To find this log, you need to have cmd prompt enabled when pxe booting.
Software Library > Boot Images > Select Boot Image > Properties > Customization > Enable Command Support (Testing Only)
When on the client, you are in the 'sccm install menu' (if I may call it that way), hit F8 and a cmd prompt opens.
Navigate to x:\windows\temp\smstslog to view the log.
you might want to copy the log file to another computer for easy reviewing, as SCCM tries to reboot the client in 15 minutes.
TIP: you can use the cmdlet "net use z: \\sccmserver\share" to create a network mapping