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This study explored the feasibility of a fast and uniform large-scale laser sintering method for sintering stretchable electrodes. A homogenized rectangular infrared (IR) laser with a wavelength of 980 nm was used in the sintering process. A highly stretchable composite electrode was fabricated using silver (Ag) microparticles and Ag flakes as the fillers and polyester resin as the binder on the polyurethane substrate. This laser-sintering method showed a sintering time of 1 sec and a very uniform temperature across the surface, resulting in enhancing the conductivity and stretchability of the electrodes. The effects of the laser power on the electrical and electromechanical properties of the electrodes were investigated. Using stretching, bending, and twisting tests, the feasibility of the laser-sintered stretchable electrodes was comprehensively examined. The electrode that was sintered at a laser power of 50 W exhibited superior stretchability at a strain of 210%, high mechanical endurance of 1,000 repeated cycles, and excellent adhesion. The stretchable electrodes showed excellent bendability and twistability in which the electrodes can be bent up to 1 mm and twisted up to 90° without any damage; thus, they are highly applicable as stretchable electrodes for wearable electronics. Additionally, the Ag composites were explored for use in a radio-frequency (RF) stretchable antenna to confirm the application of the laser-sintering method for stretchable and wearable electronic devices. The stretchable dipole antenna showed an excellent radiation efficiency of 95% and a highly stable operation, even when stretched to 90% strain.

The transfer process of moving individual micro-LED chips to a display substrate has been regarded as the biggest technical obstacle for the commercialization of displays based on micro-LEDs. To date, various candidate technologies have not been able to meet the requirements of the transfer process in terms of productivity, yield, capability, known good die compatibility, and cost. In addition, after the transfer process, bonding between the micro-LED and the display substrate is typically accomplished through a process different from the transfer process, which also requires a technical solution. Generally, the bonding process proceeds sequentially after the mass transfer. Therefore, to increase the productivity, yield, capability, and compatibility of both processes, a new method is needed that considers the characteristics of both processes. In this work, we have successfully developed a novel process in which transfer and bonding occur at the same time. This process was named SITRAB, standing for SImultaneous TRAnsfer and Bonding. The SITRAB process was developed by applying our proprietary material called SITRAB adhesive to the LABC (Laser-Assisted Bonding with Compression) process, which we proposed several years ago. The total process time of SITAB was less than 10 s. Its working area could be easily enlarged, according to the homogenized area of the LABC equipment. Because the LABC process uses an IR laser, it is much cheaper than processes that require UV laser equipment. The major features of the STRAB adhesive are that it is fume free, and it is compatible with the LABC and repair process. The adhesive used during the SITRAB process is not cured, so additional transfer and bonding is possible with the applied adhesive on the substrate. A display substrate with a $\mathrm{35}\times \mathrm{35}$ micro-LED array was developed using the SITRAB process and the adhesive. The size of the micro LED-chip was about $\mathrm{80}\ \mu\mathrm{m}\times\ \mathrm{130}\ \mu \mathrm{m}$ , for which the conventional pick-and-place process could not be applied. Two types of solder materials were adopted and proven to be applicable. The repair process was confirmed based on SITRAB technology.

We have transferred and bonded a $\mu \text{LED}$ array to the patterned substrate using SITRAB (Simultaneous transferring and bonding) and ASP (Anisotropic Solder Paste). ASP contained epoxy as a base matrix, solder powder, and a small fraction of polymer balls and was applied to the substrate using screen printing. We have arrayed $\mathrm{20}\times \mathrm{20}$ of $\mu \text{LEDs}$ on the interposer, which is PDMS laminated glass wafer. The interposer was picked-up by quartz bonder-head and aligned to the substrate. After aligning the interposer to the substrate, the bonder-head moved to the substrate and applying pressure. While applying the pressure, the homogenized laser with a wavelength of 980nm was irradiated through the quartz bonder-head for a few seconds. In this process, transferring and bonding the $\mu \text{LED}$ array are performed simultaneously, so we named this process SITRAB. After the SITRAB process, the $\mu \text{LED}$ array bonded substrate was cured at 120°C for 2 hours. Because the ASP contains no vaporizable substance, there is no fume during the SITRAB process, and the cleaning is unnecessary after the post-cure process. Using ASP material and the SITRAB process, a $\mathrm{20}\times \mathrm{20}\ \mu \text{LED}$ -array was successfully transferred and bonded to the substrate. We observed the light output from the $\mu \text{LED}$ array bonded to the electrodes and measured a cross-sectional SEM image to analyze the bonding joint.

