Method for producing a final surface and printed circuit board

Abstract

Method for producing a final surface (10) on at least one exposed metallic surface (2) of a printed circuit board (1) comprising the steps - Coating the at least one exposed metallic surface (2) with an electrically conductive adhesion promoter (4), - Applying an electrically conductive surface protective layer (5) comprising metallic components to the adhesion promoter (4) by means of an atmospheric plasma coating process, wherein - particles (8) made of metal are applied by means of an atmospheric plasma coating process and the protective layer consists of an inorganic-organic hybrid polymer with corrosion protection properties or an acylate.

Description

[0001] The invention relates to a method for producing a final surface on at least one exposed metallic surface of a printed circuit board and to a printed circuit board with a final surface that can be produced according to the method.

[0002] A printed circuit board (PCB) is a rigid or flexible substrate for electronic components. It serves for the mechanical mounting and electrical connection of these components. The substrate comprises electrically insulating material with electrically conductive surfaces attached to it, such as solderable pads, contact pads, and conductive traces. Fiber-reinforced plastic or, in the case of flexible PCBs, polyimide is commonly used as the insulating material. The electrically conductive surfaces are usually etched from a thin layer of copper, typically 35 µm thick.

[0003] Through-hole components are conventionally soldered into solder pads. Surface-mount devices (SMDs), on the other hand, are soldered directly onto solderable pads. SMD components increase packing density and allow for double-sided mounting. Furthermore, they are significantly easier to handle in automated assembly systems. With through-hole components, a challenge lies in ensuring that all leads are properly seated on the solder pads and that the permissible bending radii of the leads are maintained.

[0004] Soldering components creates a metallurgical bond between their leads and the metallic pads on the circuit board. This process involves melting a solder (soldering) to form a liquid phase. Depending on the material, the metallic pads on the circuit board tend to react with atmospheric oxygen to form metal oxides. This reaction hinders the wetting properties of the solder and the formation of an intermetallic phase between the two surfaces being joined. However, a flawless soldering of the SMD components is essential for the high quality of a circuit manufactured using SMD technology.

[0005] It is therefore necessary to protect the exposed metallic surfaces, especially the solderable contact surfaces, from oxidation after etching the conductor track structure and the connection and, if applicable, contact surfaces, as otherwise the soldering process will be impaired.

[0006] To protect the exposed metallic surfaces from oxidation, a final surface coating is applied.

[0007] The following methods are used to apply these final surface(s) to printed circuit boards (see http: / / www.leiterplattenakademie.de / publikationen / cadbg / cad_lp_bg_012.pdf dated 18.09.2017): 1. In the HAL method, also known as HASL (Hot Air Solder Leveling), the finished printed circuit board is immersed in a bath of liquid tin-lead. Subsequently, hot air is used to blow away the excess tin-lead from the surface, resulting in the smoothest possible surface on the solderable pads. This coats the copper with a layer of tin-lead, similar to conventional solder. Due to its cost-effectiveness, HAL is the most widely used method for producing a final surface finish. However, a disadvantage is that the uneven distribution of the tin-lead coating on the surface can lead to relatively large irregularities and, with very fine structures, short circuits. 2. In the immersion tin method, the top layer of copper is chemically replaced with tin using thiourea. The chemical exchange of copper atoms for tin atoms stops automatically when the copper surfaces on the circuit board are completely covered by tin to a thickness of approximately 0.7 µm to 1.2 µm. The advantages of immersion tin are the very flat surface and the avoidance of problematic substances such as lead in the final product. Disadvantages include the complexity and cost of manufacturing, the use of carcinogenic thiourea, and the potential for whisker formation on the pure tin surface. 3. The OSP (Organic Solderability Preservative) method is a surface treatment based on organic substances such as benzotriazole, imidazole, or benzimidazole. These substances form a 100 nm thick metal-organic coordinative bond with the top copper layer, thus protecting the copper from oxidation. Advantages include a flat surface and inexpensive manufacturing; however, a disadvantage is the limited shelf life. OSP should not be used on printed circuit boards with through-hole components, as surface passivation in the solder pads is not guaranteed. 4. In the ENIG (Electroless Nickel Immersion Gold) method, a layer of nickel approximately 3 µm to 6 µm thick is first electroplated onto the copper surface. This nickel layer acts as a barrier to the gold, preventing it from diffusing into the copper. A layer of gold approximately 50 nm to 100 nm thick is then electroplated onto the nickel layer, chemically replacing the top nickel layer with gold. The advantages of this method, besides the smooth surface, are its relatively long shelf life. The disadvantages are the comparatively high processing complexity and associated costs.

[0008] AU 2014 202 320 A1 discloses a method for producing a final surface on at least one exposed metallic surface of a printed circuit board.

