Allowed Mechanical Pressure when mounting Heatsink for AD9528 and ADCLK948


We are designing one whole heatsink to be mounted on the top of of AD9528 and ADCLK948, total about 3W. 

We may use compressible thermal pad for interfacing material placed between case TOP of IC and metal heatsink. Certain pressure should be applied to keep the thermal pad contact with IC surface tightly to ensure the good thermal conductivity. 

So what is the recommended Mechanical Pressure for  AD9528 and ADCLK948 chip (unit is MPa or PSI),  and what is the pressure limit so as to keep IC from being destroyed by over stressed, as well as ensure long term reliability of chip under consistive mechanical load.