An ultrasonically welded seam formed in thin, semi-crystalline films is provided. In one example embodiment, the seam may include a welded zone flanked by one or more reinforcing perimeter beads, to increase the seam's peel strength and elongation-to-break properties. In one or more embodiments, the seam may be formed by using one or more heat-carrier layers during the ultrasonic sealing of semi-crystalline film layers to generate heat and transfer excess heat across a portion of their surface area and into the semi-crystalline film layers being sealed. The heat-carrier layers enables the film seam to extend wider than the width of the sonotrode and anvil surface areas, creating a reinforcing seam perimeter bead which greatly outperforms prior ultrasonically welded seams of semi-crystalline films.