Wet chemical forming method of stable tin oxide layer for printed circuit board

Abstract

The invention relates to a method for treating a tin or tin alloy layer on a metal surface, preferably a copper surface, comprising the following steps: (i) providing a metal surface, preferably a copper surface; (ii) contacting the metal surface, preferably the copper surface, with a tin or tin alloy plating bath; and (iii) oxidizing the tin or tin alloy layer obtained by step (ii).

Description

Technical Field

[0001] The invention relates to a method comprising a wet chemical oxidation step of a tin layer as a top layer. The method is preferably used for producing circuit carriers, more particularly printed circuit boards, and in particular for producing contacts on circuit carriers, such as edge connector contacts and button contacts on printed circuit boards. Background Art

[0002] During the manufacture of the circuit carrier, an organic and / or metal layer is applied to the copper surface of the base material. These layers can perform different functions. The organic layer can be used, for example, to structure the copper surface in a subsequent method. For this purpose, a photoresist is applied to the copper surface so that it is completely covered. Afterwards, the layer is partially exposed to light using a special photomask, which images the desired line structure on the photoresist. Afterwards, the imaged structure is developed with corresponding chemicals. Depending on the type of photoresist, it can be negative or positive, and the areas that have been exposed to light or those that have not yet been removed expose the copper layer areas located below due to development. These areas can then be selectively etched or plated using copper or other metals using non-electrical, chemical or electrochemical methods.

[0003] If the metal layer is partially etched or deposited as described, the resulting circuit carrier has certain line structures. To build up complex structures, the method steps can be repeated. The individual layers are compacted together to form a multilayer circuit.

[0004] In order to allow electronic components to be mounted on a circuit carrier provided with a wire structure, additional layers (such as gold, silver, tin, nickel layers) are subsequently deposited, for example, using electroless, chemical or electrochemical methods to form surface top layers, while using solder resist. On the one hand, these surface top layers serve to form the solderable surface areas required for mounting components. On the other hand, the gold surface areas are also suitable for bonding packaged and unpackaged semiconductor components.

[0005] WO 2012 / 095334 discloses a tinning bath.

[0006] Furthermore, these surface top layers also serve as protective layers intended to prevent oxidation of the copper surface and preserve its solderability. These top layers are necessary because the manufacture of the circuit carrier and its further processing (e.g. mounting components thereon) will generally not take place at the same manufacturing site, so that further processing will only take place at a later stage.

[0007] Gold and silver layers, for example, are also formed as surface top layers for producing removable electrical contacts, for example plug connectors for plugging into a circuit carrier by inserting it into a contact socket and for producing contact areas of pushbuttons.

[0008] Once the circuit carrier has been completed, i.e. after the surface top layer has been applied, the circuit carrier is rinsed thoroughly once more, then dried and then stored or subjected to further processing in order to clean it from any ionic contaminants originating from the various process steps and more particularly from the deposition method used to produce the surface top layer.

[0009] Such cleaning is disclosed, for example, in WO 2007 / 025675.

[0010] Due to the extremely high quality requirements of the electronics industry, impurities (eg atmospheric impurities) must be avoided. Therefore, today, the products, in particular the circuit carriers with the newly produced tin layer, are immediately packaged under vacuum.

[0011] However, it has been found that these metallic tin layers show stability problems in the final assembly process. There is a tendency to dewet, which in extreme cases leads to ball formation and / or copper diffusion from the underlying copper layer through the tin layer to the surface. This effect becomes more pronounced as temperature increases. In modern lead-free solder reflow processes with peak temperatures above the melting point of pure tin, the undesirable dewetting is a continuous challenge.

[0012] Another problem is automated optical inspection (AOI), where pure tin metal surface markers are not correctly identified. This has at least two reasons. One is the brightness of the tin metal layer. Another is the undefined optical characteristics of the surface (such as, for example, the PCB surface). The undefined optical characteristics can be assigned as at least partial dewetting.

[0013] Objectives of the invention

[0014] It is therefore an object of the present invention to find a way to minimize or even avoid the above problems.

[0015] Another object of the invention is to find a method which does not negatively affect the tin layer, for example by chemical contamination, in particular ionic contamination. As a control measure, the conductivity of the medium in the final rinsing step should be kept below 10 μS / cm.

[0016] Another object of the present invention is to find a method which is cost-effective and easy to perform. Summary of the invention

[0017] The above mentioned object is solved by the method according to the invention according to claim 1. Preferred embodiments of the invention can be found in the dependent claims.

