Homebrew PCBs

Hi All,

This is a continuation of the discussion on the forum CNC_PCB-Design 
with Stefan on EDM & Etch Resist.

Stefan suggested tin as an etch resist, and I followed this up. Here 
are my tests to date.

Built a simple EDM power supply, 2 x 30V trafos with 70 Volt 6800uf 
reservoir cap, and dubious 2 x 1uF discharge caps for ablation. Used 
Don Lancaster's trick of light bulbs as ballast resistors, but only 
had 2 x 100 watt (240V), (yes I'm in the UK).

Obtained strong spark, but slow spark repetition rate.

Coated a section of copper clad pcb with tin/lead solder using 
soldering iron. Etched a line through the tin/lead solder by spark 
erosion by hand. Electrode was 1mm copper wire (all this came out of 
my junk box). Dielectric was diesel fuel (works well but smells).

Continued ablating the tin/lead until the copper showed. Copper is 
readily visible through the tin. Etched in Ferric Chloride solution. 
Result is complete etching of the EDM exposed copper.

This demonstrates to me that isolation milling by EDM is possible, 
and that pcb production by this method is a real possibility.

Spark eroding by hand was difficult, I could generate a repetitive 
spark for about 2-3 seconds, but when moving the electrode the spark 
ceased either due to over current (short circuit) or under current
(open circuit).

Next trial is to mount an electrode control on the cnc and try for an 
isolation mill on a pcb. I need to adjust the RC time constant for 
high spark rate for this application, since the objective of this 
excercise is to ablate the tin relatively quickly, and then etch the 
copper.

Will keep you posted.

Roger