Note: Descriptions are shown in the official language in which they were submitted.
AC ETCHING OF ALUMINUM CAPACITOR FOIL
This invention relates to AC etching of alumi-
num foil for electrolytic capacitors.
AC etching has been used to produce aluminum
articles; such as lithographic plates and capacitor foil.
It also has been used to electropolish aluminum articles
prior to metal plating.
The prior art has discussed the difficulties
in obtaining an even or uniform etch structure and has
overcome these difficulties in a variety of ways, e.g.,
interrupting the etch process to apply protective coat-
ings, carrying out the etch process in stages of differ-
ing degrees of aggressiveness, and using additives in the
electrolyte bath to control pit size or to increase mechan-
ical strength of the foil.
Another problem has been to prevent the precipi-
tation of aIuminum hydroxide, formed during etching, on or
into the etched surfaces.
The resolution of these problems has led to
prior art processes in which the etch conditions are care-
fully controlled to provide the desired increase in surfacearea, with little change in mechanical strength. Such a
process is described by Arora and Randall in U.S. patent
US 4,279,714 issued July 21, 1981, that utilizes a phos-
phate in the hydrochloric etch bath. Another such process
is described by Arora, Randall and Bernard in patent
'7~ ~ 1'7
-- 2
US 4,279,715 issued July 21, 1981, that utilizes a phos-
phate and chromate in the hydrochloric acid etch bath.
Still another process is described by Arora in US 4,315,806
issued February 16, 1982, that utilizes an intermittent
etch scheme.
The present invention overcomes the environmental
concerns about the prior art use of the chromate, while
improving on the results obtained from the prior art use
of the phosphate only.
A feature of this invention is the provision of
a process of etching aluminum capacitor foil which utilizes
AC current to give a uniform etch structure while maintain-
ing the strength of the foil. Another feature is the pro-
vision of a process which also provides a greater etch pit
density and size than prior art processes.
In accordance with this invention aluminum capaci-
tor foil is etched by a process utilizing aLternating cur-
rent and a chloride electrolyte bath containing phosphoric
and nitric acids.
In a drawing which illustrates the process of
this invention,
The sole figure shows foil being etched in the
present inventi.on.
In general, aluminum capacitor foil is etched in
accordance with this invention by using an etchant solution
containing hydrochloric acid, aluminum chloride, nitric
acid and phosphoric acid at 28 to 45C, alternating current
of 20 to 30 Hz frequency, and anodic current density of 1.5
to 3.0 A/in2.
The concentration of the hydrochloric acid in the
etchant is 0.9 to 1.8 M to provide strongly acidic condi-
tions that prevent the formation and precipitation of alu-
minum hydroxide on the foil. The concentration of the alu-
minu~ chloride is 0.2 to 0.5 M to initially catalyze the
reaction, especially with fresh etchant solution. The con-
centration of nitric acid is 0.05 to 0.3 M to give the
desired uniformity and etch density (number of pits/units
area of foil). It is postulated that the phosphoric acid
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_ 3 _
in concentrations of 0.03 to 0.3 M serves to passivate
sites that have already been started, so that etching
will proceed at different sites, givîng the desired etch
density, rather than concentrating at started sites. The
phosphoric acid also appears to minimize the polishing
effect and to maintain the etch structure. When phospho-
ric acid is present, capacitance is increased by about 30%
over that o~tained with a hydrochloric-nitric acid etchant
alone.
The current density is maintained at 1.5 to 3.0
A/in2 to provide the desired number of sites. If the cur-
rent density is too low, i,e., below 1.5 A/in2, there will
not be the desired number of sites; if too high, i.e.,
above 3.0 A/in2, the etch structure becomes fragile lead-
ing to a smooth polished surface rather than an etched one
and capacitance decreases. Foil is also thinned in the
latter case.
The frequency of the alternating current is main-
tained at 20 to 30 Hz, as this frequency with the electro-
lyte of this invention gives etched foil with optimum ca-
pacitance. The voltage, a function of current density and
etch cell design, is +7V. ~ith a different cell design,
the voltage will differ also.
The temperature of the etchant solution is main-
tained at 28 to 45C to provide pores of suitable size and
densit~. Lower temperatures result in fewer and larger
pores, while higher temperatures give more pores but nar-
rower pits and lower capacitance.
Aluminum foil 10 is passed over roll 20 into etch-
ing tank 30 between insulated electrodes 31 and 32, under
roll 40 and between electrodes 32 and 33, over roll 41 and
between electrodes 33 and 34, under roll 42 and between
electrodes 34 and 35, and out of tank 30 and over roll 50.
The electrodes are carried by insulated frames 43 and 44
that have openings for passage of foil 10. More electro-
des and rolls may be used than shown. In fact, it is more
efficient to use more electrodes, but enough have been
shown to illustrate the invention.
l~ 7a~:l7
Because the electrodes 31, 32, 33, 34 and 35 are
mounted in insulated frames 43 and 44, the alternating cur-
rent passed through ~hem is forced to pass through the foil
cmd not through the main body of etchant solution. In this
way, the foil is electrochemically etched during the time
l:he foil passes between a pair of electrodes and not elec-
trochemically etched when outside the frame, e.g., between
frame 44 and roll 40.
The etched foil obtained by the method of the pre-
sent invention retains a solid metallic core that givesgood mechanical properties and low-temperature capacitance
retention. There is an interrelationship among etchant
composition and concentration, temperature, AC frequency,
and current density, with the last being the least crucial,
that gives the desired results, i.e. good capacitance and
mechanical properties, not just etchant composition and
concentration alone.