An epoxy-based solder paste (ESP) is a promising alternative to conventional solder pastes to improve the reliability of fine-pitch electrical joining because the epoxy encapsulates the solder joint. However, development of an appropriate epoxy formulation and investigation of its reaction mechanism with solder powder is challenging. In this study, we demonstrate a newly designed ESP consisting of diglycidyl ether of bisphenol F (DGEBF) resin, Sn-3.0 Ag-0.5 Cu (SAC305) solder powder, and L-glutamic acid (Glu), which is a proteinogenic amino acid for biosynthesis of proteins in living systems. The mechanism of the thermochemical reaction was explored and tentatively proposed, which reveals that the products of the reaction between SAC305 and Glu function as catalysts for the etherification of epoxides and alcohols produced by chemical bonding between DGEBF and Glu, consequently leading to highly crosslinked polymeric networks and an enhancement of impact resistance. Our findings provide further insight into the mechanism of the reaction between various formulations comprising an epoxy, amino acid, and solder powder, and their potential use as ESPs for electrical joining.

We bonded 70μm-thick Si chip and PI substrate with anisotropic solder paste (ASP) using laser-assisted bonding (LAB). ASP contains 5 vol.% of Sn/58Bi type 5, 6 vol.% of nonconductive PMMA balls with diameter of 20μm and thermosetting resin. We have applied ASP on the metal pattern on PI substrate. We have aligned the 70μm-thick Si chip on the PI substrate, and applied pressure to keep the bond line thickness uniform. Then, homogenized laser of which wavelength is 980nm, was irradiated on the Si chip for 5 seconds. When the laser is irradiated to the Si chip, the Si absorbs the laser and its temperature rises. Due to the high thermal conductivity of the Si and electrodes, the heat flux is concentrated on the electrode. Thus selective-solder joint is formed only between the electrodes of the chip and substrate. After the bonding process, we have cured bonded devices at 80°C for 2 hours. Measured contact resistance was ~0.03˟. There were no vaporized substances during the LAB process and the cleaning process is not necessary because the ASP contains no conventional flux and solvent.

Light-emitting diodes (LED) based displays are highlighted because of their interesting features compared with others. Especially, the LED display with active matrix (AM) has additional advantages such as low power consumption, elimination of current cross talk, and so on. However, AM- driving LED displays are fabricated on active substrates with transistors so that there are many problems like high-cost, difficulty in enhancing yield, and repair problem. As an alternative, an AM-driving LED display on a passive substrate has been developed in this study. One pixel is composed of RGB mini-LEDs and a control IC, fabricated using the CMOS technology. It can decrease the cost of a substrate compared with other AM LED displays, and keep constant the complexity of substrate regardless of the variations of the pitch of mini-LEDs. To mount mini-LEDs and control ICs at the same time needs the technical innovations because they have different surface finishes. Moreover, there is no solder resist layer on a substrate because of the fine-pitch of mini-LEDs and control ICs in a single pixel. The Solder-on-Pad (SoP), fluxing underfill, and Laser-Assisted Bonding (LAB) technologies are developed to implement digital signage modules, successfully.

We manufactured a hybrid adhesive film capable of curable deoxidization by combining thermoplastics and thermosets. The generated acidic moieties on this hybrid adhesive film completely removed the oxide layer of various Sn-based metal solders and Cu, leading to electrical interconnection. The chemical structure, reaction rate, and glass transition temperature (Tg) of the films were examined via Fourier transform infrared spectroscopy and differential scanning calorimetry. The initial reaction rate of the hybrid deoxidizing adhesive films increased with increasing concentration of anhydride and phenoxy resin. Meanwhile, the reaction rate at the end for curing completion decreased as a function of anhydride and phenoxy resin. Tg of the films decreased with the increase in anhydride and phenoxy resin concentrations. The fabricated film resulted in complete wetting of the solder/Cu for electrical interconnection between semiconductor chips as a promising electronic application. This hybrid film can be useful...

