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etching troughs. For safety, check the tray before you use it: Fill it with hot (150 to 180 degree) water. The tray shouldn't become soft or melt.
For best results, a tray should have ripples or ridges on the bottom. This texture allows the etchant solution to flow freely under the board while the board stews in the tray. Finally, you need two plastic or bamboo tongs to handle the board. Don't use your fingers! You can buy these tongs at any photographic or arts supply store.
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 youíre 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 etchantís 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 canít 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.
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Chapter 12: Building Your Own Printed Circuit Boards 269
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 youíre 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 )8-inch thick, preferably /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 ó 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 donít respond as quickly to the etchant. However, be sure that you donít over-agitate because you can undercut the copper under the resist. Undercutting happens when etchant oozes under the resist and attacks the copper that you donít want to remove.
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270 Part V: A Plethora of Projects
Etchant, be gone!
Diluted etchant solution that you use for hobbyist applications usually doesn't pose a serious threat to plumbing, but the etchant is a pollutant and a toxin. Take the exhausted etchant to a licensed recycler or approved chemical waste
disposal site. If you live near an electronics manufacturer, you may be able to get them to dispose of it for you. Because they can reclaim copper from exhausted etchant solution, the company may not charge you for this service.
Final Prep and Drilling
If youíve gone through the steps earlier in this chapter, youíve almost created your first finished board. But before you turn off your work light for the night, you have one more phase to handle: you have to do final prep and drilling of the circuit board.
The etchant has completed etching when you canít see any traces of exposed copper. Assuming that you did the etching process correctly, the copper under the resist should remain intact. Still, the black resist for the traces and component pads remains, so first you have some clean-up duties to perform. After etching, rinse the board under cold running water for 15 to 20 seconds. Be sure to rinse the etchant from the back side of the board, as well.