Search

In the 3D integrated circuit package industry, the remelting of solder joints during repeated stacking processes can cause electrical failure and low bonding strength. Transient liquid phase sintering (TLPS) bonding based on forming full intermetallic compounds (IMCs) in the solder joint to increase the remelting point has emerged as a potential solution to this issue. Here, pressureless TLPS Cu-Cu bonding was conducted with Sn-3.0Ag-0.5Cu solder powders and various Ag flake powder content (15 wt.%, 30 wt.%, 45 wt.%, and 60 wt.%). The TLPS paste was screen-printed and the bonding process was conducted at 255°C for 2 h in an air atmosphere without bonding pressure. Additionally, this study investigated the microstructural evolution and fracture modes of the TLPS joints after the shear tests were investigated. High-temperature storage tests were conducted at 300°C for 24 h, 48 h, and 96 h, and a shear test was then performed to evaluate bonding strength. A differential scanning calorimetry analysis of the TLPS paste was conducted to investigate the thermal behavior of the paste during the bonding process. No residual solder was found in TLPS joints with an Ag flake content above 45 wt.% The highest bonding strength in a TLPS joint with full IMC layers was 27.3 MPa, representing an approximately 9% decrease after 96 h of high-temperature storage test. TLPS bonding with an optimal composition was resistant to the remelting of solder joints due to the full IMC layers, i.e., it represents a reliable interconnection method for 3D stacking.

Cu sintering bonding has been considered a promising interconnection method for high-temperature applications like power electronics because of the low cost, low electrical resistivity, and high heat endurance of copper. However, Cu sintering bonding requires high bonding temperature and pressure as well as Cu oxidation prevention. To overcome these problems, we fabricated a hybrid Cu paste consisting of sub-micron and flake fillers, mechanically mixed with various epoxy contents (0, 5, 10, or 15 wt%). Cu–Cu bonding with printed hybrid Cu–epoxy paste was performed in N2 at 250 °C for 30 min without pressure. The process temperature and pressure could be lowered to these levels because the mechanical reinforcing effect of the epoxy supplemented the weak bonding between Cu fillers. When 10 wt% epoxy was added, the shear strength of joint increased remarkably to 12.1 MPa, whereas electrical resistance was maintained at 1.8 Ω. The joint with 15 wt% epoxy was further strengthened, but electrical resistance increased drastically. A high-temperature storage test was then performed at 175 °C for 72 h to investigate the bonding reliability. Since Cu fillers were encapsulated in epoxy to prevent oxidation, the bonding shear strength of the Cu–Cu joint with hybrid Cu–epoxy remained robust even after thermal aging.

The mass reflow (MR) process is widely used in electronic packaging interconnection. However, the conventional MR process can cause thermal damage to components and requires a long process time. The laser-assisted bonding (LAB) process is considered as an alternative soldering process for overcoming these limitations due to its extremely fast process time and high thermal selectivity. The LAB process causes low thermal damage; hence, it can be used to fabricate flexible and stretchable applications. This study investigates the mechanical properties, microstructural variation, and intermetallic reaction between the SAC 305 solder and Cu electrodes. The mechanical properties are investigated using shear tests, while the microstructure is analyzed using scanning electron microscopy and X-ray scans. Furthermore, the reliability of the LAB- and MR-produced SAC 305 solders is assessed using a high-temperature storage test. As a result, the intermetallic compound layer is found to be considerably thinner, and the volume of voids in the solder matrix is lower in the LAB-produced solder than that in the MR-produced solder. The mechanical properties of the LAB-produced solders are enhanced by these microstructural advantages.

To analyze the effects of Ni(P) layer thickness and Pd layer composition on interfacial reactions and the mechanical reliabilities of Sn–58Bi solder joints, we evaluated a phosphorous-contained Ni (Ni(P)) layer thicknesses ranging from 0.3 to 1.0 μm with Au/Pd/Ni(P) or a phosphorous-contained Pd [Au/Pd(P)/Ni(P)] layer in thin-electroless-nickel electroless-palladium immersion gold (ENEPIG) with Sn–58Bi solder joint after aging test. (Pd, Au)Sn4 and Ni3Sn4 intermetallic compounds (IMCs) were dominantly formed at the interfaces of the 0.3 µm to 1.0 μm Ni(P) layers in the thin-Au/Pd/Ni(P) or thin-Au/Pd(P)/Ni(P) joints after aging at 85 °C and 95 °C for 100 h. However, the Ni3Sn4 IMC layer changed to the (Cu, Ni)6Sn5 IMC layer in the 0.3 μm Ni(P) layer contained the Au/Pd/Ni(P) joint after aging at 85 °C for 300 h, because the Cu elements in a Cu pad penetrated through the P-rich Ni layer. Otherwise, the Ni3Sn4 IMC of the 0.3 μm Ni(P) layer in the Au/Pd(P)/Ni(P) joint changed to (Ni, Cu)3Sn4 IMC after aging at 105 °C and 115 °C for 1000 h, due to the P in the Pd layer, which affects the IMC growth rate. The 0.7 µm and 1.0 μm Ni(P) layers in the Au/Pd/Ni(P) or Au/Pd(P)/Ni(P) joints were attributed to the Ni3Sn4 IMC layer for whole aging conditions because the thick P-rich Ni layer suppress Sn and Cu diffusion during aging. In a high-speed shear tests, the shear strength of the 0.3 μm Ni(P) layer in the Au/Pd/Ni(P) joints was relatively low than that of the Au/Pd(P)/Ni(P) joints after aging at 105 °C and 115 °C for 100 h. Ni3Sn4 IMC was observed at the fracture surfaces of the 0.3 μm Ni(P) layer in the Au/Pd(P)/Ni(P) joints after aging at 115 °C for 1000 h, whereas the fracture surface of the Au/Pd/Ni(P) joint was Cu substrate. Therefore, Ni(P) layer thicknesses in excess of 0.7 μm and the P-contain Pd layer in the thin-ENEPIG surface finish with Sn–58Bi solder joints are expected to be highly reliable after long-term aging treatment.

