Toe In - the leading edge of the pad should touch the rim first, before the trailing edge (front of the pad - facing the front of the bike, should touch before the rear).



Another thing that will affect squeal is the looseness of the brake arm pivot points.

If the rear touches first, that causes the pad to "chatters" against the rim as the edge lossens/holds/loosens/holds against the rim.

And sometimes....a certain combo of pad and rim, just squeals to high heaven until broken in.