Study of Nucleation and Solidification of Sn-based Lead-Free Solders

该项目的目的是研究和了解无铅焊料中BSN的形核以及对焊点中存在的反应层和相的形核过冷的影响。此外，还将研究焊料的微观结构，以了解BSN过冷对焊点凝固组织的影响。无铅焊点中BSN的形核通常表现为从液体到固体的大量过冷。在高纯样品中可以获得高达80K的过冷度，而添加某些合金元素如Ti、Ni和Co可以将BSN的形核过冷度降低到10-5K左右。由于每个焊点的尺寸较小，BSN的形核过冷对焊点的最终性能和凝固组织有很大的影响。更长的使用寿命可能与某些微结构的流行有关，这些组织是由BSN的形核过冷度决定的。这里使用的方法是将差示扫描量热法(DSC)与新的分析显微镜技术(微观结构的3D成像与结晶学测量相结合)相结合，以提取对焊点微观结构形成的新见解。这项研究有可能通过控制焊点组织来提高电子器件的可靠性。该项目旨在回答以下问题：-在Ni3Sn4金属间化合物(IMC)存在的情况下BSN的形核。Ni3Sn4是一种常见的IMC，它作为锡和镍衬底之间的反应层存在，其对形核过冷的影响是已知的，但Ni3Sn4 IMC颗粒上BSN的形核机制尚不清楚。电子背散射衍射(EBSD)研究凝固在Ni接头和Ni3Sn4初生IMC晶体上的BSN是否可以通过取向关系(OR)或任何其他机制从结晶学上解释这种形核催化作用。借助差示扫描量热仪(DSC)可以研究BSN的形核过程，观察过冷度随工艺参数的变化情况，这将与Ni3Sn4反应层的形态和特征有关。上述研究将与过去关于Cu6Sn4上BSN形核的研究一起用来解释为什么BSN在工业上都有较低的过冷度在Ni上凝固。-BSN组织对形核中间冷却的影响将在不同形核过冷值下对Sn3Ag-0.5Cu(SAC305)焊料合金的微观组织进行研究，以更详细地报告这种关系。光学显微镜结合EBSD和聚焦离子束(FIB)球磨SAC305样品将揭示不同凝固形态的三维组织，这些形态将被用来更好地理解BSN从液体中的形核和生长，并有望更好地控制工业电子接头中的BSN组织。-BSnDSC的形核动力学研究各种焊料合金和焊点中的BSN形核，可以用来开发一个关于BSN形核的定量框架，这将有助于快速评估不同合金的形核机制。这项研究的目的是将理论形核模型应用于实际工业系统，目的是开发该框架。

The aims of the project are to study and understand the nucleation of BSn in lead free solders and the influence on the nucleation undercooling of reaction layers and phases present in solder joints. Microstructures of solders will also be studied with the aim of developing an understanding of the influence of BSn undercooling on the resulting solidification microstructures in solder joints. Nucleation of BSn in lead-free solder joints commonly exhibits large undercooling of the solid from liquid. Undercooling values of as large as 80K can be obtained in high purity samples, whereas adding certain alloying elements like Ti, Ni, and Co can reduce the nucleation undercooling of BSn to around 10-5K. The nucleation undercooling of BSn, due to the small size of each solder joint, has huge influence on the final properties of the joint, and the solidification microstructure of them. Better in-service lifetimes can be linked to the prevalence of certain microstructures that are determined by the nucleation undercooling of BSn. The approach used here is to combine differential scanning calorimetry (DSC) with novel analytical microscopy techniques (3D imaging of microstructure linked with crystallographic measurements) to extract new insights into microstructure formation in solder joints. The research has the potential to improve the reliability of electronics through controlling solder joint microstructures.The project aims to answer the following questions:-Nucleation of BSn in the presence of Ni3Sn4 intermetallic compound (IMC)Ni3Sn4 is a common IMC present as a reaction layer between Sn and Ni substrates, its effect on nucleation undercooling is known, however the mechanism of BSn nucleation on Ni3Sn4 IMC particles is not known. Electron Backscatter Diffraction (EBSD) study of BSn solidified on Ni joints, and Ni3Sn4 primary IMC crystals will reveal if this nucleation catalysis can be explained crystallorgaphically through an orientation relationship (OR), or any other mechanism.Nucleation of BSn can be studied with the aid of a Differential Scanning Calorimeter (DSC) to observe how the undercooling changes with varying processing parameters, which will be linked to the morphology and features of the Ni3Sn4 reaction layer. The above research will be used, along with past research on nucleation of BSn on Cu6Sn4, to explain why BSn solidifies with a lower undercooling on Ni substrates than on Cu substrates, both of which are used in industry.- Influence of BSn microstructure on the nucleation intercoolingMicrostructures of Sn-3Ag-0.5Cu (SAC305) solder alloy will be studied at different nucleation undercooling values to report the relationship in great detail. The use of optical microscopy combined with EBSD and Focused Ion Beam (FIB) milling on SAC305 samples will reveal the 3D microstructure of the various solidification morphologies.The morphologies will be used to better understand the nucleation and growth of BSn from liquid, and hopefully allow better control of BSn microstructures in industrial electronic joints. - Nucleation kinetics of BSnDSC studies of BSn nucleation in various solder alloys, and solder joints can be used to develop a quantitative framework on BSn nucleation, which will hopefully be useful in quickly assessing the mechanism of nucleation on different alloys.The research will aim to apply theoretical nucleation models to real industrial systems, with the aim of developing the framework.