Ni decorated multi-walled carbon nanotube (Ni-MWCNT) composite solders have been fabricated using Ni-MWCNT composite materials and Sn58Bi solder alloys. Ni-MWCNT composite materials were synthesized with functionalization, reduction and electroless plating. The Ni-MWCNT composite solders were fabricated at various weight percentages (0, 0.05, 0.1 and 0.2 wt.%) of Ni-MWCNT. The mechanical properties of Ni-MWCNT composite solder were evaluated using ball shear tests. The shear strength and fracture energy were improved about 16.02% and 12.60%, respectively, by adding 0.1 wt.% of Ni-MWCNTs, which released the stress in the solder alloy. The environmental reliability was examined at 85°C with 85% relative humidity at various holding times. After environmental reliability testing, the microstructure was analyzed to characterize the grain and intermetallic layers. The growth of grains and intermetallic layer thickness were mitigated by Ni-MWCNTs in the solder matrix that affected the mechanical properties. The distribution of Ni-MWCNTs inside the solder joints was confirmed by EPMA and Raman spectroscopy.

Fan-out wafer/panel level package (PLP) is an advanced package. Compared to the conventional package, its thickness is reduced, and electrical and thermal characteristics are improved, through the substitution of a substrate/interposer with a redistribution layer (RDL). However, its thermal and thermomechanical characteristics need to be improved and controlled more elaborately. In this article, six types of Ag patterns were embedded in the upper side of the fan-out package. Thermal emission was improved by building an efficient thermal conductive path, while warpage was improved by vertical symmetry and strain distribution. Overall, the large pattern in both fan-out and fan-in zones reduced the warpage and the pattern embedded in the fan-in zone efficiently improved the thermal emission. However, the pattern area should be designed by considering the overlapped areas with the through via interconnection or the embedded component in the fan-out zone. Because the embedded pattern with a large area is not necessarily a proper approach, an optimum pattern was studied with the simulation and measurement of the thermogram and warpage. The approach of the embedding pattern in the fan-out package is expected to improve its thermal and thermomechanical behaviors.

The study of photonic sintering has gained interest based on the advantages of fast processing at room temperature. However, printed electronics made from photonic sintering with an intensive pulsed light (IPL) energy source exhibit more mechanical instability than those made from conventional thermal sintering processes. To solve the mechanical instability problems, we fabricated Ag flake hybrid pastes with a variety of concentrations of Ag flake (0, 25, 50, 75, and 100 wt.%). All of the screen-printed hybrid Ag circuits were fabricated on polyimide substrates and were sintered at 3.5 MW. Surface porosity was analyzed using the Brunauer-Emmett-Teller method. An IPC (Packaging Electronic Circuits) sliding test was performed to analyze the flexibility of the screen-printed Ag flake hybrid circuits. The adhesion strength of the hybrid circuits was evaluated with a roll-type 90° peel test. The hybrid Ag printed circuit showed improvements in both the flexibility and adhesion strength with the addition of Ag flake.

The development of ball grid array (BGA) packages, such as the chip scale package, wafer level package and package on package, has focused on creating electronics packages that are smaller, thinner, higher-performance and capable of higher functionality, among other desirable traits. Among the interconnection materials used in BGA packages, the Cu-core solder ball (CCSB) has many advantages, such as the use of finer pitch, improved electrical conductivity and better controllability of the coplanarity of the chip. In this study, we evaluated the mechanical properties of the CCSB and Sn-3.0Ag-0.5Cu (SAC) by low-speed shear tests, and the von Mises stress distribution and plastic strain distribution were simulated using a finite element method. The diameter of each solder ball was 280 μm, and the outer layer of the CCSB was a plated SAC layer. The shear strength of the CCSB was about 10% greater than that of SAC. The maximum value of the simulated von Mises stress for the CCSB was higher than that of SAC because the Cu-core is stiffer than SAC. The fracture energy of the CCSB decreased by about 50% compared to that of SAC. The maximum value of simulated plastic strain, which is associated with fracture surfaces, was higher with the CCSB than with SAC. We can thus conclude that the Cu-core in the CCSB affects the shear strength and fracture behavior of solder joints.

The suitability of Ni–Sn combination paste as a transient liquid phase sintering (TLPS) bonding material for applications in high-temperature power electronic technologies was investigated. The microstructure and mechanical properties of joints bonded through TLPS with a Sn-30 wt% Ni paste were investigated at varying bonding temperatures (250, 270, 300, and 350 °C) and times using field-emission scanning electron microscopy, field-emission transmission electron microscopy, electron probe micro-analyzer, differential scanning calorimetry, and X-ray diffraction analyses. The results indicated that Ni and Sn reacted and bonded quickly to form Ni–Sn intermetallic compounds, which primarily consisted of Ni3Sn4 and residual Ni particles. The mechanical strength analysis results show that the shear strength values of the joints tended to increase as the bonding temperature increased. As Ni3Sn4 reacted with Ni particles, the joints achieved a stable and dense microstructure. The shear strength values were determined to be 47.9 MPa (300 °C) and 41.4 MPa (350 °C) for 15 and 5 min, respectively. Furthermore, the shear strength value saturated at a bonding temperature of 350 °C for a bonding duration of 5 min.

Transient liquid phase sintering (TLPS), which is a combined bonding technology of sintering and transient liquid phase bonding, is considered to be a promising sic. die attach material owing to its excellent mechanical properties and low cost. To prevent the oxidation problem, Ni is typically plated onto direct bonded copper and sic. chip. In this study, we investigated the Ni–Ni bonding by adapting TLPS method using Ni and Sn-58Bi. The bonding temperature and time were 220 °C and 60 min, respectively. In addition, the bonding atmosphere was maintained in air without bonding pressure. To confirm the bonding reliability, high-temperature storage test was conducted at 200 °C for 1000 h. With an increase in the remelting temperature to 271 °C, the bonding strength of the TLPS joint of 20 wt% Ni case was about 15 MPa. In addition, the bonding strength decreased by approximately 32% after the high-temperature storage test for 1000 h. In conclusion, Ni–Ni bonding was successfully achieved by the TLPS of Sn-58Bi with Ni. The high-heat endurance bonding between Ni to Ni was achieved by transient liquid phase sintering bonding using Ni and Sn-58Bi. By the sintering reaction between Ni, and the intermetallic reaction of Ni, Sn, and Bi, the remelting temperature increased from 139 to 271 °C. This bonding method can be applied in SiC die attachment technology.

