Yoo-Sun Kim

Parallel machine scheduling considering a job-splitting property

This paper focuses on the problem of scheduling jobs on parallel machines considering a job-splitting property. In this problem, it is assumed that a job can be split into a discrete number of subjobs and they are processed on parallel machines independently. A two-phase heuristic algorithm is suggested for the problem with the objective of minimizing total tardiness. In the first phase, an initial sequence is constructed by an existing heuristic method for the parallel-machine scheduling problem. In the second phase, each job is split into subjobs considering possible results of the split, and then jobs and subjobs are rescheduled on the machines using a certain method. To evaluate performance of the suggested algorithm, computational experiments are performed on randomly generated test problems. Results of the experiments show that the suggested algorithm performs better than an existing one.

Flux function added solder anisotropic conductive films (ACFs) for high power and fine pitch assemblies

In this study, flux function added solder ACF was proposed in order to be used not only with the ultrasonic bonding method which can break solder using ultrasonic vibration oxides but also with conventional thermo-compression bonding method. Since the solder oxides cannot be broken only with conventional thermo-compression method, we had to remove the oxide chemically. The test vehicles were consist of polyimide based flexible substrates and FR-4 organic rigid boards which have 300 μm pitch Cu patterns with electroless nickel immersion gold (ENIG) surface finish. Solder ACFs 50 μm thick were epoxy based adhesive films containing flux functional additive. The films contain 5-15 μm diameter SAC305 (96.5Sn-3.0Ag-0.5Cu) solder particles which are used as conductive particles and 5 μm diameter Au coated Ni particles which are used as spacers. According to the experimental results, the flux functional additive did not change the curing reaction of the solder ACF. Moreover, the addition of flux functional additive caused significant improvement of electrical properties of solder ACF joints such as power handling capability and reliability. Therefore, using a flux function added solder ACFs can be used for various FOB and FOF assemblies and can provide an alternative interconnection for high power and fine pitch assemblies.

Effects of ACF Bonding Parameters on ACF Joint Characteristics for High-Speed Bonding Using Ultrasonic Bonding Method

As the use of anisotropic conductive films (ACFs) in flex-on-board bonding has increased recently, the need for high-speed bonding for higher productivity has developed. For high-speed bonding, a fast-curable acrylic resin has been used instead of an epoxy resin, and ultrasonic (US) bonding is introduced to allow for short bonding time at high bonding temperatures. However, it is possible for fast curable acrylic ACFs at 250°C to be fully cured before the resins flow out sufficiently between the electrodes. In such cases, the ACF joints could show poor electrical characteristics due to there being no conductive particle capture. Therefore, in this paper, in order to understand important factors for high-speed bonding, the effects of the bonding parameters, substrate geometry, and ACF material property on the ACF joint characteristics, such as resin flow and joint resistance, are investigated using acrylic-based ACFs and US bonding. The fast-curable acrylic resin is fully cured at a bonding temperature of 250°C within 1 s. However, the ACF joints bonded at 250°C show joint gaps larger than the diameter of conductive particles, i.e., 8 μm, due to insufficient resin flow and open failure due to no particle capture. As the bonding pressure increases from 2 to 4 MPa, the ACF joint gap at 250°C is smaller than the conductive particle size, and the joint resistance reaches about 20 mΩ within 7 s when using 3-mm-long electrodes and a high-viscosity resin. However, in order to use a shorter bonding time and the common bonding pressure of 3 MPa, the effects of substrate geometry and resin property on ACF joint characteristics are investigated. As the electrode length of the substrates are decreased from 3 to 1 mm, the ACF joints show stable ACF joint gaps and joint resistance at 3 MPa despite the bonding temperature of 250°C applied within 1 s. In terms of the ACF resin properties, when a resin with low minimum viscosity is used, the ACF joint gaps are smaller than the diameter of the conductive particles. Furthermore, the joint resistance of low-minimum-viscosity ACF is more stable than that of the ACF joints bonded with the high-minimum-viscosity resin at 250°C within 1 s, even though a 3-mm-long electrode is used. These results indicate that the substrate geometry of the printed circuit board and the ACF resin viscosity are significant factors for high-speed bonding to obtain stable ACF joints at a high bonding temperature of 250°C within 1 s.

