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Think
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| Acid Copper Through-hole Plating |
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Overview
Once the through-holes have been activated, your board is ready for acid copper
electroplating (short for "electrolytic plating"). Considering the amount of
trepidation that seems to surround the entire topic of through-hole connectivity, and the
lengths that some people will go to avoid wet chemistry, putting a uniform, reliable
sheath of copper on the insides of every hole turns out to be quite straight forward if
not downright easy. Thanks to decades of work by the major electrochemical suppliers, the
various chemical systems are well understood and readily available in most industrialized
nations. Most of the equipment, like acid
copper plating tanks, is easy to make and will last for many years if properly
maintained. The items that are not easily fabricated in a home shop are available from a
variety of sources. A high-performance plating
solution can be mixed using readily available materials.
Basic principles
An acid copper electroplating solution is a mixture of water, sulfuric acid, copper
sulfate, and a trace of hydrochloric acid. To this is added a number of organic
constituents that serve to regulate and distribute the delivery of copper to the surface
being plated. The two basic organic additives are commonly referred to as the "brightener/leveler"
and the "carrier".
A basic electroplating cell consists of a tank full of the above electrolyte with
arrays of copper anode bars arranged along two opposite sides. These bars are referred to
as the anodes, and, as you might expect, are connected to the positive terminal of a
current source. This supply must be capable of continuous sourcing into a near
short circuit load (a typical copper electroplating bath has an effective full load
operating "impedance" that ranges between 0.025 Ohms and 0.015 Ohms). Situated
halfway between these anode "banks" is the copperclad substrate that is to be
plated. It is variously referred to as the cathode (duh!) or the workpiece.
In the simplest terms, copper deposition occurs when an electrical potential is
established between the anodes and the cathode. The resulting electrical field initiates
electrophoretic migration of copper ions from the anodes to the electrically  conductive surface of
the cathode where the ionic charge is neutralized as the metal ions plate out of solution.
In the Think & Tinker process, a thin layer of conductive ink
extends the conductivity of the surface foil layers into the through holes. This ink forms
a highly reliable surface for efficient electrolytic copper depositon. The figure at right
shows a photomicrograph of the mechanically active surface that results when the ink is
cured and the uniform layer of smooth, bright copper that has been deposited inside the
through-hole.
At the anode (in a properly maintained bath), sufficient copper erodes into the
electrolyte, to exactly make up for the deposited material, maintaining a constant
concentration of dissolved copper. This all sounds quite nice, except for the annoying
tendency of electrical charges to build up on the nearest high spot, thereby creating a
higher electrical potential. This area of increased potential attracts more copper than
the surrounding areas which in turn makes the high spot even higher. If this process were
allowed to continue unchecked, the resulting plated surface would resemble a random jumble
of copper spears instead of the smooth, bright surface needed for reliable electrical
circuit formation. Inhibiting and controlling this nonlinear behavior is where the organic
additives come in to play. This situation is especially critical at the rims of the
through-holes. Here the field concentration is sufficiently high, that, in the absence of
some mediating mechanism, electrodeposition would completely close off many of the smaller
diameter holes.
Organic additives
In a well controlled plating bath, the carrier supports the formation of a black skin
on the anode material which serves to regulate the diffusion of copper ions into the
electrolyte. The material is also attracted to, but not co-deposited on the cathode
(work piece) forming a layer (film layer) in close proximity to the surface that controls
the rate of copper grain growth.
The brightener works within the film layer to control copper deposition on a
microscopic level. It tends to be attracted to points of high electro-potential,
temporarily packing the area and forcing copper to deposit elsewhere. As soon as the
deposit levels, the local point of high potential disappears and the brightener drifts
away. (i.e. brighteners inhibit the normal tendency of the plating bath to preferentially
plate areas of high potential which would inevitably result in rough, dull plating) By
continuously moving with the highest potential, brightener/levelers prevent the formation
of large copper crystals, giving the highest possible packing density of small equiaxed
crystals which resulting in smooth, glossy, high ductility copper deposition.
Mark Brelsford of QMS in Toronto, ON likens the action of the carrier to the function
of a doorman at a theater who regulates the flow of people into a theater but doesn't
really care where they go once inside. The brightener would then be the ushers who
politely lead each person to a vacant seat until the theater is uniformly filled.
Panel vs. pattern plating
There are two main approaches commonly used when electroplating PCB substrates, panel plating and pattern plating.
During panel plating, the entire copper surfaces on both sides of
the substrate, as well as the hole walls are plated up to a desired final thickness. While
this requires a fairly hefty current source for even a modest size PCB, the end result is
a smooth, bright copper surface that is easy to clean and prepare for later processing. A
major problem for folks without access to a photoplotter, is the need to use negative
artwork to expose the circuit pattern into the more common contrasting reversing dry-film
photoresists (contrast preserving films have been introduced from time to time but never
seem to stay around for very long). When you etch a panel plated board, you end up
removing most of the material that you plated, so the burden of extra erosion of the anode
banks is exacerbated by an increased copper loading in your etchant.
