I have been producing PCB's for my projects for a number of years and I have found a number of "interesting" methods suitable for building many of my QRP projects. As always, my methods are very cost conscious, ie. cheap!! Although photo-etching has been touched upon in this article, you will find more information in my illustrated photo-etch method article.
The layout of a PCB can be very important so I always lay the components out as they are in the circuit diagram to begin with. Instead of circuit symbols I draw the component pins. The drawing is made with a pencil on 1/4" squared paper. The compnents are then "shuffled around" with a pencil/eraser to get a compact layout. Each square of the paper represents 0.1" so resistor leads are spaced 4 squares appart for horizontal mounting or one square if mounted vertically. Draw lines between tracks and try various compinations that don't require tracks crossing. The drawing should be viewed from the top component side of the board.
For complex PCBs you can do quite a lot with a "DALO" or other "etch-resist" pens. The hardest part is to accurately position the components so that the final board looks reasonable. This is how I do it:
With a 0.1" matrix stripboard (veroboard etc.), insert the components in the positions decided. At this point you may need to move the odd component to take into account components with wide bodies etc. Update the pencil drawing as this will become the component "overlay".
Remove each component and mark the holes in the stripboard with an eraseable felt-tip marker. Clamp the stripboard over the copperclad board and a sheet of flat wood to form a sandwich with the copper clad board in the middle and the copper side DOWN. Drill through all the holes marked with the felt tip pen, and the copper-clad board will be drilled with perfectly regemented 1mm holes for your components. On the copper side of the copper-clad board, join the dots with a DALO or other etch-resist pen. Clean off the stripboard and re-use it for the next project. The PCB is etched in the normal way.
RF PCBs can be made very simply when they are to be used for VHF Power Amp's up to 10 watts or so. These boards nearly always have square or rectangular pads for the RF conductors. Measure and mark the copper where you want the track to be and stick SELLOTAPE (UK), DUREX (Australia) SCOTCH TAPE (rest of the world) where you want the copper to be. Make certain that there are no air-bubbles under the tape. Using a sharp knife (or scalpel) and ruler, cut around the tracks and remove unwanted tape. This will leave you with tape on the copper in the positions you want the tracks. Use a DALO pen if you need to add the odd thin track. Etch the PCB as usual.
Put the copper-clad board in the oven, pre-heated to 120 degrees Celcius and drop a small lump of candle wax in the centre. Remove the board and allow it to cool whilst keeping it horizontal. The wax is a good etch-resist, so just scrape away the wax where you do NOT want the copper to be. A flat modelling chizel-knife (Xacto etc) is ideal for this. You can use a ruler and felt-tip pen on the wax without damaging the wax surface. Etch the PCB as usual.
This method is a little more "complicated", but I do NOT believe in spending load of money on UV-exposure boxes etc. A simple 18watt DAYLIGHT flourescent tube works just as well. This means that you can use photo-etch with a simple working-man's pocket. If you can get two 8-watt UV tubes glowing with a wavelength somewhere around 340nM to 440nM then you will be able to achieve professional results in far less time.
Spray the copper side of your board with a "photo-POSITIVE" etch resist in a dimly-lit dust-free room. Electrolube PRP200 (manky green that gives poor results) but the best results I have found is from CRC POSITIVE 20. Photo-resist is available from:
|Sweden||ELFA and FARNEL|
|England||MAPLIN, RS and FARNEL|
|Middle East||ARAB ENGINEERS and MEJDAF|
|USA||RADIO SHACK and TANDY|
Whilst the board is drying, make your artwork. The artwork can be drawn:
I use the first two methods, mainly because I am not a millionaire - but I am working on it!!. The page DRAWINGS & PCBs contains a PC-Paintbrush (right-click on picture and save as BMP file) pattern with the correct component spacing for IC's and standard pads for resistors and capacitors. The little single dots give you markers on a 0.1" matrix for planting pads using cut'n-paste. Print the finished file reduced to 38%, on tracing paper (ink-jet printer) or OHP film (Laser printer). Draw the artwork so that your view of the is from the top (component side) of the PCB and expose with the printed (ink) emulsion in contact with the photoresist (eliminates areas of "penumbra"). You could print to ordinary photocopy paper and double exposure time for making prototype boards. You could also photocopy drawings onto OHP film.
Expose the PCB in a custom made printing frame. Build up a sandwich of (top to bottom) glass, artwork, copper-clad board and a bit of chip-board (wood). The artwork must be inky-side down. The copper-clad board must be sensitive side up. Expose for 2 HOURS 10cm from an 18 watt DAYLIGHT flourescent tube. You will have a even and uniform exposure if you position the tube over one side of the board for 1 hour, then the other side for the other hour. If you have access to a UV light box with two 8-watt tubes then 5 - 6 minutes is all that you will normally need.
I use one cup of Caustic Soda crystals, dissolved in two cups of water as a STOCK developer solution. Use 1ml of this to each 60ml of water to make a working strength developer. This solution should be stored at room temperature and always have undissolved crystals in the bottom of the bottle (a saturated solution). Immerse the board in working strength developer and the exposed coating will dissolve in seconds. Wash the board very well as small traces of soda can contaminate your etchant.
* WHEN MIXING STOCK SOLUTION OF DEVELOPER, MUCH HEAT IS GENERATED. ADD SODA VERY SLOWLY AND A SMALL AMOUNT AT A TIME AND DO IT IN A BOWL OF COLD WATER.
Whatever method you have used to create your artwork on the copper foil, you must now etch it. Place the board with resist in a vertical tank filled with etchant solution (normally 1/2 Kg Ferric Chloride to 1 liter of water). The board should be totally emersed and vertical. This will ensure that bubbles are allowed to rise and the sedement is allowed to fall. I usually support my boards using plastic clothes pegs (clothes pins). My etching tank is a stout plastic bag in a wooden former. The front of my tank is replaced with perspex sheet so that I can see the board etching. You could of course use a plastic "tupperware" type of bowl, but you will need constant agitation or to float the board on the surface of the etchant. Etching is complete when all the unwanted copper is dissolved. Wash the board thoroughly after etching is complete.
When you first put the board in the tank the copper will turn a dark colour where it is exposed. If bits of WANTED copper turn dark then the board may be removed, washed and touched up with a DALO pen. If UNWANTED copper stays nice and shiny then the board may be removed, washed, and carefully scraped free of the grease or resist where it should not be.
The board will normally etch within 15 minutes, less if the etchant is warm during etching. Save etchant for re-use on the next PC board in a tightly stoppered bottle. When the etchant is almost exhausted the etching time is increased. If etching takes longer than two hours then adding a little salt to the etchant will speed it up a little.
Mix together 1-part Hydrochloric Acid (HCl), 1-part Hydrogen Peroxide (H2O2) and four parts of water (H2O). Place the board in the solution copper-side up. Etching is completed when all the unwanted copper has been dissolved. This usually takes about 2 to 3 minutes. Small splashes of any of these liquids on clothing will cause holes to appear and will also burn skin. Wear rubber gloves and use photographic tongs to handle the boards. The fumes can also cause damage so use in a well ventillated area and a fan to take fumes away from you. Never try to store ready-mixed etchant - it gasses and blows bottles apart. If you should try this method you will find it is clean, cheap and quick but you have to be a little carefull with it.
Ferric Chloride etchant will stain clothes, carpets, sinks, skin, the wife, the children and pets. It is also extremely poisonous and kills 99% of household plants.
!!! FERRIC CHLORIDE CAN SERIOUSLY DAMAGE YOUR MARRIAGE !!!
Have fun, de HARRY, Upplands Vasby, Sweden.Return to INFO page