Hi there,
NO, Not saying that , but that the Battery Pads on the Xiao are SMD spec’d pads. (so thickness),i.e.heat needs to be in range when soldering. or they will be damaged, or burnt off. Don’t ask how I know 
Just make the OVAL holes are plated through in your PCB and pads are over sized a tad. Use solder paste b4 you clamp it down. Notice how the holes (for usb port)also in PCB allow it to lay flat no space or the solder won’t bridge.
Go back and look at the past Live Stream they solder a Xiao and ust the same tech on the Relay board too.
Takes Patients and good temp control, solder paste and solder roll, small tip Iron.
study the pics in those threads, you get it.
HTH
GL 
PJ 