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度的影响。焊料的微观结构也将进行研究，目的是了解BSn过冷对焊点凝固微观结构的影响。无铅焊点中BSn的形核通常表现出固体从液体的大的过冷度。在高纯度样品中可以获得高达80 K的过冷度值，而添加某些合金元素如Ti、Ni和Co可以将BSn的成核过冷度降低到约10- 5 K。由于焊点尺寸小，BSn的形核过冷度对焊点的最终性能和凝固组织有很大的影响。更好的使用寿命可以与某些微结构的流行有关，这些微结构由BSn的成核过冷度决定。这里使用的方法是结合联合收割机差示扫描量热法（DSC）与新的分析显微镜技术（3D成像的微观结构与晶体学测量），以提取新的见解微观结构形成的焊点。该研究有可能通过控制焊点微观结构来提高电子产品的可靠性。该项目旨在回答以下问题：-Ni 3Sn 4金属间化合物（IMC）存在下BSn的成核Ni 3Sn 4是一种常见的IMC，作为Sn和Ni衬底之间的反应层存在，其对成核过冷度的影响是已知的，但BSn在Ni 3Sn 4 IMC颗粒上的成核机制尚不清楚。对在Ni接头上固化的BSn和Ni 3Sn 4初级IMC晶体的电子背散射衍射（EBSD）研究将揭示这种成核催化作用是否可以通过取向关系（OR）或任何其他机制在晶体学上解释。BSn的成核可以借助于差示扫描量热仪（DSC）来研究，以观察过冷度如何随着不同的工艺参数而变化，这将与Ni 3Sn 4反应层的形态和特征相关联。上述研究将与过去对BSn在Cu 6Sn 4上成核的研究一起沿着用于解释为什么BSn在Ni衬底上比在Cu衬底上以更低的过冷度固化，这两种衬底都用于工业中。BSn微结构对形核过冷的影响本文研究了Sn-3Ag-0.5Cu（SAC 305）钎料合金在不同形核过冷度下的微结构，详细地报道了它们之间的关系。利用光学显微镜结合EBSD和聚焦离子束（FIB）铣削对SAC 305样品进行处理，将揭示各种凝固形态的三维微观结构，这些形态将用于更好地理解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微观结构。- BSn成核动力学DSC研究了BSn在各种焊料合金和焊点中的成核过程，建立了一个定量的BSn成核模型，这将有助于快速评估不同合金的成核机制，并将理论模型应用于真实的工业系统，以建立该模型。

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.