Method for producing a solderable circuit board with connecting surfaces made of aluminium

Abstract

The invention relates to a method for producing a solderable circuit board (1) comprising a substrate (2) made of an electrically insulating material, conductor paths (3) arranged on the substrate, having connecting surfaces (3.1) for SMD components, wherein the conductor paths and the connecting surfaces are made of aluminium or an aluminium alloy. According to the invention, in order to easily allow for the reproducible production of soldered connections of SMD components to the connecting surfaces of conductor paths made of aluminium on the circuit board, the connecting surfaces of the conductor paths made of aluminium are to be selectively metallised, in particular with nickel or a nickel-tin alloy, in a single-stage process, wherein the metallisation method is compatible with the commonly used plastics and composite materials of flexible and rigid circuit boards, and the metallised connecting surfaces provide a suitable surface for the subsequent soldering process, in particular in the form of reflow soldering.

Description

[0001]Method for producing a solderable printed circuit board with aluminum connection pads. The invention relates to a method for producing a solderable printed circuit board comprising a carrier made of electrically insulating material, conductor tracks arranged on the carrier, which have connection pads for SMD components, wherein the conductor tracks and the connection pads are made of aluminum or an aluminum alloy. SMD stands for "surface mounted device." Unlike through-hole components, SMD components do not have wire connections, but are soldered directly to the connection pads of the conductor tracks on a printed circuit board using solderable contact pads. The associated technology is surface mount technology (SMT). Surface mount technology (SMT) enables very dense assembly and, above all, double-sided assembly of the printed circuit board.The electrical properties of the circuits are positively influenced, especially at higher frequencies. The space required by the components is reduced. This allows the devices to be manufactured smaller and at the same time significantly more cost-effectively. In surface mounting, the SMD components are connected directly to the circuit board via the solderable contact surfaces on their undersides and solder paste previously applied to the connection surfaces of the circuit board. Reflow soldering is the standard process for soldering SMD components. SMD components, solder, and flux are already located on the connection surfaces of the conductor tracks, so that only heat energy needs to be supplied in a precisely defined temperature-time profile during the reflow soldering process.Surface mount technology (SMT) is established on printed circuit boards with copper conductors and connection pads and represents an efficient and cost-effective process. The electronics industry has long desired to utilize aluminum and its alloys – hereinafter and in the context of this invention referred to as aluminum – as a conductor material, given their advantages in terms of cost, weight, and availability. However, there is a serious problem with the use of surface mount technology (SMT), which is widespread in electronics manufacturing, in combination with aluminum surfaces. The metal is very base and reacts with air and water at room temperature in freshly cut areas to form aluminum oxide. This immediately forms a thin layer (passivation) that is impermeable to air and water, thus protecting the aluminum from corrosion.However, the aluminum oxide layer must be removed from the connection pads before soldering SMD components. A pretreatment paste from Averatec is known from the state of the art for soldering SMD components on circuit boards with aluminum conductors. It is a flux that reduces aluminum oxide. In a first step, the pretreatment paste is printed onto the connection pads. Only after the printed pretreatment paste has cured for approximately three minutes at approximately 85°C can a conventional solder paste for reflow soldering be applied in a second step. This solder paste contains a tin alloy and a flux (accessed on January 23, 2023, at https: / / www.averatekcorp.com / chemical / mina / ).The known process is always two-step, incurs high processing costs, and is incompatible with conventional surface mount technology (SMT) lines. The "Information Sheet O8 - 2nd Edition, Galvanic and Chemical Coatings, German Association of the Aluminium Industry" describes the so-called zincate process for metal plating of aluminum. The surface pretreatment required for metal plating of aluminum comprises several steps: cleaning and degreasing, pickling, and activation. Cleaning serves to remove dirt and grease residues from the surface of the aluminum workpiece. Subsequent alkaline pickling removes the aluminum's oxide layer. Subsequent pickling in an acidic solution removes any heavy metal particles present as alloy components that have not been detached from the workpiece surface.Subsequent activation using a zincate treatment prevents further surface oxidation. For this purpose, the surface is rinsed at least once