Electronic Packaging and Interconnections: On-Demand Oral Presentations
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 8:00 AM
March 14, 2022
Room: Electronic Materials
Location: On-Demand Room

In-situ Observation of Liquid Solder Alloys and Solid Substrate Reactions Using High-voltage Transmission Electron Microscopy: Xin Fu Tan1; Kazuhiro Nogita1; Flora Somidin2; Stuart McDonald1; Hiroshi Maeno3; Syo Matsumura3; 1University of Queensland; 2Universiti Malaysia Perlis; 3Kyushu University
    The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilising creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, detail understanding of the dynamics of the process is difficult due to the lack of direct observation with micro- and nano-level resolution. In this study, high voltage transmission electron microscopy (HV-TEM) is employed to observe the morphological changes that occur in Cu6Sn5 between a Sn-3.0wt%Ag-0.5wt%Cu (SAC305) solder alloy and a Cu substrate in-situ at temperatures above the solidus of the alloy. This enables the continuous surveillance of rapid grain boundary movements of Cu6Sn5 during soldering and increases the fundamental understanding of reaction mechanisms in solder solid/liquid interfaces.

Power Cycling and Thermal Cycling Performance of Pressureless Silver Sintered Silicon Carbide Power Module Compared to Lead-free Solder Joint: Won Sik Hong1; Mi Song Kim1; 1Korea Electronics Technology Institute
    1200V 200A SiC MOSFET power module for electric vehicle (EV) was developed with pressureless silver (Ag) sintering process which was 240 ℃, 75 min in vacuum and nitrogen gas. Ag finish Si3N4 active metal brazed (AMB) ceramic substrate and Ag paste were used. Also, we prepared Sn-3.0Ag-0.5Cu (SAC305) soldered power module to compare the Ag sintered module. As-sintered bonding strength, bonding layer thickness (BLT), void content and densification were 39-41 MPa, 70-80㎛, 2% under and 91-95 %, respectively. To compare the electrical performance for SAC305 soldered and Ag sintered power modules, we conducted power cycling tests (50~100 ℃ (△Tc=50℃), 50~150 ℃ (△Tc=100℃), 3000 cycles) and thermal cycling(-40~125 ℃, 30 min dwell time, 1100 cycles), and then, on resistance and switching efficiency were compared. We finally confirmed that the reliability and electrical performance degradation characteristics of the Ag sinter joint power module were superior to that of the solder-bonded module.

Characterising the Intermetallic Layer in Mixed Sn-Bi Paste/SAC BGA Solder Ball Joints: Jiye Zhou1; Qichao Hao1; Xin Fu Tan1; Stuart McDonald1; Kazuhiro Nogita1; 1University of Queensland
    The polymorphic transformation that occurs in the Cu6Sn5 intermetallic compound at 186C 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 past few years we have demonstrated how manipulating the reflow profile for Sn-Cu and SAC solders on Cu substrates can influence cracking behaviour. In this presentation, the authors investigate crack fromations in Cu6Sn5 in Sn-37Bi and Sn-58Bi solder pastes that are reflowed adjacent to a Cu substrate and SAC solder balls. The influence of peak reflow temperature and cooling conditions on cracking is discussed with respect to the solder composition.

Characterisation of η-(Cu,Ni)6Sn5 Formed between Cu-xNi/Sn Couples: Xin Tan1; Qinfen Gu2; Michael Bermingham1; Stuart McDonald1; Kazuhiro Nogita1; 1University of Queensland; 2ANSTO
    The transient liquid phase soldering (TLPS) process between Sn and Cu to form a coherent matrix of Cu6Sn5 intermetallic compounds (IMCs) can be greatly accelerated if, instead of pure Cu, a Cu-xNi alloy is used as a substrate. This enables the formation of a bond that is stable at high temperatures in a time comparable with that of conventional reflow soldering. Depending on the Ni concentration, the (Cu,Ni)6Sn5 IMCs formed are different from the well-studied Cu6Sn5 with altered morphologies and crystal structures, which may influence the mechanical and physical properties of the joint. The IMCs growth rates at different Ni concentrations also have a direct impact on the bonding process in industry applications. In this study, scanning electron microscopy, electron backscatter diffraction, synchrotron powder X-ray diffraction and density functional theory calculations were used to characterise the η-(Cu,Ni)6Sn5 formed between Cu-xNi/Sn couples and to reveal the mechanisms of the accelerated growth.

