The present paper is a study on flip-chip open bump failure mechanism. Initial electrical testing showed open circuit condition. Scanning acoustic microscope (C-SAM) identifies delamination on particular bump(s). Initial cross-sectional images suggested that the separation took place at Al – TiW interface. However, EDS analysis on the separated surface indicated the presence of Al metal at both sides of the separation, which raises a question of why the Al layer is cracked or separated instead of interface de-lamination. Research in literature and investigation at assembly line points an ultrasonic cleaning step in manufacturing process as a contributor to the open bump failure. Examining virgin dice after bump removal observed crack in nitride passivation around the bump neck, indicating high stress level during passivation film deposition and/or bump formation process. Hence it is concluded that ultrasonic cleaning in device assembly aggravates preexisting stress in weak bump(s), resulting in latent failure in field application.