The invention relates to a layer for an electrical contact element.

An electrical contact element serves to produce an electrical connection by means of a contact. The contact element is in mechanical and electrical contact with a counter-contact element. A more or less strong mechanical pressure is thereby often also applied to the contact element and especially to the surface thereof. Since the contact element is often connected to the counter-contact element for a long period of time, this pressure exists in most cases over long periods of time. For example, a pressing contact can rest for a long period of time in a counter-element and be permanently subjected to a high level of mechanical stress. A contacting with a resilient-force-reinforced contact on a contact face leads to longer-term loading.

In order to improve the properties of the connection and in order to ensure a stable connection for many connection cycles, the contact element is generally coated. Such a coating may, for instance, lower the transition resistance, have increased wear-resistance or delay or prevent a chemical change, for example, an oxidation of the substrate located below the layer. Since the crystal structures of the substrate and the layer deviate from each other to a greater or lesser extent, the presence of a layer alone can already lead to internal mechanical tensions in the layer. Depending on the coating method, differently sized regions of the layer material with different properties, for example, different orientations or crystallisation forms, may further occur, which leads to an increase of the internal tensions in the layer. Even without external mechanical pressure, the layer may already be subjected to an internal mechanical pressure. With common materials for a layer, in particular with tin, this leads in most cases to a greater or lesser extent to pronounced growth of hair- like or needle-like structures from the layers. These can become very long over time and contact other electrical components, with the result that a short-circuit occurs, or break off and bring about a short-circuit at another location.

In order to prevent the growth of such structures, the use of tin, both as a main material and as an alloy partner, is intended to be dispensed with. - -

An object of the invention is to provide a layer for an electrical contact element which is mechanically stable, abrasion-resistant and is highly electrically conductive, and which, at the same time, prevents a growth of the hair-like structures which are known as whiskers. According to the invention, this is achieved in that the layer contains bismuth and is tin- free.

When bismuth is used, the layer has good electrical conductivity and low contact transition resistance. At the same time, bismuth reacts only slightly with the atmospheric environment and with any other layer components and is consequently stable for a number of years. Since the layer is tin- free, the growth of whiskers is prevented. The term tin- free is intended to be understood in such a manner that tin is contained at the most in small quantities, for example, as an impurity. However, any tin which is present is in particular not intended to be present in quantities in which it has an influence on the properties of the layer. Advantageous developments of the invention which can be freely combined with each other are described below.

The layer may contain more than 10%, in particular more than 50%, especially more than 90% of bismuth. The percentages relate in each case to the mass. Depending on the remaining components of the layer, it may be necessary to increase the bismuth proportion. In particular, bismuth may also be more cost-effective, more simple to obtain and/or to process. The physical and/or chemical properties of bismuth may also be more suitable for the respective application than the remaining components so that a high proportion of bismuth is advantageous.

A layer may be particularly simple to produce and/or to dispose of if, with the exception of inevitable impurities, it comprises exclusively bismuth. In the production of such a layer, additional mixing or alloying steps are superfluous. In another advantageous embodiment, the layer contains no lead. Lead is poisonous to humans and can accumulate in the body. Furthermore, lead can cause environmental damage. It is therefore advantageous for lead to be completely dispensed with. In particular, the use of lead may also be subject to legal provisions. Nonetheless, it may be necessary or desirable to have lead as a component in the layer. . .

The layer may have other components, in particular such materials may be elements. For example, lead, zinc, indium, antimony, copper, nickel, silver, gold, palladium and/or ruthenium are suitable therefor. Each of these materials has specific physical and/or chemical properties so that, depending on the properties of the layer to be achieved, one or more of these materials may be components of the layer in different proportions. By adding silver and/or gold, for example, particularly good electrical conduction properties can be achieved. However, this is generally linked with reduced mechanical stability. Other materials may bring about increased mechanical stability, in particular also increased abrasion-resistance of the layer. Owing to the addition of other materials, the combination of a plurality of materials may have properties which are more favourable than may be anticipated with the purely linear interpolation of the corresponding size. For example, the electrical conductivity owing to electronic effects may be higher than the electrical conductivity of each individual material of the layer.