Ag-decorated multi-walled carbon nanotubes (Ag-MWCNT) were investigated for use in Sn58Bi solder joints for FCLED packages. Among lead-free solders, Sn58Bi solder has been identified as a candidate for widespread use because of its low melting temperature and high tensile strength. However, Sn58Bi solder is brittle, and it is difficult to relieve it from an impact shock. To overcome these limitations, the bonding characteristics of Sn58Bi solder have been improved with Ag-MWCNT. Ag-MWCNT composite solder paste is fabricated by mixing Ag-MWCNTs of various concentrations with solder paste, and the FCLED chips are bonded on the printed circuit board. The thermal, electrical, and mechanical properties of the solder joints are investigated using various tests. The bonding strength of the Sn58Bi solder with the Ag-MWCNTs increased by 16% compared to that without it. Furthermore, the thermal resistance of the solder alloy decreased by a factor of 2 with the addition of 0.1 wt% Ag-MWCNT.

The multifunctional properties of cross-liked carboxymethlycellulose (CMC) and polyethyleneimine (PEI) films with varied CMC:PEI ratios and Ag flake sizes were studied. Both the CMC and PEI were cross-linked through a sonication process to achieve dispersive equilibrium. Fourier-transform infrared spectroscopy (FT-IR) was used to identify the functional groups in the sample material, with thermogravimetric analysis and differential scanning calorimetry used to investigate the overall thermal behavior of the CMC–PEI cross-linked films. Thermal diffusivity and thermal conductivity were also analyzed using laser flash analysis. To analyze the effects of Ag flakes as a filler material, the distribution of the Ag flakes within the film was determined using scanning electron microscopy. The thermal conductivity and resistance of the CMC films increased when cross-linked with 20% PEI. The electrical and thermal properties of the films also improved with the addition of Ag flakes. The multifunctional properties of cross-liked carboxymethlycellulose and polyethyleneimine films with varied CMC:PEI ratios and Ag flake sizes were studied. The thermal, electrical and mechanical properties of solder joints were investigated by using various kinds of tests.

The microstructures and mechanical reliability of Sn–58Bi solder and epoxy Sn–58Bi composite solder joint were investigated with organic solderability preservative surface finishes. The mechanical reliabilities of Sn58Bi and epoxy Sn58Bi solder were evaluated by the board-level drop test and the 3-point bend test after temperature-humidity storage testing. The microstructure and chemical composition of the solder joints were characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. The addition of epoxy in solder paste did not affect the morphology of the intermetallic compound. The thickness of the scalloped-shaped Cu6Sn5 intermetallic compound of solder/OSP joint increased with aging time. The drop number until fail for the epoxy Sn58Bi/OSP joint was higher than that for the Sn–58Bi/OSP joint; the average numbers of drops withstood by the Sn–58Bi/OSP joint and epoxy Sn–58Bi/OSP joint following the reflow process were fewer than 10 drops and 180 drops, respectively. The drop number of solder/OSP joints decreased with increasing aging time. The result of the 3-point bend tests shows that the number of bend cycles for the epoxy Sn–58Bi/OSP joint was 30 times higher than that for the Sn–58Bi/OSP joint. The number of bend cycles for solder/OSP joints was decreased with increasing aging time.

It is difficult to maintain high adhesion strength between a printed metal circuit and polymer substrate due to the weak chemical bonding of metal and organic materials. Therefore, an additional process is necessary to improve the adhesion strength. Silane coupling agents (SCAs), which are capable of forming bonds with metals and organic materials, can provide a solution. Here, SCAs were placed on polyimide (PI) substrates and coated with various functional groups to compare the effects of various functionalized SCAs on the adhesion strength between a screen-printed copper (Cu) circuit and the PI substrate, where the printed Cu patterns were fabricated on the coated PI substrate. Chemical bonding between the coated silane coupling agent and the PI substrate interface was investigated by X-ray photoelectron spectroscopy. The adhesion strength of the screen-printed Cu circuit and PI substrate interface was measured by a roll-type 90° peel test. The surface free energy was calculated using the contact angle. The failure mode was analyzed by field emission scanning electron microscopy. The silane coupling agent containing an amine functional group significantly improved the adhesion strength because it facilitates a superior bond between organic substrates and metals via chemical reactions such as copolymerization and graft copolymerization. As a result, the adhesion strength between the printed Cu pattern and PI increased about 4.26 times with the use of SCAs.

As demanding for high performance and miniaturization of electronic devices, interconnection materials required higher reliability in mechanical, thermal and electrical. The importance of electromigration issue has increased because of these trends. We evaluated the electromigration behavior of Sn58Bi solder and Sn58Bi epoxy solder under high temperature and constant current flow. The electromigration test-kit was a designed and fabricated flip chip-type module and the diameter of the solder bump was 250 μm. A current was passed through the two solder joints, producing a current density of 3.0 × 103 A/cm2 at 100 °C. The microstructure of solder joint after electromigration test were investigated with field-emission scanning electron microscopy during electromigration, a Bi-rich layer was observed at the anode side of the solder joint and the formation of Kirkendall voids was observed at the cathode side of the solder joints. Different inorganic materials affect electromigration in the eutectic Sn58Bi solder joints.

Package structures are continually becoming more complicated to achieve high-performance devices. Thus, the 2D structure of package needs to be modified to a 2.5D or 3D structure. Therefore, standoff properties at the solder joint are required to prevent Si chip damage and electrical shorts when the solder joint is located between the substrate and interposer. Cu-core solder balls (CCSB) are the most popular interconnection choice for a 2.5D package because they have better standoff properties than the general SAC solder. The mechanical properties of CCSBs were evaluated by ball shear tests and measuring bump height as compared with Sn–3.0 wt.%Ag–0.5 wt.% Cu solder. The bump heights of CCSBs and Sn–3.0 wt.%Ag–0.5 wt.% Cu were measured under several compressive pressure conditions at room temperature, 175 °C and 235 °C. The stand-off height of the CCSBs dramatically better than that of Sn–3.0 wt.%Ag–0.5 wt.% Cu solder under high pressure and temperature. The shear strength of the CCSBs was stronger than that of SAC solder at all multiple reflows due to the finer microstructure and higher stacking of dislocations at the interface of the Cu core ball surface.

