Abstract
The Au-Sn eutectic bonding process was developed by applying micro bonding process for LSI package. The Sn-plated Cu sheet was bonded by heat-press with the bonding tool on the Au-plated Cu foil.
When the bonding, which was made under a pressure of 100 MPa using Au plating with a thickness of 1.2 μm and Sn plating with that of 3 μm, the bonding strength which was obtained by the 90° peel test was 0.5 N/mm at 250°C. With increasing the bonding temperature, the strength became up to 3.5 N/mm at 300°C. By reducing the bonding pressure from 100 MPa and using Au plating with a thickness of 0.6μm, the strengths were reduced rapidly. For example, the bonding strength which was obtained by bonding at 50 MPa was 2.5 N/mm.
The brittle intermetallic compounds as AuSn, AuSn2 and AuSn4, were formed in the bonding layer. When the bonding was made at low temperature, the Au-Sn intermetallic compound layer was formed in a thickness of 2 μm. This cross-sectional structure demonstrated that fracture occured through the thick Au-Sn layer. With the high temperature bonding above 300°C the Au-Sn-Cu layer was formed due to diffusion of Cu. The Au-Sn intermetallic conpounds existed like islands in the bonding layer at high bonding temperature. When the bonding made with Au thin plating like a thickness of 0.6μm, the Au-Sn intermetallic compound layer was formed in the bonding layer. On the case of bonding at low pressure it was found that the Au-Sn layer was formed in the bonding layer. It is assumed that the high bonding strength is achieved by eliminating the Au-Sn layer and forming Au-Sn-Cu layer contained a little Au-Sn intermetallic compound.