[0018] The method of the invention comprises a step of wet chemical oxidation of the tin layer. This has the advantage that this step can be easily integrated into existing methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1The thickness of the tin oxide layer on tin is shown as a function of different parameters: (a) concentration of the oxidant H2O2, pH and residence time (at constant temperature); (b) concentration of the oxidant H2O2, pH, ionix and residence time (at constant pH).

[0020] Figure 2 Shown are printed circuit boards treated: (a) according to Comparative Method C-1; (b) according to Comparative Method C-2; (c) and (d) according to Method P, using different concentrations of oxidizing agent.

[0021] Figure 3 show Figure 2 The lower right corner (middle column) and Figure 2 Enlarged images of the upper middle part (right column): (a) according to comparative method C-1; (b) according to comparative method C-2; (c) and (d) according to method P, using different concentrations of oxidant.

[0022] Figure 4 Shown are surface tension measurements of tin oxide layers of printed circuit boards prepared under different conditions, i.e., with and without aging; with and without an oxidation step (iii); with and without another post-dip coating step different from the oxidation step (iii). DETAILED DESCRIPTION

[0023] Unless otherwise specified, percentages throughout this specification are weight percentages (wt%). An exception is the yield, which is provided as a percentage of the theoretical yield. Unless otherwise specified, concentrations given in this specification refer to the volume of the entire solution.

[0024] The method according to the invention for treating tin or tin alloy layers on metal surfaces, preferably copper surfaces, is characterized by the following method steps:

[0025] (i) providing a metal surface, preferably a copper surface;

[0026] (ii) contacting the metal surface (preferably the copper surface) with a tin or tin alloy plating bath; and

[0027] (iii) oxidizing the tin or tin alloy layer obtained in step (ii).

[0028] Step (i) - Providing a metal surface, preferably a copper surface

[0029] In principle, any metal surface can be used in the method of the present invention. Preferably, the metal of the metal surface is selected from the group consisting of nickel, gold, palladium, copper and silver. Particularly preferably, the metal surface is a copper surface.

[0030] Step (ii) - contacting the metal surface (preferably a copper surface) with a tin or tin alloy plating bath

[0031] Contact of the metal surface with the tin or tin alloy plating bath serves as a deposition method to deposit tin or tin alloy on the metal surface.

[0032] In principle, any method known to the skilled person may be used. These methods include electroless, chemical and electrochemical methods. Preferably, the method wherein the tin or tin alloy plating bath is an immersion plating bath. In an alternative preferred method, the tin or tin alloy plating bath is an electroless plating bath.

[0033] The tin or tin alloy plating bath may additionally include at least one stabilizer.

[0034] The at least one stabilizer may be selected from the group consisting of stannates, organic acids, inorganic acids, and combinations thereof.

[0035] Examples of stannates are potassium stannate or sodium stannate.

[0036] Examples of the organic acid include organic sulfonic acid, aliphatic carboxylic acid, oxycarboxylic acid, aminocarboxylic acid and the like.

[0037] Among organic acids, organic sulfonic acid is excellent because wastewater treatment is easy, the solubility of metal salts is high, high-speed plating is feasible, electrical conductivity is high, and the like.

[0038] Examples of the organic sulfonic acid include methanesulfonic acid, ethanesulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-butanesulfonic acid, 2-butanesulfonic acid, pentanesulfonic acid, hexanesulfonic acid, decanesulfonic acid; alkylsulfonic acids such as dodecanesulfonic acid, 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid (2-propanolsulfonic acid), 2-hydroxybutane-1-sulfonic acid, 2-hydroxypentane-1-sulfonic acid and the like; alkanolsulfonic acids such as 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecane-1-sulfonic acid, 2-hydroxydodecane-1-sulfonic acid; 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, toluenesulfonic acid, xylenesulfonic acid, p-phenolsulfonic acid, cresolsulfonic acid, sulfosalicylic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, diphenylamine-4-sulfonic acid and the like.

[0039] Furthermore, the above-mentioned aliphatic carboxylic acids, oxycarboxylic acids, aminocarboxylic acids and the like effectively prevent hydrolysis of metal salts in the range of about pH 1 to 10.

[0040] Generally, a carboxylic acid having 1 to 6 carbons can be used as the aliphatic carboxylic acid, and specific examples thereof include acetic acid, propionic acid, butyric acid, sulfosuccinic acid, trifluoroacetic acid and the like.

