Abstract:
An apparatus for electroplating at least one substrate is disclosed as including a trough, two anodes, at least one bath and two poly-tetrafluoroethylene sheets or a number of substantially rigid polypropylene plates, in which the trough supports the substrate and is in an electrically conductive relationship therewith, and the bath contains the anodes, the trough and an electrolyte, in which, in operation, an electric field exists in the electrolyte between the trough and the anodes, and in which the poly-tetrafluoroethylene sheets or the polypropylene plates are movable to vary the amount of electric current passing between the trough and the anodes.

Description:
This application is a continuation-in-part of Ser. No. 998,368, filed Dec. 24, 1997, and now abandoned. 
    
    
     This invention relates to an electroplating apparatus and is, in particular, concerned with such an apparatus with improvements in the uniform distribution of metal on substrates to be electroplated, e.g. printed circuit boards. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 4,879,007 discloses a floating shield for use in an electrolytic bath. The shield includes an elongated trough in which substrates are loaded for plating. The substrates are held in the trough in a vertical plane, and with the lower edges of the substrates below the plane of the upper edges of the trough. Immersed into the bath are a pair of anodes, each extending parallel to the longitudinal axis of the floating shield. The substrates loaded in the trough are connected to a cathode bus bar by one or more clamps. In operation, electric current passes between the anodes and the cathode bus bar through electrolyte contained in the bath, and thereby to electroplate the substrates. 
     When a plating factory orders an electroplating apparatus from a manufacturer, it is necessary to specify the “board size” of the substrates intended to be plated by the apparatus. In this connection, “board size” means the vertical length of the substrate when such is held in a vertical plane. It is also a usual practice in the relevant field to so arrange the depth of the floating shield in the bath that the top edge of the substrates is of a fixed distance from the top edge of the bath. The manufacturer will then arrange the dimensions of the bath, the anodes and the depth of the trough in the bath such that there will be a satisfactory and uniform electroplating result. It is found in practice that if substrates of a board size shorter than the intended board size are electroplated in the bath, the lower edges of the substrates will be “over-plated”. This is known as “edging effect” and means that the metal deposited on or around the lower edges of the substrates is thicker than that deposited in the rest of the substrates. 
     While it is possible to reduce this “edging effect” by changing the anodes used in each electroplating task, as there may be over forty anodes in a single tank, and they are very heavy, it is very difficult and not practical to replace anodes. 
     It is therefore an object of the present invention to provide an improved electroplating apparatus in which the aforesaid shortcoming is mitigated, or at least to provide a useful alternative to the trade. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an apparatus for electroplating at least one substrate, comprising support means, anode means, at least one container and blocking means, wherein said support means supports said substrate and is in an electrically conductive relationship therewith, wherein said container contains said anode means, said support means and an electrolyte, wherein in operation, an electric field exists in said electrolyte between said support means and said anode means, and wherein said blocking means is movable to vary the amount of electric current passing between said support means and said anode means. 
     Advantageously, said blocking means may be movable by said support means. 
     Conveniently, said blocking means may be movable to vary the amount of electric current passing between a bottom part of said support means and said anode means. 
     Suitably, said blocking means may be made substantially of an electrically insulating material. 
     Said insulating material may advantageously be poly-tetrafluoroethylene. 
     Said blocking means may conveniently comprise a plurality of sheet members. 
     Said sheet members may suitably be movable away from each other. 
     Advantageously, said blocking means may be elastic. 
     Conveniently, said blocking means may comprise a plurality of substantially rigid blocking members. 
     Suitably, said apparatus may comprise at least two plates to each of which at least one blocking member is hingedly engaged. 
     Said blocking members may advantageously be pivotally movable relative to the plate to which it is hingedly engaged. 
     Said blocking members may conveniently be made substantially of polypropylene. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of an example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram showing a first embodiment of an electroplating apparatus according to the present invention in which the trough is loaded with a substrate of a vertical length of 18 inches; 
     FIG. 2 is a side view of a polypropylene diaphragm used in the present invention; 
     FIG. 3 is a schematic diagram of the electroplating apparatus shown in FIG. 1 in which the trough is loaded with a substrate of a vertical length of 21 inches; 
     FIG. 4 is a schematic diagram of the electroplating apparatus shown in FIG. 1 in which the trough is loaded with a substrate of a vertical length of 24 inches; 
     FIG. 5 is a schematic diagram showing a second embodiment of an electroplating apparatus according to the present invention in which the trough is loaded with a substrate of a vertical length of 18 inches; 
     FIG. 6 is a schematic diagram of the electroplating apparatus shown in FIG. 5 in which the trough is loaded with a substrate of a vertical length of 21 inches; 
     FIG. 7 is a schematic diagram of the electroplating apparatus shown in FIG. 5 in which the trough is loaded with a substrate of a vertical length of 24 inches; 
     FIGS. 8A and 8B are respectively a side view and an end view of a side plate used in the electroplating apparatus shown in FIG. 5; 
     FIGS. 9A and 9B are respectively a side view and an end view of a blocking plate used in the electroplating apparatus shown in FIG. 5; and 
     FIG. 10 is a perspective view showing the engagement between the side plate and the blocking plates in the electroplating apparatus shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a first embodiment of an electroplating apparatus according to the present invention is designated generally as  10 . The electroplating apparatus  10  includes a bath  12  for containing an electrolyte which, for clarity purposes, is not shown in any of these drawings. Depending from an upper edge  14  of the bath  12  are two rows of anodes  16 , of which only two are shown here. Within the bath  12  is a trough  18 , which may be lowered into the bath  12  or raised therefrom. The trough  18  includes a number of receiving plates  20 , only one of which is shown here. Each receiving plate  20  includes a V-shaped notch in which an lower edge  22  of a substrate  24  to be plated, e.g. a printed circuit board, may be received, so that the substrate  24  is held in a substantially vertical plane. In FIG. 1, the vertical length of the substrate  24  is 18 inches. On either side of the trough  18  is an electrically insulating polypropylene diaphragm  26 , details of which are shown in FIG.  2  and to be discussed below. 
