Andrew decided to write up a how-to on DIY BGA soldering. His article describes why it’s a bad idea to keep the vias unfilled with solder before reflow. Capilary force sucks up the ball of solder into the via, and there is no solder left to make contact between the IC and the pad.
After the rough cut I polished with 1200 grit sandpaper and wiped away the dust with a wet cloth. Upon looking under the microscope I saw that the failure I was hoping to demonstrate had indeed occurred – one of the balls had been sucked down into an uncapped via by capillary action, resulting in a complete lack of electrical contact. The ball at far right had been partially sucked into the via but the solder mask dam was big enough to keep it from going in all the way.
Via the forum.