with a zincate pickling solution. Zinc from the solution precipitates. Due to the high pH value, the zincate process is unsuitable for use in combination with materials such as those used as substrates for printed circuit boards. US 2014 / 0166098 A1 discloses methods for producing electrically conductive aluminum structures. An aluminum foil laminate is structured according to the desired electrode shape by punching, etching, or laser cutting. Subsequently, oxidized portions of the aluminum surface, namely where electrical connections are made, are removed.The oxide layer is preferably removed in the partial areas by laser ablation. After the oxidized partial areas have been removed, copper is preferably deposited by electroplating. EP 3647 462 A2 discloses a method for depositing a metal layer on aluminum which requires less complex surface treatment of the aluminum and which enables selective deposition of the metal on the surface of an aluminum layer. In a first step, at least a partial area of ​​the at least one aluminum layer is laser-structured. Subsequently, in a second step, the metal layer is deposited using an electroless metal exchange process on each laser-structured partial area of ​​the at least one aluminum layer in an electrolytic bath. The electrolytic bath contains copper or a copper alloy for the electroless metal deposition of a copper layer.EP 3657 914 A1 discloses a printed circuit board for an LED module, comprising a carrier element, an aluminum coating arranged on the carrier element and structured into conductor tracks, and a solder resist arranged on or above the aluminum coating. At least one window for providing a solder pad is formed in the solder resist. A tin coating is arranged within the window in the solder resist directly on the aluminum coating and is designed as a solder pad for receiving a solder paste or solder for contacting surface-mount components during production.The tin coating can be formed by surface finishing by tinning a surface of the aluminum coating or by immersing the carrier element with at least the structured aluminum coating in a tin-containing bath. Based on this prior art, the invention is based on the object of proposing a method for producing a solderable printed circuit board that enables the reproducible production of solder connections of SMD components to the connection pads of aluminum conductor tracks on the printed circuit board with minimal effort.The solution to this problem is based on the idea of ​​selectively metallizing the connection surfaces of the aluminum conductor tracks in a single-step process, in particular with nickel or a nickel-tin alloy. The metallization process is compatible with the plastics and composite materials commonly used for flexible and rigid printed circuit boards, and the metallized connection surfaces offer a suitable surface for the subsequent soldering process, in particular by means of reflow soldering. In detail, this problem is solved by a method having the features of claim 1. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 5 of 21 The application of a solder mask to all conductor tracks of the circuit board with the exception of the connection surfaces ensures that the metal deposition in the electrolytic bath takes place selectively only in the connection surfaces.The mixture of water as solvent, at least one metal salt, and phosphinic acid in the electrolytic bath is determined in such a way that, in a single-step process, the oxide layer on the connection surfaces is removed using the phosphinic acid, and an electroless metal deposition of metal atoms of the at least one metal salt takes place on the connection surfaces. The phosphinic acid is used as an etching agent to remove the aluminum oxide layer and as a reducing agent for the metal salts. In a single-step process, the oxide layer is removed, the metal salts are reduced, and the metal atoms created by the reduction process are deposed on the aluminum connection surfaces without the use of external current. The electroless metal deposition in the reduction process is based on the oxidation of the reducing agent phosphinic acid with the release of electrodes, which lead to a reduction of the metal ions of the metal salt.To enable targeted deposition from the electrolyte, metal deposition occurs under the catalytic influence of the aluminum connection pads. The residence time of each connection pad of the circuit board after insertion into the electrolytic bath is between 5 and 300 seconds. Each connection pad is completely immersed in the bath after insertion. In order to selectively metallize the connection pads of the aluminum conductor tracks with nickel, in an advantageous embodiment of the invention, the at least one metal salt is a nickel salt. The nickel salt is in particular a nickel(II) chloride, NiCl2, and / or nickel(II) sulfate, NiSO4. If the selective metallization is to be carried out with a nickel-tin alloy, the electrolytic bath additionally contains at least one tin salt.The tin salt is in particular a tin(II) chloride SnCl₂ and / or tin(II) sulfate SnSO₄. The mixture of nickel and tin salt in the electrolytic bath is determined in such a way that a nickel-tin alloy is deposited