[0009] The metallic surfaces are provided with a coating that protects them from oxidation, corrosion, or other impairments. The coating can be two-layered and include an electrically conductive adhesion promoter. Furthermore, the application of an electrically conductive surface protection layer comprising metallic components by means of a plasma coating process is disclosed.

[0010] The invention is based on the objective of creating a method for producing a final surface on at least one exposed metallic surface of a printed circuit board, which protects the metallic surface on the printed circuit board, in particular made of copper or aluminum, even more effectively against oxidation and ensures reliable soldering of the components.

[0011] This problem is solved by a method having the features of claim 1. The method according to the invention comprises two coating steps, namely 1. Coating the at least one exposed metallic surface with an electrically conductive adhesion promoter and 2. Application of an electrically conductive surface protective layer comprising metallic components to the adhesion promoter by means of an atmospheric plasma coating process, wherein particles of metal provided with a protective layer are applied by means of the atmospheric plasma coating process and the protective layer consists of an inorganic-organic hybrid polymer with corrosion protection properties or an acrylate.

[0012] At least 90% by weight, preferably at least 95% by weight, of the surface protection layer consists of metallic components. The remaining components of the surface protection layer are preferably inorganic-organic hybrid polymers with corrosion protection properties or an acrylate.

[0013] Since the exposed metallic surfaces are usually already covered with an oxide layer and exhibit poor adhesion to solders, the electrically conductive adhesion promoter is applied to the at least one exposed metallic surface in the first coating step. The adhesion promoter, preferably in the form of an adhesive or paste, compensates for the unevenness of the metallic surfaces caused by oxidation. At the same time, it ensures good adhesion of the surface protection layer applied after the adhesion promoter has cured. The paste and adhesive comprise metal particles and an organic or inorganic binder, various additives, and, depending on the curing mechanism, possibly also a solvent or a crosslinking agent. Electrical conductivity is ensured by contact points between the metal particles and by the suitable embedding of the metal particles in the binder of the adhesion promoter.The order is preferably placed using a printing process or via a dispenser.

[0014] However, the conductive paints and pastes available on the market as adhesion promoters are not designed for soldering processes.

[0015] They therefore exhibit poor wettability for solder (tin).

[0016] This disadvantage of adhesion promoters is eliminated in the second process step by applying a readily solderable metallic surface protection layer to the hardened adhesion promoter using an atmospheric plasma coating process. Surprisingly, tests have shown that the metallic surface protection layer resulting from the plasma coating process exhibits very little oxidation and maintains good wettability and adhesion strength for solder over the long term. Simultaneously, an intermetallic phase always forms between the solder and the surface protection layer produced in this way during the soldering process.

[0017] The oxidation of the surface protective layer is further reduced by applying metal particles, particularly copper, coated with a protective layer using an atmospheric plasma coating process. This protective layer also prevents the copper particles from adhering to each other in the plasma jet. Simultaneously, the coating further improves the wettability with the solder. The protective layer consists of an inorganic-organic hybrid polymer with corrosion-inhibiting properties. Such inorganic-organic hybrid polymers are also known under the brand name ORMOCER® from the Fraunhofer Society for the Advancement of Applied Research in Munich. Alternatively, the protective layer can be made of an acrylate.

[0018] When particles of an oxidation-resistant copper alloy, particularly a copper-tin alloy, are applied using atmospheric plasma coating, the surface protection layer remains solderable and stable even after a salt spray test. Simultaneously, an intermetallic phase always forms between the solder and the surface protection layer during soldering.

[0019] The generation of an atmospheric plasma, also known as a low-temperature plasma, for applying a coating to a substrate is known per se from EP 1 230 414 B1. A plasma jet of a low-temperature plasma is generated under atmospheric conditions by passing a working gas through an excitation zone. The gas plasma is generated by a gas discharge. The metallic coating material is fed into the plasma jet separately from the working gas. Unlike thermal plasmas, the temperature of a low-temperature plasma in the core of the plasma jet reaches less than 900 degrees Celsius at ambient pressure.

[0020] The plasma coating process enables the deposition of metallic coating materials from cold-active atmospheric pressure plasma directly onto the surface of the cured adhesion promoter. This process allows for the automated production of homogeneous metallic surface protection layers with reproducible thicknesses. Therefore, plasma coating is particularly well-suited for the cost-effective and high-volume production of finished surfaces on printed circuit boards.

[0021] Suitable adhesion promoters include electrically conductive pastes based on silver, carbon, or tin-antimony: • Electrically conductive, silver-based paste is characterized by very high conductivity and good adhesion to exposed metallic surfaces. The adhesion promoter cures at temperatures between 50°C and 175°C, depending on the circuit board material. • Electrically conductive, carbon-based paste is characterized by very good adhesion and high abrasion resistance. The adhesion promoter cures at temperatures between 50°C and 175°C, depending on the circuit board material. • Electrically conductive paste based on tin and antimony is characterized by good mechanical resistance and low required layer thicknesses. High flexibility and flexibility are retained after curing, making this adhesion promoter particularly suitable for flexible printed circuit boards.

