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Due to the demand for high-performance electronic products, there has been a rapid development in three-dimensional integrated circuit (3D IC) packaging technology. The 3D stacking relies on microbumps and through silicon vias (TSVs) to connect multiple Si dies vertically. These high-performance electronic products usually need to consume high power, combined with limited heat dissipation due to dense packing, resulting in the issue of Joule heating. The problem of Joule heating impacts the reliability of products. This study investigated the factors contributing to Joule heating of microbumps and its subsequent reliability issues through experimental methods and finite element analysis (FEA) simulations. Joule heating was quantified by measuring the temperature increase at stable states using the temperature coefficient of resistance (TCR) equation. We first observed that environmental temperature influences Joule heating. Higher oven temperature can exacerbate the Joule heating effect and lead to higher temperature increase under the same current density. We also examined the effect of intermetallic compound (IMC) formation on Joule heating, with a 12.5% increase in heat generation after IMC formation. Furthermore, the adverse effect of voids induced by sidewall wetting during high-temperature storage (HTS) on Joule heating using FEA was demonstrated. Additionally, we discovered severe Joule heating to cause thermomigration (TM) in the microbumps at a system level. Finally, we proposed structural optimization of the microbump to mitigate Joule heating and enhance reliability.

High-temperature pipelines are susceptible to defects due to the long-term service in high-temperature environments. The guided wave technology is promising for online non-destructive testing. Before implementing it in engineering, it is essential to better understand guided wave characteristics, especially attenuation, which significantly decreases the test distance and the resolution. However, describing the large attenuation is challenging due to the strong thermoelastic coupling, resulting in limitations of available models. To address these limitations, this paper aims to introduce a memory-dependent generalized thermoelastic model for investigating guided waves propagating in a hollow cylinder with temperature-dependent material properties. The analytical solutions are obtained using the Legendre polynomial method. The influence of the memory-dependent effect, temperature variation, and boundary conditions are studied. Some interesting findings are revealed: (1) The time delay factor is more dominant when adjusting κ and n to accurately describe guided wave propagation at high temperatures. (2): The mode conversion is accompanied by the attenuation and group velocity jump when the adjacent flexural torsional and longitudinal modes intersect, which is advisable to avoid when choosing the excitation frequency. (3) The relationship between phase velocity and attenuation versus temperature for quasi-elastic modes is nonlinear, while that for thermal modes is approximately linear.

Ball Grid Array (BGA) packaging method has been widely used in microelectronic devices, and electromigration (EM) in BGA is a crucial reliability issue, determining the performance of the products. Herein, we report a new failure mode induced by a coupling effect of EM and Joule heating in BGA structure. The test sample consists of a roll of 500 μm-BGA solder joints with upper and lower Cu under-bump-metallization (UBM) of different thicknesses, 16 μm, and 68 μm, respectively. Open failures of the thinner cathodic Cu UBM at the contacting area with solder joint were observed under four different EM conditions (130 °C-3000 A/cm2, 160 °C-2300 A/cm2, 160 °C-3000 A/cm2, and 160 °C-4600 A/cm2). Infrared (IR) results reveal the temperature at the thinner Cu UBM is higher than that in the solder joint as well as at the thicker Cu UBM. Simulation results elucidate the current density at the contacting area is about 22 times higher than the average density in the solder joint, so the effect of current crowding can be very serious in the failure site. Furthermore, severe current crowding and Joule heating promote the nucleation rate of voids at the joint location, which can further increase the current density and temperature, leading to a positive feedback effect until an open failure occurs in the circuitry. Our analysis enables the discussion of a coupling failure mode of EM and Joule heating in BGA, and provides a valuable guide of device design against EM failure in consumer electronic products.

We report here an approach to inhibit Cu–Sn intermetallic compounds (IMCs) in flip chip solder joints under thermal-gradient annealing. We reflowed two types of solders joints, Cu/SnAg/Cu and Cu/SnAg/Ni, in an oven (isothermal) and on a hot plate (with thermal-gradient) at 260 °C. During the isothermal reflow, the IMC growth rates in the top and bottom sides of the solder joint were ∼4.0 × 10−2 and 8.9 × 10−2 μm/min, respectively. However, the IMC growth rate increased to 7.8 × 10−1, 7.9 × 10−1, and 8.9 × 10−1 μm/min in the cold ends with low, medium, and high thermal gradients, respectively. Yet, the IMC thickness on the hot ends remained almost unchanged. These results indicate that the thermal gradient has facilitated the Cu–Sn IMC formation in the cold ends. But, when a Ni film was sandwiched at the cold end, the IMC growth rates in the cold end were reduced to ∼9.7 × 10−2, 1.4 × 10−1, and 1.5 × 10−1 μm/min with low, medium, and high thermal gradients, respectively, indicating that the thermomigration of IMCs was retarded by the Ni. It is speculated that the Ni layer transformed the IMCs on the hot end into stable (Cu,Ni)6Sn5 ternary IMCs, and thus the thermomigration of Cu was suppressed. A kinetic model is also proposed to address the competitive reactions of Cu-molten solder joints.

