What is this SB component on schematics
It's a solder bridge.

I'm guessing that you're looking at schematic on page 37 of this document:

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/user_manual/DM00039084.pdf

If so, then SB19 is documented in table 4.11 on page 20, under "Solder Bridges". This solder bridge allows you to add/remove the pulldown resistor on the BOOT1 signal.