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Letter 8007
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I guess I should restate my question, I am testing different voltages 1.5 volts, 3 volts and 20 volts. What relationship does the voltage have to the amount of copper plated onto a zinc plate? Is there a way to have too much voltage to where it would cause a weaker bond? or is there a limit to where there wouldn't be any better results after a certain voltage is reached? If you could explain those resistances in depth that would be great.
Thanks for your help!
Scott Ross
- Los Osos, California
Faraday's Law says you will ideally deposit one gram equivalent weight of metal for every 96,500 ampere-seconds.
When you double the voltage from 1.5 volts to 3 volts you more or less double the current because of Ohm's Law, I = E / R, so you more or less plate twice as fast.
But at 20 volts you will deplete the solution of copper ions in the area of the cathode almost instantly. But the electrons you are making available have to go somewhere--with the result that the electrons will reduce the hydrogen in the water to nascent hydrogen gas instead of depositing copper which isn't there. This formation of copious hydrogen gas, plus atoms of metal being yanked out of solution instantly instead of slowly growing as crystals, is called "burning".
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 Ted Mooney finishing.com Brick, New Jersey |
So then what is the difference when you change the voltage (1.5, 3 or 20 volts previously mentioned) but can keep the current constant the same by adding an additional resistance to the circuit?
Philip Bradford
- Raleigh, North Carolina
Hi, Philip. You can't do what you are thinking of. If you keep the current that is going through the solution constant, you are also keeping the voltage drop across the solution constant, again because of Ohm's Law.
You can set the voltage at 20 volts and add a big external resistor to eat up a big voltage drop there, but if the current through the solution is constant, the voltage across the solution is constant, and the rest of the voltage drop is across the resistor.
Regards,
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Ted Mooney finishing.com Brick, New Jersey |
Ted,
Thank you for your answer, although maybe I did not ask my question
clearly enough. When I meant extra resistance, I meant between the
cathode and anode (with no other resistors in the plating system). It
is clear to me that the resistance between the anode and cathode are
dependent on the conductivity of the solution, the shape and surface
area of the anode and cathode and the distance between them.
Please consider the hypothetical situation. The solution conductivity
is constant. The shape and surface area of the anode and cathode are
constant. The resistance between them can be changed by changing the
distance between them. If the source has a voltage of 1V and the
resistance between the anode and the cathode is 1 ohms then the
current is 1 amp. If the voltage is increased to 4V and the
resistance is changed to 4 ohms by changing the distance between the
electrodes the current is still 1 amp. In this situation you should
have equal plating rates since the current is the same and thus
shouldn't have the effect of depleting the metal ions near the
electrode like you mentioned, but your driving force is four times is
great. This is the effect of voltage that I am looking for. I know it
may not make sense to have your parts meters away from each other in
the plating bath just to increase the voltage but I just was hoping
to gain insight into the effect.
From the books I have it seems that if you apply toooo much voltage
then you get larger over potential which causes your current density
to go up exponentially. At what voltage does this start to occur for
acid copper plating? In the linear regime what effect does an
increase in voltage (assuming a constant current) have? Are there any
effects on density, morphology, macro and micro throwing power,
etc.?
Thanks for your help.
Philip Bradford
- Raleigh, North Carolina
Hi, Philip. I think you're still not quite accepting all of the implications of Ohm's Law :-)
If you put the anode and cathode further apart to keep the current constant as you increase the voltage, you're still just looking at a voltage drop across an "external resistor", although in this case the "external resistor" is the extra plating solution between the anode and cathode rather than an actual external resistor. And in fact, practical experience shows the qualities of the electroplated metal to be the same whether the anode to cathode distance is a couple of inches or several feet.
There is a diffusion reaction in the boundary layer around the cathode as the plating proceeds. If you plate very slowly you tend to get large crystals because there is copper available when the electron touches it. If you plate faster, but still in conventional plating range, you get more adatoms and smaller crystals. If you try to plate still faster, you get burning. Good solution agitation keeps plenty of copper available just outside of the boundary layer, and extreme agitation (directing a high pressure stream of fluid at the cathode) reduces the thickness of the diffusion layer and allows faster plating. Pulse plating produces some changes in plated layer properties, but some people feel that the only real mechanism behind that is that the pulses cause agitation in the boundary layer -- but maybe not; maybe if you had pulses of microsecond duration instead of millisecond duration you could engineer the plating at a nano level.
Regards,
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Ted Mooney finishing.com Brick, New Jersey |
Ted,
What you have said makes sense. Now I have more followup questions
for you!
Is there some minimum voltage/current level that you need to be able
to reduce the metal ions to make the pure metal at the cathode? I
know the standard value for the reaction is positive, +0.34 V, so I
am assuming that there is no minimum voltage required but
electrochemistry is not my strongest subject so I won't feel bad if
you tell me I am wrong. The reason I ask is that I am trying to plate
copper at an extremely low rate to be able to fill voids on the order
or tens and hundreds of nanometers wide and my thickness only needs
to be on the order of hundreds of nanometers so deposition rate is
not a concern. I will use ultrasonication to agitate the solution. I
know that using pulsed plating techniques researchers have avoided
bottlenecking to fill deep nano-voids but I am hoping to get around
buying a $5k pulsing power supply by slowing the plating to the
slowest possible rate and providing good agitation.
Do you have any more tips or starting points for the bath copper and
sulfuric acid concentrations that might give me the best results for
plating in small voids, considering the fact that I want to plate
very slowly? Thanks!
Philip
Philip Bradford
- Raleigh, North Carolina
I believe you have the half cell potential for Copper correct at +0.34 V. If you were plating copper onto silver, which has a half cell potential of +0.80 V, you would need a minimum of 0.46 V to get it to deposit. If you were plating onto iron, which has a half cell potential of -0.41, the copper will spontaneously immersion deposit onto the iron while generating 0.75 V.
These potentials are for standard conditions, and I believe that the Nernst equation can get you more realistic numbers for your actual situation.
Sorry, but I have no experience in the area that you are exploring (trying to fill nano voids). But "brighteners" have long been used in the plating industry to try to get more uniform plating. They work by (at least partially) blocking a surface so that the electricity is steered towards causing the metal to deposit elsewhere, resulting in smaller grain, brighter deposits, and more even distribution.
The following is from an engineer, not an electrochemist, and it is a simplification -- but here's the way I understand it: In order to copper plate thru-holes in thick multilayer circuit boards, where the current is very low in the hole but very high on the surface, they have developed developed special brighteners. These brighteners have molecules so large that they can't get into the hole to block deposition there, but cover everything else. The result is copper is deposited deeply into the thru-hole. Please talk to one of the major suppliers about their acid copper process for thru-hole plating. In may be what you need.
Also, please see our library article "A Review Of Copper Plating High Aspect Ratio Plated Through Hole Papers" contributed by Paul Stransky.
Regards,
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Ted Mooney finishing.com Brick, New Jersey |
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