U.S. patent application number 12/614784 was filed with the patent office on 2010-03-04 for anodes with corner and edge modified designs.
Invention is credited to James C. Bates, JR., Christian Guerrero, Randy S. Hahn, Jeffrey Poltorak, John Prymak, Yongjian Qiu, Lance Paul Thornton.
Application Number | 20100053849 12/614784 |
Document ID | / |
Family ID | 40088596 |
Filed Date | 2010-03-04 |
United States Patent
Application |
20100053849 |
Kind Code |
A1 |
Poltorak; Jeffrey ; et
al. |
March 4, 2010 |
ANODES WITH CORNER AND EDGE MODIFIED DESIGNS
Abstract
Porous sintered anode bodies for capacitors formed from valve
metals are treated by electrolysis to form a dielectric layer and
coated with cathode layers. When standard parallelepiped shapes are
used, cathode coverage at the edges and corners is non-uniform and
failures occur at those locations. Rectangular prisms, obround
prisms and cylindrical prisms are formed with transition surfaces
at edges and corners, such as chamfers and curves, to enhance
cathode layer uniformity. The transition surface greatly enhances
the application of polymer slurries.
Inventors: |
Poltorak; Jeffrey; (Fountain
Inn, SC) ; Qiu; Yongjian; (Greenville, SC) ;
Guerrero; Christian; (Travelers Rest, SC) ; Thornton;
Lance Paul; (Simpsonville, SC) ; Hahn; Randy S.;
(Simpsonville, SC) ; Bates, JR.; James C.;
(Brownsville, TX) ; Prymak; John; (Greer,
SC) |
Correspondence
Address: |
NEXSEN PRUET, LLC
P.O. BOX 10648
GREENVILLE
SC
29603
US
|
Family ID: |
40088596 |
Appl. No.: |
12/614784 |
Filed: |
November 9, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11807765 |
May 30, 2007 |
|
|
|
12614784 |
|
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Current U.S.
Class: |
361/528 |
Current CPC
Class: |
Y10T 428/249953
20150401; Y10T 428/24999 20150401; H01G 2/065 20130101; Y10T
428/24917 20150115; Y10T 428/249991 20150401; H01G 9/052 20130101;
H01G 9/15 20130101; H01G 11/48 20130101; Y10T 428/24777 20150115;
Y02E 60/13 20130101 |
Class at
Publication: |
361/528 |
International
Class: |
H01G 9/04 20060101
H01G009/04 |
Claims
1-40. (canceled)
41. Capacitor precursor bodies prepared by the process of pressing
a pellet of an anode in the shape of a regular prism, forming
transition surfaces at edges of multiple surfaces, sintering the
pellet, electrolyzing to form a dielectric oxide on the surface of
the pellet and applying a slurry of a prepolymerized intrinsically
conductive polymer to said oxidized pellet.
42. Capacitor precursor bodies according to claim 44 wherein said
forming transition surfaces at edges of multiple surfaces occurs
during pressing.
43. Capacitor precursor bodies according to claim 44 wherein said
forming transition surfaces at edges of multiple surfaces occurs
after pressing.
44. Capacitor precursor bodies according to claim 44 wherein said
sintering is done prior to forming transition surfaces at edges of
multiple surfaces or after forming transition surfaces at edges of
multiple surfaces.
45. Capacitor precursor bodies according to claim 44 wherein said
transition surfaces are substantially chamfers.
46. Capacitor precursor bodies according to claim 44 wherein said
transition surfaces are substantially curves.
47. Capacitor precursor bodies according to claim 44 having an
anode lead inserted into said pellet before pressing.
48. Capacitor precursor bodies according to claim 44 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
49. Capacitor precursor bodies according to claim 48 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
50. Capacitor precursor bodies according to claim 48 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
51. Capacitor precursor bodies prepared by the process of pressing
a pellet of a anode in the shape of a rectangular prism having
transition surfaces at more than 5 intersections of at least two
surfaces, sintering the pellet, electrolyzing to form a dielectric
oxide on the surface of the pellet and applying a slurry of a
prepolymerized intrinsically conductive polymer to said oxidized
pellet.
52. Capacitor precursor bodies according to claim 51 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
53. Capacitor precursor bodies according to claim 52 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
54. Capacitor precursor bodies according to claim 52 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
55. Capacitor precursor bodies prepared by the process of pressing
a pellet of an anode in the shape of a rectangular prism, forming
transition surfaces at all intersections of at least two surfaces,
sintering the pellet, electrolyzing to form a dielectric oxide on
the surface of the pellet and applying a slurry of a prepolymerized
intrinsically conductive polymer to said oxidized pellet.
56. Capacitor precursor bodies according to claim 55 wherein said
forming transition surfaces at all intersections of at least two
surfaces occurs during pressing.
57. Capacitor precursor bodies according to claim 55 wherein said
forming transition surfaces at all intersections of at least two
surfaces occurs after pressing.
58. Capacitor precursor bodies according to claim 55 wherein said
sintering is done prior to forming transition surfaces or after
forming transition surfaces.
