Abstract:
The object of the invention is a lead frame for electrolytic capacitor anodes characterized by the fact that it comprises, for each anode to be connected, a tab with near its free end a window, the said window being sufficiently large to stop the rise by capillary action which can occur during manufacture of electrolytic capacitors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention concerns the manufacture of electrolytic capacitors, more particularly the lead frames for electrolytic capacitor anodes. 
     At present, electrolytic capacitors are generally made using an aluminium or tantalum anode and an electrolyte which may be solid or liquid. 
     A process of manufacture of capacitors with aluminium and with a solid electrolyte comprises several stages, a similar process being usable for capacitors with tantalum and a solid electrolyte. These stages are principally: 
     construction of the anode block either by compacting an aluminium wire, or by etching, folding or rolling of a sheet of aluminium, or by sintering; 
     soldering of the anode block onto a tab of a lead frame; 
     anodization of the aluminium block to form a thin layer of alumina (oxidation); 
     formation of the solid electrolyte (impregnation and pyrolysis); 
     formation of the cathode (graphitization and/or silvering); and 
     molding or potting. 
     The manufacturing process of these capacitors is rather complicated and the problems presently encountered during the manufacture of electrolytic capacitors are of several types. 
     The connection of the anode block to the tab of the lead frame, which is performed by soldering, is subject to dimensional constraints to be respected to take into account the different phases of manufacture. 
     To deposit the electrolyte and then the graphite, the anodized blocks are soaked in different solutions which have good wettability, and it can be observed that the conducting electrolyte and graphite rise by capillary action around the solder. There is therefore a risk of short-circuiting between the non-oxidized anode and the cathode constituted by the electrolyte. This rise by capillarity is not controllable and varies depending on the materials and their surface states. 
     To prevent such short-circuiting, dimensions are imposed: a minimum height for the oxidation of the tab of the lead frame and a lower minimum height for electrolytic impregnation and graphitization, taking into account the capillary rise. There is therefore an oxidized part of the anode which is not impregnated and which is inactive from a capacitive point of view. 
     SUMMARY OF THE INVENTION 
     The present invention proposes a mechanical means to eliminate this capillary effect, while enabling the necessary dimensions to be reduced. By reducing the height of oxidation with respect to that of impregnation and graphitization, it decreases the loss of active volume. 
     The invention consists of a lead frame for electrolytic capacitor anodes, characterized by the fact that it comprises, for each anode to be connected, a tab with, close to its free end, a means preventing the rise by capillary action, without hindering the soldering of the anode block. 
     The invention also concerns a process for manufacture of an electrolytic capacitor using such a lead frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and other advantages will appear on reading the description of a preferred mode of embodiment, with reference to the appended drawings of which: 
     FIG. 1 represents a lead frame for electrolytic capacitors according to the prior art; 
     FIG. 2 represents a lead frame for electrolytic capacitors according to the invention; and 
     FIG. 3 represents a variant embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows part of a lead frame 1. This frame can be made of aluminium or tantalum or any other metal compatible with the anode of the capacitor. It is provided with tabs 2 of the same material. On one of them has been soldered the connecting wire 3 from an anode block 4. This soldering 5 is performed before soaking the anode in the various baths for oxidation of the anode block, then the deposition or impregnation of the electrolyte and finally the graphitization. After soaking in these baths, a rise by capillarity 6 is seen around the solder 5, to a height which is not controllable. If the height Y represents the tolerance for the bath level in the impregnation and graphitization phases, then the height X is the height lost for the active volume of the capacitor. For safety reasons, the oxidation must go up to the limit h on FIG. 1, which for given external dimensions reduces the active volume of the capacitor. 
     A mode of embodiment of the present invention is represented in FIG. 2. In this case, the lead frame 1&#39; has been improved over that seen previously. A rectangular window 7&#39; is cut in the tab 2&#39; leaving sufficient space between itself and the edge of the tab 2&#39; to enable soldering 5&#39; of the anode block 4&#39;. The window 7&#39; stops the rise by capillarity 6&#39; and imposes the height Z&#39;. This height Z&#39; is fixed. For this reason, the oxidized level can safely be brought nearer. As a consequence, the distance X&#39; between the soldering and the oxidation level is less than the distance X and has been reduced by the difference between Z and Z&#39;. 
     The window 7&#39; may be of various shapes. It may be rectangular, round, or of any shape as long as its dimensions are sufficient to form a barrier to the capillary rise, while still leaving a sufficiently solid connection tab. It is preferably at least 0.3 mm thick and its length is preferably 0.5 to 0.7 times the width of the tab. 
     As an example, a window was made 0.3 mm thick and 1.6 mm wide on a connection tab 2.4 mm wide. This window is 0.3 mm from the free end of the tab. For a connection made by a soldering of dimension 0.3 to 0.4 mm, of a 0.3 mm wire on such a connection tab, wetting is observed over a length of 0.9 to 1.2 mm. This wetting is not controllable and varies depending on the materials and their surface states; but it should be noted that wetting is greater with graphite than with manganese nitrate. The present invention therefore limits the extent of this wetting. 
     This invention also has other advantages. 
     The window can also be used as a visual level mark during the various steps in manufacture. 
     The window 7&#39; enables soldering of the anode block 4&#39; at the very edge of the tab 2&#39; of the lead frame 1&#39;, avoiding the waste of a length of inactive anode. 
     During the final potting phase, the resin used penetrates the window 7&#39; and thus provides better anchorage in the connection tab. 
     The window 7&#39; may be pierced before or after the soldering 5&#39; of the anode blocks 4&#39; on the tabs 2&#39; of the lead frame 1&#39;, and in either case, the window 7&#39; enables excess wire 3&#39; to be cut off above the solder 5&#39;. 
     According to a variant of the present invention represented in FIG. 3, the window 7&#34; can be relatively thick. In this case, the window serves to identify the anode 2&#34;. FIG. 3 then represents part of a lead frame 1&#34; whose tabs 2&#34; are pierced by a window 7&#34; which is thicker than that in FIG. 2. Consequently, once the capacitor 4&#34;  is finished, the anode 2&#34; can be distinguished from the cathode 8&#34;, thanks to the window 7&#34; which is not completely potted or molded.