that you use flux? give marks. and what is best for you.
I tested many brands of flux but I am not happy with the results. which I recommend.,
i am sure that interflux is the good one
also if you are lucky try to use Amtec (Original)
I recommend two brands to you, you can check from here:
http://www.ownta.com/bga-gel-flux.html
http://www.ownta.com/jovy-jv-f010-bga-gel-flux.html