Albert T Wu

• PhD
• National Central University

This study investigates the properties of two Sn-Ag alloys with distinct compositional modifications. The Sn4Ag3Bi0.05Ni (SABN) alloy was engineered to enhance the reliability of solder joints in automotive electronics under extreme conditions. The addition of Bi improves mechanical properties by forming a solid solution with Sn, while the incorpor...

Low-temperature bonding has become a significant trend in advanced electronics packaging technology. A low-reflow-temperature process can effectively reduce the risk of warpage, thus greatly enhancing device reliability. SnBi eutectic solder is one of the best candidates for low-temperature assembly. In this study, first, a layer of pre-solder (Sn-...

Background Corrosives substance in polluted air causes severe reliability issues for electronic devices. One of the critical purposes of a surface finishing layer on a printed circuit board is to increase the corrosion resistance of the conductive Cu layers. However, the currently available technologies for consumer products have difficulties meeti...

A pressureless low-temperature Cu-embedded intermetallic joint for interconnects is fabricated using a Cu-based mixed paste containing different amounts of SAC305 (SAC) particles. The highest joint's shear strength reaches 24.2 MPa. The average of the shear strength is 19.2 (±4.3) MPa, which is also high for pressureless solid-state bonding. After...

Background Low temperature bonding is the key for developing advanced electronic packaging technique. While Cu is regarded as a promising material for interconnects with high conductivity, the oxidation issue during processing needs to be solved. Methods In this study, Cu-to-Cu direct bonding was achieved at low temperature without applying pressu...

The literature has accumulated plenty of interesting findings of electromigration-induced phenomena in pure Sn. Most of the researches revealed the thermodynamically steady states of materials under electro-migration. We presented a comprehensive study of electromigration in pure Sn at 5.5-7.5 × 10 3 A/cm 2 for 5.5 h revealing the effects on crysta...

Bismuth-telluride (Bi2Te3) based thermoelectric materials are well known for their high figure-of-merit (zT value) in the low-temperature region. Stable joints in the module are essential for creating a reliable device for long-term applications. This paper used electroless Co-P to prevent a severe interfacial reaction between the joints of solder...

PbTe-based alloys are potential mid-temperature thermoelectric materials due to their excellent thermoelectric properties. Formation of intermetallic deteriorates mechanical joint strength and thermoelectric performance as well. In the present work, interfacial reaction, electrical and mechanical behaviors for both p- and n-PbTe joints with an addi...

In automotive technology, the corrosion resistance of variable surface finishes on automobile printed circuit boards (PCBs) represents a crucial topic. The key purpose of these surface finishes is to prevent corrosion between the Cu substrates in PCBs and corrosive SO2 gas in the automobile's working environment. In this study, the surface finishes...

The effects of the grain size and thickness of Sn films on the kinetics of spontaneous Sn whisker growth have been quantitatively analyzed separately. The results reveal that the length and diameter of the whiskers are related to the parameters of the Sn film. A thicker film has a stronger influence on the whisker morphology than does the grain siz...

In this study, the interfacial reactions, microstructures, and intermetallic compounds (IMCs)of 68In–32Bi, 50In–50Bi, and 33In–67Bi (in weight percent)low melting alloys reacted with Cu substrates were investigated. CuIn2, Cu11In9, and Cu2In IMCs formed at the interface in different In–Bi/Cu systems and were characterized by grazing incidence X-ray...

(Bi0.25Te0.75)2Te3 (p-Bi2Te3) is thermoelectric material that can harvest waste heat into useful electric power. A severe reaction between p-Bi2Te3 and Sn-based solder decreases the reliability of thermoelectric modules. Sn/p-Bi2Te3 and Sn3.0Ag0.5Cu (SAC305)/p-Bi2Te3 with and without electroless Co-P at the interfaces were investigated in this stud...

