Tape BGA aligment issue

Hi, I am currently facing alignment problem on TBGA. Before reflow, the alignment is good. After reflow, Most of the TBGA was off the pad by 25% or more. The peak temperature on TBGA is 210 C with liquidus time that about 50 sec. Paste thickness is 7 mils with opening diameter of 32 mils. I have rule out the possible of board warpage and machine vibration because Other BGAs like CCGA and CBGA on the same boards look fine. Any comment or experience to share? rgs, chiakl

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