The present technology discloses a system, for joining workpieces using energy, such as ultrasonic energy, where the energy concentrates at a location within a weld area, promoting sequential melting of a plurality of energy directors. The system can be configured so that the sequential melting begins at the center of the weld area and progresses outwards. Sequential melting may occur through use of a welding tip configured to reduce air pockets, a tapering the height of a plurality of energy directors, and/or tapering the energy directors themselves, all of which reduce the size of an energy transfer area produced by thermal energy. The present technology also includes a method for joining workpieces using energy such as ultrasonic energy that concentrates at a location within a weld area causing sequential melting of a plurality of energy directors using the aforementioned features.