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P.O. Box 1606, Palmer Lake, CO 80133
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Peroxy-Sulfuric Etching Module
Operation/Overview I

The bad news first.

Before you even think about setting up to chemically etch printed circuit boards at home, or in a small shop, it is a good idea to get a single ethic firmly planted in your mind.

It is fundamentally wrong to pour toxic chemicals down the drain, out the back door, or on your neighbor's property. Besides being unethical, it is downright wasteful since you end up throwing away materials that you have paid good money for.

The net result of this is that etching boards on a small scale with ferric chloride, the erstwhile standard of the hobbyist world, is out of the question. Generally speaking, it really does not make sense to use an etchant that cannot be recycled or replenished locally, without additional cost to you.

The good news!

Fortunately, recent improvements in an infinitely replenishable copper etchant commonly referred to as "peroxy-sulfuric" with its environmental compatibility and ease-of-use has come to the rescue. Peroxy-sulfuric is very aggressive oxidizer/corrosive that can be mixed on site from inexpensive ingredients, and, with proper use and maintenance, literally never wears out.

The real beauty of this mixture of hydrogen peroxide, sulfuric acid, copper sulfate and organic stabilizers is that excess copper can be removed by simple precipitation, after which, the bath is ready to consume more copper. In addition, during operation, the etchant is "self agitating". The bubbles and heat that evolve during etching, so thoroughly stir up the bath that the etchant works almost as well in a simple dip (immersion) tank as it does in a far more expensive spray etcher.

Unfortunately, in the early days, peroxy-sulfuric was tricky to use in shops requiring high throughput.

The two primary reactions that effect the erosion of copper (copper to copper oxide, copper oxide to oxygen + copper sulfate) are very exothermic. A lot of heat can build up in a bath with no provision for cooling the etchant. Generally speaking, if the bath "loading" exceeds 2 oz. of copper (1 ft² of double sided one ounce copperclad) per gallon of bath per hour, enough heat can accumulate that the stabilizers begin to fail. Once the effectiveness of the stabilizers is impaired, the peroxide reacts with the dissolved copper to spontaneously decay into oxygen and water, releasing even more heat. If left unchecked, this runaway reaction (often called "going exothermic") can melt plastic tanks and severely compromise the integrity of any plumbing attached to the system. Needless to say, it also eats up every bit of the hydrogen peroxide in the bath and, may leach enough material out of the tank walls and plumbing to thoroughly pollute the solution and render it quite useless. Advancements in catalysts/stabilizers have virtually eliminated this problem, yielding systems capable of high throughput with very manageable bath heating.

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Established 1990

On the web since 1994

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Copyright © 1994 - 2014 Think & Tinker, Ltd. Updated 2/13/2014 8:36:56 AM