There are two basic ways that you can remove unwanted copper from copperclad substrates to form electronic circuits; mechanical etching and chemical milling (etching). Since laser ablation is outside the budget of most small shops, it will not be included here.
- Mechanical Etching
- involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace. The removal of this copper electrically isolates the circuit element from the rest of the foil.
- Chemical milling (chemical etching)
- relies on the action of any one of a family of corrosive liquids to dissolve away unwanted copper in order to define the desired circuit pattern.
In response to the inexorable tightening of environmental regulations and the difficulty of obtaining strong acids and oxidizers in many locales, "mechanical etching" has seen unprecedented growth over the last 10 years. In spite of high up-front costs and lack of plated through-holes, its complete freedom from toxic chemicals has made this technique very attractive to many PCB prototyping shops.
As the density and complexity of circuit designs have increased, forming reliable electrical connectivity between the top (component) and bottom (solder) circuit patterns has evolved from a mere convenience to an absolute necessity. While some mechanical etch vendors offer manual and semi-automatic machines for inserting eyelets into all of the designated through-holes, the cost of the equipment, the penalty in PCB real-estate (holes must be drilled oversized to accommodate the eyelets), and the relative sluggishness of the process render it undesirable for many applications. As a result, many shops have adopted simplified electrolytic plating processes to accomplish this critical task.
As mentioned above, mechanical milling involves the use of a precise numerically controlled multi-axis machine tool and a special milling cutter to remove a narrow strip of copper from the boundary of each pad and trace. There are a number of configurations currently available for these special mechanical etch bits, but most users report that bits with spiral flutes (vs. a flat "spade" geometry) are the most effective at removing copper debris and tend to stay sharp longer at higher cutting rates. Tip angles of 60° and 90° are the most common, with 90° seeming to offer the best combination of minimal substrate penetration and longer cutter life. If the circuit design also requires that some (or all) of the non-circuit copper be removed (clear milling), conventional carbide end-mills can be used to accelerate the copper milling process. Typical diameters range from 0.010" (.25mm) to 0.050" (1.27mm).
Mechanically etching a PCB proceeds as follows:
- Lay out the circuit using a compatible PCB layout package (often proprietary to the milling machine vendor).
- Run the layout through a post processor to generate the boundary paths that the cutter will need to follow to define the circuit elements and any cutter paths needed for clear milling (always vendor specific).
- Mount the substrate (or flex circuit) in the machine as instructed in the user's manual.
- Insert the drill size indicated by the operating software into the chuck and adjust the height (if no set ring is present) to insure that the bit drills all the way through the substrate and a short distance into the backing material.
- Drill the first hole size.
- Repeat the previous 2 steps for each drill size.
If you will be plating the through-holes, remove the board from the machine for hole wall activation and electroplating. After the through-holes are plated, return the board to the milling machine for further processing.
- Insert a mechanical etch bit and adjust the depth setting foot to the approximate cutting depth desired for your design. The cutting depth sets the width of the channel milled through the copper foil, so you must know the tip angle of your cutter prior to setting the depth.
- Off to one side of the substrate, cut a couple of test channels and carefully measure the width of each. Adjust the height of the foot until the desired width is consistently achieved.
- Mechanically etch (and clear mill) the first side of the board.
- Carefully inspect the cutter. If the cutting edge shows signs of wear or of excess copper buildup, replace it with a new bit before proceeding.
- If your design requires a double sided PCB, flip the substrate and repeat steps 8-9 for the second side (making sure, of course, to load the second side artwork into the mechanical etch software).
- Mechanically etch (and clear mill) the second side of the board.
- Remove the "etched" board from the machine.
- Carefully examine both sides for copper debris that may have become wedged (or smeared) in the milled channels. You cant bet that any such material will short out the most expensive and unobtainable component on the entire board so you need to be very diligent during this inspection.
- If any copper strands are found shorting out circuit elements, use the tip of an X-Acto knife to remove them. Be very careful not to cut thin traces during this operation.
- After any shorts have been removed and the through holes cleared of plugs of copper and milled substrate detritus, the board is ready for further processing.
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.
- 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.
- 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".
- 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.
Chemically etching a PCB proceeds as follows
- Read the chemical handling safety procedures until you understand them.
- Analyze all components of the etchant and make additions as necessary.
- Start pre-heating the bath so that it will be at the recommended operating temperature [approx. 110°F (immersion), 120°F (air agitated or spray) or 49°C] when your board is ready to be etched.
- Thoroughly clean the copper foil on both sides of the board.
- Laminate with the desired photoresist.
- Being careful to align the artwork with the correct side of the board, image the photoresist.
- Let the board sit for at least 30 minutes to allow the exposed resist to totally cure.
- Peel the Mylar cover sheet off the photoresist and develop the board.
- Thoroughly rinse the photoresist image, gently blow dry, and set the board in a 100°C oven for 10 minutes.
- Remove the board and allow to cool to room temperature. While the board is cooling, etch a test coupon and record the time (T) required to etch the board clear (without damaging the copper protected by the resist).
- Put the board in the etcher and etch for T/2 minutes.
- Pull the board from the etcher, flip it top to bottom and left to right, and etch for another T/2 minutes.
- Pull the board from the etcher, thoroughly rinse in deionized (or distilled) water and carefully examine the etched pattern for shorts or incompletely etched areas.
If you find, small isolated shorts, but the rest of the board looks pretty good, etch the board for another 30 seconds and re-examine. If the shorts persist, it might be easier to remove them with an razor knife or other implement of destruction. If shorts seem to occur in localized groups, and there are no signs of resist failure, etch the board for another minute and re-examine.
Continue in this fashion until the etched circuit is totally free from shorts and other defects. Be careful, however, not to over-etch the board. It is generally easier to cut away a few shorts than it is to rebuild traces that have been totally etched away.
- Once you are satisfied with the quality of the etched image, thoroughly rinse the board, first in deionized water, then under cool tap water.
- Immerse the board in a heated (50°C) 5% solution of sodium hydroxide. After about 2 minutes, the resist will start to lift off of the copper. Swish the board around (or scrub it with a lightly abrasive pad) until all of the resist is removed.
- Rinse the board under warm tap water and blow dry.
- The etched board is now ready for further processing.
included by permission of Think & Tinker, Ltd copyright 1986 to 2012