I assume most of you know how to make boards using the
rustic process of drawing the tracks on a copper clad board with an etch-resistant marker
then etching and cleaning. It's a quick and simple method but, as many will have realised,
has some disadvantages:
1. Drawing anything but the most simple boards is a challenge
2. Copying complex layouts from books and magazines can be tedious.
4. Getting component spacing correct (especially ICs) is hard.
5. To duplicate a board you must draw it from scratch.
5. No two duplicated boards will be exactly the same.
6. Lacks a professional high quality finish.
I started using the photographic method of making PCBs a number of years ago and haven't looked back since. Although to start requires some special equipment and time to familiarise yourself with the process, the time saved later and resultant board quality hugely outweighs this initial hurdle. It's best to spend an afternoon making small pieces of test PCBs. A little like cooking or photographic picture developing and printing, there are many variables which will affect the outcome for each of your unique circumstances. After you find a 'routine' that works for you it's childsplay and difficult to understand why electronic hobbyists think that it is a complex process.
Extras to consider:
Although you can buy these from electronic supply stores, they tend to be expensive. Much better to build your own. Mine is made from a fuse-cabinet I picked up at my local DIY store. The inside is covered with reflective-silver sticky paper (available in art-shops or stationers) to spread the light around. Wire in a couple of UV fluorescent lamps, mine are low power types used in electric mosquito-traps. Be careful when using UV lamps - they can permanently damage your eyes. A sheet of Perspex is used to place the board on and some sponge off-cuts keep it and the layout flat against each other, when the hinged door is closed during exposure. Don't use glass - apart from being harder to work with, it also absorbs UV light, resulting in much longer exposure times. Other possible refinements are an interlock switch on the hinged door, to prevent UV eye accidents and an exposure-timer.
Trays, Tongs, Funnils and Thermometers.
Photographic equipment suppliers are a good source for developing, etching and tinning trays, tongs and a thermometer. I use common plastic containers instead of purpose made trays - they cost virtually nothing. Remember to always use the same one for each solution - use a permanent marker to lable what each one is for. Also have separate funnils to pour back etchant and tinner solutions for re-use. I use the thermometer to get the developer solution just right.
The Scene of the Crime.
I alternate between kitchen and bathroom. There's more space in a kitchen and a table top to work on but the bathroom's good for flushing away used chemicals and using the shower-head to thoroughly rinse the board at various stages of the process. Wherever you may choose to do it, make sure it's well ventilated as some of the solutions may give off unpleasant fumes.
Due to the corrosive nature of some chemicals wear old clothes or an apron. The ferric chloride etching solution burns anything it may get onto instantaneously. If you get it on your hands rinse them in cold water. This goes for the developer solution also - if your hands feel soapy, rinse them immediately as this will be the caustic soda eating away at you. Cover work tops with old newspapers and avoid splashing the solutions. Wearing goggles or glasses may be a good idea if your clumsy by nature.
Step by Step
The following is the process that works for me:
1. Converting your circuit to a PCB layout. I use one of the freeware PCB drawing utilities found on the internet. It doesn't autoroute the connections, ie: convert from a shematic circuit to a PCB layout. This means I must spend a little more time planning the layout, but for a free programme I can't complain. To use a design from a book or magazine use a scanner.
2. Making a transparency. I use a laser printer to print the layout onto a high contrast transparent film. The type I use is 'HP Laserjet Monochrome Transparency Film' and is used for making overhead projector transparencies. You will find that, although it is quoted as being 'high-contrast', better end results are obtained if you print the layout twice on the same tansparency. Passing it twice through the printer gives opaque true-black tracks.
3. Making the PCB light sensitive. Don't! Buy a pre-sensitised copper-clad board. These are slight dearer but worth the extra money. I have in the past used positive photoresist spray to coat standard copper-clad boards and, yes it works. However it takes longer (24hrs to dry), if a speck of dust falls onto the board while spraying or drying this affects the final product and if the coating of photoresist is not uniform on the copper you get different thicknesses of tracks. Pre-sensitised boards are a lot less problematic. They come in light-proof bags with black tape on the sensitive side of the board. All SRBP and fibregalss types sizes are available - I always use the fibreglass type even if not making an 'RF project' since it gives a more professional looking final board.
4. Exposing the layout. Tape your layout transparency onto the perspex sheet in your lightbox using small pieces of masking tape and make sure it's the right way round! Then, in subdued light, take the light-protecting tape off the pre-sensitised copper-clad board and place it on the transparency - make sure it's straight and sensitive side down, then fix it in place with bits of masking tape. Put a sponge off-cut (rescued from some packing material) on top of the board and close the lightbox's top. The sponge will compress and hold the board tight against the transparency preventing bent tracks and shadows. Make sure the box is closed and switch on the lamps - never look at UV-light as it causes permanent eye damage. How long to expose for depends on a number of factors including the types of lamps used, their power, their distance from the board, the type of photoresist paint the board has and its thickness. Take an afternoon to experiment on small scrap pieces of board. Under-exposing will take too long to develop later on (if at all) and over-exposing will take too little. Four minutes is about the time my setup takes.
5. Developing the board. There will be no visible difference to the board when you carefully remove it from the lightbox. It needs to be developed. Handling only by the edges, place it in the photoresist developer solution. You can make your own if you don't want to buy the purpose made stuff. I use a couple of teaspoons of caustic soda (drain unblocker) dissolved in warm (40degC) water. Use gloves as it burns skin. Development should take a couple of minutes and is complete when the shiny copper shows through on the non-track areas. Careful not to over develop as it results in dull tracks which will be eaten away during etching - again, experiment to start with. Rinse thoroughly in cold water to halt development.
6. Etching. An etching solution
is made by dissolving Ferric Chloride crystals, available at your local electronic
component supplier, in water. The developed board should be submersed in this until all
the unwanted copper is eaten away. Agitating the solution occasionally will speed up the
etching process as will warming the solution and increasing the concentration of ferric
chloride. After etching, rinse the board with cold water and dry. Don't discard the
solution as it can be re-used, although etching will take longer the more times you use
it. Ferric chloride is highly corrosive - always use gloves, an apron and plastic or glass
I personally don't use ferric chloride, preferring to mix up my own etching concoction from household products. It's a superior etchant but a touch volatile if you don't know what your doing - a friend of mine managed to kill both his pet budgies using it and had a difficult job convincing his XYL they died in the name of science... remember, only use chemicals in well ventilated areas!
7. Cleaning. Acetone (nail-polish remover) and some paper towels will remove the remaining etch-resist paint. Then degrease with washing-up liquid detergent, rinse thoroughly and dry.
8. Tinning. Optional but gives a nice silver-like professional finish. Small bottles of tinning solution are expensive but can be re-used - mine's lasted for years. Just follow the instructions on the bottle - which are usually to soak the board a minute in the tinner, then rinse in running water and leave to dry.
9. Drilling holes. Use 0.8mm drills with wider shanks, available at model supply shops, for most component holes. For better control I prefer a hand-drill to the small electric modellers drills.
You should now have a finished board with a
semi-professional look about it.
Easy when you get the hang of it!
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