Introduction: Is the Best PCB Etchant in Every Kitchen ?

About: I'm a 19 years old physics undergraduate who spends his time making experiments and asking questions. Otherwise I play the bass guitar and the saxophone, I love jass music ! Sometimes I also pick up the solde…
After a lot of reading and inspiring from Instructables for two years now I decided to step ahead and do my first Instructable on two subjects I particularly like : PCB making & The scientific method.
Feel free to comment and criticize my work, I think scientific method is also about improving and discussing the best way to experiment :) !
English is not my native language so please excuse my hesitating grammar.
5/30 Edit : I would like to thank you all for supporting this Instructables in the Scientific Method contest =D See you for other experiments !

Most of us Instructables users etch PCBs occasionally if not on a daily basis. This process is not anodyne as it may have a great impact on the quality of the future circuit, the environment and, last but not least, on your wallet !

Ferric chloride is commonly used to etch PCBs as it is both reliable and efficient. However, it's a dangerous product which requires special care in handling. Recently, some alternatives have been found to accomplish the same task. Among them is an astonishing hydrogen peroxide, salt and vinegar mix which seems to be a “Mac Gyverish” way to print a circuit board.

Let's assess the “Mac Gyverish” hydrogen peroxide/vinegar/salt mix using the “tried and tested” ferric chloride solution as a control !

To etch a PCB one can either immerse the board into a bath of etchant (most common technique) or rub it with an impregnated sponge. I have tested both ways with each etchant so that makes a total of 4 experiments.

The techniques and etchants will be compared according to four criteria :

  • Time/Handling
  • Quality of resulting circuit
  • Cost
  • Environmental impact

Step 1: The Theory Behind Pcb Etching

A copper board is made of an epoxy resin board covered with a thin layer of copper. The etchant reacts with the layer of copper and dissolves it, except where a mask protects the copper. The epoxy is left intact as it does not interact.

Here are the two chemical reactions taking place : (1) is ferric chloride’s (aka Iron (III) chloride) and (2) is vinegar/hydrogen peroxide/salt mixture’s.

(1) FeCl3 + Cu --> FeCl2 + CuCl 
(2) H2O2 + 2(CH3COOH) + Cu  --> 2(CH3COO-) + Cu2+ + 2(H20)

The reactant is the result of the combination of acetic acid (which represents 80 grams per liter of white vinegar according to the bottle [which says 8° acidity]) and hydrogen peroxide (which is at 3% mass concentration).

We can calculate the optimal ratio of each component in order to get a maximum amount of it.

Hydrogen peroxide is 34 grams per mole and acetic acid is 60 grams per mole.

So, in a liter of vinegar there are 80 grams of acetic acid which represents 80/60 or 4/3 moles of acetic acid. In a liter of hydrogen peroxide, which approximately weighs a kilogram, we have 3% x 1000 grams = 30 grams, nearly a mole, of pure hydrogen peroxide.

Because the reaction uses twice the number of hydrogen peroxide molecules of acetic acid molecules we can say that we need to put roughly a 2/3 ratio of hydrogen peroxide/vinegar in volume to get things optimal.

Now you may be asking yourself “Why do we want to add salt to that ?”.

Well, one possible explanation (I'm not actually sure it is the right one) is that the reaction brings neutral Cu atoms of the board to Cu2+ ions in solution that would form copper acetate (it is the combination of one Cu2+ ion and two CH3COO- "acetate" ions). And, at one point the bath would reach an equilibrium (because there is as much Cu2+ forming from Cu than Cu2+ getting back at a solid Cu form).

Generally this happens before your board gets its beautiful tracks, which is quite sad. Table salt, or NaCl, brings chloride ions Cl- to which Cu2+ ions will bond to form cupric chloride or CuCl2 instead of being left in solution (those ions would endlessly come back and forth from Cu to Cu2+). You can see this during the reaction (if you leave the reaction with no salt it will turn blue which is the color of copper acetate and stop, whereas if you put salt it will turn green, the color of cupric chloride, and carry on).

2Cl- + Cu2+ --> CuCl2

By doing so, the equilibrium point is pushed forward and more copper can be dissolved, so that, if you are generous on salt (if you introduce table salt in excess) you may get your PCB in the end ! One tablespoon of salt on the board is generally sufficient.

Hypothesis : Given the concentrations of hydrogen peroxide and acetic acid in our "recipe" ferric chloride should be way more efficient. However the Salt/Vinegar/Hydrogen peroxide seems to be both less expensive and safer.

Now comes the fun part, let's put up the lab coat and etch PCBs !

Step 2: Setup & Experiment

The experiment aims at comparing, in a "normal" situation, what etching method is the most efficient (immersed or rubbed ? Ferric chloride or Hydrogen peroxide/Vinegar/Salt mix ?)

