EP1082471A1

EP1082471A1 - Method for coating surfaces of copper or of a copper alloy with a tin or tin alloy layer

Info

Publication number: EP1082471A1
Application number: EP99927684A
Authority: EP
Inventors: Hartmut Mahlkow; Petra Backus
Original assignee: Atotech Deutschland GmbH and Co KG
Current assignee: Atotech Deutschland GmbH and Co KG
Priority date: 1998-04-23
Filing date: 1999-04-15
Publication date: 2001-03-14
Also published as: CA2326049A1; WO1999055935A1; CN1297490A; JP2002513090A; KR20010042625A

Abstract

Sufficiently good soldering results can not be obtained using known methods for tinning copper surfaces. In particular, the surfaces remain incapable of being soldered after thermal treatment. To eliminate this problem, a method is used for coating surfaces of copper or of a copper alloy with a layer made of tin or of a tin alloy. Said method comprises the essential following steps: a) treating the surfaces with a solution containing at least one noble metal compound in order to deposit noble metal; b) treating the surfaces which are coated with noble metal according to step a) with a solution containing at least one tin compound, at least one acid and at least one complexing agent for copper from the group comprised of thiourea and the derivatives thereof in order to form the tin or tin alloy layer.

Description

Process for coating surfaces on copper or a copper alloy with a layer of tin or tin alloy

Description:

The invention relates to a method for coating surfaces on copper or a copper alloy, for example on brass, with a tin or tin alloy layer, further a layer combination including a tin or tin alloy layer and applications of the method for producing solderable even after thermal treatment Surfaces on copper or a copper alloy and for the production of layers to protect such surfaces against corrosion. The invention also relates to a bath solution for the electroless deposition of a tin layer or a tin alloy layer.

For the surface treatment of workpieces on copper or copper alloys with the aim of creating corrosion-resistant surfaces, tin layers have long been applied in an electroless process in which the base metal dissolves in favor of the deposited tin ions. The tin layers can in principle also be deposited on other metals, such as iron. In order to form a sufficiently thick layer, the workpiece must be treated at 90 to 100 ° C for two to three hours.

For example, US Pat. No. 2,891,871 A describes a process for the direct coating of workpieces on copper or a copper alloy, in which a tin layer is formed on the workpiece by immersion in a solution of a tin salt, a carboxylic acid and thiourea or a derivative of thiourea by charge exchange becomes. US Pat. No. 2,282,511 A describes a process for coating copper surfaces in which a solution is used which contains compounds containing tin (II) ions, dissolved thiocarbamide and a small amount of an alkali metal carbonate. With this solution, too, a tin layer can be formed directly on the copper surfaces. For example, such solutions can be used for electroless deposition of tin on the inner surfaces of copper pipes in order to avoid the dissolution of copper.

US 2,369,620 A also describes an electroless tinning process for the coating of copper surfaces. The aqueous coating solution described in this document is acidic and, in addition to a tin-II salt, preferably SnCl ₂ , also contains thiourea.

According to DE-AS 1 521 490, an aqueous immersion bath for the deposition of tin also contains hypophosphorous acid or its alkali salts in order to improve the stability of the deposition solution and to obtain purer, lighter, denser and more resistant to etching tin layers than with the deposition solutions known hitherto . It is also stated that the deposition solution can also contain an organic acid such as acetic acid, citric acid, malic acid, maleic acid and similar aliphatic mono-, di- and tricarboxylic acids. This solution also contains thiourea and a wetting agent. With this bath, layers with a thickness of a few μm could be deposited.

DE 30 11 697 A1 describes an acidic chemical tinning bath, for example for coating copper surfaces, which contains a source of tin (II) ions, thiourea and, as a further constituent, an inhibitor, an organic sulfonic acid preferably being used as the inhibitor. The pH of the solution is kept below 1. This solution also contains a hypophosphite.

For use in the manufacture of printed circuit boards, a method is described in US 4,657,632 A in which a part of the copper surfaces on the base material is removed by etching, in that an etching resist is applied to the regions of the copper surfaces which are not to be removed and the parts of the copper surfaces which are to be removed are left free. The etching resist layer is formed by electrolessly depositing a tin layer on the copper layers. In addition to the tin-II salt and thiourea or a thiourea derivative, the deposition solution used for this purpose also contains urea or a derivative of urea. Furthermore, the solution can also contain a chelating agent, for example amino and hydroxycarboxylic acids, a reducing agent, for example an aldehyde, and an acid. A wetting agent may also be present in the solution.

