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.
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:
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.
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!
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:
The etchant has a deep green transparent color.
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.
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.
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.
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.
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.
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.
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
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:
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.
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).
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 cm(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
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.
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)