Wire Bonding Novel 3D Air-metal Dielectric Structures with ISIG Passivation: Process Development and Reliability: Yipin Wu1; Pichaya Sommai1; Joyce Christiansen-Salameh2; Jim Clatterbaugh1; Leyla Hashemi-Sadraei1; 1Keysight Technologies; 2Keysight Technologies, Cornell University
     PolyStrata® is a 3D air-metal-dielectric copper structure which allows miniaturization of complex RF components including filters, combiners and baluns integrated into devices such as signal analyzer frequency extenders and arbitrary waveform generators. For PolyStrata®to meet RF performance at 100+ GHz, standard NiAu electroplating with 1 um Au no longer meets test specifications due to the high loss in the Ni layer. Instead, a thin layer of Immersion Silver Immersion Gold (ISIG) is introduced with only 20 nm Au. Wire bonding process development explores silver oxidation, plating adhesion and Au-Au weld formation, ultimately enabling a repeatable and reliable process for a range of IG thicknesses starting at 20 nm. Additionally, an autocatalytic plating process is examined to achieve a consistent 100 nm IG metallization which further allows compatibility with oxygen plasma cleaning in microcircuit manufacturing processes.

Low Temperature Solder Interconnect Board Level Shock Performance at Elevated Temperature: Tae-Kyu Lee1; Gnyaneshwar Ramakrishna1; Young-Woo Lee2; Edward Ibe3; Karl Loh3; 1Cisco Systems; 2MK Electron; 3Zymet
    The adaptation of low melting temperature for solder interconnection comes with significant benefits to less warpage and component defect risk due to the lower assembly temperature, but counter with inferior shock performance due to less ductility in Sn-Bi eutectic alloy system. 12x12 mm chip array BGA (CABGA) components on 62mil think boards were shock tested at room temperature and elevated temperature with 1500G shock input. The shock performance between edgebond applied and no-edgebond shock is compared at room temperature condition and at 100oC environment. The correlation between crack propagation and localized recrystallization were compared in a series of cross section analyses using polarized imaging and Electron–backscattered diffraction (EBSD) imaging. The correlation between the shock induced straining at the solder joints, effect if the microalloy elements and the effect of the edgebond adhesive on the joint microstructure will be presented and discussed.

Thermal Expansions of βSn and Bi in Sn-Bi Alloys: Qichao Hao1; Xin Fu Tan1; Qinfen Gu2; Stuart McDonald1; Kazuhiro Nogita1; 1The University of Queensland; 2Australian Synchrotron
    During the soldering process and the daily operation of the electronic devices, solder alloys experience temperature variation frequently. The mismatch in volume expansion of the solder alloys and the interconnected components can result in stresses which lead to failure. In an alloy system with high solubility of one element in another, the effects of thermal expansion and the solubility limits are both important contributing factors to the thermal induced volume change. In this study, Sn-57wt%Bi and Sn-37wt%Bi alloys which are promising material for low-temperature solders were investigated by in-situ heating synchrotron X-ray diffraction (XRD) to reveal the changes of the lattice parameters of Sn and Bi. Density functional theory (DFT) calculations was adopted to reveal the influence of the solid solution of Bi on the lattice parameters of βSn, thereby decoupling the effects of thermal expansion and solid solution of Bi on the thermal induced volume change of Sn.

