The person who did your brakes should have put a layer of anti-squeal paste on the backs of the front pads, that will usually cure the squealing, but there are some pads which will always squeal, the standard Volvo pads are the best - dusty but do not squeal.
The shims which everyone talks about are for the rear pads and are cut away to cure a low speed vibration when the bakes are applied hot.
Your rotors could easily be warped, if someone did not torque the wheels correctly that would warp them also running through a deep puddle when the brakes are hot will also cause them to warp. I would have your mechanic do a run out check on them, the run out should not be greater than .003 on the rear and .0015 on the front. The minimum thickness for the rear should be 0.331 and on the front 0.905.
Good Luck,
John Scales 1996 850 R.