Inlaid Logic : a Giant Circuit Board Inlay Made Out of Waterjet Electronic Traces

This Instructable demonstrates how to make a giant inlaid electronic circuit board out of stainless steel traces cut on an OMAX Waterjet and Acrylic CNC milled on a Shopbot.

These steps extend to build any electronic circuit, or any type of inlay using a Waterjet and CNC mill.

Inlaid Logic is part of my CMOS Logic Sketch Series and was made as an AiR (artist in residence) at Pier 9. The work in this series focuses on CMOS Logic Gates as the fundamental building blocks of digital electronic technology, and renders them so that they can be seen and appreciated.

Start by designing your circuit using traditional PCB (printed circuit board) design software and have some boards made just to make sure they work. For this project, I used Eagle to design my schematics and a local PCB fab house (Bay Area Circuits) to make the boards.

The circuits I made were of the following 8 digital logic gates (Inverter, Buffer, NAND, AND, NOR, OR, XNOR, XOR). These gates are constructed using individual N-Type + P-Type Mosfets(metal-oxide field effect transistors). This type of transistor and their complementary (CMOS) arrangement is significant because this type of transistor is what makes digital electronics as we know them today possible. The input of each circuit is a switch and the output is an LED.

Attached are the logic gates circuit schematics and the board design files. Feel free to play with these circuits, or whatever circuit you'd like!

Once you have your circuit figured out (functionally), it is time to redraw it so that it works with the waterjet inlay fabrication process.

The two most important considerations when designing your circuit is the waterjet kerf (width of the cutting stream) and the diameter of your End Mill. At Pier 9, the nozzle on the Omax Waterjet has a kerf of 0.042". The smallest End Mill that I was using was 1/32" (0.03125").

Since the electronic traces cut on the waterjet must fit precisely into their CNC milled negatives, it is important for the design files to consider both of those tools.

• When cutting the traces on the waterjet, the kerf applies a radius to interior corners.
• When cutting the negatives of the traces on the CNC shopbot, the End Mill diameter applies a radius to exterior corners.

If you don't compensate for the waterjet kerf and the End Mill diameter for all of your design files the waterjet inlay won't fit nicely into its milled negative.

It is best to do this in your design file, before you get to your waterjet and CNC milling toolpathing step. This way you only have to worry about it once. Above are two diagrams to help explain why it is important to compensate for your tools. The above diagrams show an example of an electronic trace. One design does not compensate for tool width and kerf, the other does.

When the waterjet pierces your material, the cut will be the widest at the surface where the jet first pierces the material and it will be the narrowest where the stream exits through the bottom of the material.

The result of this creates a tapered cut that is actually perfect for fabricating electronic trace inlays!

When you prepare your files to waterjet, you should mirror your inlay so that the top surface is facing down when you cut. This way, when you hammer in your inlays, they will be their widest where they meet the top surface of your material.

Another tip is when you are creating your toolpath, it might be a good idea to add tabs to your tiny pieces so that they don't fall down into the waterjet bath. I experimented with several ways of trying to avoid using tabs, but ultimately tabs were the best option. It is best to place tabs along a straight edge so that they are easier to remove later with a cutting tool.

Even though we know our Waterjet currently has the 0.042" diameter nozzle, we still need to do some calibration.

• Over time/use, the diameter of the stream increases
• We aren't really sure where that 0.042" diameter shows up within the depth of the cut. Does it happen where the jet first pierces the material? when it exits? or somewhere in between?

For this step, make a little test piece that has all of your geometry (or as much of it as you feel like). Run a couple test cuts where you vary the kerf offset. In Omax Make, there is an option to vary the kerf radius in the screen right where you load your file. The goal here is to find the correct offset where the widest part of your inlay (the bottom of the cut) matches your desired width.

For my piece, I varied the kerf offset from 0.021" (what you would assume is the correct offset for a 0.042" diameter nozzle) down to 0.018". For my material (0.12" stainless steel) the correct offset was 0.019".

Cut your traces!

The only thing to watch out for here are the places where your piece is over a slat in the waterjet bed. Sometimes when the stream hits a slat it bounces back and damages the underside of your material. For us the underside is the visible surface. Take a look at all of your traces after cutting and re-cut anyones that were damaged.

Parallel alignment brackets

Cut a square of MDF (about 12"x12"-longer is better) and screw it into the shopbot bed near the 0,0 corners of your piece. Once in place, mill out the 0,0 corner of where your material will sit. Don't forget to use a dogbone. Your material should nest into this corner and sit parallel to the x,y axis of the shopbot bed. You can optionally repeat this for the opposing corner, assuming you know exactly what you material size is. Otherwise, just make a right angle cutout in another piece of MDF and press it up against the opposing corner.

Fixture your material to the bed
I bought my stock material extra long so that I could drill fixturing holes in the corners of the material that I could cut off at the end using a Festool. These corner holes get screwed in once the material is aligned in place. It doesn't really matter where these holes are, as long as you know you can cut them off in the end.

Alignment Holes

With a 2-sided piece, the most important part of alignment is drilling alignment holes. These alignment holes are in the same place regardless of whether you are working on the top or bottom of your piece, so they only have to be drilled once in your actual material. These holes will mark the centerline of your piece, regardless of its overall dimensions. Once your material is fixtured to the bed, drill alignment holes using the shopbot through your material using a 3/8" bit or End Mill.

When it is time to flip your piece, before re-loading your material onto the bed, first drill new 3/8" alignment holes directly into the sacrifical material. Lay your material on top of this and place 3/8" alignment dowels through the existing holes in your material and into the bed to hold your piece in alignment.

Once you flip your piece, rely on these alignment holes to position your material. Use your old corner brackets to help hold the material in place, but it is really these holes that will keep it square.

