In the present research, in order to fundamentally investigate the strength and interfacial microstructure of the joints between Sn–3.5Ag–0.7Cu (SAC) and Sn–8Zn–3Bi (SZB) solders and Cu, examinations using the standardized solder joint specimen for measuring the shear strength were performed. The effects of the addition of Ag, Bi and Pb to the solders, which are the ingredients of possible lead plating materials of quad plat package, solder layer thickness and aging treatments ranging from 358 to 423 K were investigated. The SAC base solder joints, whose strength hardly degraded after aging, were more reliable than the SZB base solder joints. Since no detrimental effect of the Bi or Pb addition was recognized, the lead plating materials are found to have no significant effect on the strength of the SAC solder joints. Although the SZB base solder joints had higher strength than the SAC base solder joints in as-soldered condition, a significant degradation of the joint strength occurred after aging. In the case of thinner solder layer of 20 μm, the Cu6Sn5 layer that formed at both the solder/Cu5Zn8 and Cu5Zn8/Cu interfaces after aging caused the fracture of the solder joints and resulted in the degradation of the joint strength. In the case of thicker solder layer of 100 μm, the Cu5Zn8 interfacial layer continuously grew during aging, which caused the fracture of the solder joints and resulted in the decrease in the joint strength. As for the effect of the addition elements, while Pb addition caused a marked degradation of the joint strength, Ag addition and Bi addition had no detrimental effect on the strength of the solder joints. In particular, the joint with the Ag addition solder had the better strength in the case of thinner solder layer. Thus, as for lead plating for the SZB solder, Sn–Pb should be avoided and Sn–Bi or Sn–Ag as Pb-free plating has no detrimental effect on the joint strength. In particular, Sn–Ag plating is preferable in the viewpoint of the joint strength.

The purpose of the present study was to verify the effectiveness of dry laser peening (DryLP), which is the peening technique without a sacrificial overlay under atmospheric conditions using femtosecond laser pulses on the mechanical properties such as hardness, residual stress, and fatigue performance of laser-welded 2024 aluminum alloy containing welding defects such as undercuts and blowholes. After DryLP treatment of the laser-welded 2024 aluminum alloy, the softened weld metal recovered to the original hardness of base metal, while residual tensile stress in the weld metal and heat-affected zone changed to compressive stresses. As a result, DryLP treatment improved the fatigue performances of welded specimens with and without the weld reinforcement almost equally. The fatigue life almost doubled at a stress amplitude of 180 MPa and increased by a factor of more than 50 at 120 MPa. DryLP was found to be more effective for improving the fatigue performance of laser-welded aluminum specimens with welding defects at lower stress amplitudes, as stress concentration at the defects did not significantly influence the fatigue performance.

The evolution of the transient liquid-phase sintered (TLPS) Cu–Sn skeleton microstructure during thermal aging was evaluated to clarify the thermal reliability for die-attach applications. The Cu–Sn skeleton microstructure, which consists of Cu particles connected with Cu–Sn intermetallic compounds partially filled with polyimide resin, was obtained by the pressure-less TLP sintering process at 250 °C for 1 min using a novel Cu-solder-resin composite as a bonding material in a nitrogen atmosphere. Experimental results indicate that the TLPS joints were mainly composed of Cu, Cu₆Sn₅, and Cu₃Sn in the as-bonded state, where submicron voids were observed at the interface between Cu₃Sn and Cu particles. After thermal aging at 150, 175, and 200 °C for 1000 h, the Cu₆Sn₅ phase fully transformed into Cu₃Sn except at the chip-side interface, where the number of the submicron voids appeared to increase. The averaged shear strengths were found to be 22.1 (reference), 22.8 (+3%), 24.0 (+9%), and 19.0 MPa (−14%) for the as-bonded state and specimens aged at 150, 175, and 200 °C for 1000 h, respectively. The TLPS joints maintained a shear strength over 19 MPa after thermal aging at 200 °C for 1000 h because of both the positive and negative impacts of the thermal aging, which include the transformation of Cu₆Sn₅ into Cu₃Sn and the formation of submicron voids at the interface, respectively. These results indicate an excellent thermal reliability of the TLPS Cu–Sn skeleton microstructure.

We have proposed a low-temperature bonding technology utilizing the sintering of Cu particles with transient liquid-phase of Sn-based solder, called transient liquid-phase sintering (TLPS), as a die-attach solution for high-temperature power modules. A copper-intermetallic compound-resin (Cu-IMC-resin) microstructure, which consists of Cu particles connected with Cu–Sn intermetallic compounds (IMCs) partially filled with polyimide resin, is obtained by the pressureless TLPS process at 250 °C for 1 min using a novel Cu-solder-resin composite as the bonding material in a nitrogen atmosphere. Macro- and micro-deformation properties of the unique microstructure of the TLPS Cu-IMC-resin are evaluated by finite element analysis using a three-dimensional image reconstruction model. The macroscopic computational uniaxial tensile tests of the Cu-IMC-resin model reveal that the utilization of the IMCs and the addition of the easily-deformable resin facilitates the temperature-stability and low-stiffness of the mechanical properties. The microstructure exhibits a significantly low homogenized Young’s modulus (11 GPa). Microscopic investigations show that the local stresses are broadly distributed on the IMC regions under uniaxial macroscopic tensile displacement, indicating highly reliable performance of the joint within a specific macroscopic strain condition. Numerical and experimental investigations demonstrate the excellent thermal cyclic reliability of die-attached joints between silicon carbide chips and directly bonded copper substrate.