Faster, Safer, and Simpler etching with cupric chloride

I describe here a method for etching printed circuit boards (PCBs) using a regenerable cupric chloride etchant. The general idea is decribed in the german patent DE2942504A1 from 1979 and a lot of the writing here is inspired by the work "Etching with Air Regenerated Acid Cupric Chloride" by Adam Seychell, a copy of which is available here.

Summary

In the cupric chloride etching solution proposed here, potassium chloride is used to replace the free hydrochloric acid often used for etching with cupric chloride. This has several advantages:

faster: etching a 35 μm Cu PCB in below 5 minutes at room temeprature and in below 3 minutes at 50°C
safer: no free hydrochloric acid in the etch bath so no aggressive HCl fuming
simpler: only specific gravity needs to be measured for bath control, no effortful titration or other special equipment needed

For me this method is the optimal way for PCB etching at home. The etch speed is very fast even when just using just a simple container, so no fancy equipment is needed. The etch speed is the same each time, generating comparable results not dependig on the freshness of the etchant with only minimal effort in bath control. The solution can be stored away for long times without degradation but can also easily be used once or twice a week. And finally it generates very little waste to dispose, or even none, when increasing the amount of etchant.

Warning

Caution: Chemicals mentioned on this page are potentially dangerous. Handle with care and always wear protection equipment (especially gloves and googles)! Read and follow safety data sheets!
The etchant is very corrosive towards metals with violent reactions especially for less noble metals (e.g. aluminum). Do not to use metal tools or containers!

Etchant composition

The patent DE2942504A1 from 1979, thus long expired, proposes etchants containing 130 to 320 g/l CuCl and 70 to 250 g/l KCl. I focus on the one highlighted in the patent, which has a concentration of 210 g/l CuCl and 160 g/l KCl.

Thus, 1 liter of etchant, which has a specific gravity of 1.27 kg/l consists of:

210 g CuCl₂ (1.55 mol)
160 g KCl (2.15 mol)
900 g water (50.0 mol)

The molar concentrations of the ions are:

1.55 mol/l Cu²⁺ (100 g/l)
5.25 mol/l Cl^- (187 g/l)
2.15 mol/l K⁺ (83.9 g/l)

The etchant has a deep green transparent color.

Etching

The etching works according to the following reaction:

Cu + CuCl₂ → 2 CuCl

Cu(II) ions oxidizing Cu(0) to Cu(I) being reduced to Cu(I) themselves.

Remark: In reality neither CuCl₂ nor CuCl are very much present at these concentrations but instead a mixture of (aqua) chloro complexes of Cu(I), Cu(II), mixed valence complexes, and copper potassium complexes. For simplicity we will stick to the simpler but not perfectly accurate notation. However, this fact has great implications for etching speed and observed colors.

During the etching the etchant changes its color to a olive/very dark green and later brownish color which is very hard to see through. This happens for very low concentrations of Cu(II) already.

fresh etchant
starting to etch
etching...
etching...
after stirring

When etching a 35 μm PCB in a simple container at room temperature (22-23°C), after about 3 minutes the substrate can already be seen in some places, and before 5 minutes are up the PCB etching is usually finished. To achieve these times, agitation is crucial: the PCB (or the etchant) needs to be moved the whole time. When leaving the PCB just sit in the etchant, it will easily take more than 15 minutes. However, due to the short etching times, even moving the PCB around by hand is not an issue.
When heating the etching solution to 50°C, the PCB is finsihed in less than 3 minutes.

Etch rate

This section shall give some more details on the influences on the speed of etching and document several conducted experiments.
For most experiments, a copper wire of 0.25 mm diameter was immersed into the solution and the time measured until the wire is dissolved indicated by a change in ohmic resistance.

0.25 mm copper wire connected to measure ohmic resistance placed into the etchant
just after breaking of the wire the time is stopped automatically

Comparison of different chloride donors

Four etching solutions are compared here. All consist of 210 g/l CuCl₂ and 2.15 mol/l of additional chloride ions, differing just in the donor of these additional chloride ions:
"KCl": The etchant with the composition proposed here (3 different measurement series)
"NaCl": The "KCl" etchant with KCl replaced by the same amount of NaCl
"HCl": Cupric chloride etchant with 2.15 mol of free acid
"CuCl₂": An etchant consisting of only CuCl₂ (2.63 mol/l = 354 g/l) - This was disarded after the initial measurement due to its poor performance.
Etch rate measurements were performed with the fresh etchant and with an increasing amount of copper already etched indicated by the corresponding Cu(I) concentration.

Using potassium chloride to replace the free hydrochloric acid more than doubles the etch rate. Even with a Cu(I) concentration of 30 g/l it is faster then a fresh etchant with the same amount of free hydrochloric acid.
Sodium chloride on the other hand perfoms very similar compared to hydrochloric acid. So it can be also used to get rid of the free hydrochloric acid and its disadvantages.