TSV chips were designed and fabricated to investigate the effects of the types of the stacking process on the manufacturability. Two processes were compared: thermocompression and laser bonding. The pitch of UBMs, solder bumps, and TSVs were 50μm. A fluxing underfill was developed and used as a pre-applied underfill during the stacking process. The bonding profiles and the solder joint morphology based on the thermocompression and the laser bonding were compared.

As a state-of-the-art interconnection technology, laser assisted bonding process has been applied to the LED bonding process with PET substrates. In conventional reflow processes, when PET substrate is exposed to the 150° C environment, the PET substrate is easily deformed due to the low glass transition temperature of 70° C. Therefore, it is necessary to carry out the semiconductor bonding process without changing the dimension of PET substrate during the bonding process. In this study, laser assisted bonding process was carried out using solvent free epoxy based solder paste which was newly developed. LED device (2mm x 2mm x 1mm) was bonded to a 0. 2mm thickness PET substrate with a laser of 980nm wavelength. The absorption, transmission and reflection of the laser were examined on the PET substrate. The power and time of the laser were precisely tuned to optimize device and substrate temperatures during the laser reflow process and prevent deformation of the PET substrate. Temperature of LED device and the PET substrate were measured by IR camera and thermo-couple during laser bonding process. The laser bonding process using the newly developed solvent free epoxy based solder paste was successfully performed on LED device and PET substrate without any damage. Laser reflow bonding process are expected to be extensively used in semiconductor packaging in the near future.

A LABC (Laser-Assisted Bonding with Compression) bonder and NCF (Non-Conductive Film) were developed to increase the productivity of the bonding process for the advanced microelectronic packaging technology. The design features of a LABC make its UPH above 1,000. The NCF was applied to both of LAB and TCB (Thermal Compression Bonding Technology). The 780µm-thick daisy chain top and bottom chips with the minimum pitch of 30µm and bump number of about 27,000 were prepared and tested to verify the LABC and NCF technology. The effects of the laser power on the joints quality after the LABC bonding process were investigated and compared with the joints formed by the TCB technology. Finally, the SAT (Scanning Acoustic Tomography) images of the test vehicles before and after the TCO (Pressurized oven) were observed to check the voids in the NCF after the LABC bonding process.

Integration technologies involving flexible substrates are receiving significant attention owing the appearance of new products regarding wearable and Internet of Things technologies. There has been a continuous demand from the industry for a reliable bonding method applicable to a low-temperature process and flexible substrates. Up to now, however, an anisotropic conductive film (ACF) has been predominantly used in applications involving flexible substrates; we therefore suggest low-temperature lead-free soldering and bonding processes as a possible alternative for flex-on-flex applications. Test vehicles were designed on polyimide flexible substrates (FPCBs) to measure the contact resistances. Solder bumping was carried out using a solder-on-pad process with Solder Bump Maker based on Sn58Bi for low-temperature applications. In addition, thermocompression bonding of FPCBs was successfully demonstrated within the temperature of 150 °C using a newly developed fluxing underfill material with fluxing and curing capabilities at low temperature. The same FPCBs were bonded using commercially available ACFs in order to compare the joint properties with those of a joint formed using solder and an underfill. Both of the interconnections formed with Sn58Bi and ACF were examined through a contact resistance measurement, an 85 °C and 85% reliability test, and an SEM cross-sectional analysis.

Flexible displays and electronics are emerging technology. A PET substrate is one the strong candidate substrate materials for the applications, however, its low thermal stability limited its penetration depth in the market. We propose the interconnection process using laser and hybrid underfill as a technical solution to prevent the thermal deformation of a PET film during the interconnection process. The LED module array is successfully fabricated on a PET substrate using laser and hybrid underfill. The process features a very short cycle time and tiling process for the high throughput.

The requirement for success of via filling is its ability to fill via holes completely without producing voids or seams. Defect free via filling was obtained by optimizing plating conditions such as current mode, current density and additives. However, byproducts stemming from the breakdown of these organic additives reduce the lifetime of the devices and plating solutions. In this study, the relationship between JGB and current density on the copper via filling was investigated without the addition of other additives to minimize the contamination of copper via. AR 4 with 15 µm diameter via were used for this study. The pulse current was used for the electroplating of copper and the current densities were varied from 10 to 20 mA/cm 2 and the concentrations of JGB were varied from 0 to 25 ppm. The map for the JGB concentration and current density was developed. And the optimum conditions for the AR 4 via filling with 15 µm diameter were obtained.

