Chemistry of the Cupric Chloride and Ammoniacal Etchants
Replenishment of the Cupric Chloride Etchant
The Cupric Chloride etchant generates large quantities of Cuprous ion as the Copper is being etched. It is important that this be gotten rid of as soon as possible, to maintain etch speed. The Cuprous ion is “gotten rid of” by converting it to Cupric ion, which becomes the etch feedstock for the next set of Copper to be etched. There are a number of systems for doing this. We will discuss three of them.
Chlorine as a Replenisher
The simplest replenisher, from a chemical point of view, and arguably one of the most difficult to implement in the real world, is Chlorine. The use of Chlorine as a replenisher is elegant in many ways, because the etchant then becomes, in theory, steady state. The overall reaction is this:
There is only one reagent that is consumed, in theory, and this can be monitored by an Oxidation Reduction Potential (ORP) meter. Some Hydrochloric Acid (HCl) will be lost due to evaporation, and dilution, and this will have to be monitored. However, since the HCl is not consumed by the etch/replenishment reaction, it will vary in concentration much more slowly.
Further with the use of Chlorine as a replenisher, it is possible to achieve the widest possible operating window on the etching parameters. It is possible to etch with, and maintain etch solution Baume
s as high as 40+, and thus achieve very high etch rates, with very low etch factors (see chapter 12). And it turns out, that the economics of the use of Chlorine are also the most attractive.
However, the practical difficulties of using Chlorine for replenishment, not just the actual safety issues, but more importantly, the perceived problems, in the heavily legislated, risk-averse, environment of today, make the use of Chlorine not very common, even in the largest facilities.
It should be noted here, that much of the perceived safety of alternate replenishers is just that, perceived. Any replenishing system in use today, if the controls do not function correctly, can generate significant quantities of free Chlorine gas. Chlorine gas can, of course, be very dangerous to any form of life, even to process engineers.
In the ideal system, the ORP of the etchant is continuously monitored, and Chlorine is injected to raise the ORP to the point where there is just the barest trace of free Chlorine. The HCl can be controlled either by an automatic titrator, conductivity monitor or by manual titration. Water is added automatically by a device that continuously measures the specific gravity of the solution, (“Baume Controller”) and adds it to keep it below a set point. This maintains the Cupric Chloride content at a point below saturation, so that the etcher remains sludge free. Excess etchant overflows the etch sump, and is pumped to storage where it is later sold. The Cupric Chloride produced this way is very pure, and is more readily sold, and can get a better price than etchant contaminated with Sodium Chloride.
The Sodium Chlorate/colorimetric Replenisher
Perhaps the most popular method of control and replenishment of the Cupric Chloride etchant today in the PCB industry, is the colorimetric Sodium Chlorate 
method. This method has some inherent safety features, but these features come at a price. The overall etch/replenishment reaction, using this method becomes:
Sodium Chloride is also added to complex with the Cupric Chloride generated, and form the complex 
. This is obviously a more complicated reaction, but to the credit of the colorimetric control technology, this is all made invisible, and easy. There are two problems that come attached to this type of system, the obvious one is that the spent Cupric Chloride etchant is contaminated with Sodium Chloride, and thus becomes more difficult to get rid of. In fact, most facilities using this technology in the United States are forced to dispose of spent etchant to hazardous waste disposal/recycling companies, who actually charge to remove it from the facilities.
Further the fact that the etchant contains Sodium means that the Baume (which means Cupric Chloride concentration) must be kept fairly low (<22 Be), so as not to precipitate Sodium Chloride from the etchant solution. This makes the etch chemistry slower than it would be at a higher Baume (concentration), and further increases the volume of spent etchant, by as much as 100%, thus increasing disposal costs.
An incidental advantage to running the etchant with zero free acid, is that if, or as more experienced people would say “when”, an excess of Sodium Chlorate is added to the etch sump, (beyond that required to convert the then existing Cuprous to Cupric), the excess Chlorate reacts only slowly to form Chlorine gas, and is thus a safer chemistry.
Hydrogen Peroxide as a Replenisher
The Hydrogen Peroxide 
system of replenishing has been unpopular in the PCB industry, for no outstanding reason, but not in the closely related Photo-Chemical Milling industry. Perhaps the real reason why it is not popular, is that a reliable control system, for both components required to replenish the system, Hydrogen Peroxide, and Hydrochloric Acid, has only recently become available. Previously, it was probable that those people who took on the challenge of devising their own control system would likely have chosen the least expensive, and easiest system to control replenishment system, which is the Chlorine system. The overall etch/replenishment reaction for Hydrogen Peroxide is:
The use of Hydrogen Peroxide also produces a purer Cupric Chloride, without Sodium Chloride (NaCl) impurities, but the Hydrochloric Acid level must be carefully monitored, as it is consumed in the regeneration part of the reaction. This replenishment method also suffers from the fact that over-replenishing with the Hydrogen Peroxide can generate free Chlorine gas.
