hole had no sleeve-shaped metal layer 3, for example). In both cases, successful through-connection was observed. However, in the tests in which the holes were galvanically pre-metallised, it was found that a more uniform filling had formed in the holes.

[0082] The tests performed so far have been carried out without applying a dry film as a protective layer 4 on the PCB surfaces. However, the use of a dry film can be considered to avoid an on-growth of electrolytic metal on, for example, the top and/or bottom of the board and/or, for example, when using silver pastes (for example, in galvanic silver plating) to avoid a deposition of silver on the copper layer.

[0083] The experiments carried out so far were also carried out with a paste electrolyte cation concentration that was equal to the cation concentration of the electrolyte in the electroplating bath. By diluting the electrolyte in the electroplating bath, the cation concentration of the electrolyte in the electroplating bath can be easily adjusted to a lower value than the paste electrolyte cation concentration. Alternatively or additionally, a paste electrolyte cation concentration can be increased by adding an appropriate metal salt into the electrolyte of the paste (for example, before mixing with metal powder), and/or a separate, higher concentrated electrolyte can be used.

[0084] The galvanolysis can also be carried out, for example, with a reverse pulse current method for electroplating and/or in a galvanic ultrasonic bath. This is expected to produce a uniform and rapid bonding between the introduced copper particles.

[0085] Initial measurement results show that the fillings of holes in printed circuit boards produced in this way are superior to conventional paste fillings (in particular, resin-based pastes, especially epoxy-based pastes) in terms of electrical conductivity and thermal conductivity. Furthermore, the filling produced according to the present invention is expected to have a high peel strength compared to that of resin-bonded (e.g., epoxy-bonded) metal pastes, and the paste according to the invention is easy to store.

[0086] Furthermore, the measured properties are comparable to those of purely galvanic metal fillings of holes in PCBs. However, the latter are not universally applicable (for example, for all types of holes in PCBs), and purely galvanic filling of holes can sometimes take a very long time (for example, several days compared to less than 60 minutes for the method according to the present invention).