Engineering transport properties of thermoelectric (TE) materials leads to incessantly breakthroughs in the zT values. Nevertheless, modular design holds a key factor to advance the TE technology. Herein, we discuss the structures of TE module and illustrate the inter-diffusions across the interface of constituent layers. For Bi2Te3-based module, s...

Co-sputtered Cu(Ti) films on SiO2 substrates exhibit the phase separation of Cu and Ti after annealing at 400 °C. In this study, the diffusion behavior and phase separation were investigated using X-ray photoelectron spectroscopy (XPS), grazing angle incident X-ray diffraction (GIXRD), and transmission electron microscopy (TEM). The GIXRD pattern r...

This paper investigates the interfacial reaction between Sn and Sn3Ag0.5Cu (SAC305) solder on n-type Bi2Te3 thermoelectric material. An electroless Ni-P layer successfully suppressed the formation of porous SnTe intermetallic compound at the interface. The formation of the layers between Bi2Te3 and Ni-P indicates that Te is the dominant diffusing s...

This study explores and compares two new alloys, namely 50In-50Bi and 17Sn-26In-57Bi (wt%), with melting peak temperatures of 95 and 82 °C, which are substantially lower than the melting points of lead-free solders commonly used for low-temperature applications. The microstructures and intermetallic compounds (IMCs) of the new alloys, which were re...

This work describes an in-depth study on the electroless deposition of Co–P via the addition of a reductant to a solution on a roughened, sensitized, and activated n-PbTe thermoelectric substrate. The Co–P layer facilitates the bonding of the thermoelectric module with the solder paste and serves as a diffusion barrier that prevents severe interact...

This article addresses the reliability challenges regarding electromigration in developing three-dimensional integrated circuits (3D-ICs). The line-type sandwich structure of Ni/Sn3.5Ag(15 μm)/Ni was used to simulate microbumps to examine the reliability of electromigration in 3D-IC technology. The solder strip of Ni/Sn3.5Ag(15 μm)/Ni was stressed...

Flow-rate interruption (FRI) atomic layer deposition (ALD) technique was adopted to fabricate AZO/Al2O3/AZO thin film on a ZnO nanorod array template at low temperature. The high quality amorphous dielectric Al2O3 layer was deposited at 50 °C. The template with an average of 0.73 μm in length was made by a simple hydrothermal method on a c−plane sa...

Through a simple hydrothermal method, well-aligned and periodic honeycomb-like ZnO nanorod arrays were fabricated on a c-plane sapphire with an aluminum-doped ZnO (AZO) seed layer. Vertical and highly ordered ZnO nanostructures with the <0002> orientation were synthesized by employing a self-assembled monolayer of polystyrene (PS) microspheres as a...

In the present study, Cu bonds were fabricated through face-to-face thermocompression by co-sputtering Cu(Ti) films. The diffusion behaviors and bonding strengths at various Ti concentrations were analyzed. Phase separation occurred when Ti diffused toward the substrate and Cu moved to the surface with limited oxidation. A high affinity between Ti...

Due to the increase of air pollution in recent years, the sulfur content increases in the atmosphere. Creep corrosion occurs on the electronic components caused by high sulfur (S) environment and high moisture. This phenomenon enhanced the failure time for the Printed Circuit Boards (PCBs) in the consumer products, automotive engine and air polluta...

An in situ electromigration study has been conducted on U-groove Cu/Sn-3.5Ag/Cu and Ni/Sn-3.5Ag/Ni sandwich structures; the results were used to simulate microsolder joints passing current density of 1 × 10⁴ A/cm² at 150°C. The solder gap was only 15 μm, shorter than the critical length of Sn-3.5Ag solder. Backstress was proved to exist at critical...

P-N junction diodes with excellent rectifying characteristics were prepared by segregating the Ge nanocrystals of Si1−xGex thin films deposited by co-sputtering. The current-voltage characteristics in darkness and under illumination were studied. The correlation between the p-n junction performance and the microstructure of the films is discussed,...