The fact that the replenisher ingredients are supplied in water solutions, as well as the fact that the replenishment reaction produces water, limits the maximum Cupric Chloride content of the etch, but it is still possible to get to as high as 
, depending on the concentration of the Peroxide and HCl replenishers.
The Chemistry of the Ammoniacal etch
Just like the Cupric Chloride etchant, the Copper that is already in solution in the etchant, dissolves the copper metal on the board.
Key things to remember to sort this explanation out:
- The metal is dissolved by being “Oxidized”, which means that it loses electrons, which changes it from the orange metal to a water-soluble blue compound.
- The electrons are initially removed from the Copper on the panel by the copper that is already in solution. This seemingly unlikely idea can only happen because copper can exist in 3 forms:
- Orange Metal - copper with a “correct amount of electrons”
- Cupric salts,
(blue) — copper missing two electrons. This is the blue, water-soluble form we are all familiar with. - Cuprous salts,
(white) — copper missing one electron, or half way between form 1. and form 2. This form is usually insoluble, unless it is chelated, and is not often seen in a board shop in large amounts.
The actual etch reaction is:
Reaction 1.
The cuprous salts are then (supposed to be) immediately oxidized (electrons removed) by the Oxygen in the air which is being pulled through the etcher. In the process, the spent Oxygen is converted to water. (See reaction 2.)
Reaction 2.
All of this is facilitated by the ammonia and chloride in the etchant, because the cuprous () compounds are not soluble, except in the presence of the ammonia (
). The overall reaction is this:
Reaction 3.
After the etch reaction produces the Cuprous copper, the air reacts with the Cuprous created to change it to the blue Cupric form, where it is ready to etch more metal:
Reaction 4.
This is why the ammoniacal etchant is (or should be) inexpensive to run, the etchant replenisher is really only a “receiver” or carrier for the copper, the real “fuel” driving the etch is air!
The real complexity in running this etchant comes from the fact that the etchant absolutely must have air blown through it to supply the air to oxidize the Cuprous to Cupric, to continue the etching process. However, if too much air is put through, too much ammonia will be lost by evaporation, and the pH will drop excessively, which slows the etching speed, and ultimately can cause the etchant to precipitate, or “sludge”. Too much air being pulled through the etcher can also cause the pH to drop because the Carbon Dioxide 
in air reacts with the ammonia in the etcher to form Ammonium Bicarbonate, thus consuming free ammonia, and lowering the pH, (see reaction 5).
Reaction 5.
The theoretical amount of air that must go through an etcher to convert the Cuprous to Cupric is only 0.94 cubic foot per square foot of one ounce copper that is etched. Because all the Oxygen coming in to the etcher does not react with Cuprous ion, the actual amount of air required is larger. The actual amount of air required is dependent on many factors, eg. where the air comes in, and the droplet size in the etcher. It is likely that most etchers have more air pulled through them than is required for oxidation of the Cuprous ion. This is because excess air keeps the odor of ammonia in the area of the etcher down. This can cause loss of ammonia, and cause the pH to fall.
Too low a pH causes etching problems. One of the most important control factors of the ammoniacal etch is the pH. The reason for this is that the pH controls this reaction:
Reaction 6.
In other words, the ammonium ion is the spent ammonia after it is reacted with acid. What this means is that the pH controls the “free” ammonia. Ammonia that is in the form of the ammonium ion (
) cannot complex with the cuprous ion (see Reaction 3.), which is necessary for etching to occur; this is why ammoniacal etchants etch faster at higher pH (but, also lift photoresists).
At pH 9.25, half the Ammonia present is in the free ammonia () form, and half is in the Ammonium ion () form. Notice that this is a reversible reaction, and a particular ammonia molecule can exist in either form, depending on the pH. At pH 8.25, only one tenth of the ammonia present is as free ammonia, the rest is as ammonium ion. And at pH 7.25, only 1/100 the Ammonia is present as free ammonia, and 99% is present as the inactive ammonium ion. Given the fact that the etch rate is so dependent on the level of free Ammonia, it becomes obvious why the pH is so critical to the etch rate.
The bottom line on running the ammoniacal etchant is that it is inexpensive to run, but it must be carefully controlled to perform at its best. Checking only the Baume does not give enough control to result in uniform etching.