Hi,
 your asics failed and can't use solder to place your hit sink on chip.
You need to replace with new asic BM1397AG or BM1397AI and need Stencil for chip.
Then use a little solder on chip or hit sink and then put hit sink on chip with heater and make sure solder between chip and hit sink totally melted.
if your board is running well and you cant replace your asics you can put your hit sink by kafuter 704 but this isn't recommended way and after some day or month your bord severely damaged.