Myung-Jin Yim

Design and understanding of anisotropic conductive films (ACF's) for LCD packaging

Anisotropic conductive film (ACF) composed of an adhesive resin and fine conductive fillers such as metallic particles or metal-coated polymer balls are key materials for fine pitch chip-on-film (COF) and chip-on-glass (COG) LCD packaging technologies. To understand and design better quality ACF materials, the theoretical electrical conduction model with physical contact mechanism was simulated and experimentally proven. To understand the contact area changes, two pressure dependent models (1) elastic/plastic deformation; (2) finite element method (FEM) model were developed, and experimentally proven by various ACFs fabricated in our laboratory. Experimental variables were applied bonding pressure, number, size, mechanical and electrical properties of nickel powders and Au-coated polymer conductive particles. It was found that the models were in good agreement with experimental results except at higher bonding pressures. In general, as bonding pressure increases, a sharp decrease of contact resistance followed by a constant value is observed after reaching the critical bonding pressure. However, an excessive bonding pressure rather increased the connection resistance of ACF interconnection. If more conductive particles-were added, the connection resistance rapidly decreased and then became constant. This is because the counter-effect of two opposing factors, the resistance increase caused by a decrease of contact area per one particle and the resistance decrease caused by increasing number of conduction path. In addition, environmental effects on contact resistance such as thermal aging, high temperature/humidity aging, and temperature cycling were also investigated. As a whole, better design of ACF materials can be achieved by understanding the ACF conduction mechanism.

The contact resistance and reliability of anisotropically conductive film (ACF)

The effect of bonding pressure on the electrical and mechanical properties of anisotropic conductive film (ACF) joint using nickel particles and metal-coated polymer ball-filled ACFs was investigated. The contact resistance decreases as the bonding pressure increases. Contact resistance of ACF is determined by the contact area change between particles and contact substrates. Electrical conduction through the pressure engaged contact area between conductive particles and conductor substrates is the main conduction mechanism in ACF interconnection. In addition, environmental effects on contact resistance and adhesion strength such as thermal aging, high temperature/humidity aging and temperature cycling were also investigated. Interestingly, the contact resistances of the excessively bonded samples deteriorated more than those of optimally bonded ones. Increasing contact resistance and decreasing adhesion strength after harsh environmental tests were mainly due to the loss of contact by thermal stress effect and moisture absorption, and also partially due to the formation of metal oxide on the conductive particles.

Thermal Cycling Reliability and Delamination of Anisotropic Conductive Adhesives Flip Chip on Organic Substrates With Emphasis on the Thermal Deformation

a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejon 305-701, South Korea b MEMS Lab., Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, South Korea c CARE-Electronic Packaging Laboratory, Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, South Korea

Highly reliable non-conductive adhesives for flip chip CSP applications

Non-conductive adhesives (NCA), widely used in display packaging and fine pitch flip chip packaging technology, have been recommended as one of the most suitable interconnection materials for flip-chip chip size packages (CSPs) due to the advantages such as easier processing, good electrical performance, lower cost, and low temperature processing. Flip chip assembly using modified NCA materials with material property optimization such as CTEs and modulus by loading optimized content of nonconductive fillers for the good electrical, mechanical and reliability characteristics, can enable wide application of NCA materials for fine pitch first level interconnection in the flip chip CSP applications. In this paper, we have developed film type NCA materials for flip chip assembly on organic substrates. NCAs are generally mixture of epoxy polymer resin without any fillers, and have high CTE values un-like conventional underfill materials used to enhance thermal cycling reliability of solder flip chip assembly on organic boards. In order to reduce thermal and mechanical stress and strain induced by CTE mismatch between a chip and organic substrate, the CTE of NCAs was optimized by filler content. The flip chip CSP assembly using modified NCA showed high reliability in various environmental tests, such as thermal cycling test (-55/spl deg/C/+160/spl deg/C, 1000 cycle), high temperature humidity test (85/spl deg/C/85%RH, 1000 h) and high temperature storage test (125/spl deg/C, dry condition). The material properties of NCA such as the curing profile, the thermal expansion, the storage modulus and adhesion were also investigated as a function of filler content.

