Milled PCBs for the M1

For a long time I've been been etching circuit boards as needed using ferric chloride.  The process is simple enough (though tedious):  Print a mask using a regular laser printer, melt the toner onto a blank copper clad board, remove the paper, and eat away the non-masked areas of the board by dipping it in ferric chloride.

I mentioned with my last post that I had ordered six 1 Mb MRAM ICs for the M1.  The trouble comes in how to assemble the memory for the system.  I had originally planned to put down six DFN pads in FreePCB and let the board auto-route, but with the clearances that I have to work with due to the limits of my PCB manufacturing method it just wasn't working.  The router was choking with quad channel memory, yet alone six-channel.

Those thoughts have prompted me to investigate (once again) building a small CNC mill for PCB manufacture via trace isolation.  It's more or less the mechanical equivalent to what I do now chemically.  The benefits are numerous, however, in terms of labor required and resolution achieved.

The mask & etch process goes like this:

1. Put on clean, un-powdered latex or nitrile gloves and wet-sand the surface of a copper clad board first with ~150 grit until the copper loses its initial texture and fingerprints are no longer visible.
2. Wet sand the surface with 200-220 grit until the copper becomes smooth again.
3. Wipe the board with a clean, oil-free cloth of some sort while under water until the cloth quits accumulating copper dust.  DO NOT TOUCH THE SURFACE.
4. Remove from water and wipe dry with a different clean, oil-free cloth.  Leave out in the sun to dry completely.  Store in a plastic bag with a desiccant, wrapped in a clean, oil-free cloth until needed.
5. Print a photomask onto photopaper with a laser printer.  Repeat if necessary until a good print (free of pinholes, scratches) is achieved.  DO NOT TOUCH THE PRINTED SURFACE.
6. Carefully scribe the paper to roughly fit the outline of the printed area or the PCB.  They're usually the same size.
7. Melt-transfer the mask to the copper board by putting the printed side of the mask against the copper board, applying even pressure, and heating to 250-300F (depending on toner brand and series) for 2-3 minutes.  20 PSI for 2m45s at 275F seems to work well for black, genuine Samsung toner.  An industrial heat press or a lamination machine works.
8. Soak the paper in water until soft.  Scrub the paper with a stiff plastic brush until it is wholly removed, leaving just toner.  If your transfer wasn't perfect or if you scrub -too- hard you'll start to lose flakes of toner around edges.  This will happen to a small degree even on a "perfect" transfer.  Some areas will also have run more than others, reducing realized resolution.
9. Double-glove using nitrile.  This chemical is bad for your health.  Etch the board by immersing it in ferric chloride and agitating the solution for 5-20 minutes while the board etches.  Remove after the minimum amount of time required to remove all unwanted copper.  Excess time leads to undercutting of traces since the walls of copper tracks and pads aren't covered by toner.
10. Rinse and clean throughly with water.  Ferric chloride is toxic.
11. Remove the toner mask with acetone while wearing butyl rubber gloves.  Rinse the board again. 
12. Drill all necessary holes one-by-one with a drill press.  Go slow and carefully so as to get as good of hole alignment as possible. 

Milling removes all of those steps.  All of them.  They are instead replaced by:

1. Export gcode from your CAD package for your design.  This is used by the mill controller to route the engraving bit around.
2. Export a drill file from your CAD package.  This is used by the mill controller for drilling the holes.
3. Mount a copper clad board to the work table and define the "home" position.
4. Chuck in an engraving bit, press start, and go have a beer.
5. Change the engraving bit for a drill bit.  Press resume.  More beer.

That's it.  No more careful pre-cleaning is required, no handling concerns exist, no toxic chemicals and protective equipment is needed, no manual labor during production is needed, etc.  You just press a few buttons and come back to find a superior board.

In a few months, I'm hoping to have my small mill project finished so that I may resume work on the M1.