[0041] Examples of the oxycarboxylic acid include lactic acid, citric acid, gluconic acid, tartaric acid, malic acid and the like.

[0042] Aminocarboxylic acids include ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), hydroxyethylethylenediaminetriacetic acid (HEDTA), triethylenetetraaminehexaacetic acid (TTHA), glycine, alanine, N-methylglycine, lysine, glutamic acid, aspartic acid, and the like.

[0043] Examples of the inorganic acid include phosphonic acid, phosphoric acid, sulfuric acid, hydrochloric acid, boric acid, hydrofluoric acid, silicic acid, sulfamic acid and the like.

[0044] Various known additives such as surfactants, complexing agents, stabilizers, brighteners, semi-brighteners, pH adjusters, buffers and the like may be appropriately mixed with the tin plating bath of step (ii) of the method according to the objective.

[0045] Step (iii) - Oxidizing the tin or tin alloy layer obtained in step (ii)

[0046] Step (iii) of the method is carried out by contacting the tin or tin alloy layer obtained from step (ii) with a composition comprising at least one solvent and at least one oxidizing agent.

[0047] Preferably, the at least one solvent comprises water in an amount of 50% by volume or more. The remainder is selected from the group consisting of water-miscible organic solvents. Preferably, the water-miscible organic solvent is ethanol.

[0048] More preferably, the at least one solvent is water.

[0049] The at least one oxidizing agent is selected from non-decomposable and decomposable oxidizing agents, such as peroxides, such as hydrogen peroxide, metal peroxides (such as sodium peroxide, potassium peroxide); metal superoxides (such as potassium superoxide); copper ions and iron ions.

[0050] However, ionic species have a negative impact on conductivity and are less preferred.

[0051] Preferably, the at least one oxidizing agent is a decomposable oxidizing agent, more preferably, the at least one oxidizing agent comprises hydrogen peroxide, and most preferably, the at least one oxidizing agent is hydrogen peroxide.

[0052] To prevent decomposition of the peroxides, further additional compounds may be added, such as p-phenolsulfonic acid, phosphoric acid, stannates and / or organic acids and the respective derivatives, such as oxalic acid and the corresponding esters, α-ketocarboxylates or aldehydecarboxylates.

[0053] The concentration of at least one oxidizing agent in the composition generally ranges from about 0.1 to about 20 g / l, preferably from about 0.2 to about 10 g / l, more preferably from about 0.3 to about 5 g / l, even more preferably from about 0.5 to about 3 g / l.

[0054] The pH of the composition generally ranges from about 3 to about 12. Preferably, the pH of the composition ranges from about 3.5 to about 10. More preferably, the pH of the composition ranges from about 4.0 to about 8.0. Even more preferably, the pH of the composition ranges from about 4.5 to about 6.5. Most preferably, the pH of the composition ranges from about 4.5 to about 5.5. The pH can be adjusted by any chemical compound known to the skilled artisan. However, nonionic compounds are preferred. Ammonia is particularly preferred.

[0055] The residence time of step (iii) is generally from about 0.1 minutes to about 5 minutes, preferably from about 0.2 minutes to about 2 minutes, more preferably from about 0.3 minutes to about 1.5 minutes, even more preferably from about 0.4 minutes to about 1 minute.

[0056] The temperature of step (iii) is usually above 40°C, preferably, the temperature is from about 40°C to about 90°C, more preferably from about 50°C to about 80°C, even more preferably from about 60°C to about 75°C.

[0057] In a preferred embodiment, the method of the invention is used for producing circuit carriers in vertical and / or horizontal lines.

[0058] In a preferred embodiment, the method according to any of the preceding embodiments is used for producing electrical contacts on a circuit carrier.

[0059] The method generally further comprises rinsing the tin or tin alloy layer with deionized water at least once before and / or after step (iii).

[0060] In a preferred embodiment, the method further comprises a step (ii-a) between step (ii) and step (iii), wherein step (ii-a) comprises treating the tin or tin alloy layer with an aqueous solution, the solution comprising:

[0061] a) at least one first compound selected from the group consisting of ethanolamine compounds and salts thereof;

[0062] b) at least one second compound selected from the group consisting of alcohol solvents; and

[0063] c) preferably at least one third compound selected from the group consisting of guanidine compounds and salts thereof.

[0064] It has been found earlier that this step has an overall positive effect in reducing conductivity. It is assumed that this step acts as a surface cleaner and simultaneously as a swelling agent.