In particular, another component of the layer may also constitute a large proportion of the layer, in particular the mass. Another material may, for example, constitute more than 50% of the mass of the layer. However, the materials added are generally more expensive so that the respective proportions are intended to be as low as possible. Typically, the other components would then constitute approximately from 1-10% of the mass. Often, a very small proportion of other materials is also already sufficient to achieve positive effects. A doping, that is to say, an addition of 1% or much less, may also, for example, already be sufficient. If bismuth forms the layer together with other materials, it is advantageous for bismuth to be the main component, that is to say, when the proportion of bismuth as a mass percentage is the highest of all the proportions. If, therefore, in addition to bismuth, for example, a further ten additional elements are used, each at a mass percentage of nine percent, bismuth at a proportion of 10% may already be the main component. This particularly ensures that the properties of the layer are primarily determined by the properties of the bismuth proportion.

The thickness of a layer for an electrical contact element may extend from an atomic to comparatively thick layers of approximately 1 mm thickness. Typically, the layer thickness for an electrical contact element is approximately 2 μιη. In particular when it is ensured that . . the layer is mechanically stable and abrasion-resistant, however, a layer thickness of 1 μιη and lower may also be sufficient.

The layer may be applied to a copper substrate. Copper is a widely used and cost-effective material for contact elements. Furthermore, copper has sufficiently good mechanical, thermal and electrical properties.

Between the substrate, for example, a copper substrate, and the layer according to the invention, other layers may be provided. For example, a nickel layer which is known as a pre-coating of nickel may be provided between the copper substrate and the layer according to the invention. This nickel layer may assume, for example, a barrier function and prevent the diffusion of copper atoms into the layer. Such a diffusion could have a negative effect on the properties of the layer. In particular, the layer may be directly applied to a copper substrate. In particular when high proportions of bismuth are used, in particular with a pure bismuth layer, no negative influence of the layer owing to the copper which is directly in contact therewith is to be anticipated. The otherwise conventional nickel barrier layer can thus be dispensed with. Since bismuth is highly diamagnetic, a layer which contains bismuth may further have additional, positive electrical and/or magnetic properties. It may, for example, be used for electromagnetic shielding or as a waveguide. In particular, when the layer completely or almost completely surrounds a contact element, there occurs a selective influence of the transmission properties or the shielding effect.

The layer according to the invention may also be part of a layer system which comprises a plurality of layers. In particular, such a layer may be the outermost layer, in particular also a contact layer, which comes into contact with a counter-element and/or the environment. However, it may also act as an intermediate layer.

A layer according to the invention or a layer system according to the invention can be produced by means of electroplating. In particular, only a portion of the layer or a portion of the layer system may also be produced by means of electroplating. This method is a widely used coating method, with the result that there is a broad knowledge base relating to the - - equipment and operating methods in existence in this regard. Electroplating is in most cases also simple and cost-effective. The layers produced thereby have generally advantageous properties, for example, suitable sizes of the domains produced, a good surface structure and a uniform distribution. The electroplating method can be carried out continuously or in batch production.

Particularly for very thin layers or layer systems, the layer or the layer system can be produced by means of physical vapour deposition. In this method, a larger range of alloy partners is possible. Furthermore, this method allows a plurality of layer components to be applied sequentially and/or at the same time with a very wide range of mixing ratios, or allows very pure layers to be produced.

A layer according to the invention can be used as a contact layer in a plug type connector contact. In particular, it can be used as a contact layer for the insertion region, the connection region and/or the pressing region of a plug type connector contact.

Advantageously, this is a pure bismuth layer so that bismuth is used as a contact layer for the insertion region, the connection region and/or the pressing region of a plug type connector contact. The invention is explained below with reference to exemplary embodiments. The features and embodiments set out therein may be freely combined with each other, dependent on how this is advantageous for the respective application. In the drawings:

Figure 1 is a schematic sectioned view of a layer according to the invention on a substrate; Figure 2 is a schematic sectioned view of a layer according to the invention together with an intermediate layer and a substrate;

Figure 3 is a schematic cross-section of a contact element having a layer according to the invention;

Figure 4 is a schematic cross-section of a contact element having a layer system according to the invention, together with a counter-contact element,

Figure 5 is a schematic cross-section of a pressing contact element having a contact layer according to the invention;

Figure 6 is a schematic cross-section of a layer system according to the invention; - -

Figure 7 is a schematic graph of an EDX (energy dispersive X-ray spectroscopy) analysis of a galvanically produced bismuth layer.