EP 0 503 389 A2 describes a method for electroless coating of workpieces with surfaces on copper or a copper alloy with tin or a tin / lead alloy. In addition to the metal salts, the acidic coating bath contains a reducing agent, such as hypophosphite or its acid, and a complexing agent, for example organic carboxylic acids or thiourea or its derivatives.

EP 0 521 738 A2 describes a solution for electroless plating with tin or with a tin / lead alloy of copper surfaces preferably contained on printed circuit boards, which in addition to tin salts, thiourea, an acid and a reducing agent such as hypophosphite, also one or more nonionic Contains wetting agents, preferably a polyoxyalkyl ether, for example polyoxynonylphenyl ether.

DE 40 01 876 A1 describes a composition for a bath for electroless coating with tin or with a tin / lead alloy, the alkane or alkanolsulfonic acids and their tin and lead salts and also thiourea and thiourea derivatives and mono-, di-, Contains tricarboxylic acids or their salts as chelating agents for tin and lead. The composition is used to coat copper or copper alloys. The composition can also contain, inter alia, nonionic wetting agents, for example polyoxyalkylene alcohol ethers. The use of tin layers as a solderable layer was also repeatedly tested for the production of printed circuit boards. The conductor tracks obtained after the structuring of the copper layers must be solderable for the assembly of components, with the requirement that the exposed metal surfaces have good wettability with the solder, usually tin / lead solder, even after a long storage period of several days to weeks. must have.

However, it has been found that the wettability of the tin layers when soldering is usually much worse than when using tin / lead alloys. By storing printed circuit boards with conductor tracks that are coated with tin layers, it was found that large areas of the printed circuit board could not be wetted after their storage, while other areas did not cause any such problems. Electroless deposition of tin / lead layers is, however, complex, so that the use of pure tin layers as a soldering aid appears advantageous.

For example, attempts have been made to avoid this phenomenon by applying temporarily applied organic protective lacquers. This protective coating should dissolve in the tin / lead solder within the first few minutes. However, such varnishes are expensive. They also cause problems during processing because they get into the solder bath and contaminate it during soldering. In addition, tin layers on printed circuit boards provided with a temporary protective lacquer have disadvantageous properties. In addition, it has been found that the known methods were unable to obtain sufficiently high-quality layer surfaces made of tin or a tin alloy, so that problems arose in subsequent processing steps for the printed circuit boards.

The present invention is therefore based on the problem of avoiding the disadvantages of the prior art and, in particular, of finding a method and a bath solution with which it is possible to add tin and possibly also tin alloy layers to surfaces on copper or a copper alloy form, which are easily wettable with liquid tin / lead solder even after a long storage period. This problem is solved by the features of claims 1 and 8, 9, 12 and 13.

Preferred embodiments of the invention are specified in the subclaims.

It has been shown that the formation of an intermetallic phase is responsible for the poor solderability of tin layers on printed circuit boards that have been stored for several days to weeks. The storage forms a copper / tin alloy at the phase interface between the copper base and the tin layer, the thicker the longer the printed circuit board is stored and the higher the temperature during storage. These intermetallic phases form very quickly. The solderability drops rapidly when the entire tin layer, which has a thickness of 0.7 μm, for example, is converted into the intermetallic phase.

If the electroless plating of tin is closely observed in the first seconds of the plating process, it can be determined that "states" that were already visible on the copper surface before the tin plating are "frozen" by the tin layer. The tin or tin alloy layer is deposited more rapidly on certain morphological structures that are located on the copper surface than on locations on the copper surfaces where these structures are not. For example, structures are formed on rinsing water lines or oxidized areas of the copper surfaces that differ at least optically from neighboring areas. Will the

If the circuit board is annealed after tinning such spots, intermetallic phases form very quickly at these spots. These are more easily oxidized in air than pure tin layers or copper surfaces. When oxidized, these intermetallic phases lose their ability to be wetted with liquid solder.

