Electronic Packaging and Interconnections: Low Melting Temperature Solder
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Tae-Kyu Lee, Cisco Systems; Albert T. Wu, National Central University; Won Sik Hong, Korea Electronics Technology Institute; Kazuhiro Nogita, University of Queensland; Govindarajan Muralidharan, Oak Ridge National Laboratory; David Yan, San Jose State University; Luke Wentlent, Plug Power

Monday 8:30 AM
February 28, 2022
Room: 303B
Location: Anaheim Convention Center

Session Chair: Praveen Kumar, Indian Institute of Science

8:30 AM  Invited
Comparing Intermetallic Compound Formation in Solder Interconnects under Different Current Load Conditions: Yi Ram Kim1; Allison Osmanson1; Mohsen Tajedini1; Choong-Un Kim1; Patrick Thompson2; Qiao Chen2; 1University of Texas at Arlington; 2Texas Instruments, Inc.
    The comparative studies of intermetallic compound (IMC) formation by electromigration (EM) in SnAgCu (SAC) solder interconnects are done under direct current (DC), and low and high frequency pulsed DC and alternating current (AC). The different features of IMC formation are observed. The DC and pulsed DC load conditions form a typical biased IMC under EM at the anode side of SAC solder interconnect by diffusing Cu from the cathode side. However, high frequency AC forms a relatively thin IMC at both sides. The mechanism of such IMC formation differs between DC and pulsed DC because the Cu diffuses into SAC but does not reach the other side of Cu, forming IMC at its originating side. In addition, the high frequency AC exaggerates a growth of IMC in a finger-like shape and forms IMC at the middle of solder interconnect. The details of the findings will be discussed in this report.

8:55 AM  
Length Scale Dependent Mechanical Behaviour of Elastically Constrained Sn Rich Solder Joints: Anwesha Kanjilal1; Praveen Kumar1; 1Indian Institute of Science,Bangalore
    Mechanical behaviour of ductile metal layers confined between elastic substrates is influenced by constraints imposed by substrates, leading to pronounced size effects. This occurs in micro-bumps of Sn-rich solder/Cu joints having different form factor of ductile solder. In this study, the size effect was captured experimentally from tensile tests and creep tests on Sn-based solder/Cu joints, wherein systematic transition in tensile strength, overall creep rate and failure strain by necking and cavitation was observed as joint size decreased from 1mm to 150Ám. The role of geometric constraints was modelled using finite element analysis(FEA). A distinct difference was observed between experimental results and FEA as joint size reduced. This is attributed to size dependent microstructural effects which was further confirmed by EBSD and crystal plasticity(CP) modelling. Results from CP modelling agreed well with experimentally observed trend. Subsequently, the length-scale dependent mechanical behavior was quantified to predict the overall deformation of joints.

9:15 AM  
Microstructural Evolution of Bi Phase during Reflow and Thermal Aging in Sn-58Bi Solder: Amey Luktuke1; Sridhar Niverty1; Hamidreza Torbati-Sarraf1; Fengjiang Wang2; Aniket Tekawade3; Viktor Nikitin3; Rajkumar Kettimuthu3; Nikhilesh Chawla1; 1Purdue University; 2Jiangsu University of Science and Technology; 3Argonne National Laboratory
    Sn-58Bi solder alloy is considered an ideal alternative to Sn-Pb and SAC solder alloys due to its significantly lower melting point. Bi phase plays a crucial role in determining the mechanical behavior and reliability properties of Sn-58Bi solder joint. The mechanisms governing the evolution of the Bi phase during solidification and thermal aging are not well understood. In this study, we investigate the microstructural and morphological evolution of the Bi phase during reflow in Sn-58Bi solder using x-ray tomography. A novel approach of real-time data streaming, reconstruction, and segmentation at microtomography beamline 2-BM of the Advanced Photon Source (APS) is used to understand the formation of the Bi particles and Sn dendrites during solidification. Coarsening of Bi phase during room temperature and high-temperature aging is studied using SEM, EBSD, EDS, and nanoindentation. Unique insights into the evolution of eutectic microstructure and the growth of the Bi particles will be discussed.

