Thanks for your reply. So any clue what is wrong in my case? Thanks in advance
If you have mount holes, mount the sink with high quality thermal paste. If you have an older unit without mount holes, use a high quality thermal epoxy ( but do it right, google it, practice on something else, you have pretty much one shot with epoxy).
The adhesive pads do not work well for fpga applications. I would guess 90% of issues when people are unable to achieve decent speed is directly related to how well the job was done mating the heat sink on to fpga core. This has been very true in my own experience, regardless of brand or type.
