Preparation of sea urchin-like silver microparticles with high sintering activity for power electronics packaging

With the continuous advancement of wide-bandgap semiconductor technology, its potential has significantly expanded in fields such as new energy vehicles and 5G communication. In these applications, the chip service process is often accompanied by extreme environments such as high temperature, high voltage, high current density, etc., which impose stricter requirements on the performance of die-attach materials, necessitating better thermal and electrical conductivity as well as superior mechanical strength to ensure the reliability during chip service. Sintered silver connecting materials using Ag nanoparticles (AgNPs) as silver paste have emerged as strong contenders to traditional solders due to their capability for low-temperature connecting and high-temperature service. However, AgNPs owing to their extremely high specific surface area and activity, were prone to severe agglomeration. To mitigate this issue, a thick organic coating layer was typically applied to their surfaces, which, in turn, hindered the sintering process at low temperatures. In contrast, Ag microparticles (AgMPs) exhibited better dispersibility, but their larger size resulted in lower sintering activity, making sintering at low temperatures more challenging. To address these limitations, sea urchin-like AgMPs were synthesized via an aqueous-phase reduction method. Based on the oriented attachment (OA) approach, the concentrations of silver ions, reducing agents, and coating agents during the liquid phase reduction reaction were strictly controlled, so that silver ions were rapidly reduced and nucleated to obtain silver seeds. Due to the selective coating effect, these seeds grew anisotropically into silver nanorods, which subsequently self-assembled into sea urchin-like AgMPs. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to characterize and analyze the micromorphology of the particles. The thermal properties of the particles were analyzed in detail with the assistance of differential scanning calorimetry (DSC) and Thermogravimetric (TG). Brunauer-Emmett-Teller (BET) surface measurements were conducted to determine the specific surface area of particles. The synthesized sea urchin-like AgMPs exhibited uniform morphology and excellent dispersibility. The nano-structured spines on their surfaces contributed to a high specific surface area with 8.21 m²/g and enhanced sintering activity. AgMPs demonstrated superior dispersibility while maintaining comparable sintering activity. The sea urchin-like AgMPs were mixed with glycol to produce a paste as die-attach materials. Pressure sintering was carried out under 15 MPa pressures in an air atmosphere at 175 for 10 minutes to fabricate the joints, achieving a maximum shear strength of 62.8 MPa. The microstructure of fracture surfaces of the joints was analyzed and characterized. The influence of temperature variation on the evolution of fracture morphology during pressure sintering was discussed. In general, porous sea urchin-like AgMPs were successfully prepared by a simple liquid phase reduction method, exhibiting a high specific surface area and enhanced sintering activity. The silver paste obtained using this particle can achieve a reliable connection between the chip and the substrate at a lower temperature and has relatively excellent shear strength. These sea urchin-like AgMPs provide a more diverse choice for electronic die-attach materials.