On my first flexwatt install I melted the back a little as you described, and have no ill effects from it. If they are heating up and not getting blazing hot at the solder joints themselves then I think your connections should be fine. Don't worry about them not getting as hot as you think they should... remember you are testing them in an open room (likely under 70 degrees) and not under a tub, or even in the back of an enclosed rack. It's kinda like turning on an oil/electric space heater outside (doesn't do much), but put it in a small room and it will get hot enough to the touch to almost burn you.