Pattern plating, as the name implies, involves masking off most
of the copper surface and plating only the traces and pads of the circuit pattern. Due to
the reduced surface area, a much smaller capacity current source is generally needed.
Further, when using contrast reversing photopolymer dry-film plating masks (the most
common type), a positive image of the circuit is all that is needed. For many prototype
PCBs, this artwork can be reliably produced on a relatively inexpensive laser printer or
pen plotter. Pattern plating consumes less copper from the anode bank and requires that
less copper be removed during etching reducing bath analysis and maintenance. The downside
of the technique is that it requires that the circuit pattern be plated with either
tin/lead or an electrophoretic resist material prior to etching and then stripped prior to
soldermask application. This increases the complexity and adds another set of wet chemical
baths to the process.
Panel plating copperclad substrates proceeds as follows:
- Calculate the total plating time.
An acid copper plating bath based on the Lea Ronal PCM+ additive system
deposits 0.0011" (1.10 mils, 28 microns, 0.81 oz) of high ductility copper in 1 hour
at 20 ASF(Amps per Square Foot). Plating up "one ounce" of copper (i.e. plating
1 oz. of copper onto 1 square foot of board) is equivalent to plating a thickness of
0.0013" (1.3 mils or 34 microns).
Example: If you are starting with
"half ounce" copperclad and want to plate up to a finished thickness of
"one ounce", you will need to add .65 mils (0.00065"). The total plating
time at 20 ASF will be:
[0.65 mils / (1.1 mils/hr.)] x 60
min./hr. = 35.5 minutes = TC
- Calculate the required plating current.
Convert the total area of the substrate being plated into square feet (remember both sides!) and multiply the result by 20.
Example: If you are plating a 12" by
9", double-sided board, you will need:
[(12" x 9" x 2
sides)/144] x 20 = 30 Amps = C
- Carefully inspect the substrate for deep scratches and nicks that might impair the
quality of the finished circuit.
- Format the drilling stack to minimize burr formation during
drilling.
- Drill the through-holes and mounting holes, and mill/router
any slot or cavity that is to be plated.
- Activate the hole-walls.
- While the ink is curing, take a few minutes to analyze the electrolyte. If you have a hull cell, this is a good time to run a test to insure that the
organic components of the bath (which are very difficult to test directly) are in
balance and present in the proper concentrations.
- After activation and curing, both sides of the substrate should be thoroughly cleaned to remove any trace of conductive ink from both surfaces.
Any ink that is not removed will almost certainly show up in the worst possible place so take
your time cleaning the board and make a good job of it!
An abrasive pad (e.g.
Scotchbrite® pad) can be used to remove ink that proves to be too stubborn for
conventional cleaning, but be careful. You must be certain that you do not break the
electrical contact between the conductive ink on the inside of the holes and the copper
foil on the surface of the board or the holes in question will not plate properly.
- Rinse the board thoroughly in deionized water before proceeding.
- Dip the board into a 10% solution of sulfuric acid to minimize the introduction of
contaminants into the copper plating tank.
- Attach the cathode clip to the board, making certain that both copper surfaces have good
electrical contact to the negative terminal of the plating power supply.
- Turn the power supply on.
Note: The power supply
should be adjusted so that, at its lowest setting, it establishes an electrical potential
of about 0.25 Vdc when the board is first lowered into the bath. This will help prevent
the formation of a low adhesion "electroless" copper layer that might lead to
trace peeling and cracking during soldering.
- Lower the board into the plating tank halfway between the two anode banks until the top
edge is at least 1" below the surface of the electrolyte.
- Swish the board gently back and forth to drive any trapped air
bubbles out of the through holes.
- Turn on the air compressor and adjust the air flow until a uniform blanket of agitation
roils the top of the bath on both sides of the board. You only need about 2 CFM (Cubic
Feet per Minute) of air flow per square foot of bath surface.
- Slowly ramp up the current (take about 20 sec.) to the value C
calculated above.
- Plate the board for ½ the total time (TC) calculated above.
- Turn the current down and flip the board top to bottom and left to right. This will help
minimize any plating non-uniformity that results from asymmetric, inconstant plating
conditions.
- Reconnect the cathode clip and lower the board back into the bath.
- Plate the board for ½ TC
- Remove the board from the bath and thoroughly rinse in the rinse tank to remove most of
the electrolyte. Rinse the board under running tap water to remove the rest.
Note: If no outside contamination is
introduced, the water in the primary rinse tank can be added back into the plating bath to
make up for drag out and evaporative losses. This is crucial to reducing the effluent from
this process to near zero.
- Blow dry.
- The plated board is now ready for further processing.
