The present invention relates to the field of solid state capacitors. The invention particularly relates to capacitors of the type in which a powder-formed valve action metal forms a highly porous anode body portion of a capacitor, an electrically insulating dielectric layer is formed through the porous structure of the anode body, and a conducting cathode layer is formed on the dielectric layer which layer is then electrically connected to a cathode terminal, the anode body being electrically connected to an anode terminal.
U.S. Pat. No. 5,357,399 (Salisbury) describes a method for simultaneously manufacturing multiple such capacitors from a porous tantalum layer sintered to a tantalum substrate. The layer is machined to form anode body portions of each capacitor. After processing a top plate (substrate lid) is bonded to the processed anode body top ends. The plate forms a lid which, after machining of the substrate/anode body/plate sandwich, becomes the cathode terminal of each capacitor. United Kingdom patent application no. 9824442.9 concerns a modified version of the Salisbury method in which the volumetric efficiency of the capacitors produced is optimized by removing the need for a substrate lid as the cathode terminal of each capacitor, thereby increasing the specific capacitive volume.
The foregoing methods permit the manufacture of very small but highly efficient capacitors. However the continued pressure of electronic circuit board design towards miniaturization of components and ease of assembly of such boards maintains a continued need for capacitors of improved volumetric efficiency and reduced component windows (or footprint) on the circuit board. There is a particular requirement for a method of producing capacitors which have a low profile on a circuit board, in some cases having a thickness of less than 1 mm.
The present invention seeks to provide improved capacitors and improved methods of manufacturing such capacitors.
According to one aspect of the present invention there is provided a method of manufacturing solid state capacitors comprising: providing an electrically conducting substrate; forming a plurality of porous bodies comprising valve action material on a surface of the substrate, the bodies each having an upper surface distal to the substrate; forming an electrically insulating layer over the bodies; forming a conducting cathode layer over the insulating layer applied to the bodies; and dividing the substrate into capacitor portions, each portion comprising a body and a portion of substrate, characterized in that an end region of each body portion distal from the substrate is provided with a platform which is locally raised with respect to the rest of the end region, the platform providing a cathode terminal site in the final capacitor, and the substrate portion providing an anode site.
The platform may consist of or be comprised of the porous valve action material from which the bodies are made. In this case the platform may be formed by green or post-sintered forming of the bodies integrally with the platform. Preferably the platform is formed by green molding of the bodies.
Alternatively the platform may be formed by machining of the sintered bodies. In yet another method the platform is formed by green forming of the platform onto pre-formed bodies.
In another aspect of the invention the platform comprises a solid conducting material. In this case the platform may be applied as a planar frame or lattice applied to upper end regions of the bodies, which frame or lattice is subsequently formed into individual platforms for each body.
In yet another aspect, the platform is formed from a conductive material applied as a coating or paste which solidifies to form the platform.
The method of the present invention will almost always further comprise encapsulating each capacitor body portion with a protective insulating material, leaving exposed an outer surface portion of the substrate and an outer surface portion of the platform.
It may be desired to form a capacitor with anode and cathode poles on a common face or side of the capacitor. Hence in yet another aspect of the invention a conducting bridge is applied to each capacitor, extending the anode terminal site over at least a portion of the encapsulation layer. Preferably the portion of the encapsulation layer to which is applied the conducting bridge includes a region adjacent the cathode site platform, whereby electrical anodic and cathodic terminal contact with the capacitors may be made on a common side of each capacitor at terminals corresponding to the platform and the encapsulated region adjacent the platform. The conducting bridge maybe formed by a conducting end cap applied to a sidewall of the capacitor portion, which end cap overlaps the substrate end of the capacitor and encapsulated portion of the platform end of the capacitor. In a preferred arrangement the conducting bridge comprises two end caps applied to opposite sidewalls, thereby to form two anode terminal contacts, one at side of the capacitor and overlapping the platform end of the capacitor.
Typically, the platforms take the form of rectilinear, circular or oval taps or steps. Usefully, the platform is generally centrally located on a substrate-distal end of each body. In one embodiment the platform is located at one side region of the substrate-distal end of each body.
In another aspect of the invention, two or more locally raised platforms are formed on the distal end of each anode body, thereby to form two or more cathode terminal sites on each body.
Generally, the bodies are arranged on the substrate in an array of rows and columns, and the dividing comprises cutting along the rows and columns as is conventional in the art.
The final processing step is usually a termination process. This applies solder-compatible coatings onto the cathode terminal sites on each platform and the cathode terminal sites. The termination process may comprise liquid coating of each terminal contact surface with conducting paste, and allowing the coating to solidify. In addition, or in the alternative, the termination treatment comprises metal plating to form a layer of metal or metals on the respective terminal sites.
The present invention also seeks to provide structurally novel and inventive capacitors, which may be manufactured according to the method of the present invention.
Hence according to a further aspect of the invention there is provided a solid state capacitor comprising: an electrically conducting substrate member; a porous body comprising valve action metal provided on a surface of the substrate; an electrically insulating layer formed over the free surface of the cathode and anode; a conducting cathode layer formed over the electrically insulating layer on the anode body and the cathode body; wherein the body has an upper surface distal to the substrate, which surface is formed with a locally raised platform with respect to the adjacent upper surface of the body, the locally raised portion providing a cathode terminal site and the substrate providing an anode terminal site.
Preferably the body is encapsulated by a sleeve of a protective insulating material, leaving exposed an outer surface portion of the substrate as the anode site and an outer surface portion of the platform as the cathode site.
