KR20010042625A - Method for coating surfaces of copper or of a copper alloy with a tin or tin alloy layer - Google Patents

Abstract

Known methods of tinning copper surfaces do not yield sufficiently good bonding results. In particular, the surface is not bondable after heat treatment. To solve this problem, a method of covering the surface of copper or copper alloy with a layer made of tin or tin alloy is used. The method comprises: a) treating the surface with a solution containing at least one precious metal compound to deposit the precious metal; b) tin or tin by treating the surface coated with the 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 consisting of thiourea and derivatives thereof It is necessary to form an alloy layer.

Description

METHODS FOR COATING SURFACES OF COPPER OR OF A COPPER ALLOY WITH A TIN OR TIN ALLOY LAYER}

The present invention relates to a method of coating a surface of copper or a copper alloy, for example brass, with a tin or tin alloy layer, and more particularly the invention relates to a combination of layers comprising a tin or tin alloy layer, as well as after heat treatment. A method for producing a surface of copper or a copper alloy suitable for soldering, and a method for producing a layer for protecting against corrosion on such a surface. The present invention also relates to a bath solution for electroless deposition of a tin layer or a tin alloy layer.

For the surface treatment of copper or copper alloy workpieces for the purpose of forming corrosion resistant surfaces, electroless methods by basic metal decomposition have been used for a deposited tin ions for a long time. The tin layer may also be deposited on other metals, such as iron, in general. To form a sufficiently thick layer, the workpiece must be treated for 2 to 3 hours at a temperature of 90 to 100 ° C.

For example, US 2,891,871 A discloses a method of directly coating a workpiece of copper or a copper alloy, in which the workpiece is immersed in a solution consisting of tin salt, carboxylic acid and thiourea or thiourea derivatives. Charge exchange forms a tin layer on the workpiece.

US 2,282,511 A discloses a method for coating a copper surface using a solution containing a compound containing tin-II ions, dissolved thiocarbamide and a small amount of alkali metal carbonate. With this solution, a tin layer can likewise be formed directly on the copper surface. For example, a solution of this kind can be used for the electroless deposition of tin on the inner surface of the copper pipe to prevent the decomposition of copper.

US 2,369,620 A also discloses an electroless tin-coating method for coating copper surfaces. The aqueous coating liquid disclosed in this document is an acid and, in addition to the tin-II salt, preferably further contains SnCl ₂ , thiourea.

According to DE-AS 1 521 490, aqueous immersion baths for the deposition of tin also improve the stability of the deposition solution and are more transparent, clearer, more resistant to etchant than conventional deposition solutions, Hypophosphoric acid or its alkali salts are contained in order to obtain a denser tin layer. It is also described that the deposition solution may also contain organic acids such as, for example, ethanolic acid, citric acid, malic acid, maleic acid and similar aliphatic mono-, di- and tricarboxylic acids. Thiourea and further wetting agent are also included in the solution. A layer having a thickness of several μm may be deposited into the bath.

DE 30 11 697 A1 contains acid-based chemical tin-platings for coating the surface of copper, for example containing tin-II ion sources, thioureas and inhibitors (organic sulfonic acids are preferably used) as further components. The bath is starting. The pH value of the solution is kept below 1. In addition, the solution further contains hypophosphite.

A method for use on a printed circuit board is disclosed in US Pat. No. 4,657,632 A, which removes a portion of the copper surface of the base material by etching, and adds an etch-resist to the copper surface for the portion that will not be removed. The glass surface area to be removed is left in a glass state. An etch-resist layer is formed by electroless deposition of the tin layer of the copper layer. The deposition solution used for this purpose further contains urea or urea derivatives, in addition to tin-II salts and thiourea or thiourea derivatives. In addition, the solution may further contain chelating agents such as amino- and hydroxycarboxylic acids, reducing agents such as aldehydes and acids. Wetting agents may also be contained in the solution.

EP 0 503 389 A2 discloses a method for electrolessly coating a workpiece with tin or tin / lead alloys on the surface of copper or copper alloys. In addition to the metal salts, the acid coated bath further contains a reducing agent such as hypophosphite or an acid thereof, and a complexing agent such as an organic carboxylic acid or thiourea or a derivative thereof.