Pattern plating copperclad
substrates proceeds as follows:
- Calculate the total plating time.
An acid copper plating bath based on the Lea Ronal PCM+ additive system
deposits 0.0011" (1.10 mils, 28 microns, 0.81 oz) of high ductility copper in 1 hour
at 20 ASF(Amps per Square Foot). Plating up "one ounce" of copper (i.e. plating
1 oz. of copper onto 1 square foot of board) is equivalent to plating a thickness of
0.0013" (1.3 mils or 34 microns).
Example: If you are starting with
"half ounce" copperclad and want to plate up to a finished thickness of
"one ounce", you will need to add .65 mils (0.00065"). The total plating
time at 20 ASF will be:
[0.65 mils / (1.1 mils/hr.)] x 60
min./hr. = 35.5 minutes = TC
- Calculate the required plating current.
Convert the total area of the substrate being plated into square feet (remember both sides!) and multiply the result by 20.
Example: If you are plating a 12" by
9", double-sided board, you will need:
[(12" x 9" x 2
sides)/144] x 20 = 30 Amps = C
- Carefully inspect the substrate for deep scratches and nicks that might impair the
quality of the finished circuit.
- Format the drilling stack to minimize burr formation during
drilling.
- Drill the through-holes and mounting holes, and mill/router
any slot or cavity that is to be plated.
- Activate the hole-walls.
- While the ink is curing, take a few minutes to analyze the electrolyte. If you have a hull cell, this is a good time to run a test to insure that the
organic components of the bath (which are very difficult to test directly) are in
balance and present in the proper concentrations.
- After activation and curing, both sides of the substrate should be thoroughly cleaned to remove any trace of conductive ink from both surfaces.
Any ink that is not removed will almost certainly show up in the worst possible place so take
your time cleaning the board and make a good job of it!
An abrasive pad (e.g.
Scotchbrite® pad) can be used to remove ink that proves to be too stubborn for
conventional cleaning, but be careful. You must be certain that you do not break the
electrical contact between the conductive ink on the inside of the holes and the copper
foil on the surface of the board or the holes in question will not plate properly.
- Rinse the board thoroughly in deionized water before proceeding.
- Dip the board into a 10% solution of sulfuric acid to minimize the introduction of
contaminants into the copper plating tank.
- Attach the cathode clip to the board, making certain that both copper surfaces have good
electrical contact to the negative terminal of the plating power supply.
- Turn the power supply on.
Note: The power supply
should be adjusted so that, at its lowest setting, it establishes an electrical potential
of about 0.25 Vdc when the board is first lowered into the bath. This will help prevent
the formation of a low adhesion "electroless" copper layer that might lead to
trace peeling and cracking during soldering.
- Lower the board into the plating tank halfway between the two anode banks until the top
edge is at least 1" below the surface of the electrolyte.
- Swish the board gently back and forth to drive any trapped air
bubbles out of the through holes.
- Turn on the air compressor and adjust the air flow until a uniform blanket of agitation
roils the top of the bath on both sides of the board. You only need about 2 CFM (Cubic
Feet per Minute) of air flow per square foot of bath surface.
- Slowly ramp up the current (take about 20 sec.) to the value C
calculated above.
- Plate the board for ½ the total time (TC) calculated above.
- Remove the board from the bath and rinse thoroughly to remove any electrolyte.
- Clean the board and laminate both
sides with plating resist (photoresist used for etching will also works pretty well).
- Image both sides of the board, being careful that the correct
pattern in aligned on each side (component side on the top and solder side on the bottom).
Note: Be sure to leave a bare copper area on both
sides of the board so that you can insure good electrical contact with the cathode clip.
- Develop the circuit pattern.
- Reconnect the cathode clip to the board.
- Turn the power supply on.
- Lower the board into the plating tank halfway between the two anode banks until the top
edge is at least 1" below the surface of the electrolyte.
- Swish the board gently back and forth to drive any trapped air bubbles out of the
through holes.
- Turn on the air compressor and adjust the air flow until a uniform blanket of agitation
roils the top of the bath on both sides of the board.
- Slowly ramp up the current (take about 20 sec.) to the value C calculated above.
- Plate the board for ½ the total time (T).
- Turn the current down and flip the board top to bottom and left to right. This will help
minimize any plating non-uniformity that results from asymmetric, inconstant plating
conditions.
- Reconnect the cathode clip and lower the board back into the bath.
- Plate the board for ½T.
- Remove the board from the bath and thoroughly rinse in the rinse tank to remove most of
the electrolyte. Rinse the board under running tap water to remove the rest.
Note: If no outside contamination is
introduced, the water in the primary rinse tank can be added back into the plating bath to
make up for drag out and evaporative losses. This is crucial to reducing the effluent from
this process to near zero.
- Blow dry.
- The plated board is now ready for further processing.
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