59. Capacitor precursor bodies according to claim 55 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
60. Capacitor precursor bodies according to claim 59 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
61. Capacitor precursor bodies according to claim 59 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
62. Capacitor precursor bodies prepared by the process of oxidizing
a pellet of a valve in the shape of a rectangular prism having
transition surfaces at more than three intersections of three
surfaces.
63. Capacitor precursor bodies according to claim 62 further
comprising an intrinsically conductive polymer wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
64. Capacitor precursor bodies according to claim 63 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
65. Capacitor precursor bodies according to claim 63 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
66. Capacitor precursor bodies prepared by the process of pressing
a pellet of an anode in the shape of an obround prism wherein a
prism shape is changed at least one intersection of two surfaces to
create transition surfaces, sintering the pellet, electrolyzing to
form a dielectric oxide on the surface of the pellet and applying a
slurry of a prepolymerized intrinsically conductive polymer to said
oxidized pellet.
67. Capacitor precursor bodies according to claim 66 wherein said
prism shape is changed at least one intersection of two surfaces to
create transition surfaces during pressing.
68. Capacitor precursor bodies according to claim 66 wherein said
prism shape is changed at least one intersection of two surfaces to
create transition surfaces after pressing.
69. Capacitor precursor bodies according to claim 66 wherein said
sintering is done prior to said prism shape being changed at least
one intersection of two surfaces to create transition surfaces or
after said prism shape being changed at least one intersection of
two surfaces to create transition surfaces.
70. Capacitor precursor bodies according to claim 66 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
71. Capacitor precursor bodies according to claim 70 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
72. Capacitor precursor bodies according to claim 70 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
73. Capacitor precursor bodies prepared by the process of pressing
a pellet of an anode in the shape of a cylindrical prism wherein
the edge of at least one flat surface has been changed to create a
transition surface, sintering the pellet, electrolyzing to form a
dielectric oxide on the surface of the pellet and applying a slurry
of a prepolymerized intrinsically conductive polymer to said
oxidized pellet.
74. Capacitor precursor bodies according to claim 73 wherein said
edge of at least one flat surface has been changed to create a
transition surface during pressing.
75. Capacitor precursor bodies according to claim 73 wherein said
edge of at least one flat surface has been changed to create a
transition surface is after pressing.
76. Capacitor precursor bodies according to claim 73 wherein said
sintering is done prior to edge of at least one flat surface has
been changed to create a transition surface or after edge of at
least one flat surface has been changed to create a transition
surface.
77. Capacitor precursor bodies according to claim 73 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
78. Capacitor precursor bodies according to claim 77 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
79. Capacitor precursor bodies according to claim 78 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
80. Capacitor precursor bodies prepared by the process of pressing
a pellet of an anode in the shape of a rectangular prism, forming
transition surfaces at more than five intersections of two
surfaces, sintering the pellet, electrolyzing to form a dielectric
oxide on the surface of the pellet and applying a slurry of a
prepolymerized intrinsically conductive polymer to said oxidized
pellet.
81. Capacitor precursor bodies according to claim 80 wherein said
forming transition surfaces at more than five intersections of two
surfaces occurs during pressing.
82. Capacitor precursor bodies according to claim 80 wherein said
forming transition surfaces at more than five intersections of two
surfaces occurs after pressing.
83. Capacitor precursor bodies according to claim 80 wherein said
sintering is done prior to forming transition surfaces at more than
five intersections of two surfaces or after forming transition
surfaces at more than five intersections of two surfaces.
84. Capacitor precursor bodies according to claim 80 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 0.25 micrometers.
85. Capacitor precursor bodies according to claim 84 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 1.0 micrometers.
86. Capacitor precursor bodies according to claim 85 wherein said
intrinsically conductive polymer is built up on said pellet to a
thickness of at least about 3 micrometers.
87. Capacitor precursor bodies according to claim 51 wherein said
transition surfaces are formed during pressing.
88. Capacitor precursor bodies according to claim 51 wherein said
transition surfaces are formed after pressing.
89. Capacitor precursor bodies according to claim 51 wherein said
sintering is done prior to forming transition surfaces or after
forming transition surfaces.
Description
FIELD OF THE INVENTION
[0001] This invention relates to optimized geometries for anodes in
solid electrolytic capacitors. More particularly, the invention
relates to modifications to the geometries of capacitor anodes to
facilitate coating of the total surface with a conductive polymer
and avoid uncoated areas where surfaces meet.
[0002] A rectangular prism-shaped anode with rounded, chamfered, or
cut out corners allows for improved coating of the corners and
transition surfaces by cathode layers in an electrolytic capacitor.
Edgeless rectangular, cylindrical, elliptical or obround anodes
allow for improved coverage and reduced stress on the anode. More
particular, the present invention allows corners or edges to be
covered by cathode layers applied by dipping the anode into a
liquid slurry or suspension of the cathode material followed by a
drying or curing step, saving processing steps. The present
invention also provides a means of reducing mechanical stress on
the edges of cylindrical anodes used in hermetically sealed solid
electrolytic capacitors.
BACKGROUND OF THE INVENTION
[0003] The anode of a typical solid electrolytic capacitor consists
of a porous anode body with a lead wire extending beyond the anode
body and connected to the positive mounting termination of the
capacitor. The anode is formed by first pressing a valve metal
powder into a pellet. Alternatively, the anode may be an etched
foil, for example aluminum foil as is commonly used in the
industry. Valve metals include Al, Ta, Nb, Ti, Zr, Hf, W, and
mixtures, alloys, nitrides, or sub oxides of these metals. NbO may
also be used as an equivalent to a valve metal. The pressed anode
is sintered to form fused connections between the individual powder
particles. All anodes are anodized to a pre-determined voltage in a
liquid electrolyte to form an oxide of the valve metal which serves
as the dielectric of a solid electrolytic capacitor. A primary
cathode material, such as a conductive polymer or manganese
dioxide, is subsequently applied via a multi-cycle liquid dipping
process. In order to minimize the equivalent series resistance
(ESR) of solid electrolytic capacitors the devices subsequently are
dipped in a silver paint, which when dried provides a highly
conductive cathode terminal coating. A carbon layer, usually
applied between the primary cathode material and terminal silver
layer, serves as a chemical barrier to isolate the two layers. The
silvered anodes are then assembled and encapsulated to form the
finished devices. The encapsulation process may be a transfer
molding process or conformal coating process to manufacture surface
mount capacitors. Conformal coating with a plastic sealant is often
used to manufacture leaded devices. The industry standard case
sizes for surface mount capacitors are rectangular solids, thus
rectangular anodes or parallopipeds are used in these devices to
maximize volumetric efficiency. In hermetically sealed capacitors
the silvered anodes are placed in cylindrical cans containing a
solder plug. Heat is applied to the can to reflow the solder. After
reflow the solder secures the anode in place and forms an
electrical connection between the cathode and the metallic can. The
anodes used in these devices are cylindrical.
[0004] The reliability of all such devices is highly dependent on
the quality of the external cathode layers.
[0005] The ability to isolate flaws in the dielectric is a
requirement of the primary cathode material chosen for
manufacturing solid electrolytic capacitors. This property of the
primary cathode material results from a process termed `healing`.
The application of voltage to the capacitor causes current to flow
through flaw sites in the dielectric, resulting in an increase in
the temperature at the defect site due to Joule heating. As current
flows through the flaw site the counter electrode material
immediately adjacent to the flaw site is rendered nonconductive.
The temperature of the cathode layer immediately adjacent to the
flaw site also increases due to conduction. When manganese dioxide
is employed as the cathode material, the manganese dioxide
immediately adjacent to the flaw site is converted to manganese
sesquioxide at the decomposition temperature of manganese dioxide
(500-600.degree. C.), thus isolating the flaw. Since the
resistivity of manganese sesquioxide is several orders of magnitude
greater than that of manganese dioxide, leakage currents through
the flaw sites decrease according to Ohm's law. A similar mechanism
is postulated for conductive polymer counter electrodes. Possible
mechanisms to account for the healing mechanism of conductive
polymer films include complete decomposition of the polymer
adjacent to the flaw site, over oxidation of the polymer, and
dedoping of the polymer at the flaw site. At temperatures above
600.degree. C. the amorphous tantalum pentoxide which serves as the
dielectric in tantalum capacitors is converted to a conductive
crystalline state. Thus, in order to be an effective primary
cathode material for tantalum the material must convert to a
nonconductive state at temperatures below 600.degree. C. The
maximum withstanding temperatures of other valve metal oxides is
similar to that of tantalum.
[0006] Since the graphite and silver layers do not decompose to
form nonconductive materials at temperatures below 600.degree. C.,
continuous coating of all dielectric surfaces by the primary
cathode material is essential to prevent the graphite or silver
layers from contacting the dielectric. If the graphite or silver do
contact of the dielectric the device there will be a short
circuit.
[0007] Conductive polymer coatings are applied to the anode using a
variety of methods as described in U.S. Pat. No. 6,072,694. The use
of polymer slurries or liquid suspensions containing
pre-polymerized conductive polymer as an alternative to the monomer
is very attractive due to the simplicity of manufacturing, the
reduction in waste, and the elimination of costly and time
consuming washing steps after each coating step as directed in U.S.
Pat. No. 6,391,379. Although this process approach is attractive it
has not yet been implemented on a production scale. One of the
principle technical obstacles to the successful implementation of a
polymer slurry to serve as the primary cathode layer is the
difficulty coating edges and corners of the anode with slurry.
These materials tend to pull away from corners and edges due to
surface energy effects. The resulting thin coverage at corner and
edges results in reduced reliability of the device. The magnitude
of the force pulling the liquid away from the edge is given by the
Young and Laplace Equation:
.DELTA.p=.gamma./r
Wherein
[0008] .DELTA.p=the pressure difference causing the liquid or
slurry to recede from an edge
[0009] .gamma.=the surface tension of the liquid or slurry; and
[0010] r=the radius of curvature of the edge.
This effect is illustrated in FIG. 1.
