3D render of the MCU PCB, from an isometric view, back bottom corner, showing how the LED and MCU PCBs would stack up. The MCU PCB has twelve gold pads around the circumference, each with a small hole. The square black outline of the ATmega328P QFN package can be seen on the back.
https://cdn.masto.host/ledssocial/media_attachments/files/112/072/752/367/501/010/original/91e8ae62b98adac0.png