What screw or fastener have others used to secure heat transfer plates to the underside of the subfloor?

We are using a heavy 0.06" thick aluminum heat transfer plate with 1/2 PEX. Installed about 30 plates, cleaned up at the end of the day and everything looked good. Went home that night and come back to the job the next day and found two screw heads had broken off over the night. Now I am concerned that when we start heating the system more screws will break.