[0011] During dipping the liquid phase of a suspension will enter
the pores of the anode. If the particles in the suspension are
larger than the pores, they will be prevented from entering the
anode body and can buildup on the external surface of the anode.
Thus external buildup on the faces and corners of the anode after
dipping in the slurry is somewhat dependent on the void volume
(i.e. density) of the anode. Variations in local density of the
anode can result in non-uniform coating, especially on the corners
and edges of an anode.
[0012] The reliability of a solid electrolytic capacitor is also
degraded due to differences in coefficients of thermal expansion
between the anode bodies and encapsulates material. These
mismatches lead to thermo-mechanical stresses on the
anode/dielectric during surface mounting. These stresses are
greatest at the edges and corners of the anode body. Capacitor
manufacturers rely on the external cathode coatings of carbon and
silver paint to reduce or distribute the stress, especially at high
stress points like corners and edges of the anode. However, the
external cathode layers often are applied in the form of liquid
slurries or suspensions which produce thin coverage at corners and
edges resulting in reduced reliability of the device.
[0013] Capacitor manufacturers have also employed rectangular prism
anode designs with designated rounded or chamfered edges in order
to reduce the thermo-mechanical stress on edges of surface mount
devices after encapsulation. An anode with chamfered edges at the
top of the anode was described by D. M. Edson and J. B. Fortin in a
paper published in the Capacitor and Resistor Symposium in March
1994 entitled "Improving Thermal Shock Resistance of Surface Mount
Tantalum Capacitors." These authors used finite element analysis
and failure site identification techniques to demonstrate that most
failures which occurred during surface mounting were along the top
edges of the anode (surface where the lead projects). A modified
anode design as depicted in FIG. 2 was reported to reduce the
surface mount failure rate.
[0014] An anode with a chamfered portion at the top edge of the
anode (see FIG. 3) is described in U.S. Pat. No. 5,959,831 (Maeda,
et al.). The purported purpose of this design is to reduce the
likelihood of the primary cathode layer wicking up the anode lead
wire during dipping. In U.S. Pat. No. 7,190,572 the inventor claims
that excess edge buildup of manganese dioxide can be avoided by
chamfering the bottom edges of the anode (see FIG. 4). The buildup
of manganese dioxide at the corners is the opposite phenomenon to
that observed when conductive polymers are applied. Also, some
rounding of side edges of pressed anodes has been observed in
capacitors on the market. (see FIG. 5).
[0015] One of the drawbacks of rounded side edges as depicted in
FIG. 5 is the difficulty in pressing reproducible anodes with these
geometries using a radial press design. A radial press design is
defined as a press which compacts the powder in a direction
perpendicular to the anode lead wire (typically the long axis of
the compact). Axial presses are defined as a press which compacts
the powder in a direction parallel to the anode lead wire. Although
axial pressing allows for greater flexibility in anode geometries
of interest to capacitor manufacturers, it often leads to other
problems such as smearing of the powder at the surface as it slides
inside the die cavity and density gradients in the anode in the
axial direction of the anode lead. The high density regions and
powder smearing make it more difficult for the liquid phase of a
slurry or suspension to enter the pores of the anode, exacerbating
the problems of poor cathode coverage. Although powder smearing and
density gradients also occur with radial pressing, they occur to a
considerably lesser degree since the longest dimension of the anode
is typically along the length of the anode lead wire.
[0016] Although rounded and chamfered edge rectangular anodes have
been described and utilized in the industry for years, the concept
of corner chamfering has not been explored. In fact, since the
edges represent a line drawn between two points, the corners,
corner rounding would not be expected to provide benefits beyond
that of edge rounding. However, analysis of many electrically
failed conductive polymer anodes indicates that the dielectric
breakdown mainly occurs on the corners of the anodes, as shown in
FIG. 6.
[0017] Another approach to improving corner coverage would be to
eliminate the corners through the use of cylindrical or obround
anode geometries. However cylindrical anodes are volumetrically
inefficient when used in industry standard case dimensions for
surface mount product. Obround anodes are more volumetric
efficient, but pressing these anodes is generally done on an axial
style press. This leads to density gradients and high densities at
either the top or bottom edge of the anode. These high density
edges are as difficult to fully coat with a slurry as a corner on a
rectangular shaped anode.
[0018] Axial leaded hermetically sealed solid electrolytic
capacitors are extremely reliable capacitors. Because the only heat
introduced in the solder attach process is to the leads, on the
opposite side of the printed circuit board (PCB) as the part, the
temperature rise is small, and damaging forces (mismatch in
coefficients of thermal expansion) created by this process are
minimal. Compared to the forces created in the solder process for
surface mount capacitors (SMT) where the entire capacitor package
is immersed into the high thermal profile of the solder, theses
forces should never create failures. This fact is born out in the
recommended applications of these capacitors. Leaded capacitors may
be used up to 80% of its nameplate voltage, whereas the product is
limited to 50% of its nameplate voltage.
[0019] The big disadvantage for these leaded products is the
susceptibility to mechanical forces created in handling of the
parts. As loose pieces are handled, there is a potential of
dropping the device, crimping the device, or pressing on the device
in the process of moving from package to place in circuit that is
not detectable. If the piece survives the initial electrical
testing, the flaw created by the physical force can grow and become
a circuit failure at a later point.
