Eutectic growth and microstructure selection in electronic solder alloys
Creators: name: Hou, Ning
Publisher: name: Imperial College London
Year: 2018

description: This thesis develops the understanding of eutectic growth and microstructure selection in electronic solder alloys, including the Sn-Ni, Sn-Cu and Sn-Ag binary systems, and alloys from the ternary Sn-Ag-Cu system. These systems are relevant to Pb-free soldering and the development of optimised Pb-free solder compositions. Past research has reported the presence of both stable Sn-Ni3Sn4 eutectic and metastable Sn-NiSn4 eutectic in Sn-Ni alloys and Sn-3.5wt%Ag/Ni solder joints. The present work follows up on these initial findings with a detailed investigation into competitive growth between these stable and metastable eutectics. As no information was available for the metastable Sn-NiSn4 eutectic phase diagram, the eutectic points of the two eutectics and the Ni3Sn4 liquidus line were measured in this thesis. Controlled Bridgman solidification experiments were then used to explore eutectic growth mechanisms of the stable and metastable eutectics and investigate the origins of metastable eutectic formation in this system. The dynamics of eutectic growth and the transition between the two eutectics were investigated by synchrotron radiography, and the crystallography of eutectic growth was measured by EBSD. The competition between Sn dendrite growth and metastable Sn-NiSn4 eutectic growth was then measured and compiled into a microstructure selection map in the range of C0 = 0.05 wt%Ni – 0.26 wt%Ni and V = 0.5 µm/s – 1000 µm/s. Sn-Cu, Sn-Ag and Sn-Ag-Cu (SAC) alloys are commonly used as soldering materials and past researchers have focussed on microstructure selection maps in these systems. However, there has been limited research on the eutectic crystallography and growth mechanisms. In the present work, laboratory and synchrotron Bridgman solidification is used to observe the eutectic growth front and its response to changes in pulling rate, and to analyse the eutectic crystallography and growth mechanisms in Sn-Ag, Sn-Cu and Sn-Ag-Cu alloys. First, the binary Sn-Ag3Sn and Sn-Cu6Sn5 eutectics are investigated. The growth of the ternary Sn-Ag3Sn-Cu6Sn5 eutectic is then compared with the two binary eutectics. Next, research explores the more complex case of Sn-1Ag-0.9Cu (wt%) which lies on the Sn-Cu6Sn5 univariant eutectic groove where there is competition between Sn dendrite growth, univariant SnCu6Sn5 growth, and invariant ternary Sn-Ag3Sn-Cu6Sn5 growth. Orientation relationships between Sn and the intermetallic compounds (IMCs) and the IMC growth directions are determined in all systems. The experimental observations of the growth microstructure as a function of velocity and composition in the Sn-Ni, Sn-Ag and Sn-Cu systems were combined with literature data to construct experimental eutectic coupled zones for the three binary systems and for the Sn-Pb system. The transition between fully eutectic growth and Sn dendrite growth ahead of a eutectic front was considered using the criterion that the microstructure whose tips can grow at highest temperature is selected (i.e. wins). Since eutectic growth often did not occur at the extremum for these nonfaceted-faceted eutectics, the eutectic growth temperature versus growth velocity was measured for the Sn-NiSn4 and Sn-Ag3Sn eutectics in this thesis, and literature data for this relationship in the Sn-Pb system was used. To calculate the fully eutectic / Sn dendrite transition, a combination of the measured eutectic growth temperature response function (TE* vs. V) and Lipton, Glicksman and Kurz (LGK) calculated dendrite tip growth temperature response function (Td* vs. V) were used to calculate eutectic coupled zones. With this approach, reasonable agreement between the calculated and measured coupled zones was obtained.