Abstract:
A surface treatment of electrical contact pieces in electrolytic plants, in particular of machines for the treatment of circuit boards and conductor sheets is discloses, wherein the contact pieces are made of titanium or some other oxidizing and therefore chemically and electrochemically resistant material. To eliminate unwanted metallization of the contact pieces as a result of an inadequate demetallization on account of the insulating oxide layer and in order to enhance protection against wear, an electrically conductive diamond coating is applied at least on the contact-making areas of the contact pieces.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of prior filed copending PCT International application no. PCT/DE2006/001232, filed Jul. 16, 2006, which designated the United States and has been published but not in English as International Publication No. WO 2007/009441 A1 and on which priority is claimed under 35 U.S.C. §120, and which claims the priority of German Patent Application, Serial No. 10 2005 034 419.4, filed Jul. 19, 2005, pursuant to 35 U.S.C. 119(a)-(d), the contents of which are incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates, in general, to the field of electrical contacting of items in assemblies for electrolytic treatment. 
         [0003]    Nothing in the following discussion of the state of the art is to be construed as an admission of prior art. 
         [0004]    Electrolytic treatment of items, such as circuit boards and conductor sheets in immersion bath assemblies and continuous line assemblies having a vertical or horizontal plane of conveyance and assemblies for guiding items from wheel to wheel, may involve electrolytic etching or metallization. A main application is electroplating. Therefore, the following description relates primarily to the field of electroplating by way of example. 
         [0005]    During electroplating, the items to be plated are polarized as a cathode against electrodes which are anodes. Contact wheels or contact rollers are typically used for electrical transfer of a corresponding potential or of a treatment current onto the items. These elements are electrically conductive at least at their corresponding surfaces or contact areas. As a consequence, metallization occurs not only of the items to be plated but also of those surfaces or contact areas. The presence of metallization is disadvantageous and must be removed from the surfaces and contact areas from time to time. Demetallization or stripping is electrolytically performed if circuit boards in a copper bath are to be treated. The contact pieces are hereby temporarily or permanently polarized as an anode against a secondary electrode polarized as a cathode. In order to prevent a dissolving of contact material of the contact pieces polarized as an anode, it must be electrochemically resistant, i.e. chemically resistant in case of anodic polarity. Examples of contact material include titanium, niobium or tantalum. These materials quickly create in an electrolyte an oxide layer which prevents an anodic erosion of the material since the oxide layer is an electrical insulator. This, however, is unwanted, when a contact piece is involved, because it requires application of a sufficient contact force of e.g. 30 Newton, in combination with a rubbing movement during closing of the contact piece or rolling of a roller contact in order to penetrate the thin and porous oxide layer for conduction of an electric current. This has proven unreliable and causes increased wear of the contact pieces. Furthermore, not only the items but also the contact pieces are plated through the porous oxide layer and/or through the cracked oxide layer. In addition, such an undesired metallization or deposition affects also the insulating oxide layers. 
         [0006]    Electrolytic demetallization or stripping of contact pieces typically involves the application of a high cell voltage in order to completely strip metallic coated areas. This is disadvantageous for two reasons. First metallic material which is electrochemically resistant at low voltage starts to dissolve at high anodic potential. Titanium, for example, starts to dissolve in an acid copper bath at a voltage of approximately 10 V. Second, the metallization of the contact pieces results in a loss of electrical contact because an increase in metallization necessitates an increase in the effective treatment current. There are only few and small electrical connections through the oxide layer to the contact piece which connections are interrupted like a fuse by the high current during stripping and closed again during subsequent metallization. 
         [0007]    However, a high current for stripping is necessary in order to curb metallization of the contact pieces. The balance between metallization and stripping can only be maintained for a relatively short duration of time. Thereafter, an unbalance between metallization and stripping manifests itself by an intensive metallization of the contact pieces having a surface of electrically insulating oxidized metallic material. After only a few hours of electroplating of a circuit board, a lasting copper layer develops on the surface of the contact pieces, whereby the extent of metallization depends on the applied current density for metallization and stripping. Periodically, the copper layer on the contact pieces must be removed outside of the electroplating assembly to be able to continue the electroplating process for circuit board production. 
