Artik 5 already have an OS fedora 22. I followed the link to boot from SD card, and got success:
https://developer.ubuntu.com/en/snappy/start/samsung-artik-iot-modules/
But I cannot flash Ubuntu-core to Artik 5. Instead of writing to eMMC, it boots from SD card when SW2 (1:on 2:off) or (1:on 2:on).