[0020] Through diligent research the present inventors have found
that polymer coverage and leakage are improved by various
techniques of modifying the corners of anodes to improve the corner
coverage with the primary cathode material. Problems of poor edge
coverage on obround or cylindrical anodes can be overcome by
modifying the edges of these anode designs. The reliability of
hermetically sealed devices can be improved by similarly modifying
the edges of cylindrical anodes used in this style capacitor.
SUMMARY OF THE INVENTION
[0021] It is an object of the present invention to provide anode
designs which facilitate corner and edge coverage by the cathode
layers, especially the primary cathode layer.
[0022] It is another object of this invention to provide anode
designs with modified corner geometries which are readily pressed
using conventional radial presses.
[0023] Another object of this invention is to achieve corner
rounding with minimal loss of anode volume.
[0024] It is yet another object of this invention to provide anode
designs suitable for use with conductive polymer slurries.
[0025] Yet another object of this invention is to provide anode
designs with improved leakage characteristics relative to
conventional anodes.
[0026] These and other advantages are provided in anodes with
modified edge and corner geometrics through the use of transition
surfaces such as rounded or chamfered corners and edges and by an
anode with a groove cut in each corner of the anode.
[0027] Yet another embodiment of the present invention is provided
by a cylindrical or obround anode with grooves cut along either the
top or bottom edge, by an edgeless rectangular anode, by an
edgeless obround anode, and by an edgeless cylindrical anode.
DESCRIPTION OF DRAWINGS
[0028] FIG. 1 is an illustration of a liquid receding from an edge
due to surface tension effects.
[0029] FIG. 2 is a depiction of prior art top edge chamfered
anode.
[0030] FIG. 3 is a depiction of prior art top edge chamfered
anode.
[0031] FIG. 4 is a depiction of prior art bottom edge chamfered
anode.
[0032] FIG. 5 is a cross sectional view of prior art rounded side
anode.
[0033] FIG. 6A and FIG. 6B indicate the failure site location of
anodes following a breakdown voltage test.
[0034] FIG. 7 is a depiction of a corner chamfer anode design.
[0035] FIG. 8 is a depiction of an edgeless rectangular anode.
[0036] FIG. 9 is a depiction of an edgeless obround anode.
[0037] FIG. 10 is a depiction of an edgeless cylindrical anode.
[0038] FIG. 11 depicts the mechanical forces acting on a
hermetically sealed device.
[0039] FIG. 12 depicts edge rounding of cylindrical anode in
hermetically sealed construction.
[0040] FIG. 13 depicts a rectangular prism for illustration of
surfaces, edges and corners.
[0041] FIG. 14 depicts a rectangular prism in perspective view.
[0042] FIG. 15 is a depiction of a corner cut anode design
[0043] FIG. 16 depicts polymer coverage of corner cut anodes
[0044] FIG. 17 depicts polymer coverage of conventional anode with
dielectric show through evident at corners due to incomplete
polymer coverage.
[0045] FIG. 18 plots leakage for standard and corner cut anodes
[0046] FIG. 19 depicts polymer coverage on anodes with rounded
corners.
[0047] FIG. 20 depicts polymer coverage on anodes with conventional
corners
[0048] FIG. 21 depicts polymer coverage at the top of an obround
anode pressed on an axial press.
[0049] FIG. 22 depicts polymer coverage at the bottom of an obround
anode pressed on an axial press.
DETAILED DESCRIPTION OF THE INVENTION
[0050] A porous pellet is prepared by pressing a powder and
sintering to form a porous body. The pellets may be made from any
suitable material such as tantalum, aluminum, niobium, hafnium,
zirconium, titanium, or alloys of these elements, nitrides and
suboxides. Tantalum and ceramic niobium oxide are the preferred
materials. Tantalum is the most preferred material. The sintered
pellet is then anodized to form the oxide film which serves as the
dielectric of the capacitor. The internal surfaces of the anodic
oxide film are next coated with a primary cathode layer. Manganese
dioxide may be applied as a primary cathode layer by dipping in
manganous nitrate solution and converting the nitrate to manganese
dioxide via heating in a pyrolysis oven. Typically the conversion
step is carried out between 250.degree. and 300.degree. C.
Alternatively, an intrinsically conductive polymer can be employed
as the primary cathode layer. The conductive polymer material is
typically applied as a monomer using either a chemical oxidative
process or by dipping in a preformed polymer slurry. In the case of
a chemical oxidative process, byproducts of the reaction are
removed by washing and typically multiple dips and washings are
required prior to a reanodization process used to isolate the
defect sites in the dielectric. The pellets are then placed in
suitable electrolyte bath, for instance a dilute aqueous phosphoric
acid solution with a conductivity in the range 50 to 4000 micoS/cm.
