If you build your own PCB, please make sure that there are no short circuits, otherwise you may damage your computer/printer.
Please see multiple tutorials as there are many versions of the BMCU.
An assembling video (In chinese)
https://www.bilibili.com/video/BV1PuPCehEP3/?vd_source=e9b8d46574a1915d5f3cd835b2b34359