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Is the best PCB etchant in every kitchen ?

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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
 
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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

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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

Picture of The results
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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

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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...

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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

Picture of 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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JesusGeek20 days ago

I use 1:1 ratio of vinegar and hydrogen peroxide, and add salt as I go. It works pretty well, but my main issues are

(1) What do I do with all this copper acetate! (main leftover solution, no I do not want to electroplate with it)

(2) Effective way to transfer traces onto board (soon to try using glossy magazine paper, gloss photo paper leaves broken traces all over, and using a laser printer)

what if i add a few drops of hcl in it

hey its not working jst wrkd fr the first time and irrespective of adding more of vinegar, h2o2&saltsalt

Quesnelquack2 months ago

I like the way you approached the subject with rigorous attention to detail and a scientific frame of mind. A few comments if I may:

1) The number on the vinegar bottle I think is in degrees Baume (density) rather than % concentration. I am not sure how this converts with acetic acid. For instance 20 degrees Baume HCl is about 35% concentrated.

2) The fact that you got better results with wiping than immersion demonstrates an important principle with all etching baths: There has to be agitation of the etchant. I run an aquarium pump through mine to create bubbles.

3) Ferric Chloride works better at higher temperature - I run mine close to 40 degrees C.

4) In terms of economics one has to point out that Ferric Chloride can be re-used - many times!

5) Environmental effects of disposal are determined by the copper ion content: For some reason even the small concentrations such as in your little set-up would be frowned upon. Ferric chloride by itself consists of chemicals which are naturally occurring in soil. OTOH if you want to get rid of roots in your garden or septic tank what do they sell you? Copper sulphate! Go figure.

6) Finally, having said all that about FeCl3, I have more or less stopped using it in favour of electro-etching with copper sulphate as electrolyte. However, for the occasional PCB that may way too much hassle.

With rock salt as a close second for the roots,but no doubt the Copper sulphate works better,and is longer lasting in this old plumbers opinion.

Many many valid points- and a very helpful article overall! Disposal of dissolved copper is always tricky; the electrochemical properties of copper make it a very good accumulator of other metals within a system that we otherwise don't want accumulating, such as Zinc or Magnesium. Caution is never overdone in the lab or workshop! I'd love to put some of these ideas to the test myself :)

crazypj2 months ago

Can the dissolved copper be used for copper plating steel/iron/ferrous materials?

That way it's kind of reclaimed and made into something 'useful'

Nice instructable mate

Feynmaniac (author)  syedhamzahasan2 months ago

Thank you =)

michaelmacnz2 months ago

Well done you... This makes it super simple to make a decision (choice) ... Thank you.

Feynmaniac (author)  michaelmacnz2 months ago

I thought it was a way to get an "objective" answer to this question of pcb etchant. It really seems to split people ^^

Thank you =)

weibbed2 months ago

Great Instructable! I'm just trying to make sure I got the solution proportions right for the hydrogen peroxide/vinegar/salt. Twothirds liter of hydrogen peroxide with one liter of vinegar and 1 TBSP salt? I got a little lost in the chemical formulas (which I am very glad you figured out and explained, but I'm just an end user, not a scientist!). I want to try etching some copper sheet for jewelry, not PCB boards. I plan on trying a Sharpie marker as the mask, since I know that works for PCB etchant on copper sheets.
Thanks for the idea to try. I rarely do metal etching since the disposal of the etchant intimidates me, but this would be great!

0x5c weibbed2 months ago

It won't work well. The hydrogen peroxide/vinegar/salt mix will dissolve the sharpie ink. It will be better for you to use the electroetching method (http://www.instructables.com/id/How-To-Electro-Etch-a-Solid-Metal-Plaque/?ALLSTEPS)

weibbed 0x5c2 months ago

Thanks for suggesting an alternative.

fezick2 months ago

So of course I wonder, can you extract the etched copper from the solution for later recycling or jewerly uses? Maybe through electrolysis?

Feynmaniac (author)  fezick2 months ago

I'm pretty sure you can recycle it for electrolysis. If you want you can check out this instructable : http://www.instructables.com/id/High-Quality-Coppe...