[0065] Example

[0066] A. A method for treating a tin or tin alloy layer on a metal surface (preferably a copper surface), comprising the following steps:

[0067] (i) providing a metal surface, preferably a copper surface;

[0068] (ii) contacting the metal surface (preferably the copper surface) with a tin or tin alloy plating bath; and

[0069] (iii) oxidizing the tin or tin alloy layer obtained in step (ii).

[0070] B. The method of embodiment A, wherein the tin or tin alloy plating bath is an immersion plating bath.

[0071] C. The method according to embodiment A or embodiment B, wherein step (iii) is carried out by contacting the tin or tin alloy layer obtained by step (ii) with a composition comprising at least one solvent and at least one oxidizing agent, wherein preferably the at least one solvent comprises water, and more preferably the at least one solvent is water.

[0072] D. The method according to embodiment C, wherein the composition further comprises at least one stabilizer.

[0073] E. The method according to embodiment C or embodiment D, wherein the at least one oxidizing agent is selected from non-decomposable and decomposable oxidizing agents, preferably selected from decomposable oxidizing agents, more preferably the at least one oxidizing agent comprises hydrogen peroxide, and most preferably the at least one oxidizing agent is hydrogen peroxide.

[0074] F. A method according to any one of the preceding embodiments C to E, characterized in that the concentration of the at least one oxidizing agent in the composition ranges from about 0.1 to about 20 g / l, preferably from about 0.2 to about 10 g / l, more preferably from about 0.3 to about 5 g / l, and even more preferably from about 0.5 to about 3 g / l.

[0075] G. A method according to any one of the preceding embodiments C to F, characterized in that the pH range of the composition is between about 3 to about 12, preferably about 3.5 to about 10, more preferably about 4.0 to about 8.0, even more preferably about 4.5 to about 6.5, even more preferably about 4.5 to about 5.5.

[0076] H. A method according to any of the preceding embodiments, characterized in that the residence time of step (iii) is from about 0.1 minutes to about 5 minutes, preferably from about 0.2 minutes to about 2 minutes, more preferably from about 0.3 minutes to about 1.5 minutes, and even more preferably from about 0.4 minutes to about 1 minute.

[0077] I. A method according to any one of the preceding embodiments, characterized in that the temperature of step (iii) is higher than 40°C, preferably the temperature is about 40°C to about 90°C, more preferably about 50°C to about 80°C, even more preferably about 60°C to about 75°C.

[0078] J. The method according to any of the preceding embodiments, for producing circuit carriers in vertical and / or horizontal lines.

[0079] K. The method according to any of the preceding embodiments for producing an electrical contact on a circuit carrier.

[0080] L. The method according to any one of the preceding embodiments, characterized in that the method further comprises rinsing the tin or tin alloy layer with deionized water at least once before and / or after step (iii).

[0081] M. The method according to any one of the preceding embodiments, characterized in that the method further comprises a step (ii-a) between step (ii) and step (iii), wherein step (ii-a) comprises treating the tin or tin alloy layer with an aqueous solution, the solution comprising:

[0082] a) at least one first compound selected from the group consisting of ethanolamine compounds and salts thereof;

[0083] b) at least one second compound selected from the group consisting of alcohol solvents; and

[0084] c) Preferably, at least one third compound selected from the group consisting of guanidine compounds and salts thereof.

[0085] N. A method according to any one of embodiments A to M, characterized in that the surface tension of the tin oxide layer after drying and aging at 90°C for 14 hours is 70 mN / m or higher, preferably 75 mN / m or higher, more preferably 80 mN / m or higher, and even more preferably 85 mN / m or higher.

[0086] O. A method according to any one of embodiments A to M, characterized in that the surface tension of the tin oxide layer after drying and aging at 90°C for 14 hours is 70mN / m to 100mN / m, more preferably 80mN / m to 95mN / m, and even more preferably 85mN / m to 95mN.

[0087] P. An article comprising a metal layer, preferably a copper layer, a tin layer and a tin oxide layer, characterized in that the tin oxide layer has a thickness of 1 to 3 nm, as determined by SERA.

[0088] Q. An article according to embodiment P, characterized in that the surface tension of the tin oxide layer after dry aging at 90°C for 14 hours is 70 mN / m or higher, preferably 75 mN / m or higher, more preferably 80 mN / m or higher, and even more preferably 85 mN / m or higher.