Electromigration Behaviors of Nanotwinned Ag Interconnects: Chi-Shen Chen1; Tsung Lin1; Fan-Yi Ouyang1; 1National Tsing Hua University
    Advanced packaging techniques would be key bottleneck to enable heterogeneous integration in WBG semiconductors applications and metal direct bonding is regarded as a promising solution for 3D IC in advanced packaging technology. Ag interconnects will play an important role in advanced packaging because they possess unsurpassed thermal and electrical conductivity. In this study, we successfully fabricated nanotwinned (nt) Ag structure with a twin spacing around 9 nm. Compared to the regular Ag, the nt-Ag exhibits highly (111) preferred orientation, and its hardness can reach 1.88 GPa, being twice higher than the regular Ag ones. The electromigration (EM) behaviors were tested at 2 10^5 A/cm^2 at 150 C. We found the nt-Ag possessed higher EM resistance than the regular Ag. The detailed microstructural evolution of nt-Ag during electromigration and its corresponding failure mechanism will be discussed and compared to regular ones in this talk.

Transient Liquid Phase (TLP) Bonding in Cu-Ni/Sn-0.7Cu/Cu-Ni Joints: Nurul Razliana Abdul Razak1; Xin Fu Tan1; Stuart McDonald1; Michael Bermingham1; Hideyuki Yasuda1; Kazuhiro Nogita1; 1School of Mechanical and Mining Engineering, The University of Queensland
    Bonding methods involving a transient liquid phase (TLP) have potential applications for high-operating temperature Pb-free solder electrical interconnection, including power modules in hybrid and electric vehicles. These methods typically involve the formation of high-melting temperature phases via the interdiffusion of low and high melting temperature phases. In Sn/Cu based systems, slow kinetics and uncontrolled porosity are significant challenges that must be overcome. The addition of Ni to Cu can significantly accelerate the TLP process. Through a synchrotron X-ray microradiography technique, the TLP soldering process for Sn-0.7Cu with different compositions of Cu-Ni substrates was successfully observed in real-time. This allowed the kinetics of the reaction to be quantified and provided insight into the formation and distribution of porosity.

Corrosion Mechanism of Co-based Surface Finishing Layer: Si-Wei Lin1; Albert T. Wu1; 1National Central University
    The reliability of electronic devices in harsh conditions becomes significant for rapid development of 5G network and automobile industry. When devices are exposed to air-polluted environment, corrosion of the surface finishing layer could damage the reliability of the devices. In the study, corrosion behaviors of Co-based films on printed circuit boards (PCBs) are investigated. Electroless-Co and Electroless-Co/Electroless-Pd/Immersion Au layers were deposited on PCBs for comparison. Samples were placed in a chamber at 80 。C and 100% relative humidity (RH) with different concentration of SO2 for corrosion tests. After corrosion tests, the morphology and components of corrosion products were characterized. The electrochemical analysis provides the understanding of the corrosion mechanism. The results show that the Co-based layers can effectively enhance the corrosion resistance of the PCBs.

Interfacial Reaction between Cu and In-48Sn Alloy: Fu-Ling Chang1; Han-Tang Hung1; C. Robert Kao1; 1National Taiwan University
    Nowadays, many applications require low soldering temperature process to avoid chip warpage during soldering. Two low melting temperature alloys, In-48Sn and Sn-58Bi, are receiving the most attentions. Sn-58Bi, with superior fatigue resistance and tensile strength, has been widely used in the industry. However, the brittle property of Bi-containing phases restricts the application of this materials. In-48Sn, with excellent mechanical properties and thermal conductivity, has not been fully investigated yet. Therefore, this study focused on the interfacial reactions between Cu and liquid/solid In-48Sn solder, especially the appearance of the Cu2In3Sn phase at low temperature. In order to obtain an artifact-free microstructure that is not damaged by mechanical polishing, cryogenic broad Ar beam ion polishing was applied. Electron probe micro-analyzer and X-Ray diffractometer were also used to identify the composition and phase structure at the interface.

Development of Silver─Tin Alloy Paste for High Power IC Packaging by High Energy Ball Milling: Wei Chen Huang1; Chin-Hao Tsai1; C. Robert Kao1; 1National Taiwan University
    Ag–Sn alloy paste for die attachment of high power devices packaging is developed. The alloyed powders are prepared by high-energy ball milling of Ag and Sn powders in an argon atmosphere. The mechanical alloying process is performed in a planetary ball mill with a series of milling speeds for 2, 6, and 10h to determine the optimal milling parameters. Characterization of the powders is carried out by XRD and SEM. The die bonding is carried out between metalized Cu substrates and the sintering process is analyzed by cross-sectional observation. Structural integrity of the Ag–Sn paste is investigated by a high-temperature storage test. The joint is comprised of Ag(Sn) solid solution, and Cu(Sn) solid solution formed at the interface between joint and Cu substrate. The die shear strength is approximately 40 MPa obtained at 10 wt.% Sn alloyed powders, and maintain its strength after 2000h of aging at 300⁰C.

