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High density interconnection structures using an area-arrayed small bump structure have been employed in multi chip packages and modules. Both the diameter and the pitch of the bump will be decreased from a few hundred mum to dozens mum. Copper thin film interconnection and lead-free solder (mainly consists of tin) is used for the micro joint structures. As a result, the intermetallic compounds (IMC) such as Cu6Sn5 and Cu3Sn easily grow at the interface between the copper interconnections and the lead-free solder bumps. Since physical properties of the IMC are quite different from tin and copper, the increase of thickness of the IMC in the joint structures may affect mechanical and electrical quality of the joint structures with the miniaturization of solder bump. One of the important change is the increase of the stress and strain fields in the bump structures. Since the stress and strain fields affect the fracture mode of structures, the change of the stress and strain fields due to the IMC growth was analyzed in small bump structures. In addition, the effect of the miniaturization of the bump size on the increase of the electrical resistance of the bump at high frequencies was also analyzed considering the existence of the IMC layer.