EP1230034A1 - Process for the non-galvanic tin plating of copper or copper alloys - Google Patents

Abstract

The invention describes a process for non-galvanic tin plating of copper and copper alloys by precipitation of tin from methanesulphonic acid and tin-containing electrolytes, containing a complexing agent. In describing a process by which a durable tin layer which can be soldered is created, which, at the same time, prevents liberation of the base material, this invention discloses that the electrolytes have at least one foreign metal added to form a diffusion barrier in the tin layer.

Description

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In order to avoid the above disadvantages it is the purpose of the invention to devise a process for the non-galvanic tin plating of copper or copper alloys by which a durable tin layer which can easily be soldered can be produced which, at the same time, prevents the liberation of the basic material. As a solution it is proposed by the invention that at least one foreign metal is added to the electrolyte to form a diffusion barrier in the tin layer.

With the process described in the invention, a tin bath is suggested for the formation of a tin layer by chemical precipitation, which contains at least one foreign metal. The addition of foreign metals to the tin bath achieves an advantageous suppression of the diffusion processes, and thus a diffusion barrier is built which prevents the liberation of metals from the base material to a large extent. The advantages thus gained are good soldering characteristics at the surface and good durability of the tin layer.

The formation of a tin layer by the above process therefore not only creates the possibility to produce effective corrosion protection but, moreover, by the use of foreign metals a diffusion-stable tin layer is produced which prevents the liberation of materials from the base layer to a large extent. This is an advantage especially in view of the copper liberation from potable water carrying copper tubing. However, an out-diffusion of lead and zinc from basic brass materials is prevented by a diffusion-stable tin layer.

In accordance with one feature of the invention, a metal of the group silver, bismuth, nickel, titanium, zirconium and indium is suggested as the foreign metal, whereby the use of indium has shown to be especially effective. For the formation of a diffusion barrier within the tin layer, at least one of the above metals is added to the tin bath as a foreign metal.

In accordance with an additional feature of the invention, thiourea and/or its derivative is used as the complexing agent. Thiourea as the complexing agent enables the liberation of positively charged copper ions. A copper thiourea complex forms which is soluble in electrolytes at a temperature of >28°C. As a result of the complexing of the copper, its potential compared to that of tin is reduced. The then more noble tin precipitates, forming a layer of tin on the copper. The liberated copper ions concentrate in the electrolyte, whereby at a 3 copper concentration above 7 g/1 economical working is no longer possible since at these concentrations tin is no longer precipitated at satisfactory rates. It is therefore suggested to remove the copper by precipitation of the copper-thiourea compounds in solution in the electrolyte. In this manner, a substantial increase in the useful life of the tin bath may be achieved. The precipitation of the copper- thiourea compounds can be achieved by means of another feature of the invention by filtration.

For the application of the non-galvanic precipitation of diffusion-stable tin layers in accordance with the invention a tin bath is of advantage, which preferably contains the following components:

1. A source of tin, preferably a bivalent tin salt, for example tin methanesulfonate, with 1 to 30 g/1 of tin in the tin bath;

2. An acid, preferably methanesulphomc acid with 5 to 200 g/1 in the tin bath, whereby the tin bath assumes a pH value of 0 to 3 ;

3. A complexing agent, preferably thiourea or a derivative in quantities of 10 to 200 g/1;

4. A wetting agent in quantities of 1 to 10 g/1; 5. At least one foreign metal, preferably a metal in the group Ag, Bi, Ni, Ti, Zr and In in a proportion of

1 to 500 mg/1 in the tin bath.

To apply the process described in the invention, a working temperature of the tin bath of 35 to 80°C is suggested. In addition, already known measures common to the state of the art can be taken when using the process described. This includes, for example, rinsing, pickling and drying of the work pieces.

Further details regarding the invention follow from the examples below in each of which an electrolyte composition is suggested. Example 1

Thiourea 100 g/1

Methanesulphomc acid 100 g/1

Tin methanesulphonate 5 g/1 tin

Wetting agent 5 g/1

Bismuth 30 mg/1

Example 2

Thiourea 100 g/1

Methanesulphomc acid 100 g/1

Tin methanesulphonate 15 g/1 tin

Wetting agent 3 g/1

Antioxidant 5 g/1

Titanium 5 mg/1

Example 3

Thiourea 120 g/1

Methanesulphonic acid 140 g/1

Tin methanesulphonate 15 g/1 tin

Wetting agent 5 g/1

Antioxidant 5 g/1

Indium 50 mg/1

The process described by the invention makes it possible to produce diffusion-stable tin layers by means of chemical precipitation, whereby the diffusion barrier generated by the addition of foreign metals prevents the liberation of metals from the base materials in an advantageous manner. In addition, by using thiourea as complexing agent it becomes possible to remove the 5 copper ions liberated from the copper from the electrolyte by filtration, and thus to achieve a substantially extended useful life. Furthermore, in this manner a substantial acceleration of the process is achieved.

Claims

Claims

1. Process for non-galvanic tin plating of copper and copper alloys by precipitation of tin from methanesulphonic acid and tin-containing electrolytes, containing a complexing agent, characterized by the electrolyte having at least one foreign metal added in order to build a diffusion barrier in the tin layer.

2. A process in accordance with claim 1, characterized by a metal of the group Ag, Bi, Ni, Ti, Zr and In being used as the foreign metal.

3. A process in accordance with claims 1 and 2, characterized by thiourea and/or its derivative being used as a complexing agent.

4. A process in accordance with one of the preceding claims, characterized by the electrolytes being reconditioned by having the copper in solution removed through precipitation of the copper-thiourea compound.

5. A process in accordance with claim 4, characterized by the copper- thiourea compound being filtered out.