[0022] Electrically conductive adhesives, also known as conductive adhesives, can also be used as adhesion promoters. These are metal-filled adhesives (approximately 60-80 wt.% metal components), particularly epoxy resin-based, that cure in a temperature range between 120 °C and 180 °C. Their electrical conductivity is due to the formation of conductive pathways through the statistical distribution of the metallic components within the organic matrix. Conductive adhesives are insensitive to temperature fluctuations. They can be applied using pressure printing, dispensing, and direct blasting systems.

[0023] A conductive adhesive that is particularly preferred due to its good electrical conductivity is a silver conductive adhesive with silver particles embedded in the organic matrix.

[0024] The final surface is preferably applied selectively to only some of the exposed metallic surfaces, and in particular only to the connection pads of the printed circuit board where components are soldered. The remaining surface of the printed circuit board, i.e., the conductor tracks, the insulating areas of the printed circuit board, and any contact surfaces present, is preferably coated with an adhesion-repellent material before the final surface is applied to prevent adhesion of the adhesion promoter and / or the metallic coating material.

[0025] The adhesion-repellent material is, for example, a solder resist; it protects the metallic areas of the conductor track not covered with the final surface, and in particular the conductor tracks themselves, from oxidation.

[0026] The conductor track structure, including the exposed connection pads, is preferably made of aluminum or copper. However, the conductor track structure, including the connection pads, can also be made of a silver conductive paint, graphite, or steel. Electrically conductive organic and / or transparent materials are also possible for the conductor track structure.

[0027] As an alternative to rigid printed circuit boards, thin flexible printed circuit boards, e.g., based on polyimide films, PET films, or composite films such as aluminum / PET, are also used. While the printed circuit boards constructed in this way are more expensive, they can be folded into very tight structures to save space and are used, for example, in cameras, video cameras, or smartphones. The final surface can also be produced on a flexible printed circuit board using the method according to the invention.

[0028] The invention is explained in more detail below with reference to the figures. They show: Fig. 1 a schematic representation of a printed circuit board, Fig. 2A)-E) a schematic representation of the process for producing a final surface on a printed circuit board, Fig. 3 A schematic representation of the production of a solder joint between an SMD component and a printed circuit board according to Fig. 2 as well as Fig. 4 A schematic representation of a device for carrying out an atmospheric plasma coating process.

[0029] Fig. Figure 1 schematically shows a printed circuit board (1) with a conductor track structure comprising connection pads (2) to which contact pads (13) of SMD components (12) are soldered.

[0030] A final surface (10) is produced on the exposed connection surfaces (2) of the printed circuit board (1) using the method according to the invention. The final surface (10) comprises two layers, namely a layer comprising an electrically conductive adhesion promoter (4) and a metallic surface protection layer (5) (see Figure 1). Fig. 2B) 2C)).

[0031] The manufacturing process takes place in the following steps: Fig. Figure 2A shows the coating of the surface of the printed circuit board (1) with a solder mask (3), except that no solder mask (3) is applied to the copper pads (2). The remaining surface of the printed circuit board (1), i.e., the conductor tracks between the pads (2) and the insulating areas between the conductor tracks and pads, is coated with the solder mask (3). The solder mask (3) serves as an adhesion-resistant material so that the final surface (10) can be selectively produced only on the exposed pads (2) in an automated process. The solder mask (3) can be applied to the remaining surface of the printed circuit board (1) by screen printing, roller application, or spraying. Fig. Figure 2B) shows the coating of the contact surfaces (2) with the electrically conductive adhesion promoter (4) by means of a printing process (not shown). In the illustrated embodiment, the adhesion promoter is a conductive silver adhesive with 70–80% silver components by weight, embedded in an epoxy resin matrix. The electrically conductive silver adhesive is cured by the controlled application of heat within a temperature range of 150–180°C. Fig. Figure 2C) shows the application of a metallic surface protection layer (5) made of copper to the cured adhesion promoter (4) using an atmospheric plasma coating process. To carry out the atmospheric plasma coating process, a plasma coating device (6) is used that is arranged to be relatively movable relative to the surface of the printed circuit board (1). Its design and operation are described in Figure 2C. Fig. 4 is explained in more detail. The plasma coating process enables the deposition of metallic coating material directly onto the surface of the cured adhesion promoter (4). In the illustrated embodiment, the coating material is copper.