We investigated the lattice strain of nano-twinned Cu (nt-Cu) and regular Cu redistribution lines (RDLs) at 215, 300, 375 and 434 K using x-ray nanodiffraction; this technique has spatial resolution 100 nm. We found that the largest thermal strain or stress appeared at the corner of a Cu line at which the line turns 90°. As the maximum thermal stress in both Cu lines did not exceed the yield strength of Cu, there was no plastic deformation at the highest temperature, 434 K. There was, however, a stress gradient in the corner, which might cause early failure in thermal cyclic tests. In nt-Cu lines, the stress value in the corner is 4.5% greater than the neighbor area at 375 K, and 7.4% greater at 434 K. The maximum stress of nt-Cu is 344.6 MPa at 434 K; the maximum stress of regular Cu is 363.8 MPa.

Highly (111)-oriented nanotwinned copper (nt-Cu) and non-conductive paste (NCP) were employed to fabricate hybrid Cu–Cu bonding. We tailored and correlated the fracture modes, bonding strengths, and microstructures of the joints. A non-flow underfilling process was performed, and low temperature bonding was achieved in a single heat treatment at 180 °C for 120 min without vacuum. We found that under a post-annealing treatment, recrystallization and grain growth occurred. The bonding interfaces were partially removed and the joints were further strengthened. The fracture modes of the hybrid bonding structure were characterized using pull tests and correlated with their bonding properties. Such hybrid Cu–Cu microbumps with high bonding strength and low thermal budget can be widely used for advanced ultra-fine-pitch packaging.

With downward scaling of the micro-bumps in three-dimensional integrated circuits, surface inter-diffusion becomes dominant, changing the kinetic path of intermetallic compounds (IMC) formation and causing serious reliability issues. However, an in-depth understanding of the surface inter-diffusion process and the corresponding influence on the formation mechanism of IMC in a micro-bump remain unclear. We conducted annealing at 170 ℃, over 16 h for pillar type Sn/Cu micro-bumps and observed a unique 2-step sidewall Cu3Sn IMC formation phenomenon on the FIB-cut clean surface of the micro-bumps. It is found the two-step sidewall IMC formation is dominated by the surface inter-diffusion of Sn and Cu atoms. Density functional theory calculations reveal that the activation energy barrier of nucleation for sidewall Cu3Sn IMC is about 1/3 of that of sidewall Cu6Sn5 on corresponding interfacial IMC layers, making the formation of sidewall Cu3Sn dominant. Moreover, we proposed a kinetic model that can predict the mean lateral growth rate of the sidewall IMC which may cause fatal short-circuit failure.

A surface creep model is presented for analyzing Cu-to-Cu direct bonding under thermal compression. The driving force is a pressure gradient, which squeezes layers of atoms to fill voids at the bonding interface. The link among the key parameters of surface roughness, experimental bonding time, temperature, and pressure is presented in a kinetic equation. The theoretical bonding time can be calculated through these key parameters. The benchmark of the theoretical bonding time is the time required for the bonding area to reach 95%, which is measured from TEM images of the bonding interface. The experimental bonding time might be longer, and the bonding area might larger than 95%. Because the bonding mechanism includes surface diffusion and grain boundary diffusion, it is difficult to distinguish their individual influence. Therefore, the effective diffusivity (Deff) is calculated from the bonding conditions of other research and the equation of bonding model. If the bonding surface is highly (111)-oriented, the Deff is close to the (111) surface diffusivity. From the surface creep bonding model, the theoretical bonding time of grain boundary diffusion is very sensitive to the bonding temperature, while that of (111) surface diffusion is not. Additionally, at low temperature bonding (

