In recent years, the rapid development of the new energy industry has driven continuous upgrading of high-density and high-power devices. In the packaging and assembly process, the problem of differentiation of the thermal needs of different modules has become increasingly prominent, especially for small-size solder joints with high heat dissipation in high-power devices. Localized soldering is considered a suitable choice to selectively heat the desired target while not affecting other heat-sensitive chips. This paper reviews several localized soldering processes, focusing on the size of solder joints, soldering materials, and current state of the technique. Each localized soldering process was discovered to have unique characteristics. The requirements for small-size solder joints are met by laser soldering, microresistance soldering, and self-propagating soldering; however, laser soldering has difficulty meeting the requirements for large heat dissipation, microresistance soldering requires the application of pressure to joints, and self-propagating soldering requires ignition materials. However, for small-size solder junctions, selective wave soldering, microwave soldering, and ultrasonic soldering are not appropriate. Because the magnetic field can be focused on a tiny area and the output energy of induction heating is large, induction soldering can be employed as a significant trend in future research.

With the continuous development of high-power electronic devices, the traditional tin-lead brazing materials no longer meet the conditions of use, and sintered nanometal solder paste is promising for a new generation of packaging materials. The mechanism of microstructural changes of nanoparticle sintering during the sintering process has not been well studied at present. Molecular dynamics (MD) simulations can effectively track the diffusion process of metal atoms during the sintering process and help to reveal the dynamic evolution of nanoparticles. This review presents many MD simulations of nanoparticle sintering, including the growth mechanism of nanoparticles, the effect of different sintering parameters on the performance of sintered joints, the connection mechanism between the reinforced phase and nanoparticles and the performance of composite sintered joints. The low temperature and low pressure sintering of nanopaste are still in face of some problems, and MD simulations are very helpful for improving the sintering process and verifying the mechanism of the reinforcing phase.

Active soldering of 5A06 Al alloy was performed at 300 °C by using Sn-1Ti and Sn-1Ti-0.3Ga active solders, respectively. The effects of soldering time on the microstructure and mechanical properties of the joints were investigated. The results showed that the Sn-1Ti solder broke the oxide film on the surface of the Al substrate and induced intergranular diffusion in the Al substrate. When Ga was added to the solder, severe dissolution pits appeared in the Al substrate due to the action of Sn-1Ti-0.3Ga solder, and many Al particles were flaked from the matrix into the solder seam. Under thermal stress and the Ti adsorption effect, the oxide film cracked. With increasing soldering time, the shear strength of 5A06 Al alloy joints soldered with Sn-1Ti and Sn-1Ti-0.3Ga active solders increased. When soldered for 90 min, the joint soldered with Sn-1Ti-0.3Ga solder had a higher shear strength of 22.12 MPa when compared to Sn-1Ti solder.

The electronic product has gravitated towards component miniaturization and integration, employment of lead-free materials, and low-temperature soldering processes. Noble-metal aerogels have drawn increasing attention for high conduction and low density. However, the noble metal aerogels with outstanding solderability were rarely studied. This work has successfully synthesized an aerogel derived from silver nanowires (AgNWs) using a liquid phase reduction method. It is found that the noble metal aerogels can be made into diverse aerogel preformed soldering sheets. The influence of bonding temperature (150−300 ℃), time (2−20 min), and pressure (5−20 MPa) on the joint strength of the AgNWs aerogel affixed to electroless nickel/silver copper plates were investigated. Additionally, the AgNWs aerogel displays almost the same shear strength for substrates of various sizes. In a word, this study presents a flux-free, high-strength, and adaptable soldering structural material.

The traditional nano-sintering or TLP techniques are generally expensive, time-consuming, and hence unsuitable for realizing practical mass production. Herein, we have developed an improved TLP process to rapidly produce IMC-skeleton structures across the bonding region by initiating a localized liquid-solid interaction among micron particles at traditional soldering temperatures. The developed IMC skeletons can reinforce solder alloys and provide remarkable mechanical stability and electrical capabilities at high temperatures. As a result, the IMC-skeleton strengthened interconnections exhibited higher thermal/electrical conductivity, lower hardness and almost doubled strength than traditional full-IMC joints, attaining 87.4 MPa and 30.2 MPa at room condition and 350 ℃. Meanwhile, the necessary heating time to form metallurgical bonds was shortened, one-fifth of nano-sintering and one-tenth of TLP bonding, and the material cost was significantly reduced. This proposed technique enabled the fast, low-cost manufacturing of electronics that can serve at temperatures as high as 200−350 ℃. Besides, the interfacial reactions among particles and the correlated phase evolution process were studied in this research. The formation mechanism of IMC skeletons was analyzed. The correlated influencing factors and their effect on the mechanical, thermal and electrical properties of joints were revealed, which may help the design and extensive uses of such techniques in various high-temperature/power applications.

In this work, the ultrasonic assisted active metal soldering of SiO₂ glass and Al was successfully achieved using Sn-2Ti solder filler at a low soldering temperature of 250 ℃ in ambient atmosphere. A nano-crystalline α-Al₂O₃ layer with the average thickness of 13.9 nm and a nano-crystalline R-TiO₂ layer with the average thickness of 16.2 nm are formed at the interface of Al/Sn and SiO₂/Sn respectively because Al elements did not diffuse from Al alloy side to SiO₂ side, which verified that a sono-oxidation reaction had occurred during the ultrasonic assisted active metal soldering process. The soldered butt joints exhibited an average tensile strength of 25.31 MPa.