The thing that I most like about dabbling in electronics is that I’ve been able to create circuit boards at a resolution that should be very difficult to do at home.  In fact, I’ve managed to do so with relative consistency.  Despite my success, I’m not entirely thrilled with the lithographic process.  It requires a lot of discipline (which I have not got) and chemicals.  If a board doesn’t turn out, there are a myriad of possible reasons.  There may have been a fingerprint on the mask, the etchant may not have been up to temperature, the developer could be nearing the end of its useful life, I might forget to set a timer and over-expose or develop the board, etc.  The chemical process is also a bit time consuming, and it doesn’t lend itself toward the rapid development of small boards.

The “holy grail” of home board production, has managed to elude me.  I’ve got a small CNC mill which I use for drilling the boards before I etch them.  Once the board has been drilled, I use the drill holes to line up the lithographic mask for exposure.  For quite some time, I’ve thought that it would be really nice to be able to isloation mill small boards instead of relying entirely upon the lithographic process.  My efforts have been largely in vain, though.

Consider for a moment the dimensions of a small surface mount board.  The Atmel chips that I’ve been working with generally have a pitch of about .7 or .8 mm.  That means that the pins are .7 mm apart on center.  My nephew recently dropped off a Microchip PIC that is .65mm pitch, and I offered to make a DIP adapter for him.  At .65MM pitch, the distance between pins is in the neighborhood of .3mm.  Finding a cutter that size is no small achievement.  A stepper motor has about 200 positions, so once you combine that with a 10 TPI screw, your maximum resolution is 1/2000 of an inch.  The good news is that resolution isn’t the big problem, however if you have any backlash at all you’ll have a difficult time cutting at this size.

M y first attempt resulted in the near instant destruction of $25 worth of end mills.  After some stewing I purchased a set of engraving bits from E-Bay.  The aren’t precise, and the plunge might be a problem…but the price was right.  This afternoon I dropped a bit into my mill and destroyed it seconds later.  The feed speed was 20 inches per minute and the depth of the cut was .01”.  With a V-shaped engraving bit, a .01” cut is really deep (and wide).  Combine that with a fast feed speed and you can imagine what I did to my cutter.

I reset the program to feed at only 3 inches per minute, which is terribly slow except that I’m only feeding a total of about 10 inches anyway.  Also, I set the depth to only .005” and then I loaded a new cutter.

The results were, well, surprisingly good.  I think all but two of the traces will test out just fine.  I also think I can solder the chip onto the board.  In the picture to the left, you can see that the traces are pretty clean.  What you cannot get a sense of in this picture is the actual size of the circuit board.  Take a moment to visualize it in your mind, then click here to see another picture of it.