Once your material is in place, put a small endmill in and manually travel to coordinates on your material where there are existing features (like holes) to verify that everything is properly aligned. Repeat this after fixturing your material to the bed in case something moved.

Before moving on to this step, it is a good idea to re-measure your waterjet inlay parts to make sure they are in fact what you expecting. As mentioned before, your profiles for the CNC shopbot should measure exactly the widest part of your inlaid pieces. This way when you hammer them in they will fit nice and snuggly. If your waterjet parts aren't what you are expecting, offset your lines to fix.

For this step we will be CNC milling pockets in acrylic to accept the waterjet inlay parts. We will first be using the biggest possible End Mill to remove as much material as possible. For me this was the 1/16" Onsrud O-flute. Then I used a 1/32" End Mill for doing the smaller features. The O-flute end mill allows for maximum material removal through a wide flute. I couldn't find an O-flute in 1/32", but 2 flutes did the trick.

Use a 3/32" long reach drill bit to drill holes for the vias. These holes can be bigger than the actual via. They aren't what will hold your via rods in place, and it is better to make them a bit bigger to allow for some extra wiggle tolerance.

Things to watch out for...

• Have an airhose nearby to keep your cutting tool as cold and free of debris as possible.
• If you are milling acrylic, make sure to mill in as small areas as possible. Cast acrylic is not perfectly flat. My piece was 1.25" thick and varied from 1.1" to 1.28". For inlays that are .12" thick, this difference is very significant. Make sure to zero your z-axis as close to where you are milling as possible. Don't assume 1 z-axis zero will work for your whole piece!

Ream your vias

If you have a circuit with multiple layers, you likely have vias (connections between layers). The hole that the waterjet cuts has a taper on it, and is probably not sized for the rod that you will eventually stick through.
Take a correctly sized reamer, I used 1/16" and use a drill press at a low speed to correctly size your via holes. The easiest time to do this is while your waterjet parts are still attached with tabs to your material. So do this before you remove your tabs. You can also use a drill to ream your holes if that is easier if your material is really big.

Remove your tabs

For this step you will need :

• Dremel
• Several Metal cutting wheels with EZlock fitting to quickly change out (depending on your material these wear out quickly)
• Hand File
• Vice
• Sandpaper
• Tape
• Scrap wood or other material

Clamp your waterjet piece down to a table on top of a piece of wood or some other sacrificial material. Make sure you have a good grip on your dremel and start cutting away the tabs to free your circuit traces. My material thickness was .12", so I normally would cut about halfway through on one side and then flip to the other side to finish the cut.

Once each of the traces are free, secure them one at a time in your vice and use a hand file to get rid of any remaining material from your tab.

For really big pieces (like the ground circuit pictured above that is in my lap), I support it delicately on my lab while I hand file away excess tab material

Sand your pieces

Depending on the surface finish that you'd like for your traces, you might want to sand them. For this step, tape stages of sandpaper with incrementing grit to the table. Hold your piece carefully and sand in a single direction. You might want to be consistent depending on the orientation of your piece within your circuit.

For some of the milled pockets it was necessary to go back afterwards and pick away excess material. You want to make sure that all of your pockets are clear of any material before hammering in your inlays. This was a big pain, and I recommend a finishing pass on the shopbot if you have time for this instead. I ended up picking away at the excess material and spraying it with an air hose.

If your inlaid circuit has multiple layers that need to be connected, this step will show you how to size tiny rods and then hammer them in.

In a previous step, we reamed our via holes to be exactly 1/16". The rod I selected measures 1/16" in diameter, so it's a tight fit, but it works.

• Cut a bunch of rods to the approximate size (bigger than you need) for it to be flush with the thickness of your material. Make extra!
• File one side of each rod so that it is nice and flat, sometimes when you cut the rods they get crushed and this looks bad
• Take all of rods and line them up with their one finished side against a flat surface. Once they are inline, put a piece of tape down to hold them in place.
• Use a sharpie to mark the rod length you need
• Put another piece of tape down to use to mark each rod with a sharpie. This will show you how much you need to file away to get to the proper length.

Make sure that everything is clean before you hammer in your traces. Use a hammer and a wooden block to protect the surface of your piece and distribute the force.

This is the most fun part! Enjoy!

Remember those tiny rods of metal you sized perfectly? Now is the time to hammer them in.

• Be careful so that you don't damage your nice surface.
• If the rod doesn't fit, take some sandpaper and try adding a tiny chamfer to the top edge of the rod.

Use a transparent epoxy to hold your components in place before applying the conductive epoxy to the joints between the part and the piece. This step makes it way easier to do the next step, and it also hold the parts on more securely.

Use a needle or a blunt tip syringe to apply your epoxy.

Use tweezers to carefully place your componets.

I use Atom Adhesives silver conductive epoxy. It has a 90-minute cure time, so leaves a lot of time to work. It is a bit pricey, but you need very little of it to make your conductive joints between your component leads and the traces.

Use a needle or a blunt syringe tip to apply the epoxy. Don't overdo it. This stuff wants to be able to thoroughly dry so that it is maximally conductive.

Before powering your circuit on, it's a good idea (probably) to do some conductivity tests on your joints. Also check for shorts! Conductive epoxy can be pretty finicky, especially while it is in the process of drying. For this step I went and checked to make sure the leads on all of my components were connected where they were supposed to be.

When I did this, I found about 20 or so places where they were not connected, even thought they were based on a visually inspection. Like I said, conductive epoxy is finicky.

Take some pieces of masking tape to mark off these troublesome parts, wait for the joints to dry more and retest. If the joints are still not conductive, I used the end of a tweezer to scrape away conductive epoxy from the joint. Sometimes this helps. If it doesn't, then add some more conductive epoxy and see if that works.