Design Constraints

Know your design constraints before you start. A design constraint can be anything from the shape required, to the location of headers and mounting holes.

A good example is the Raspberry Pi HAT. The HAT form-factor is a specific size and shape, with mounting holes at the corners. The shape a HAT must be is well documented. You can find details here: https://github.com/raspberrypi/hats/blob/master/hat-board-mechanical.pdf

Another example is the Arduino Shield, which has a certain layout and placement of headers. All single board computers and micro-controller boards have similar requirements for addons.

The HAT form-factor also requires specific components for the EEPROM. Take these into account when designing your layout.

Raspberry Pi HAT Considerations

When is a HAT a HAT? You don't need the camera and display flex cut-outs to call your board a HAT. You do need the EEPROM.

A couple of my own designs, Piano HAT and Drum HAT, make use of the entire size of the HAT board. They include an EEPROM, 40-way header and mounting holes but no slots or cutouts. They're still HATs.

If you want your final board to fit inside a Raspberry Pi case, be wary of large components or parts that overhang the edge. Try to stay within the confines of your chosen form-factor.