I printed and ironed a toner "barcode" with strips of decreasing width (5/4/3/2/1.7/1.4/1.1/0.7/0.4 millimeters) on four identical boards, those will show the resolution achieved by each method (I chose resolution as quality indicator).

In my lab (aka shed), each board is then immersed in a small bath (approx. 200mL at room temperature : 70°F) of etchant or rubbed with an impregnated sponge up to the point where I can’t see any unmasked copper left. The board is then rinsed and the circuit is wiped with acetone to remove the mask.

In this experiment :

  • Control variables (aka variables that do not change throughout the experiment) are lab temperature, size of copper board, masking technique, quantity of etchant in the bath/sponge.
  • Dependent variables (aka variables that may vary because of tested factors) are achieved resolution, duration of etching process, cost and environmental impact.
  • Independent variables (aka tested factors) are nature of etchant (Ferric chloride or Salt/Vinegar/Hydrogen peroxide) and etching technique (rubbed or immersed).

Step 3: The Results

The experiment went very well even if got a little bit frustrated by my copper boards' poor quality and persistent toner ! ^^

Maximal resolution (0.4 millimeter) was obtained for each experiment which shows that the etchant/technique itself does not change the final quality of a regular PCB. I got some scratches on the rubbed PCBs because of the sponge (not very soft).

However the duration of etching process was not even : the shortest method is Ferric chloride rubbed (3 minutes) followed by Ferric chloride bath (10 minutes), Salt/Vinegar/Hydrogen Peroxide has the longest etching duration (11 minutes when rubbed and 20 when PCB is immersed).

Step 4: Cost and Environmental Impact

Salt/Vinegar/Hydrogen peroxide mix is, by far, the cheapest etchant (Hydrogen peroxide 3% is 2$ for 16 oz, Vinegar is 2$ for a liter and salt is almost negligeable) with a price of less than 4$ per liter.

Ferric chloride costs around 10$ for 16 oz which is arround 20$ per liter, five times more than the first !

In terms of environmental impact "sponge technique" is, by far, the less dangerous for both etchants (and the most economic with only 10mL of etchant used). You only need to rinse your sponge once finished, a tablespoon of etchant is not harmful (I've asked local authorities).

Ferric chloride bath needs to be stored and brought where it can be treated. I'm not sure for Salt/Vinegar/Hydrogen peroxide mix as copper precipitates (the solution gets a pale blue tint only).

In terms of handling Salt/Vinegar/Hydrogen peroxide requires a small preparation before each etching session (the solution does not stay efficient very long) but it is safe whereas Ferric chloride is corrosive (wear gloves ;)).

Step 5: And the Winner Is...

Salt/Vinegar/Hydrogen peroxide (immersed) and Ferric chloride (rubbed) are 1st ex-aequo. The first has the advantage of being both readily available in the kitchen and quite safe whereas the second is both cheap and quick =D !

Then comes Salt/Vinegar/Hydrogen peroxide (rubbed) which has all the advantages above plus the fact of being extremely cheap and green, however you need to rub the sponge for 11 minutes which is not that funny (with all the risks of destroying your tracks if you rub too hard) !

Finally "old fashioned" Ferric chloride (immersed) comes last as it is relatively both expensive and difficult to dispose of (if you are responsible).

Step 6: Conclusion and Possible Improvements

The experiment in itself did not separate the two etchants which are almost as efficient for a regular hobbyist use. However factors such as cost, handling or environmental impact played a key role in putting the vinegar/salt/hydrogen peroxide mix forward. The setup and experiment are easy to make which is good for reproducibility. Don’t hesitate to cross check my results ;).

Though, as all experiments this one has limits. In fact I tested only two etchants whereas dozens exist and may be a middle ground such as Hydrochlorhydric acid + Hydrogen peroxide (which is more powerful than the Vinegar/Salt/Hydrogen peroxide mix) or Cupric chloride (whose effectiveness is comparable to the one of ferric chloride).

However Ferric chloride had the advantage of being the most common method for etching PCB so I thought it was the reference to which I had to compare other methods and Vinegar/Salt/Hydrogen peroxide was amazingly inoffensive and available at minimal cost. I was also skeptical about this “Mac Gyver” method : this experiment helped me to figure out that it is reliable =D !

One could also say that I only tested one temperature (21°C). Although this is true those chemicals’ reaction rate have the same dependence upon temperature. This means that if one method is relatively more efficient at 5°C, it will also be relatively more efficient at 40°C (but the reactions will take less time). The same goes about reaction rate in a bubbling tank or other setup improvements.

Please feel free to comment and help me improve this experiment with new factors and/or new methods ! =D

I had a great moment creating this first 'ible, I hope you liked it too ;)

Those great pages helped me making this 'ible :

Etch PCBs in One Minute !

Stephen Hobley's blog

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