Weisheng Xia

Fabrication and electrochemical performance of solid oxide fuel cell components by atmospheric and suspension plasma spray

Abstract The theory of functionally graded material (FGM) was applied in the fabrication process of PEN (Positive-Electrolyte-Negative), the core component of solid oxide fuel cell (SOFC). To enhance its electrochemical performance, the functionally graded PEN of planar SOFC was prepared by atmospheric plasma spray (APS). The cross-sectional SEM micrograph and element energy spectrum of the resultant PEN were analyzed. Its interface resistance was also compared with that without the graded layers to investigate the electrochemical performance enhanced by the functionally graded layers. Moreover, a new process, suspension plasma spray (SPS) was applied to preparing the SOFC electrolyte. Higher densification of the coating by SPS, 1.61%, is observed, which is helpful to effectively improve its electrical conductivity. The grain size of the electrolyte coating fabricated by SPS is also smaller than that by APS, which is more favourable to obtain the dense electrolyte coatings. To sum up, all mentioned above can prove that the hybrid process of APS and SPS could be a better approach to fabricate the PEN of SOFC stacks, in which APS is for porous electrodes and SPS for dense electrolyte.

Fabrication and characterization of Cu-Sn-Ni-Cu interconnection microstructure for electromigration studies in 3D integration

Purpose The purpose of this paper is to fabricate a new Cu-Sn-Ni-Cu interconnection microstructure for electromigration studies in 3D integration. Design/methodology/approach The Cu-Sn-Ni-Cu interconnection microstructure is fabricated by a three-mask photolithography process with different electroplating processes. This microstructure consists of pads and conductive lines as the bottom layer, Cu-Sn-Ni-Cu pillars with the diameter of 10-40 μm as the middle layer and Cu conductive lines as the top layer. A lift-off process is adopted for the bottom layer. The Cu-Sn-Ni-Cu pillars are fabricated by photolithography with sequential electroplating processes. To fabricate the top layer, a sputtered Cu layer is introduced to prevent the middle-layer photoresist from being developed. With the final Cu electroplating processes, the Cu-Sn-Ni-Cu interconnection microstructure is successfully achieved. Findings The surface morphology of Cu-Sn pillars consists of densely packed clusters which are formed by an ordered arrangement of tetragonal Sn grains. The diffusion of Cu atoms into the Sn phases is observed at the Cu/Sn interface. Furthermore, the obtained Cu-Sn-Ni-Cu pillars have a flat surface with an average roughness of 13.9 nm. In addition, the introduction of Ni layer between the Sn and the top Cu layers in the Cu-Sn-Ni-Cu pillars can mitigate the diffusion of Cu atoms into Sn phases. The process is verified by checking the electrical performance using four-point probe measurements. Originality/value The method described in this paper which combined a three-mask photolithography process with sequential Cu, Sn, Ni and Cu electroplating processes provides a new way to fabricate the interconnection microstructure for future electromigration studies.

Thermal warpage analysis of PBGA mounted on PCB during reflow process by FEM and experimental measurement

Purpose – The purpose of this paper is to study the thermal warpage of a plastic ball grid array (PBGA) mounted on a printed circuit board (PCB) during the reflow process. Design/methodology/approach – A thermal-mechanical coupling method that used finite-element method software (ANSYS 13.1) was performed. Meanwhile, a shadow moire apparatus (TherMoire PS200) combined with a heating platform was used for the experimental measurement of the warpage of PBGA according to the JEDEC Standard. Findings – The authors found that the temperature profiles taken from the simulated results and experimental measurement are consistent with each other, only with a little and acceptable difference in the maximum temperatures. Furthermore, the maximum warpage measurements during the reflow process are 0.157 mm and 0.149 mm for simulation and experimental measurements, respectively, with a small 5.37 per cent difference. The experimental measurement and simulated results are well correlated. Based on the validated finite element model, two factors, namely, the thickness and dimension of PCB, are explored about their effect on the thermal warpage of PBGA mounted on PCB during the reflow process. Practical implications – The paper provides a thorough parametrical study of the thermal warpage of PBGA mounted on PCB during the reflow process. Originality/value – The findings in this paper illustrate methods of warpage study by combination of thermal-mechanical finite element simulation and experimental measurement, which can provide good guidelines of the PCB design in the perspective of thermal warpage during the reflow process.

Microstructural evolution of the intermetallic compounds in the high density solder interconnects with reduced stand‐off heights

Purpose – This paper aims to investigate the microstructural evolution rules of the intermetallic compound (IMC) layers in high‐density solder interconnects with reduced stand‐off heights (SOH).Design/methodology/approach – Cu/Sn/Cu solder joints with 100, 50, 20 and 10 μm SOH were prepared by the same reflow process and isothermally aged at 150°C. The IMC microstructural evolution was observed using scanning electron microscopy.Findings – The whole IMC layer (Cu3Sn + Cu6Sn5) grew faster in the solder joints with lower SOH because of the thinner IMC layer before aging. Also, the IMC proportion increased more rapidly in solder joints with the lower SOH. In all solder joints with different SOH, the growth rates of the Cu3Sn (ϵ) layers were similar, and slowed down with increasing aging time. The Cu6Sn5 (η) was consumed by the Cu3Sn (ϵ) growth at the beginning of the aging stage; while it turned to thickening after a period of aging. Finally, the Cu6Sn5 thickness was similar in all the solder joints. It is i...