In 3D IC integration, a critical demand of interfacial joints in high-end devices is ultra-fine pitch, below 2 μm. The pitch cannot be fabricated by using solder joints because of its low melting point. Microelectronic industry prefers a solid state process, but the processing temperature cannot be too high, preferable near 250 °C. Thus, Cu-to-Cu direct bonding has attracted much attention. We have bonded the highly (111)-oriented Cu microbumps to highly (111)-oriented Cu films as well as to randomly-oriented Cu films for comparison, at temperatures from 200 to 350 °C. We examined the microstructures of bonded interfaces and conducted shear tests to evaluate the mechanical strength of the Cu joints. The bonded interfaces for the (111) Cu microbumps to (111) Cu films had fewer voids than those bonded to randomly oriented Cu films. In addition, the bonding strength of the latter is much less than the former. We propose that the fast surface diffusivity on the (111) surfaces play a critical role on surface creep, which increases the bonding strength of the Cu joint. A surface creep model and its creep rate are presented, and a comparison to the experimental data has been made; the agreement is reasonable.

Polyimide (PI) has been adopted to protect the Cu from oxidation in the packaging industry. Electromigration (EM) of Cu redistribution layers (RDLs) capped with PI was investigated at 160 °C under 1.0 × 106 A/cm2. The results indicated that failure of fine-pitched (2-µm) is different from that of 10-µm RDLs, which is void formation. The failure of the 2-µm RDLs is mainly caused by severe oxidation during EM. To analyze the EM failure of the Cu RDLs with different pitches, the oxidation area of nanotwinned copper (nt-Cu) and regular Cu RDLs during EM was compared. We propose an oxidation equation to estimate the resistance increase of RDLs with various widths.

Electromigration (EM) mechanisms are well studied in damascene copper lines capped with inorganic dielectrics. However, EM in Cu redistribution lines (RDLs) capped by organic layers is not well-understood. In this study, nanotwinned copper (nt-Cu) is adopted, which has high mechanical strength as well as excellent electrical resistivity. We designed EM test vehicles with four-point probes to measure the lifetime of RDLs and examine the failure mechanisms of the Cu RDLs. The length of the RDLs was 800 µm, the width was 10 µm, and the height was 5 µm. The samples were tested at 200 °C, with current densities of 1.0 × 106 A/cm2. The results indicated that there were two main failure mechanisms, including EM-induced voiding and oxidation of Cu RDLs. The measured EM lifetime of nt-Cu RDLs is 4 times longer than that of regular Cu RDLs.

In the big data era, Si chips are integrated more and more to satisfy the fast growing demand from customers. In addition, in the post-COVID-19 virus era, the trend of distance teaching and home office has increased greatly the need of advanced consumer electronic products. To provide more processing tolerance and to build a wider temperature window in manufacturing, it becomes necessary to develop low melting temperature solders because a hierarchy of solders is needed in the packaging technology. Much research has been on Pb-free and Sn-based solders with a melting point from 180 to 230 °C. In this review, we will concentrate on low melting point solder alloys with a melting point lower than 180 °C and even below 100 °C. We review eutectic SnBi, eutectic SnIn, and the alloys with trace addition of a third element to them. Eutectic Sn-Bi solder is too brittle, and Sn-In solder is too soft. However, third element addition can only improve the solder properties partially; thus, we will further review the properties of Sn-Bi-In ternary alloys and the effect on the addition of a fourth element to them. This approach, when we include the Cu substrate, becomes a 5-element system, which leads us to consider high entropy alloys for soldering. With these Sn-, Bi-, and In-based alloys, we hope to develop new ways to design industrial applicable low melting point solders.

SnBiInZn-based high-entropy alloy (HEA) was studied as a low reflow temperature solder with melting point around 80 °C. The wetting angle is about 52° after reflow at 100 °C for 10 min. The intermetallic compound (IMC) growth kinetics was measured to be ripening-control with a low activation energy about 18.0 kJ/mol; however, the interfacial reaction rate is very slow, leading to the formation of a very thin IMC layer. The low melting point HEA solder has potential applications in advanced electronic packaging technology, especially for biomedical devices.

Thermomigration (TM) has become a critical reliability issue in advanced electronic packaging because of Joule heating. A temperature gradient is required to conduct heat away, and only 1 °C of temperature difference across a 10 μm thick microbump produces a temperature gradient of 1000 °C/cm, which can cause TM, especially in low melting eutectic Sn-Bi solder interconnects. We report here that Bi atoms moving from the hot end to the cold end of the temperature gradient, are the dominant diffusing species. Under the assumption of constant volume, the Sn atoms are squeezed by the Bi atoms at the cold end and have to accommodate for the Bi atoms, which makes them move to the hot end. Consequently, the opposing fluxes of Bi and Sn are found to be about the same. Moreover, the growth of Cu-Sn intermetallic compound (IMC) layers at the cold and the hot end were symmetrical, and were unaffected by TM. Additionally, finite-element-method (FEM) simulations showed that the phase separation of Bi and Sn reduced the current crowding regions which affect the electromigration of the eutectic Sn-Bi solder interconnects. Keywords: Thermomigration, Sn-Bi solder, Phase segregation, Finite element method (FEM), Current crowding

