# Electronic Circuits and Projects Forum

## Black dots on PCB - what am I doing wrong?

1. The rate limiting step(s) is oxidation of the copper at the surface. That is one reason spray etching is so fast. I don't think catalyzing those other reactions would help speed up the etching process.

John

2. Oh. Dang.

3. Lots of equations can be written for the reactions of copper, HCl, and H2O2.
Again, I totally agree! Just try to google "hcl h2o2 reaction"! Here is one link for example:
I studied physics, so I'm not too good in chemistry, but I know that, except Cl2 gas, and H2 gas also O2 can be produced (if you add too much peroxide)!

About GIF "decomposition": I'm using very good "Easy GIF Animator" - you can freely watch every individual image from GIF, also you can adjust time for each frame and extract images you want. I extracted 3 pics from PCBWING and merged them into one JPG, just for demonstration!
Formule-extracted.jpg
I'm still awaiting some practical recipe for my chemicals: 19% HCl and 12% H2O2! I found one "calculator" on this page:
http://www.dr-lex.be/hardware/tonertransfer.html
Thus, I should mix about 1,5 : 4 : 4 proportion (for peroxide, HCl and water, respectively) - is it a good mixture?

4. There are lots of recipes for HCl/peroxide etchant. I don't use it for the reason already mentioned and can't recommend a particular recipe. The link and calculator you give is probably as good as any.

It seems you have obtained fresh and/or have decided your HCl concentration is 19%. That is probably pretty close as constant boiling HCl is 20% by weight.

A word of caution about hydrogen peroxide. It, of course, also decomposes with time. More important, I found the concentration in consumer products is sometimes impossible to decipher from the bottle. In the USA, the antiseptic or drug store variety is about 3% or so by weight. Wikipedia gives another system of measurement (http://en.wikipedia.org/wiki/Hydrogen_peroxide) based on volume of oxygen released. There is also a system based on activity in bleaching hair. As I recall, those solutions go up to "40%", but that is not 40% by weight. It is 40% the activity of some standard beauty parlor activity. So, just be sure of the concentration you have.

Also, it is generally recommended to add the acid to the peroxide-water solution. Some people do add additional peroxide, if that appears to become depleted. That is probably safe, as the acid is considerably diluted at that point.

Do you have a reference for the production of hydrogen gas in this reaction? That seems pretty unlikely.

John

5. Originally Posted by John
It seems you have obtained fresh and/or have decided your HCl concentration is 19%. That is probably pretty close as constant boiling HCl is 20% by weight.
I just bought the new bottle of HCl (because, allegedly, the shelf life has expired some 11 years ago for my old bottle, although I'm not sure that HCl has shelf-life if stored properly (it was in a plastic bottle with some kind of seal on the cap)). I measured pH (with universal indicator) of my old HCl and it shows pH below +1 !? As I know, 20% (or 6M) HCl have pH of about -0,8 (so, it should be OK). Same result for the new bottle. On that new bottle of "technical HCl" stays 19%, but I saw before that manufacturers mark that "concentration range" as 19-21%. So, it is, in fact, azeotropic solution (about 20±1%). I bought pharmacy grade peroxide of 12% (in my state all the pharmacists MUST have master's degree (one level below doctor's degree), so I can be confident about peroxide concentration. Btw. I simply can't understand why people demand 30 (or even more!)% peroxide??? There is no any need for that concentration, and that is POISON! I can buy (not allways!) in the pharmacy that concentration of peroxide, but only if I give my IDs and put signature in special book (also known as "poison book")! Adults only!
Originally Posted by John
Also, it is generally recommended to add the acid to the peroxide-water solution.
You forgot that I have pretty much laboratory experience (as I said I studied physics) - of course, never add water in any acid (except if it is diluted enough)! But, we allways have to repeat that warning, if nothing, because of "young chemists" that make stupid things over and over again (like mixing acetone and H2O2 or so)! And if you have concentrated acid and peroxide (why?) - peroxide is like water - so I agree, first goes water (or peroxide) and than, carefully, add acid. About hydrogen gas production: frankly, I don't remember when I saw that; I personally think that predominant gas in our case is oxigen (as you pointed before), than Cl, HCl(g)...

6. Originally Posted by ChildOfVision
....When I put PCB in the solution it immediately turns bright-green, but etching took too long, over 45 min.!

Im' using muriatic/hydrogen peroxide for etching and a 4"x6" SS board striped clean flat 3 minutes..

7. ## What went wrong here ?