[0089] R. An article according to embodiment P, characterized in that the surface tension of the tin oxide layer after drying and aging at 90°C for 14 hours has a value of 70mN / m to 100mN / m, more preferably 80mN / m to 95mN / m, and even more preferably 85mN / m to 95mN.

[0090] The following non-limiting examples further illustrate the present invention.

[0091] Examples

[0092] SERA measurement: Place the dry sample on QC-100. Use Bor Acid / Borat buffer with pH around 9 and -30 μA / cm 2 The current density of the Sn(II) oxide layer will be measured by coulometric method with a sealed opening diameter of 1.6 mm. The results will be verified by the current accumulated between the reduction voltages of -0.8 to -1.1 V.

[0093] Bright tin measurement: The dry samples were aged in a reflow oven with a reduced oxygen concentration of about 100 ppm and the reflow profile of Senju M31-GRN360-KV solder paste (peak 245°C). After aging, a digital image of the sample was created using a flatbed scanner. Because of the typical angle of about 45° between the light source and the RGB-sensor of the flatbed scanner, only diffuse reflected light can be detected by the RGB-sensor. Each total emitted light (mirror) on the tin surface will not reach the RGB sensor. Depending on the optical characteristics of the tin surface, more or less light reaches the RGB sensor. A normal tin surface produces a bright white color. Bright tin is detected as a dark area with much less brightness.

[0094] Surface tension measurement: Contact angle measuring device OCA15 - using water and Young's equation: cosΘ=(σ S -σ LS ) / σ L Measurement; where σ L =72.85mN / m and σ LS = 12.3 mN / m; drop size: 1 μL; measurement time: 3 seconds Alternatively, commercially available water / ethanol test inks can be used in the range of 30 to 72 mN / m.

[0095] Example 1:

[0096] Printed circuit boards with a solder mask (Taiyo PSR-4000AM03TS) were treated with a tin deposition method according to Table 1 and applied with a 1 μm thick layer of immersion tin. The tin deposition bath contained tin(II) methanesulfonate, methanesulfonic acid and thiourea.

[0097] After tin deposition, different parameters were applied in a post dip coating step and the samples were investigated by SERA. The parameters and the respective tin oxide thickness are shown in Figure 1 middle.

[0098] Table 1. Method order

[0099]

[0100]

[0101] Dl water: deionized water RT: room temperature

[0102] (*)Similar method to 60243

[0103] (**) Similar method to 60243 (comparative example)

[0104] Example 2

[0105] Printed circuit boards with a solder mask (Taiyo PSR-4000AM03TS) were treated with a tin deposition method according to Table 1 and applied with a 1 μm thick layer of immersion tin. The tin deposition bath contained tin(II) methanesulfonate, methanesulfonic acid and thiourea.

[0106] Figure 2 Printed circuit boards treated as follows are shown: (a) according to Comparative Method C-1; (b) according to Comparative Method C-2; (c) according to Method P with a concentration of 0.3% H2O2 (post-dip coating) and (d) according to Method P with a concentration of 1.0% H2O2 (post-dip coating). Figure 3 show Figure 2 The lower right corner (middle column) and Figure 2 Respective enlarged images of the upper middle part (right column).

[0107] The results are shown in Figure 2 / Figure 3 and Table 2.

[0108] sample method Ionix Post dip coating Brightness (a) C-1 - - all (b) C-2 yes - Almost bright (c) P yes <![CDATA[Yes (0.3% H2O2)]]> no (d) P yes <![CDATA[Yes (1.0% H2O2)]]> no

[0109] Example 3

[0110] Printed circuit boards with a solder mask (Taiyo PSR-4000AM03TS) were treated with a tin deposition method according to Table 1 and applied with a 1 μm thick layer of immersion tin. The tin deposition bath contained tin(II) methanesulfonate, methanesulfonic acid and thiourea.

[0111] Processing is performed involving different combinations of:

[0112] -aging / not aging;

[0113] - Application / non-application of PD2020 in the dip-coating step following Table 1, i.e., oxidation step (iii) of method P;

[0114] - In the dip coating step following Table 1, with / without application of PD270, an established method for finishing the tin surface.