Energy Harvesting Devices Research Section, Information & Communications Core Technology Research Laboratory218, Gajeong-ro, Yuseong-gu, Daejeon 305-700, Korea(Received June 10, 2015: Corrected June 17, 2015: Accepted June 19, 2015)Abstract: Novel low-volume solder-on-pad (SoP) process is proposed for a fine pitch Cu pillar bump interconnection.A novel solder bumping material (SBM) has been developed for the 60 µm pitch SoP using screen printing process. SBM,which is composed of ternary Sn-3.0Ag-0.5Cu (SAC305) solder powder and a polymer resin, is a paste material to performa fine-pitch SoP in place of the electroplating process. By optimizing the volumetric ratio of the resin, deoxidizing agent,and SAC305 solder powder; the oxide layers on the solder powder and Cu pads are successfully removed during thebumping process without additional treatment or equipment. The Si chip and substrate with daisy-chain pattern are fab-ricated to develop the fine pitch SoP process and evaluate the fine-pitch interconnection. The fabricated Si substrate has6724 under bump metallization (UBM) with a 45 µm diameter and 60 µm pitch. The Si chip with Cu pillar bump isflip chip bonded with the SoP formed substrate using an underfill material with fluxing features. Using the fluxing underfillmaterial is advantageous since it eliminates the flux cleaning process and capillary flow process of underfill. The optimizedinterconnection process has been validated by the electrical characterization of the daisy-chain pattern. This work is thefirst report on a successful operation of a fine-pitch SoP and micro bump interconnection using a screen printing process. Keywords: solder-on-pad, fine pitch bumping, micro bump interconnection

Recently, we have witnessed the gradual miniaturization of electronic devices. In miniaturized devices, flip-chip bonding has become a necessity over other bonding methods. For the electrical connections in miniaturized devices, fine-pitch solder bumping has been widely studied. In this study, high-volume solder-on-pad (HV-SoP) technology was developed using a novel maskless printing method. For the new SoP process, we used a special material called a solder bump maker (SBM). Using an SBM, which consists of resin and solder powder, uniform bumps can easily be made without a mask. To optimize the height of solder bumps, various conditions such as the mask design, oxygen concentration, and processing method are controlled. In this study, a double printing method, which is a modification of a general single printing method, is suggested. The average, maximum, and minimum obtained heights of solder bumps are , , and , respectively. It is expected that the HV-SoP process will reduce the costs for solder bumping and will be used for electrical interconnections in fine-pitch flip-chip bonding.

A novel interconnection technology based on a 52InSn solder was developed for flexible display applications. The display industry is currently trying to develop a flexible display, and one of the crucial technologies for the implementation of a flexible display is to reduce the bonding process temperature to less than . InSn solder interconnection technology is proposed herein to reduce the electrical contact resistance and concurrently achieve a process temperature of less than . A solder bump maker (SBM) and fluxing underfill were developed for these purposes. SBM is a novel bumping material, and it is a mixture of a resin system and InSn solder powder. A maskless screen printing process was also developed using an SBM to reduce the cost of the bumping process. Fluxing underfill plays the role of a flux and an underfill concurrently to simplify the bonding process compared to a conventional flip-chip bonding using a capillary underfill material. Using an SBM and fluxing underfill, a pitch InSn solder SoP array on a glass substrate was successfully formed using a maskless screen printing process, and two glass substrates were bonded at .

An electro-plating technology on a cured isotropic conductive pattern with a hybrid Cu paste composed of resin matrix, copper, and solder powders has been developed. In a conventional technology, Ag paste was used to perform a conductive pattern on a PCB or silicon substrate. From previous research, the electrical conductive mechanism and prin- ciple of the hybrid Cu paste were concisely investigated. The isotropic conductive pattern on the PCB substrate was per- formed using screen-printing technology. The optimum electro-plating condition was experimentally determined by processing parameters such as the metal content of the hybrid Cu paste, applied current density, and time for the electro- plating in the plating bath. The surfaces and cross-sections were observed using optical and SEM photographs. In con- clusion, the optimized processing conditions for Cu electro-plating technology on the conductive pattern were a current density of 40mA/cm 2 and a plating time of 20min on the hybrid Cu paste with a metal content of 44 vol.%. More details of the mechanical properties and processing conditions will be investigated in further research.

