Etchant solution

[psycho]

Hey all, anyone used Muriatic acid and Hydrogen Peroxide to make an etchant solutiohn? I know it will work (pretty well, too) but I have read conflicting instructions on mixing the two together. Some say to add the acid TO the peroxide and some say add the peroxide TO the acid... Which is correct? PLEASE don't guess! I plan on doing it outside but I don't want any mishaps even in the open spaces.

Anyone mixed this before?

Thanks,
Kevin

[reloadron]

We start with the old add acid to water or add water to acid:

A large amount of heat is released when strong acids are mixed with water. Adding more acid releases more heat. If you add water to acid, you form an extremely concentrated solution of acid initially. So much heat is released that the solution may boil very violently, splashing concentrated acid out of the container! If you add acid to water, the solution that forms is very dilute and the small amount of heat released is not enough to vaporize and spatter it. So Always Add Acid to water, and never the reverse.

Though it holds true mainly for very concentrated acid the best safety practice would be to add your acid to your peroxide. Hydrogen peroxide itself is a weak acid, so we can assume that the acid concentration of the peroxide is weaker than your concentration of what I would guess to be sulfuric acid as an etchant. When mixing two acids you want to mix the stronger to weaker.

Ron

[psycho]

The acid I have is from my local pool supply store. It is called muriatic acid but it's actually Hydrochloric Acid (~30%).

I am making a Wiggler type JTAG cable circuit. I bought a Brother HL-2140 (I think) laser printer for $50 at officemax. I have read that toner transfer doesn't work with some Brother printers. I can say that it worked great for me with this printer. I have yet to actually etch the board but the design transferred to it quite easily! A few touchups with a sharpie and it should work great (I hope).

Instead of using photo paper, I used label sheets with the labels removed (the really smooth side after the labels are pulled off). I have also read that you can use magazine paper but I have not tried that.

Thanks for the info!
Kevin

[CeaSaR]

Psycho,

Are you using the method shown here? Look around 3:10.

Amazingly enough, I was going to ask about this earlier this week, but other duties called.

CeaSaR

[psycho]

Unfortunately I cant look at any videos right now as our internet is FAP's (hughesnet fair access policy). This means that I am going to be below dial-up speed for the next 24 hours.

But, the basic toner transfer method can be seen here: http://www.5bears.com/pcb.htm

Here are some google results: http://www.google.com/search?q=toner+tr ... US267US268

If you have not tried it, you should. I don't want to spend the money for a board house for simple boards... It is quicker and cheaper to do at home (messier, too).

Kevin

[jimmy101]

Always add the acid to the water. 30% HCl is pretty concentrated acid and will generate a fair amount of heat when mixed with water. The hydrogen peroxide is probably 3% so that basically water from a mixing standpoint.

Any source that says to add the peroxide to the acid is wrong.

Add the acid slowly with stiring.

[haklesup]

All the etches I know of that use peroxide use the really strong 30% or 40% solution which you can't easily buy retail. Since 9/11 it's considered bomb making material (the subject of why we can't bring liquids on a plane). Muriatic acid is basically pool supplies and so is easy to get.

Whats the recipe you are looking at. At 30% its not water and will burn you faster than the acid. Add acid to water may not be a valid rule of thumb.

There are safer copper etches and more stable ones as well. An acid-Peroxide etch would only have a shelf life of a few hours max. I flipped thorogh my etch book and Peroxide-HCL is only listed for metals like Aluminum, Molybdnum, Chrome but not for copper. I would think it would be too agressive. Usually peroxide acid etches because the peroxide component oxidizes the metal and the acid consumes the oxide. A very very agreessive etch would be Sulphuric and peroxide, that one is called pirhanna because it etches almost any metal fast even gold.

For solder we used Acetic and peroxide and copper we used amonium hydroxide and peroxide. In every case it was 30% peroxide.

[psycho]

It uses standard 3% over the counter Hydrogen Peroxide (2 parts) with (1 part) Muriatic Acid (from a pool supply or hardware store).

Kevin

[jimmy101]

The heat of dilution for 30% H2O2 is pretty small, only about 0.2Kcal/mol. For 30% HCl it's more than ten times bigger at 2.7kcal/mol. So you need to add the HCl to the H2O2.

http://www.h2o2.com/intro/properties/thermodynamic.html
http://www.jsia.gr.jp/data/handling_02e.pdf

For this particular etchant it's just 3% H2O2, so it's basically water (heck, it's used as a mouth wash). 3% will bleach hair and clothes so try to to splash it about too much.