When I first saw the H2O2/HCl method I liked because it used easy to source chemicals as well as cheap. My first try was a small PCB like 2 X 1 inch and it was etched like in 10 minutes or so. However, when I tried a bigger PCB ( about 4 X 7 inches) and most of the copper had to be etched, it happened that a lot of areas seemed not be attacked or attacked at a very slow rate. The first try I had to leave the PCB for almost one hour but then EVERYTHING has got attacked (the copper under the exposed photo resist as well as the photo resist itself). I blame it on a poor copper cleaning job ( in spite I did it very thoroughly using detergent and the Scotch Brite pad).

too_much_etch.png

So the second time I doubled the time scrubbing and degreasing the copper. I also verified that running water made a "film" over the copper surface with no beading at all. I' ve prepared 2,100 cc of etchant made out of 1,400 cc of H2O2 ( 3%) and 700 cc of HCl bubble assisted and same result, after about 25 minutes , still plenty of copper being attacked at a very slow rate. I stopped there and below is the picture of it.

Could it be I need more solution (like a gallon or so) or what could be causing this?? I completed the PCB recurring to the good old Ferric Chloride and it also had some areas that took longer time to etch but I've got below PCB in about 15 minutes

PCB1.png

Thanks for any help guidance. I've made an acrylic container with the air pump but I can not use it with Ferric Chloride.

Acrylic_Cont.png

8. @jofre: at first, welcome to our forum (I see this is your first post here)!
Originally Posted by jofre
Could it be I need more solution (like a gallon or so)
NO!, afaik, that really doesn't matter!
Originally Posted by jofre
I completed the PCB recurring to the good old Ferric Chloride and it also had some areas that took longer time to etch but I've got below PCB in about 15 minutes
Do you want to say that with use of FeCl3 everything is OK? If so,what is the problem? Because you can't use your bubble container?
Edit: it is quite normal that some areas took longer to etch (copper-pour areas for example), it is important that etchant does not attack protected copper!

I found muriatic acid & peroxide very aggressive to laser-toner (if you use toner-transfer) and also to POSITIV-20 (i.e. photo resist)! Other members of this (and other) forum also noticed (in painful way) that! On the bottle of Positiv-20 is explicitly stated that FeCl3 should be used for etching! My very first boards made by HCl turned out good, but that (ancient) time I used nail-lacquer - but it is very resistant to all kinds of etching solutions! Now I'm turning to FeCl3 - it is messy, but for now is the best way.

Originally Posted by jofre
I've made an acrylic container with the air pump but I can not use it with Ferric Chloride.
Why you can't use it with ferric chloride? I want to build one such container, so it is important to me!

9. Thanks , yes in spite I read and learnt a lot of stuff in this forum since long ago, above was my first posting.

1)
Yes, everything is OK with FCl3 although the bigger poured copper areas took a longer time they were the latest to finish etching at about 15 minutes ( solution was at 60C).

2) The Acrylic container is nice with a clear solution where you can see. With the FCl3 you will need to keep taking the PCB in and out of the etchant. Other thing is that I keep the FCl3 at about 60C on a hot plate , then I use a Pyrex tray and I slosh it once a while. But apart from this, the vertical acrylic container can be used with FCl3 too.

Coming back to the problem with the peroxide / HCl . The problem on waiting a long time is that it will attack the exposed areas as shown in my first picture.

I am using Negative dry photo resist ( available on Ebay) and the last picture on above post shows my BEST PCB ever so, I have adopted this system over the toner.

I may do another try , now sanding the copper with a 400 grit sand paper , also scrubbing and perhaps using NaOH to rinse and degrease and try again to see if the copper is attacked in a more uniform way and get an overall etching time at or under 15 minutes.

If anybody has a better and proven way to clean / sand / degrease the copper just let me know and I will give a try!

thanks,

JV

10. Ok, I will not argue too much - we will see what others think about this, BUT I still affirm that HCl+H2O2 is too strong and aggressive solution for ANY photo resist and should be avoided! I have more than 20 years experience in making PCBs, and muriatic acid is cheap, always available and that's all. This etchant penetrates also through TRF-Green - I tried this more than once!
Originally Posted by jofre
With the FCl3 you will need to keep taking the PCB in and out of the etchant.
I agree. But you can try "sponge-etching" or maybe etching in foam!
Originally Posted by jofre
Other thing is that I keep the FCl3 at about 60C...
It is unnecessary high temp. if you ask me - many people work under room-temperature, eventually with bubble agitation. Btw. you can put aquarium heater in your tank and agitate solution with appropriate air-stone and stronger pump (I plan to do this).
Originally Posted by jofre
The problem on waiting a long time is that it will attack the exposed areas as shown in my first picture.
Again, it is only my opinion: it is not about long etching time (maybe in a smaller degree) but in solution itself - it is simply too aggressive. Some people have success even with toner-transfer, but that is IMHO a big frustrating lottery.
I repeat: HCl+H2O2 is not suitable for toner-transfer as well for (any) photo-resist method in at least 95% of cases (IMO)!
Originally Posted by jofre
If anybody has a better and proven way to clean / sand / degrease the copper just let me know and I will give a try!
Do you try with fine steel-wool with neutral soap (it looks like a small cushion) for (manual) washing of dishes? It does everything needed: sand, clean and degrease (in one go) and doesn't leave any "remnants". After that let the PCB dry and than clean it with acetone! So far, I didn't find better method.

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