on the connection surfaces, which preferably consists of 70-90% nickel and 10-30% tin, particularly preferably 90-98% nickel and 2-10% tin. The above percentages refer to the deposited mass fractions of nickel and tin in relation to the mass of the nickel-tin alloy deposited on the connection surfaces. For the selective coating of the connection surfaces with nickel or a nickel-tin alloy, the electrolytic bath is a mixture of substances with the following mass fractions: - water 50-95%, preferably 70-80% - phosphinic acid 1-30%, preferably 10-20% Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx 27.November 2023 Page 7 of 21 - Nickel salt 1-10% - Tin salt 0-10% - Additives 0-10%, preferably 1-2% The mass fraction indicates the relative proportion of the mass of the individual substances to the total mass of the mixture. The mass fractions of the individual substances add up to 100% within the ranges specified above. Additives, also known as fluxes and / or catalysts and / or stabilizers and / or buffer substances and / or accelerators, are particularly suitable. Tests have shown that a mass fraction of phosphinic acid of 15% of the mixture is optimal for removing the oxide layer and metallizing. If the electrolytic bath contains sodium tetraborate Na2B4O7 (borax) and / or disodium hydrogen phosphate Na2HPO4 as additives, metallization is accelerated.The manufacturing method according to the invention is suitable both for the selective metallization of rigid printed circuit boards with a rigid carrier made of electrically insulating carrier material for the conductor tracks and for the selective metallization of flexible printed circuit boards with a flexible carrier made of electrically insulating carrier material for the conductor tracks. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 8 of 21 The rigid carrier preferably consists of a thermally conductive CEM3 material with a thickness of 0.5-2 mm, and the conductor tracks have a layer thickness between 20-200 µm. The flexible carrier preferably consists of a carrier film made of PET or polyimide with a thickness of 10-200 µm, and the conductor tracks consist of an aluminum layer bonded to the carrier film with a thickness of 5-200 µm.In an automated environment, the flexible printed circuit board offers the advantage that the flexible carrier provided with the conductor tracks is pulled through the electrolytic bath as a strip, preferably in a roll-to-roll process. A constant strip speed ensures that the residence time of each connection pad in the electrolytic bath between its immersion and exit from the electrolytic bath is consistent. The metallized connection pads on the strip can be fed directly to the reflow soldering process after leaving the electrolytic bath; alternatively, the strip with the flexible circuit boards and metallized connection pads is temporarily stored on the roll and fed to the reflow soldering process at a later time. The conductor tracks of both rigid and flexible printed circuit boards are preferably made of aluminum alloys from groups 1xxx and 8xxx.Preferred aluminum alloys of group 1xxx are the aluminum alloys of type 1100, 1200, and 1245. A preferred aluminum alloy of group 8xxx is the aluminum alloy of type 8079. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 9 of 21 The solder mask fulfills various functions; it protects the areas of the conductor tracks coated with the solder mask from corrosion and mechanical damage, and prevents the surfaces on the circuit board coated with the solder mask from becoming coated during metallization. The solder mask thus prevents short circuits resulting from metal bridges between adjacent conductor tracks. In this respect, a solder mask is used that is resistant to phosphinic acid at least during the metallization process in the electrolytic bath. For example, the following materials are used to produce the solder mask:LPI (Liquid Photo Imageable) coatings or photoimageable solder masks (Liquid Photoimageable Solder Mask [LPI]) in various colors are considered. For circuit boards equipped with LEDs as SMD components, white solder masks were used; however, solder masks of other colors can also be used. The following table lists the suppliers, the type designations of suitable solder masks, and their colors: Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 10 of 21. The method according to the invention is explained in more detail below with reference to the figures. They show: Figure 1 a schematic representation to illustrate the method for producing a printed circuit board with connection areas for SMD components, Figure 2 the assembly of the connection areas with an SMD component, Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 11 of 21 Figure 3 a schematic representation of a roll-to-roll process for producing flexible printed circuit boards with connection areas for SMD components, Figure 4 a schematic representation to illustrate a method for producing rigid printed circuit boards with SMD components in batches. Figure 1 shows in the left half of the figure a printed circuit board 1 comprising a rigid or flexible carrier 2, conductor tracks 3 arranged on the carrier 2 with exposed connection areas 3.1.In a first step, a