Create Printed Circuit Board
This is a simple beginner’s guide how to make EZo boards or PCB for any general purpose. I describe my experience with pictures to make professional quality PCBs on a modest budget.
1. How to get PCB layout in size 1:1 ?
- EZoFlash programmer and adapters PCB’s are presented in bitmap format, scale 100pixel=1cm.
2. Printing the artwork
- Positiv Photo-Resist Developing is my method of choice for making high quality boards every single time.
- Print the designed artwork onto clear transparency.
- I use H&P LaserJet 5P printer and Transparency film for LaserJet printers (product 3M, Apli or other).
- It is very important that the printer produces a good solid black with no toner pinholes. The printer used must have at least 600dpi resolution, set the intensity to darkest.
Always use manual paper feed, and set the straightest possible paper output path , to keep the artwork as flat as possible and minimise jamming.
- When the film is printed, the traces should be sharp and completely opaque black. I usually put the film up in and against a bright light source to carefully examine the image. I always make a new copy if in doubt. You can multiply more artworks or move the image sideward on the Word source list. Small transparent segments can be retouched with Permanent Marker size 0.75mm or less (for example, Edding 404)
- Cut PCB image from the transparency list with borders 0.5-1cm.
3. Preparing the board
- Blank PCB material can easily be obtained from parts suppliers and come in a number of varieties. I use single sided fibreglass (glass-epoxy) copper clad boards , 1.0-1.5mm thick.
- Cut the copper board at the dimensions of the printed layout leaving an extra centimetre on each side.
The ideal cutting tool is a sheet metal shear or small guillotine. Hacksaw also will do the job with more work. You should also file off the edges to make sure there are no bits of copper poking up.
- Be certain to make sure that there is not grease on the board or oxidized surface. Clean the board with some mild abrasive to obtain a shiny surface. I scrub the copper board with a fine (size number 320) sandpaper in all directions to remove oxidation, stains, scratches, etc.
- Scrub the board with a paper towel soaked in acetone. Keep doing it until no more discoloration is seen on the new paper towel. Acetone is flammable liquid, use in a well ventilated area.
- When you are sure the copper surface is very clean, give it a good rinse under fast –flowing water and then stand the board vertically in the warm to dry completely, protect it from dust and fingerprints. Don’t wipe it clean because this could leave lint or fibres on the surface.
- I use a spray Positiv 20 from Kontakt Chemie. Photoresist is widely distributed, technical datasheet is available.
- A dark room is not required for the application of the photoresist. This must be done however in subdued light with no sunlight or fluorescent light entering the room. Very important that the work is carried out in dust-free atmosphere.
- Place the plate in a horizontal or slightly inclined position and apply the spray from distance of approximately 20 cm. It is best to make the application continuous (not intermittently) in serpentine lines beginning at the upper left. This will give an even coverage. A beaten effect of dark blue peel appearance results and spraying could cease. After a short time a thin, light sensitive uniform layer will be formed. If to much spray is applied edges and coats of varying and undesirable thickness will form requiring a longer exposure time.
- After application of the sprayed-on coating, board must be dried immediately in dark. It can be dried at room temperature which will take at least 24 hours. Accelerate drying can be done in a thermostatically controlled oven , slowly raise the temperature to 70C maximum.
- My boards are dried at room temperature about half an hour, then I use air heater or hot radiator (in wintertime) in clean place to avoid dust.
- Until exposure store prepared boards in a completely dark place.
- The photoresist board needs to be exposed to ultra-violet light through the artwork, using a UV exposure unit. It can easily be made using standard fluorescent lamp ballasts and UV tubes. They look white or occasionally black when off and light up with a light purple.
- Don’t use short-wave UV lamps like EPROM eraser tubes or germicidal lamps, which have clear glass- these emit short wave UV which can cause eye and skin damage , and are not suitable for PCB exposure.
- I use old family quartz lamp with heater. The lamp reaches full intensity normally in 2 minutes from being switched on. Exposure time is approximately 3 minutes on the lamp-to-film distance about 30cm.
- I expose the board in a custom made frame. It is build up like a sandwich of top and bottom glass, artwork and sensitised copper-clad board between, held together by large metal spring clips. Align the artwork on the board, for example dust point on the board hits to the transparent area. Clean the top glass from the dust and fingerprints before use.
- You will need to experiment to find the required exposure time for a particular UV unit, distance and layer type - expose a test board fragments from 2 minutes and the next with time interval increment. Mark the board into vertical sections, place a paper on top of the frame to block the light for the time strip and move it to the next marker every minute. Develop the board and use the time which gave the best image. Overexposure is better than underexposure.
- Caustic Soda (Natrium Hydroxide, NaOH) is a good developer for the positive coated boards.
I prepare the developer with a solution of 7gram crystals in 1litre water , temperature approximately +20C. For correctly exposed layers development time ranges from 10 to 30 sec. Be careful about the temperature and concentration of the dissolution, because the developing is sensitive to this parameters.
- I mix up a fresh batch of developer for each batch of boards being done.
- Use plastic photographic container and enough developer to cover the board. Sink the board completely in the developer, copper side up. Shake the tray softly. You will see purple ‘smoke’ loosen from board. The traces over the board and the exposed copper areas comes visible.
- Avoid skin contact with the developer , photoresist and etching solutions, latex gloves are recommended. Wash off any skin splashes immediately.
- Be careful not to over-develop the board. In cases of overdosed developer or the photoresist not dried enough, the image of the circuits will appear for a short time but will eventually be removed by developer.
- When process completed, rinse the board thoroughly with clean water and carefully dry between a paper towel.