In advanced 3D IC assembly, one of the key processes is to thin down the wafer for multiple-layer stacking vertically. Warpage is commonly seen in the IC chips due to the differences in coefficient of thermal expansion of the materials. The conventional peak temperatures for reflowing Sn-base Pb-free solders are above 230 °C. High temperature therm...

This paper discusses how the microstructures of Sn films affect the kinetics of spontaneous Sn whisker growth. Thin films and those with small grains grew longer whiskers at higher rates than did thick films and those with large grains. Biaxial stresses in the films were measured using synchrotron radiation X-ray; the stress evolution during anneal...

This study presented the disruption of the Sn and Ag3Sn lattice structures of Sn3.5Ag solder induced by electric current at 5–7 × 103 A/cm2 with a high resolution transmission electron microscope investigation and electron diffraction analysis. The electric current stressing induced a high degree of strain on the alloy, as estimated from the X-ray...

This study investigated the effect of massive spalling of intermetallic compounds (IMCs) on the microstructural evolution and mechanical properties of solder joints after multiple reflows. SAC305 solder was reflowed on substrates with four Co-based surface finishes: electroless Co (EC), electroless Co/immersion Au (ECIG), electroless Co/electroless...

A dislocation density of as high as 1017 /m2 in a tin strip, as revealed by high resolution transmission electron microscope, was induced by current stressing at 6.5 x 103 A/ cm2. The dislocations exist in terms of dislocation line, dislocation loop, and dislocation aggregates. Electron Backscattered Diffraction images reflect that the high disloca...

A new ternary solder alloy Sn62Bi32Ga6 (in wt.%) with melting point of 128°C, which is 10°C lower than that of eutectic Sn-Bi, has been developed. The composition was obtained by adding the maximum possible amount of Ga without causing Ga segregation and the formation of a liquid phase inside the solder. Solders were reflowed on Cu substrates to in...

Ni/Sn-2.5Ag/Ni samples were used to simulate the microbumps in three-dimensional (3D) packaging. The annealed test was adopted to observe the microstructure of intermetallic compound formation at 100°C, 125°C, and 150°C up to 1000 h. In the Ni/Sn-2.5Ag/Ni, predominant phases of layer-type Ni3Sn4 and Ag3Sn particles could be seen under the thermal t...

This paper investigates massive spalling that occurs in the intermetallic compounds at the interfaces between Pb-free SAC 305 and Co-based surface finishes. The SAC 305 solders were reflowed on the Co-based substrates and aged at different conditions. The samples were cross-sectioned to study the interfacial morphology. The occurrence of massive sp...

Amorphous Si1-xGex films were prepared by co-sputtering on an oxidized Si wafer, followed by rapid thermal annealing to form nanocrystal films. The formation of Ge nanocrystals was not at thermodynamic equilibrium formed in the amorphous Si1-xGex matrix. High-resolution transmission electron microscopy was used to characterize the increase in the s...

Three-dimensional packaging provides an acceptable solution for miniaturized integrated circuits. Because of the technological flexibility required for combining various modules to form a functional system, miniaturization can be achieved by using embedded techniques that could enhance the reliability of assembled systems. Because the mismatch of t...

The variations in the crystal structure of Sn9Zn alloy due to electrical current stressing were investigated with in situ synchrotron XRD analysis. The XRD (X-ray Diffraction) orientation peaks of both Sn and Zn crystals diminished rapidly upon current stressing. The behavior of peak diminishing indicated the electrodisruption of the crystal struct...

This study sought to clarify the relationship between cracks in surface oxide layers and the growth behavior of tin whiskers. The number, length, and total volume of extrusions were precisely calculated and residual stress was measured using synchrotron radiation x-ray diffractometry. The aim was to elucidate the influence of stress on the driving...

Because of the miniaturization of electronic devices, the reliability of electromigration has become a major concern when shrinking the solder dimensions in flip-chip joints. Fast reaction between solders and electrodes causes intermetallic compounds (IMCs) to form, which grow rapidly and occupy entire joints when solder volumes decrease. In this s...

