Characterization of sintered Cu nanopaste for micro-bumping with Injection Molded Solder technology

We have previously developed a novel plating-free bumping process using Cu nanopaste and Injection Molded Solder (IMS) technology. In the present study, we investigated the further detail about the microstructural and mechanical properties of sintered Cu nanoparticles formed into a pillar shape. By analyzing cross-sections of Cu nanoparticle pillars sintered in various conditions, we clarified how the sintering conditions affect the microstructural features, including the size and numbers of Cu grains and voids inside sintered Cu nanoparticles. In addition, we conducted the shear testing for the obtained Cu pillars to evaluate relationships between the mechanical strength and the microstructural features. We found that the results of the shear testing were consistent with the microstructural features of the sintered Cu nanoparticles. Finally, we injected molten solder onto the Cu nanoparticle pillars to evaluate the overall feasibility of the developed process. It was confirmed that the molten solder injected by IMS process has good wettability against the sintered Cu nanoparticles, which resulted in the successful bump formation without solder missing. In addition, The IMC layer between the sintered Cu nanoparticles and injected solder was formed well. These results proved the quality of microbumps fabricated by the novel bumping process using Cu nanopaste and IMS.

CFD Analysis of Molten Solder Flow Behavior and Bridging Mechanism During Solder Bump Formation

Technology of fine pitch interconnect with lead-free solder joint has been developed to enhance the performance of flip-chip high density packages. This study presents an investigation of solder bump forming behavior by means of CFD simulation analysis. The flow motion of molten solder is analyzed with 3D model we developed, and the simulation result is validated with the experiment. Moreover, the investigation of factors affecting solder bridging across adjacent pads is also performed. It is revealed that wettability between liquid solder and organic insulator, which is represented as contact angle in the calculation has large effect on the solder bridging phenomena. The simulation result suggests that worsening the wettability of the insulator can reduce the occurrence of bridging.

Effect of Surface Roughness and Electroless Ni–P Plating on the Bonding Strength of Bi–Te-based Thermoelectric Modules

In this study, electroless-plating of a nickel-phosphor (Ni–P) thin film on surface-controlled thermoelectric elements was developed to significantly increase the bonding strength between Bi–Te materials and copper (Cu) electrodes in thermoelectric modules. Without electroless Ni–P plating, the effect of surface roughness on the bonding strength was negligible. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric elements and defects such as pores were generated at the bonding interface owing to poor wettability with the solder. However, defects were not present at the bonding interface of the specimen subjected to electroless Ni–P plating, and the electroless Ni–P plating layer acted as a diffusion barrier toward Sn and Te. The bonding strength was higher when the specimen was subjected to Ni–P plating compared with that without Ni–P plating, and it improved with increasing surface roughness. As electroless Ni–P plating improved the wettability with molten solder, the increase in bonding strength was attributed to the formation of a thicker solder reaction layer below the bonding interface owing to an increase in the bonding interface with the solder at higher surface roughness.

Dissolution behavior of the Ni substrate and Ni3Sn4 phase in molten lead-free solders

The dissolution behavior of the Ni substrate and Ni3Sn4 phase was studied in the following molten lead-free solders: Sn, (wt.%) Sn–3.0Ag–0.5Cu (SAC), Sn–0.7Cu (SC), Sn–58Bi (SB) and Sn–9Zn (SZ) at 240[Formula: see text]C, 270[Formula: see text]C and 300[Formula: see text]C. The dissolution rate of the Ni substrate in solder decreased from Sn, SAC, SC, SB, to SZ. The thick Ni5(Zn, Sn)[Formula: see text] phase formed at the SZ/Ni interface hindering the Ni dissolution. Ni3Sn4 phase dissolution rate in molten solder decreased from Sn, SZ, SAC, SC, to SB at 240[Formula: see text]C and SC, Sn, SAC, SZ, to SB at 300[Formula: see text]C. The (Cu, Ni)6Sn5 phase spilling was observed at the SAC/Ni3Sn4 and SC/Ni3Sn4 interfaces. Zn in the SZ solder reacted with the Ni3Sn4 phase to form the (Ni, Sn)5Zn[Formula: see text] phase at the interface.