BACKGROUND OF THE INVENTION
The present invention relates to dielectric formation in the manufacture
of solid state electrolytic capacitors, i.e. tantalum capacitors, and more particularly
relates to selectively controlling the thickness of a dielectric formed on portions
of an anodized electrode.
Solid state electrolytic capacitors are well known in the art and
are typically fabricated by forming a pellet of powdered metal (e.g. tantalum)
about an anode rod, sintering the pellet to form a porous mass bonded to the rod,
forming a dielectric coating through the interior of the mass and exposed portions
of the rod, e.g. by an anodizing step in a phosphoric acid bath, forming a conductive
counter-electrode or cathode by overcoating the dielectric via manganizing or
conductive polymer dipping, and terminating as by forming graphite and silver layers
over the exterior surface of the counterelectrode.
The step of anodizing is a crucial step in the formation of a dielectric
layer for use in solid state electrolytic capacitors, such as aluminum and tantalum.
When, for example, an anodization is performed on a sintered anode in an electrolyte
solution containing, for example, phosphoric acid, the thickness of the dielectric
formed is in direct proportion to the time and applied voltage. The dielectric
layer formed will be uniform throughout the anode as long as all portions of the
anode are fully wetted by the electrolyte used in the process. In other words,
the above-described conventional anodization process is non-selective, i.e., provides
no mechanism for varying a depth of the dielectric on various portions of and within
the anode body.
The inner body of the anode is responsible for contributing most
of the finished capacitor's capacitance value. Consequently, the thickness of dielectric
interposed between the anode and counter electrode directly defines the resulting
capacitance. As a thickness of dielectric increases, the capacitance decreases
but with consequent increase in working voltage of the capacitor. An increased
dielectric thickness on the anode lead and the external anode surface (as distinguished
from the inner portion) of the anode body provides increased protection against
stress-related and other mechanical damages generated in post-anodization processes,
such as in a step of manganizing the dielectric-coated anode. A dilemma therefore
exists utilizing conventional anodization methods, which are non-selective, whether
anode lead and body surface will receive a thicker dielectric layer and, hence,
greater mechanical and electrical integrity at the expense of decreased capacitance.
It would thus be desirable to provide a means for forming an increased dielectric
thickness on external portions of the capacitor without increasing the thickness
of the interior capacitance forming dielectric.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process
for manufacturing a solid state electrolytic capacitor which overcomes the above-described
shortcomings of the prior art.
It is another object of the present invention to provide a process
for manufacturing a solid state electrolytic capacitor by which a thicker dielectric
layer may be provided on portions of the anode's lead and external body surface
without substantially decreasing the capacitor's capacitance value.
It is yet another object of the present invention to provide a process
for manufacturing a solid electrolytic capacitor where a dielectric thickness on
different anode portions is selectively controlled.
It is still yet another object of this invention to provide a solid
electrolytic capacitor which is formed utilizing a selective anodizing process
such that a dielectric thickness on anode lead and anode body surface is selectively
increased without substantially further impregnating the anode body with the dielectric.
A method for manufacturing a solid electrolyte capacitor is provided
which enables the selective control of a dielectric thickness at various portions
of an anode during an anodization process. In a preferred form of the given method,
the anode is anodized twice, once in a non-selective process to render the desired
dielectric thickness within and over the porous anode body and then selectively
anodized a second time such that only the thickness of dielectric covering the
lead and body surface are substantially increased.
In a preferred form, the method includes a step of anodizing an anode
using a conventional method to form a capacitance defining dielectric coating thereon
at desired thickness. A step is then implemented where the anodized anode is soaked
with a solvent to prevent the bulk or body of the anode from direct contact with
an electrolyte utilized in a subsequent anodization step. Solvent is removed from
portions, i.e. external surfaces and the solvent-treated anodized anode is then
anodized in an aqueous-based electrolytic solution, whereby selective anodization
occurs only at the lead and anode surface, i.e. the surfaces free from solvent.
In a final step the solvent is evaporated, rendering an anode with a greater amount
of dielectric material layered on its anode lead and surface, than the amount
layered within the anode body.
The solvent utilized within the process is preferably an organic
solvent that is water-immiscible. In addition, the organic, water-immiscible solvent
preferably displays a reasonably high boiling point and has a low viscosity to
facilitate penetration. In addition, it is preferred that the solvent is easily
impregnated into the dielectric coated anode body and easy to remove. One group
of solvents fitting the above limitations includes benzene, dichlorobenzene, trichlorobenzene,
trimethylpentane, heptane, propylene carbonate, toluene, xylene, etc.; where toluene
and xylene are preferable.
Capacitors formed according to the improved process of this invention
show strong capacitance values as well as better imperviousness to stress and mechanical
damage at the lead and anode surface particularly during the subsequent steps
of manufacturing, i.e. counter-electrode formation via MnO2 formation and dipping.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to a method of selectively anodizing
a capacitive anode body for the manufacture of solid state electrolytic capacitors,
for example, capacitors formed of powdered and subsequently sintered metals, i.e.
tantalum, niobium and aluminum. An ideal result of the process of this invention
is a capacitor or capacitor preform, the anode lead and body surface of which
is strengthened by a dielectric layer, which is thicker than the thickness of the
capacitance forming dielectric provided within the anode body without capacative
value loss as a result of additional dielectric thickness within the anode body.
