High Reliability Interconnects: New Methodologies for Lead-free Solders

电子产品的可靠性在很大程度上取决于互连电路的焊点的可靠性。大多数焊接接头含有锡作为主要相，以使焊接温度能够耐受电子元件，但由于使用中的电阻加热，锡必须在高达其熔点的80%下工作。作为熔点的百分比，这与航空发动机中的涡轮机叶片的要求一样高，并且存在类似的提高操作温度的持续需求。在使用中，由于电阻加热和自然冷却的循环，接头经常在熔点的~ 50%和80%之间循环，这导致所有相的热膨胀和收缩，因此，由于界面处的热膨胀系数（CTE）不匹配而引起的热疲劳。焊点也可能会受到冲击、振动和电流密度波动的影响，所有这些都必须经受住，以确保成功运行。焊点仅含有少量锡颗粒，并且具有高度异质性和各向异性。因此，为了理解和预测焊点的性能，有必要（i）将机械测量与接头中的微观结构和晶体学取向联系起来，以及（ii）开发晶体级变形和损伤模型，明确说明不断变化的微观结构，并链接到热循环、冲击冲击等的组件和PCB级模型。为了利用这种方法所产生的理解，有必要开发在焊接过程中可再现地产生预期在使用中提供最佳性能的微结构和取向的能力。为了实现这一愿景，我们汇集了控制焊料合金凝固动力学的专业知识，晶体滑移和界面滑移转移的原位微机械测量，缺陷成核和生长，以及晶体和微观结构层面以及组件和板级的微机械建模。通过这个团队，我们寻求在理解、预测和制造焊点方面的一个阶跃变化改进，这些焊点在英国高价值行业和消费电子行业中被优化为高可靠性。这项工作致力于使用固化处理来生成单晶和结构代表性单元（例如，具有所需刻面的金属间晶体（IMC）、β-Sn微柱或具有单个已知β-Sn取向的BGA接头等）。这些都是要仔细研究仪器的微观力学测试，以提取关键的材料性能，并在晶体水平上的缺陷成核的机械理解。性能和缺陷成核机制将在晶体塑性模型中实施，必要时，离散位错塑性模型提供热机械和冲击载荷下焊料性能的有效定量预测。然后利用这些模型来设计焊料微结构，以获得最佳性能。然后，该工作将开发在焊接过程中制造这些最佳微观结构的方法，并建立在该团队在微观结构控制方面的最新进展基础上。然后将对这些优化的接头进行测试和建模，以便最终实现优化设计的高可靠性接头。

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.