[0032] The in Fig. The plasma coating device (6) shown in Figure 4 serves to generate an atmospheric plasma in an atmospheric plasma head (16). In the plasma head (16), a plasma jet is generated under atmospheric conditions by passing a working gas through an excitation zone (not shown). The plasma is generated within the excitation zone by a gas discharge. The metallic coating material (copper) is fed into the plasma jet via a separate feed (17). By a relative movement (19) of the plasma coating device (6) in the direction of the arrow to the surface of the printed circuit board (1), the metal particles (8) made of copper are deposited by the plasma jet (18) onto both the surfaces coated with solder resist (3) and the exposed contact surfaces (2). The metal particles (8) deposited on the contact surfaces (2) by the plasma coating process form the copper surface protection layer (5).Due to the non-adherent properties of the solder mask (3), the metal particles (8) can adhere to the other surface areas of the printed circuit board, as is particularly the case in . Fig. 2D), 2E), can be easily cleaned off, leaving only the copper surface protection layer (5) on the connection surfaces (2). Cleaning is done, for example, by brushing the surface. Fig. Figure 2E shows the cleaned surface (9) of the solder mask after brushing off the metal particles (8). The process for producing the final surface on the contact pads (2) of the printed circuit board (1) is thus completed.

[0033] Based on Fig. 3 below explains how an SMD component (12) with its contact pads (13) is soldered onto the circuit board (1):

[0034] The connection surfaces (2) provided with the final surface (10) are treated as shown in Fig. Figure 3B) shows a solder paste (11) applied. The solder paste (11) is a pasty mixture of solder metal powder and flux and is used to solder the SMD component (12) by means of reflow soldering.

[0035] After applying the solder paste (11), the solder is applied in the Fig. In step 3C), the printed circuit board (1) is populated with the SMD component (12). For this purpose, the contact pads (13) of the SMD component (12) are placed onto the contact pads (2) which are coated with the final surface (10) and solder paste (11). The populated printed circuit board (1) is then heated sufficiently so that the solder contained in the solder paste melts. At the same time, the increased temperature activates the flux in the solder paste.

[0036] After the solder has cooled, the soldering of the SMD component (12) to the circuit board (1) is complete, as shown in Fig. 3D) is indicated. Reference symbol list 1 circuit board 2 Connection area 3 Solder resist 4 liability mediators 5 Surface protection layer 6 Plasma coating machine 7 - 8 metal particles 9 Cleaned surface 10 Final surface 11 Solder paste 12 SMD components 13 contact surfaces 14 - 15 - 16 Plasma head 17 Feed-in 18 Plasma beam 19 Relative motion

Claims

[1] Method for producing a final surface (10) on at least one exposed metallic surface (2) of a printed circuit board (1) comprising the steps - Coating the at least one exposed metallic surface (2) with an electrically conductive adhesion promoter (4), - Applying an electrically conductive surface protective layer (5) comprising metallic components to the adhesion promoter (4) by means of an atmospheric plasma coating process, wherein - particles (8) made of metal are applied by means of an atmospheric plasma coating process and the protective layer consists of an inorganic-organic hybrid polymer with corrosion protection properties or an acylate. [2] Method according to claim 1, characterized by , that the adhesion promoter (4) is an electrically conductive paste or adhesive. [3] Method according to claim 2, characterized by , that the adhesion promoter (4) is a silver conductive adhesive or a silver paste. [4] Method according to any one of claims 1 to 3, characterized by , that copper particles (8) coated with the protective layer are applied by means of the atmospheric plasma coating process. [5] Method according to any one of claims 1 to 3, characterized by , that particles (8) made of copper alloys provided with the protective layer are applied by means of the atmospheric plasma coating process. [6] Method according to claim 5, characterized by that the copper alloy is an oxidation-resistant copper alloy, in particular a copper-tin alloy. [7] Method according to any one of claims 1 to 6, characterized by , that the final surface (10) is produced only on some of the exposed metallic surfaces (2). [8] Method according to claim 7, characterized by, that the exposed metallic surfaces (2) comprise solderable connection surfaces (2) and the final surface (10) is produced exclusively on the connection surfaces (2). [9] Method according to claim 7 or 8, characterized by , that the remaining surface of the printed circuit board (1) is coated with a material (3) that is resistant to adhesion for the metallic surface protection layer before the production of the The final surface (10) will be coated. [10] Method according to any one of claims 1 to 9, characterized by , that the exposed metallic surfaces (2) of the circuit board (1) are made of aluminium, copper, a silver conductive paint, graphite or steel. [11] Method according to any one of claims 1 to 10, characterized by , that the final surface (10) is produced on a flexible printed circuit board (1). [12] Method according to any one of claims 1 to 11, characterized by, that the adhesion promoter (4) is cured, dried or cross-linked before the application of the surface protective layer (5). [13] Printed circuit board (1) having at least one exposed metallic surface (2) and an end surface (10) on the at least one metallic surface (2) can be produced by a method according to any one of claims 1 to 12.

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