Voltage is applied to drive the process which causes isolation of
the dielectric flaw sites This process may not be required in the
case of application of the conductive material by dipping the
anodes in a preformed (prepolymerized) polymer slurry. The process
is repeated to insure complete coverage of the internal and
external dielectric surfaces. The components are subsequently
dipped in a carbon suspension to coat the external surfaces of the
primary cathode material. A silver layer is formed by dipping the
device in a silver paint to form an external coating.
[0051] FIG. 1 depicts the manner in which a liquid or slurry pulls
away from an edge or corner due to surface tension effects. FIG. 2
depicts prior art in which the top edges of a surface mount
capacitor were chamfered to reduce stress on those edges. FIG. 3 is
a depiction of an anode with chamfered top edges as described in
U.S. Pat. No. 5,959,831 (Maeda, et al.). FIG. 4 depicts chamfering
of bottom edges as depicted in US 2005/0231895 A1. FIG. 5 is a
picture of a commercial capacitor which has been cross sectioned to
reveal rounded edges. FIG. 6 is a picture of capacitors following a
breakdown test indicating the failures occurred on the corners of
the anode. In a breakdown voltage test, a power supply, resistor,
fuse, and capacitor are placed in series. The voltage applied to
the capacitor is increased until the capacitor breaks down as
indicated by the blown fuse. Especially in the case of capacitors
with polymer slurry cathode the failure site occurs predominantly
on the corners of the anode which are poorly coated by the polymer
slurry.
[0052] For purposes of understanding the invention, reference is
made to FIG. 13. FIG. 13 shows a rectangular prism or a
parallelopiped. The X, Y, and Z axes are defined with respect to
origin "O." The exposed surfaces are labeled XY, XZ, and YZ. An
edge is defined as the intersection of two surfaces. A corner (or
point) is defined as the intersection of three surfaces or three
edges.
[0053] Modification of an edge can be defined by reference to FIG.
14. FIG. 14 represents an anode in perspective view. A surface XZ
with a length X' and width Z' represents a first external surface
of an anode. A surface YZ with a length Y' and width Z' represents
a second external surface of an anode. For conventional anodes XZ
and YZ meet to form a right angle at an edge. In an edge modified
design the first surface XZ will deviate at point a and distance
X'' from the edge, e, which is the projected intersection of XZ and
YZ. The second surface of the anode will deviate from YZ at point b
and distance Y'' from e. This deviation creates at least one
additional surface, herein defined as a transition surface, TS. In
one embodiment the deviation is a straight diagonal line between
points a and b wherein the transition surface creates a chamfer. In
another embodiment the transition surface is a non-linear, curved,
or radiused edge. Edge modified designs as defined here refers to
any deviation of the external surface from XZ and YZ such that:
0.03 mm<X''<0.5X'
and
0.03 mm<Y''<0.5Y'.
[0054] The concept can be extended to a third dimension of a
conventional rectangular prism. A corner, c, is defined by the
projected intersection of three surfaces YZ, XZ and XY. The surface
XZ with a length X' and width Z' representing an external surface
of an anode. In a corner modified design the surface XZ will also
deviate at point d and distance Z'' from c. A corner modified
design as defined herein refers to any deviation of the external
surfaces such that:
0.03 mm<X''<0.5X'
and
0.03 mm<Y''<0.5Y'
and
0.03 mm<Z''<0.5Z'.
[0055] In a conventional SMT the anode shape is a regular
rectangular prism as illustrated in FIG. 13. The surfaces all
intersect at right angles (or approximations thereof), providing
six surfaces and twelve edges.
[0056] According to this invention, most or all of the edges are
modified to form transition surfaces. The transitions may be flat
as in a traditional chamfer or bevel. Alternately, the transition
may form multiple chamfers including, in the limit, a curved
surface such as would be obtained using a corner round router
bit.
[0057] When rounded edges intersect, a quarter of a hemisphere is
formed which maybe regular, as when all radii of generation are
equal or compound when the radii of the generating curves
differ.
[0058] Referring again to FIG. 14, it is apparent that the size of
a straight bevel or chamfer can be defined in terms of X'', Y'',
and/or Z''. Since there are twelve edges and eight corners formed
by six surfaces, a great variety of shapes can be formed when the
lengths X, Y and Z differ from each other or when different edges
are chamfered or when only corners are chamfered. Depending upon
the size of the anode-case size-different transition surface shapes
and sizes are found to be preferred.
[0059] As a first example of a body having a transition surface,
reference is made to FIG. 7. Anode body 71, having six planar sides
73, 74, 75, 76, 77, and 78 and an anode lead 79 has been chamfered
at each corner to provide transitional planar surfaces 81, 82, 83,
84, 85, 86, 87, and 88. This shape directly addresses the problem
with corner coating as illustrated in FIG. 6A and FIG. 6B. This is
a corner chamfer anode.
[0060] When edges and corners are all curved, the result is an
edgeless shape as shown in FIG. 8. Three transitional surfaces are
present, a short side curved transitioned surface 91, a long side
curved transitional surface 93 and a corner quarter hemisphere 95.
In the preferred embodiment, all radii of generation are equal but
such is not necessary. For small case sizes a greater radius in the
Z direction may be preferred.