But for this electroplating you use copper acetate which means you'll have to avoid putting salt into the etching bath and adapt the mixture (you can stick to this instructable and simply put your board instead of copper scrubbers).

Whiternoise2 months ago

After staining more things than I care to count with ferric chloride I started using Sodium Persulphate. It requires a water bath to be quick, but this is fairly straightforward (just place the etch container in a bowl of very hot water). It's non-staining and the byproduct is copper sulphate solution. The main advantage is that it requires no agitation, the copper simply dissolves into the solution so you can easily check if it's done and there's no gunk to continually scrape off the boards. It's reasonably cheap, a small bag can be bought for £5/$5 on eBay and it lasts for a long time if you make up the solution as you go.

Don't fool yourself that persulfate is safe. I once spilled some on a blue bathroom rug. It left the fiber bleached yellowish white in no time.
Hmm wasn't aware of that, though wikipedia says it's essentially a bleach. May be just the thing to get rid of Ferric Chloride stains on my lab coat!

Also wasn't aware that it's used in hair bleaching products. Maybe a cheap source?
inkspot0992 months ago

I know that when using Hydrochloric Acid with Hydrogen Peroxide that using an air bubbler replenishes the oxygen and accelerates the action. I suspect that it would help with this Vinegar/Salt/H2O2 procedure also if doing a large board for example, or just to speed up the process.

Feynmaniac (author)  inkspot0992 months ago

I think that would make a great experiment =)

shobley2 months ago

This is fascinating... Thanks for taking the time to explain the process.

I don't know if I "invented" the process... but adding the salt seemed to do the trick for me, and created a bit of a debate when I published my findings.

I get quite a bit of email still from people who can't get the mixture to etch, so this will be a great resource to point them to.

Feynmaniac (author)  shobley2 months ago

Thank you for sharing this process. Without your blog I think I would never have imagined that this mix could work given the concentration of acetic acid in vinegar ! =)

I'm glad my main source actually likes this 'ible =) !

kbyrne2 months ago

Along this line is lowes muratic acid and hydrogen peroxide. Very low cost, foam brush used for board but no salt???

jhall30 kbyrne2 months ago

Muriatic acid is another name for hydrochloric acid, and is usually sold as a pool cleaner. I've found and purchased a 900ml bottle of a higher concentration on Amazon for just this purpose, but haven't tested it yet.

Feynmaniac (author)  kbyrne2 months ago

I think (I'm not sure) that HCl (muriatic acid) + Hydrogen peroxide does not need salt (NaCl) because it already has the chloride ions we are looking for to form cuppric chloride (CuCl2) :)

However I think this reaction creates toxic fumes, plus I feel more confident with a vinegar bottle than with a strong acid one ^^ !

Interesting result. I have experiment with both method, but with some what different result. What I find is it is very hard to get consistent result with peroxide/vinegar/salt. Sometimes it will etch quickly and sometimes not (or not at all), which is most of the time. Also it is not Sharpie friendly, ie. it dissolve the ink, which is not good if you make any correction on your board. Although clear varnish will work, but it is less convenience. The long etch time leads to under cutting of tracks and pitting on large copper area which is not good. Compare this with Ferric chloride, my initial impression was this is bad stuff; it stain everything, hard to dispose of and expensive. But then I came across this, the Edinburgh Etch:

http://www.nontoxicprint.com/etchcopperandbrass.ht...

By adding Citric Acid (lemon juice if you like) to the Ferric chloride, it improve the efficiency and the longevity of your etchant, ie. number of time you can use it. if you use Ferric chloride before, you know the more you use it the less effective it is as it form a layer of sediment at the bottom of the bottle, the citric acid remove this. Also, I find the temperature does matter. I usually have the Ferric chloride sitting in a warm water bath and it etch fast! normally 25-30C is good enough.

I don't do a lot of PCB, but the time I do, I like it to work first time, so I need something that will have a long shelf life and consistent in result. The Edinburgh Etch does give me that. BTW, proper way to neutralise and dispose of Ferric chloride is also well known; Calcium Bicarbonate or baking power. I do consider

peroxide/vinegar/salt as my secondary method, as say, you can get the stuff any where and it is not toxic.

