Using cupric chloride to etch copper alloys

My name is Ben Renard. I am a model railroad enthusiasts. One of the aspects of the hobby that I have dove into recently is photo etching printed circuit boards and thin parts from copper alloys. 

The basic process begins by coating the metal sheet or PC board with a etch resist film that when exposed to UV light hardens and cures. Transparency  film containing the art work is laid over the sheet and exposed to UV light. After exposure, the sheet is "developed" to removed the uncured resist, revealing the bare metal which matches the original 
artwork pattern. 

The sheet is then put in a bath of etchant. In my case I chose to use cupric chloride. If all goes right, all the exposed metal is removed. 

On thicker metal sheets, etch resist is applied to both sides. Etching occurs from both sides and hopefully meets in the middle. 

After doing a lot of research on the web, I had decided to use cupric chloride as my etchant of choice for a number of reasons.  

1) Spent etchant can be easily regenerated by introducing oxygen into the mix. This can be accomplished by bubbling in air from an aquarium pump, adding a little h2o2 into the mix, or simply let the solution absorb air from atmosphere. Typically, I use the h2o2 method. 

2) No extensive waste. Since the solution can be renewed, disposal is not an issue. 

3) Over time, the volume of the etchant grows since regeneration chemicals are added. 

During the etching process, the cupric ions become cuprous ions. The more cuprous ions, the less effective the solution is. 

I have complied a lot of reference material from the web detailing optimal conditions for etching. 

Below are some links to some of my base material

http://www.chemcut.net/pdf/Cupric-Chloride.pdf
http://members.optusnet.com.au/eseychell/PCB/etching_CuCl/index.html

I have also constructed an appropriate machine to safely execute the process. This machine sprays the solution onto the workpiece from both sides.

I am now at a point where web research and my personal knowledge have become virtually exhausted. 

My questions for you are how to maintain the optimal conditions of the etching solution before, during, and after the etching process based on the readings from my instruments. 

Here are my questions?

Spcific gravity:

My solution is heated and maintained at 120F.  I am using a glass hydrometer calibrated at 60F for my specific gravity measurment. The target is to maintain a specific gravity of 1.2 to 1.3.  How does 120 f affect the readings of the hydrometer?

Acid concentration:

The material states that the free acid level should be between 1-3N.  Is there a way I can use a PH meter to moitor this?  Or is taking a sample of the solution and perfroming a titration with NaOH the best way? Would the PH of the cupric cloride change as the copper ions change from the cupic state to the cuprous. Would this PH reading differ from a straight 2N HCl solution? I would like to moitor the solution live so I can make adjustements as the free HCL is consumed.

Regenerating the solution:

When the solution needs to be regenerated, I drop 35% H2O2 into the bath until it changes from an opaque olive color to a bright emrald green clear color.  When this happens, the sprayer pump seemed to load up more.  What is happening here when the solution is regenerated.

Thanks in advance.

Ben Renard 



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