[0061] When the curvature at the edges in the YZ surface is
expanded to become a continuous curve, the resultant figure is an
obround prism as shown in FIG. 9. The YZ surface has been replaced
with a curved surface, such as semicircular in cross-section. In
the preferred embodiment, the transition surfaces form the XY
surface and from the semicircular side are radiused into the XZ
surface (cf. 103) and the transition surface from the XY surface
(cf. 105) are radiused into the XZ surface. Such an anode has no
sharp edges save for some flashing at the points of juncture of the
dice employed. The XY surface of an oblong prism may be flat or
curved.
[0062] Extrapolation of the edgeless obround shape of FIG. 9 is the
edgeless cylinder of FIG. 10. A cylindrical anode has traditional
round sides 107, but the transition surface 111 to the flat top 109
(and bottom, not shown) is chamfered or, in the drawing, rounded or
curved to make a smooth transition from side to top.
[0063] When the basic prism shape is obround, the edges and corners
may have consistent or changing radii, but the chord for the curve
is defined using the same criteria as for a chamfered surface. When
the figure is a cylinder, the radius of the circle of origin
becomes one length, and the height of the cylinder becomes the
other length, i.e., the intersection of planar surface and
circumferential surface is characterized as 0.03 mm<R<r and
0.03 mm<H<h/2 where r and h are the radius of the circle of
origination and H the height of the cylinder.
[0064] The edgeless cylinder has particular application in
hermetically sealed leaded devices.
[0065] Failure site analysis reveals that the vast majority of
failures, up to 95%, will appear on the edges of the cylindrical
anode. These edges are most susceptible to any outside forces
applied to the case wall (FIG. 11). In between these edges, the
pellet structure offers a strong resistive structure that will
spread the force and absorb it. In between the edge and the sealed,
top of the case, the case can compress to absorb the force. At the
edges, the forces can create a fracturing force on the pellet. The
relative stresses are in the order S.sup.1, <S.sup.2,
<S.sup.3. The top edge (nearest the anode seal) is more
susceptible than the bottom edge (nearest the cathode lead) as the
closed end of the barrel adds stiffness here.
[0066] In order to mitigate this failure mechanism the edges of the
pellet can be rounded. By eliminating the sharp edge (FIG. 12), the
amount of force required to fracture or chip the pellet increases
tremendously. Once the pellet is soldered in the case, the sharp
edges would have been eliminated and replaced with a tapering
solder thickness. The radiused or rounded elements nearest the
outer diameter have capabilities of spreading blunt forces through
the case. The radiused elements furthest from the outer diameter
have thicker solder which creates additional buffering.
[0067] It has been found that a second approach to enhancing
coverage is surprisingly effective. An anode having cut-away
portions at the corners--hereinafter a corner cut anode--is
effective in collecting conductive polymer at the corners during
the coating process. FIG. 15 shows a preferred corner cut anode
sintered body. At the juncture of three surfaces 73, 75 and 78, two
cuts are made to create two additional transitional surfaces, 121
and 123. This pattern is repeated at the other seven corners to
form "pockets." The improvement may be seen in FIG. 16 when
contrasted with FIG. 6 and FIG. 17. While not being bound by any
theory, it is seemed that monomer, and subsequently polymer,
accumulates on the surfaces of the transition surface 121, 123 and
compensates for the thin or incomplete layers found in standard
rectangular parallelepiped shape for anodes. The corner cut anode
seems particularly suitable for dipping in polymer slurries.
[0068] Polymer slurries of intrinsically conductive polymers are an
alternative coating methodology to the formation of polymer from a
monomer and catalyst on the surface of the oxidized pellet.
Slurries may be applied using a cross-linking agent as disclosed in
U.S. Pat. No. 6,451,074. The use of slurries reduces the number of
coating steps when making the capacitor and reduces the loss of
monomer due to contamination. U.S. Published Application No.
2006/02336531 discloses polythiophene particles with filler as a
coating material of conductive polymer. Any intrinsically
conductive polymer may be used. Polyaniline is preferred due to
ease of handling. Coating thickness should be at least 0.25
micrometers, preferably at least 1 micrometer and optimally at
least 3 micrometers to obtain complete coverage of all edges. The
use of anode pellets with transition surfaces at the end and/or
sides away from the anode lead allows reliable mechanical dipping
into the slurry with minimal deposition of polymer on the anode
lead. The capacitor precursor then may be coated with graphite and
Ag, a cathode lead attached and final assembly performed.
[0069] A fluted anode is one which has surfaces which are not
substantially flat. The variations in the surface may be, but are
not necessarily symmetrical or repeated in a pattern. Examples of
fluted anodes may be found in U.S. Pat. Nos. 7,154,742; 7,116,548;
6,191,936; and, 5,949,639. The capacitors disclosed in these
references are pressed to have substantially flat ends where anode
lead projects and at the opposite end. Most have flat sides except
for the penetrations into the body of the anode. Multiple sharp
edges are present and present challenges when coatings are
applied.
