Printed circuit board milling

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New for January 2023.  Printed circuit board (PCB) milling is used for making fast prototypes of circuits, including microwave circuits.  The "mill" is basically three-axis computerized numerical-controlled positioner (CNC), and a router.  If English is not your primary language, be aware that "router" has two engineering definitions: it is what controls your data flow to the internet, or it is a type of high-speed drill that can be used to remove material as it is dragged across a surface. Tools bits used for milling are called "end mills", and they are specifically sharpened on the side surface and have flat bottoms that can also cut material.  A "drill bit" is sharpened specifically for removing material only from its pointed tip.  Here's an Amazon link to an example router for milling, it appears to be the same one used in the video that appears later on this page. 

You can buy a fancy new PCB milling machine, or you can work with an older one which may have some quirks but will save you some money.  One key feature to look for is automatic tool-change capability; without this you will have to create multiple files to run and change the tool manually in between.

Here are some definitions that will help you understand PCB milling:

  • Runout: this is a measure of how loose the router is in the X and Y directions.  In microwave circuits, you will want less than 1 mil (25um) of runout to be able to control line impedances.
  • Isolation geometry: here, a small router bit is used to mill out the outline of your copper features.  It is impractical to use a small bit to clear off larger areas of copper, hence the next bullet point:
  • Rub-out geometry: a series of larger router bits are used to clear off large areas of copper.  Also known as non-copper clearing, or NCC.
  • Route-out: this is the geometry where the part you are building is cut from a larger panel.
  • Mouse-bites: when you fabricate one or more printed wiring circuits on a larger panel, you may wish to leave part of the cutout intact. this is done by leaving gaps in the route-out. When you snap the part off the panel, the remnants of the area that held the part to the panel kind of look like a mouse bit the boards.
  • Registration of backside to frontside of board: A pair of precision steel dowel pins are used to hold down the copper circuit that is being milled.  When the board is flipped, the dowel pins ensure that the computer can register the origin from one side to the other.
  • Drill file: the CNC mill is used to create via holes through the board in a drilling operation (plunging an actual drill bit, versus dragging an end-mill). These holes will later become low-inductance ground connections by stitching wires through the board and soldering to metal traces on both sides.

There is one compromise in milling versus etching a microwave PCB.  Invariably, some dielectric material will be removed around the traces, causing less capacitance to ground than intended in a design. This will vary from unit to unit, depending on how physically flat the board is. You can't make an omelet without breaking some eggs...

In the featured video below from Graybeard Microwave, you can learn how Gerber artwork is imported into shareware called FlatCAM, which generates multiple output files for multiple tools.  The machine used is an awesomely-repurposed unit, but it is a manual affair, requiring manual tool changes.  You can see that it is possible to make a fatal mistake by mounting the wrong tool for a particular run.

"GBM 004 Inverted F Antenna (IFA" PCB Milling" by Graybeard MIcrowave

Graybeard Microwave, a.k.a.Lou, is a good supporter of Microwaves101 (a.k.a MadEngr on the Microwaves101 forum).  Check out his other videos over on his Graybeard Microwave Youtube Channel; we recommend you subscribe. There is also a companion video on designing the inverted F antenna he fabricates in his PCB milling video, for you antenna types!


Author : Unknown Editor