|Custom Etched Circuit Boards|
It is relatively easy to design, etch, and drill a custom circuit board for
your next electronics project. The initial investment is under $50.00, and
once you are going, each new board will cost just a few dollars.
For a board with more than one or two ICs, it takes me less time to make a circuit board
than it would to hand wire the circuit. The final result is much prettier and easier
to repair, as well!
To show you how easy it is to make your own circuit boards, I'll take you through the entire process, from design through final assembly, using a circuit I needed recently.
Our household stereo system (Bose Box) does not have an input for a turntable or a record changer. Ancient technology, I guess. I've still got a significant collection of vinyl, and from time to time I want to play it, but there was no simple way to use my old turntable with the brand new Bose Box. Connecting the turntable outputs to the aux inputs of the Bose Box was physically possible, but I knew that it wouldn't sound good at all. Vinyl records, and the cartridges that turn the physical bumps into electrical signals, are not perfect flat bandpass signal sources.
To match the characteristics of the cartridge to the aux inputs, I needed a pre-amplifier with a built-in RIAA filter. (RIAA stands for Recording Industry Association of America). To see what I wanted to build and why, have a look at the RIAA Preamp page in the "Circuits" section of this site.
Step 0: Obtain Supplies and Equipment
(I refer to this as step 0 since you will not need to repeat this setup process for each board.)
At my local electronics parts store, I purchased the following items:
Step 1: Circuit Design
This step has two objectives: 1. Create a reference document against which all further steps will be compared, 2. Organize the circuit as nearly like the physical layout as possible.
Refer to the circuit diagram on the RIAA Preamp page. Here are a few points to notice:
Step 2: Graphical Layout
Planning: Using the circuit diagram as a guide, prepare a preliminary floor plan for your circuit board. This is a lot like playing with doll house furniture. You want to position all of the components (chairs, tables, etc) so that the room looks nice, there is plenty of space to walk through, etc. In our situation we want to arrange all of the components so that they can be connected together as the circuit requires, with minimum length conductors, a little physical space between their bodies, and so on.
Cross-overs: Sometimes, regardless of all the careful planning, one trace needs to cross over another trace in order to connect to the correct point in the circuit. Since you are designing a single-sided board, you'll have to use two pads and a wire on the component side to achieve this cross-over. In the real-world these are called zero-ohm resistors (and yes you can actually purchase them). They are the same size as 1/4W resistors. For home-made boards, you are not restricted as to the length of these cross-overs. Make them what ever length you need.
Choose Your Enclosure: Now that you have a preliminary layout, choose the box or enclosure into which you will put this circuit. Plan the location of batteries, switches, indicators, and external signal connections. Make sure that the enclosure has plenty of room near each item; don't plan to put a connector in the same place as the screw that holds the top and bottom together.
Final Plan: Using the locations of the connectors, switches, etc. make a final layout plan. Just a sketch is enough, but be sure you note the sizes of the various physical components and leave enough room for them. To the right is the component layout that I used for the RIAA pre-amp board. Note that your components may not be the same size and shape as mine.
Actual Layout: Create the actual circuit board layout graphics using the tool of your choice. Specific details will vary by tool, but here's a general plan: 1. Make a component layer, and place all components on it. Use a reference grid, and place all components on grid (if you must choose, place their holes on grid rather than worrying about their outlines). 2. Make a pads layer (you can share this layer for both sides of a 2 sided board). 3. Make a traces layer for each side of the board. If you are making a single sided board remember to put all of the cross-overs on the components layer. 4. Make sure to back-annotate the schematic when you decide which portion of a multi-function device to use for each circuit section.
Production Graphics: Print out a transparency of the pads and traces, not showing components, jumpers, or other stuff that won't be on the actual copper. If you are using the MGC process, you'll need a full size transparency with very opaque black areas. A lazer printer works well for this. I have not tried an ink-jet printer.
Here's the RIAA pre-amplifier layout, final form, ready to use in the board exposure process. Note that the lettering will be facing downwards on the solder side of the board, whereas the graphics were prepared looking at the component side of the board. That's why the lettering looks backwards in the picture -- you are looking at it through the board.
If you want a copy of the artwork to make this circuit for yourself, download it here (300 DPI GIF)
Step 3: Expose Board
(These steps are specific to the MGC photo-sensitive process.) Before starting the exposure process, put on a pair of rubber gloves and mix some developer and warm water together in the tupperware container labeled "developer". Be sure to dilute the developer with water, using the ratio specified by MGC on the developer bottle. If you local tap water is not very pure, use distilled water. (In the Portland Oregon area where I live tap water is about as pure as distilled water!) Keep the gloves on for the next step.