3D multi-stack integration with through silicon via (TSV) and non-conductive paste (NCP) is one of the candidate packaging technologies that can successfully meet the requirements of high-end electronic products. A clear relationship should be established between the material properties of NCP and reliability of 3D TSV multi-stack chips. This research aims to characterize the mechanical properties of NCP with 0, 20, 30, and 40 wt% silica filler and examine the influence of different silica contents in NCP on the reliability of 3D TSV multi-stack chips under thermal shock test. Results show that adding silica filler does not influence the curing behavior of the NCP material, although it decreases the coefficient of thermal expansion and increases Young's modulus and viscosity. The thermal shock test reveals that the specimens bonded with NCP without silica filler fail before those bonded with NCP of 40 wt%. The failure mechanism for all specimens is recognized as a crack in the TSV/redistribution layer interface. These results suggest that the NCP is beneficial to the reliability of 3D TSV multi-stack chips.

A variety of underfill materials as film-and paste-types have been developed for electronic package applications, as the die/pitch becomes thinner and finer, recently. The paste-based adhesion such as isotropic conductive pastes (ICPs) is hard to precisely control the required amount, resulting in surplus resins at edges and a laborious process. Common film-based adhesion is deficient in flux, limited to non-oxidizable electrode elements, like Au stud bumps. This film formation also requires solvents or additional UV-curable resins. Here, to overcome these drawbacks, we generated flux-infiltrated adhesive films via a solvent-free, thermally-induced film formation, based on our previous research involved with fluxing underfill pastes. Thermoplastic materials and heat-curable epoxy resins were utilized for newly designed films as a matrix and an adhesive, respectively. They were mixed together at a temperature between the thermoplastic melting and the beginning of epoxy cure to prevent solidification and reaction. The melt-mixture was coated and at the end cooled down onto plastic release films. This solvent-free, facile film fabrication is a promising pathway in terms of convenient processing conditions allowing a solvent-free process during film production. This film also provides a simultaneous fluxing/underfill function during reaction at a temperature where solder components are melted. This solvent-free non-conductive film (NCF) showed good solder wettability and transmittance ∼90%. Toward this end, the combination of thermoplastics and curable resin facilitates flux-adhesive film formation without a solvent and thus has potential to be used for a wide range of electronic package applications.

For a high-power device of hybrid electrical vehicles requiring high thermal reliability, the concept of low temperature sintering material composed of Ag coated Cu, Sn/Ag/Cu ternary solder and chemical components removing oxide on the surface of metal fillers was studied under a processing temperature of 240oC. Due to the limited thermal reliability of conventional Tin based solder systems, the high-power device applications with a good thermal performance have been hindered because these solder materials were possibly re-melted under extreme operating conditions. Isotropic conductive paste based on thermosetting resin was previously investigated. With increase of temperature during processing, the Sn/Ag/Cu ternary solder in low temperature sintering material was melted at the melting temperature (220oC) of solder, and then, Sn was reacted with the adjacent Cu particles, producing intermetallic compound such as Cu 6 Sn 5 or Cu 3 Sn. The metallic reaction between Tin and copper increased the melting temperature of the yielded metal filler from 220oC to 360oC. Therefore, we expect the enhanced reliability and thermal stability of low temperature sintering material, compared to the solder-based conventional conductive interconnection materials, because the created metal filler product (intermetallic compound) in low temperature sintering material after processing will not be re-melted at the similar operating temperature to the processing temperature, 240oC. After sintering processing of low temperature sintering material in present research, 3.7 wt.% of the residue chemicals was detected by TGA because most of chemical components was evaporated during sintering process. In conclusion, low temperature sintering material is believed to be a good candidate material for the high-power device packaging due to the phenomena of the increased melting temperature of metal filler.

