Showing You My Etchings: Etching the Circuit Board
Creating the resist pattern on a new sheet of printed circuit board material really only gets your circuit board one third of the way done. For the next step, you need to etch that board to remove the unwanted copper. The copper that remains forms the printed circuit that makes your project work.
You use something called etchant to etch your board. Etchant is a caustic (meaning it can burn you) chemical that dissolves copper. It's not like some acid that a monster in a B-movie oozes out dissolving everything in its path; etchant doesn't fizzle away the copper on contact. The etching process actually takes several minutes. The copper that the resist pattern doesn't protect dissolves away first. The etchant finishes its job when it gets rid of all the copper in the exposed, resist-free areas.
First step: Inspecting the board
Think of etching as an unforgiving process. In the steps leading up to this process, you can modify or redo your work, to a certain extent. But when you reach the etching stage, you're making a commitment: After you etch, if you have an error in your circuit, you probably have to chuck the whole thing and start again.
That's why you really, really need to inspect the board for errors in layout, missing traces and pads, skips in the resist pattern, and other gremlins that produce a poor result before you actually etch the board:
- If you created the resist pattern from artwork appearing in a book or magazine, compare your board with the printed layout. Follow the traces from pad to pad and note any discrepancies. If you find any, you'll have to redo the artwork or fix any problems before you begin etching.
- If you created the resist pattern from your own design, or by using the direct-etch method, carefully review your work and compare it against a schematic or paper drawing. Be sure that pads and traces aren't too close together. At a minimum, all pads and traces should be 1/32nd of an inch apart, but more is always better here.
Repairing a board after a shoddy etching job — if you can do it at all — is time-consuming and frustrating.
Cleaning the board — carefully, please!
After you inspect the board, wet a cotton ball with isopropyl alcohol and gently clean the exposed parts. Don't apply much alcohol because some types of resist may melt or distort when exposed to alcohol. Also, let the alcohol dry completely before immersing the board in the etchant fluid.
Use isopropyl alcohol with a minimal water content. General purpose isopropyl alcohol that you buy at the drug store can have 30- to 40-percent water content. The more water mixed in with the alcohol, the more chance you have that the water will damage the resist. Look for so-called technical grade isopropyl alcohol, available at chemical supply outlets and school lab suppliers.
Kvetching about etching
Etching can be dangerous — not only to your health, but also to your wardrobe. Most circuit board etchants, whether in liquid or powder form, are toxic and highly caustic. Never allow the etchant chemical to come into contact with your skin or your clothes. If you do get some etchant on your fingers or hands, wash it off immediately.
Because etchant stains skin and clothing, avoid wearing your best party clothes when etching. Instead, wear a smock, your least favorite pair of pants, and old shoes. Also, wear eye protection to prevent the etchant from injuring your peepers if it splashes onto your face.
Wear gloves to protect your hands against burns and stains. Choose gloves that let you work almost as well as if you didn't have gloves on at all. (So don't use those old gardening gloves for this kind of work.) Disposable plastic or latex gloves do a good job.
Prolonged exposure to etching solution fumes can seriously injure you, so be sure to etch your circuit boards only in a well-ventilated area. All etchant solutions give off fumes, which can do serious harm to the mucous membranes in your nose and throat. You don't necessarily notice the effect right away. You may etch one or two boards and not be aware of the fumes. But an hour or two later, you feel an intense burning in your nose or throat that can last up to several days.
Store unused etchant solution in a dark-colored plastic bottle designed for photographic chemicals and keep the bottle in a dry, dark, cool place. Clearly label the bottle with its contents and keep it away from children.
Mixing the etchant
If the previous section didn't scare you away from ever touching etchant, even with a ten foot pole, check out this section for the mad scientist portion of the process — mixing the etchant.
You find etchant, whether ferric chloride or ammonium persulfate, in three popular forms:
- Liquid, not concentrated
- Liquid, concentrated
- Powder (sometimes this comes as a semi-glutinous paste)
You can get liquid, unconcentrated etchant at Radio Shack and most electronics stores. It comes in a plastic bottle ready for use. Just open the bottle, pour the etchant solution into a plastic (remember, never metal) tray, and you're ready to go.
You can use unconcentrated liquid etchant to make more than one board, depending on the size of the boards. The etching action reduces as you increase the surface area of the board.
For example, if the board measures 4 x 6 inches, with one side to etch, the board has 24 square inches of copper clad. Check the bottle for your etchant's recommended usage. Your particular solution may be able to etch up to 50 square inches of copper clad. This estimate assumes that you use the entire contents of the bottle. If you use less etchant, you also reduce the expected amount of coverage.
The size and number of boards that you make determines how long the etchant lasts before it just can't etch anymore. You need to throw out weaker etchants after you use them to make just one 2 x 3-inch board; you can use stronger etchants to make several large boards. Using weak etchant, you may have to wait ages for the etching to finish, and this weak etchant can lead to voids in the copper pattern.
Here are some tips to keep in mind when mixing and using etchants for making printed circuit boards:
- You must dilute concentrated liquid etchant before you use it. For best results, dilute the etchant with hot water; this addition increases the etching action. Typical dilution ratios are 2:1, 3:1, and 4:1. The higher the ratio, the longer the concentrate lasts. For best results, though, balance the thrifty use of the concentrate with your tolerance for longer etching times. The weaker the etchant, the longer it takes to remove the excess copper.
- You have to mix powder (or paste) etchant before you use it. One packet of powder etchant generally makes one or two quarts of unconcentrated etchant. You can mix the powder to make a smaller amount of liquid and then dilute the mixture when you're ready to use it.
Now that you're itching to etch . . .
After you go through all the preliminaries in the preceding sections, you get to actually etch your printed circuit board.
Follow these steps to etch the board:
1. Pour the etchant into the plastic tray carefully, avoiding spills and splashes.
Pour enough etchant to create a pool at least 1/8-inch thick, preferably 1/4-inch thick.
2. Dunk the board into the tray and continually rock it back and forth.
3. Keep the board in the soup for 10 to 30 minutes (depending on the type and strength of the etchant) or until the etchant has removed all the excess copper. Keep that tray a-rockin — but gently!
4. Use the plastic or wooden tongs to lift the board out of the tray from time to time to check progress.
The etchant removes the copper, starting from the edges and areas close to the resist. Large, open areas of copper can be stubborn and take 2 to 3 times as long to etch completely. You may want to agitate those areas of the copper that don't respond as quickly to the etchant. However, be sure that you don't over-agitate because you can undercutthe copper under the resist. Undercutting happens when etchant oozes under the resist and attacks the copper that you don't want to remove.