Here is how I would layout this board with an explaination of the changes:
rpi_ws2812_shield_3.fzz (75.1 KB)
Breadboard:
converted from perf board to a breadboard as I find making reliable connections easier. Ran power wires as they will be (and are) on PCB. The 2 pin connector that provides power is the base connection of a star layout. One line from the star (both power and ground) goes to the RPI and level translator. A Schottky diode and tantalum cap was added for 2 reasons: it isolates the RPI power source from voltage spikes caused by the LEDs drawing power all at once and if the USB is connected, protects the USB from trying to power the leds (the diode will block it), there isn’t such a diode in the RPI schematic I looked at. If a current spike drops the 5V line, the diode will block the LEDs from drawing current from the bypass caps on the RPI leaving it with a stable supply of 5V and avoiding software crashes due to undervoltage. That may or may not be needed but it is cheap insurance. In pcb view, there are separate traces from the 5V input pin on the 2pin connector to the RPI diode and both of the led output connectors. That way any current induced voltage drop in the high current traces (the power to the LEDs) only affects the LEDs not the RPI and the diode and cap should filter them out anyway.
Rather than do ground fill I cheated because it is possible to type in a number up to around 128 in to the trace width box in Inspector and get extra wide traces. In this case I used 100 as being about as large as is useful. You could use ground fill to do the same type of thing and I think @opera_night has some other tricks for creating large controlled copper areas in pcb (I don’t usually do pcbs, so I’m somewhat out of my area of expertise). The RPI connector has only one connection for 5V and one for ground to avoid ground loops. The multiple pins on the connector should have internal connections on the board and thus an external connection to multiple pins can form a ground loop. Finally I moved the connectors so the base of them are entirely on the board (you may have a reason for placing where they are and thus have to move them back though). The reason for this it to give them the maximum mechanical stability in the case where someone pulls on the wires connecting to them by accident. The more area supported by the board the stronger they will be. Hope this helps!
Peter