The shims that are supplied with some OEM pads are for heat dissipation not squeaking.
If I have to fix squeaking brakes I use copper grease on the back plate and the edges which cures most cases. Never have I had a customer come back with contaminated pads due to this practice but I'm sure some brake gurus out there from ebc, brembo may disagree but this is real world experience and not in a lab.
Squeaking is my opinion is down to bad fitting pads, you may not agree with me but different manufacturered pads are always slightly different in size (as are calipers) although all will fit and it's this that generates vibration so, when you grease the back plate and edges this stops the two surfaces rubbing together at high frequency.
Also a good practice is to clean the disc/rotor, you wouldn't believe how dirty they get making the surface uneven. Just spray some brake cleaner on a clean cloth and give them a wipe , never spray pads with brake cleaner either .