AA6061-T4 composites containing SiC and B4C particles were fabricated by friction stir processing (FSP) with an SKD11 tool. The microstructures and mechanical properties of the composites were investigated with various test methods. With the inclusion of ceramic particles, refined grains in the stir zone (SZ) were observed using a scanning electron microscope (SEM) and tunneling electron microscope (TEM). Because the ceramic particles facilitated grain refinement in the SZ via the pinning effect, the SZ with the particles had a much smaller grain size than the SZ without the particles. Vickers hardness test, tensile test and Charpy impact test were conducted to evaluate the mechanical properties. Mechanical properties of the SZ with the ceramic particles were improved relative to those of the SZ without the particles. Vickers hardness (from 50 to 90 HV), tensile strength (from 117 to 253 MPa) and Charpy impact absorbed energy (from 4.2 to 5.6 J) of the SZ increased with the addition of ceramic particles.

An epoxy Sn–58wt.%Bi solder joint was evaluated by a three-point bending test with electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG) surface finishes aged at 85 °C and 85% relative humidity. Scanning electron microscopy and electron probe microanalysis were carried out to study intermetallic compound variation. The morphology, total thickness, and chemical composition of intermetallic compound in epoxy Sn58Bi solder joints were the same as those of Sn–58wt.%Bi solder joints with each surface finish. The average number of bending-to-failure cycles for the epoxy Sn–58wt.%Bi solder/ENIG joints and epoxy Sn–58wt.%Bi solder/ENEPIG was more than 4000 and 5000, respectively. The average number of bending-to-failure cycles of the epoxy Sn–58wt.%Bi solder joint decreased with increasing age. Three-point bending reliability of epoxy Sn–58wt.%Bi solder joints was higher than that of Sn–58wt.%Bi solder with both surface finishes. Cracking of all solder joints subjected to as-reflowed was propagated through the solder matrix. However, after aging for 1000 h, cracking occurred primarily between intermetallic compound layers.

The fan-out wafer level package (FOWLP) is the most common advanced package technology due to its higher I/O density, ultra-thin profile, high electrical performance, and low power consumption. However, warpage induced by the coefficient of thermal expansion (CTE) mismatch between different kinds of materials is a mechanical issue in FOWLPs. We investigated the warpage of fan-out package (FO package) components molded by EMC (with 3 kinds of materials) for two mold thicknesses. The fan-out package component was fabricated with Si chips (three thicknesses). The warpage of the fan-out package component from room temperature to 260 °C decreased with increasing chip thickness and mold thickness. Finite element method (FEM) analysis showed that the warpage of the fan-out package at 25 °C decreased when the CTE mismatch between the EMC and Si chip decreased. The warpage of the fan-out package at 260 °C decreased with decreasing modulus of the EMC due to a lower stress relaxation. The electrical resistance of the FOWLP component increased more than 2.5 times after temperature–humidity and thermal shock testing. The warpage decreased with increasing EMC thickness, decreasing chip thickness, a lower CTE of the EMC, and a lower modulus of the EMC.

The microelectronics packaging industry, although rapidly growing, faces several challenges including 3-D integration, issues with multifunctional capability, and fluctuating input/output (I/O) density, among others. Better-performing microelectronics assemblies for mitigating these challenges require alloys with superior solderability and minimal metallization layer thickness. To this end, in this study, we investigated two kinds of electroless-nickel electroless-palladium immersion gold (ENEPIG) with 0.3 μm Ni, 0.1 μm pure Pd or Pd-phosphorous (Pd(P)), and 0.1 μm Au layers plated on a printed circuit board (PCB) substrate. To analyze the effects of the pure Pd and Pd(P) layers in the thin ENEPIG, we evaluated the interfacial reactions and mechanical properties of the SAC305 solder with a pure Pd or Pd(P) layer in the thin ENEPIG joints after aging at 150 °C. Needle-type and chunky-type (Cu,Ni)6Sn5 IMCs were formed at the interfaces of the pure Pd and Pd(P) joints, respectively. The (Cu,Ni)6Sn5 IMC of the pure Pd joint was thinner than that of the Pd(P) joint after reflowing and aging for 100 h. However, the total IMC of the Pd(P) joint was thinner than that of the pure Pd joint from 250 to 1000 h. In a low-speed shear test, the shear strength of the Pd(P) joint was higher than that of the pure Pd joint for the entire aging time. Most fractures occurred at the Sn-rich surface with a ductile mode, regardless of the different substrates and aging times. After high-speed shear testing, the shear strength of the pure Pd joint was higher than that of the Pd(P) joint until aging for 100 h. After aging for 250 h, the shear strength of the Pd(P) joint was higher than that of the pure Pd joint. The results for brittle fracture rate were similar to those for high-speed shear strength. Hence, Pd(P) joints are expected to demonstrate higher reliabilities than pure Pd joints after long aging treatment.

Cu nanopaste is a candidate to replace Ag nanopaste, but the oxidation problems associated with Cu nanopaste must be overcome. In this study, we fabricated Cu nanopaste with various contents of sodium dodecylbenzene sulfonate (SDBS), and the oxidation behaviors of Cu nanopaste with various contents of SDBS were investigated. Then, Cu nanopastes were printed using a screen-printing machine and sintered with infrared energy. The fabricated circuits were evaluated using high-temperature storage tests and steady-state temperature humidity bias life testing. The printed Cu pattern with SDBS showed better oxidation-resistant properties than that without SDBS. After the reliability test, 2 wt% of SDBS shows less oxide based on X-ray diffraction, microstructure, and electrical resistivity measurements, and the optimized content of SDBS was 2 wt% in the Cu nanopaste.

