"Helmet to helmet" is not a rule in the NFL.

Helmet to the head or neck area of a defenseless player (including a QB in the pocket) is prohibited and specifically excludes incidental contact.

Rule 12-2-9

c) In covering the passer position, Referees will be particularly alert to fouls in which defenders impermissibly use the helmet and/or facemask to hit the passer, or use hands, arms, or other parts of the body to hit the passer forcibly in the head or neck area (see also the other unnecessary roughness rules covering these subjects). A defensive player must not use his helmet against a passer who is in a defenseless posture—for example, (1) forcibly hitting the passer’s head or neck area with the helmet or facemask, even if the initial contact of the defender’s helmet or facemask is lower than the passer’s neck, and regardless of whether the defensive player also uses his arms to tackle the passer by encircling or grasping him; or (2) lowering the head and making forcible contact with the top/crown or “hairline” parts of the helmet against any part of the passer’s body. This rule does not prohibit incidental contact by the mask or non-crown parts of the helmet in the course of a conventional tackle on a passer.