Ultrasonic-assisted thermo-compression bonding method for high-performance solder anisotropic conductive film (ACF) joints

In this study, in order to improve the reliability of ACF joints in the electronic packaging with minimized mechanical damages, solder ACF bonding using ultrasonic-assisted thermo-compression (TC) bonding was demonstrated. This technology utilizes small ultrasonic vibration with TC bonding to break solder oxide layers for better wetting of solder on the electrodes in the ACF joints. Ultrasonic-assisted TC bonding was also performed with various bonding parameters in comparison with conventional TC bonding methods. Bonded solder ACF joints were investigated in terms of solder joining behavior, and reliabilities. According to the experimental results, ultrasonic-assisted TC bonded ACF joints showed approximately two times larger diameter of solder wetting area on electrodes than that of conventional TC bonded ACF joints. Moreover, when bonded to ENIG finished Cu pads, the Au contents of solder particles in the ACF joints bonded with ultrasonic-assisted TC bonding was 12 atomic % which was three times higher than those bonded with conventional TC bonding. In the unbiased autoclave test (121°C, 2atm, 100%RH), ultrasonic-assisted TC bonded ACF joints showed open circuit failure 3 times lately than those with conventional TC bonding due to good interconnection between solder particles and electrodes by sufficient solder wetting. These results mean that solder ACF bonding using ultrasonic-assisted TC bonding would be one of the solution for high reliable ACF joints in the package without mechanical damages.

Reliability improvement methods of solder anisotropic conductive film (ACF) joints using morphology control of solder ACF joints

As the use of ACFs increased in various areas of electronic packaging such as semiconductors and flexible devices, demands for highly reliable ACFs have been also increased. For this demand, solder ACFs which use solder particles as conductive particles were introduced. In the solder ACF bonding, the solder ACF joint morphology should be controlled because the failures of solder ACF joints by crack propagation have close relationship with morphology of solder ACF joints. In this study, in order to improve the reliability of solder ACF joints in electronic packaging, the morphologies of solder ACF joints were controlled and the reliability depending on the morphologies was investigated. For the investigation of ACF joint reliability, unbiased autoclave tests were performed at 121°C, 2atm, and 100% relative humidity. According to the results, as bonding pressure increased from 2 MPa to 6 MPa, aspect ratio (Joint area/joint gap) increased by increased joint area and decreased joint gap. In unbiased autoclave tests, some of solder ACF joints with bonding pressure of 2MPa showed electrical open failures after 60 hours because tensile stress was applied to solders due to polymer resin expansion by water absorption. On the other hand, solder ACF joints with bonding pressure of 6 MPa showed no open failures for 60 hours due to higher tensile strength by higher aspect ratio compared with those with bonding pressure of 2 MPa. Solder ACF joints with bonding temperature of 250°C showed hourglass shape by large spreading of solders and 5 times higher radius of curvature of stress concentration region than that of solder joints with bonding temperature of 200°C which showed barrel shape. Solder ACF joints with bonding temperature of 250°C showed higher reliability than those with bonding temperature of 200°C due to smaller amount of concentrated stress by hourglass shape. In terms of ACF resin materials, despite of bonding temperature of 200°C, low curing rate acrylate ACFs showed hourglass shape and higher reliability than that of solder joints with conventional acrylate ACFs. The reason of that was low curing rate acrylate ACFs showed lower degree of cure of resin around solder at solder melting point than that of conventional ACFs. These results indicate that solder ACF joint morphology can be controlled by adjusting bonding conditions and ACF materials. Furthermore, the morphologies of solder ACF joints can be significantly important factors for highly reliable ACF joints in high temperature and high humidity.

