Advanced Microelectronic Packaging, Emerging Interconnection Technology and Pb-free Solder: Pb-free Solder Alloys II
Sponsored by: TMS Functional Materials Division, TMS: Electronic Packaging and Interconnection Materials Committee
Program Organizers: Christopher Gourlay, Imperial College London; Kazuhiro Nogita, University of Queensland; David Yan, San Jose State University; Mike Wolverton; Babak Arfaei, Ford Motor Company; Andre Delhaise; Mehran Maalekian, Mat-Tech; Mohd Arif Salleh, Universiti Malaysia Perlis
Wednesday 8:30 AM
February 26, 2020
Room: Palomar
Location: Marriott Marquis Hotel
Session Chair: Nilesh Badwe, Intel Corporation; Mehran Maalekian, Mat-Tech, Netherlands
8:30 AM Invited
Micro-mechanical Testing of Grain Boundary Sliding in a Tin Alloy: Junnan Jiang1; Richard Todd1; Angus Wilkinson1; 1University of Oxford
Grain boundary sliding (GBS) is an important deformation mechanism that contributes to creep and superplastic forming. In tin-based lead-free solders GBS can make significant contributions to in service performance. We have used FIB to cut short micro-scale cantilevers across targeted grain boundaries and then used a nano-indenter to drive shear deformation on the grain boundary plane. This approach allows GBS to be uncoupled from accommodation mechanisms that complicate interpretation in polycrystals. Effects of displacement-rate, and grain boundary character on the shear stress required for GBS have been examined. Post-test observations in SEM confirm that deformation if localised on the grain boundary and reveal the morphology of the slid boundary. Deformation rates and shear stresses obtained from the micro-scale test are compared to results from bulk tests on fine grained polycrystals.
8:50 AM
Micropillar Compression Test of Tin-solder Microstructural Units to Reveal Slip Activity: Tianhong Gu1; Finn Giuliani1; Ben Britton1; 1Imperial College London
We are interested in direct measurement of the microstructural components of tin-based solders, to provide inputs into phenomenological crystal plasticity models. Using focused ion beam machining, we have cut micropillars in high purity tin with specific crystal orientations. We explore the effect of strain rates, crystallographic orientations, and holding times on deformation mechanisms of β-Sn. Electron backscatter diffraction (EBSD) is used before and after compression to reveal the evolution of the crystallographic orientations and assist in resolving the stress state within the pillar. Imaging of the pillars before and after each test are used to resolve the slip activity, including the use of slip trace analysis and the potential for digital image correlation. Our data is being used to support a UK based integrated research programme of microstructure engineering during solder fabrication, local property extraction, and reliability modelling with crystal plasticity.
9:10 AM
Determination of β-tin Slip Properties using Micro-pillar Tests and Crystal Plasticity Modelling: Yilun Xu1; Tianhong Gu1; Ben Britton1; Fionn Dunne1; 1Imperial College London
The mechanical properties of a high-purity β-tin crystal have been investigated at room temperature and with various loading rates utilizing a coupled micro-pillar test and crystal plasticity finite element (CPFE) modelling. Square cross section samples were prepared by focused ion beam (FIB), and each sample contained a single β-tin crystal with grain orientations [100]-like and [001]-like with respect to the nano-compression direction. The favourable slip systems in the two grains have been identified by analysing the slip traces. Reasonable agreement of force-displacement curves obtained in the experiments and the crystal plasticity modelling with different displacement rates has been achieved, which validates the mechanical properties, including the slip strength, hardening term and rate-sensitivity governing parameters, extracted for β-tin single crystals. The properties will be implemented in multi-phase, polycrystal crystal plasticity models with explicitly modelled intermetallic crystals (IMCs) to predict faithful quantitative behaviour of solder joint performance under thermo-mechanical and impact loading.
9:30 AM
Molecular Dynamics Study of the Effect of Ultrasonic Vibration on Evolution of Crystal Defects: Milad Khajehvand1; Henri Seppänen2; Panthea Sepehrband1; 1Santa Clara University; 2Kulicke & Soffa Industries, Inc.
Ultrasonic wire bonding is an interconnection technique used in microelectronics packaging. Despite the well-studied general aspects of this process, the fundamental physics of the bonding mechanisms remain unknown. In our previous works, through Molecular Dynamic (MD) simulations, we have shown that contaminant-free surfaces, in close vicinity, can jump together and make bonds, a phenomenon known as Jump-to-Contact (JC). JC is found to result in dislocation generation/multiplication, which can significantly affect the diffusional mechanisms and phase transformations following contact. In this work, MD simulations are used to study the evolution of crystallographic defects during ultrasonic vibration of two FCC metallic substrates. For this purpose, two substrates are brought into a close vicinity until JC happens and contact forms. Then, the effect of vibration (magnitude and speed) and substrates’ thickness on the generation of dislocations and deformation twins is investigated. Vibration is expected to affect the bond characteristics through changing dislocation density/distribution.