Figure 1 shows a contact element 1 which comprises a substrate 2 and a contact layer 3 according to the invention applied to the substrate 2. The contact layer 3 is located between the substrate 2 and the environment 4 so that it shields the environment 4 and the substrate 2 from each other. The contact layer 3 serves to produce a contact with a counter-contact element (not shown). The contact layer 3 shown in this instance was applied to the substrate 2 by means of electroplating. In this instance, it grew from the surface 2a of the substrate outwards in the growth direction G until it had reached a thickness DK. The coating operation was then interrupted. The typical thickness DK of such a contact layer 3 is between 1 μιη and 10 μιη. In order to achieve a more stable layer, however, the thickness may also be selected to be greater. In particular, the selected coating method may also allow the production of thicker layers only. On the other hand, a thinner layer is often also sufficient. For example, layers with a thickness of less than 10 μιη, in particular also of less than 1 μιη, may already be sufficient. The contact layer 3 shown here contains bismuth, but no tin. It may be a pure bismuth layer. However, the layer may also contain other elements so that the bismuth proportion is reduced. Under some circumstances, a bismuth proportion of > 10% may be sufficient to exclude undesirable whisker formation. Preferably, the layer comprises more than 50%, in particular more than 90% of bismuth. The percentages in each case, as in the remainder of the text, relate to the mass. With higher proportions of bismuth, the properties of the layer may be primarily determined by the bismuth. The crystal structure, the morphological, electrical, physical and/or chemical properties are mentioned as such properties purely by way of example. In contact layers, the electrical conductivity and the abrasion resistance are of prime importance.

The example shown in this instance of a contact layer 3 is arranged directly on the substrate 2. In particular there is therefore no intermediate layer between the substrate 2 and contact layer 3. The substrate may in particular be copper. The contact layer 3 according to the invention may be applied directly to the copper, which would not be possible with other - - contact layers, for example, with tin layers, since a diffusion of the copper atoms into the tin layer would take place, which would have a negative influence on the properties of the contact layer 3. With the contact layer according to the invention, such an intermediate or barrier layer may be dispensed with. Nonetheless, such a nickel layer could also be present in this case.

The substrate 2 is at least completely covered over the surface-area by the contact layer 3 which, owing to the diamagnetic properties of the bismuth, can produce a shielding effect. Figure 1 shows only a cutout. A contact element 1 could, for example, be a pin contact having a square cross-section. The contact layer 3 could extend completely round the cross- section so that the contact layer 3 surrounds the substrate 2 in a tunnel-like manner. Owing to the diamagnetic properties of bismuth, a waveguide which conducts specific frequencies or a specific frequency range in an almost loss-free manner can thereby be produced. Figure 2 is a schematic section through a second embodiment of a contact layer 3 according to the invention. The contact layer 3 is part of a layer system 5 which, in addition to the contact layer 3, further contains an intermediate layer 6 and is arranged on the substrate 2. Such an intermediate layer 6 may, for example, influence the transition resistance between the substrate 2 and the intermediate layer 6 in a desired manner. In particular, the

intermediate layer 6 may lower the transition resistance so that the electrical conductivity of the entire system is low.

The intermediate layer 6 may also act as an intermediate layer 6b when, for example, a growth of the contact layer 3 on the substrate 2 is not possible without an intermediate layer. Such an intermediate layer 6b may therefore be able to be connected to the substrate 2 well and to the contact layer 3 well. It may further have lattice constants between the individual atoms, which is between the values of the lattice constants of the substrate and the lattice constants of the contact layer. An internal mechanical pressure or tension and/or the increased occurrence of defects, as would be the case with direct application of the contact layer 3 to the substrate 2, can thereby be prevented.

The intermediate layer 6 may also simply be a barrier layer 6c which prevents diffusion of components of the substrate 2 into the contact layer 3 or vice- versa. - -

The thickness DZ of the intermediate layer 6 and the thickness DK of the contact layer 3 can be selected to be of different sizes depending on the application. For example, the intermediate layer 6 may have only a small thickness DZ, whilst the thickness DK of the contact layer 3 is rather large. On the other hand, the thickness DK of the contact layer 3 may also be small and the thickness DZ of the intermediate layer 6 may be large.

In particular, the intermediate layer 6 may also be an alloy layer which comprises the components of the substrate 2 and the contact layer 3. This may be brought about intentionally or occur by chance.

As with the other embodiments, the contact layer 3 shown in this instance, in addition to bismuth, may also contain other materials, in particular other elemental materials. In particular, it may contain zinc, indium, antimony, copper, nickel, silver, gold, palladium and/or ruthenium. This can be varied, depending on the application and desired properties to be achieved. So that bismuth remains the determining element for the properties, it must in particular be the main component, that is to say, the proportion of bismuth is larger than the proportion of any other element alone. Lead can also be added but it is advantageous for the contact layer 3 to contain no lead since lead is poisonous to humans and can harm the environment.

