Emerging Interconnect and Pb-free Materials for Advanced Packaging Technology: Intermetallic Compound and Microstructural Evolution of Pb-free Materials
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Fan-Yi Ouyang, National Tsing Hua University; C. Robert Kao, National Taiwan University; Albert T Wu, National Central University; Fay Hua, Intel Corporation; Yan Li, Intel Corporation; Babak Arfaei, Binghamton University; Kazuhiro Nogita, The University of Queensland

Wednesday 8:30 AM
March 1, 2017
Room: 30E
Location: San Diego Convention Ctr

Session Chair: Kazuhiro Nogita, The University of Queensland; Sergey A Belyakov, Imperial College London

8:30 AM  Invited
Nucleation and Growth of Primary Cu6Sn5 in Solder Joints: Christopher Gourlay1; J.W. Xian1; M.A.A. Salleh2; Sergey Belyakov1; Kazuhiro Nogita2; 1Imperial College London; 2University of Queensland
    Primary Cu6Sn5 solidifies in the bulk solder when common Pb-free alloys are soldered on copper substrates. This talk will overview the factors affecting the nucleation and growth of primary Cu6Sn55 including strategies to minimise their size. 3D characterisation and crystallographic studies of the growth morphologies will be presented, spanning from faceted Cu6Sn5 growth forms to Cu6Sn5dendrites and the faceted to nonfaceted growth transition. Next, the role of peak reflow temperature and cooling rate on the morphology of Cu6Sn5in solder joints will be examined using Sn0.7Cu/Cu joints as an example. This will include consideration of the competition between growth from the pre-existing Cu6Sn5 layer versus nucleation in the liquid ahead of the layer, and an outlook on how primary Cu6Sn5solidification is likely to differ in emerging interconnection technologies.

8:50 AM  
Growth Behavior of Interfacial Intermetallic Compound at ENIG and Sn-Ag-Cu Solder Joint with Plating Temperature of Ni(P): Wonil Seo1; Young-Ho Kim2; Sehoon Yoo1; 1Korea Institute of Industrial Technology; 2Hanyang University
     Interfacial intermetallic compound (IMC) growth behavior of ENIG/Sn-3.0Ag-0.5-Cu solder (SAC305) joint was investigated with varying temperature of Ni(P) electroless plating solution. The temperature of Ni(P) electroless plating solution for the ENIG finish was varied from 75 to 85C. SAC305 solder balls with diameters of 450 μm were mounted on the ENIG finish and reflowed at the peak temperature 250C. The brittle fracture behavior was evaluated with a high speed shear (HSS) test. After HSS test, fracture surface was observed to understand failure modes. A (Cu, Ni)6Sn5 IMC was formed at the interface between Ni(P) and SAC305. The grain size of the (Cu, Ni)6Sn5 IMC increased with increasing temperature of Ni(P) plating solution. The thickness of the IMC decreased with increasing the temperature of Ni(P) plating solution. In addition, the percentage of brittle fracture decreased with increasing the temperature of Ni(P) plating solution.

9:10 AM  
Study of Al−Cu Compounds as Soldering Bond Pad for High-power Device Packaging: Yan-Hao Chen1; Cheng-Yi Liu1; 1National Central University
    In this talk, we will report the wetting reaction of Pd-free solders on the Al-Cu compound substrates. The four equilibrium Al-Cu phases, Al2Cu, AlCu, Al3Cu4, and Al4Cu9 were produced by annealing the solid state Al-Cu diffusion couples. With a proper polishing process, Al-Cu compounds were fabricated as soldering substrates. We found that the pure Sn solder ball can wet properly on the Al3Cu4, Al4Cu9 substrates, but cannot wet on the Al2Cu and AlCu substrates. Interestingly, Sn can wet on the Al4Cu9 substrate as well as on the Cu substrate. In addition, a dark layer were found below the interfacial Cu6Sn5 compound by the back scattering electron (BSE) microscope image, which is analyzed to be an Al-rich Al4Cu9 phase by x-ray photoelectron spectroscopy (XPS). Compare to the thickness of Cu6Sn5 formed by Sn/Cu and Sn/Al4Cu9, we found the Al-rich Al4Cu9 layer can retard the inter-diffusion of Sn and Cu atoms.

9:30 AM  
Thermodynamic and Microstructural Evaluation of the Sn-Si-Ge Ternary System for Advanced Pb-Free Solder Design: Kathlene Reeve1; Carol Handwerker1; 1Purdue University
    The majority of today’s lead-free solders are Sn-rich alloys that depend on both the anisotropic properties of β-Sn, as well as the size, distribution, and morphology of β-Sn grains and solder intermetallics (Cu6Sn5 and Ag3Sn) for overall mechanical performance. This study analyzes the thermodynamic attributes and as-solidified microstructures in the Sn-Si-Ge ternary system. The Si-Ge binary system consists of a continuous solid solution with a diamond cubic (DC) crystal structure and a lattice parameter ranging from 0.543 – 0.566 nm with composition. The lattice parameter of the body centered tetragonal (BCT) β-Sn phase in the a-axis is 0.583 nm, varying only 5% from the lattice parameter of the Si0.5Ge0.5 composition. The relationship between the predicted solidification paths (via Thermo-Calc) and the resulting microstructures will be discussed. The potency of varying compositions and orientations of Si-Ge as nucleation sites for β-Sn will be analyzed via microscopy and electron backscatter diffraction.