The size dependency of full IMC solder joint for 3D interconnection

The further miniaturization of electronic devices brings the challenge to the electronic packaging, especially for 3D interconnection as the miscrostructure of the solder joint could be replaced by IMC fully without any solder bulk remain. In this paper, both FE modeling and experimental testing were applied to investigate the stress distribution and the fracture mode of the solder joint under tensile load by considering the effect of IMC type and portion. Then, the size dependency regarding the IMC properties on grain size, defects, and crystallographic orientation has been pointed out to help understanding the reliability of full IMC joints.

Research on the microstructure and shearing property of microbumps with one Sn grain for high density solder interconnects

In order to meet the requirement of the higher density interconnects, the volume size of the solder microbumps continues to be reduced. In the present research, the microstructure of the microbumps with a diameter of 40μm was studied using Electron Backscatter Diffraction (EBSD). When the microbumps are smaller than a Sn grain in size, only one Sn grain is found to be contained in the microbumps. Shearing tests were carried out to study the mechanical behavior and the fracture mode. The strain rate sensitivity exponent of the microbump with one Sn grain is about 0.1. And a typical sliding fracture of the Sn grain is observed at different shearing speeds, which is different from the usual microvoids accumulation fracture displayed by BGA solder joints. The research findings are useful to assess the reliability of the high density solder interconnects, such as fine pitch flip chip, 3D-TSV, copper pillar bumps, and so on.

Effect of heat sink on electromigration lifetime of Ni thin film

Thermal design and thermal management are the key issues for electronic products during the trend of miniaturization. Heat sink is one of the promising and popular choices to relieve the thermal problem. In this paper, the effect of heat sink on electromigration (EM) lifetime of Ni thin films is investigated. It is proved that heat sink had no influence on the temperature coefficient of resistance (TCR), but could reduce the temperature rise (TR) and raise the breakdown voltage efficiently. The failure data with time obtained by the accelerated lifetime test were analyzed by ALTA software based on the Temperature-Non Thermal (T-NT) lognormal model. The maximum allowable voltage under the service condition could be obtained based on 5 years lifetime and 100% duty cycle with 0.05% cumulative distribution function (CDF). In summary, heat sink could obviously improve the EM lifetime of Ni thin film due to its excellent heat dissipation ability.

Preparation, microstructure and properties of Sn-Ag-Cu solder reinforced with Al 2 O 3 nanoparticles

Sn3.0Ag0.5Cu(SAC)-XAl<inf>2</inf>O<inf>3</inf>(X = 0.2, 0.4, 0.6, 0.8 wt.%) composite solders reinforced with Al<inf>2</inf>O<inf>3</inf> nanoparticles were prepared through the powder metallurgy route (ball milling, compacting and sintering). After ball milling, the morphology of composite powder was studied by scanning electron microscopy (SEM). In addition, the prepared composite solders were characterized in terms of their microstructure, wettability and microhardness. After mixing by ball milling, the Al<inf>2</inf>O<inf>3</inf> nanoparticles were found embedded into the surface of Sn-Ag-Cu solder powder. Moreover, microstructure observation proved Al<inf>2</inf>O<inf>3</inf> nanoparticles were successfully incorporated into the SAC solder. Finer microstructure with smaller β-Sn grains was achieved with the addition of Al<inf>2</inf>O<inf>3</inf> nanoparticles. On the other hand, wettability was improved by moderate addition of Al<inf>2</inf>O<inf>3</inf> nanoparticles. 12.6% decrease in contact angle was achieved with 0.4 wt.% Al<inf>2</inf>O<inf>3</inf> nanoparticles addition. Microhardness of composite solders increased with the increasing content of Al<inf>2</inf>O<inf>3</inf> nanoparticles. This improved mechanical property can be attributed to the refined microstructure and the dispersed Al2O3 nanoparticles.

Electromigration — Induced failure mechanism and lifetime prediction in NiCu thin film

NiCu is one of the widely used thin-film materials, which is now commonly used as sensor and resistor in very-large-scale-integration (VLSI) because of its high resistivity and stability. However, with the scale decreasing and the temperature increasing in service condition, the limited reliability of NiCu thin-film material has been a key issue in microelectronics and packaging industry mainly due to the current induced electromigration (EM). In this paper, a sandwich-like NiCu thin-film resistor (Ta/NiCu/Ta) with the thickness of 120 nm was fabricated on the glass substrate by sputtering technique. The resistance of the thin-film structure was proved to increase with time during test. The mean-time-to-failure (MTTF) analysis based on the Blacks equation found that the activation energy (Ea) was 0.99 eV and the model parameter for current density of Blacks equation was close to 3. Additionally, multi-voids were observed in the thin-film, which can be explained by the Ni diffusion along the barrier layers of Ta crystals. The results in this study proved that multi-voids formation by Ni diffusion is the main reason that leads to EM failure.

Effect of SiC whiskers addition on microstructure, microhardness and wettablility of Sn-Ag-Cu solder

A novel SiC doped SAC305 composite solder was developed by mechanically mixing various amount of the SiC whiskers with Sn-3.0Ag-0.5Cu solder paste. The effect of SiC whiskers addition on microstructure, microhardness and wettability of the composite solders was investigated. It was found that the eutectic structure was refined in the composite solder, and both of the microhardness and wettability of the composite solders were improved. Specifically, 0.1 wt.% of SiC whiskers addition in composite solder lead to 29.2% increase in hardness while 0.2 wt.% of SiC whiskers addition cause a 6.3% decrease in the contact angle.