Yoshikazu Takaku

Pb-free high temperature solders for power device packaging

Abstract Reliabilities of joints for power semiconductor devices using a Bi-based high temperature solder has been studied. The Bi-based solder whose melting point is 270 °C were prepared by mixing of the CuAlMn particles and molten Bi to overcome the brittleness of Bi. Then, joined samples using the solder were fabricated and thermal cycling tests were examined. After almost 2000 test cycles of −40/200 °C test, neither intermetallic compounds nor cracks were observed for CTE (Coefficient of Thermal Expansion) matched sample with Cu interface. On the other hand, certain amount of intermetallic compound such as Bi 3 Ni was found for a sample with Ni interface. In addition, higher reliability of this solder than Sn-Cu solder was obtained after −40/250 °C test. Furthermore, an example power module structure using double high temperature solder layers was proposed.

Thermodynamic Assessment of the Ni-Sb Binary System

The Ni-Sb binary alloy system was thermodynamically assessed using CALPHAD approach in this article. Excess Gibbs energies of solution phases, liquid and fcc phases, were formulated using the Redlich-Kister expression. The intermediate phases were modeled by the sublattice model with (Ni, Va)0.5(Ni, Sb)0.25(Ni)0.25 for Ni3SbHT phase and (Ni, Va)0.3333(Sb)0.3333(Ni, Va)0.3333 for NiSb phase. The other phases including Ni3Sb, Ni7Sb3, and NiSb2 were treated as stoichiometric compound owing to their narrow composition ranges. Based on the reported thermodynamic properties and phase diagram data, the thermodynamic parameters of these phases were optimized, and the obtained values can reproduce the available experimental data well.

Thermodynamic assessment of phase equilibria in the Sn-Au-Bi system with key experimental verification

The phase equilibria at 200 °C, 250 °C, 300 °C, and 400 °C and the phase transformation of the Sn-Au-Bi system were investigated by using the electron probe micro-analyzer (EPMA) and differential scanning calorimeter (DSC), respectively. It is found that there is a new ternary intermetallic compound with a possible AuSn structure (called the ϕ phase in the present work), which has a limited solubility of Au in the Au-rich portion, and the ϕ phase decomposes peritectically at about 313 °C. Based on the experimental data reported in the previous papers and new experimental data determined by the present work, thermodynamic assessments of the Sn-Au-Bi system were carried out by the calculation of phase diagrams (CALPHAD) method. The thermodynamic parameters for describing the Gibbs free energy of each phase were optimized, and reasonable agreement between the calculated results and experimental data was obtained in the Sn-Au-Bi ternary system.