Always maintain a flat, neutral spine when squatting. If you round your lower back, you’ll put dangerous stress on your lumbar spine. In the squat community, this is called a “butt-wink” because, as you descend towards parallel, your butt tucks underneath and your lower back rounds.
Do NOT arch your lower back harder to combat the butt wink. (I made that same mistake for years.) Instead, unrack the bar, get into your stance, take a deep breath, and exhale as hard as you can. You’ll feel your ribcage drop and your pelvis point tilt upward. Now squat while maintaining this new ribcage and pelvic position; you’ll fire your entire core and relieve the pressure on your back.