If your back is hurting More than likely you don't have great hip mobility (assuming you form starts out good). What then happens is when you get to a certain depth your pelvis tilts forward (Google butt wink) resulting in shearing forces on your spine and pulling on the ligaments which cause the pain - you risk doing damage. You can see proper squat form and the hip,mobility work on
www.mobilitywod.com
(also vids on the butt wink). Get someone to video your squat and see when your butt tucks under. Just above that point put a box or bench and do box squats i.e. You squat to the point of touching the box to stop you going beyond the point where your mobility is the limiter. As you losen up your hips go lower. A squat should be ass to grass not this half/ quarter movement most people seem to think is a squat. Get mobile then squat. 15kg above body weight should be easy all the way down for reps.