EP 0 521 738 A2 discloses a solution for electrolessly coating a copper surface on a printed circuit board with a tin / lead alloy, preferably tin or tin / lead alloy, which solution, in addition to tin salt, Thiourea, acids and reducing agents such as hypophosphoric acid and further one or more nonionic wetting agents, preferably polyoxyalkylethers such as polyoxynonylphenol ethers.

DE 40 01 876 A1 discloses mono-, di-, tricarboxylic acids or their chelating agents as alkanes or alkanol sulfonic acids and tin and lead salts thereof, and further thiourea and thiourea derivatives, as well as tin and lead chelating agents. A composition for baths for electroless coating with tin or tin / lead alloys containing salts is disclosed. The composition is used to coat copper or copper alloys. In the composition, it may contain, among other things, nonionic wetting agents and others, for example polyoxyalkylene alcohol ethers.

For the production of printed circuit boards, the use of tin layers as solderable layers has been repeatedly tested. The conductive tracks obtained after the production of the copper layer should be solderable for the bonding of the components and the exposed metal surface should show good wettability for the solder, which is mostly tin / lead solder even after long days of storage for several days to weeks. The condition exists.

However, it has been found that the wettability of the tin layer during soldering is generally considerably worse than when using tin / lead alloys. Through storage of printed circuit boards with conductive tracks coated with a tin layer, it has been found that much of the printed circuit board cannot be wetted after storage, while other parts do not exhibit this problem. Electroless deposition of tin / lead layers is expensive, but using a pure tin layer would be beneficial as a solder aid.

For example, attempts have been made to avoid this phenomenon by temporarily applying an organic protective varnish. This protective coating is for dissolving the tin / lead solder in 1 minute.

However, varnishes of this kind are not only expensive, but also cause problems during processing because they reach and contaminate the solder bath during soldering. In addition, the tin layer of the printed circuit board provided with the temporary protective varnish has disadvantageous characteristics.

It has also been found that known methods do not allow the surface of layers of sufficiently high quality to be obtained from tin or tin alloys and problems arise with printed circuit boards in subsequent processing steps.

Accordingly, the problem underlying the present invention is to find a method and bath solution which is capable of forming tin on the surface of the copper or copper alloy and, in particular, a tin alloy layer, if necessary, avoiding the disadvantages of the prior art, said surface Is easily wettable with liquid tin / lead bonds even after long term storage.

The problem is solved by the aspects of claims 1 and 8, 9, 12 and 13. Preferred embodiments of the invention are illustrated in the dependent claims.

Inadequate bonding of tin layers on printed circuit boards stored for days to weeks has been found to be due to the formation of phases between the metals. Through storage, at the interface between the copper substrate and the tin layer, thicker copper / tin alloys are formed, and the longer the printed circuit board is stored, the higher the storage temperature. The phase between the metals is formed very quickly. For example, if the entire tin layer with a thickness of 0.7 μm is converted into a phase between the metals, the bonding ability is easily degraded.

If the electroless deposition of tin is properly observed at the first moment of the deposition process, the "state" that was visible before tin plating on the copper surface can be determined to be "frozen" by the tin layer. For certain morphological structures located on the copper surface, tin or each tin alloy layer deposits faster than the point on the copper surface where the structure is not located. For example, at the site of oxidation on the wash water pipe or copper surface, a structure is formed that is at least visually distinct from the adjacent inert. If the printed circuit board is tempered after tin plating at this point, an image between the metals is formed very quickly at this point. They oxidize more easily in air than pure tin layers or copper surfaces. During oxidation, the phase between the metals loses their ability to wet the liquid bond.

The above problem is solved by the method according to the invention with the following essential steps:

a. In order to deposit the precious metal in the first step, the copper or copper alloy surface is treated with a solution containing at least one precious metal compound. The thickness of the metal can be very small, for example small enough to form a layer of precious metal that is not visible to the naked eye. Solving the problem in performing the first method step and subsequent tin plating alone indicates that treatment with the noble metal leads to the formation of a noble metal layer on the surface of the copper or copper alloy.

b. Complexing the surface coated with the noble metal according to step a with at least one tin compound, and optionally at least one further metal compound to be deposited, and at least one acid and thiourea and derivatives thereof for at least one copper Treatment with a solution containing the agent forms a tin or tin alloy layer. During the treatment, a tin or tin alloy layer is formed.

