Saturday, March 14, 2009

Chemical Etching

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 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, peroxy-sulfuric can be tricky to use in shops requiring high throughput. The two primary reactions that effect the erosion of copper (copper -> copper oxide, copper oxide -> 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.

As you might imagine, throughput can often be increased by pumping the etchant through a heat exchanger during etching to remove excess heat before it poses a threat. In a limited sense, you can also increase throughput (as well as the etch rate) by agitating the bath with compressed air (a.k.a. sparging). The benefits of "air sparging" are three fold.

  1. The turbulence in the wake of the bubbles breaks up the depletion layer immediately adjacent to the board's surface, delivering fresh etchant to the unprotected copper. As a result, the etch rate can be significantly enhanced.
  2. This same turbulence has the added benefit of similarly "freshening" the etchant down in the nooks and crannies of the resist pattern, effectively increasing the etching resolution of the bath. Resist geometries with 0.008" (0.2mm) traces and 0.008" spaces can be routinely etched with such a "bubble etcher".
  3. Although the total air volume is fairly low, air bubbles tend to carry away some of the heat generated during etching. This cooling effect is further enhanced by the evaporative phase change that occurs at the bubble walls as they rise through the heated solution.

The primary downside of bubble etching is that it generates a significant quantity of corrosive aerosol. Effective fume collecting with active scrubbing must be implemented if a bubbler is used.

Please note that using an air agitation system with an aggressive etchant chemistry does not remove the danger of the bath going exothermic. It will however, significantly increase the size of the safe operating window and improve the overall performance of your etcher.

Peroxy-sulfuric etchants are most effective, and safe when used in spray etching equipment. Spraying:
  • increases the etch-rate by increasing the delivery of fresh etchant and removal of depleted etchant,
  • enhances the effective resolution by improving the delivery of fresh etchant into finer resist geometries,
  • cools the etchant before it impacts the copper, rendering a runaway exothermic reaction virtually impossible
  • can produce far greater uniformity of copper removal from large area panels

As with the bubbler above, spray etching with peroxy-sulfuric generates a significant quantity of corrosive aerosol. Effective fume collecting with active scrubbing must be implemented if spraying is used.

Preparing etching test coupons

Regardless of whether you use immersion, bubble-assisted, or spray etching, always etch a test sample to see how long it takes to totally etch copperclad with the same weight foil as you will be using. If possible, it is a good idea to image, and develop a set of copperclad test-coupons whose resist geometry is representative of the minimum sized feature in your circuit design. In most cases, mixed blocks (1" x 1") of horizontal, vertical and crossed (gridded) 0.010" (0.25mm) traces on 0.020" (0.51mm) centers act as very effective probes for measuring many facets of etchant performance.

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