Etching the PCB

The APL has a tank of nasty copper chloride etchant (the polypropylene cereal box with nuclear green liquid inside). This functions via the following reactions. The obvious attraction of this process is that it produces no waste stream: The result of etching copper is more etchant!

Cu(s) + CuCl2 -> 2CuCl

CuCl + chlorinating oxidation -> CuCl2

---

Aside:

The tank contains a not insignificant amount of hydrochloric acid (the unmistakeable smell of which is detectable when it is open). However, the HCl is not free: It is almost entirely bound up in complexes with the copper chloride. Transition metal salts are known for being extremely colorful, and copper is no exception.

Dilute solutions of copper chloride in water are a serene blue (the copper ion coordinates with 6 waters). Intermediate concentrations are yellowish, and the addition of hydrochloric acid to a concentrated solution results in the copper ions forming a complex with two waters and four chloride ions (CuCl_4^{-2} \cdot 2H_2O), and it is this complex which gives our etchant its beautiful and almost hypnotically intense green hue.

The second reaction above is a summary of what is actually a rather complicated series of ion-complex reactions. The first reaction proceeds rapidly, while the second is the rate limiting factor (the insertion of a powerful oxidizer, like hydrogen peroxide, to the solution can speed this up).

This can be observed if the board is withdrawn entirely from the etchant solution to examine: Immediately, brownish liquid can be seen draining off of all exposed copper areas - these drops are etchant in which all available acid has been used up and copper (II) has saturated the solution with brown copper (I) ions.

This underscores why turbulent action is essential to the etching process: Like photographic development, etching is a surface reaction whose components come from a bulk fluid, and the bulk must be turbulently washed over the surface to continuously bring "new" etchant, unladen with copper I ions, within diffusion range of the surface.

---

The action is slow at our room temp (22C) and may require as long as 30 minutes of agitation. Agitation *must* be continuous (and currently, by hand as we lack a bubbler). The etching process occurs at the surface and not in the volume, thus it is a diffusion limited process - agitation is *essential* or the metal simply forms a depletion zone adjacent to itself devoid of etching ions. Etching is complete once close inspections shows NO remaining exposed copper.

Note that the board cannot be left to etch unattended even with automatic agitation - the acid etches across as it cuts down (at a fairly small angle), but if left indefinitely it will eat *all* the copper off the board.

Once etching is complete, remove the board from the etchant. Try to shake as much etchant as possible back off into the tank before putting the board into a prepared small tray of sodium bicarbonate solution. This will immediately halt the etch action, and remove all remaining copper ions from the etchant solution that remained to the board. This is important because putting copper ions down the drain is very illegal:

https://nature.nps.gov/water/ecencyclopedia/assets/contaminant-pdfs/copper.pdf

Copper ion concentrations measured in single-digit parts per million are dangerous to aquatic life, and the required dilutions for copper ion contaminated water to go down the drain are concomitant: Our tank of etchant would contaminate several olympic swimming pools of water beyond legal release limits.

Under no circumstances allow any amount of green etchant solution to go down the drain! If any does, dump the sodium bicarbonate in immediately, chase it with a torrent of water, and pour additional bicarbonate in the sink straight from the box to 'kill it' before it enters the wastewater stream. If substantial spillage (more than a thimble full) occurs, get Dr. Boggs and call EHS immediately. A large release of copper ions down the drain threatens the entire city's wastewater treatment plant.

Gentle brushing will take any copper bicarbonate off the board. Once the board is clean, a remarkably small amount of poured acetone will strip the resist off of the copper exposing the beautiful tracks.