This study investigates electromigration in Bi2Te3 thermoelectric (TE) material systems and the effectiveness of the diffusion barrier under current. The Peltier effect on the interfacial reaction was decoupled from the effect of electromigration. After connecting p- and n-type Bi2Te3 to Sn3Ag0.5Cu (SAC305) solders, different current densities were...

To decrease the size of portable devices, this study incorporates a stacked three-dimensional integrated circuit architecture into advanced packaging techniques. The traditional FR-4 substrate was substituted with silicon interposers. Because silicon is rigid and highly resistant to deformation, this minimizes thermal stress caused by thermal expan...

A back-fill Sn flow is reported against the current-stressing at Sn/Cu micro-joint interface. Upon current-stressing, as the cathode Cu 6Sn5 compound is dissolving into adjacent Sn solder matrix, Cu in the dissolving Cu6Sn5 compound region would be highlydepleted, which would likely transform to highly-vacant Sn grains. Thus, a large Sn concentrati...

This study investigates the interfacial reactions and microstructures of SAC305 solders on four different Co-based surface finishes, electroless Co (EC), electroless Co/immersion Au (ECIG), electroless Co/electroless Pd (ECEP), and electroless Co/electroless Pd/immersion Au (ECEPIG). The evolution of microstructure at different reflow conditions re...

Various microstructural zones were observed in the solidified solder of flip-chip solder joints with three metal bond-pad configurations (Cu/Sn/Cu, Ni/Sn/Cu, and Cu/Sn/Ni). The developed microstructures of the solidified flip-chip solder joints were strongly related to the associated metal bond pad. A hypoeutectic microstructure always developed ne...

High-texture surface tin oxide transparent conductive oxide thin films were directly deposited using the atmospheric pressure chemical vapor deposition technique on nanoparticle-coated glass substrates. A simple nozzle-spraying process was developed for the tin oxide nanoparticle coating process for the high-texture tin oxide thin films. The nozzle...

Antimony-doped tin oxide (SnO2:Sb) thin films were prepared by atmospheric pressure chemical vapor deposition. Precursors were mixed with gaseous SnCl4, SbCl5, and oxygen. Both antimony and chlorine ions became involved in doping and reduced resistivity. The figure of merit suggested that films deposited at 500 °C with the ratio of SnCl4/SbCl5 equa...

An electroless Ni layer is often chosen as an effective diffusion barrier in thermoelectric modules. Thermal aging can form a thick layer of NiTe. This study investigates the growth kinetics of NiTe intermetallic compounds (IMCs). The apparent activation energy was determined to be 70.9 kJ/mol. Ni/Co was also selected as an alternative barrier beca...

This study investigates the stability of the interfaces between an n-type thermoelectric material, Bi2Te3, and pure tin solders in a thermoelectric module. The module was stressed by a current at elevated temperatures. A map of the failure criteria of the module was constructed. Ni was used as the diffusion barrier between the solder and the thermo...

Antimony-doped tin oxide (SnO2:Sb, ATO) films have been deposited on glass substrates using atmospheric pressure chemical vapor deposition (APCVD) method. The precursors are mixed with SnCl4, SbCl5, and O2 to prepare the films. This study used synchrotron grazing incidence X-ray diffraction (GIXRD) to investigate the film microstructure. Our result...

Antimony-doped tin oxide (SnO2:Sb, ATO) films have been deposited on glass substrates using atmospheric pressure chemical vapor deposition (APCVD) method. The precursors are mixed with SnCl4, SbCl5, and O2 to prepare the films. This study used synchrotron grazing incidence X-ray diffraction (GIXRD) to investigate the film microstructure. Our result...

Bi-metallic thin film redistribution layers, deposited in the Through-Silicon-Via (TSV), were stressed by electrical current for failure analysis. A new mechanism suggested that the formation of intermetallic compounds, at the corners of the vias, redistributed and reduced the electrical current at the current crowding region. This study proposes a...

