Evaluation of bonding characteristics of thermal compression bonded solder joints via nanoindentation test

We used nanoindentation method to evaluate the mechanical properties of two solders, Sn-57.6Bi-0.4Ag (melting point 138°C) and Sn-3.0Ag-0.5Cu (SAC305, melting point 217°C), used in thermal compression bonding. The following conditions were used to determine the optimum bonding conditions: 190, 210, and 230°C and 2, 2.5, and 3 MPa for Sn-57.6Bi-0.4Ag; and 280, 300, and 320°C and 3, 5, and 7 MPa for SAC305. The solder hardness decreased as the bonding temperature increased, which demonstrated that bonding strength decreases with grain growth, in accordance with the Hall-Petch equation. Increased stiffness and elastic modulus were also observed alongside larger grains. Nanoindentation, which can accurately measure small parts for fine pitch packaging, is expected to be an effective technique for determining mechanical properties of a sample.