The Supreme Court of Pennsylvania has held that any participation by a trial judge in the plea bargaining process prior to trial is forbidden.
Commonwealth v. Evans, 434 Pa. 52, 252 A.2d 689 (1969).
Stanley M. Stein,
Criminal Procedure - Plea Bargaining,
Duq. L. Rev.
Available at: https://dsc.duq.edu/dlr/vol8/iss2/13