- At this point you can still make corrections to the pattern (before the etching) using permanent marker or by scrapping off resist from excess sections.
- Classical and widely used method for dissolving of copper foil is using of ferric chloride (FeCl3) solution. I use the hexahydrate type of ferric chloride (FeCl3*6H2O) obtained from chemical laboratory. I mix 250g crystals in small doses slowly to 500g warm water.
- Now Hydrated Ferric Chloride is available in component shops , it comes as yellow-green powder or granules and schould be dissolved in warm water according the instructions on the package.
Anhydrous Ferric Chloride is sometimes encountered, which is dark brown-green crystalline powder. Use extreme caution, as it creates a lot of heat when dissolved - always add the powder very slowly to water, do not add water to the powder .
- Always take extreme care to avoid splashing when dissolving and using either type of FeCl. Ferric Hlorid has the habit of staining anything it touches !
- It takes a while to remove the copper. Depending on copper size and chemical temperature (the warmer the better , but not hot), this could take from 10 minutes to 1 hour.
- I warm the etchant placing its storage bottle in a larger tray filled with boiling water . If you are warming up your etchant, have it done under a fume hood and be careful how hot you get it and how you heat it.
- Use plastic container to put the etchant. Place the PCB completely in the etchant. Shake the container softly. You will see that the traces remain over the board, and the exposed copper areas that slowly will dissolve. Small size boards I etch vertically in the jar.
- When etching is complete, rinse the board in a flush of tap water , dry with paper towels.
- Remove the remaining photoresist coat with paper towel soaked in acetone.
- Etchant may be reused and stored in a plastic or glass bottle. When etchant is obviously taking much too long to etch, it has expired and a fresh batch is needed.
- I recommend that all PCB drilling be done with a drill stand. Accurate drilling with a handheld drill is nearly impossible.
- You need a good light on the board when drilling to ensure accuracy.
- The diameter of the holes varies for different components. I use a 0.7mm drill bit for all holes. For larger components holes are redrilled with 0.8, 0.9 and 1.0 mm drill bits.
A series of 1.0mm drills is used for large holes (for example DC jack and shield pins of parallel connector) , then cleared with drill bit by accurate sideward motions. Check fit the component pin in the hole.
- Be sure to get spare bits because the bits tend to break easily and come dull after hundred of drills.
- Caution. Broken drill bits can damage your eyes or hands, wear safety glasses. Don’t breath fiberglass dust.
- Cut down the board to final size and use a medium sized file to smooth out the edges for nice finish.
- Scrub the board down with extremely fine sandpaper to remove burrs from the holes and refresh the traces before tin plating.
- Make sure you thoroughly inspect your board . I use magnifier, good light and tester-beeper.
- The most common two problems are a short circuit between two traces (where some copper didn’t get removed) or a break in a copper trace (where the etch resist failed and the copper was removed).
Fixing the break is easy enough, just solder a jumper wire across the copper. Mark the place and do it after the tin plating.
Fixing a short is a little more tricky . Carefully cut with the scalpel the copper foil on either side where you want to remove it, then carefully lift the copper with the blade.
10. Tin plating
- The tin-plating makes PCB a lot easier to solder and effectively protects the circuits paths from oxidation..
- Clean and degrease the surface with paper towel soaked in acetone.
- Spread a thin layer of flux pasta over the PCB. I use a FluxPlus (No Clean Paste Flux) from EFD.
- Use ordinary solder and 25-40W soldering iron with a flat surface tip. Add each time a small quantity of the tin to iron. Spread all the tin from up to down over traces in a thin layer.
- Remove the flux with acetone .
- I use a standard 25W soldering iron and Multicore solder with flux, wire size 0.5mm. Extra flux for board components not applied.
- Install and solder all wires according to wire or component layout.
- Use a side cutter to reduce the length of a wire or pin.
- Install and solder all resistors.
- Install and solder remaining components.
Strip connectors for my boards usually are obtained in larger size pins (1x40, 2x40). I cut them to the board required size.
- Hot air soldering iron and soldering iron or station with fine tips are required to solder and remove flash chips in TSOP or SOP packages on adapters and on your device. Extra flux pasta should be applied.
- Inspect your board, be sure no soldering shorts or bad soldering. The short can be removed with the desoldering braid.
- Clean the board with a cotton pad soaked in acetone.
12. More ideas …
- If you have an artwork printed on paper, use spray Transparent 21 to do a paper transparent to the UV light.
- Transfer a mirrored layout (Paint, Image>Flip/Rotate>Flip horizontal) to the Word document. The tonerside of the printed film will be turned in direct contact with the sensitive board surface and parasitic UV light will be reduced between the artwork and the photoresist layer.
- Inkjet printers can be used for layouts printing, apply Transparency film for Inkjet Printers.
- Positive Photo Resist Pre-Sensitised PCBs are available in electronic component shops, skip the guide sections 3. and 4.
- Any bright UV light (mercury vapour lamp, table top fluorescent lamp or sun) source may be used , exposure time varies.
- The silicate based developer is NaOH free and less sensitive to temperature and over-develop.
- Etching can be done by using ammonium persulphate or hydrochloric acid.
- Carbide drill bits are more suitable for hard fibreglass boards. Bits are expensive and only on very precise vertical drills can be used, otherwise they will break almost instantly.
- After all components are soldered to the board, spray copper with a protective coating (for example transparent Plastic Spray 70 from Kontakt Chemie).
Yours suggestions , corrections or helpful comments and pictures will be added here:
And finally , the process described here works for me.
After the first two or three times, you’ll start getting the knack. It takes time to learn and have fun !