Homebrew PCBs

Stefan Trethan wrote:


> 
> Well, if i can get current densities that would be fine too.
> I just want to know if we are talking 10ampere or 100mA here, i have 
> absolutely no idea.

If you don't know the electrode surface area then the answer can 
be anywhere from less than 100mA to above 10A.

To take a guess I would start with cathode C.D from 10 to 50 
mA/cm^2 and anode C.D from about 2 to 10mA/cm^2.



> Also i wonder which supply is needed, i mean is it necesarry to be very 
> stable current?

A constant current source is not essential. Often the voltage can 
be set and the cell ohmic resistance is enough to get reasonable 
consistent current. When experimenting use a electronic power 
supply so you know for sure that your power source is stable. If 
you get into full scale then you should worry about power supply 
requirments.


> 
> 
> I also wondered if it would do any good to connect the board  to the anode 
> at the beginning.
> Maybe the electrolytic action would aid in taking off the copper. of 
> course you can't etch
> the entire board this way because there is no way of connecting it but it 
> takes a long
> time until the first patches break free. (certainly more than half the 
> etch time i think).
> 
> Would this work? Can there be significant speedup expected? Would a 
> electrolytically aided etch
> be even or not?

Well, one coulomb will plate 0.00034 grams of copper. Typical 
double sided "1 oz copper foil" PCB with 50% track area usage and 
10 X 15cm size will have total etched copper mass of 4.6 grams.
Total number of coulombs is 4.6 / 0.00034 = 13500

So to electrically etch in 15 minutes will require 15 amps,
and the current density is 15A / (10cm*15cm) = 100mA/cm^2

100mA/cm^2 current density is very high and it might also 
decompose water. The free hydrochloric acid content will need to 
me very high to keep up with the reaction;
Cu + 2e -> Cu+2

Most plating baths operate around 10 to 30mA/cm^2.




> I think i might get titanium sheetmetal for the anode, from which i could 
> cut stripes
> to mount to the pcb holder for connecting to the pcb if it works.
> This would allow a big anode area.

Graphite is often used for anode material in chloride solutions. 
I thought titanium anodes forms an oxide layer that gets thicker 
with time and eventually the insulation resistance gets so high 
the cell becomes too inefficient. Its not unsual for a titanium 
anodizing cell to run at 100 Volts !. Titanium anodes are usually 
a titanium substrate coated in a precious metal, like platinum. 
The advantage being if the titanium substrate is exposed (some 
precious metal coating chipped off) then the oxide layer forms 
and protects it from further oxidation.

Adam.