This is no easy task. There are two separate and distinct answers.
Due to the internet, in Pennsylvania has forced all of the localities to migrate their previous varied systems to a unified system for the whole state. Some of the magistrates and/or townships and/or boroughs didn't do such a good job. This is how you can have a record with the Pennsylvania State Police, but cannot locate the conviction to get it expunged.
You go to the Court of Common Pleas and ask to have a miscellaneous docket created. You use the miscellaneous number to file a petition for expungement. You have to go in and argue this type of case and these petitions are difficult to argue. In larger counties, this happens all of the time so the clerk at the criminal counter usually is very helpful. In small counties, this can be very difficult because the clerk may have never had this happen, so you may run in to trouble. In jumbo counties, Allegheny and Philadelphia counties, there are expungement rules and a miscellaneous docket can confuse a rubber-stamper sitting at a counter.
The second route is a challenge of the your criminal history through the state attorney general's office. In this sense, the county or the attorney general will force a recreation of your criminal record in the county if you win. This method is very difficult to achieve; the actual legal term we use is a CHRA appeal.