PCB fabrication (homebrew)

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Homebrew PCB fabrication with the emphasis on quickly and efficiently producing consistently high quality PCBs rather than on reducing the cost. Making PCBs at home is much quicker, with results the same day instead of waiting weeks for a commercial fabricator, and cheaper (if you don't take the time into account). However it involves working with messy chemicals, drilling will be tedious and the overall quality wont be as good.

PCB layout

Main article: PCB layout design

Although possible with tape and dry transfers, or permanent marker pen EDA software will offer schematic capture, PCB layout, ERC (electrical rule checks), DRC (design rule checks) and other features, e.g. simulating the circuit with SPICE.[1]

Cutting PCB to size

With SRBP use a craft knife and straight edge. Score it deeply on both sides. Clamp one or both sides between wood to prevent the board being scratched. Then snap it off. A craft knife works especially well for perforated prototyping board. Score down the center of a row of holes, line the score line up with the edge of the table, and press down quickly on the overhanging piece.[2]

FR4 boards are much harder to cut and very hard on tools.[3] A PCB guillotine is the right tool for the job. Failing that use a bench shear or an office guillotine that can cut through more than 2 mm of paper. Use a nibbling tool for non-straight cuts.[4][3] If there is going to be any dust a conventional dust mask might be insufficient protection as the dust is extremely fine. Use a vacuum cleaner to catch the dust at source. Safety glasses should always be worn.[2]

Alternately take a straight edge and secure it along the line to cut. Then take a chisel (one set aside for this), and run one of the corners along the straight edge. First pass go lightly to just get a bit of a groove dug in for the tool to follow so it wont jump out on subsequent passes. Next pass or two press hard, and dig in for a deep groove. Score on both sides. Secure the board and snap the piece off. To finish the edge make a pass or two along it with a fine file.[5][3]

Masking

There are two popular methods of masking the copper laminate from the etchant, the toner transfer method is cheap and simpler but takes a lot of patience and fiddling about. The more expensive photo resist method can be much more accurate and one printout can be used multiple times but also requires an UV exposure box and developing the board.[6]

Toner transfer

Main article: Toner transfer

Using a laser printer or copier, this does not work with an inkjet printer, to print an image of the PCB tracks on glossy paper. Laying this printed side onto the de-oxidised and de-greased copper laminate and using an iron at a high temperature, on the rear of the paper to transfer the toner from the paper onto the copper. After soaking in water to remove the paper, the toner now transferred to the copper laminate acts as an etch resist.[7] This can also be used to print component side parts and legends. Press'n'Peel is similar to this method. If you don't tin the board, coat the copper laminate with rework flux to prevent it oxidising.[8]

Photo resist

Print the PCB layout onto clear or translucent film – this is the mask. Plastic transparency might deform from the heat of the printing. Place the printed side against photo-resist coated PCB and expose to ultra-violet light. Pre-sensitised copper clad boards are easier than spray photo-resist where it's difficult to attain consistent thickness needed to estimate a correct exposure time. For double sided boards, first tape the two masks printed sides together then tape the board in place between them and expose both sides to the UV.[9] Use the recommended developer not sodium hydroxide, this removes the coating where it has been exposed to UV. Follow the manufacturers directions for correct concentration and temperature. Leaving the photo-resist on the board prevents the copper oxidising and it acts as half reasonable a solder flux,[9] unless the board is going to be tinned.

Etching

Etching the PCB in Ferric Chloride (FeCl3) solution removes the copper not masked by the etch resist. Take precautions because FeCl3 will deeply stain anything it comes in contact with.

Safe disposal of FeCl3

FeCl3 is toxic and harmful to the environment – do not flush spent etchant down the drain. After use leave the FeCl3 to stand still for at least a couple of days, the copper precipitates to the bottom, leaving still useful etchant on top. Pour this into another container. Add pure sodium bicarbonate (NaHCO3) aka baking soda to the thick mud at the bottom. The NaHCO3 will react with ferric chloride rendering the solution relatively harmless to the environment. Do only a little bit at a time, waiting for the reaction to settle down each time. The reaction will produce a substance 7 to 10 times the volume of the solution, any spillage will contain still active FeCl3 that will stain whatever it comes in contact with. Keep adding the NaHCO3 until the rust-coloured mess is fairly dry in texture. It can then be disposed of along with the household waste.[10]

Alternative etchant to FeCl3

Cupric chloride etchant is very similar to ferric chloride, but is simple to regenerate, there are no disposal problems,[11] and doesn't stain everything it comes in contact with. It does use more dangerous chemicals. Open Circuits wiki discusses a variety of other chemical etchants.

Tinning

Tin-plating a PCB makes it a lot easier to solder and makes working with SMDs much easier. Use room-temperature tin plating crystals e.g. Seno Immerse Tin Crystals, these produce a good finish in a few minutes but can be expensive. Only make enough tinning solution to cover a PCB in the tinning tray. Keep the solution in a concertina-type bottle to exclude air. Also avoid contamination with metals other than copper. Thoroughly rinse and dry the PCB before tinning. Use a separate tray and pair of tongs specifically for tinning, and rinse them after use. If the solution stops tinning, discard it, clean & rinse the tray, and make up a fresh solution. A cool tinning solution will usually prevent tinning so ensure the temperature of the tinning solution is at least 25ºC, but not more than 40ºC, put the bottle in hot water to warm it up.[8]

Strip the etch resist thoroughly. Rub the copper surface with a plastic scourer until it is bright and shiny all over, wipe with a paper towel and immediately immerse the board in the tinning solution. Take care not to touch the copper surface after cleaning, as fingermarks will impair plating. Within about 30 seconds the copper should turn a silver colour, leave the board for about 5 minutes, agitating occasionally. For double-sided PCBs ensure the solution can get to both sides. Rinse the board thoroughly, and dry with paper towel to remove any tinning crystal deposits, which can spoil the finish. If the board isn't going to be soldered for a day or two, coat it either with a rework flux spray or a flux pen.[8]

Drilling

Use a drill press and a drill with a collet not a chuck. Use a 0.75 mm drill bit for most holes and a 0.9 mm for headers.[8]

To use a CNC drill first generate an Excellon or NC drill file.

See also

References

  1. ^ PCB design, Software options, ladyada.net
  2. ^ a b How do you cut PCB?, Electrical Engineering Stack Exchange
  3. ^ a b c Easiest way to cut PCB's, diyAudio forum, August 2005
  4. ^ Cutting PCB, Homebrew Printed Circuit Boards Yahoo! group
  5. ^ Cutting FR4 boards, EEVblog forum, August 2012
  6. ^ Re: DIY etching PCB's – Toner Method or Photo Resist Method? by R O Tiree, 27 May 2013
  7. ^ Easy Printed Circuit Board Fabrication, Using Laser Printer Toner Transfer by Thomas P. Gootee, 2007
  8. ^ a b c d How to make really really good homemade PCBs by Mike Harrison
  9. ^ a b Mega PCB Instructional Video
  10. ^ Re: how do you all dispose of Ferric Chloride? by Mark Hammer, 15 October 2006
  11. ^ Etching Circuit Boards Using Cupric Chloride and Acid Solution by Stephen Kasten

Further reading

  • How to Design and Make Your Own PCBs by R. Penfold, Babani, 1983, ISBN 0-85934-096-1
  • Fabricating Printed Circuit Boards' by Jon Varteresian, Newnes, 2002, ISBN 1-878707-50-7
  • Build Your Own Printed Circuit Board by Al Williams, Tab, 2003, ISBN 978-0-07-142783-8

External links