In low-temperature soldering technology, the microscopic bond strength at the interface between the solder ball and the substrate directly impacts soldering reliability. Especially as electronic devices advance towards higher density and miniaturization, interfacial bonding quality becomes a key factor limiting product lifespan. Improving bond strength requires coordinated breakthroughs in material design, process optimization, and interface modification. By manipulating atomic diffusion behavior, enhancing mechanical interlocking, and optimizing interfacial chemical reactions, the mechanical properties of low-temperature solder joints can be significantly improved.
Innovative material system design is fundamental to improving bond strength. For example, low-temperature sintering nanosilver solder paste features fine and uniform particle size, enabling the formation of high-density necking structures. The high surface energy of the nanosilver particles provides the driving force for low-temperature sintering, enabling dense connections to be achieved at temperatures below 200°C, far below the melting point of conventional high-temperature solders. Furthermore, by adjusting the silver particle morphology (e.g., spherical, flake, or dendritic), the particle packing density can be optimized, porosity can be reduced, and the bulk strength of the sintered body can be enhanced. When the solder paste contacts the substrate, the silver nanoparticles penetrate the microscopic concave and convex structures on the substrate surface, forming a mechanical interlock and further strengthening the interfacial bonding.
Precise control of low-temperature soldering process parameters is crucial for interfacial bonding strength. Temperature, time, and pressure are three key factors. Taking ultrasonic welding as an example, high-frequency vibration (20-40 kHz) generates a localized high temperature (although below the melting point of the material) at the interface between the solder ball and the substrate. Simultaneously, static pressure (0.1-1 MPa) is applied to break up the oxide film on the metal surface, allowing clean metal atoms to come into direct contact and form a metallurgical bond. During this process, the matching of vibration frequency and pressure directly affects the depth of atomic diffusion at the interface: excessively high frequency can lead to material embrittlement, while insufficient pressure cannot effectively break down the oxide film. Furthermore, controlling the soldering time requires balancing heat input with material deformation to avoid substrate warping or solder ball collapse due to excessive heating.
Substrate surface pretreatment is an effective means of enhancing bonding strength. The electroless nickel-gold (ENIG) process is widely used in low-temperature soldering because it forms a uniform, dense nickel-phosphorus alloy layer and a thin gold layer on the substrate surface. The nickel layer acts as a diffusion barrier, preventing excessive diffusion of silver atoms into the substrate and causing de-densification. Its amorphous structure resists oxidation during the sintering process, facilitating the formation of metallic bonds. The gold layer ensures long-term stability of the solder joint by providing low contact resistance and corrosion resistance. Furthermore, low-pressure plasma cleaning technology bombards the substrate surface with reactive particles, removing contaminants such as oxides and oil. It also introduces polar groups, improving surface wettability, allowing the solder paste to spread more easily across the substrate surface and enhancing interfacial adhesion.
Manipulating interfacial chemical reactions is a key mechanism for improving bond strength. During low-temperature sintering, silver nanoparticles undergo solid-phase diffusion with metal atoms (such as nickel and copper) on the substrate surface, forming intermetallic compounds (IMCs). For example, silver and nickel form an Ag-Ni compound, whose crystal structure is similar to that of silver, effectively transferring stress and reducing interfacial crack propagation. By controlling the sintering temperature and time, the thickness and distribution of the IMC can be controlled. Excessively low temperatures can lead to insufficient IMC growth and weak bonding strength; excessively high temperatures can result in an excessively thick IMC layer, making it brittle and hard, which in turn reduces bonding strength. Therefore, optimizing the process window to form a thin, continuous IMC layer is key to improving interfacial bonding strength.
Mechanical interlocking can be enhanced through substrate surface microstructural design. For example, micron-scale grooves or holes can be created on the substrate surface, which solder paste can fill during the sintering process, creating an "anchoring" effect. When the solder ball is subjected to external forces, the silver filled in the grooves must overcome greater resistance to detach, significantly improving interfacial bonding strength. Furthermore, the substrate surface roughness must be precisely controlled: too low a roughness level results in insufficient contact area and weak bonding strength, while too high a roughness level can induce stress concentration and reduce joint fatigue life.
In low-temperature soldering technology, improving the microscopic bonding strength between the solder ball and substrate requires a coordinated approach encompassing material design, process optimization, surface pretreatment, interface reaction control, and mechanical interlocking enhancement. Through the densification of nano-silver solder paste, metallurgical bonding of ultrasonic welding, interface protection of chemical nickel-gold plating, precise control of the IMC layer and microstructure design of the substrate surface, the mechanical properties of low-temperature solder joints can be significantly improved to meet the needs of high-reliability electronic packaging.
