Kingbo RMA 218 flux the best flux to use both reflows and reballs.
It remains active all the way to 245c, the higher temps is where oxidation occurs so use kingbo flux to prevent oxidation from occuring. We use
http://kingboflux.com
For ball placement you can use Kingbo combined with Insat flux. Or just use Kingbo alone but keep in mind you want to use a tiny minuscule amount, if you think your not using enough then you actually are.