The bath solution for electroless deposition of the tin layer or tin alloy layer according to the invention is characterized by containing:

a. One or more tin compounds and, if desired, one or more of the compounds of the additional metal to be deposited,

b. One or more acids, and

c. At least one complexing agent for copper from the group consisting of thiourea and its derivatives, and

d. If desired, one or more wetting agents.

Like the tin or tin alloy layer, the noble metal layer is formed by charge exchange as a result of copper ions moving into solution simultaneously with deposition of the metal. The precious metal coating formed is believed to prevent further oxidation of copper and formation of a phase between the metal between copper and tin. As a result, further oxidation of the tin layer is suppressed, and the wettability of the surface to the solder is maintained even after a long heat treatment.

During the soldering process itself, the tin and precious metal layers decompose very quickly by the liquid bond. The process proceeds very quickly, leaving enough time for the tin / copper phase between the liquid tin / lead bond and copper to form a preferred metal that is representative of the bonding process.

The following method steps are preferably carried out:

1. The copper or copper alloy surface is cleaned or etched.

2. Rinse the treated surface thoroughly.

3. Precious metals are then deposited by carburization.

4. The workpiece is then contacted with an aqueous acid solution.

5. The tin is then deposited at about 60 ° C. in an electroless manner and the treatment time is preferably about 4 minutes to about 30 minutes.

To clean the copper or copper alloy surface, conventional cleaning and etching solutions are used, for example solutions containing wetting agents, and solutions that may further contain, for example, hydrogen peroxide and sulfuric acid. In baths used for the deposition of precious metals, one or more precious metal compounds from the group consisting of silver, gold, platinum, palladium, ruthenium, rhodium, osmium and iridium are used. The concentration of the noble metal in the solution is preferably about 0.1 to 2000 ppm (parts by weight of noble metal per million parts by weight of solution), preferably about 1 to about 100 ppm.

The printed circuit board is processed for only a short time in the precious metal bath, for example, for about 60 seconds to about 120 seconds.

Long processing times can be selected without difficulty. The treatment temperature during the deposition of the noble metal is preferably about 20 to about 30 ° C.

After deposition of the precious metal, the substrate is in contact with the acid solution. The acid contained in the tin plating bath can preferably be used as the acid. Different acids can be used without difficulty. During the treatment, copper or each copper alloy surface is prepared for tin plating, while the acid forms a protective film for the tin bath followed prior to dilution.

The electroless tin bath should have as high stability as possible to degradation. In particular, known baths tend to form precipitates for a very short time (a few days).

As the acid contained in the tin bath, mineral acids, organic acids and sulfonic acids are preferably selected.

The tin plating bath is operated at a temperature of about 50 to about 70 ° C. Under these conditions, an adhesive and an evenly clear tin layer of about 0.6 to about 1.4 μm in thickness may be deposited on copper or each copper alloy.

As tin alloys, for example tin / lead alloys can be deposited. The immersion bath in this case contains a lead-II salt, for example PbCl ₂ or Pb (OCOCH ₃ ) ₂ .

In order to carry out the process, the printed circuit board in the standard method is in turn immersed in a container containing the individual treatment solution. Because the processing time is very short, the printed circuit board may be processed in a continuous processing plant, which allows the substrate to be in a horizontal or vertical orientation and to be processed in the horizontal movement direction.

The method according to the invention is not only particularly suitable for the production of surfaces of copper or copper alloys suitable for joining even after heat treatment, but also for the production of layers for protecting the surfaces of copper or copper alloys against corrosion. The method can also be used to form combination layers according to the invention on workpieces other than printed circuit boards, for example in the coating of pipes for corrosion protection.

The following examples are intended to further illustrate the present invention.