Since the layer contains no tin, undesirable whisker growth is prevented. In Figure 3, the layer 10 according to the invention is again a contact layer 3. This time, the surface 2a of the substrate 2 does not extend linearly, however, but instead in a curved manner and the contact layer 3 which is located thereon surrounds the substrate 2. The example shown in this instance of a contact layer 3 has, in the growth direction G, a thickness DK which remains constant. The contact element 1 may be, for instance, a pin- like contact element la which comes into contact with a flat surface. Such a contact layer 3 may, for example, be produced by means of an immersion coating method. Figure 4 shows another example of a layer 10 according to the invention. This layer 10 does not form the contact layer 3 this time, but instead is covered by a separate contact layer 3a. Between the layer 10 and the substrate 2, there is further another intermediate layer 6, for example, an intermediate layer 6, as already described in Figure 2. The contact layer 3 which is applied to the layer 10 in the growth direction G may, for example, serve to reduce - - the transition resistance. It may also be necessary in order to prevent soldering or welding of the layer 10 to the counter-element 7. Furthermore, the separate contact layer 3 a may also prevent a chemical reaction of the layer 10 with the counter-element 7 or the environment 4, such as oxidation in the air.

The counter-contact element 7 presses in a contact direction C on the contact element 1 so that the layer system 5 is under mechanical pressure 9 which extends from the tip 7a of the counter-contact element. The mechanical pressure 9 has, in particular in the vicinity of the tip 7a of the counter-contact element 7, a component 9a parallel with the contact direction C. The greater the distance from the tip 7a, the greater a component 9b which extends perpendicularly relative to the contact direction C and parallel with the layers becomes. This mechanical pressure 9 normally increases the tendency to form tin whiskers, which grow, for example, in the growth direction G from a layer 10. With a layer 10 according to the invention, however, this does not occur since it contains bismuth and was produced in a tin- free manner. An additional separate contact layer 3 a may also further prevent such growth of whiskers, but does not necessarily have to contain bismuth.

The density DZ of the intermediate layer 6, the thickness DS of the layer 10 and the thickness DK of the contact layer 3 can be adapted to the respective applications. In particular, individual thicknesses may be very much smaller or very much larger than other thicknesses.

In this example, only a single separate contact layer 3 a is shown. Other layers in the growth direction towards the layer 10 may also be present.

Figure 5 shows a layer 10 according to the invention, which acts as a contact layer 3 and which extends around a substrate 2. Figure 5 shows a forked pin lb which acts as a contact element 1 and is pressed in a counter-contact element 7, for example, a coated plate hole. With normal contact elements 1 having contact layers 3 not in accordance with the invention, this would lead to metal hairs growing from the contact layer 3 over time, which hairs can break off and/or cause electrical short-circuits. In the layer 10 according to the invention shown in this instance, which is used as a contact layer 3, this phenomenon does not occur since it contains bismuth. - -

Figure 6 shows another layer 10 according to the invention, in the form of a contact layer 3. However, the contact layer 3b shown in this instance extends over only a part-region 2b of the substrate and over a part-region 6f of the intermediate layer 6. Such a spatial selection may, for example, be achieved by covering the substrate in the case of galvanic production of the layer 10 or the intermediate layer 6. A removal of non-desired part-regions of layers after the actual production process, for instance, by mechanical processing or by means of etching, also leads to such a configuration

Figure 7 shows the EDX spectrum (energy-dispersive X-ray spectroscopy) of a galvanically produced sample. The substrate is a copper alloy. The bismuth layer was applied with an additive-free bismuth electrolyte directly to the copper substrate, that is to say, no intermediate layer.

- -

List of reference numerals

1 Contact element

la Pin-like contact element

lb Pin

2 Substrate

2a Surface of the substrate

2b Part-surface of the substrate

3 Contact layer

3a Separate contact layer

4 Environment

5 Layer system

6 Intermediate layer

6b Intermediate layer

6c Barrier layer

6f Part-region of the intermediate layer

7 Counter-contact element

7a Tip of the counter-contact element

9 Mechanical pressure

9a Component of the mechanical pressure in the contact direction

9b Component of the mechanical pressure perpendicularly relative to the contact direction

10 Layer

C Contact direction

DZ Thickness of the intermediate layer

DS Thickness of the layer

DK Thickness of the contact layer

G Growth direction