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I've designed about 20 PCBs in Cadsoft Eagle, and for the board outline I've always used the "20 Dimension" layer, using "wires" of width 0 to enclose the PCB. I did this regardless of whether the PCB was rectangular (thus it could be separated from the panel by V-cuts), or non-rectangular (necessitating break routing).

As an example of non-rectangular PCB, see the part below. I've pressed "properties" on the long arc next to the dialog: Cadsoft Eagle wire width=0 example

As you can see, the width is 0.

I've always assumed that

  • A) the dimension defines what is the "inside" of your PCB shape, e.g. if you define your Dimension layer as a polygon (0", 0") -> (1", 0") -> (1", 1") -> (0", 1"), you should receive a PCB that is exactly 1-inch-sided square, differing up to manufacturing tolerances (and, importantly, the same size should be produced by either scoring or break routing)
  • B) this definition is a bit abstract, and the PCB fab is expected to figure out the details. E.g., if they use 2mm break routing, they'd need to extend this shape by 1mm (essentially a Minkowski sum of my shape + 1mm circle) and that would give them the path for their router. They'd also need to figure out where to place the break tabs - it's not "my job" to select this. If they used 2.4mm break routing, they'd need to extend by 1.2mm. In any case, it seems wrong to assume what exact steps would the fab do to prepare my board for their manufacturing process.

In a debate with a coworker, he said my use of 0-width dimension is wrong, because the gerbers it generates are also a bit wrong, as e.g. CAM350 would complain about the zero width. One easy fix is to increase the width to e.g. 0.25", but this is not a no-op, as it actually shrinks the PCB a little. So in essence, he said I should fix my design by doing the Minkowski sum myself, using some non-zero width, if I wanted to get my PCBs the way I initially envisioned them.

I'm not convinced by his arguments, because

  • the warning message could be just a peculiarity of CAM350
  • I've sent at least 10 such designs to batching fabs like OSH Park and Ragworm, and I've never received a single complaint that my PCBs are not well-specified (but maybe they are lenient to noobs)

Questions

  1. Are my assumptions A & B correct?
  2. Is dimension of width 0 allowed / normal / accepted in the industry?
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1 Answer 1

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Talk with your PCB manufacturer.

Gerber files are for photoplots of copper layers, silk screen with part numbers and solder resist.

Excellon or Sieb und Meyer drill files are used to drill the board at the defined positions and with the drill diameter needed to get the hole size you want after copper plating.

For the board outline router files are needed. It is common practice to correct the router path by the router radius outwards. It is the job of the manufacturer to select a router with a proper diameter, for instance 2 mm. To produce the proper route file a line width of 0 is allowed, normal and accepted. If gerber files are submitted they have to be transformed into a router file format needed by the routing machine. Routing machine controllers are capable to correct the path with the router radius. There is a tool definition file containing the router diameter, the router rotation speed, the move speed in horizontal direction and the router feed speed in vertical direction.

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