         [0008]    Attempts have been made to protect the surface of the contact pieces by a coating of noble metal. For example, an electrolytic gold coating or platinum coating having a thickness of e.g. 5 micrometers has been applied onto the surface of a contact piece so that the surface is free of insulating oxides and able to provide contact with a low electrical resistance. However, it has been discovered that there is weak adhesion of an electrolytically applied coating on metals having a surface with insulating oxides. Moreover, the circuit boards have sharp edges as a result of the use of glass fibers to strengthen the material. In continuous line assemblies for electroplating, contact rollers may roll on circuit board edges. As a result of its small thickness and low hardness, the metal coating is abraded and worked off by the surface of a contact piece after a short time. 
         [0009]    The progression from a coated contact piece to an uncoated contact piece having the afore-mentioned difficulties takes place gradually. As a result, the undesired metallization of the contact pieces increases again. Therefore, electrolytic treatment of circuit boards is inadequate. Furthermore, an exchange of contact pieces with abraded coating with contact pieces with a new coating of noble metal is time-consuming and expensive, and in addition a coating of contact pieces with a noble metal is costly. 
         [0010]    German Offenlegungsschrift DE 198 40 471A1 discloses a contact device for electrolytic metallization of circuit boards. Contact wheels have segments which are located in the areas for contacting and made of titanium. Electrolytic stripping of the segments is hereby monitored by a control system with the assistance of a voltage-current curve in a respective stripping cell to provide information about the progress of stripping. In a predetermined situation, electrolytic stripping is disabled. A horizontal continuous line assembly for electrolytic treatment of circuit boards includes many, e.g. 60, rotary contact wheels. Thus, the electrolytic stripping of contact pieces becomes very complicated. 
         [0011]    German Pat. No. DE 36 45 319 C2 discloses clamps which mechanically grasp and electrically contact items such as panel-like objects at their lower ends, thereby electroplating at least the lower ends of the clamps. Such an undesired electroplating or metallization of clamps must be removed by a subsequent stripping device. The same problems as described above are also experienced here. The insulating oxide layer of an electrolytically resistant material of the contact pieces adversely affects contacting and stripping of contact pieces. 
         [0012]    German Offenlegungsschrift DE 196 33 797 A1 discloses a rotatable contact and transport unit of a continuous line assembly for electrolytic treatment of circuit boards. The contact and transport unit has a circumference which has at least some areas provided with a metallic coating. By means of a secondary electrolysis, metal on contact pieces, deposited during undesired metallization process, is deposited on secondary cathodes. This arrangement has the same disadvantages as described above for electrical contacting of items to be plated. 
         [0013]    Common to all prior art proposals is the need for a compromise between metallization, stripping and service life of contact pieces. Noble metal coatings are too expensive, have a too low hardness and therefore are not enough abrasion-resistant. 
         [0014]    It would therefore be desirable and advantageous to provide an improved device for electrical contacting of items to be plated in an electrolytic immersion bath assembly, horizontal continuous line electroplating assembly, vertical continuous line electroplating assembly and assemblies supplied with a transport system using wheels, which device obviates prior art shortcomings and is yet cost-effective and easy to realize. 
       SUMMARY OF THE INVENTION 
       [0015]    According to one aspect of the present invention, a device for electric current transfer to an item to be plated in an electrolytic machine includes a contact piece which is made of a chemically and electrochemically resistant metal has an electrically insulating oxide layer at anodic polarity, with the contact piece having a first surface which is intended for contact with the item to be plated, and an electrically conductive diamond coating for application on the first surface of the contact piece. 
         [0016]    According to another aspect of the present invention, an electrically conductive diamond coating for surface treatment is used as protection layer on an electrical contact piece which is made of a chemically and electrochemically resistant metal having an electrically insulating oxide layer at anodic polarity and which is adapted to operate for electric current transfer to an item to be plated in an electrolytic machine. 
         [0017]    In accordance with the present invention, the surfaces of the contact-making elements such as e.g. segmented contact-making wheels, contact wheels, contact brushes or clamps, are electrically conductive in the absence of an insulating oxide layer. The surfaces are abrasion-resistant and protected against wear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0018]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0019]      FIG. 1  is a schematic perspective view of a sequential contact-making wheel; 
           [0020]      FIG. 2  is a side view of a clamp for grasping and contacting panel-like items; and 
           [0021]      FIG. 3  is a side view of an exchangeable clamp jaw. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0023]    Turning now to the drawing, and in particular to  FIG. 1 , there is shown a schematic perspective view of a contact piece, such as a contact wheel  2 . The contact wheel  2  comprises a chemically and electrochemical resistant metal having an insulating oxide layer. A first contact surface  1  of the contact wheel  2  is provided with a diamond coating  3  which is made electrically conductive by means of adequate doping. The coating  3  is thus chemically and electrochemically resistant, exhibits high mechanical stability, is abrasion-resistant, and temperature-resistant up to 600° C. In  FIG. 1  the thickness of the coating  3  is not to scale but greatly exaggerated. 