Low temperature camera module assembly using acrylic-based solder ACFs with ultrasonic-assisted thermo-compression bonding method

In this study, in order to improve thermal stability and reliability of camera module assembly for high yield rate, the camera module assembly using low temperature curable acrylic solder ACFs with ultrasonic-assisted thermo-compression (TC) bonding has been introduced. In general, the acrylic-based ACFs are cured at lower bonding temperature than conventionally used epoxy-based ACFs. And, the recently adopted ultrasonic-assisted TC bonding utilizes ultrasonic vibration during vertical thermo-compression to break solder oxide layers. Bonded solder ACF joints of camera modules for this study were investigated in terms of curing behavior, peel adhesion strength, and solder wetting. In addition, thermal stability of internal camera module assembly at this bonding condition was also investigated. According to the experiment results, acrylic-based solder ACF joints showed peel adhesion strength above recommendation with full degree of cure and excellent solder wetting on the electrodes at the low bonding temperature of 150°C and the ultrasonic amplitude of 6 μm. At the bonding temperature of 150°C, the internal temperature of a camera module was about 110°C which was lower than 137°C, the glass transition temperature of the lens. Therefore, the low temperature curable acrylic-solder ACFs with ultrasonic-assisted TC bonding is appropriate for the camera modules, since the ACFs were optimized in terms of degree of cure, peel adhesion strength and solder wetting and the camera modules were thermally stable at the low bonding temperature of 150°C.

Reliability improvement of solder anisotropic conductive film (ACF) joints by controlling ACF polymer resin properties

As the use of ACFs has increased in electronic packaging applications, demands for highly reliable ACFs has been also increased. For this demand, solder ACFs which use solder particles as conductive particles of ACFs has been previously introduced combined with an ultrasonic bonding method. In the solder ACFs bonding, solders are melted simultaneously with polymer resin curing reaction and the solder particles between two metal electrodes form various solder joint morphologies depending on the ACF resin curing characteristics. Furthermore, the solder joint morphologies between two metal electrodes can affect the ACF joint reliability at high temperature and high humidity reliability conditions. Therefore, the study on the morphology control of solder ACF joints and understanding the solder crack initiation and propagation during the reliability tests are needed for the highly reliable solder ACFs bonding. In this study, to understand the formation of solder morphology during solder ACFs bonding process, in-situ ACF resin viscosity was investigated. In-situ ACF resin viscosity can be predicted by the fitted equation at the specific temperatures. Resin viscosity at specific temperatures increased as degree of cure of ACF resin increased with the S-shaped growth curve, which was fitted with the Boltzmann equation. From the fitted equation, the ACF resin viscosity can be predicted at solder melting point (MP). Using this prediction, the effects of resin properties on solder morphologies were investigated. Slow curing acrylic ACFs showed concave shaped solder joints with larger spreading of solder particles due to the lower viscosity of resin surrounding solder particles. The degree of cure of slow curing acrylic ACFs at the solder MP was lower compared with that of conventional acrylic ACFs. In terms of resin viscosity, lower viscosity epoxy resin showed concave shaped solder joints because of lower viscosity at the solder MP compared with that of conventional acrylic resin which showed convex shaped solder joints. Solder joints bonded with slow curing acrylic ACFs and low viscosity epoxy ACFs showed higher reliability than that of solder joints with conventional acrylic ACFs, because solder joints with slow curing ACFs and lower viscosity epoxy ACFs formed concave shaped solder joints resulting in lower stress concentration due to larger curvatures of stress concentration regions than that of conventional acrylic ACFs.

Effects of Bonding Pressures and Bonding Temperatures on Solder Joint Morphology and Reliability of Solder ACF Bonding

