You can find Pennsylvania's expungement statutes here. You seem primarily interested in sections 9106 and 9122.
In addition to those exceptions outlined by statute, it is possible that the record will have been disseminated to private databases which are not required to remove the information based on expungement, or that information has been submitted to federal databases (although that seems unlikely for misdemeanors). There's no way of being 100% certain that a background check won't reveal an expunged record. (And if you're dealing with high level security clearances, they often will.)