The problems described above were solved by the method according to the invention, which has the following essential method steps: a. The copper or copper alloy surfaces are treated with a solution which contains at least one noble metal compound in order to deposit noble metal in a first step. Its thickness can be extremely small. For example, it is sufficient to form a layer of precious metal that is not visible to the naked eye. That alone

Solving the above-mentioned problems when carrying out this first process step and subsequent tinning indicates that the noble metal treatment leads to the formation of a noble metal layer on the surfaces on copper or a copper alloy. b. The according to step a. surfaces coated with noble metal are then treated with a solution which contains at least one tin compound and optionally at least one compound of a further metal to be deposited, at least one acid and at least one complexing agent for copper from the group consisting of thiourea and its derivatives. With this treatment the

Tin or tin alloy layer.

The bath solution according to the invention for the electroless deposition of a tin layer or a tin alloy layer is characterized in that it a. at least one tin compound and optionally at least one compound of a further metal to be deposited, b. at least one acid, c. at least one complexing agent for copper from the group consisting of thiourea and its derivatives and d. optionally contains at least one wetting agent.

Just like the tin or tin alloy layer, the precious metal layer is formed by charge exchange, through which copper ions dissolve at the same time as the metal deposition. The noble metal coating formed is believed to hinder the further oxidation of copper and the formation of the intermetallic phase between copper and tin. This probably suppresses the further oxidation of the tin layer, so that the wettability of the surfaces with solder is retained even after prolonged heat treatment. During the soldering process itself, the tin and precious metal layers are dissolved extremely quickly by the liquid solder. This process proceeds very quickly, so that there is sufficient time between the liquid tin / lead solder and the copper to form the intermetallic tin / copper phase that is typical and desired for the soldering process.

The following process steps are preferably carried out:

1. The copper or copper alloy surfaces are cleaned or etched.

2. The treated surfaces are rinsed thoroughly.

3. Subsequently, precious metal is deposited cementatively.

4. The workpieces are then brought into contact with an aqueous acid solution. 5. Then tin is deposited without current, preferably at a temperature of about 60 ° C; the treatment time is preferably about 4 minutes to about 30 minutes.

To clean the copper or copper alloy surfaces, customary cleaning and etching solutions are used, for example solutions containing wetting agents, which can also contain, for example, hydrogen peroxide and sulfuric acid.

One or more noble metal compounds from the group consisting of silver, gold, platinum, palladium, ruthenium, rhodium, osmium and iridium are used in the bath used for the noble metal deposition. The concentration of the noble metals in the solution is preferably about 0.1 to about 2000 ppm (parts by weight of noble metal per million parts by weight of solution), preferably about 1 to about 100 ppm.

The circuit boards are only treated briefly in the precious metal bath, for example within a period of about 60 seconds to about 120 seconds. Of course, longer treatment times can also be selected. The treatment temperature when depositing noble metal is preferably about 20 to about 30 ° C.

After the precious metal has been deposited, the plates are brought into contact with an acid solution. An acid contained in the tinning bath can preferably be used as the acid. Another acid can of course also be used. In this treatment, the copper or copper alloy surfaces are prepared for tinning; at the same time, the acid protects the subsequent tin bath from dilution.

The electroless tin bath must have the highest possible stability against decomposition. In particular, in known baths there is a tendency for precipitates to form within a short time (within days).

Mineral acids, organic acids and sulfonic acids are preferably selected as the acids contained in the tin bath.

The tinning bath is operated at a temperature of about 50 to about 70 ° C. Under these conditions, firmly adhering and uniformly bright tin layers with a thickness of approximately 0.6 to approximately 1.4 μm can be deposited on copper or a copper alloy.

For example, a tin / lead alloy can be deposited as the tin alloy. In this case, the deposition bath additionally contains a lead II salt, for example PbCI ₂ or Pb (OCOCH ₃ ) ₂ .

To carry out the process, the printed circuit boards are immersed one after the other in containers in which the individual treatment solutions are contained. Since the treatment times are extremely short, the circuit boards can also be treated in a continuous system through which the boards are passed in horizontal or vertical orientation and in the horizontal transport direction. The method according to the invention is particularly suitable for the production of surfaces on copper or a copper alloy that can be soldered even after thermal treatment, and for the production of layers for protecting surfaces on copper or a copper alloy against corrosion. The method can also be used to form the layer combination according to the invention on workpieces other than printed circuit boards, for example for coating pipes against corrosion.

