I tend to go off on a tirade sometimes, I realized I ignored your question.....Yes, burning of the lower back during a squat is bad, and should not occur. Squats in an indirect way hit the low back and cause the erectors to be strengthened, but it is not a direct low back exercise and should not cause burning....I will go out and guess that at some, maybe you're even unaware of it, that the bar comes forward past your knees at some point, just double check this fine form point and if thats the problem, correct it and you should be squatting pain free......a lot of guys and gals do almost a hybrid goodmorning with max squat weight, or they pop up their hips too soon and lock their legs out and then straighten out their back, these are fine points, but it may be aggrivating your back, or it could be weak abs, and don't get all bent out of shape over this, you may have a visible 8 pack, and still have weak abs, a lot of people who do "crunches" look great in a speedo, but they have weak abs, try doing full range of motion sit-ups for the abs, also do your not so heavy sets with no weight belt if u don't already do this......I am trying to bring up some things u may be doing, so don't take the post the wrong way brother, it is just that full squats are the shit, and if I could get somebody who is having a problem with them to just re-evaluate their form and assistance work and see if the problem can be corrected in favor of scrapping squats for a sissy exercise or doing 1/8 of a rep on your squats, I think it is fucking great, so just look at all this stuff before you count out full squats, a lot of the things I mentioned I have seen a million times and I have seen problems corrected a million times.