9:35 AM  
NOW ON-DEMAND ONLY - Stress Development in Solder Interconnects under Pulsed Electric Current: Allison Osmanson1; Yi Ram Kim1; Mohsen Tajedini1; Choong-Un Kim1; Patrick Thompson2; Qiao Chen2; Sylvester Ankamah-Kusi2; 1University of Texas at Arlington; 2Texas Instruments
    The theoretical mechanism behind the stress effect on electromigration (EM) in solder interconnects integrated in wafer-level chip scale packages (WCSPs) is investigated in this study. This research is spurred by our recent findings suggesting that the failure which occurs in the device under test (DUT) in pulsed-DC conditions is assisted by thermal fatigue. This conclusion was deduced from samples tested under varying duty factors (DF) of low-frequency pulsed-DC conditions, which were then subjected to cross-sectional analysis. Samples tested under high DF conditions had the shortest mean-time-to-failure. A crack was also observed along the solder bump/under-bump metallization (UBM) interface, suggesting that significant plastic deformation, dislocation gliding, and strain hardening occurs with fluctuating temperature and stress during pulse “on”/“off” cycles, leading to thermal fatigue and thus enhancing EM failure. The finite element method (FEM) of the thermal fatigue mechanism which occurs in pulsed-DC conditions is implemented and presented in this study.

9:55 AM Break

10:15 AM  
Surface Precipitation and Growth of Bismuth Particles in Sn-Ag-Cu-Bi Solder Joints: John Wu1; Amey Luktuke1; Nikhilesh Chawla1; 1Purdue University
    Since the widespread of Pb-free solders, many different types of elements have been added into solders to form alloys with improved mechanical behavior. It has been previously reported that a low percentage Bi composition in SAC305 solders would result in Bi precipitation on the polished exposure surface. However, the mechanisms of the Bi precipitates at the early stages of surface exposure have not been carefully observed. The cause of surface Bi precipitation has also not been carefully explained. In order to study the growth mechanisms of Bi precipitates in SAC305 solders, samples were prepared and polished and re-exposed multiple times to reset the precipitation cycle. Once the samples surfaces were exposed, Bi precipitation coarsening was observed via scanning electron microscopy (SEM). Our results indicate that precipitation in this system is driven by surface diffusion effects. A parametric correlation between growth rate and growth mechanism is presented for this unique phenomenon.

10:35 AM  
Enhancing Mechanical Properties via Adding Ni and Zn in Cu/Sn3.5Ag/Cu TLP Soldering by TCB Process: Zih You Wu1; Junq Gong Duh1; 1National Tsing Hua University
    Recently, transient liquid phase (TLP) bonding process has become a promising method in advanced electronic packaging. Full intermetallic compounds (IMCs) joints provide good strength and reliable high-melting-point phase after bonding. However, Kirkendall voids and preferred orientation of Cu6Sn5 may deteriorate the reliability in conventional Cu/Sn/Cu bump. To resolve these problems and to enhance the mechanical proprieties, Ni and Zn are used to modify the overall microstructures of the TLP bonding. After the addition of Ni and Zn, the strength of Cu18Ni/Sn3.5Ag/Cu and Cu18Ni18Zn/Sn3.5Ag/Cu bump increased significantly, as compared to Cu/Sn3.5Ag/Cu. Both Cu18Ni/Sn3.5Ag/Cu and Cu18Ni18Zn/Sn3.5Ag/Cu bump demonstrated favorable strength and toughness. Moreover, microstructure, grain and mechanical analyses are employed to elucidate the mechanisms behind the strengthening effect of Ni and Zn in Cu18Ni/Sn3.5Ag/Cu and Cu18Ni18Zn/Sn3.5Ag/Cu bump.