Friction stir welding (FSW) is a novel solid-state welding technique. This process has many advantages, such as no consumable requirement, low distortion, low shrinkage, and excellent mechanical properties. Recently, FSW has been successfully used to fabricate metal matrix composites (MMCs), as the process overcomes environmental problems caused by other MMC fabrication processes. In previous studies, the goal was to reinforce the mechanical properties of base materials. In this study, the FSW technique and Ag-decorated carbon nanotubes (Ag-CNTs) were used to enhance the thermal properties of copper, and the microstructures of joints of Ag-CNT-reinforced copper fabricated by FSW were evaluated. The microstructures were analyzed using an optical microscope, and the presence of Ag-CNT in the joint was observed using a scanning electron microscope. Additionally, we confirmed the distribution of Ag-CNTs by electron probe microanalyzer, and the thermal properties of joints were evaluated by laser flash analysis. The Ag-CNT-reinforced Cu composites fabricated by FSW have improved thermal properties compared with conventional copper MMCs.

The mechanical properties of Sn-58 wt.% Bi solder with different amounts (0 wt.%, 0.05 wt.%, 0.1 wt.%, and 0.2 wt.%) of Ag-decorated multiwalled carbon nanotube (MWCNT) nanoparticles under 85°C/85% relative humidity environmental conditions for 0 h to 1000 h was investigated. Sn-58 wt.% Bi solder is a lead-free option for use in solder joints due to its low melting temperature and good creep resistance; however, it is brittle and has reliability issues. Ag-decorated MWCNT nanoparticles were used to improve these weaknesses of Sn-58 wt.% Bi solder. A ball shear test was performed using a bond tester to investigate the solder's mechanical properties. The microstructures of the solder joints and fracture mode were analyzed using a field-emission scanning electron microscope. The results demonstrated that the addition of Ag-decorated MWCNT nanoparticles to Sn-58 wt.% Bi increased the shear strength and fracture energy by approximately 15% and 14%, respectively, compared with Sn-58 wt.% Bi alone. After a high-temperature, high-humidity test for 1000 h, the shear strength and fracture energy of Sn-58 wt.% Bi with 0.1 wt.% Ag-decorated MWCNT nanoparticles were 13% and 21% greater than for Sn-58 wt.% Bi alone.

Electronic packaging has been miniaturized for high density and performance. Therefore, the formation of reliable interconnections has become an important issue. However, the conventional reflow process depends on a high process temperature and long process time. Those process limitations of the reflow process cause problems with advanced package structures, such as chip delamination and warpage problems in soldering interconnections. Laser-assisted bonding (LAB) has been highlighted as an advanced soldering interconnection process because of an ultrafast bonding process and thermal selectivity. However, the use of a laser homogenizer is necessary to improve laser irradiation area accuracy and it reduces price competitiveness. To overcome the problems of the reflow and the LAB processes, the intense pulsed light (IPL) soldering process has been considered as the next-generation answer for soldering interconnections. The IPL soldering process could reduce thermal damage to polymer components with its extremely short bonding time, thus it could be a solution to the warpage problem. Furthermore, the grain size of solder alloys could be controlled, depending on IPL conditions. In addition, IPL could irradiate in a desired wavelength range using a filter because IPL includes light in a wide wavelength range from ultraviolet to infrared radiation. We investigated the microstructure evolutions and mechanical properties of the IPL soldering process using Sn-3.0Ag-0.5Cu (SAC305) solder. Test-kits for evaluating mechanical properties were designed according to the JEDEC-B117A standard. A copper (Cu) electrode was used with two kinds of surface finish, organic solderability preservative and electroless nickel electroless palladium immersion gold. The IPL soldering process was conducted using different pulse widths (2, 2.25, and 2.5 ms). Other IPL conditions were fixed, such as pulse frequency (3 Hz), pulse number (10), and pulse power (4 kW). The Reflow process was conducted at a peak temperature of 270°C for 5 min. After the soldering process, a high-temperature storage test was performed at 150°C to evaluate the mechanical reliability of the solder ball. The IPL soldering process generated a thinner and flatter-shaped intermetallic compounds layer than the reflow process at the interface between the solder and the Cu electrode. Moreover, small grain size and microstructure region aligned in the same orientation were shown in the solder alloy that was fabricated using the IPL soldering process. Shear strengths of IPL soldering are 6%-11% higher than the reflow process in all high-temperature storage test conditions. The novel soldering process using an IPL energy source is suggested as a solution to the warpage problem and to improve mechanical properties.

We used nanoindentation method to evaluate the mechanical properties of two solders, Sn-57.6Bi-0.4Ag (melting point 138°C) and Sn-3.0Ag-0.5Cu (SAC305, melting point 217°C), used in thermal compression bonding. The following conditions were used to determine the optimum bonding conditions: 190, 210, and 230°C and 2, 2.5, and 3 MPa for Sn-57.6Bi-0.4Ag; and 280, 300, and 320°C and 3, 5, and 7 MPa for SAC305. The solder hardness decreased as the bonding temperature increased, which demonstrated that bonding strength decreases with grain growth, in accordance with the Hall-Petch equation. Increased stiffness and elastic modulus were also observed alongside larger grains. Nanoindentation, which can accurately measure small parts for fine pitch packaging, is expected to be an effective technique for determining mechanical properties of a sample.

Nickel-decorated multi-walled carbon nanotubes (Ni-MWCNTs) were synthesized using a colloidal method and an electroless plating method to form Ni nanoparticles on the surface of MWCNTs. The Ni electroless plating process was optimized with solutions at various pH (5, 6, 7, and 8) levels. The Ni-MWCNTs fabricated at pH 7 show the most stable Ni nanoparticles on the surface of the MWNCTs. After optimization of the Ni plating process, the Ni-MWCNTs/Sn-58Bi composite solder were fabricated with varying contents of (0, 0.05, 0.1 and 0.2 wt%) Ni-MWCNTs. The experimental results show that Ni-MWCNTs enhance the mechanical properties of Sn-58Bi solder. The shear strength of Sn-58Bi solder improves by about 14% with 0.1 wt% Ni-MWCNTs. Furthermore, the microstructure evolution after a high temperature storage test shows that Ni-MWCNTs have an effect on grain refining and interrupt growth of the Intermetallic compound layer. The distribution of Ni-MWCNTs could be recognized with the results of EPMA.