In the examples below, soft foil of 99.99% purity
was used. Hard foil can be etched by this process, as can
foil of different purity, but satisfactory capacitance is `
obtained without resorting to hard foil and the processing
problem associated therewith.
The etch scheme used is the intermittent etch
described by Arora in the above-identified US 4,315,806.
Since the foil is etched intermittently, the total charge
passed to the foil is a more meaningful unit than etching
time.
Example 1
Soft aluminum foil of 2.9 mil initial thickness
was etched intermittently using alternating current and an
etchant electrolyte of a mixture of hydrochloric acid, phos-
phoric acid, aluminum chloride, and nitric acid.
In Table la, electrolyte composition and molar
concentration, M (moles/liter), is given for each run.
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Table la
Run HCl AlCl3 H3PO4 HN03
1 1.2 0.35 0.11 0.116
2 1.3 0.30 0.14 0.154
3 1.6 0.25 0.11 0.04
4 1.3 0.25 0.11 Q.31
1.3 0.30 0.08 0.154
6 1.4 0.40 0.20 0.195
7 1.5 0.35 0.03 0.154
8 1.5 0.35 0.11 0.154
9 1.5 0.35 0.11 0.154
lQ 1.6 0.35 0.11 0.154
In Table lb, etching conditions and results are
presented. The temperature is degrees Celsius (~C), the
total charge passed is in coulombs/in2 of foîl, alternating
current frequency is in Hertz (Hz), anodic current density
is in amperes/in2 of foil (A/in2), thickness is foil thick-
ness in mils after etching, and lOV and 30V capacitance is
capacitance/unit area (~F/in2).
Table lb
Run Temp. Charge Freq. Current Thick- Wt.Loss Capacitance
passed density ness % @ lOV @ 30V
1 32 500 25 2.6 2.80 33.3 27075.9
2 36 520 25 2.6 2.87 34.4 338 109
3 32 520 25 2.6 2.82 34.2 267 90
4 32 470 25 2.6 2.84 32.4 23365.2
36 520 25 2.6 2.87 33.3 333103.5
6 40 616 30 2.8 2.82 40.8 346102.2
7 28 465 25 1.94 2.85 3Q.0 30287.7
8 32 500 25 1.75 2.85 32.7 336 100
9 32 450 20 1.94 2.82 34.5 30777.5
32 500 25 1.55 2.83 30.0 32890.3
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Example 2
Soft aluminum foil was etched in a pilot plant
using as electrolyte: 1.15 M hydrochloric acid, 0.26 M alu-
minum chloride, 0.11 M phosphoric acid; and 0.154 M nitric
acid at 34C, AC current at 26 Hz, 2.9 A/in2 anodic current
density, and 652 coulombs/in2 total charge passed. The
weight loss was 30.1%, and 10 and 30V capacitance was 306
and 90.9 ~F/in2, respectively.
An added advantage of this electrolyte is that
the concentration of the constituents can vary somewhat
without adversely affecting results. The preferred concen-
tration of the hydrochloric acid is 1.3 to 1.6 M, with
0.9 M as the lower limit below which not enough etching
took place and 1.8 M the upper limit above which macro-
thinning of the foil took place.
Some phosphoric acid is needed, with 0.03 M being
the lower limit. However, if too much phosphoric acid is
present, above about 0.3 M, capacitance decrea~es. The
preferred amount is about 0.11 M.
As is known, aluminum chloride initially cat-
alyzes the etching reaction, and for that reason at least
0.2 moles/liter must be present inîtially. Higher concen-
trations of aluminum ion are preferred, as electrolyte re-
cycle and recovery rate are a function of aluminum ion con-
centration. However, above about 0.5 M aluminum ion concen-
tration, the character of the etching changes to a uniform
corrosion rather than preferential tunnel etching. There-
fore, 0.5 M aluminum ion concentration is the upper limit
with about 0.35 M preferred.
The presence of nîtric acid is critical. Without
it, capacitance is about one-third lower and weight loss is
4-5~/O higher leading to poorer mechanical properties of the
etched foil. The use of nitric acid in a nitric:hydrochlo-
ric acid ratio of 1:8-10 permits~ etching at lower tempera-
tures and permits a wider latitude in current density and
concentration of electrolyte components, particularly hydro-
chloric acid and aluminum ion, without adversely affecting
capacitance, foil thickness, and weight loss. With lower
11 f~ 7
-- 7 --
nitric acid concentrations, the temperature i5 increa$ed
up to 45C to get the desired results, but below about
0.05 M nitric acid capacitance begins to fall off. Above
0.3 M nitric acid, capacitance also falls off. The pre-
ferred amount is about 0.15 ~.
Frequency and current density parameters areinfluenced by machine design. In the laboratory, optimum
current density was found to be between 1.5 A/in2 and
3.0 A/in2 , while pilot plant equipment was operated satis-
factorily at up to 3.6 A~in2. The higher current densitiesare particularly useful for cathode foil etching. Optimum
frequency was found in the laboratory to be 25 Hz with 20
and 30 Hz as the lower and upper limits, while the pilot
plant machine was operated at 26 Hz.
Nevértheless, the presence of relatively small
but definite amounts of both phosphoric and nitric acids in
a hydrochloric acid etch solution gives an etched foil with
higher capacitance and lower weight loss than are obtained
with the hydrochloric acid with either alone.
