you probably got a desi service on the brakes. - aka useless.
the correct way to service disc brakes starts with unbolting the caliper and the pad carrier, removing the pads and clips then buffing the abutment surfaces with a wire wheel grinder.
reinstall the abutment clips with silicone grease on the pad ears so they can slide easily.
use copper or moly antisieze on the pad backer plates and shims so they dont chatter (squeal)
reinstall the caliper, lube the pins with silicone grease, flush the brakes and pack the job.
never use WD40 anywhere near the brakes.