Microwave model of anisotropic conductive film flip-chip interconnections for high frequency applications

Microwave model and high-frequency measurement of the anisotropically conductive film (ACF) flip-chip interconnection was investigated using a microwave network analysis. The test integrated circuits (ICs) were fabricated using a 1-poly and 3-metal 0.6 /spl mu/m Si process with an inverted embedded microstrip structure. As flip chip bumps, electroless Ni/Au plating was performed on Al input/output (I/O) pads of test IC chips, As an interconnect material, several ACFs were prepared and flip-chip bonded onto the Rogers(R) RO4003 high frequency organic substrate. S-parameters of on-chip and substrate were separately measured in the frequency range of 200 MHz to 20 GHz using a microwave network analyzer HP8510 and cascade probe, and the cascade transmission matrix conversion was performed. The same measurements and conversion were conducted on the test chip mounted substrates at the same frequency range. Then impedance values in flip-chip interconnection were extracted from cascade transmission matrix. The extracted model parameters of the 100 /spl mu/m/spl times/100 /spl mu/m interconnect pad show the resistance increases due to skin effect up to 8 GHz. Above this frequency, conductive loss of epoxy resin also increases. Reactance is dominantly affected by inductance of Ni/Au bumps and also conductive particles in the ACF interconnection over the measured frequency range. The inductance value of ACF flip chip interconnection is below 0.05 nH and the contact resistance is below 0.9 R. In addition, the effects of different ACF conductive particle materials on high frequency electrical behavior in GHz range were also investigated, Different ACF conductive particle materials show difference in the reactance, resistance, and resonance frequency behavior up to 13 GHz. Our results indicate that high frequency electrical performance of ACF combined with electroless Ni/Au bump interconnection is acceptable for use in the high frequency flip chip application up to 13 GHz. Finally, 80-ps rise time digital signal transmission with small dispersion low loss reflection was demonstrated through the flip-chip interconnection with combination of ACF and Ni/Au bump.

Effects of silica filler and diluent on material properties and reliability of nonconductive pastes (NCPs) for flip-chip applications

In this paper, thermomechanical and rheological properties of nonconductive pastes (NCPs) depending on silica filler contents and diluent contents were investigated. And then, thermal cycling (T/C) reliability of flip chip assembly using selected NCPs was verified. As the silica filler content increased, thermomechanical properties of NCPs were changed. The higher the silica filler content was added, glass transition temperature (T/sub g/) and storage modulus at room temperature became higher while coefficient of thermal expansion (CTE) decreased. On the other hand, rheological properties of NCPs were significantly affected by diluent content. As the diluent content increased, viscosity of NCP decreased and thixotropic index increased. However, the addition of diluent deteriorated thermomechanical properties such as modulus, CTE, and T/sub g/. Based on these results, three candidates of NCPs with various silica filler and diluent contents were selected and used as adhesives for reliability test of flip chip assemblies. T/C reliability test was performed by measuring changes of NCP bump connection resistance. Results showed that flip chip assembly using NCP with lower CTE and higher modulus exhibited better T/C reliability behavior because of reduced shear strain in NCP adhesive layer.

High-frequency SPICE model of anisotropic conductive film flip-chip interconnections based on a genetic algorithm

This paper firstly reports on the high-frequency SPICE model of the ACF flip-chip interconnections up to 13 GHz. The extraction process is based on an optimization procedure, called a genetic algorithm, which is known as a robust optimization tool. The proposed equivalent circuit model of the ACF interconnection can readily be used in SPICE circuit simulations for signal integrity analysis of high-frequency packages. Two different ACF interconnections were studied using the Au-coated polymer ball and Ni-filled ball. The extracted models of the two ACFs were found strongly dependent on not only size and rigidity of the conducting balls, but also on their magnetic permeability.

