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I know that there are multiple processes and lots of variables regarding homemade PCB manufacturing. Using the right tools it is possible to achieve professional levels.

I'm concerned for students their make themselves their first board. What could be a safe recommendation to reduce problems?

Edit: On regarding the process they'll probably use termal transfer (laser printing) and ferric chloride on the etching process.

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  • \$\begingroup\$ That certainly depends on the actual process, well established diy setups can go really small. Can you talk a bit more about your process and involved tools and chemicals etc.? \$\endgroup\$
    – PlasmaHH
    Jun 30, 2015 at 13:41
  • \$\begingroup\$ Absolute beginners? 10/20 trace to clearance at the least. Preferably 15/30. That then scales down to 8/8 with experience over time, anything below that nobody should want to do at home anyway. \$\endgroup\$
    – Asmyldof
    Jun 30, 2015 at 13:41
  • \$\begingroup\$ Indeed with toner transfer, make them traces fat, especially for beginners getting sharp edges (and maybe even with an iron instead of a laminator) is a problem here. People have reported to get their process reliably down to ~2mil but well, rarely necessary. \$\endgroup\$
    – PlasmaHH
    Jun 30, 2015 at 13:49
  • \$\begingroup\$ With artwork done on an inkjet printer and photo-sensitive board I routinely used 10/10 mil and could use 8/8. 6/6 worked OK as a test. \$\endgroup\$ Jun 30, 2015 at 13:54

1 Answer 1

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I'll turn my comment(s) into an answer then.

Numbers I mention are mil, where 1 mil = 25.4 μm, of course. Trace to clearance is expressed in this message (and many indications elsewhere) as #/# where the first number is trace width and the second is clearance. So 15/20 would be 15mil (0.381mm) trace with 20mil (0.508mm) clearance.

As @LeonHeller I have gone down to 6/6 mil, even 3/5 mil with thermal transfer, but afterwards never thought "gee this investment in time and attention was worth it rather than paying a Chinese proto-fab $35 to do it in 5days and send it with DHL express".

For "learning the tricks of the trade" start with at the very least 10/20 mil, preferably, to get students to not give up at first try start with 15/30. A common problem is flowing out of toner on over-heating, so a good clearance to start the practise with is not unwise. 10 to 20 mil traces will stick nicely to the copper, so damage should in most cases be small enough for the trace to survive and the tacks will be "fat enough" to repair with a run-of-the-mill Steadtler or Edding fineliner.

You can progress down to 10/10 if you have the time, but for an introductory course I would only decrease further if there happens to be a talented student that wants to push the limits, or the less talented ones will become frustrated easily with shorts and missing traces.

For the thermal transfer I advise you to get some soft, unwoven linen or cotton cloths or whipes to use between the source of the heat and the PCB+transfer foil, as this is one of the best ways to equalise pressure and heat. Woven fabrics will put the pressure onto the strands, so that's no good. And cotton and linen can take a lot of heat before they start smelling or browning, so the thickness can be compensated by increasing or decreasing the heat. If you first do a little experiment to find approximate settings and application time, given a certain thickness of cloth, this will also make it a lot easier for the students to replicate your process.

I use standard unwoven cotton wipes of no-brand, they are approximately 0.1mm thick and I fold them over once or twice to get 2 or 4 layers. Works a treat for nice and even sticking of the toner. I also use them below the PCB, by the way, which seemed pointless, but still appears to help a teensy tiny amount, but if you have a rolling heat source in stead of my heat-bed that may be fiddly work and might not be worth it.

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