In this paper, in order to improve the reliability of anisotropic conductive film (ACF) interconnections, solder ACF joints were investigated in terms of solder joint morphology. ACFs are film-type interconnection adhesive materials that consist of polymer adhesive resins and randomly dispersed conductive particles. Recently, in order to obtain high reliability of ACF joints, solder ACFs that use solder particles as conductive particles of ACFs have been introduced combined with an ultrasonic bonding method. However, further researches are needed in the area of crack initiation and the propagation of solder ACF joints at high temperature and high humidity conditions due to the stress generated by hygroscopic expansion of ACF resin. Therefore, in order to solve the crack initiation of solder ACF joints, solder ACF joint morphology should be controlled and optimized to minimize the solder crack-related reliability problems. In this paper, solder ACF joint reliability was investigated depending on the solder morphologies of ACF joints bonded with various bonding pressures and temperatures. According to the results, as bonding pressure increased from 2 to 6 MPa, aspect ratio (joint diameter/joint gap) increased due to the increased joint area and decreased joint gap. In the pressure cooker test (PCT) reliability tests, as solder aspect ratio increased, electrical resistances were more stable after 60 h of the tests due to higher joint strength. In the target bonding temperature profile of 250 °C, solder joints showed a concave shape. However, joints bonded at in the target bonding temperature profile of 200 °C showed a convex shape. It is mainly due to the lower degree of cure of resin when ACF temperature reached to solder Melting Point (MP) in the target bonding temperature profile of 250 °C compared with that in the target bonding temperature profile of 200°, because ACF temperature reached to solder MP in shorter time in the target bonding temperature profile of 250 °C. Concave-shaped solder joints showed higher PCT reliability than the convex-shaped solder joints due to the lower stress concentration. These results indicate that solder ACF joint morphology was a significantly important factor for highly reliable solder ACF joints in high temperature and high humidity reliability conditions.

The effects of self-fluxing additives in solder anisotropic conductive films (ACFs) on solder wettability and joint reliability of flex-on-board (FOB) assemblies

In this study, self-fluxing additives were added in solder anisotropic conductive films (ACFs) in order to eliminate solder oxide resulting in an excellent solder wetting on metal pads during ACF bonding. Since the solder oxide causes poor wettability of solder particles, solder oxide was chemically removed by self-fluxing additives in solder ACFs. The test boards were 25 μm-thick polyimide based FPCs and 1 mm-thick FR-4 organic rigid PCBs which have 400 μm pitch Cu patterns with electroless nickel and immersion gold (ENIG) surface finish. Newly formulated solder ACFs were acrylic based adhesives film which can be fully cured above 150°C containing self-fluxing additives. The film contained 25 μm diameter Sn58Bi particles which has 138°C melting point and 8 μm diameter Ni particles as a spacer to maintain the gap between metal electrodes. According to the experimental results, the addition of self-fluxing additive caused significant improvement of solder wettability and reliability of solder ACF joints. Therefore, using a flux function added solder ACFs can be used for various applications such as FOB and FOF assemblies, and can provide an alternative interconnection method for high power and fine pitch assemblies for many other applications.

Ultrasonic-Assisted Thermocompression Bonding Method of Solder Anisotropic Conductive Film Joints for Reliable Camera Module Packaging

In this paper, to improve the reliability of anisotropic conductive film (ACF) joints in the camera module packaging without any mechanical damages, ultrasonic-assisted thermocompression (TC) bonding was investigated using solder ACFs. This technology uses small lateral-direction ultrasonic vibration during the TC ACF bonding to break solder oxide layers for sufficient solder wetting on the electrodes at ACF joints. Bonded solder ACF joints were investigated in terms of solder ACF joint morphology, resistance, adhesion strengths, and reliabilities. In addition, the stability of camera module components after bonding process was also observed. Ultrasonic-assisted TC bonded solder ACF joints showed excellent solder wetting compared with conventional TC bonded solder ACF joints due to broken solder oxide layers by applied ultrasonic vibration. The Au contents diffused from electroless nickel immersion gold-finished Cu pads to solders was about 12 atomic% in the ultrasonic-assisted TC bonding. The Au contents were three times higher than that of the solders in the conventional TC bonding. In the ultrasonic-assisted TC bonding, ultrasonic amplitude was optimized. At the optimized amplitude, solders were well wetted and showed no mechanical damages of camera module components. Adhesion strength of ultrasonic-assisted TC bonded solder ACF joints was almost the same as that of TC bonded ACF joints due to fully cured resins in these samples. During the unbiased autoclave test (121 °, 2 atm, 100%RH), ultrasonic-assisted TC bonded ACF joints showed no open-circuit failure compared with conventional TC bonded Ni ACF joints due to metallurgical alloy joints between solder balls and electrodes by sufficient solder wetting. These results prove that ultrasonic-assisted TC bonding using solder ACFs can be applied for highly reliable ACF assembly in the camera module packaging without any mechanical damages.