The source of the squeal is usually from where the backside of the pads (metal) meets the piston (also metal). When you apply the brakes these two metal surfaces vibrate and rub together resulting in a squeal.
I went to a local auto shop, they had a material specially made for this purpose, i.e. to stop squealing breaks. Itīs glued to the backsides of the pads, and voila, the squeal is gone.