PCB Alignment for double-sided milling

I wanted to be able to mill the top, flip the board, mill the bottom, drill the bottom, all without having to re-set the X and Y home position.

First off, I use the non-commercial CADsoft Eagle license, which limits me to a 3x4" board.  I decided to use board mounting holes to do "double duty" - mounting holes and  aligning the boards for milling.  I pre-drilled a bunch of PCB 'raw stock' with two precisely placed mounting holes (more on that later).

Alignment pins need to be placed on the top side of the sacrificial board.  To do this, mount the X-Y home block and home the X & Y axis.  Then drill four 0.125" holes 0.3" deep into the top of the sacrificial board at:

1. X =  3.80, Y = 0.15
2. X =  0.15, Y = 0.15
3. X = -0.15, Y = 0.15
4. X = -3.80, Y = 0.15

I cut-off some old, broken drill bit shanks to 0.4", chamfered the ends a bit to ensure no burs and epoxied them into the holes.

Sacrificial board, top view.
(click for larger image)

The image shows the sacrificial board, mounted on the milling table.  I could extend the life by drilling half of my boards via the top, and half via the bottom.  In the image, you can see all the drilling so far is via the bottom of the board.  No particular reason.

PCB mounts to the left two pins (negative X coordinates) for bottom etching/drilling and flips over to the right two pins (positive X coordinates) for top etching/drilling.  Thin double-sided tape is used to hold the PCB securely to the sacrificial board.

Setting the tool home point quickly

Home Block Top
(click for larger view)

Now that we can quickly and accurately mount and re-mount the sacrificial board to the milling table, let's turn our attention to the top side of the sacrificial table.  First, we need a method to quickly, accurately and repeatedly 'nail' the X=0 and Y=0 points, without having to "eyeball it".

To do this, I made an "L" shaped jig, with conductive brass edges that pins very snugly onto the top of the sacrificial board.  The conductive strips are wired into the X-home and Y-home inputs of my optical interface board (see earlier post on Option Isolation Board).

The idea is to set the zero point one time, good for etching and drilling both sides of the board without having to reset anything.

Home Block Bottom
(Click for larger view)

The alignment pins are 1/4" aluminum rod, epoxied into place and are on a 3.0 inch center-to-center.

The square brass bar is set into small dados so they are sticking out just a little bit proud of the wood surface. This ensures the tool will touch the bar before it touches any wood.

This block fits snugly into the holes drilled in the sacrificial table in the previous step.

Home Block mounted to
sacrificial board (click for lager image)

Preparation for double-sided PCB - sacrificial surface

A replaceable backing system is necessary since the drilling will damage whatever is behind the PCB.
To hold the PCB, I use a 3/4" MDF (Medium Density Fiberboard).  The PCB backing board needs:

• flat and true
• inexpensive
• easy to replace
• fast alignment
• fast leveling

MDF clearly meets the first three requirements.  To meet the last two, I decided to use three 1/4" registration pins and "T" nuts with set screws for leveling.

0.250" Registration holes in milling table
(Click for larger image)

I drilled 3 holes into my machine table. I made an "L" shaped layout.  The pattern (in inches) for the three holes are:

• 0,0
• 12,0
• 0,8.5

These holes are small and do not interfere with any operations when I'm milling large stock.

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The actual sacrificial table is 3/4" MDF, 10" x 14" and is routed from the back to accommodate:

Underside of sacrificial table
(Click for larger image)

• Three alignment pins - the reverse of the dimensions above (1/4" aluminum rod stock)
• Three "T" nuts with inset hex head set screws for leveling
• Two alignment holes for homing the tool (setting x=0 and y=0)

This part is milled from the back side (as shown in the image the above) - this allows all holes to be milled at one time.

Close-up of alignment pin and
leveling screw
(Click for larger image)

Here is a close up of one of the alignment pins (epoxied into place) and one of the "T" nuts with the leveling set screw visible.

Double-Sided PCB Isolation Routing

I've been thinking about this for a very long time.  My design requirements:

• Easy-to-setup
• Easy to get the PCB 'flat'
• Accurate and fast
• Automatic setting of home
• Jigged to make it super-easy to mill both sides without changing my X & Y zero points

Click to enlarge

I am excited to say I think I got it.  My recent test had trace widths down to 0.012 inches and they were razor sharp. The image to the right (20x) shows four of the 0.012" traces ending in pads for a 25pin D connector.  The board only needed very light sanding and the isolation paths were excellent.

Front-back alignment was very good (some issues during the test may have contributed to a less-than-perfect alignment).

I will document the enhancements I made.  I added an anti-backlash feature - it is both easy and, based on my recent results - razor straight 0.012" traces, quite effective.  I have a table/jig that makes flipping the PCB a zero-time exercise - without having to reset my X & Y home points!.  I made a simple jig to nail homing my X and Y axis