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LW1DSE > TECH 14.08.22 05:27l 189 Lines 9140 Bytes #999 (0) @ WW
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Subj: Copper electroplating
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From: LW1DSE@LU7DQP.#LAN.BA.ARG.SOAM
To : TECH@WW
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Copper Plating (Extracted from the Internet)
ΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝΝ
Source 1:
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The chemistry is quite important if you want a good result. While one
can use a variety of acids for the bath, not all of them work equally well.
Also, the copper anode is important. Dunno about the standards in your country,
over here in Germany, electric permanent wiring (single stranded wires in the
walls, for example) are usually made of almost pure copper. Also, copper pipes
for water installations are pretty pure copper too.
The used current is important as well, better too low a current than
too high. If it bubbles, you basically destroy the electrolyte.
Since I do electroplating quite a lot, for my self made through-plated
PCB's, I can give you the recipe for a nice bath that i work with:
1700 ml H2O (distilled)
490 ml H2So4 38% (battery acid)
280 gr CuSo4 (copper sulfate)
300 mg NaCl (salt)
1 ml Tween 20
The NaCl is used to allow for a smoother deposit of the copper onto
the target, giving a more shiny surface. The Tween 20 is used to lower the
surface tension of the solution, so that it easily enters into holes. For the
actual plating I use about 3.5 amperes on a PCB of 160x100mm size.
About the copper: a lot of copper stuff is actually an alloy of copper
and other metal. During electroplating, the copper transfers to the cathode,
but not the other metal. Instead, the other metal end's up as very fine
"grains" in the bath. Those "grains" then stick to the cathode, giving a very
uneven and fragile surface.
You may want to constantly move the cathode around, to get an more
even deposit. This is because the electrolyte in close proximity to the
cathode will "deplete" if it isn't moved around. If you are unsure about the
copper anodes, first pack them into soft paper tissue, then into a cotton bag.
This will filter out the unwanted "grains" that i spoke of above.
Start out with a really low current first. Increase it slightly if you
want. If it is too high, the cathode starts to get dark. With even more current,
you get bubbles.
To get a really strong bond between the surface of the cathode and the
copper you are going to e-plate, you may want to micro-etch the cathode. That
is, place it into a strong etchant for a few seconds, rinse it, then put it in
the plating bath. This process will roughen up the cathode's surface, giving a
bigger surface area.
Source 1:
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Acid Copper Solutions
Acid copper baths are simple formulations, containing copper ions,
additives and either sulfate or fluoborate ions along with the corresponding
acids. Because of their acidity, they cannot be plated directly onto active
metals, such as zinc die castings and steel, for they will produce non-adhe_
ring immersion deposits.
The chemical cost of acid copper baths is low, and they can have a
wide range in composition. When compared with cyanide and alkaline non-cyanide
baths, their effluent control is simpler, they are easier to control and they
are more stable. Their anode and cathode efficiencies are high, close to or
equal to 100%. With high agitation they can tolerate high current densities.
However, because of their low cathode polarization, the acid baths do
not have throwing power as good as that of alkaline solutions, making them
poor strike baths.
Chemistry
Table III shows the chemical makeup and operating conditions of typi_
cal acid copper sulfate baths. General purpose baths are used for decorative
plating, while high-throw and high-speed baths are for special applications
such as printed circuit board and strip plating. High-throw baths are formu_
lated to plate more copper in the very low current density holes and less on
the surface of circuit boards than other acid copper processes. A high-speed
bath can plate about twice as fast as conventional baths while retaining all
the desired deposit properties.
The concentration of copper sulfate helps determine the properties of
the baths. At higher concentrations, the resistivity of the bath is greater,
and the anode and cathode polarization are slightly reduced. At lower copper
sulfate concentrations, throwing power increases. A concentration of less than
60 g/L of copper sulfate decreases cathode efficiency. The solubility of copper
sulfate decreases with increasing sulfuric acid concentration.
Sulfuric acid gives the bath its high conductivity, reduces anode and
cathode polarization and prevents precipitation of basic copper salts. A prac_
tical minimum sulfuric acid concentration is about 45 g/L.
Agitation, anodes.
To ensure brightness and to prevent high-current-density burning, agi_
tation of acid copper baths is essential. Air agitation from an oil-free blo_
wer is best for decorative copper plating. For PC boards, mechanical agitation
in which the PC board’s movement forces solution through the board’s holes, is
good to obtain maximum throwing power. For high-speed copper plating at high
current density, high-velocity solution flow and/or part movement, perpendi_
cular to the cathode, has been successful.
A necessary black cupric oxide film forms on the anodes. If it is dis_
turbed, such as with excessive high anode current density, brightener consum_
ption and roughness will increase, deposit ductility will decrease and leve_
ling of deposits will be reduced. If the anodes have either a pink or a light
gray appearance, too low anode phosphorus content or electrical problems may
be the cause. Low-current-density electrolysis, starting at 0.5 and building
up to 2.5 A/dm2, should develop the desired film if the correct anode material
is used.
Contamination.
Nickel, cobalt, chromium and iron will not readily co-deposit with
copper but will reduce the solution’s conductivity when a total of about 1,000
ppm are present. These metals cannot be removed.
Iron will also cause the copper concentration in the bath to increase
by a reaction with the anode during idle periods.
Calcium and lead will precipitate as sulfates and cause roughness if
not removed, but they do not affect the deposit.
Tin can co-deposit to cause rough dark deposits if present above 60
ppm. Lead and tin are usually introduced into the bath by carry-over on solder
plated racks. Stripping the racks after solder plating will minimize this.
Medium-current dummying at 1.5~2.0 A/dm2 will remove tin.
Antimony and bismuth will co-deposit if they are in the 20~100 ppm
range, causing brittle deposits. Antimony is usually introduced as an impurity
in the copper anodes. It can be removed by low-current-density dummying. Alu_
minum in amounts greater than 50 ppm may cause dullness in recesses.
Troubleshooting, purification.
Acid copper sulfate baths are easy to maintain. Use quantitative ana_
lysis to control copper, sulfuric acid and chloride. Add proprietary addition
agents, which control brightness, ductility, and leveling, on the basis of
amp-hr. You can also regulate their addition by using Hull cells and a copper
analysis.
Even though acid copper baths are very tolerant to contamination, they
must be purified occasionally. Organic contamination, recognized by a green
tint to the solution, is probably the most common, since it is easily intro_
duced into the bath from cleaners, oils, greases, brightener-breakdown pro_
ducts and brightener overloads. Most proprietary systems can tolerate an over_
dose of additives for a short time but if overdosing continues a light carbon
treatment might be necessary.
Pack the filter with activated carbon on non-cellulose filter aid and
circulate the solution through the carbon for about three or four turnovers.
This should remove enough organics to permit the bath to continue to operate
satisfactorily.
Most acid copper baths will eventually require a carbon/hydrogen pe_
roxide treatment to remove organics that cannot be removed by a light carbon
treatment. The organic contaminants will cause the bath to have a narrow
bright plating range and to produce a dull and burned copper deposit.
Source 3:
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If you want high-quality coating of copper dense and crumbly.
Then:
copper sulfate - 60 χ 80 g/l
Sulfuric acid - 180 χ 200 g/l
Sodium chloride - 30 mg/l
The current density of 1 A to start and gradually rising to 4 A.
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