[0115] The different combinations can be described as follows:

[0116] a) Ionix - rinse - no post-dip (= Table 1, method C-2)

[0117] b) Ionix - rinse - post dip PD270 - rinse (= Table 1, method C-2, in which the inserted post dip step is different from the post dip step of the present invention)

[0118] c) Ionix – rinse – post-dip PD2020 – rinse (= Table 1, method P)

[0119] d) Ionix - rinse - post dip 270 - rinse - post dip PD2020 - rinse (= Table 1, method P, wherein the post dip step inserted before the post dip step of the present invention is different from the post dip step of the present invention)

[0120] The surface tension of the tin oxide layer of the PCB has been measured. The results are shown in Figure 4 middle.

[0121] Metals and metal oxides show high surface tension, while organic substances tend to show lower surface tension. In particular, surfactants have very low surface tension values. Therefore, they are advantageous in achieving and maintaining high surface tension.

[0122] The measurements show that the chemical oxidation of the printed circuit boards not only has a positive effect on the formation of the tin oxide layer, but also increases the chemical purity of the surface by reducing the risk of organic substances migrating to the tin / tin oxide surface during storage and thereby contaminating the surface. This migration can occur during storage or transport from the PCB factory to the assembler, as well as during the assembly process itself.

[0123] It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications and changes thereof and combinations of features described in this application will be suggested to those skilled in the art and are to be included within the spirit and purview of the invention and within the scope of the appended claims. All publications, patents and patent applications cited herein are incorporated herein by reference.

Claims

1. A method for treating a tin or tin alloy layer on a metal surface, preferably a copper surface, comprising the following steps: (i) providing a metal surface, preferably a copper surface; (ii) contacting the metal surface, preferably the copper surface, with a tin or tin alloy plating bath; and (iii) oxidizing the tin or tin alloy layer obtained in step (ii).

2. The method according to claim 1, wherein the tin or tin alloy plating bath is an immersion plating bath.

3. The method according to claim 1 or claim 2, wherein step (iii) is carried out by contacting the tin or tin alloy layer obtained by step (ii) with a composition comprising at least one solvent and at least one oxidizing agent, wherein preferably the at least one solvent comprises water, and more preferably the at least one solvent is water.

4. The method of claim 3, wherein the composition additionally comprises at least one stabilizer.

5. The method according to claim 3 or claim 4, wherein the at least one oxidant is selected from non-decomposable and decomposable oxidants, preferably from decomposable oxidants, more preferably the at least one oxidant comprises hydrogen peroxide, most preferably the at least one oxidant is hydrogen peroxide.

6. A method according to any one of claims 3 to 5, characterised in that The concentration of the at least one oxidizing agent in the composition ranges from about 0.1 to about 20 g / l, preferably from about 0.2 to about 10 g / l, more preferably from about 0.3 to about 5 g / l, even more preferably from about 0.5 to about 3 g / l.

7. A method according to any one of claims 3 to 6, characterised in that The pH range of the composition is from about 3 to about 12, preferably from about 3.5 to about 10, more preferably from about 4.0 to about 8.0, even more preferably from about 4.5 to about 6.5, even more preferably from about 4.5 to about 5.

5.

8. A method according to any one of the preceding claims, characterised in that The residence time of step (iii) is from about 0.1 minutes to about 5 minutes, preferably from about 0.2 minutes to about 2 minutes, more preferably from about 0.3 minutes to about 1.5 minutes, even more preferably from about 0.4 minutes to about 1 minute.

9. A method according to any one of the preceding claims, characterised in that The temperature of step (iii) is above 40°C, preferably the temperature is from about 40°C to about 90°C, more preferably from about 50°C to about 80°C, even more preferably from about 60°C to about 75°C.

10. The method according to claim 1, for producing circuit carriers in vertical and / or horizontal lines.

11. The method according to claim 1, for producing electrical contacts on a circuit carrier.

12. A method according to any one of the preceding claims, characterised in that The method further comprises rinsing the tin or tin alloy layer with deionized water at least once before and / or after step (iii).

13. A method according to any one of the preceding claims, characterised in that The method further comprises a step (ii-a) between step (ii) and step (iii), wherein step (ii-a) comprises treating the tin or tin alloy layer with an aqueous solution, the solution comprising: a) at least one first compound selected from the group consisting of ethanolamine compounds and salts thereof; b) at least one second compound selected from the group consisting of alcohol solvents; and c) Optionally, at least one third compound selected from the group consisting of guanidine compounds and salts thereof.

14. An article comprising a metal layer, preferably a copper layer, a tin layer and a tin oxide layer, characterized in that The tin oxide layer has a thickness of 1 to 3 nm as measured by SERA.

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