The shrinking solder dimensions greatly impact the reliability of devices and increase entire failure modes. Limited solder volumes can be consumed completely and transformed into intermetallic compound (IMC) microbumps. Microvoids surface when microbumps are formed and may be attributed to a mismatch of the thermal expansion coefficient between th...

This study achieved controlling the positions of spontaneous growth of tin whiskers. We surmounted the unpredictable growing nature of such whiskers and performed accurately quantitative analyses of the growth kinetics and yielded precise measurement of the growth rate. Furthermore, using synchrotron radiation x-ray, this study determined the stres...

Sol-gel SiO2 anti-reflection (AR) coating on solar glass is known to increase the current output by a few percent, but its mechanical durability is of concern. To improve its strength, the amorphous SiO2 may be replaced by zeolite, which is a microporous aluminosilicate crystalline material. Scratch-resistant AR coating has been prepared by the dip...

Ni3Sn4 grains were formed on Ni by reactive wetting between molten eutectic SnPb and thermally annealed Ni foil. Using synchrotron white beam micro x-ray diffraction analysis, two kinds of preferred orientation relationships between Ni3Sn4 and Ni were found. The existence of preferred orientation with large interfacial misfit is suggested as a gene...

In recent years, transparent conductive oxides (TCO) films have been widely used for solar cell due to its high transmittance, low sheet resistance and texture structure that could increase the efficiency. The purpose of this study is to analyze the amount of antimony doped in tin oxide thin films by synchrotron radiation grazing incidence X-Ray di...

The intermetallic compound SnTe rapidly formed at interfaces between p-type bismuth telluride (Bi0.5Sb1.5Te3) thermoelectric materials and lead-free solders. The intermetallic compound influences the mechanical properties of the joints and the reliability of the thermoelectric modules. Various lead-free solder alloys, Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-0.7...

As used in wafer-level bonding in microelectromechanical system (MEMS) devices, the eutectic Al/a-Ge bilayer is characterized by its remarkable hermetic sealing after annealing. For MEMS packaging, this study investigates metal-induced crystallization (MIC) of the amorphous Ge and the layer exchange of Al and Ge, mainly by scanning electron microsc...

This study investigated the effects of adding Bi and In to Sn-3Ag Pb-free solder on undercooling, interfacial reactions with Cu substrates, and the growth kinetics of intermetallic compounds (IMCs). The amount of Sn dominates the undercooling, regardless of the amount or species of further additives. The interfacial IMC that formed in Sn-Ag-Bi-In a...

For the feature of "slim and light" in portable devices, stacked 3D-IC architecture was introduced in the advanced packaging techniques. The traditional FR-4 substrate was substituted by Si substrates. In general, the thickness of Si chip and substrates would be larger than 300 micron. However, silicon is rigid and has high resistance of deformatio...

Due to high performance demand, the dimensions of chips should be reduced so the stacked 3D-ICs are introduced into semiconductor industry. For small substrate dimensions and high interconnects requirements, the FR-4 substrates are substituted by the Si substrates. The Si chips and substrates are thinned down. A rigid material like Si suffers much...

This investigation elucidates stress evolution in situ in tin strips under electromigration using synchrotron radiation x-ray. Minute variations in stress are determined precisely using intense x-rays. Back stresses gradient with the values of 5.5 and 16.5 MPa/cm, which are induced by the current densities of 1 × 103 and 5 × 103 A/cm2, respectively...

Despite the extensive use of Sn-Ag-Cu as a Pb-free solder alloy, its melting point is significantly higher than that of eutectic Sn-Pb solder. Sn-Ag-Bi-In solder is an alternative Pb-free solder, with a melting temperature close to that of eutectic Sn-Pb. This study elucidates the electromigration behavior of Sn-Ag-Bi-In solder and then compares th...

