Homebrew PCBs

On Sat, 20 Aug 2005 02:38:22 +0200, <JanRwl@...> wrote:

> Can someone here PLEASE outline the procedure and chemistry to  use for  
> this
> etch-method?  I have never seen ANYWHERE a list like,
> "1.  Boil the bare blank in swamp-water;
>   2.  Incant;
>   3.  Swish the board around in a 30% CuCl solution  over 80\ufffdC for 15  
> minutes;
>   4,  Etc. etc."
> With FeCl, you can omit steps 2. and 4., and the result is  fine, but  
> messy.
> Please?


I'll try.
Let's divide that into 3 parts:
Etching boards, maintainance, and first "brewing" of CuCl.


For normal etching, you'd need to do nothing much. You'd just put a board  
in, and add H2O2 if the etchant is too dark in color. a few ml is enough  
to oxidize it. The color is all you need to determine if it is ok or needs  
something. Then you turn on the bubbler and simply wait until done.  
Nothing needs to be done after etching.

The adding of hydrogen peroxide (H2O2) takes the hydrogen away from some  
of the hydrochloric acid (HCl). That means the chlorine is freed and can  
"take" one copper from Cu2Cl (the dark brown stuff) to form more CuCl  
(light green), and water. CuCl is now prepared to take a second copper  
again, thus etch the board. You can also introduce the oxygen from other  
sources, like pumping air through it (takes long), or even chlorine gas  
directly, but that's poisonous.


maintainance:
 From time to time you might want to measure the molarity of the HCl in  
there. That is, how many hydrochloric acid is still "free" (not carrying  
copper) and can be used to form CuCl. Because every time you add H2O2 some  
of the HCl is converted into CuCl. At some point, if all HCl is used up,  
adding H2O2 will not bring on the expected brigtening of the etchant and  
increase in etch speed. Also, the etchant will have a bluish color ad  
deposit a blue sludge on boards. This is a sure indicator there is not  
enough HCl, but you sould have added some by then.

To measure the HCl molarity, you need 4 things. A alkaline solution of  
known molarity (NaOH), some indicator (like methylorange), and two  
eyedroppers. You can get your NaOH solution by solving a weighed amount of  
developer in water. It should be a 1 molar solution. How much you need to  
weigh is said in one of the pages in the links section. When you have that  
the procedure is simple. Put 10 drops of etchant in a small container, add  
some water if you like, and one drop of indicator. It's now orange. Then  
add the alkaline solution drop by drop, all the time swishing the  
container to mix it. When it turns green stop and your number of alkaline  
solution drops added is your molarity after dividing by 10.
I don't do that often, and usually only add HCl by feel. A wide range of  
molarity will be ok. I keep it around 1. Still every few months i feel  
like checking the molarity.

There's also the density to be measured. Adding the copper makes the  
etchant more "heavy". In a away this is good - because you'll have more  
CuCl do do the etching. But if it gets too much you need to remove some  
etchant and add water. I have never encountered this, i guess the water in  
the HCl and H2O2 added is enough. You can measure the density with a  
hydrometer, which you can simply by in a car parts store, as a tool for  
checking car battery acid. It comes in a neat huge "eyedropper" in which  
the hydrometer swims.


First brewing:
Some tutorials call for adding copper in some form.
I have found you can start just as well with only HCl and H2O2, and water.
At first, you don't have any CuCl, but those two chemicals will etch all  
by themself. You can make etchant up by diluting HCl to about 10% and  
adding a swig of H2O2. Then put in the board and go by color.
At first there is none or little CuCl, so you have to add some H2O2 each  
time you etch. As copper content increases you get more and more CuCl,  
which does the etching and must only be re-oxidized from time to time. I'm  
now at a stage where i need to add H2O2 maybe each third or fourth board.

Here you go.

ST