Double-Sided Boards

While single-sided PCBs are simple and ideal to use when possible, many circuits require two sides. Double-sided boards offer a wide variety of possibilities for circuit board designers because they have more surface area and flexibility when routing traces. Using two sides instead of one also allows for a smaller board size overall, which can be important when building small-scale, integrated devices.

This guide contains all the information you need to get started milling double-sided PCBs using Otherplan.

Double-Sided PCBs and the Othermill

Process Overview

Milling double-sided PCBs is a straightforward process: mill the top of the board, then mill the bottom. But in order to achieve perfect registration between the two sides, there’s a process that you must follow, which is covered in more detail further down. Here’s an quick overview:

  • Install and locate the Othermill’s alignment bracket.
  • Align a piece of double-sided FR-1 to the left corner of the alignment bracket.
  • After importing your design file, mill out the traces and holes (but not the outline) of the top side of the board.
  • With the top side milled, flip the board over and tape it down — this time aligning to the right side of the bracket. By aligning to the right side, we’re using the same corner of the board to align on both sides, which reduces the possibility of alignment errors.
  • Mill out the rest of the board including the outline.

Double-sided PCB A milled double-sided PCB.

Materials

When milling double-sided PCBs on the Othermill, we recommend using double-sided FR-1 blanks. These boards are copper clad on both sides, with a thin sheet of phenolic resin in the middle holding them together.

Design Considerations

Unlike double-sided PCBs produced by a board house, PCBs made on the Othermill are bare, with no plated vias. As a result, you’ll need to connect vias manually. Because this is a manual process, we recommend that you limit the number of vias on your board to save time during the assembly process.

To hook up vias, thread a piece of wire through the vias on your board, as shown below. Then, solder each side of the wire. Finally, snip the excess wire on each side of the via.

Vias threaded with tinned wire Vias threaded with tinned wire.

Soldered and trimmed vias Soldered and trimmed vias.

Step-by-Step Walkthrough: Milling a Double-Sided PCB

Before starting this walkthrough, make sure to install and locate the alignment bracket. This will ensure that both sides of the circuit board are properly aligned.

  1. Clear off the Othermill’s spoilboard with a small brush or vacuum, taking care to remove all chips from the inside of the alignment bracket. This will help ensure that your FR-1 is precisely aligned.
  2. Using a pair of digital calipers, measure the dimensions of a piece of double-sided FR-1. Enter these dimensions into Otherplan’s Material panel, and set the material type to Double-Sided FR-1. Entering material dimensions into Otherplan Entering material dimensions into Otherplan.
  3. Cover one side of the FR-1 with double-sided tape, using Nitto tape if available and taking care not to let the tape overlap. Align the FR-1, tape side down, to the left side of the bracket, then move it down, toward the front of the bed, until it’s aligned to the corner of the bracket. Push down on the FR-1 to make sure that it’s fully attached. PCB aligned to lower left corner of bracket PCB aligned to lower left corner of bracket.
  4. Import a double-sided PCB file. EAGLE .brd and Gerber files are both supported. A new plan panel will appear on the right side of the Otherplan window and will show the settings for the top side of the board.
  5. Configure the milling parameters for the top side of the board. After selecting the appropriate tools, make sure to deselect Outlines by clicking the Outlines button. (We’ll cut out the outline of the board as our final step on the bottom side of the board.) Imported file with Outlines deselected Imported file with Outlines deselected.

    You may wish to add an extra offset to the plan to prevent accidentally milling into the alignment bracket. To do this, expand the Placement option and add an offset in the x and y direction (we suggest 0.2”). 0.2-inch offset added for x and y 0.2-inch offset added for x and y.
  6. Take a final look at the preview to make sure that you have the appropriate tools selected and that there is enough offset to make sure your tools will not mill into the alignment bracket. Click “Start Milling” to mill the top side. If you don’t have the appropriate tool installed, Otherplan will prompt you to switch tools before milling begins.
  7. When the top side has finished milling, clean the board by vacuuming away the chips. Remove the FR-1 from the Othermill and peel off all tape.
  8. Apply a new layer of tape to the top side of the board (the side with the newly milled traces). Attach it to the mill, this time aligning it to the right side of the alignment bracket and moving it toward the front so that it’s precisely aligned to the right corner of the bracket. Milled PCB flipped and aligned with lower right corner Milled PCB flipped and aligned with lower right corner.
  9. Click the Top/Bottom toggle switch to switch to the bottom side. In the preview, you’ll see the bottom side of the board design, with the plan and material aligned to the right. As you did with the top side, select the appropriate tools and milling settings for the bottom side. Make sure to re-enable traces, holes, and outlines. This will ensure that the outline of your board is fully milled out. Imported PCB plan flipped to bottom side, ready to mill Imported PCB plan flipped to bottom side, ready to mill.
  10. Verify your configuration using the the 3D preview. Click “Start Milling” to mill the bottom. If you need to switch tools, Otherplan will prompt you to do so.
  11. When the bottom cut is finished, including the outline, you should end up with a nicely aligned, double-sided board ready for you to place your components.

    Milled double-sided PCB Finished double-sided PCB.

Troubleshooting

The two sides of my board are not aligned.

Make sure to register the top side of the board with the left corner of the alignment bracket and the bottom side of the board with the right corner of the bracket. If you followed this procedure and still have misregistration issues, please contact our support team.

Do I really need to mill the holes twice?

No, but this ensures that the holes are fully milled out and registered.

If you run into trouble, feel free to email us at support@othermachine.co and we’ll give you a hand.

This guide refers to Otherplan. Otherplan Classic instructions are available here.