We investigated the interfacial reactions and mechanical reliabilities of different Ni layer thicknesses and P-containing Pd layers in thin electroless-Ni electroless-Pd immersion gold (thin-Au/Pd(P)/Ni(P)) surface-finished printed circuit board (PCBs) with Sn-3.0Ag-0.5Cu (SAC 305) solder joints. To analyze the optimal Ni layer thickness in the thin-Au/Pd(P)/Ni(P) for aging, we evaluated 0.3- to 1.0-μm Ni layers in thin-Au/Pd(P)/Ni(P) PCBs with SAC 305 solder joints aged at 75, 100, 125, and 150 °C for 1000 h. A scallop-type (Cu,Ni)6Sn5 intermetallic compound (IMC) dominantly formed at the bottom and top sides of the interfaces of all Ni joints under all aging conditions. Furthermore, a P-rich Ni layer formed at the interface between the (Cu,Ni)6Sn5 IMC and Cu substrate during aging regardless of Ni layer thickness. The (Cu,Ni)6Sn5 IMCs of the joints with 0.3- and 0.5-μm Ni layers aged at 125 and 150 °C for 1000 h were thicker than those of 0.7- and 1.0-μm Ni layers. In high-speed shear tests, the shear strength reduction rates of the joints with 0.3-μm Ni layer aged at 125 and 150 °C were higher than those of the 0.7- and 1.0-μm Ni layers. The brittle fracture rates of the joints with 0.3- and 0.5-μm Ni layers aged at 150 °C for 1000 h were higher than those of the 0.7- and 1.0-μm Ni layers. We determined that these trends arose from the diffusion barrier role of the relatively thick P-rich Ni layers maintained at the interfaces of the joints with 0.7- and 1.0-μm Ni layers for all aging temperatures and times. In low-speed shear tests, the shear strengths of the joints with 0.3-μm Ni layer were slightly lower than those of the 0.5- to 1.0-μm Ni layers. The low- and high-speed shear strengths of the joint with 0.7-μm Ni layer were similar to those of the 1.0-μm Ni layers for each condition. Therefore, Ni joint thicknesses of over 0.7 μm are expected to provide high reliability for aging.

High heat-endurance bonding has become an important technology requisite for the demands of SiC die attachment or other high-temperature packaging applications. Transient liquid phase sintering (TLPS), which is a promising candidate among high heat-endurance bonding technologies, has been widely studied in recent years. Because the advantages of TLPS bonding are short bonding time, low bonding pressure, and low cost. In this study, in order to bond in air condition and low temperature, TLPS pastes using Ni, Sn–58Bi, and flux were fabricated with various weight ratios. Pressureless TLPS bonding processes were conducted in an oven at 220 °C for 60 min in air. The thermal behaviors of TLPS pastes (before bonding) and TLPS joints (after bonding) were analyzed by differential scanning calorimetry (DSC). The shear test was conducted for investigate about mechanical properties. The microstructures of cross-sectional and fracture surfaces were analyzed by field emission scanning electron microscopy (FESEM). Also, the atomic distribution of cross-sectional micrographs was analyzed by electron probe micron analyzer (EPMA) and electron dispersive spectrometry (EDS). After bonding, the residual eutectic Sn–Bi lamellar structure was observed only in Ni–90(Sn–58Bi). To verify the reliability of TLPS joints, the samples were stored in an oven at 200 °C for 100, 500, and 1000 h, and the shear test was conducted. The initial shear strengths of Ni–90(Sn–58Bi), Ni–80(Sn–58Bi), and Ni–70(Sn–58Bi) TLPS joints were 23.56, 17.23, and 5.47 MPa, respectively. After the high-temperature storage test, the decrease in shear strength was smallest in Ni–70(Sn–58Bi) and largest in Ni–90(Sn–58Bi). In conclusion, the optimum weight ratio of Ni and Sn–58Bi was Ni–80(Sn–58Bi) because of increased re-melting temperature (270 °C) and high bonding reliability compared to other conditions.

The mechanical reliability of Sn–MWCNT composite solder containing various content of Sn-decorated MWCNTs (0, 0.05, 0.1, and 0.2 wt%) and Sn–58Bi solder were investigated. The Sn-decorated MWCNTs nanoparticles were used to improve the mechanical reliability of Sn–58Bi solder, which is a representative, low-temperature, lead-free solder. The Sn-decorated MWCNT nanoparticles were synthesized using the polyol method, and the Sn–MWCNT composite solder paste was fabricated by mechanical mixing. The shear and bending tests were conducted to evaluate the mechanical properties of the solder joints. We identified the microstructure of solder to investigate the intermetallic compound and failure mechanism of Sn–MWCNT composite solder. Furthermore, a high-temperature storage test at 100 °C for 1000 h was performed to determine long-term reliability. The shear strength and fracture energy increased about 21% and 23%, respectively, with 0.1 wt% Sn-decorated MWCNTs. In addition, the bending reliability of Sn–58Bi solder increased approximately 25%.

Printed electronics have been used because of various advantages such as low cost, productivity, and eco-friendly processing. However, for fabricating printed electronics, the conventional sintering process conducted at high temperatures has problems such as thermal damage and long processing time. To solve these problems, low-temperature processes such as intensive pulsed light (IPL) have been widely studied. Research of composite materials has become important in supporting these low-temperature processes. We fabricated Ag-decorated, multi-walled carbon nanotubes (Ag-MWCNTs) in composite pastes and sintered printed circuits using IPL energy. We investigated the electrical resistivity, microstructure, and flexibility of the Ag-MWCNT composite circuit with various durations of IPL energy irradiation and Ag-MWCNT content (0, 1, 2, and 3 wt%). The electrical resistivity of the circuit sintered with IPL energy decreased with irradiation time, but it had an unstable microstructure from the drastic sintering process. The Ag-MWCNTs could solve this problem using the net structure of the MWCNTs. The Ag-MWCNT composite nanoparticles could be a solution to overcome the problems in the photonic sintering process using IPL energy.

