Electronic Packaging and Interconnections 2021: Solder Joint Intermetallics
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Mehran Maalekian, Materials & Metallurgy Expertise; Christopher Gourlay, Imperial College London; Babak Arfaei, Ford Motor Company; Praveen Kumar, Indian Institute of Science; Sai Vadlamani, Intel Corporation; Kazuhiro Nogita, University of Queensland; David Yan, San Jose State University

Tuesday 8:30 AM
March 16, 2021
Room: RM 22
Location: TMS2021 Virtual

Session Chair: Kazuhiro Nogita, The University of Queensland; Xin Fu Tan, The University of Queensland

8:30 AM
On Interface Formation in Zr-based BMG /6061 Al Interconnects Joined by µFSSW: David Yan¹; Logan Vahlstrom¹; ¹San Jose State University
    Bulk metallic glasses (BMGs) are very attractive to a range of microelectronic applications including sensing elements, precision optics and micro-geared motors due to its high strength, elasticity, corrosion resistance and soft magnetic properties. However, joining BMGs to dissimilar materials to manufacture interconnects for microelectronic devices is a great challenge. Micro friction stir spot welding (µFSSW) is a novel solid-state joining process which makes it a strong candidate for joining BMGs or BMGs to other crystalline materials to fabricate various types of interconnects. But studies on the dissimilar µFSSW of BMGs to Al alloys are limited. In this paper, experimental investigations are conducted on µFSSW of Zr-based BMGs, i.e, LM105 to 6061 Al alloys. The effect of µFSSW conditions on the weld zone intermetallics formation is studied in relation to the processing force, torque and temperature.

8:50 AM
Real-time Observation of the Accelerated Growth of (Cu,Ni)₆Sn₅ on Cu-xNi: Xin Tan¹; Sergey Belyakov²; Te-Cheng Su²; Stuart McDonald¹; Christopher Gourlay²; Hideyuki Yasuda³; Syo Matsumura⁴; Kazuhiro Nogita¹; ¹University of Queensland; ²Imperial College London; ³Kyoto University; ⁴Kyushu University
    The time required to form a bond by transient liquid phase soldering (TLPS) with a mix of Sn and Cu is determined by the rate at which the Cu reacts with the Sn to form a coherent matrix of Cu₆Sn₅ intermetallic compound (IMC). The growth rate of the IMC is greatly accelerated if, instead of pure Cu, a Cu-xNi alloy is used, making possible formation by TLPS of a bond stable at high temperatures in a time comparable with that of conventional reflow soldering. The (Cu,Ni)₆Sn₅ formed has the additional benefit of resistance to the polymorphic transformation and Cu₃Sn formation is suppressed. The dynamics of the accelerated growth were studied by real-time observation of the formation of (Cu,Ni)₆Sn₅ between Cu-xNi alloys and liquid Sn using Synchrotron X-ray imaging. A range of electron microscopy techniques were used to reveal the mechanisms of the accelerated growth.

9:10 AM
Influence of Indium on the Microstructure and Properties of Interfacial IMC in Sn-rich Solder Joints: Experiments and First Principle Calculations: Amey Luktuke¹; Arun Sundar¹; Nikhilesh Chawla¹; ¹Purdue University
    The composition of the interfacial intermetallic (IMC) layer (Cu6Sn5) in Sn-rich solder joints plays a critical role in determining mechanical properties, layer stability during thermal aging, and the damage due to electromigration stressing. Alloying of Indium gives rise to the substitution of Sn atoms by In atoms forming the IMC layer of Cu6(Sn,In)5. However, the effects of such a compositional change on mechanical properties, thermal aging, and electromigration behavior are not well understood. Microstructural and mechanical properties of the interfacial IMC were investigated, with solder composition ranging from Sn-2-10 wt% In. Using WDS precise compositional analysis of IMC layer was carried out. SEM and EBSD were used for microstructural characterization of IMC. To obtain the mechanical properties, nanoindentation analysis was performed. Effect of In substitution on the modulus of Cu6Sn5 was also analyzed by performing first principle calculations. The correlation between experiment and calculations was established and will be discussed.