Dependance on temperature

As expected the etch rate significantly increases with temperature. An increase from 25°C to 50°C doubles the etch speed. As there is no free acid in the solution, HCl fuming is not an issue compared to other etching solutions and even higher temperatures could be used. Keep in mind that the etchant becomes more aggressive also to more noble metals at higher temperatures and temperature itself can be an issue depending on the etching device.
As a 35 μm PCB is also etched in 5 minutes at room temperature, this is often faster than heating the etching solution.

Dissolution capacity

The concentration of Cu(I) in solution has a negative impact on the etch rate. This means two things: First it should be kept at a minimum. So the etching solution should be regenerated as soon as possible.
And second, this determines the amount of etchant needed to etch without regeneration. However, as there is no drop in etch rate until 80 g/l but rather a steady decrease, there is no clear Cu(I) concentration to base this calculation on.
For example, one single sided euro size (100x160mm) PCB with 35 μm copper thickness has 5 g of copper. If this was to be removed completely it would generate 10 g of Cu⁺ ions. To have only a minimal increase of the etching time (less than 10%) while starting with a fresh etchant, one would need to keep the final Cu(I) concentration below 12 g/l. This is achieved using 0.83 l of etchant. If, however, allowing the etch time to double for the first etchings, one would only need to keep the final Cu(I) concentration only below 56 g/l, which is achieved with only 0.18 l of etchant. Keep in mind, that using regeneration and bath control the amount of etchant is increasing step by step, so this problem will solve itself anyway.

Regeneration and bath control

One of the major advantages of using cupric chloride for etching is the ability to regenerate the etchant. So instead of using the etching solution to exhaustion, it can easily be renewed in the time between etching. The chemicals used in bath control are only KCl and HCl, which are both readily available and which are usually used to generate the initial etchant.

This section describes how to restore the original concentrations starting with the used etchant. It consists of several steps which should be followed in order as they depend on each other.

1. Cu(I) concentration - regeneration

The used etchant can be regenerated by letting itself react with O₂ according to the reaction:

12 CuCl + 3 O₂ + 6 H₂O → 4 Cu₂(OH)₃Cl + 4 CuCl₂

etchant with ~40g/l Cu(II)
shortly after etching
already a little precipitation at the bottom
after 1 day
right after shaking
after 1 day
20 min after shaking
after 2 days
without prior shaking
Cu₂(OH)₃Cl generated at both the top
and precipated to the bottom
after 9 days
right after shaking
after 9 days
2 hours after shaking
after 14 days
no prior shaking
completely regnerated
Cu₂(OH)₃Cl seperated at the bottom

The olive / dark brown color will change back to the original green color, while the created dicopper chloride trihydroxide Cu₂(OH)₃Cl precipitates from the solution and gives a light greenish precipate. The speed of this reaction is very slow (several days) and needs O₂ from the atmosphere. There are several factors influencing the reaction speed:

Cu(I) concentration: The more Cu(I) having to be oxidized to Cu(II), the longer it takes.
Surface vs. volume: For a wide range of conditions, the reaction speed depends linearly on surface area.
Free acid: adding HCl to have free H₃O⁺ ions available will increased the speed of the regeneration. Even only a little free acid to just have enough to be consumed during regeneration can double the regeneration speed. However, this counteracts the advantages of using KCl in terms of simple bath control and no HCl fuming.
KCl concentration: an increased concentration of KCl will slightly reduce the regeneration reaction.

There are several suggestions aerating the etchant using some special equipment. Yes, this will speed up the regeneration reaction, but one should decide depending on the usage whether the extra equipment is necessary.
For me, it is convenient enough to store the etchant in a rather big container where there is a big surface between etchant and residual air. I even use a closed container, which I open just once a day (or every two days) for several days after etching. This way, after one week the etchant is regenerated completely.

The patent DE2942504A1 mentions copper hydroxide Cu(OH)₂ to precipitate during regeneration. I have never observed this with the concentrations I used. However, everything to be done for bath control would still work in the same way.

As even a low concentration of Cu(I) already gives the etchant an olive green or brownish very dark color, it is easy to check whether the etching solution is completely regenerated. When the regeneration is completely finished, the liquid part consists of only Cu²⁺, K⁺, and Cl^- ions and Cu₂(OH)₃Cl is seperated at the bottom.

2. Cu₂(OH)₃Cl precipitation - redissolving using HCl

after redissolving
Cu₂(OH)₃Cl using HCl

Cu₂(OH)₃Cl reacts with HCl according to:

Cu₂(OH)₃Cl + 3 HCl → 2 CuCl₂ + 3 H₂O.