Electromigration of Cu-Cu Joints Fabricated by of Highly (111)-oriented Nanotwinned Cu: Shih Chi Yang1; Chih Chen1; 1National Yang Ming Chiao Tung University
     As the size of joints scales down below 10 μm, side wetting and bridging failure would occur in solder joints. Cu-Cu bonding has appeared to be a solution to the limitation of scaling down of solder joints and retard RC delay in 3D IC. However, the strength and reliability of Cu joints are still of concerns. In this study, the Cu joints are fabricated by highly (111)-oriented nanotwinned Cu . We performed instant and post-annealing bonding steps. Tensile tests were conducted to characterize the bonding strength of the Cu joints. Electromigration tests were performed to investigate the reliability of the Cu joints. The failure mechanism and the influence of nanotwinned structure were analyzed after the joint resistance increased 20% of its initial value. The Cu-Cu interface has high electromigration resistance when grain growth takes place across the bonding interface, and the weakest link may locate in the redistribution Cu lines.

Cu-Cu Bonding with Silver Thin Film Capping Layer: Hsiang Hou Tseng1; Chih Chen1; 1National Yang Ming Chiao Tung University
     Cu-Cu bonding has replaced solder joints for ultrahigh density packaging due to its scaling ability and low resistance. However, copper can be oxidized easily and it can lead to poor quality. A thin passivation capping layer is a method to resolve this problem. We adopt Ag passivation to protect Cu from oxidation, because Ag oxides can be reduced to Ag over 180 C and Ag has the lowest resistivity. 100 nm Ti and 200 nm Cu were deposited by sputtering on Si (100) substrate as adhesion layer and seed layer, respectively. A highly (111) nanotwinned copper (nt-Cu) film was deposited on the substrate by electroplating and chemical mechanical planarization (CMP) was conducted. After that, a 20 nm silver film was sputtered on the nt-Cu film. Two specimens were bonded together under 1MPa and bonded at 200 ℃for 60 minutes successfully, and the results will be presented in the conference.

Low Temperature Direct Bonding in Atmosphere on Highly (111) Oriented Nanotwinned Silver: Ching-Yao Cheng1; Po-Hsien Wu1; Fan-Yi Ouyang1; 1Dept. of Engineering & System Science, National Tsing Hua University
    In the development of 3D IC technology, metal-to-metal direct bonding has been regarded as an important technique to to enable heterogeneous integration. In this study, we proposed to adopt highly (111) oriented nanotwinned Ag films as bonding materials due to their good electrical and mechanical properties. The resistivity and hardness of Ag films is around 2.03 μΩ-cm and 1.9 GPa, respectively. The Ag-to-Ag direct bonding using highly (111) oriented nanotwinned Ag films and bumps can be achieved by thermo compression process at 200 ℃ under air atmosphere within a short time. The bonding ratio could be higher than 90 %, and the bonding strength could reach 70.0 MPa in the shear test. Furthermore, the Ag films and bumps remained nanotwinned structure after bonding. Meanwhile, the Ag-to-Ag bonding samples could provide good specific contact resistance. The corresponding bonding mechanism would be discussed in more details in this work.