The use of synchrotron radiation x-rays is a powerful and novel technique for determining the strain distribution in flip-chip dies. In this study, thermal and electrical effects on the strain of such dies were investigated insitu using synchrotron radiation x-rays. Intense light and small beam size allow precise measurement of minute strain variat...

The melting point of SnAgBi system solders exceeds that of SnPb solder. Adding In reduces the melting point without reducing the mechanical strength of the joints. In this study, different amounts of In were added to SnAgBi solder systems to investigate the formation of intermetallic compounds at the interfaces and in the bulk solders under reflows...

Tin oxide nano particles dispersed in water solution were sprayed on the tin-plated copper surface and served as coating layer in order to study its effect on the prevention of tin whisker formation. The results indicated that tin oxide nano particles could inhibit the growth of tin whiskers to certain extent. Many hillocks instead of long whiskers...

The growth behavior of the intermetallic compounds that formed at the interfaces between Sn-Ag-Bi-In solders and Cu substrates during solid-state aging is investigated. The compositions of the intermetallic compounds are Cu3(Sn,In) near the Cu substrates and Cu6(Sn,In)5 near the solders; very little Bi or Ag was dissolved in the compounds. The agin...

84Sn-3Ag-3Bi-10In and 89Sn-3Ag-3Bi-5In solders were subjected to reflow and solid state aging. The intermetallic compounds that formed at the interfaces between the solders and the Cu substrates and those that formed in the bulk solders in both test conditions were investigated. In the as-reflow samples, the intermetallic compounds were identified...

The resistance of β-tin decreases by 10% under electromigration because of the anisotropic properties of the material during electromigration and the consequent rotation of grains. The diameter of the grains was measured ex situ and the angle of the rotating grains was directly calculated. The angle of rotating grains increased in proportion to the...

Electromigration in Sn-8Zn-3Bi flip chip solder bumps on Cu pads has been studied at 120°C with an average current density of 4×103A/cm2 and 4.5×104A/cm2. Due to the polarity effect, the thickness of the intermetallic compound Cu-Zn (γ-phase) formed at the anode is much greater than that at the cathode. The solder joint fails after 117h of stressin...

Spontaneous Sn whisker growth is a surface relief phenomenon of creep, driven by a compressive stress gradient. No externally applied stress is required for the growth, and the compressive stress is generated within, from the chemical reaction between Sn and Cu to form the intermetallic compound Cu6Sn5 at room temperature. To obtain the compressive...

Eutectic SnPb solder has been widely used in packaging for several decades. The stability of the interface between solder and under-bump metallization (UBM) is an important issue that has led to many studies. Even though Ni atoms dissolve much slower into SnPb solder than Cu, the intermetallic compound, Ni3Sn4, which forms when eutectic SnPb solder...

Electromigration in beta-Sn has shown a 10% drop of resistance due to the anisotropic properties of the material. The drop was proposed due to reorientation of grains to reduce the resistance. The driving force as well as the atomic mechanism of grain rotation under electromigration has been considered in this letter. We propose that the anisotropi...

Under constant current electromigration, white tin (P-Sn) exhibited a resistance drop of up to 10%. It has a body-center tetragonal (BCT) structure, and the resistivity along the a and b axes is 35% smaller than that along the c axis. Microstructure evolution under electromigration could be responsible for the resistance drop. Synchrotron radiation...

White Sn (ß-Sn) thin film stripe shows a voltage drop about 10% when subjected to electromigration testing. Since ß-Sn has anisotropic crystal structure, it possesses different resistivity along a-, b- and c axis. The direction of the axes determines the resistance in each grain. Under electromigration, low resistance grain tends to grow in the exp...

Under constant current electromigration, white tin exhibited a resistance drop of up to 10%. It has a body-centered-tetragonal structure, and the resistivity along the a and b axes is 35% smaller than along the c axis. Microstructure evolution under electromigration could be responsible for the resistance drop. Synchrotron radiation white beam x-ra...