Has someone the schematic of the sata board of the DS412+?

I can not say with 100% certainty; NO. My experience with Synology concerns a DS1813+. If it is under warranty they will replace your NAS for free. If it is out of warranty they will suggest that you buy a new one and send the old one to your local land fill. After 6 weeks of exchanging emails with their tech department they did admit that they may be able to supply a power supply. NO mother boards are available and NO schematic or repair manuals. On a Fixit friendly scale I would give Synology a 1.