Low Temperature and Presureless Cu-to-Cu Direct Bonding by Sintering of Green Synthesized Cu Nanoparticles: Albert T. Wu1; Wei Liu1; Chang-Meng Wang2; 1National Central University; 2SHENMAO Technology Inc.
    Cu nanoparticle (NP) sintering is a promising technique to form Cu bond that can be applied to advance electronic packaging technique for its possible excellent physical properties and reasonable cost. In this study, Cu NPs are prepared by chemical reduction method with non-toxic reducing agent without capping agent. Particle size of the Cu NPs can be controlled by adjusting the pH values of the reducing solution. The synthesized Cu NPs are mixed with a bio-compatible solvent to form Cu NP paste and bonds Cu substrates at low temperature without pressure. An iterating reduction mechanism is proposed. The oxide layer on the Cu NP acts as a catalyst which triggers the decomposition of the solvent at low temperature. The decomposed products further reduce the oxide layer and allow the sintering of Cu NP without reducing atmosphere. A high quality Cu-to-Cu bond can be achieved by choosing proper synthesizing parameters.

Synchrotron X-ray Study of Cu Electromigration in A Blech Structure: Pei-Tzu Lee1; Cheng-Yu Lee2; Shao-Chin Tseng3; Mau-Tsu Tang3; C. Robert Kao1; Cheng-En Ho2; 1National Taiwan University; 2Yuan Ze University; 3National Synchrotron Radiation Research Center (NSRRC)
    Electromigration of Cu interconnects has been a critical reliability concern in semiconductor package for past decades and a dynamic, nondestructive characterization on this issue is still quite lacking to date. In this study, we conducted a real-time analysis on the Cu electromigration in a Blech structure via nano-X-ray fluorescence (nano-XRF) microscopy using synchrotron light source at the beamline 23A, Taiwan Photon Source. The Cu depletion at the cathode and Cu hillock/extrusion formation at the anode upon electron current stressing of 8.3 × 105 A/cm2 were in-situ characterized using nano-XRF in a vacuum environment (5 × 10-7 torr). Furthermore, the detailed crystallographic microstructure of Cu interconnects after electromigration were analyzed using electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The research findings would greatly advance our fundamental understandings of Cu electromigration mechanism, which is very beneficial to the interconnect reliability reinforcement. Details will be presented in this talk.

Effect of Geopolymer Ceramic Reinforcement to the Microstructure Formation and Mechanical Properties of Sn-0.7Cu Solder Joint: Mohd Izrul Izwan Ramli1; Mohd Arif Anuar Mohd Salleh1; Nur Nadiah Izzati Zulkifli1; Nur Syahirah Mohamad Zaimi1; Mohd Mustafa Albakri Abdullah1; 1Universiti Malaysia Perlis (UniMAP)
    This paper investigates the effect of kaolin geopolymer ceramic (KGC) reinforcement to the microstructure formation and mechanical properties of Sn-0.7Cu lead-free solder. Besides conventional cross-section techniques, advanced characterization techniques such as synchrotron micro-XRF and tomography imaging were used to study the microstructure of the kaolin geopolymer ceramic and the composite solder. Experimental results show that the kaolin geopolymer ceramic refines the β-Sn area while increases the eutectic area. Synchrotron micro-XRF also indicated that some Al and Si element, which major in kaolin geopolymer system were in the solder matrix area. The result of the mechanical properties of this composite solder also showed that the addition of kaolin geopolymer ceramic has higher strength and can be proposed as a potential reinforcement material in composite solder.

Low-thermal-budget Hybrid Cu-Cu Bonding with Highly <111>-oriented Nanotwinned Cu and Polyimide: Pin-Syuan He1; Kai-Cheng Shie1; Chih Chen1; 1National Yang Ming Chiao Tung University
    In this study, highly <111>-oriented nanotwinned Cu (nt-Cu) was combined with polyimide (PI) to achieve the low thermal budget hybrid bonding (<250 ℃). We adopted three bonding scenarios to confirm the possibility of this approach: (A) PI-PI, (B) Cu/PI-PI, and (C) Cu/PI-nt-Cu bonding. For the PI-PI bonding, the imidization degree (DI) increased significantly and bonding interface was eliminated if the PI partial curing condition was 230 ℃ and below 0.3tfull curing. For the Cu/PI-PI bonding, the bonding interface of PI/PI was removed and an excellent adhesion between Cu pad and PI film was obtained. For the Cu/PI-nt-Cu bonding, we found an obvious gap between Cu pad and nt-Cu film due to Cu pad dishing and dome shape PI caused by hybrid structure co-planarization issue. Thus, to achieve the nt-Cu/PI hybrid bonding, we can predict the required Cu protrusion with the mismatch of Cu and PI in linear expansion coefficients.

