Across the board and Not to mention

Across the board adverb - Including all.

Not to mention and across the board are semantically related. In some cases you can use "Not to mention" instead an adjective phrase "Across the board".

Not to mention adverb - Much less.

Across the board and not to mention are semantically related. Sometimes you can use "Across the board" instead an adjective phrase "Not to mention".