Hybrid solder joints –new promising soldering strategy

Employing metal nanoparticles in the soldering process is currently a research focus in development of next generation electronic devices. The properties achieved by the addition of nanoscale metallic inclusions, such as improved time and temperature dependent mechanical stability, can meet the high reliability requirements of solder joints in electronic components. The main focus of research is on lead-free nanocomposite solder pastes, which consist of microscopic solder powder, flux and a small amount of nanoscale metallic inclusions. The behavior of metal nanoparticles in bulk solder and their effect on the microstructure and properties of the produced solder joints are relatively well established. However, the essential problem that a significant part of the nanoscale inclusions is removed from the solder joint together with the flux during the reflow process remains still unsolved. In addition, the high chemical activity of metal nanoparticles and the existing strict guidelines for their safe handling and disposal seriously hinders a potential industrial application of Sn-based nanopowders or nanocomposite solders. The present research is focused on two main strategies. The first strategy is to replace commonly used metal nanoparticles in the soldering process by nanoparticles with a metal core and an oxide shell. This outer shell should prevent an uncontrolled oxidation of the metal in air. The second strategy is to produce solder joints by using fluxes doped with various amount of nanoparticles together with a lead-free Sn-Ag-Cu solder foil. This can prevent the rejection of nanoparticles from the liquid solder during the soldering process. It is expected that the flux doped with metal nanoparticles can be successfully used for an in-situ targeted alloying at the solder/substrate interface, where the achieved effect strongly depends on the type of nanoparticles employed. Therefore, the behavior of various metal nanoparticles with oxide shell in flux will be investigated. Finally, the mechanical reliability of produced hybrid solder joints will be investigated with respect to time, temperature, and current density. On the basis of the experimental data, material and lifetime models will be developed using the finite element methods. This research will lead to a deeper understanding of the behaviour of solder materials with metal nanoparticle inclusions during synthesis and processing. A link will be established between the microstructural features of the developed novel hybrid solder joints, the reflow process, and the thermo-mechanical reliability of the final product. The knowledge gained will provide the essential information needed for the use of NPs in the soldering industry to produce high quality materials for advanced electronic applications.

Reliable solder joints are crucial for maintaining mechanical stability and ensuring secure electrical connections in modern electronic systems. From smartphones and laptops to cars, airplanes, and renewable energy systems, millions of tiny solder connections ensure that devices function safely and reliably. With the global phase-out of toxic lead-based solders, lead-free alternatives such as Sn-Ag (SA) and Sn-Ag-Cu (SAC) alloys have become the industry standard. Yet these alloys face a critical challenge as their higher melting temperatures accelerate the growth of brittle intermetallic compounds (IMCs) at the solder-substrate interface, which can weaken joints and shorten product lifetimes. Improving the durability of lead-free solder joints is therefore essential not only for consumer electronics but also for safety-critical applications in industry. This project developed a scalable approach to strengthen solder joints by adding nanoparticles at the flux stage of soldering. Unlike traditional methods that mix nanoparticles directly into the solder, this technique concentrates them at the interface-the weakest point of the joint-where they can most effectively suppress harmful IMC growth and reinforce the connection. Nanoparticles can be added at the flux stage without modifying the base solder; effective implementation requires controlled dosing, agglomeration mitigation, and tuned flux chemistry. The method scales to high-throughput reflow and existing quality assurance processes, delivering cost-effective improvements in joint strength and electromigration resistance that reduce field failures. Scientific investigations focused on iron (Fe) and cobalt (Co) nanoparticles, alongside comparative studies with ceramic and carbon nanostructures. Results showed that optimized nanoparticle concentrations refine interfacial structures, reduce IMC thickness, and improve mechanical strength. Ultrasonic mixing proved particularly effective, ensuring stable dispersion and retention of nanoparticles during reflow. Advanced microscopy revealed that Fe nanoparticles form FeSn phases and slow the growth of brittle CuSn/CuSn layers, while Co nanoparticles form (Cu,Co)Sn and nanoscale precipitates that suppress IMC thickening. Mechanical testing confirmed that Fe-doped joints consistently achieved higher shear strength, especially at elevated temperatures, while Co performed best at low concentrations. Nanoindentation further demonstrated that Fe nanoparticles increased interfacial hardness to ~10 GPa. Stress relaxation experiments and finite-element modeling validated that optimized nanoparticle levels delay damage onset, while long-term electromigration tests showed that Fe nanoparticles strongly inhibit Cu dissolution and stabilize the solder/substrate interface under high current loads.