The following examples serve to further explain the invention:

Example 1 :

A wiring board provided with copper structures and in particular with connection points for electrical components was treated with an aqueous solution of sodium peroxodisulfate in a silver complex solution containing 50 ppm silver after 1 minute at room temperature after cleaning and etching the copper. After the plate was rinsed with water and subsequently treated with a solution of 2% by weight acid in water, it was tinned in an electroless tin plating bath at 60 ° C. for 15 minutes. The tinning bath had the following composition:

Tin II fluoroborate 15 g tin

Fluoroboric acid 100 ml

Thiourea 100 g

Na lauryl sulfate 2 mg

Make up to 1 1 with water

The tin layer obtained after the treatment with the bath was light metallic and had a thickness of 1.05 μm. The plate was then annealed in air at 155 ° C for eight hours and then subjected to a wave soldering process. As a fluxing agent, a low solid content (2%) no-clean Flux was used (product Kester ^® Litton-Kester, USA). The soldering result at the connection points was excellent because the wetting behavior on the tinned copper surfaces was flawless. The passage of the solder into the drill holes contained in the plate was 80 to 90% flawless (the circuit board was not equipped with components).

Example 2:

A circuit board pretreated as in Example 1 was coated with platinum in a platinum solution containing 15 ppm of platinum for 1 minute at room temperature and then further treated as in Example 1.

The bright tin layer obtained had a thickness of 1 μm after a 30 minute immersion in the tinning bath at 55 ° C.

The plate was then tempered at 155 ° C. for four hours and subjected to a soldering test analogously to Example 1. There were neither errors in wetting with the liquid solder nor problems with solder penetration in the holes; this was 100%.

Example 3:

A circuit board pretreated as in Example 1 was treated in a ruthenium solution containing 50 ppm ruthenium for 2 minutes. The copper layers, which were covered with the extremely thin ruthenium layer, were in a chemical tin bath of the composition: Tin (II) chloride 5 g

N-methylthiourea 55 g

Sulfuric acid conc. 20g

Isopropyl alcohol 500 ml water 500 ml

tinned at 50 ° C.

The tin layer was subjected to a solder spread test. The solderability was excellent with 9 ° contact angle.

Claims

Claims:

1. Process for coating surfaces on copper or a copper alloy with a tin or tin alloy layer with the following essential process steps:

a. Treating the surfaces with a solution containing at least one precious metal compound to deposit the precious metal; b. Treat the according to step a. coated with the precious metal

Surfaces with a solution containing at least one tin compound, at least one acid and at least one complexing agent for copper, selected from the group consisting of thiourea and its derivatives, in order to form the tin or tin alloy layer.

2. The method according to claim 1, characterized in that compounds of the noble metal selected from the group consisting of silver, gold, platinum, palladium, ruthenium, rhodium, osmium and iridium are used.

3. The method according to any one of the preceding claims, characterized in that the solution for depositing the noble metal additionally contains at least one compound of a further metal to be deposited.

4. The method according to any one of the preceding claims, characterized in that the concentration of the noble metal in the solution according to step a. is set to 0.1 to 2000 ppm (weight / weight), preferably 1 to 100 ppm (weight / weight).

5. The method according to any one of the preceding claims, characterized in that the solution for depositing the noble metal contains, in addition to the noble metal compounds, at least one solvent selected from the group consisting of water and an organic solvent.

6. The method according to any one of the preceding claims, characterized in that for the solution for forming the tin or tin alloy layer acids selected from the group consisting of mineral acids, organic acids and sulfonic acids are selected.

7. The method according to any one of the preceding claims, characterized in that a wetting agent is additionally added to the solution for depositing the tin or tin alloy layer.

8. Layer combination on a surface on copper or a copper alloy, consisting of a precious metal layer lying on the surface and a tin or tin alloy layer applied thereon.

9. Bath solution for electroless plating of a tin or tin alloy layer, containing

a. at least one tin compound b. at least one acid and c. at least one complexing agent for copper, selected from the group consisting of thiourea and its derivatives.

10. Bath solution according to claim 9, characterized in that the solution additionally contains at least one compound of a further metal to be deposited.

11. Bath solution according to one of claims 9 and 10, characterized in that the solution additionally contains at least one wetting agent.

12. Application of the method according to one of claims 1 to 7 for the production of surfaces that can be soldered even after thermal treatment on copper or a copper alloy.

13. Application of the method according to one of claims 1 to 7 for the production of layers for protecting surfaces on copper or a copper alloy against corrosion.