I can't exactly answer that, But here is something that might help.
This is for plating but the chemistry is there.
Chemical Reactions
Alkaline etch is basically cupric chloride with the copper complexed with ammonia to keep it in solution at a higher pH. In the correct pH range the etchant will still attack copper but is much less aggressive towards tin, solder and nickel.
The etching reaction:
__2Cu0 + 2Cu+2(NH3)4Cl2 →4Cu+1(NH3)2Cl
The regeneration reaction:
__4Cu+1(NH3)2Cl + 4NH3 + 4NH4Cl + O2 →4Cu+2(NH3)4Cl2 + 2H2O
__The ammonia (NH3) and ammonium chloride (NH4Cl) are provided by the replenished solution. The oxygen comes from the air brought into the etch chamber by the ventilation system.
Example: How much air must be pulled into the etch chamber in the previous example to regenerate the etchant?
Solution: From the regeneration reaction it can be seen that each 4 moles of copper etched requires 1 mole of oxygen (O2) for regeneration (4 TO 1) ratio. From the prior example we know that the amount of copper etched per hour is 34119 grams. ( 1219 oz )
Moles Cu etched =
34119 gm 63.54 gm / mole Cu = 537 moles Cu
Moles oxygen needed for regeneration = 537 moles Cu
4 moles Cu /mole oxygen = 134.2 moles oxygen
134.2 moles O2 x 32 gm O2 / mole = 4294 gm O2 needed for regeneration
Air is 20% O2 so 4294 gm O2 x 5 = 21472 gm of air is needed to supply the necessary oxygen.
1 mole of air weighs approximately 30 gm and one mole of air occupies a volume of 22.4 liters
Air flow needed to supply enough oxygen for regeneration:
= 21472 gm air
30 gm air / mole x 22.4 liters / mole = 16032 liters / hr = 566 cubic ft / hr
If the system were airtight then 566 ft3 / hr would be all the flow needed
2.15 cubic feet of air per oz of copper.
one of these can produce about 2 cfh. http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/6509/cid/1785
Chemical Processing Parameters for Alkaline Etch
There are four chemical factors that contribute to etch rate and undercut. They are:
__pH
__Copper content
__Chloride concentration
__Temperature
All of these factors must be analyzed and controlled in order to find the best compromise between the fastest etch rate and the least amount of undercut. The effects of each are discussed in the following paragraphs.
pH
The pH of the etchant is a measure of the relative amount of free ammonia (NH3) that is available to the etching process. Most alkaline etch baths are designed to work in a pH range of 8.0 to 8.5 but there exist specific low pH alkaline etch formulations designed for fine line etching with pH as low as 7.8. The etch rate of the bath increases as the pH changes within these limits but the pH of the bath also has an important effect on the undercut. Under ideal etch conditions, the
diamine monovalent copper complex [Cu(NH3)2+] formed during the etching reaction forms a film on the sidewall. This film acts as a natural banking agent protecting the sidewall from lateral etch. At the upper end of the pH range,
however, the film is rapidly dissolved away in the presence of free ammonia and dissolved air. Typically the undercut is 30% to 40% more at a pH of 8.5 than it is at a pH of 8.1. For this reason Chemcut recommends that the etch bath be run as close to the lower pH limit as possible for best undercut results. Operating below the
recommended range will cause the enchants to attack the tin in tin-lead resists. Even worse, running under the recommended pH minimizes the effectiveness of the buffering system and the etchant could enter a condition known as “sludge out” where the copper-ammonia complex precipitates out of solution. Once out of solution the copper-ammonia complex will not redissolve and
the etcher must be emptied and recharged with fresh etchant.
Chloride Concentration
The chloride concentration indicates the amount of ammonium chloride (NH4Cl) present in the system. As the chloride concentration increases, more copper metal can be held in solution, allowing a decrease in the amount of undercut. The chloride component also acts as a buffering agent in the etchant, permitting a narrow pH window. The ratio of chloride concentration to copper concentration is important in the etch bath. The etch solution will become more corrosive as the amount of chloride above the stoichiometric balance (2 moles of ammonium chloride per mole of copper) increases. In this condition the etch solution will readily dissolve away the protective film on the sidewalls formed by the diamine copper complex.
:arrow: :arrow: :arrow:
