Heating pad having a phase change material

A heating pad includes an envelope containing a heating unit and a phase change material which encloses the heating unit. The heating unit includes a metal plate and a resistance heating element attached to the metal plate. Flexible strips are disposed within the envelope by attachment to the envelope, and the metal plate is positioned within the envelope through the strips. A pair of electrodes are connected to the heating element and extend outwardly of the envelope. The phase change material can undergo a phase change process through the heating unit so that no external heat source is needed to heat the phase change material.