[0070] Modifications of the external surfaces to remove sharp
angles results in improved coating. The edges and/or corners may be
chamfered or curved in the manner of FIGS. 7 and 8 to achieve a
more uniform coating of the polymer. Triangular corners as shown in
FIG. 7 and notched corners such as shown in FIG. 15 are also
preferred. Internal surfaces, meaning those wholly within the
interstices of the flutes need not be modified. In preferred
embodiments, multiple flat wires are used as anode leads.
Example 1
[0071] Commercial electronic grade 22,000 CV/g tantalum powder was
pressed to form anodes to a density of 5.5 g/cc with dimensions
4.70.times.3.25.times.1.68 mm using a radial action press. The
punches of the press were modified to create a notch or v-cut in
each corner of the anode as depicted in FIG. 15. This modification
to the corners is referred to as corner cut anode designs. The
sintered anodes were anodized at 100 volts in an aqueous phosphoric
acid electrolyte maintained at 80.degree. C. The parts were
subsequently dipped in liquid suspensions containing
pre-polymerized polyethelyenedioxthiophene (PEDT). Photomicrographs
were taken to determine the degree of polymer coverage on the
corners of the anodes (FIG. 16). After application of a conductive
polymer slurry the parts were dipped in a carbon suspension used
for commercial tantalum conductive polymer capacitors. The anodes
were dipped in an electronics grade silver paint prior to assembly
and encapsulation to form surface mount tantalum capacitors. After
encapsulation 25 volts was applied to the capacitors and leakage
was read through a 1 k ohm resistor after allowing 60 seconds for
the capacitors to charge. The results were plotted in FIG. 17.
Comparative Example 1
[0072] Commercial electronic grade 13,000 CV/g tantalum powder was
pressed to a density of 5.5 g/cc with dimensions
4.70.times.3.25.times.1.70 mm using a radial action press.
Conventional punches were used which created well defined corners
typical of anodes used in the industry. The sintered anodes were
anodized to 130 volts in an aqueous phosphoric acid electrolyte
maintained at 80.degree. C. The parts were subsequently dipped in
liquid suspensions containing pre-polymerized
polyethelyenedioxthiophene (PEDT). Photomicrographs were taken to
determine the degree of polymer coverage on the corners of the
anodes (FIG. 18). After application of the conductive polymer
slurry the parts were dipped in a carbon suspension used for
commercial tantalum conductive polymer capacitors. The anodes were
dipped in an electronics grade silver paint prior to assembly and
encapsulation to form surface mount tantalum capacitors. After
encapsulation 25 volts was applied to the capacitors and leakage
was read through a 1 k ohm resistor after allowing 60 seconds for
the capacitors to charge. The results and comparison were plotted
in FIG. 18 wherein DCL is direct current leakage and PE is
post-encapsulation. A comparison of the polymer coverage and
leakage distributions after encapsulation demonstrates the
improvements obtained with the corner cut anode design relative to
prior art.
Example 2
[0073] Commercial electronic grade 13,000 CV/g tantalum powder was
pressed to a density of 5.5 g/cc with dimensions
4.57.times.3.10.times.1.63 mm using a pill style press. The lead
wire is attached after pressing with this type of press. The action
of this style press generates anodes with rounded corners on one
side of the anode. The corners on the opposite side of the anode
are sharp, well defined corners. The sintered anodes were anodized
to 130 volts in an aqueous phosphoric acid electrolyte maintained
at 80.degree. C. The parts were subsequently dipped in liquid
suspensions containing pre-polymerized polyethelyenedioxthiophene
(PEDT). Photomicrographs were taken to determine the degree of
polymer coverage on the rounded corners of the anodes (FIG. 18).
Photomicrographs taken of the opposite side of the anode
demonstrates the poor polymer coverage on the sharp well defined
corners of the anode (FIG. 19). These pictures clearly indicate the
need to modify the corners of the anodes in order to obtain
sufficient coverage using slurries or suspensions to apply cathode
layers.
Comparative Example 2
[0074] In order to eliminate the corners completely an axial press
was used to press obround anodes. Commercial electronic grade
22,000 CV/g tantalum powder was pressed to an average density of
5.5 g/cc with dimensions 4.70.times.3.25.times.0.81 mm. An obround
shaped die was used to press an anode without corners. The sintered
anodes were anodized to 100 volts in an aqueous phosphoric acid
electrolyte maintained at 80.degree. C. The parts were subsequently
dipped in liquid suspensions containing pre-polymerized
polyethelyenedioxthiophene (PEDT). Photomicrographs were taken to
determine the degree of polymer coverage on the anodes. Polymer
coverage at the top of the anode, where the density was less than
5.5 was acceptable (FIG. 20). However, at the bottom of the anode
where the press density was greater than 5.5 the edges of the anode
were not covered with polymer (FIG. 21). The density gradient
observed in these anodes is characteristic of anodes produced on an
axial press.
[0075] The invention has been disclosed in regard to preferred
examples and embodiments which do not limit the scope of the
invention disclosed. Modifications apparent to those with skill in
the art are subsumed within the scope and spirit of the
invention.
INDUSTRIAL UTILITY
[0076] The disclosed invention improves quality and durability of
capacitors in electronic devices
* * * * *