First Good PCB with the MF70 CNC
Posted by retromaster on January 21, 2010
I’ve finally managed to manufacture a good-looking single-sided PCB using my CNC MF70 and toner transfer. I first drilled the holes using the CNC, then I applied toner transfer aligning the transper paper with the holes. I’ve etched the board afterwards but not cleaned the toner yet. The PCB is a parallel port breakout board. I’ll use it for the new parallel port-based stepper drives. Tracks are 20mil with 10/12mil clearance. This time, I’ve used ink jet transparency film, as it’s a bit easier to align with the board holes. The results are quite good (the photo does not do it justice), though I am not sure how it’ll perform with finer traces. The text at the bottom turned out all right so I guess it could work. Soon I’ll try out another board with PnP Blue.
The most significant problem I’ve faced so far was the alignment of the drilled holes with the toner transfer printout. It seems that the printout function of the PCB layout software I am using was inaccurate, as it introduced a kind of shearing transformation to the printed pattern. The effect was very slight but enough to cause mismatch. It would not be fair to fully blame the software though, as the printout function was mainly intended for documentation purposes and laser printers can be inaccurate anyway. The solution I found was to first export Gerber files, then use the excellent gerber2pdf.py utility by Joseph C. Chavez. Printing the resulting pdf file solved the shearing issue, but there was still a linear mismatch in the Y axis of about 1%. I’ve done some searching on the net and it seems that this kind of mismatch can be expected from laser printers. The gerber2pdf utility accepts a scale factor as input, so this was easily fixed, and the result is the board I mentioned earlier in this post. The alignment is now almost perfect, although the scale coefficient could probably use some finetuning.