Anyway, well done as I was waiting for someone to do a more qualitative work on peroxide/vinegar/salt method.

Keep up the good work!

I did not refer to this in my experiment but it’s true that efficiency really depends upon your Vinegar/Hydrogen Peroxide. Once I used what remained of an old vinegar bottle and I can tell you the result was quite sad ^^


However did you try to sprinkle some salt over your board while in the bath ? :)

I did, but the result again vary so much (from adding a tea spoon to half a bag) that it is hard to get consistent result, ie. a consistent repeatable recipe.

Pader2 months ago

Good instructable, we all need to make circuit boards at some time.

I have used a slight variation in the past which has given fairly good results and I find superior to ferric chloride etching. Instead of the vinegar and salt, I use ordinary, proprietary stone and patio cleaner which is available in 5ltr drums for around £12 (around US$20) in most garden centres or builder's merchants.

The cleaner that I have used contains 15% hydrohloric acid. This combined with the Hydrogen Peroxide provides me with the Cupric Chloride etchant. Better still, I do not have to throw the used etchant away because the more copper dissolved in the solution, the faster the etching qualities.

I have found that the faster a board is etched, the better the resolution as the etchant does not have time to seep under the mask. to help with this, I use a loop of PVC tube with small 'pinholes' along it's length in the bottom of the etching tray and connect it to a cheap aquarium type air pump. I also heat the etchant to around 60 degrees Fahrenheit.

I have no idea about the chemistry data involved, I picked the idea up at sometime during my advanced years. Perhaps you could do a similar analysis to your etchant for this one?

cburg2 months ago

Very Nice write up. I have tried to use Hydrogen Peroxide with small amounts of muriatic acid. I found it to work well; however, it was not until I began to use stronger Hydrogen Peroxide that I got even greater results. Turns out at any beauty shop supply store as the stuff in quarts and gallons up to 40%. the cost is still not what Ferric Chloride cost and there is no shipping problems.

Now I want to use you figures and look into the use of salt. I think we have a winner!

Thanks again for come great information.

Cburg

Necesito saber que cantidad de cada uno utilizas! Gracias!

rayj00072 months ago

For the sake of repeatability, it would be good to provide the exact materials you started with.

The dimensions of the blanks you started with, including the thickness of the copper coating and perhaps post the image of the bar code you used for masking.

I'll be trying the vinegar solution next time I get a chance. I don't make boards very ofter and it'll be easier to find this stuff in the kitchen than to figure out where I stashed the bottle last time I used it. Thanks for doing the instructable.

Green Reaper2 months ago

Being an "ole tyme" photographer, another, more potent form of Acetic Acid is used in the print and film development process and might still be available in some photo stores that still deal in paper, film and chemistry. We used glacial acetic acid and at home I used indicator stop bath. Usually an 8% solution, if I'm not mistaken. I will defiantly try this as I'm always looking at etching PCB's. Keep smiling and nice presentation.

Feynmaniac (author)  Green Reaper2 months ago

If you've got it I think it is worth a try. What's nice with this method is that it works also with citric acid contained in lemon juice (it is really a kitchen based etchant ^^).

Thanks :)

rachel2 months ago

I love this instructable! Very clear experiment, well thought out and well documented. Also highly useful information.

Feynmaniac (author)  rachel2 months ago

Thank you, it's great to see how supportive are instructables' members :)

leven2 months ago

Nice instructable, going to have to try it real soon =)

Also, I had an idea, what about putting the container with the bord and Vinegar/Salt/Hydrogen peroxide mix in a waterbath in a ultrasonic cleaner? Going to try that too, should speed up the process quite a bit? Could wash away the resist maybe, but i will try it with my spraypainted copper boards and laser etching, so shouldnt be a problem.

Feynmaniac (author)  leven2 months ago

Sure you should try to derivate this method, I'm sure more can be done to improve it :)

Tell me if you've got some results ;)

billbillt2 months ago

got my vote(s)...

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