Influence of bump geometry, adhesives and pad finishings on the joint resistance of Au bump and A/NCA flip chip interconnection

Flip chip assembly using non-conductive adhesives (NCAs) and anisotropic conductive adhesives (ACAs) is gaining importance and acceptance in electronics packaging industry. For this packaging technology, a variety of material combinations is possible involving different bump types and adhesives. One of the challenges is to select a robust flip chip joint configuration to meet desired reliability requirements. In this paper, we report the influence of Au bump types, adhesives and substrate finishings on assembly feasibility and reliability of the flip chip joints. Results showed that bump type and its geometry has a major impact on the daisy chain resistances. The lowest and most consistent joint resistances were obtained with Au stud bumps. The incompatible combination of geometrically flat bumps and silica-filled adhesives was also identified. Resistance stability as a function of stress test was most dependent on the adhesive type used. Substrate pads with Au plating showed greater stability compared to pads with organic solderability preservative (OSP) coating. Based on the results, Au stud bump with non-conductive paste (NCP) has been identified to be the most suitable combination for high I/O and large die application.