Power amplifiers used for high-speed radio frequency (RF) consume a considerable amount of power and create thermal burden. The heat flux per module of power amplifiers that are based on gallium nitride (GaN) component devices, which have recently entered maturity in the market and shown excellent RF power output, is approximately 190 W/cm2, thereby causing thermal management burden. When the number of arrays increases, and the power consumption is large, liquid cooling methods, such as maximizing cooling performance by optimizing the liquid flow path design or designing the cooling liquid line as close as possible to the heat source in the power package module, are mainly considered. However, the close contact of the liquid lines to the heat source may form a dewdrop in the package module, thereby decreasing reliability, especially in a radar system with power array modules. As an alternative, dry cooling methods that use heat pipes have been investigated. However, radar array package modules have not elicited much research attention. In this study, an aluminum flat heat pipe (AFHP) was developed for a 1 × 4 3D array module for the RF power amplifier of the next-generation X-band radar. This module is lightweight and is based on aluminum material, has a flat shape, and facilitates antenna interconnection through its hole structures. Subsequently, an AFHP with an antenna interconnection structure was designed and fabricated. Moreover, the temperature limit in the GaN power device and the temperature distribution in the entire package module were evaluated by testing the 1 × 4 antenna array module structure at 193 W/cm2 of heat flux per module. The results obtained in this study could confirm the possibility of using AFHP as a dry cooling method to high heat flux application.

Popular solder-bumping mechanisms such as electroplating and stencil printing suffer from either high process costs or technical limitations. A low-cost solder-on-pad (SoP) process has been developed to meet the requirements of fine-pitch solder bumping. This paper focuses on the characterization and estimation of the SoP process. To form a solder bump without soldering defects, optimum process conditions should be carefully designed. A model to estimate the bump volume and predict the bump height is suggested. By optimizing the composition of solder paste material called solder-bump-maker, and by adjusting the process conditions, Sn-Ag-Cu solder bumps with different heights are obtained. The experiments and analysis to understand the impact of parameters were based on test vehicles with 80 μm pitch size. Then, the measured heights of solder bumps are compared with the model to see how they fit the estimation. Finally, a similar process has been conducted to test vehicles with pitch sizes of 150 and 40 μm, to confirm the scalability of the SoP process in different pitch sizes.

For the fine-pitch application of flip-chip bonding with semiconductor packaging, fluxing and hybrid underfills were developed. A micro-encapsulated catalyst was adopted to control the chemical reaction at room and processing temperatures. From the experiments with a differential scanning calorimetry and viscometer, the chemical reaction and viscosity changes were quantitatively characterized, and the optimum type and amount of micro-encapsulated catalyst were determined to obtain the best pot life from a commercial viewpoint. It is expected that fluxing and hybrid underfills will be applied to fine-pitch flip-chip bonding processes and be highly reliable.

A cost-effective and simple solder on pad (SoP) process is proposed for a fine-pitch microbump interconnection. A novel solder bump maker (SBM) material is applied to form a 60-μm pitch SoP. SBM, which is composed of ternary Sn3.0Ag0.5Cu (SAC305) solder powder and a polymer resin, is a paste material used to perform a fine-pitch SoP through a screen printing method. By optimizing the volumetric ratio of the resin, deoxidizing agent, and SAC305 solder powder, the oxide layers on the solder powder and Cu pads are successfully removed during the bumping process without additional treatment or equipment. Test vehicles with a daisy chain pattern are fabricated to develop the fine-pitch SoP process and evaluate the fine-pitch interconnection. The fabricated Si chip has 6,724 bumps with a 45-μm diameter and 60-μm pitch. The chip is flip chip bonded with a Si substrate using an underfill material with fluxing features. Using the fluxing underfill material is advantageous since it eliminates the flux cleaning process and capillary flow process of the underfill. The optimized bonding process is validated through an electrical characterization of the daisy chain pattern. This work is the first report on a successful operation of a fine-pitch SoP and microbump interconnection using a screen printing process.

For the formation of solder bumps with a fine pitch of 130 μm on a printed circuit board substrate, low-volume solder on pad (LVSoP) technology using a maskless method is developed for SAC305 solder with a high melting temperature of 220°C. The solder bump maker (SBM) paste and its process are quantitatively optimized to obtain a uniform solder bump height, which is almost equal to the height of the solder resist. For an understanding of chemorheological phenomena of SBM paste, differential scanning calorimetry, viscosity measurement, and physical flowing of SBM paste are precisely characterized and observed during LVSoP processing. The average height of the solder bumps and their maximum and minimum values are 14.7 μ m, 18.3 μm, and 12.0 μm, respectively. It is expected that maskless LVSoP technology can be effectively used for a fine-pitch interconnection of a Cu pillar in the semiconductor packaging field.