9:30 AM
Atomic Insights into the Role of Dopants in η-Cu6Sn5 toward Its Structural Stability: Wenhui Yang¹; Xuan Quy Tran¹; Tomokazu Yamamoto¹; Kazuhiro Nogita²; Syo Matsumura¹; ¹Kyushu University; ²University of Queensland
    Chemical modification using only small amounts of elements has proven to be an effective means to control the desired crystal structure of hexagonal η-Cu6Sn5 over a wide thermally operating window, typically found in Pb-free Sn-based soldering. Though appealing, the underlying mechanisms on the role of these dopants remain incomplete and their atomic arrangements within the η-Cu6Sn5 lattices have not yet been experimentally determined. In the current study, we directly reveal the atomic positions of Zn, In, and Sb at the Sn sites of η-Cu6Sn5 via XEDS maps utilizing advanced Cs-corrected STEM. The use of advanced statistical algorithms including Poisson NLPCA and lattice-averaging enables the fine resolution of weak XEDS maps from trace dopant elements. Our first-principles calculations further identify the influence of dopants at these atomic sites on the overall energetics, electronic structures, as well as local bonding environments, leading to the most favorable situations for η-Cu6Sn5 stabilization.

9:50 AM  Invited
Reducing Cracking in BGA Solder Joint Cu6Sn5 by Controlling the Reflow Profile: Kazuhiro Nogita¹; Flora Somidin²; Keith Sweatman³; Tetsuya Akaiwa³; Tetsuro Nishimura³; Syo Matsumura⁴; Xiaozhou Ye¹; Stuart McDonald¹; ¹University of Queensland; ²Universiti Malaysia Perlis (UniMAP); ³Nihon Superior Co. Ltd.; ⁴Kyushu University
    The polymorphic transformation that occurs in the Cu6Sn5 intermetallic compound at 186°C has the potential to generate stresses that could lead to cracking of that phase in soldered joints. This cracking may occur during the multiple reflow cycles of a typical printed board assembly process or during the thermal cycles to which electronic assemblies are exposed during service. In this paper the authors report a method of reducing cracking in interfacial Cu6Sn5 in solder joints made with solder alloys Sn-3.0Ag-0.5Cu, Sn-0.7Cu-0.05Ni-1.5Bi, and Sn-37Pb to Cu-OSP substrates by controlling the cooling stage of the reflow profile. It was found that modified reflow profiles reduced cracking in the interfacial Cu6Sn5 layer and this resulted in improved resistance of the reflowed solder ball to failure in high speed impact shear testing.

10:10 AM
The Formation and Growth Kinetics of a Peculiar Cu₆Sn₅/Ag₃Sn Composite Intermetallic Layer at the Cu50Ag/Sn Interface during Solid-state Aging: Chien-Lung Liang¹; Tsung-Chieh Chiu²; Kwang-Lung Lin¹; ¹National Cheng Kung University; ²Conquer Electronics
    In this study, Cu-50wt%Ag alloy was designed as an electrical fuse, metallization, or substrate material for soldering for the potential interfacial microstructure modification. The interfacial reaction between pure Sn and Cu50Ag substrate formed isolated Cu₆Sn₅ scallops and Ag₃Sn particles. The solid-state aging, which was performed at 130^oC (0.35 T_m), 155^oC (0.37 T_m), and 188^oC (0.40 T_m) for 0.5-4 h, induced the formation and growth of a peculiar planar Cu₆Sn₅/Ag₃Sn composite intermetallic layer. The interfacial composite intermetallic layer was composed of the Ag₃Sn phase embedded uniformly within the Cu₆Sn₅ matrix. The growth kinetics of the interfacial intermetallic layer as well as the consumption of the pure Sn and Cu50Ag substrate follows a diffusion-controlled mechanism. The divergent growth kinetics of the Cu₆Sn₅/Ag₃Sn composite intermetallic layer observed at the designed aging temperatures was proposed to result from a possible diffusion mechanism transition from the surface diffusion into the grain boundary diffusion.