The preferred process begins with a step of non-selectively anodizing
an anode to form a dielectric layer on the lead, in the body or bulk of the anode,
as well as the anode's surface. The thickness of the dielectric is uniformly controlled
to the specified thickness by the conventional anodizing process. A second step
requires that the dielectric-coated and impregnated anode be soaked in a solvent
to prevent the internal capacitance forming dielectric components from direct contact
with an aqueous electrolyte in a subsequent anodization process. Concomitantly,
due to the removal of surface solvent coatings the anode lead and surface will
not be prevented from direct contact with the electrolyte.
Preferably, the solvent is an organic solvent. One method of implementing
the soaking step is by dipping the anode in a solvent bath. The conventionally
coated anode should be dipped in a solvent for around 10 seconds. In addition,
it is preferable that the solvent be water immiscible and have a boiling point
which is higher than 100°C, and be easily removable from the anode. Of the many
solvents which fit the defined solvent category, several are: benzene, dichlorobenzene,
trichlorobenzene, trimelhylpentane, heptane, propylene carbonate, toluene, xylene,
The next step in the preferred process includes preferential removal
of solvent from surface areas intended to be coated more thickly with dielectric
and subjecting the solvent-soaked dielectric coated anode to a second selective
anodization process within the preferably aqueous-based electrolyte solution.
The portions of the anode impregnated with solvent, i.e., the anode's interior,
will be impervious to application of the second dielectric coating due to the
barrier formed by the solvent. A thicker dielectric is thereby formed only on the
lead and anode surface areas from which solvent is removed. The added thickness
is derived in proportion to a higher applied formation voltage. The result is
an anode with a lead and anode surface that is thicker than the non-selectively
formed dielectric in the anode body or bulk, minimizing possible capacitance lose
while increasing ruggedness. A final step includes heating the anode to a temperature
above the boiling point of the organic solvent. Accordingly, the impregnated organic
solvent is removed by evaporation.
The following examples evidence a beneficial increased anode lead
and anode body surface thickness without detrimental capacitance value decrease.
A batch of sintered tantalum anodes were anodized by submersion in
a 0.1% phosphoric acid bath at 85°C and a formation voltage of about 85 Volts for
one hour. A counter electrode was conventionally applied to examples of the batch
and the capacitance measured at an average of about 10 microfarads. Other examples
of the batch (without counter electrode) were soaked in toluene for approximately
ten seconds. Thereafter, the saturated items from the batch were again anodized
following solvent removal from the surface (i.e., selectively anodized) under the
above-described conditions, but at a formation voltage of about 110 volts for
thirty seconds. Due to the presence of the solvent, substantially no dielectric
was added within the anode body. The anodes were thereafter heated to 150°C for
10 minutes to release impregnated solvent from the bulk of the anode, i.e. the
Physical observations of the finished anode body surface and lead
found that the second, selective anodization resulted in a change in color from
green to gold. The color change evidenced a thickening of the dielectric layer
thereon of about 450 angstroms. No substantial amount of dielectric material was
found to have been added to the interior of the body. Capacitance measurement
of the counterelectroded, twice anodized, capacitor revealed no noticeable capacitance
loss despite the high voltage applied during the second anodization.
Manipulation of the twice anodized devices demonstrated that they
were significantly more rugged and less susceptible to damage then the batch members
subjected to a single anodizing procedure.
A batch of sintered tantalum anodes were anodized by submersion in
a bath of 0.1% phosphoric acid based electrolyte at 85°C and subjected to a formation
voltage of 55 Volts for an hour. Examples of the batch were counterelectroded
and the capacitance obtained by this non-selective anodization averaged around
10 microfarads per finished capacitor. Other examples from the batch were then
soaked in a xylene bath for around 10 seconds in order to fully impregnate the
examples with the solvent. The saturated examples were then subjected to an 85
volt formation voltage for 20 seconds at 85°C and thus selectively anodized. The
selectively anodized anodes were heated to 150°C for ten minutes to evaporate impregnated
solvent. Analysis of the twice anodized examples indicated that the dielectric
on the exposed lead and anode surfaces was significantly thicker than on the single
anodized items from the batch and the twice anodized examples were clearly mechanically
superior to the once anodized examples. No significant capacitance change was measured
between capacitors formed from the once and twice anodized examples.
The process disclosed herein is of particular benefit in respect
of capacitors formed of pressed and sintered porous anodes where a thick, robust,
dielectric external coating following initial dielectric formation improves the
ruggedness of the preform and resultant capacitor. Though there are theoretically
no limits on the applied range of formation voltages during the anodization process,
the process has been observed to be thoroughly effective on parts anodized at
a voltage range of 10-150 Volts for the first anodizing. The preferred formation
voltage range used in the selective anodization step disclosed herein is generally
10 to 30 volts higher than the voltage used in the first anodizing step.
Although the invention has been described herein with reference to
a preferred embodiment, the description was for explanation purposes only and is
not meant to limit the scope and spirit of the invention as defined by the following