The study uses a very thick Cu column bump combined with a shallow solder interconnect to analyze and observe a string resistance against current crowding induced electromigration failure, which is one of the major problems in fine pitch flip chip solder joints. The analysis shows that the electromigration, if combined with thermomigration can be used to replace current crowding as a serious reliability issue in using Cu column based interconnects.

The polarity effect of electromigration on kinetics of intermetallic compound (IMC) formation at the anode and the cathode in solder V-groove samples is investigated. Details about the influence of both chemical force and electrical force, and their combined effect on the IMC growth with electric current are discussed.

A problem of optimizing the subwavelength microrelief of a binary cylindrical transmissive diffractive lens (DL) with a 300-mm focal length for a wavelength of λ=141 μm was considered. High-resistivity silicon was chosen as the DL substrate material. The angle of incidence of the illuminating beam was taken to be π/6. The optimization parameters were the height of the DL profile and the fill factor of the groove. The main goal of optimizing the design was to increase the diffraction efficiency of the lens. The DL diffraction efficiency was calculated using a Fourier mod method. The DL was fabricated by plasma-chemical etching (Bosch process) of the surface of a silicon substrate. The diffraction efficiency of the calculated lens was estimated to be 70%. However, a full-scale experiment showed the real efficiency to be much lower. These differences are related to both errors in the manufacturing process of the DL and non-ideal thickness parameters of the silicon wafers.

The thermomigration behavior of eutectic tin-lead flip chip solder joints at an ambient temperature is examined in terms of microstructural evolution, atomic transport and numerical simulation. The atomic transport due to thermomigration should be considered in the total atomic flux as thermomigration has played a key role in solder joints of microelectronic devices.

The electromigration behavior of e-SnPb was investigated at 100 degrees Celsius and at different current densities, varying within the range of 5 x [10.sup.3]-5 x [10.sup.4] A/[cm.sup.2]. The results showed that Pb is the dominant diffusing species in e-SnPb solder at 100 degrees Celsius.

The electromigration behavior of flip chip Sn96Ag3.5Cu0.5 solder joints on flexible structures is investigated under a constant current density at room temperature. A possible failure mechanism in Pb-free flip chip on flexible substrates is proposed and discussed.

Electromigration behavior in the eutectic SnAg(sub 3.8)Cu(sub 0.7) solder stripes was investigated in the vicinity of the device operation temperature of 100 degree C by using the edge displacement technique. Measurements were made for relevant parameters for electromigration of the solder, such as drift velocity, threshold current density, activation energy, as well as the product of diffusivity and effective charge number.

An electromigration-induced failure in the composite solder joints consisting of 97Pb-3Sn on the chip side and 37Pb-63Sn on the substrate side was studied. The crowding inside the Cu under-bump metallization (UBM) by simulation of distribution was confirmed.

Electromigration studies in SnPb flip chip solder joints on Cu/Ni(V)/Al thin-film under-bump-metallization and their mean-time-to-failure (MTTF) in temperatures ranging from 100 to 140 degrees C were done. The MTTF was much smaller at higher current density than the expected equation.

The electromigration in Al, Cu, and solder are compared on the basis of the ratio of their melting point to the device operating temperature of 100 degree C. It is found that in mean-time-to-failure analysis, to nucleate a void is more important in Al interconnects and flip chip solder joints than on a Cu surface interconnect.

The electronic industry is actively searching for Pb-free solders due to environmental concerns over Pb-containing solders. The metallurgical reliability issues or microstructure changes of flip chip solder joints when they are subjected to chemical, mechanical, electrical and thermal forces are discussed.

A method of measuring the contact temperature using Seebeck potential is presented. This method shows that the contact temperature increases with increasing current and decreasing contact dimension.

A study was conducted by using the ion beam imaging and transmission electron microscopy to examine the spontaneous growth of Sn whiskers on the eutectic SnCu finishes. The result found the precipitates of Cu6Sn5 in the grain boundaries of the finishes.

Experimental results of the investigation of self-healing properties of terahertz Bessel beams with orbital angular momentum (OAM) with topological charges of l=3 and l=4 in free space after passing through a dispersive medium are presented.