A conducting bridge may extend between the anode site to an encapsulated surface portion of body, thereby to form an anode terminal extension contact on the encapsulated body portion. Preferably, the encapsulated surface portion at which the anode terminal contact is formed is located adjacent the cathode site corresponding to the platform, whereby both anode and cathode terminal contacts to a printed circuit board may be made on a common side of the capacitor.
The conducting bridge may comprise one or more conducting end caps applied to one side of each capacitor. The caps may be applied as a liquid conductive paste coating, by for example dipping.
The raised platform portion may be formed by molding of the porous bodies on the substrate. The molding may comprise pressing with a female die and punch arrangement. Alternatively, or in addition the raised portion may be formed by machining of pre-formed bodies.
In a preferred embodiment, the bodies each have a generally flat upper surface and the raised portion takes the form of a step in on the surface. The step may be generally centrally located on the upper surface. Alternatively the step may be located at one side region of the upper surface.
The method may further comprise providing a termination treatment to the exposed anode and cathode terminal contacts, the treatment facilitating soldered electrical connection of the capacitor with an electrical circuit.
Typically, the bodies are arranged on the substrate an array of rows and columns, and the dividing comprises cutting along the rows and columns.
The termination treatment may comprise liquid coating of each terminal contact surface with conducting paste, and allowing the coating to solidify. The termination treatment may further comprise electro-plating, sputter coating or vapor phase deposition on each solidified coating to form a layer of metallic material the respective terminals.
The method may include providing termination means on the terminal portions of the capacitors, thereby to facilitate electrical connection of the anode and cathode bodies to an electrical circuit.
The anode bodies may be arranged on the substrate in rows and columns, and the dividing may comprise cutting along rows and columns to separate the capacitors. The cutting is preferably carried out through a plane or planes perpendicular or substantially perpendicular to the plane of the substrate. The cutting may comprise grinding by for example a grinding wheel.
The capacitor bodies may each be formed from a pre-form layer of porous valve action material that has been applied to the substrate. The pre-form layer may be machined to form the bodies.
Preferably, before dividing, the substrate treated with a protective insulating material which infiltrates in between the bodies to encapsulate sidewalls of the bodies. The dividing process comprises cutting along the channels filled with protective material, thereby to leave a sidewall of protective material around each anode and cathode body of each cathode portion. The insulating material preferably also covers the upper surface of the bodies, other than the raised portion. Alternatively the encapsulation material may be allowed to completely cover the bodies. In this case a top layer of the encapsulation material is removed (by for example machining) to reveal the platform cathode sites.
The protective material may be a resin material which is infiltrated as a liquid and subsequently allowed to set. One suitable material is epoxy resin.
The termination coating may comprise a layer of material comprising a solid dispersion of conductive particles within a carrier matrix. The termination coating may further comprise a layer of metallic plate, such as nickel and tin layers.
Preferably respective exposed terminal sites on capacitors having common side terminals are generally coplanar so that the capacitor may stand on a flat surface with the cathode terminal and anode terminal contacting the flat surface. This makes the capacitor very well adapted for placement on and attachment to a circuit board.
The anode terminal body and the anode body may each be formed from a pre-form layer of porous valve action material that has been applied to the substrate. The pre-form may be applied by laying a green, un-sintered mixture of valve action metal powder and binder/lubricant on the substrate. The green mixture may then be sintered to fuse the powder into a solid highly porous pre-form, the binder/lubricant being removed by washing/dissolution of binder from the bodies before sintering.
The pre-form layer may be machined to form the anode terminal body and the anode body. Typically longitudinal and lateral grinding cuts may be employed in order to produce a network of rectilinear anode and cathode bodies on the substrate, separated xe2x80x9cstreetsxe2x80x9d corresponding to the path of the grinding cut. Naturally more complex shapes can be produced by conventional machining techniques, as required.
The insulating layer may be a dielectric layer of an oxide of the valve action metal, applied for example by conventional anodization techniques in order to build up gradually an oxide of the required thickness and integrity. In one example in which the valve action layer is tantalum, a layer of tantalum pentoxide is built up on the bodies.
The cathode layer may be applied by dipping of the anode and cathode bodies into a precursor solution of, for example manganese nitrate solution. The layer of manganese nitrate formed on the bodies may be heated to oxidize the nitrate to manganese dioxide. Repeated dipping steps may be necessary in order to build-up the optimum cathode layer.
Isolation cutting is typically carried out to remove all cathode layer material bridging the anode and cathode terminal portions of each capacitor. This may conveniently be achieved when the capacitors are undivided on the substrate by a machining cut along the channels separating individual bodies which extends through the cathode layer, and inevitably through the insulating dielectric layer also. To avoid the cutting process a masking resist layer may be laid in the region of the substrate between anode and cathode bodies, before the cathode layer is applied. Subsequent removal of the masking resist also removes unwanted excess cathode layer material.
Typically the termination process involves the application of a first layer of conducting carbon paste which is then cured. Next a second layer of conducting silver paste is applied, and cured. A further layer of metal plate may be applied, for example by electrodeposition. Typically layers of nickel and tin; a layer of tin/lead alloy; or a layer of gold is applied. This provides a solder-compatible surface for electrical connection.
Dividing of the substrate may be achieved by machining by for example a grinding cut. If necessary a rigid backing support may be provided for the substrate to as to provide the necessary structural rigidity to permit cutting without damaging the capacitors.
The material from which the capacitor is made is typically a valve action metal, especially tantalum. However other valve action materials are not excluded from the invention, and these may include metal oxide materials or other materials which the skilled person will recognize as being suitable for use in the processes of the present invention.
Following is a description by way of example only and with reference to the accompanying drawings of methods of putting the present invention into effect.
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