there is the following note in the pin information document for Intel/Altera MAX V device family (https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/dp/max-v/5m40z.pdf):
The E64 package has an exposed pad at the bottom of the package. This exposed pad is a ground pad that must be connected to the ground plane on your PCB. This exposed pad is used for electrical connectivity and not for thermal purposes.
I have seen your case 6 days ago and i believe Martin's answer have address your query. I will now proceed to close this case. Do not hesitate to open a new case if you have any queries.