10:30 AM
Reconciling Phase Equilibria and Crystal Structures for the Cu₆Sn₅ Intermetallic in the Cu-Sn System: Andreas Leineweber¹; ¹Technical University Bergakademie Freiberg
     The Cu₆Sn₅ region of the Cu-Sn phase diagram is perhaps the most basic part of fundamental knowledge for understanding the processes in solder joints involving Sn-based solders and Cu-based substrates. The most prominent phase diagrams imply a high-temperature η and a low-temperature η’ phase. The η phase has a Ni₂In/NiAs structure whereas η’ is an ordered variant thereof. Later reported structures called η⁸, η⁶ and η⁴⁺¹ were not connected to the phase diagram.New insight on the phase diagram obtained by X-ray diffraction and microstructure analysis on dedicatedly heat-treated Cu-Sn alloys are presented. The analyses reveal a significant and temperature dependent homogeneity range of the disordered eta phase as well as existence of an incommensurate η’’ phase closely related to η’ which develops from Cu-rich eta phase upon low-temperature annealing. Implications of the crystallographic and constitutional insight for the heat treatment of solder joints will be discussed.

10:50 AM
Interfacial Reaction of Ni-In System and Mechanical Properties of Ni3In7: Jia-Yi Liao¹; C. Robert Kao¹; H. T. Hung¹; ¹National Taiwan University
    Low temperature bonding process has become an indispensable bonding process in current trends. Since Pb-based solders are prohibited due to the raise of the environmental awareness, it is essential to find other new solder materials with low melting points. In this study, indium was chosen as the solder material due to its unique properties, such as low melting point, good electrical conductivity, excellent ductility, fatigue resistance, and high thermal conductivity, to bond with nickel substrate. The interfacial reaction between molten In and solid Ni was studied. By using ion milling machine, we could get an artifact-free surface then further used SEM and EPMA to investigate the interface. For preliminary result, a continuous layer of Ni3In7 showed up at the interface with some small grooves growing between Ni3In7 and Ni. Other than interfacial morphology, mechanical properties of the intermetallic compound forming at the Ni/In interface was also investigated by indenter.

11:10 AM
Microalloying Effects on Intermetallic Compound Growth and Mechanical Reliability of Sn-Bi Solder Joints: Yaohui Fan¹; Yifan Wu¹; Travis Dale¹; Sukshitha Achar¹; Hannah Fowler¹; Nilesh Badwe²; Raiyo Aspandiar²; John Blendell¹; Ganesh Subbarayan¹; Carol Handwerker¹; ¹Purdue University; ²Intel Corporation
    Low temperature interconnect technologies based on Sn-Bi alloys are being considered for substituting Sn-Ag-Cu (SAC) solders to form joints with significantly lower melting temperatures than homogeneous SAC joints. Microstructure development studies of reflow and aging are important in understanding the relationship between alloy composition and mechanical reliability, failures paths, particularly in comparison with SAC alloys. This study focused on intermetallic growth between several SnBi eutectic and off-eutectic alloys with different microalloying elements (Ag, Sb, Cu) soldered on electroless nickel immersion gold (ENIG) metallization. In this study, Au from the ENIG finish catalyzed rapid (Au,Ni)Sn4 intermetallic growth at solder-Ni interface in SnBi and SnBiAg homogeneous joints during solid-state annealing. Formation of (Au,Ni)Sn4 led to significant embrittlement of the interconnects during mechanical loading. Further study found that the growth rate of (Au,Ni)Sn4 intermetallic could be reduced by Ag and Sb alloying of SnBi solders, and could be totally eliminated with Cu addition.