Ultrasonic Bonding Using Anisotropic Conductive Films (ACFs) for Flip Chip Interconnection

In this paper, a novel anisotropic conductive film (ACF) flip chip bonding method using ultrasonic vibration for flip chip interconnection is demonstrated. The curing and bonding behaviors of ACFs by ultrasonic vibration were investigated using a 40-kHz ultrasonic bonder with longitudinal vibration. In situ temperature of the ACF layer during ultrasonic (U/S) bonding was measured to investigate the effects of substrate materials and substrate temperature. Curing of the ACFs by ultrasonic vibration was investigated by dynamic scanning calorimetry (DSC) analysis in comparison with isothermal curing. Die adhesion strength of U/S-bonded specimens was compared with that of thermo-compression (T/C) bonded specimens. The temperature of the ACF layer during U/S bonding was significantly affected by the type of substrate materials rather than by the substrate heating temperature. With room the temperature U/S bonding process, the temperature of the ACF layer increased up to 300degC within 2 s on FR-4 substrates and 250degC within 4 s on glass substrates. ACFs were fully cured within 3 s by ultrasonic vibration, because the ACF temperature exceeded 300degC within 3 s. Die adhesion strengths of U/S-bonded specimens were as high as those of T/C bonded specimens both on FR-4 and glass substrates. In summary, U/S bonding of ACF significantly reduces the ACF bonding times to several seconds, and also makes bonding possible at room temperature compared with T/C bonding which requires tens of seconds for bonding time and a bonding temperature of more than 180degC.

Wafer-Level Flip Chip Packages Using Preapplied Anisotropic Conductive Films (ACFs)

Recently, wafer-level packaging (WLP) has become one of the promising packaging technologies due to its advantages, such as fewer processing steps, lower cost, and enhanced device performance compared to conventional single-chip packaging. Many developments on new WLP design, material, and process have been accomplished according to performance and reliability requirement of the devices to be packaged [1], [2]. For a lower cost, higher performance, and environmentally green packaging process, anisotropic conductive film (ACF) flip chip assembly has been widely used, such as in ultrafine-pitch flat panel display (FPD) and general semiconductor packaging applications, too. However, there has been no previous attempt on the wafer-level flip chip assembly using ACFs. In this paper, wafer-level flip chip packages using preapplied ACFs were investigated. After ACF prelamination on an electroplated Au bumped wafer, and subsequent singulation, singulated chips were flip-chip assembled on an organic substrate using a thermocompression bonding method. Au-plated bumps were well assembled on Ni/Au pads of organic substrates. The electrical, mechanical properties and the reliabilities of wafer-level flip chip assemblies (WL-FC As) were evaluated and compared with conventional ACF flip chip assemblies using the thermocompression method. Contact resistance measurement was performed after thermal cycling, high temperature/humidity, and pressure cooker test. ACF joints between electroplated Au bumps and substrate metal pads showed stable contact resistance of 5 mOmega per a bump, strong bump adhesion, and similar reliability behaviors compared with conventional ACF flip chip joints using a thermocompression bonding. As a summary, new wafer-level packages using preapplied ACFs were successfully demonstrated for flip chip assembly. The new wafer-level packages using preapplied ACFs can be widely used for many nonsolder flip chip assembly applications such as chip-on-board (COB), chip-on-flex (COF), and chip-on-glass (COG).

Over 10 GHz equivalent circuit model of ACF flip-chip interconnect using Ni-filled ball and Au-coated polymer balls

In this paper, we firstly present the equivalent circuit model of an anisotropic conductive film (ACF) flip-chip interconnect using Ni-filled balls and Au-coated polymer balls. The models were extracted up to the microwave frequency region over 10 GHz. The extracted model parameters of the Ni-filled ball interconnect are compared to those of the Au-coated polymer ball interconnect with respect to impedance and resonance frequency. The ACF using the Ni-filled ball can be manufactured with reduced manufacturing cost and a simplified process, while it still has comparable electrical performance to that of the Au-coated polymer ball. Thus far, the Au-coated ball interconnect has been most widely used. S-parameter measurements and subsequent microwave network analysis were used for the extraction procedure for the impedance parameters.