A new bonding mechanism of 50 μm pitch TAB-ILB with 0.25 μm Sn plated Cu lead

A 50 μm pitch TAB has been developed by applying newly developed TAB tape and inner lead bonding technology. A new electrodeposited Cu foil which has a high tensile strength and 18 μm thickness was adopted as the lead material. Sn plating on the inner leads has been thinned to 0.25 μm from the conventional 0.6 μm to avoid excess alloy formation. The inner leads were gang-bonded to the Au bumps. The cross sections of the bonded region were observed by SEM and EPMA for the specimens before and after high temperature storage (HTS), The Au-Sn fillets formed by the bonding supported the inner leads at the initial state. Ternary Au-Cu-Sn alloy was also formed at the interface between the inner lead and the bump. After HTS, cracks formed between the fillet and the inner lead, and the fillets could not contribute to supporting the inner lead. The ternary Au-Cu-Sn alloy which was formed at the bottom of the inner lead in ILB processes transformed to binary Cu-Au alloys after HTS. Sn was driven away from the lead-bump interface. The binary Cu-Au alloys kept the bonding strength after HTS. A destructive lead pull test was performed before and after HTS. The failure mode was a lead fracture in all cases