Vertical pressure in Altera devices is described in the document linked below.
Thermal Management for FPGAs URL:
    https://www.altera.com/en_US/pdfs/literature/an/an358.pdf

Page 16, Package Loading Specifications has the following description.
Maximum: 700 kPa = 7.138 kgf/1 square cm

The recommended pressure is described below on Page 18.
Clip-on heat sinks use a mechanical clip to hold the package edges.
Altera recommends a clipping force ranging from 3.0 to 5.0 lb vertical force, per the heat sink vendor's recommendation.

* Please be sure to refer to the latest version of the above application note as the figures on this page are subject to change.