Not long ago, I designed an Arduino compatible clock board. The board has 12 NeoPixel (digital addressed RGB LEDs) arranged around the board to act as hour hands. The minutes and seconds are represented by an external ring of 60 NeoPixels.
How did I go about positioning the 12 NeoPixels, and what does it matter? For aesthetic reasons, I do want each NeoPixel in the proper place. If any are off a bit, I’ll notice every time I look at the clock.
I created a triangle, with all of the correct distances, and drew in in my CAD software’s Document layer. The Document layer looks just like a silk screen layer, when visible, but it won’t be printed on the board. You can use this layer to put in extra information for yourself, or for the manufacturer.
You’ll notice that I also wrote in the document layer “No tabs here.” That’s an instruction to the board fabricator to not put a panel tab where the micro USB connector goes. If it did, the board wouldn’t be buildable when panelized.
Some create a fabrication document layer and an assembly document layer. An example might pertain to reference designators. If the board is too compact for reference designators, of if, for aesthetic reasons, you want to leave them off the finished board, You can put the reference designators in an Assembly Documentation layer. Then be sure to let your assembler know what you’ve done.
The other things I did here is to keep all the LEDs aligned with the baseline of the PCB. In theory, you can place a component at any rotation angle you want. But, like any system, manufacturing works better when there are fewer variables.
You reduce the probability of error if you keep components aligned at factors of 90 degrees. It also helps to keep polarities oriented the same way, as much as possible. For example, if you can, have all the diode polarities facing the same direction.
Time flies like an arrow; fruit flies like a banana