Example 1

Wiring substrates provided with copper structures and in particular connections for electrical components were washed and etched with an aqueous solution of sodium peroxodisulfate in a silver complexing solution containing 50 ppm of silver, and then treated at room temperature for 1 minute. do. The substrate is washed with water and then treated with 2% by weight acid solution in water and then tin plated at 60 ° C. for 15 minutes in an electroless tin plating bath.

The tin plating bath has the following composition:

Tin-II Fluoroborate 15 g Tin

100 ml of fluoroboric acid

100 g of thiourea

Sodium Lauryl Sulfate 2 mg

Filling water up to 1 l

The tin layer after the treatment with the bath is vivid metallic and has a thickness of 1.05 mu m. The plate is then tempered in air at 155 ° C. for 8 hours and finally subjected to a wave soldering process. As the flux, a low-solid (2%) no-clean flux (Kester®, manufactured by Litton Kester (USA)) is used.

Since the wetting properties on the tin plated copper surface are perfect, the joining results of the joints are excellent. Rinsing of the bond to the bore contained in the printed circuit board is 80 to 90% perfect (the printed circuit board does not match the components).

Example 2

The printed circuit board pretreated as in Example 1 was coated with platinum in platinum solution containing 15 ppm of platinum for 1 minute at room temperature and then treated as in Example 1. The obtained clear tin layer has a thickness of 1 μm after being immersed in a tin plating bath at 55 ° C. for 30 minutes.

Subsequently, the substrate was tempered at 155 ° C. for 4 hours, and then the bonding test was performed similarly to Example 1. Errors during wetting with the liquid conjugate and problems with the conjugate in the bore are not apparent; 100%.

Example 3

The printed circuit board pretreated as in Example 1 was treated in a ruthenium solution containing 50 ppm ruthenium for 2 minutes. The copper layer coated with a very thin ruthenium layer is tin plated at 50 ° C. in a chemical tin bath having the following composition:

5 g of tin (II) chloride

55 g of N-methyl thiourea

20 g concentrated sulfuric acid

500 ml of isopropyl alcohol

500 ml of water

A bonding test (bond coating test) is applied to the tin layer. Bonding ability is excellent at a contact angle of 9 °.

Claims (13)

In the following essential steps, a method for covering the surface of copper or copper alloy with a tin or tin alloy layer: a. Treating the surface with a solution containing at least one precious metal compound to deposit the precious metal; b. The surface coated with the noble metal according to step a is treated 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 derivatives thereof to form a tin or tin alloy layer. formation. A method according to claim 1, wherein a precious metal compound selected from the group consisting of silver, gold, platinum, palladium, ruthenium, rhodium, osmium and iridium is used. The method of claim 1 or 2, wherein the solution for depositing the noble metal further contains one or more compounds of the additional metal to be deposited. The process according to any one of claims 1 to 3, wherein the concentration of the noble metal compound in the solution according to step a is adjusted to 0.1 to 2000 ppm (weight / weight), preferably 1 to 100 ppm (weight / weight). Characterized in that the method. The method according to any one of claims 1 to 4, wherein the solution for depositing the precious metal contains, in addition to the precious metal compound, at least one solvent selected from the group consisting of water and an organic solvent. The process according to claim 1, wherein an acid selected from the group consisting of mineral acid, organic acid and sulfonic acid is selected for the solution for forming the tin or tin alloy layer. 7. The method of any one of claims 1 to 6, wherein a humectant is further added to the solution for depositing the tin or tin alloy layer. Combination layer on the surface of copper or copper alloy consisting of a layer of precious metal adjacent the surface and a layer of tin or tin alloy applied thereto. Bath solutions for electroless deposition of tin or tin alloy layers containing: a. One or more tin compounds, b. One or more acids, and c. At least one complexing agent for copper selected from the group consisting of thiourea and derivatives thereof. 10. The bath solution of claim 9 further comprising one or more compounds of the metal to which the solution is to be further deposited. The bath solution according to claim 9 or 10, wherein the solution further contains one or more wetting agents. Use of the method according to any one of claims 1 to 7 for producing a surface of copper or copper alloy phase suitable for solder even after heat treatment. Use of the method according to any one of claims 1 to 7 for producing a layer for protecting the surface of the copper or copper alloy phase against corrosion.