The effects of the thickness of the Ni layer in thin electroless-nickel electroless-palladium immersion gold (thin-ENEPIG) surface-finished printed circuit boards (PCB) with Sn-3.0Ag–0.5Cu (SAC305) solder joints during aging treatment were investigated. We evaluated the interfacial reactions and mechanical properties of 0.3–1.0 µm thick Ni layers in thin-ENEPIG PCBs with SAC305 solder joints at aging temperatures of 75, 100, 125, and 150 °C for 1000 h. A needle-type (Cu,aNi)6Sn5 intermetallic compound (IMC) layer formed at the interface of the SAC solder and Cu substrate. In addition, P-rich Ni layers were formed at the interface of the (Cu,Ni)6Sn5 IMC and Cu substrate for the 0.5–1.0 µm thick Ni joints. The thicknesses of the (Cu,Ni)6Sn5 IMCs with 0.3 and 0.5 µm Ni joints were strongly affected by the aging time and temperature. In contrast, the IMC growth rates of the 0.7 and 1.0 µm thick Ni joints were significantly lower than that of the 0.3 µm thick Ni joint. The reason for this behavior is that the P-rich Ni layer acts as a barrier for Sn and Cu diffusion at the joint and was maintained at the IMC/Cu interface after aging. In high-speed shear tests, the shear strengths of all Ni joints aged at 75 °C were similar. However, the rate of reduction in shear strength decreased with increasing Ni layer thickness as the aging time and temperature increased, but the shear strength of the 0.3 µm Ni joint rapidly decreased with increasing aging temperature and time. In contrast, the shear strengths of the 0.7 and 1.0 µm Ni joints slightly decreased with increasing aging temperature and time. The brittle fracture rates of the joints decreased with increasing Ni thickness, and we determined that this occurred because of the remaining P-rich Ni layer, which acts as a diffusion barrier during aging, at the interface of the 0.7 and 1.0 µm Ni joints. In low-speed shear tests, the shear strengths of the 0.7 and 1.0 µm Ni joints were slightly higher than those of the 0.3 and 0.5 µm Ni joints after aging at 125 and 150 °C for the entire aging period. Most fractures occurred via a ductile mode regardless of the Ni layer thickness. Therefore, a Ni layer thickness of more than 0.7 µm in thin-ENEPIG finished PCB with SAC305 solder joints is expected to yield high reliability on aging.

Flip-chip light-emitting diode (FCLED) packages were interconnected with various contents of Sn-decorated multi-walled carbon nanotube (Sn–MWCNT) composite solder. The Sn–MWCNT was synthesized with a Sn solution using polyol to decorate Sn nanoparticle on the surface of MWCNTs. The Sn-58Bi solder pastes containing Sn–MWCNTs were printed on the metal-printed circuit boards, and then bonded with a flip-chip bonder. The mechanical property of FCLED packages with various contents of Sn–MWCNTs were evaluated with a bond tester. Thermal property was measured with IR Camera and laser flash analysis. The shear strength of an LED chip with 0.1 wt% Sn–MWCNT solder increased about 25% over that without Sn–MWCNT. The LED chip temperature with 0.1 wt% Sn–MWCNT nanocomposite solder decreased about 7% compared to that of the Sn-58Bi solder.

Highly reliable bonding materials have attracted tremendous interest due to a growing demand for high-temperature electronics. We developed a fluxless and binder-free paste comprising Cu nanoparticles (NPs), Sn-58Bi (SnBi) particles, and polyvinylpyrrolidone (PVP) dispersing agent, which enables pressureless, low-temperature (190–250 °C) formation of robust joints (over 7 MPa) by means of transient liquid phase sintering (TLPS). Microstructural evolution of the joint was investigated under variations in PVP molecular weight (MW; 10,000, 55,000, 360,000, or 1,300,000) and bonding conditions including temperature and time. In a die-shear test, the joint formed with PVP MW 360,000 was the strongest due to its proper particle dispersion and the formation of intermetallic compounds (IMCs). Conditions of excessive PVP MW, bonding temperature, and time impeded the bonding characteristics of the TLPS joint, with formation of voids and increasing brittleness. TLPS bonding with the optimal dispersing agent enabled pressureless die attach without chip damage, demonstrating applicability as a simple, robust interconnection for high-temperature electronics.

We investigated the effect of Ag-coated multi-walled carbon nanotubes (Ag-MWCNT) on the microstructures and electromigration behaviors of a Sn58%Bi solder and organic solderability preservative (OSP) surface finished on the FR-4 printed circuit board (PCB) joint under a current stress of 3000 A/cm2 at 100 °C. Electromigration of Ag-MWCNT Sn58%Bi composite solder was investigated by daisy-chain test-kit. Reaction layers formed at the anode side and cathode side of the Sn58%Bi solder joints consisted of three microstructures; Bi-rich layer, Sn-rich layer, and intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn). The Bi-rich layer was mainly formed at the anode side in the couple of the Sn58%Bi solder joint with various times of applying current stress. The Bi-rich layer of the Ag-MWCNT Sn58%Bi composite solder joint was approximately 2 times thinner than that of the Sn58%Bi solder joint because the Ag-MWCNT acts as a diffusion barrier. Also, the Cu6Sn5 and Cu3Sn IMCs that formed at the interface between the Ag-MWCNT Sn58%Bi composite solder joints were thinner than those of the Sn58%Bi solder joints. The time to failure (TTF) was longest at the 0.05% Ag-MWCNT Sn58%Bi composite solder joint. Therefore, Ag MWCNT is expected to improve the reliability of electromigration in the Sn58%Bi composite solder joint.