Effect of Grain Size and Stress Relaxation on Whisker Growth under Applied Pressure: Nupur Jain1; Piyush Jagtap1; Allan Bower1; Eric Chason1; 1Brown University
    Sn whiskers are a well-known problem affecting the reliability of electronic components. Stress is generally believed to provide the driving force behind whisker formation, yet their growth mechanism is still not fully understood. Through experiments, we have observed real-time whisker growth in Sn films under an applied pressure. In this work we extend previous studies to different film thicknesses and grain sizes to quantify whisker density and growth kinetics in systems with different strain relaxation rates. Using a finite element model, we analyze the evolution of stress in Sn films of different thicknesses and grain sizes. We report on the predicted whisker growth kinetics from the model and compare with the experiments.

In-situ Observations of Sn-58Bi/SAC305 Joints by Synchrotron Imaging: Mohd Arif Anuar Mohd Salleh1; M.I.I. Ramli1; D.S.C. Halin1; N. Saud1; T. Nishimura2; H. Yasuda3; K. Nogita4; 1Universiti Malaysia Perlis; 2Nihon Superior Co. Ltd; 3Kyoto University; 4The University of Queensland
    The interaction and process of mixed assemblies facilitated by the use of low temperature solders is of increased importance. This paper investigates the solid-liquid interaction between liquid Sn-58Bi solder paste and solid Sn-3.0Ag-0.5Cu (SAC305) solder alloy during soldering on Cu organic soldering preservative (Cu-OSP) substrate. The effect of reflow time and temperature (lower than liquidus temperature of SAC305) on the solid/liquid interaction in Sn-58Bi/SAC305 mixed assemblies have been studied using an in-situ synchrotron imaging technique. The dissolution rate of solid SAC305 alloy into Sn-58Bi and microstructures in the solder joints is discussed with respect to the soldering time and temperature.

The Role of Lengthscale in the Creep of Sn-3Ag-0.5Cu Solder with Controlled Microstructure: Tianhong Gu1; Christopher Gourlay2; Ben Britton3; 1University of Birmingham; 2Imperial College London; 3The University of British Columbia
    Sn-3Ag-0.5Cu wt.% is widely used as a solder alloy in electronic interconnections, where the life of the components can be limited through thermomechanical failure of the solder joints. Here we assess the effect of secondary dendrite arm spacing (λ2), eutectic intermetallic spacing (λe) and intermetallic compound (IMC) size in SAC305 solder on creep in samples containing a single crystal of beta-Sn with near-<110> orientation along the loading direction. Creep is investigated under constant load tensile testing at a range of temperatures (298 - 473 K) to quantify the role of these lengthscale effects on the secondary creep strain rate and activation energy. The deformation mechanisms are investigated using electron backscatter diffraction (EBSD) and strain heterogeneity is identified between beta-Sn in dendrites and beta-Sn in eutectic regions containing Ag3Sn and Cu6Sn5 particles. This motivates our hypothesis about the role of these microstructural features and creep performance.

Thermal Cycling Reliability Tests of Cu-Cu Joints by Two-step Bonding Processes: Jia Juen Ong1; Kai Cheng Shie1; Chih Chen1; 1National Yang Ming Chiao Tung University
    This study shows the relationship between the reliability of Cu joints and their microstructures. We adopted a 2-step bonding process to fabricate Cu-Cu joints, which includes a short-time initial bonding at 300 C for tens of seconds, and a post-annealing process at 300 C for 1 h We investigate failure modes for the thermal cycling tests after 1000 cycles. Results show that the resistance of samples without post-annealing increased significantly. While the resistance of the 2-step bonding slightly increased. We found cracks formed at the bonding interface of the as-bonded samples and such a cracking failure causes the increase in resistance. However, the cracks do not lead to failure of the joints in the post-annealed samples due to the growth of oriented grain across the bonding interface. Failure analysis show that cracks propagated along the zig-zag grain boundaries and slow down crack propagation and enhance the bonding quality and reliability.