9:50 AM  
Microstructure Formation in Reinforced Sn-Cu Lead-free Solder Alloys: M. A. A. Mohd Salleh1; Stuart McDonald2; Christopher Gourlay3; Kazuhiro Nogita2; 1Universiti Malaysia Perlis; 2University of Queensland; 3Imperial College London
    Electronics manufacturers are pushing the limits in reducing the physical size of circuitry while simultaneously increasing the number of transistors to satisfy Moore’s Law. This includes investing in new materials in electronic packages with a focus on high reliability. A viable method to enhance the properties and performance of solder joints is the incorporation of reinforcement particles to the solder matrix, either by intrinsic or extrinsic methods. In this study, a series of Sn-Cu Pb-free solder alloys were manufactured with a variety of reinforcing phases and the microstructure and soldering behavior were investigated in detail using advance characterisation techniques such as in-situ synchrotron X-ray radiography imaging. Shear strength of the reinforced solder joints was also evaluated. The collective results of this study demonstrate a detailed understanding of the manufacture of reinforced Sn-Cu Pb-free solder alloys and the mechanisms of microstructure development.

10:10 AM Break

10:30 AM  
The Grain Orientation Evolution of Mixed Solder Joints with Single-crystal Grain at the Same Position of BGA Packages during Thermal Shock: Jing Han1; Fu Guo1; Shihai Tan1; 1Beijing University of Technology
    The mixed solder joints in BGA packages were made by soldering Sn3.0Ag0.5Cu solder balls on the Printed Circuit Board, and the solder paste was eutectic SnPb. And BGA packages were cross-sectioned prior to thermal shocks to obtain grain orientations of the initial state, and the grain orientation after thermal shock were detected by in-situ observation using Electron Backscattered Diffraction. In this paper, a few solder joints with single-crystal grain at the same position of BGA packages were selected to investigate the grain orientation evolution and recrystallization. The results showed that the orientation evolution and recrystallization maybe correlated with the angle between c axis of Sn crystal and the load direction of thermal stress caused by mismatched coeffient of thermal expansion during thermal shock. And the orientation evolution and recrystallization became more serious with the decreased angle between c axis of Sn crystal and the load direction.

10:50 AM  
Subgrain Rotation Behavior of SnAgCu-SnPb Mixed Solder Joints in BGA Components during Thermal Shock: Fu Guo1; Shihai Tan1; Jing Han1; 1Beijing University of Technology
    BGA samples with Sn3.0Ag0.5Cu solder joints were soldered on the Printed Circuit Board with the eutectic SnPb solder paste to form mixed solder joints. Then BGA samples were cross-sectioned prior to thermal shock to obtain the grain orientation of the initial state by using the Scanning Electron Microscope equipped with Electron Backscattered Diffraction system. Therefore, in this paper, several mixed solder joints which had different angles between the c axis of Sn crystal and chip at the corner of the BGA package were selected to investigate the subgrain rotation behavior of mixed solder joints during thermal shock. And the solder joints which were selected in this paper were single-crystal solder joints and had the similar position. The results showed that the subgrain rotation behavior were variable among the mixed solder joints with different angles between c axis of Sn crystal and chip.

11:10 AM  
Advances in High Temperature Pb-Free Composite Solder Paste Research: Stephanie Choquette1; Iver Anderson1; 1Ames Lab
    A promising Cu-Ni/Sn lead-free alternative to high-Pb solder has continued to be developed toward improvement of the reliability of PCB/SMT solder joints at temperatures greater than 300˚C. Previous issues with porosity have been addressed by decreasing the viscosity of the powder composite paste blend. Also, the Ni addition has been optimized by adjusting the Cu-10Ni fraction. The prototype paste reflows at 250˚C and forms the ductile IMC (Cu,Ni)6Sn5, dictating the solder paste’s post-reflow melting temperature of 525C. New research on optimizing the powder ratio of the paste blend, exploring limits of the paste’s reflow profile, decreasing void formation in the joint, and variations in IMC composition will be reported. Results suggest that this composite paste could be an excellent “drop-in” replacement for the Pb-based high-temperature solders soon to be eliminated by RoHS. Support from the Iowa State Research Foundation and Nihon-Superior, Inc. Ltd., through Ames Lab (DE-AC02-07CH11358) is gratefully acknowledged.