The development of epidermal electronics, which provides precise measurement of various human physiological signals without reducing the quality of life, has encountered a serious delay in its practical application because of potential damage experienced during fabrication. Such damage occurs mainly because fabrication of the epidermal device is typically carried out after attaching an ultrathin or stretchable film onto a stiffer carrier substrate, and eventually peeling it from the carrier. Substrates used in epidermal electronics suffer from very low stiffness to bending and stretching, resulting in severe strain during stripping from the carrier substrate, where devices formed on its surface are also subjected to significant damage. When a device is fabricated using an adhesive with an excessively low adhesion strength, however, some parts of the soft film can be prematurely peeled from the carrier during device fabrication. We propose a UV-responsive transparent pressure-sensitive adhesive (PSA) to fabricate ultrathin devices without process-induced damage. We synthesised an acryl-based PSA and mixed it with UV-curable oligomers. The soft UV curable PSAs contract and become rigid when the oligomers in the PSAs are cured by UV irradiation. This dramatically reduces the adhesion strength to less than 1% of the initial value and consequently enables peeling from the carrier without deteriorating the properties of the devices formed on the ultrathin film. In this case, it is not necessary to fabricate the device by using more materials than necessary considering the performance degradation in advance. Using this method, we developed a transparent bending-induced pressure sensor, with light transmittance higher than 93.5% at 550 nm.

The mechanical properties and microstructures of Sn58Bi (in wt.%) composite solders with Sn decorated multiwalled carbon nanotube (Sn MWCNT) particles were investigated. The contents of Sn MWCNT particles were 0, 0.05, 0.1, and 0.2 wt.%. Sn58Bi composite solder pastes were printed on printed circuit board (PCB) substrates and bonded to the substrates using reflow processes (1, 2, 3, 5, and 7 times). This study describes the effect of Sn MWCNTs content and the number of reflow processes on the mechanical properties and microstructures of Sn58Bi composite solders. Mechanical properties were investigated using a low speed ball shear test. In the shear test, the shear strength increased by 7.07%, and the fracture energy increased by 14.4% with 0.1 wt.% of Sn MWCNT particles after 1one reflow. The number of reflows did not significantly affect the shear strength, but the fracture energy increased with increasing content of Sn MWCNT particles. Cross-sectional microstructures and fracture surfaces were observed by scanning electron microscopy (SEM). Sn MWCNT particles in the solder matrix were observed on polished surfaces of SEM images and were also identified by a Raman spectrometer. After the shear test, brittle failure occurred in all joints, indicating no influence of MWCNT particles.

Intense pulsed light sintering of printed metallic circuits on polymer substrates is an attractive technique. However it still does not yield the mechanical quality required for industrial applications. This technology must improve adhesion and flexibility to produce highly reliable flexible electronic devices. The effects of atmospheric pressure oxygen plasma and acrylic acid polymerization treatments upon the adhesion and flexibility properties were investigated. Modified polyimide surfaces were characterized by surface free energy measurements, and adhesion and flexibility properties were evaluated by means of roll-type 90° peel tests and IPC sliding tests. The strength of adhesion with oxygen-treated and acrylic acid polymerized samples increased about 17- and 20-fold compared with that of untreated samples, and the flexibility was improved by about 2 fold, respectively. The evident improvements in surface free energy, adhesion and flexibility were attributed to the creation of oxygen functional groups on the polyimide surface after atmospheric pressure plasma treatment.

Mass reflow (MR) has been widely used in the interconnecting process of electronic packaging. However, use of conventional MR shows a limitation in the miniaturization trend of electronic devices because of thermal damage to heat- sensitive polymer components from the high process temperature and long bonding time. Furthermore, MR cannot avoid warpage issues because it cannot provide thermal selectivity to the bonding area. These representative problems of MR indicate the need for an advanced bonding process with selectivity and mass product possibility. To meet the requirements of electronic packaging, advanced bonding processes were needed. Laser-assisted bonding (LAB) has been spotlighted as a next-generation interconnection technology to overcome the problems of MR. LAB has extremely fast ramping up speed with accurately controlled wavelength and excellent thermal selectivity. The LAB process could be applied in bonding of flexible, foldable, and even stretchable applications involving a polymer component with a low glass transition temperature. Furthermore, the short process time of LAB could provide not only enhanced productivity, but improvement in mechanical properties as a result of finer grain size and reduction in intermetallic compound (IMC) formation.SAC305 solder paste was printed on two kinds (OSP and ENEPIG) of surface-finished Cu electrode, and MR and LAB processes were performed to create solder joints. The mechanical reliability of solder joint was evaluated with shear test according to the JEDEC standard. Formation and growth of an IMC layer at the interface between SAC solder and Cu after soldering were identified with FESEM. A high- temperature storage test was performed to evaluate the longterm reliability of LAB compared to that of MR. Voids in solder joints were identified by x-ray scanner to compare the soldering processes and surface finish depending on aging time. Smaller IMC layer between solder joint and electrode thickness and fewer voids in solder joints were produced with LAB compared to MR.

Transient liquid phase sintering (TLPS) bonding has been widely researched for high temperature interconnection. At TLPS bonding, intermetallic compounds (IMCs) form by solid-liquid interdiffusion between molten low–melting point (MP) metals and high–MP metal particles. This IMC formation reaction occurs simultaneously across the whole joint. Therefore, the growth rate of IMCs is higher than conventional TLP bonding, and the bonding process can be performed in low pressure condition for short time.We investigated hybrid TLPS paste using SAC305 with various size of Ag particles (0.5 – 10 μm in diameter) for pressureless bonding. Hybrid TLPS paste was fabricated using SAC305 powder, hybrid Ag particles, and organic solvent by paste mixer. Then, the pastes (hybrid TLPS paste, SAC305) were printed on Cu substrate, and Cu chip was placed onto printed pastes. Pressureless bonding process was carried out at 300℃ for 2 h in air. In cross–sectional micrograph of hybrid TLPS joint, Sn was almost replaced by IMCs, while Sn residue remained after the bonding at SAC305 joint. The mechanical property of joint was evaluated by die shear test. Shear strength of hybrid TLPS joint was 2.5 times higher than that of SAC305. The high–temperature storage test (200 °C) and salt spray test (5 % NaCl at 35 °C) were performed to evaluate the bonding reliability under harsh environment. In both tests, hybrid TLPS joint showed more mechanically and electrochemically robust properties than conventional SAC305 joint.