High Reliability Interconnects: New Methodologies for Lead-free Solders

The reliability of electronics depends to a large degree on the reliability of the solder joints that interconnect the circuitry. Most solder joints contain tin as the majority phase to enable soldering at a temperature tolerable to the electronic components, but the tin must then operate at up to ~80% of its melting point due to resistance heating in service. As a percentage of melting point, this is as demanding as a turbine blade in an aeroengine and there is a similar ongoing desire to increase the operation temperature.

In service, the joints regularly cycle between ~50 and 80% of melting point due to cycles of resistance heating and natural cooling, which causes thermal expansion and contraction of all phases and, therefore, thermal fatigue due to the mismatch in the coefficient of thermal expansion (CTE) at interfaces. Joints can also experience shock impacts, vibration and surges in current density, all of which must be withstood to ensure successful operation.

Solder joints contain only up to a few tin grains and are highly heterogeneous with anisotropic properties. Therefore, to understand and predict the performance of solder joints it is necessary (i) to link mechanical measurements to the microstructure and crystallographic orientations in the joint and (ii) to develop crystal-level deformation and damage models that explicitly account for the evolving microstructure and link through to component and PCB-level models of thermal cycling, shock impact etc. Furthermore, to capitalise on the understanding generated by such an approach, it is necessary to develop the capability to reproducibly create the microstructures and orientations during the soldering process that are predicted to give optimum performance in service.

To deliver this vision, we bring together expertise in controlling solidification kinetics in solder alloys, in-situ micromechanical measurement of crystal slip and slip transfer across interfaces, defect nucleation and growth, and micromechanical modelling at the crystal and microstructure level and at the component and board-level. With this team, we seek a step change improvement in the understanding, prediction and manufacturing of solder joints that are optimised for high reliability in high value UK industry and in the consumer electronics industry.

The work addresses using solidification processing to generate single crystal and structurally representative units (e.g. intermetallic crystals (IMCs) with the desired facets, beta-Sn micro-pillars, or BGA joints with a single known beta-Sn orientation etc.). These are to be studied in carefully instrumented micromechanical tests to extract key material properties, and mechanistic understanding of defect nucleation at the crystal level. The properties and defect nucleation mechanisms are to be implemented in crystal plasticity models and, where necessary, discrete dislocation plasticity models to provide validated quantitative prediction of solder performance under thermo-mechanical and impact loading. The models are then to be exploited to design solder microstructures for optimal performance. The work will then develop methods to manufacture these optimum microstructures within the soldering process, building on recent advances in microstructure control made by the team. These optimised joints will then be tested and modelled such that optimally designed, high reliability joints may ultimately be achieved.

The proposed methodology integrating processing-microstructure-properties-performance has the potential to provide optimal microstructures for solder joint performance tailored to the specific demands of the application. Performance and reliability of interconnects is crucial in safety-critical applications but equally in the mass consumer product markets where small efficiency gains are vital. For example, improved joint reliability can enable an increase in operation temperature capability which can contribute to automotive/aerospace lightweighting by allowing shielding to be removed and electronics to be located closer to sensors with shorter wires, both of which save mass. In safety-critical applications, the critical loading differs significantly from potential impact and accident mitigation in the nuclear energy sector through to elevated temperature high cycle fatigue superimposed on low-cycle thermal cycling in the aero-engine industry. One of the key challenges for the growing UK small modular reactor (SMR) programme led by Rolls-Royce is interestingly in instrumentation and control (as opposed to nuclear core development) for which the impact strength, endurance and reliability of solder interconnects is crucial. In the aero-engine industry, an aspiration is the full electrification of the plant, leading to massive pollutant reduction. In this safety-critical application, miniaturisation and light-weighting is key along with (mechanical) high and (thermal) low cycle fatigue endurance. In consumer mobile and computing needs, light-weighting and cost reduction are differentiators but the loading regimes for interconnects are different again to the previous two examples. Hence the ability to design and manufacture bespoke but optimally microstructured solders and to provide the generic methodology and tools with which to do so are potentially transformational. We believe our unique multi-disciplinary team and technique approach is world leading and has the potential to impact across the globe.