PCB Milling Tips

pcb-milling
#1

Has anyone been tired of having messy wires all over the place with perforated breadboards or non-solder breadboards? Has anyone wanted to make a design smaller and more professional? This post will focus on general tips for PCB milling. I used the Bantam desktop miller for my project, so I will be referencing the hardware and software in this post. For reference, the milling machine is Digi-Key part number 1932-1000-ND (manufacturer’s part number OM1002). The device comes with several tool options along with some development boards for starters and there is plenty of support for troubleshooting.

Since I am new to PCB milling, I decided to make a simple Audio Visualizer that works with an Arduino. There were a few mishaps when trying to mill my board, consider the following suggestions.

  1. Avoid making pads for direct wires that use solid copper or try to avoid soldering wires directly to the PCB in general.
  2. Even though one can technically solder directly to the pads using wires, I would advise against this. Pads end up pretty small in the first place, so soldering doesn’t provide a very strong bond to the copper board’s surface for wires. I found this often caused wires to pull out way too easily. It would actually be better to have stranded copper wires for this type of soldering, but I’d still recommend using wire to board headers or board to board headers to make soldering easier.

  3. If possible, try to use larger solder pads and aim for slightly bigger traces.
  4. I found that small solder pads that connect to smaller tracks caused issues. The boards that I used have a 1oz thick copper layer, my small pads plus small tracks caused problems where the copper layer would break if I scratched it or kept solder iron on too long. Small width tracks can break if too much heat or if it is scratched enough while soldering causing open circuits. Small pads and traces can be really hard to solder to anyway if the size of the soldering tip is limited (some may not have small solder tips).

  5. Be aware of how each part is designed and plan ahead for solderability of pads.
  6. I found that certain parts are quite difficult to solder based on their design. Male pin headers are easier to solder from the top down instead of from the bottom. Soldering these parts from the bottom can cause misalignment. Other parts that have the pins under some sort of plastic housing are easier to solder from the bottom if they are through hole parts. It might be necessary to create a via for traces if connecting to items that are soldered from the top. If the item is a surface mount and the pads are impossible to reach with a standard soldering iron, solder paste with reflowing pads will have to be used. Multiple sides to solder may need more than one ground plane reference.

  7. Aim for a one-sided board by reorienting parts in the layout.
  8. Everything is easier if all the items are on one side and in the long run boards will cost less.

  9. Use the appropriate drill bit to reduce milling time.
  10. I ended up using the wrong tool a few times causing a much longer milling time and I even broke a drill bit by using the wrong size on a layer that would work better with a larger drill bit. Using KiCad’s default trace clearance, the .005" PCB milling bit will work just fine for traces. Larger through-hole sizes will work just fine with a 1/32nd bit while smaller holes will require the 1/64th bit. Edge cuts (aka. outer border) a 1/16th bit will be suitable to not waste as much material as well as reduce the time for milling. The Bantam milling software, for example, will tell if a certain bit will work with the cutting layer chosen. If it works, no red marks will show up. Be aware of the estimated time shown on Start Milling before clicking it. Try to do one layer at a time (holes, border, and traces).

  11. Buy the PCB probing tools and always probe for board height.
  12. Measuring the board with calipers or going off the datasheet can sometimes cause an error in the final mill. I recommend getting the PCB Probe tool: Digi-Key part number 1932-1004-ND , manufacturer’s part number UP1007. Use the Bit Breaker menu and follow the prompts before cutting to probe board thickness. Move the probe tool out of the way before cutting. These tools are specific to the Bantam miller, so make sure to get the appropriate tools for your application.

  13. Fully secure the board with the guide in place with double sided sticky tape.
  14. A board guide (bracket) is a handy tool that helps the machine locate where to cut. Locate the tool by using a larger cutting tool upside down in the mill and click “Locate”. If the board isn’t fully stuck on the plate with sticky tape, the bit will move the board around when it starts to cut. Make sure the spoil board is clear from debris before taping a new board on. The locate functionality may be different for different milling machines.

  15. Measure three times, cut once.
  16. Be absolutely sure pad dimensions along with board layout dimensions are accurate before milling. I mismeasured my first attempt at making a shield for Arduino Uno and ended up being about 2.44mm off on the left side causing the whole side missing the pin holes. Even though having the mill and boards is free and cutting a new one is relatively low cost, in this case, professional boards cost much more to re-mill.

  17. Inspect the drill bits before using them on a cut.
  18. Make sure that the bits don’t look broken or dull before cutting the board. Broken or dull bits may end up going further into the board than desired or even destroy everything around what it is cutting causing a re-cut of the whole board.

  19. There is a process to cutting a board and steps should follow a general order.
    1. Home the board
    2. Attach the bracket and "Locate" it
    3. Choose if using a custom size board or standard single/double side board
    4. Load the full plan: Front, Back, Border, and Drill Holes
    5. Load the copper board that has double sided sticky tape on it
    6. Change the milling tool (choose traces for one side first) to the appropriate tool and let it find the position, make sure the tool will touch the bottom of the spoil board NOT the copper board or what is being cut
    7. Probe for the board height using the Bit Breaker Menu
    8. Begin milling the traces and change bits if needed (the mill will tell if a certain bit won't work)
    9. Mill the through holes after (if applicable) with recommended bits
    10. Mill the edge cut (border) last with a larger tool
    11. The same steps apply for a double-sided board, however, it is recommended to just do traces on one side and then traces, holes, and edge on the other side. If the drill doesn’t make it all the way through the board for the drill holes, it can be reversed to cut again, but it is easier to poke out the rest with the same drill bit that the tool cut with as it will have made it most of the way through anyway. The above process is specific to my experience using the Bantam desktop milling machine.

  20. Make sure the spoil board is clear along with the loaded copper board.
  21. If the spoil board or copper board is not clear and has material on it, the incorrect height can be measured and cause errors when milling. Vacuum out the machine after each use.

  22. The Bantam software cannot process an Opcode for a slot cut.
  23. I found out that using KiCad’s oval holes causes an error when loading .drl files into the Bantam milling software. The board will not cut and produce an error message on the screen. The same kind of cut can be achieved by using the same drill cut in the same axis overlapping the cut by a little over half the diameter of the through hole in question.

#2

Great tips!

One thing to add. Slot holes are doable in some EDA suites! Simply move slot holes to the outline layer. They will then be cut with the board’s outline. Otherwise, yes, the overlapping holes trick works in a pinch.

#3

It isn’t the EDA that made the issue to my knowledge, it is an software error I experienced using Bantam’s software. KiCad can make oval holes, but the desktop tool says there is a bad G-Code or OPCODE and causes error in the cutting process and the board won’t even cut. Unless you mean that there is a tricky work around by making the machine think it is a outline cut, which is very clever around G-Code issues.