Kyoung-Lim Suk

Nanofiber anisotropic conductive adhesives (ACAs) for ultra fine pitch chip-on-film (COF) packaging

Nanofiber ACFs composed of adhesive resins, conductive particles, and nanofibers were demonstrated for ultra fine pitch COF packages. PAN nanofibers and PAN nanofiber containing conductive particles inside the fibers were successfully produced using electrospinning methods. The effects of nanofiber thickness and structure on electrical properties of nanofiber ACFs were investigated.

Study on Fine Pitch Flex-on-Flex Assembly Using Nanofiber/Solder Anisotropic Conductive Film and Ultrasonic Bonding Method

A new concept of anisotropic conductive film (ACF) called nanofiber/solder ACF combined with an ultrasonic bonding method can overcome limitations of fine pitch flex-on-flex (FOF) assembly using conventional ACF, such as short circuit issues, low current-handling capability, and poor reliability. To fabricate the nanofiber/solder ACF, SAC305 (96.5% Sn, 0.5% Cu, and 3% Ag) conductive solder balls are added to polyacrylonitrile polymer solution and ejected as nanofibers containing SAC305 conductive solder balls by an electrospinning method. This new concept of nanofiber/solder ACF successfully prevents short circuits between neighboring conducting electrodes by limiting the free movement of conducting balls and insulating individual conductive solder balls with coated nanofiber layers. Moreover, SAC305 conductive solder balls in nanofiber/solder ACF are completely melted and made metallurgical alloy contact between FOF electrodes during the ultrasonic bonding, resulting in 41% more electrical power being carried, and showing significant improvement in reliability performance compared with conventional nickel ball ACF.

Embedded chip-in-flex (CIF) packages using wafer level package (WLP) with pre-applied anisotropic conductive films (ACFs)

For maximizing space efficiency and reducing process steps, embedded chip-in-flex (CIF) packages using wafer level package (WLP) with pre-applied anisotropic conductive films (ACFs) are one of the innovative packaging technology. This study was focused on the demonstration of CIF packages and their reliability evaluation. WLP was successfully performed in case of void-free ACF lamination on a 50 µm thin wafer, wafer dicing without ACF delamination, and flip-chip assembly which showed stable bump contact resistances. After flex-on-flex (FOF) assembly conditions were optimized, the CIF packages were successfully fabricated. The reliability of the packages such as high temperature/ humidity test (85 °C/85% RH), high temperature storage test (HTST), thermal cycling test (T/C) was evaluated. As a summary, the CIF packages showed excellent 85 °C/85% RH reliability.

Advancing Electronic Packaging Using Microsolder Balls: Making 25-nm Pitch Interconnection Possible

Electronic packaging technology has advanced in the direction of integrating diverse components into one package to satisfy market demands for multifunctionality as well as portability. For this reason, various packaging structures have been introduced, such as multichip modules, package on package, package in package, and eventually three-dimensional (3-D)-chip stacks. All of these approaches require increased input/output (I/O) counts, resulting in fine-pitch assembly. Therefore, the most critical issue in current electronic packaging is how to assemble fine-pitch components while avoiding an electrical short circuit in the x-y direction. Much research has been done on fine-pitch interconnecting technology using microsolder balls smaller than 200 nm, but the problems of solder-ball handling and low yield remain. In addition, there have been few reports so far about the fine-pitch interconnection below 25-nm pitch using microsolder balls. Three-dimensional-chip stacks require an additional microsolder and copper hybrid bumping and patterning processes on through silicon via (TSV), which increases the processing cost.

Effects of nanofiber materials of nanofiber anisotropic conductive adhesives (nanofiber ACAs) for ultra-fine pitch electronic assemblies

We investigate the effects of material properties of nanofiber on fine pitch ACA interconnection stability in terms of thermal and mechanical properties. Three kinds of nanofiber materials, polyvinylidene fluoride (PVDF), ethylene vinyl alcohol (EVOH), and Nylon 6, which have different tensile properties and melting temperatures, are coupled with conductive particles to form PVDF-CPIN, EVOH-CPIN, and Nylon 6-CPIN, respectively, and the three nanofiber ACAs are fabricated and characterized.

Measurement and Analysis for Residual Warpage of Chip-on-Flex (COF) and Chip-in-Flex (CIF) Packages

A flip-chip package using adhesive interconnection consists of materials which have different coefficients of thermal expansion (CTE). The package experiences temperature higher than room temperature during the assembly process and is also exposed to the thermal cycling load during its lifetime. As a result, flip-chip packages have residual warpage after completion of the assembly process. Excessive warpage causes various reliability problems. Therefore, residual warpage is an essential factor for evaluating the reliability of electronic packages. In this paper, we evaluated the warpage of chip-on-flex (COF) packages using the moiré methods. A chip-in-flex (CIF) package developed to increase the binding force between the chip and the substrate was also evaluated with the same methods. Finite element analysis (FEA) was also performed for comparison with the experimental results. Based on the FEA result, effective design parameters for the CIF package were found to reduce the residual warpage.