[kheston]

I just finished a chemistry course at school, I asked my instructor the same question. I know it's been mentioned above already, but for emphasis:

"do what you ought'a, add acid to wata' (water)" with a Bostonian accent...

[sofaspud]

Where did this idea originate?
Not being a trained chemist, I wanted to verify my hunch and found this:
"Aluminum reacts analagously to zinc: quite lively with concentrated hydrochloric acid. Copper, on the other hand,
is completely unreactive. This can be explained by looking at its position in the electrochemical series of metals.
Copper belongs to the noble metals. These stand in the row after hydrogen and are not affected by non-oxidizing
acids, which is why they are not dissolved by hydrochloric acid."

Edit: Ahhh, the hydrogen peroxide is the oxidizing catalyst.
Told ya I wasn't a trained chemist...

[jimmy101]

the hydrogen peroxide is the oxidizing catalyst

Actually, it is the oxidizing agent and not a catalyst. The peroxide is consumed during the reaction, a catalyst is not consumed.

Peroxide is a stronger oxidizing agent than oxygen (O2). Both are strong enough oxidizing agents to "rust" copper, the peroxide just does it much more quickly.

The two redox half reactions;
Cu0 --> Cu+2 + 2e-
H2O2 + 2H+ + 2e- --> 2H2O

(This forum does not appear to support the html or BBCode subscript and superscript tags. )

Actually, Copper can also be oxidized to Cu+1 but acid + peroxide usually gives Cu+2. The peroxide is consumed and converted to water, the copper metal goes to cupric ions, and acid is consumed.

There is also a side reaction. Cu+2 (or copper metal) catalyzes the disproportionation of peroxide to oxygen and water. The Cu+2 is not consumed in the reaction and is acting as a catalyst.
2H2O2 --> 2H2O + O2
The oxygen bubbles out of solution once the water is saturated.

It is pretty easy to calculate how much acid and peroxide is needed for a particular weight of Copper foil. As you can see in the equations above, you need 2 H+ and 2 H2O2 per copper. The typical recipe is 1 part ~30% HCl and 2 parts 3% hydrogen peroxide. The ~30% HCl is about 9M (molar), diluting gives ~3M. The 3% peroxide gets diluted to 2% which is about 0.6M. So the peroxide is the limiting agent in the reaction.

The molecular weight of copper is 63.5g/mol and we need two moles peroxide per mole of copper. So for X grams of copper;
(Xg Cu)(mol Cu/63.5g Cu)(2mol peroxide/mole Cu)(L/0.6mol) = 52mL of etchant per gram copper.

I believe a common copper clad board is 1 oz copper per square foot of board ("1oz copper clad"). One square foot of board is 28.4 grams of copper (X in the equation above). Plugging that into our equation we get that 1 square foot of board needs 1.5L of the etching solution. That's what is calculated to remove all the copper from 1 square foot of single sided 1oz board.

Chemist typically use at least a 10% excess of reagent to make sure a reaction goes to completion. In this case, I would go with at least a two fold excess, ~3L (~3/4 gallon) etchant per square foot of 1oz board. There are a couple reasons for the large excess; (1) the peroxide and acid are cheap compared to the time and cost to prepare and etch the board. (2) There are side reactions that consume some of the peroxide. (3) The large excess of the etchant means the reaction doesn't slow down too much as the peroxide is consumed.

Since I would use twice as much etchant as required it isn't necessary do the calculations all that precisely. So the "~"s in the description and equations don't really matter.

[Bob Scott]

Hmmm. What was that again . Lessee,

Do like an otter, into acid pour the water. no, no

oh no, oh heck, no

Like, do some acid. Put the otter in the water.

[CeaSaR]

Be on acid, put an otter in water?

Huh? Watch the multicolored otter... wow...


CeaSaR

[dr_when]

Why bother with any of this. Take a small foam sponge and dip it into Ferric Chloride etchant (with gloves on) and go over and over the copper for a few minutes and you can remove the copper from a small board with less than a capfull or two of etchant. Doesn't seem possible but it works. I had to try it because I was skeptical.