     Attached to each of the polypropylene diaphragms  26  is an elastic and electrically insulating poly-tetrafluoroethylene sheet  28  which extends across the length of the bath  12 . The poly-tetrafluoroethylene  28  sheets assume a generally horizontal configuration in the bath  12 . The poly-tetrafluoroethylene sheets  28  are of a thickness of about 0.5mm. 
     The trough  18  includes two side boards  30  which, when substrates  24  of a vertical length of 18 inches are loaded into the receiving plates  20 , just contact and bear on the upper surface of the poly-tetrafluoroethylene sheets  28 . 
     Turning to FIG. 2, a polypropylene diaphragm  26  is shown as including an outer impermeable electrically insulating polypropylene frame  32 , with a perforated cloth  34  made of electrically insulating polypropylene mesh material stretched across the inner open area. In this figure, for clarity purposes, only some areas (the five circular areas) of the cloth  34  are shown as perforated. It should however be understood that the whole cloth  34  is perforated. It is apparent that, with such an arrangement, while an electric field can exist across the perforated cloth  34 , no electric field can exist across the polypropylene frame  32 . The polypropylene diaphragm  26  effectively divides the bath  12  into a central “cathode compartment” and two outer “anode compartments”. The holes in the perforated cloth  34  are so sized as to prevent particles such as anode sludge from passing into the central “cathode compartment”, while allowing free movement of the electrolyte therethrough. 
     As can be seen in FIG. 1, although an electric field (as denoted by the hashed lines) can exist across the polypropylene diaphragm  26 , no electric current can pass from under the bottom part  36  of the trough  18  since such is blocked by the poly-tetrafluoroethylene sheets  28  and/or the polypropylene frame  32 . Since the amount of electric current reaching the lower end of the substrate  24  is reduced, less metal will be deposited in this area, thus reducing the “edging effect”. 
     Turning to FIG. 3, the same electroplating apparatus  10  according to the present invention is shown. The substrate  24  received by the receiving plate  20  of the trough  18  is now of a vertical length of 21 inches. It can be seen that while some electric current passes between the anodes  16  and the lower part of the substrate  24 , part of the current is blocked by the impermeable electrically insulating polypropylene frame  32  of the polypropylene diaphragm  26 . It can also be seen that when the trough  18  is lowered into the bath  12 , the side plates  30  push the poly-tetrafluoroethylene sheets  28  away from each other. Due to their elasticity, when the trough  18  is raised from the bath  12 , e.g. after plating, the poly-tetrafluoroethylene sheets  28  will resume their position as shown in FIG.  1 . As shown in FIG. 3, the side boards  30  are in contact with the poly-tetrafluoroethylene sheets  28 , so that there are no gaps between the side boards  30  and the poly-tetrafluoroethylene sheets  28  through which electric current can pass through. However, as compared with the situation in FIG. 1, more electricity passes between the anodes  16  and the lower part of the substrate  24 . 
     As to FIG. 4, again, the same electroplating apparatus  10  according to the present invention is shown. The substrate  24  received by the receiving plate  20  of the trough  18  is now of a vertical length of 24 inches. It can be seen that the trough  18  is much nearer to the bottom of the bath  12 . As the trough  18  is lowered further towards the bottom of the bath  12 , two rows of fingers  38  (only two are shown here for clarity purposes) of the trough  18  push the poly-tetrafluoroethylene sheets  28  further away from each other, so that gaps exist between the side boards  30  and the poly-tetrafluoroethylene sheets  28 . As shown clearly in FIG. 4, more electric current can pass between the anodes  16  and bottom part  36  of the trough  18 . 
     A second embodiment of an electroplating apparatus is shown in FIG. 5 as generally designated as  100 . The structure of the electroplating apparatus  100  is essentially the same as that in the electroplating apparatus  10  shown in FIGS. 1 to  4 , except that the elastic poly-tetrafluoroethylene sheets  28  are replaced by a number of rigid electrically insulating blocking plates  102  which are made of polypropylene, the structure of which will be discussed below. 
     As shown in FIGS. 5 to  7 , the blocking plates  102  operate in essentially the same way as the elastic poly-tetrafluoroethylene sheets  28 , and can be pushed away from each other when a trough  104  is immersed downward. The density of the blocking plates  102  is such that they will float and assume the position as shown in FIG. 5 in the electrolyte unless they are acted upon by the trough  104 . 
     FIGS. 8A and 8B show a side plate  106  with which the blocking plates  102  may be engaged. The side plate  106  includes a back plate  108  fixed with four rods  110 . The side plate  106  includes four holes  112  for enhancing securing thereof to the apparatus  100 . FIGS. 9A and 9B show the blocking plate  102  as also including a rod  114 . As shown in FIG. 10, the rod  114  of each of the blocking plates  102  may be engaged with one of the rods  110  so as to enable the blocking plates  102  to pivot relative to the side plate  106  when acted upon by the trough  104 . 
     From the foregoing discussion, it can be clearly seen that the performance of an electroplating apparatus according to the present invention is versatile and flexible, and can provide satisfactory plating performance of substrates over a wide range of board sizes. 
     It should, however, be noted that the above only describes an embodiment whereby the present invention may be carried out, and modifications and alterations may be made thereto without departing from the spirit of the present invention.