         [0024]    The contact wheel  2  has a second surface  4 , which is not intended for making contact with the material to be plated but intended for exposure to an electrical field in an electrolytic cell of an electrolytic device. The second surface  4  is provided with an electrically insulating layer  5 . 
         [0025]      FIG. 2  shows a side view of a clamp  6  having an upper clamp part  61  and a lower clamp part  62  for mechanically grasping an electrically contacting panel-like item  8 . In this embodiment, the clamp  6  has a first contact surface which is provided with a diamond coating  3 , and a second surface which is not intended to contact the item  8  and provided with an insulating layer  5 . 
         [0026]      FIG. 3  shows a side view of an exchangeable clamp jaw  7  of the upper clamp part  61  of clamp  6 , with the inner surface of the clamp jaw  7  being coated with a diamond coating  3 , whereas the remaining outer surface of the clamp jaw  7  is provided with an electrically insulating layer  5 . 
         [0027]    Examples of an electrically conductive diamond coating for electrodes are described e.g. in M. Fryda “Eigenschaften und Anwendungen von Diamantelektroden” ( Properties and Applications of Diamond Electrodes ), Journal for Oberflachentechnik ( Journal for Surface Technology ) JOT, 1999/05 VI-IX. During a CVD (chemical vapor deposition) process an electrically conductive coating on a base material is created by means of doping with boron. 
         [0028]    Surprisingly, it has been found that only a very thin coating of approximately 1 to 10 micrometers is satisfactory for coating contact pieces. Currently preferred is a coating thickness of 2 to 5 micrometers. Therefore, the specific electrical conductivity of such a coating is sufficient for the present application. It may reach 100 (Ωcm) −1 , which is close to specific electrical conductivity of metals. 
         [0029]    Further properties of an electrically conductive diamond coating are suited for contact pieces used in electrolytic processes: 
       A) Chemical and Electrochemical Resistance. 
       [0030]    Even at anodic polarity, there is no insulating oxide layer on the surface of a diamond coating and furthermore no abrasion occurs. A high cell voltage of stripping cell may be used, e.g. 12 V and more, wherein the coating and/or the base material protected by the coating is not dissolved at anodic polarity. Since the surfaces of the coated contact pieces do not create an electrically insulating layer, a metallization of the contact pieces is realized evenly across the entire surface of the contact pieces. The same applies for the following stripping process. A break off of the electrical connection between undesired metal deposition on contact pieces, produced during metallization process of circuit boards, and corresponding base material of contact pieces does not occur. 
       B) Very High Mechanical Abrasion Resistance. 
       [0031]    This property ensures a long service life even when electrolytic treatment of sharp-edged circuit boards reinforced with glass fibers is involved. 
         [0032]    By using such a coating provided at least on contact-making surfaces of contact pieces, the afore-described drawbacks of prior art proposals are eliminated. In case the diamond coating is used especially for a clamp, the temperature-resistance of up to 600° C. is also beneficial. The clamp is only provided with the electrical conductive diamond coating on those surfaces that come into contact with a circuit board to be plated. After providing the clamp with the diamond coating, the clamp may be provided with a synthetic coating in areas not intended to come into contact with the circuit board. Such a synthetic coating may be e.g. ECTFE (Ethylene-Chlorotrifluoroethylene). In an electrolytic cell areas that are not intended to come into contact with a circuit board are exposed to an electrical field. The synthetic coating prevents a metallization in these areas. A production of such an ECTFE coating is carried out at temperatures of approximately 500° C., which does not cause damage to the diamond coating. 
         [0033]    Such a high temperature has the consequence that a noble metal coating at a lower end of a clamp according to prior art can not be applied before the production of the synthetic coating is carried out since the noble metal coating will be damaged at this high temperature. Especially, adhesiveness of the noble metal coating on a metal with insulating oxide layer is reduced during high temperature treatment. 
         [0034]    The clamp may extend up to a length of 0.4 meters. In order to use small parts, so that a cost-effective diamond coating is achieved, the clamp may be provided with exchangeable clamp jaws mounted at the lower end of the clamp, e.g. by means of screws. 
         [0035]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
         [0036]    What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein: