Abstract:
A method for producing an electrical contact element comprises the steps of providing a base material, and applying at least one electrically conductive contact layer to the base material. The contact layer has an outer surface which is elevated by roughness and which is suitable for receiving a lubricant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119(a)-(d) or (f) to German Patent Application No. 102014005941.3, filed Apr. 24, 2014. 
       FIELD OF THE INVENTION 
       [0002]    The invention is generally related to a method for producing an electrical contact element, and, more specifically, a method for producing an electrical contact element that prevents the formation of tin whiskers. 
       BACKGROUND 
       [0003]    The use of tin as an electrically conductive material in electronic circuits often results in the formation of tin whiskers or filaments. The tin whiskers are needle-like formations of tin which form primarily on surfaces with lead-free, tin coatings. The whiskers may have a length of up to 1 mm and, in extreme cases, up to 2 mm. They are electrically conductive and can therefore result in short-circuits on assembled printed circuit boards. Currents of 10 to 50 mA may flow through a whisker. Generally, when current flows through a whisker, the whisker does not melt, but instead, breaks off at a maximum tolerable current density. Under high current conditions, this break-off phenomenon is known as self-cleaning. 
         [0004]    As a result of the increasing miniaturisation, spacings between component connections have decreased to a few hundred micrometres, which is a distance that the tin whiskers can readily cross. As a result of the increasingly small current consumption of the electronic circuits, there is also no self-cleaning effect since the tin whiskers are no longer generally destroyed by the current flow in the event of short-circuits. 
         [0005]    The occurrence of tin whiskers is particularly promoted by the action of mechanical stresses such as internal mechanical stresses in the tin layer or in the connections, which are then transmitted in turn to the tin layer by high temperatures and a high level of air humidity. 
         [0006]    Therefore, a component failure as a result of tin whiskers occurs particularly often if a tin coating in the fitted state is exposed to a permanent mechanical pressure loading. This regularly occurs, for example, in the case of press-in contacts, film conductor contacts and injection-moulded tin surfaces. 
         [0007]    A conventional method of reducing the formation of tin whiskers involves the addition of lead to the tin metal-coating. However, as a result of the introduction of the ROHS Guideline (Guideline 2011/65/EU of the European Parliament and the Council of 8 Jun. 2011 for limiting the use of specific dangerous substances in electrical and electronic devices, Official Journal EU (2011) No. L174, pages 88-110), such a low-whisker tin/lead alloy is now only permitted in exceptional cases, and is completely prohibited for applications in the plug type connector sector. Although other tin alloys, such as silver/tin, and the use of various tempering steps, have been found to inhibit the growth of whiskers, they fail to prevent whisker formation completely, so a residual risk of short-circuiting always is present. 
         [0008]    In many industries involving human safety devices, particularly in the automotive sector, the residual risk of whisker formation is unacceptable, and electrical connectors must comply with high levels of reliability requirements. 
         [0009]    Another conventional approach is to use a contact layer comprising nickel. However, it has been found that conventional nickel coatings require excessively high insertion forces when a press-in contact is pressed in to a printed circuit board. Such an excessively high insertion form often results in the deformation of the press-in contact itself or results in the printed circuit board hole becoming excessively damaged. 
         [0010]    Therefore, electrical contact elements that have substantially reduced tin whisker formation, while having a sufficiently large retention forces and low press-in forces, would meet the required high levels of reliability. 
       SUMMARY 
       [0011]    In order to address the above or other problems, embodiments of the invention provide a method for producing an electrical contact element comprising the steps of providing a base material, and applying at least one electrically conductive contact layer to the base material. The contact layer has an outer surface which is elevated by roughness and which is suitable for receiving a lubricant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention will now be described by way of example, with reference to the accompanying Figures, of which: 
           [0013]      FIG. 1  is a cross-sectional view of a contact layer positioned on a base material; 
           [0014]      FIG. 2  is a cross-sectional view of the contact layer positioned on an intermediate layer, which is positioned on the base material; 
           [0015]      FIG. 3  is an electron scanning microscope image of a contact layer surface before being coated with a lubricant; 
           [0016]      FIG. 4  is a plan view of a press-in contact before being pressed into a printed circuit board; and 
           [0017]      FIG. 5  is a plan view of the press-in contact in an assembled state. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The invention will be explained in greater detail below with reference to  FIGS. 1-5 . 
         [0019]    The starting point for the method is a base material  100 . The base material  100  may be either a strip conductor or a plug pin or the like, and may be made of copper or a copper alloy, such as tin bronze, brass, high-strength brass or a CuNi, CuMn, CuNiSi, CuAl alloy. 
         [0020]    In the embodiments shown in  FIGS. 1 and 2 , only the region which is important for the electrical contacting is shown, that is to say, for example, the region of a press-in plug pin which is in contact with a contact hole. A tin-free metal coating which forms a contact layer  102  is applied to the base material  100  in this region. 
         [0021]    A surface of the contact layer  102  is textured, having defined indentations  104  and elevations  106 . The textured surface allows a lubricant (not illustrated in the Figures) to be retained on the surface of the contact layer  102  so as to be able to be stored for a long time. The elevations  106  may have extremely different shapes depending on how they are produced. They may be constructed, for example, to be pyramid-like, conical, spherical, clod-like, or columnar. In an embodiment, a height difference between the indentation  104  and the elevation  106  is generally between 0.4 μm and 10 μm. In an embodiment, the height different between the indentations  104  and the elevations  106  may be between 0.8 μm and 5 μm. If a lubricant having a viscosity between 1.5 mm 2 /s and 680 mm 2 /s is used in conjunction with the textured surface of the contact layer  102 , an application of oil is capable of being stored for a long period of time without negatively influencing subsequent pressing-in operations. The lubricant may comprise, for example, a highly viscous oil. For application, the lubricant is mixed with a highly flowable solvent which acts as a carrier substance and which at least partially volatilises after application. 
         [0022]    A significant aspect for preventing tin whiskers is that, in the region of the actual contacting (shown in  FIGS. 1 and 2 ), there is no tin coating at the direct surface. To this end, the contact layer  102  is formed, for example, from iron, cobalt, nickel, rhodium, iridium, palladium, silver or an alloy comprising such elements. 
         [0023]    In an embodiment, the tin-free contact layer  102  is deposited by means of electroplating technology. The parameters of the electroplating deposition, such as a composition of the bath, the addition of catalysts or acids, and current strengths and voltage values, may be adjusted in such a manner that the desired surface roughness is produced. 
         [0024]    Those of ordinary skill in the art would appreciate that there may also be other deposition techniques which allow sufficient surface roughness with good adhesion, for the formation of the tin-free contact layer  102 . 
         [0025]    In an embodiment, a smooth metal coating layer may firstly be deposited, and then be roughened accordingly in a second process step, such as through an etching step. The roughened metal coating layer forms the tin-free contact layer  102  and the roughness is high enough to bond a lubricant therein in a durable manner. 
         [0026]    In an embodiment shown in  FIG. 2 , the tin-free contact layer  102  is not applied directly to the base material  100  but instead is applied to an intermediate layer  108  positioned on the base material  100 . The intermediate layer particularly facilitates the deposition of the contact layer  102  and improves the adhesion to the base material  100 . In an embodiment, a smooth intermediate nickel layer  108  may be used for a nickel contact layer  102 . In other embodiments, other metals and alloys may also be used as an intermediate layer  108 . 
         [0027]    It should be noted that the thickness relationships of  FIG. 2  are not intended to be interpreted to be true to scale in any manner. In an embodiment, an intermediate layer  108  made of nickel has a thickness of 1 to 2 μm to the base layer  100 , and the subsequently applied contact layer  102  is then formed from nickel having a thickness of 0.5 μm. 
         [0028]      FIG. 3  is an electron scanning microscope image of a nickel contact layer  102  with the corresponding elevations  106  and indentations  106 . 
         [0029]    In an embodiment shown in  FIG. 4 , a press-in contact pin  110  is shown prior to insertion into a contact pin receiving hole disposed on a printed circuit board  114 . In particular, the printed circuit board  114  has a socked receiving opening  116 , in which an electrically conductive socket  112  is fitted. The press-in pin  110  has a surface portion  118  that is curved in a convex manner in the pressing-in direction (arrow P), being curved over a predetermined longitudinal length L. The curved surface portion  118  is formed on mutually opposing contacting members  118 A,  118 B which are formed on the press-in pin  110  in the region of the longitudinal length L, curve outwards from cylindrical longitudinal portions of the press-in pin and enclose a central empty space. 
         [0030]    In the embodiment shown, the socket  112  is formed from substantially pure copper. The press-in pin  110  is also formed from copper, and, in the region of the curved surface portion  118 , the contact layer  102  has a coating of oil prior to an assembly of the press-in pin to the socket  112 . 
         [0031]    In order to produce the press-in connection, the press-in pin  110  is moved relative to the printed circuit board  114  and the socket  112  in a mating direction (arrow P). The press-in pin  110  is firstly centered in the socket  112  with the mating end tip (not labelled), whose outer diameter is smaller than the inner diameter of the socket. With continuing movement in the mating direction P, the curved longitudinal portion  118  is finally introduced into the socket  112 . In this instance, first the spacing between the outer peripheral surface of the press-in pin  110  and the inner peripheral face  120  of the socket  112  is reduced until both surfaces move into contact with each other. With continuing movement in the mating direction, the contacting members  118 A,  118 B are first resiliently deformed in a radially inward direction and finally, with further continuing movement, the material of the outer layer is plastically deformed. 
         [0032]    In this state, the oil application retained at the surface of the contact layer  102  becomes effective as a lubricant so that a necessary insertion force remains sufficiently low during further movement in the mating direction P. With continuing movement in the mating direction P, material from the contact layer  102  is sheared off from the outer peripheral surface of the press-in pin  110 , and a metallic plug  122  is formed between the rear flank of the curvature  118  in the mating direction P and the inner peripheral face  120  of the socket  112 . The press type connection produced in this manner is, on the one hand, retained by resilient restoring forces of the contacting members  118 A,  118 B which are biased radially outwards and, on the other hand, by a possible cold welding between the surface material of the press-in pin  110  and the socket  112 . The lubricant is also displaced outwards in this operation so that a reliable electrical contacting is ensured. 
         [0033]    If no tin is involved in the region of the electrical contacting between the surface portion  118  and the socket  112 , which is mechanically loaded by the pressing action, that is to say, the printed circuit board  114  also, does not contain any tin, the occurrence of tin whiskers can be prevented with complete certainty. If the printed circuit board  114  does contain tin, the growth of whiskers is substantially prevented by using press-in pins  110  with the contact layer  102 . 
         [0034]    The contact layer  102  may also advantageously be used with a large number of other electrical connections. For example, contact springs in plug type micro-connectors may be provided with the contact layer  102 . Furthermore, the contact layer  102  is suitable for use for injection-moulded contact elements, solder connections, or film conductor contacts. 
         [0035]    As a result of the complete omission of tin in the contact region, a technology which does not have any tin whisker growth has been shown. Therefore, the shearing of tin whiskers can also be prevented, as can short circuit bridging between adjacent electrically conductive structures.