Expose the board in a room which can be completely darkened. Set up the black-light to shine downwards, about 3 to 4 inches above a flat working surface. Turn off the black-light, and all room lights; use a small dim flashlight to see what you are doing. Don't shine it directly on the board. You can use a red or yellow 'safe light' to see what you are doing. I made a red 'safe light' with several ultra-bright red LEDs. In the dark, open the bag containing the sensitized board. Peel the protective layer from the board. Place the board on the work surface, sensitized side up, directly under the black-light. Place the transparency on the board and arrange it to be centered and aligned with the board edges.
I made a fixture out of plastic sheet and strips to make this alignment easier. Two strips are taped to a backing board in an L shape to hold one corner of the board in the correct location. Two other strips are taped to the clear glass plate (see next paragraph) and are meant to be diagonally opposite of the L on the backing board. I trim the transparency on two edges and tape it to the glass plate (against the plastic L strips). Orienting the circuit board and the art work is then simple. It is more a matter of touch than of appearance. When both L's are against the corners of the un-etched board, everything is in place.
Place the clear glass plate on top of the transparency and recheck the alignment. At this point, with the copper side of the board upwards, the lettering on the transparency should read correctly, not reversed. Turn on the black-light for 3 to 5 minutes. Make sure all parts of the board are receiving light. When the time is up, turn off the black-light, and with the room still darkened, unstack the glass, transparency, and the exposed board.
Keep the gloves on. Put the exposed board into the developer, turn on the room lights, and stir. The developed resist starts to turn black right away. Stir the developer a bit and you'll see the resist coating washing away from all exposed areas. Green resist coating will remain on the traces and pads areas. Remove the board from the developer after 1 to 2 minutes total. Turn on the lights. Wash the board using clear, warm, tap water.
The board is now developed and ready for etching. It is also reasonably stable; if you like you can wait a day or two before you etch it.
Step 4: Etch to Remove Unwanted Copper
Put on a pair of rubber gloves. Pour some Ferric Chloride etchant into the tupperware container marked "etchant". A depth of 1/2" is about right for most boards. Place the developed/un-etched board into the etchant and close the lid. Gently rock the tupperware container to cause the etchant to slosh back and forth across the board. If you are impatient like me, you can speed up the etching process by dipping the tupperware container into warm water. I do this in the laundry sink, with about 1" of hot water in the bottom of the sink.
From time to time look into the container and estimate the progress of the etching (you did buy a clear container didn't you!) If you can't see what's going on, open the lid and have a look. Don't worry about over-etching a little bit. Under-etching is more of a pain, because the un-etched copper connects the traces together in hard to find ways. Using my hot water technique and continuous sloshing, the etching process usually takes about 5-6 minutes.
When the board is fully etched, take it out of the etchant, and wash it thoroughly in warm water. The developed resist can be scrubbed off now, if you want, using a bright-boy or a slightly abrasive cleaner.
Step 5: Plate the Board with Tin (optional)
A while ago I notice that MGC sells a tin plating solution. I use this solution at this point in the process to deposit a thin layer of tin plating on all of the copper (traces and pads). Copper oxidizes rather quickly, and when it is oxidized it is hard to solder to. In actuality, tin oxidizes too, but a normal soldering iron is hot enough so that the tin essentially melts into the solder, even though it is oxidized. An unplated board becomes unusable in a matter of a week or so (unless you scrub all of the oxide off). A tin plated board can be stored for months before it is used (and doesn't require a second scrubbing).
Put the newly etched (and shiny) board, copper side up in another container. Pour a 1/4" thick layer of tin plating solution into the container. Slosh it around (gently) for a couple of minutes. Pour the tin plating solution back into the original container - it can be used again and again. Wash the plated board in warm water and dry it off.
Step 6: Drill holes for Components
Place the fully etched board, copper side up, on a stack of paper, cardboard, or a wooden block. Drill each component hole using a small drill motor (a Dremel moto-tool, for example). Small components need a small hole; use a 0.030" drill bit. Select larger drill bits according to the size of the lead or pin that must pass through the hole. The pad under the board protects your desk or table, and prolongs the life of the drill bit. If you have difficulty centering the drill bit on the holes, start each hole by hand, using a drill bit in a pin-vise. Keep the holes as small as possible, and make sure they are centered on the pads.
Step 7: Assemble The Board
Print out a copy of the component layer from the graphic layout process. If you used my circuit board layout then you can also use this component placement diagram as a guide, while inserting components. Insert a few components at a time, solder them, and clip their leads.
Check your work frequently using a bright light and a magnifying glass. Single sided boards are prone to poor solder joints, since they do not have plated holes. Somewhere on the board there will be a few tight places that you didn't notice during layout. Make sure you don't accidentally bridge such places with solder.
My RIAA Pre-Amp worked correctly the first time. Careful work pays off!
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