Warpage Behavior and Life Prediction of a Chip-on-Flex Package Under a Thermal Cycling Condition

Flip-chip assembly has been widely adapted to various electronic devices due to advantages, such as miniaturization of electronic devices and high density integration. The chip-on-flex (COF) package used in this paper is a flip-chip package with an anisotropic conductive adhesive flim (ACF) interconnection and shows flexible features and reduced thickness compared with chip-on-board (COB) packages. All electronic packages experience temperature variation during service conditions and under environmental changes. Under temperature variation, stresses emerge due to the differences in the coefficient of thermal expansion among components. In order to evaluate the thermomechanical reliability of a COF package, a thermal cycling (TC) test was conducted. A moiré experiment using Twyman/Green interferometry was performed to observe the warpage behavior of the package under a TC condition. Through the experiment, the rate of change of chip warpage with respect to temperature (dw/dT) as a parameter of the thermal damage model was obtained. A finite element analysis (FEA) was also performed to calculate the maximum shear stress at the ACF layer as another parameter of the model. From the experiment and FEA results, the thermal damage model can accurately represent the TC life of the COF package. However, based on observations of different warpage behavior of the COF package compared with a COB package from the moiré experiment, a modified thermal damage model that can predict the TC life of both packages more accurately was proposed.

Microsolder ball incorporated nanofiber anisotropic conductive adhesives (microsolder/nanofiber ACAs)

We suggest and investigate a new concept of nanofiber ACA that incorporates microsolder balls into nanofiber to obtain stable three-dimensional electrical properties of fine pitch electronics. This adhesive offers many advantages, such as suppressing microsolder ball movement during resin flow, perfect X-Y axes insulation at 25 μm fine pitch, and easy fine solder ball handling. Microsolder balls can be successfully incorporated into a nanofiber structure through an electrospinning process, and they have good solderability within the nanofiber/epoxy matrix.

Studies on various 2-metal chip-on-flex (COF) packaging methods

Various chip-on-2-metal flex (2-metal COF) packaging methods using such as ACF and NCF adhesives, and AuSn metallurgical bonding methods, were investigated in terms of electrical characteristics, flip chip joint quality, and reliability performances. 2-metal flex substrate and test chip were designed to include different pitches, 35 um, 25 um, and 20 um pitch. Thermal cycling test (TC test, −40 °C ~ +125 °C, 1000 cycles), and high temperature storage test (HTS test, 125 °C, 1000 hrs) were conducted to verify reliability of the 2-metal COF packages by various bonding methods. All the COF packages showed good TC and HTS reliability, whereas electrically shorted joints were observed during reliability tests only at the 20 um pitch of ACF joints. Therefore, for less than 20 um pitch of 2-metal COF packages, NCF adhesives bonding and AuSn metallurgical bonding methods are recommended, while all the ACF and NCF adhesives bonding and AuSn metallurgical bonding methods can be applied for larger than 25 um pitch applications.

20 µm-pitch complaint-bump-bonded chip-on-flex by pre-applied wafer level adhesives

A novel process which combined wafer level package technology with ultra-fine pitch chip-on-flex (COF) by sidewall-insulated compliant bumps was developed. We laminated two types of adhesives, single-layer non-conductive adhesive (NCA) and double-layer of non-conductive adhesive/anisotropic conductive adhesive (NCA/ACA), on wafers, respectively. After wafers with laminated adhesives were diced into chips, thermo-compression bonding process was executed by using high accuracy flip-chip bonder. Proper bonding parameters, such as bonding temperature, time, and pressure, were performed during bonding process. Both mechanical and electrical tests were carried out to evaluate bonding quality of adhesive-bonded COFs. 90-degree of peeling test was conducted as mechanical test to evaluate adhesion of bonding interface. Measurements of electrical insulating resistance and 606 I/Os daisy chain resistance were conducted. The reliability of pre-applied adhesive COFs with 20 εm-pitch sidewall-insulated compliant bumps was also examined. Samples that passed electrical tests were then performed to reliability test in terms of thermal humidity storage test (THST) with 85% related humidity (RH) at 85°C. From the results presented, 20 εm-pitch sidewall-insulated compliant-bump-bonded COF packages with pre-applied NCA and NCA/ACA chips were successfully demonstrated. It showed high reliability and potentiality for ultra-fine pitch COF interconnections.