solder mask 4 is applied to the conductor track 3 of the circuit board 1, with the exception of the connection pads 3.1. Since the conductor tracks 3 and the connection pads 3.1 are made of aluminum or an aluminum alloy, the surfaces of the conductor tracks 3 in the area of ​​the exposed connection pads 3.1 react with air and water to form an oxide layer 5. The solder mask 4 has various functions; it protects the areas of the conductor tracks 3 provided with the solder mask 4 from corrosion and, during subsequent metallization, prevents the surfaces on the circuit board 1 coated with the solder mask 4 from becoming coated. The solder mask 4 thus prevents short circuits resulting from metallization between adjacent conductor tracks 3. After the application and curing of the solder mask 4, patent attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx 27.November 2023 Page 12 of 21 the printed circuit board 1 provided with the solder mask 4 is placed in an electrolytic bath 6. The insertion takes place in such a way that at least the connection surfaces 3.1 of the conductor tracks 3 are completely immersed in the electrolytic bath 6. The electrolytic bath 6 contains a mixture of the following substances: - water as solvent 70 - 80% - phosphinic acid as etching and reducing agent 15% - nickel salt 1 - 10% - tin salt 0 - 10% - additives 1 - 2%, in particular sodium tetraborate Na2B4O7 (borax) and disodium hydrogen phosphate Na2HPO4 as flux. After the printed circuit board 1 has been placed in the electrolytic bath 6, the oxide layer 5 is removed from the connection surfaces 3.1 by etching 7 (solvating) the oxide layer 5 using the phosphinic acid. Subsequently, a currentless metal deposition of metal atoms 8 takes place in the electrolytic bath 6, with position number 8.1 illustrates the deposition of nickel metal atoms on the reduced aluminum connection surfaces 3.1, and section 8.2 illustrates an optional deposition of SN metal atoms on the reduced aluminum connection surfaces 3.1, provided that a nickel-tin alloy is to be deposited; in this case, the electrolytic bath contains tin salt in addition to the nickel salt. If only nickel metal atoms 8.1 are to be deposited, it is sufficient if the electrolytic bath contains nickel salt. The residence time of the connection surfaces 3.1 to be metallized in the electrolytic bath is between 5 and 300 seconds. After the process step of electroless metal deposition of the nickel metal atoms 8.1 and optionally the SN metal atoms 8.2 on the connection surfaces 3.1 of the printed circuit board 1, the latter is removed from the electrolytic bath 6.The circuit board 1 shown in the right half of the image, removed from the electrolytic bath 6, is provided with a metal layer 9 on the connection pads 3.1 made of nickel or a nickel-tin alloy and is fed directly or with a time delay to an SNT process for populating the circuit board 1 with an SMD component 10, as schematically indicated in Figure 2. The solderable contact surfaces of the SMD component 10, for example an LED component, are soldered directly onto the connection pads 3.1 provided with the metal layer 9. Before the SMD component 10 is placed, solder and flux 11 are applied to the connection pads 3.1 provided with the metal layer 9, so that in the reflow soldering process only heat energy needs to be supplied in a precisely defined temperature-time profile to solder the SMD component 10. Figure 3 shows a preferred method for producing a flexible printed circuit board 1 with a flexible carrier film 2.1 made of PET, which is designed as a composite film with the conductor tracks made of aluminum. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 14 of 21 The flexible printed circuit board 1 is continuously fed through the electrolytic bath 6 as a strip in a roll-to-roll process. The flexible printed circuit board 1 with an oxide layer 5 on the connection surfaces 3.1 is arranged on a take-off roll 12 on the left side of the electrolytic bath. The connection surfaces 3.1 provided with a metal layer 9, in particular made of nickel or a nickel-tin alloy, are wound onto a storage roll 13 after passing through the electrolytic bath 6. The electrolytic bath 1 contains water as a solvent, nickel salt and optionally tin salt, as well as flux and catalyst as additives, as described with reference to Figure 1.Figure 4 shows an alternative manufacturing method in which the printed circuit boards 1 with an oxide layer 5 on the connection surfaces 3.1 are introduced in batches 14 into the electrolytic bath 6 containing water, nickel salt, and optionally tin salt, phosphinic acid, as well as flux and catalyst as additives. After the batch 14 has remained in the electrolytic bath 6 for a period of time, the batch 14 of printed circuit boards 1 with metal layers 9 made of nickel or a nickel-tin alloy on the connection surfaces 3.1 is removed from the electrolytic bath. In all exemplary embodiments, a basin 6.1 is provided for holding the electrolytic bath 6 and the printed circuit boards 1 to be introduced therein. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 15 of 21 List of Reference Symbols. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx 27 November 2023 Page 16 of 21