9:50 AM Break
10:10 AM Invited
Effects of Bismuth, Antimony, and Indium Alloying Elements on Microstructure of High Reliability Pb-Free Solders Alloys: Richard Coyle1; Babak Arfaei2; Christopher Gourlay3; Sergey Belyakov3; Keith Sweatman4; Keith Howell4; 1Nokia Bell Laboratories; 2Binghamton University; 3Imperial College London; 4Nihon Superior, Co. Ltd.
Pb-free solder alloy development is being driven by rapidly evolving application requirements to resist damage from thermal fatigue, drop/shock and vibration loadings in aggressive product use environments. To address these performance requirements, numerous commercial Pb-free solders based on SnAgCu alloy system are being developed. These alloys have significant solute additions of bismuth (Bi), antimony (Sb), and indium (In) to promote solid solution and precipitate strengthening mechanisms. The effect of these elements on various aspects of microstructure such as the identification of specific phases in the β-tin matrix have not been fully determined. The current research work uses samples from a major industrial project to study the effects of Bi, Sb, and In on Sn grain morphology and to identify phases in ball grid arrays with different solder volumes. The effects of these alloying elements on thermal fatigue and the evolution of the microstructure resulting from thermal cycling will be discussed.
10:30 AM
Compression Creep Behavior of Sn-Ag-Cu-Bi Pb-Free Solder Alloy: Andre Delhaise1; Juan Tupac-Yupanqui Cardoso2; Fae Azhari2; Doug Perovic2; 1Celestica; 2University of Toronto
Compression creep testing was conducted on the lead (Pb)-free solder alloy “Violet” (Sn-2.25Ag-0.5Cu-6.0Bi). Testing was conducted at one of four temperatures (-25°C, 25°C, 75°C, or 125°C). A applied load of either 5MPa, 10MPa, 15MPa, 25MPa, 30MPa, 40MPa, or 50MPa was used, and samples were tested in either as-fabricated conditions or after aging (125°C for 24 hours). Three samples of each set of variables were tested, according to the ASTM E9 standard. Tests were halted after either two days had elapsed, or the sample reached a strain of 0.12.Strain-time curves were used to determine steady-state creep strain rate, allowing for analysis of creep deformation kinetics. The results from this testing, along with planned time-independent (stress-strain) testing, will allow for the development of a constitutive thermomechanical model. This is a necessary step towards the commercialization of the alloy and adoption by the industry as a viable Pb-free alloy solution.
10:50 AM
Modification of Traditional Pb-free Solders with Bi, Sb and In for Improved Reliability: Sergey Belyakov1; Tetsuro Nishimura2; Keith Sweatman2; Jingwei Xian1; Christopher Gourlay1; 1Imperial College London; 2Nihon Superior Co., Ltd
There has been a significant industrial and scientific interest in the development of next-generation Pb-free solders with improved thermal cycling properties and ability to provide adequate performance in increasingly aggressive environments. Traditional Sn-Ag-Cu and Sn-Cu-Ni-based solders are modified with various major alloying additions such as Bi, Sb, In etc. to improve their mechanical response in emerging applications. In this study we explore the role of Bi, Sb, In or their combined additions on microstructure formation and mechanical properties of Sn-Cu-Ni/Cu and Sn-Ag-Cu/Cu solders joints. The focus is on (i) new phases caused by the additions of Bi, Sb, In to the base solders and the overall microstructures, (ii) mechanical response of obtained microstructures during shear impact and fatigue tests at a range of temperatures. Finally, the influence of the additions on solder joint failure mechanisms will be addressed.
11:10 AM
A Preliminary Study of the Dissolution of Bi in β-Sn using In Situ Scanning Electron Microscopy: Andre Delhaise1; Peter Banh2; Dian Yu2; Hong Ran Wang2; Jason Tam2; Jane Howe2; Doug Perovic2; 1Celestica; 2University of Toronto
The inclusion of bismuth (Bi) in lead (Pb)-free solder alloys causes stabilization in mechanical properties after aging. Bi precipitates dissolve into the β-tin (Sn) matrix upon heating, and precipitate out uniformly upon cooling, creating a homogenous microstructure. Bi-bearing alloys also outperform traditional Pb-free alloys in accelerated thermal cycling (ATC). However, it is unknown whether Bi strengthens the alloy via solid solution or precipitation during each phase of the thermal cycle.A preliminary study was conducted to understand the kinetics of Bi dissolution in β-Sn using in situ Scanning Electron Microscopy (SEM). Binary Sn-Bi samples with 3, 6, or 9wt% Bi were sectioned, then heated at either 65°C or 90°C in SEM. Sn-6Bi samples were also pre-aged to look at the effects of Bi precipitate distribution. Images were taken periodically during heating. A mathematical model is proposed to define the visible precipitate area as a function of heating time.