So to get rid of the Cu₂(OH)₃Cl precipitation, we just have to (slowly) add HCl until all the precipitation is redissolved again. In preparation for the next step, we weight the amount of HCl needed. Using HCl of concentration 11.6% or higher ensures that the etchant is not diluted.

The amount of HCl needed could also be calculated starting from the amoung of copper etched. For example, one single sided euro size (100x160 mm) PCB with 35 μm copper thickness has 5.01 g (78.7 mmol) of copper. If this is removed completely it generates 10.1 g (157 mmol) of Cu⁺ ions and after regeneration 11.2 g (52.5 mmol) of Cu₂(OH)₃Cl, which will need 5.74 g (157 mmol) of (pure) HCl to dissolve.

In summary: etching, regeneration, and redissolving combined:

Cu + ½ O₂ + 2 HCl → CuCl₂ + H₂O.

So, the etched copper is converted via Cu₂(OH)₃Cl to cupric chloride CuCl₂, the main component of our etchant (and water).

3. KCl concentration - adding KCl

after adding KCl

As we added more CuCl₂ to the solution, we also have to add KCl to restore the original ratios of concentrations. For each 1.55 mol of CuCl₂ added, we also need to add 2.15 mol (160g) of KCl.
Fortunately, we have weighted the mass of HCl needed m(HCl) in the last step and know its concentration c_m(HCl). As 3.1 mol (113 g) of pure HCl are needed to generate 1.55 mol of CuCl₂, we can do the math and know that the mass of KCl m(KCl) to add is:

m(KCl) = m(HCl) * cm(HCl) * 1.4

4. CuCl₂ concentration / specific gravity - adding water

In its original composition the etchant has a specific gravity of 1.27 kg/l. To restore the original concentration water has to be added until the specific gravity is lowered again to 1.27 kg/l.
Note that when using HCl with a concentration of 11.6%, theoretically no water would need to be added. But due to evaporation the specific gravity has to be adjusted nevertheless.

For measuring specific gravity a hydrometer is quite usefull. Several battery hydrometers (for lead-acid automobile batteries) cover the required range and are widely available for a low price. As the original specific gravity only has to be recovered one could also build a simple hydrometer oneself by using a small glass tube, filling it with some sand, and marking the height it sinks in the original etching solution. Using a measuring cup and a scale is of course also possible, just not as convenient.

Preparation of the initial etchant

The initial etchant can be obtained by simply solving KCl and CuCl in water. For 1 liter of etchant 160 g of KCl and 210 g of CuCl need to be solved in 900g of water.

Potassium chloride - KCl

KCl is usually readily available in good quality as it is used frequently in the food industry as a preservative or flavor enhancer (E 508).

In case KCl is not available, the removal of HCl fuming and simpler bath control are also achieved by using NaCl. The 160 g/l of KCl need to be replaced by 125 g/l of NaCl and 35 g of water, as the specific gravity is also 1.27 kg/l. Keep in mind that many salts (esp. table salts) contain anticaking agents, which you might not want in the etching solution. The etching rates with NaCl are significantly lower, only comparable to cupric chloride with free hydrochloric acid. (see comparison)

Cupric chloride - CuCl₂

For preparing the cupric chloride, there are several options.

Cupric chloride can be bought directly. However, it is not easy to find and might be expensive. It is usually available only as cupric chloride dihydrate CuCl₂ · 2 H₂O, where you would need 238 g instead of 210 g and 28 g less water for 1l of etchant.
Cupric oxide CuO can also be used. It dissolves moderately quickly in HCl according to
CuO + 2 HCl → CuCl₂ + H₂O.

Caution: This reaction can realease chlorine gas! This reaction is strongly exothermic and can heat up to boiling! Ensure proper ventilation. Do not mix the full quantities of the substances at once.

For 1 l of etchant 124 g of CuO and 982 g of 11.6% hydrochloric acid (114 g pure HCl) is needed. Either add the CuO bit by bit to 11.6% hydrochloric acid (in case of higher concentrated hydrochloric acid dilute first). Or mix the CuO with the amount of water needed for dilution (keep stirring it sediments rather quickly) and slowly add the HCl. Keep an eye on temperature and creation of gas bubbles.
Copper and HCl works, when exposed to air and time is not an issue. This boils down to
Cu + 1/2 O₂ + 2 HCl → CuCl₂ + H₂O.
This reaction takes very long (several weeks). Due to the long time needed and the need for atmospheric oxygen, hydrochloric fumes cannot be avoided which somehow counteracts the idea of this etchant. As I have never used this method myself check Seychell2005 for more details.
For 1l of etchant 99g of copper and 982g of 11.6% hydrochloric acid (114g pure HCl) would be needed.

Currently ongoing investigations

influeance of aerating, ventilating, KCl concenctration, CuCl₂ concentration on regeneration speed
absorption spectra for bath control (idea as in Lambert2017)