Claims

1. A method for producing a solderable printed circuit board (1) comprising a carrier (2) made of electrically insulating material, conductor tracks (3) arranged on the carrier (2), which have connection pads (3.1) for SMD components (10), wherein the conductor tracks (3) and the connection pads (3.1) are made of aluminum or an aluminum alloy, comprising the following steps: - applying a solder resist mask (4) to all conductor tracks (3) of the printed circuit board (1) with the exception of the connection pads (3.1), - introducing the printed circuit board (1) provided with the solder resist mask (4) into an electrolytic bath (6) containing water, at least one metal salt, and phosphinic acid, wherein an oxide layer (5) on the connection pads (3.1) is removed by means of the phosphinic acid, and a metal deposition of metal atoms of the at least one metal salt on the connection pads (3.1) takes place without the use of external current.Method according to claim 1, characterized in that the residence time of each connection surface (3.1) of the printed circuit board (1) after introduction into the electrolytic bath is between 5 and 300 seconds. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 17 of 21.

3. Process according to claim 1 or 2, characterized in that the at least one metal salt is a nickel salt.

4. Process according to claim 3, characterized in that the nickel salt is a nickel(II) chloride, NiCl2, and / or nickel(II) sulfate, NiSO4.

5. Process according to claim 3 or 4, characterized in that the electrolytic bath (6) additionally contains at least one tin salt.

6. Process according to claim 5, characterized in that the tin salt is a tin(II) chloride SnCl₂ and / or tin(II) sulfate SnSO₄.

7. Method according to one of claims 3 to 6, characterized in that the electrolytic bath (6) is a mixture of substances with the following mass proportions: - water 50-95%, - phosphinic acid 1-30%, - nickel salt 1-10% - tin salt 0-10% - additives 0-10% 8. Method according to one of claims 1 to 7, characterized in that the electrolytic bath (6) contains sodium tetraborate Na2B4O7 (borax) and / or disodium hydrogen phosphate Na2HPO4 as flux.Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 18 of 21.

9. Method according to one of claims 1 to 8, characterized in that a printed circuit board (1) with a rigid carrier (2.2) made of electrically insulating carrier material is used for the conductor tracks (3).

10. Method according to claim 9, characterized in that the rigid carrier (2.2) consists of a thermally conductive CEM3 material with a thickness of 0.5-2 mm and the conductor tracks (3) have a layer thickness between 20-200 µm.

11. Method according to one of claims 1 to 8, characterized in that a printed circuit board (1) with a flexible carrier (2.1) made of electrically insulating carrier material is used for the conductor tracks (2).

12. The method according to claim 11, characterized in that the flexible carrier (2.1) consists of a carrier film made of PET or polyimide with a thickness of 10-200 µm, and the conductor tracks (3) consist of an aluminum layer bonded to the carrier film with a thickness of 5-200 µm.Method according to claim 11 or 12, characterized in that the flexible printed circuit board (1) is continuously drawn as a strip through the electrolytic bath (6). Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 19 of 21.

14. The method according to any one of claims 1-13, characterized in that the conductor tracks (3) consist of aluminum alloys of groups 1xxx and 8xxx.

15. The method according to any one of claims 1-14, characterized in that a solder mask (4) resistant to phosphinic acid is used. Patent Attorney Dipl.-Ing. Kai Kohlmann S:\Mandanten\Plasma Innovations\22228-02\anm 01.docx November 27, 2023 Page 20 of 21

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