Chang-Kyu Chung

Material properties of anisotropic conductive films (ACFs) and their flip chip assembly reliability in NAND flash memory applications

In this paper, the material properties of anisotropic conductive films (ACFs) and ACF flip chip assembly reliability for a NAND flash memory application were investigated. Measurements were taken on the curing behaviors, the coefficient of thermal expansion (CTE), the modulus, the glass transition temperature (Tg), and the die adhesion strength of six types of ACF. Furthermore, the bonding processes of the ACFs were optimized. After the ACF flip chip assemblies were fabricated with optimized bonding processes, reliability tests were then carried out. In the pressure cooker test, the ACF with the highest adhesion strength showed the best reliability and the ACF flip chip assembly revealed no delamination at the chip-ACF interface, even after 96 h. In the high temperature storage test and the thermal cycling test, the reliability of the ACF flip chip assembly strongly depends on the Tg value of the ACF. In the thermal cycling test, in particular, which gives ACF flip chip assemblies repetitive shear stress, high value of CTE above Tg accelerates the failure rate of the ACF flip chip assembly. From the reliability test results, ACFs with a high Tg and a low CTE are preferable for enhancing the thermal and thermo-mechanical reliability. In addition, a new double-sided chip package with a thickness of 570 μm was demonstrated for NAND flash memory application. In conclusion, this study verifies the ACF feasibility, and recommends the optimum ACF material properties, for NAND flash memory application.

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.

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).

Effect of Conductive Particle Properties on the Reliability of Anisotropic Conductive Film for Chip-on-Glass Applications

This paper describes how the material properties of conductive particles in anisotropic conductive films (ACFs) affect the electrical conductivity and the reliability of ACF interconnections for chip-on-glass (COG) applications. For the conductive particles, Au/Ni-coated polymer particles with a 5-diameter were used. Two different types of conductive particles were characterized with respect to their mechanical and electrical properties, such as ball hardness, recovery behavior, and electrical resistance. In addition, two ACFs were fabricated in the form of a double-layered structure, in which the thickness of the ACF and a nonconductive film (NCF) layer were optimized to have as many conductive particles as possible on the bump after COG bonding. The electrical contact resistance of an ACF interconnection in a COG structure depends mainly on the electrical properties of conductive particles in the ACF. The electrical reliability of an ACF interconnection in a COG structure also depends more on the electrical properties than the mechanical properties of conductive particles under a high-temperature and humid condition. Conductive particles with a lower electrical resistance, higher mechanical hardness, and lower recovery rate show better reliability than conductive particles with a higher electrical resistance, lower mechanical hardness, and higher recovery rate. Cross-sectional scanning electron microscopic (SEM) pictures of a COG interconnection show the deformation of two different conductive particles after the reliability tests. The ACF interconnections in the edge or corner of a driver IC show less reliable joints due to high absorption of moisture.

Effects of Conductive Particles on the Electrical Stability and Reliability of Anisotropic Conductive Film Chip-on-Board Interconnections

The effects of conductive particles on the electrical stability and reliability of anisotropic conductive film (ACF) joints for chip-on-board applications were investigated. In this paper, two types of conductive particles were prepared. One was a conventional Ni/Au-coated polymer ball, and the other was a Ni/Au-coated polymer ball with Ni/Au-projections. According to the results of a nano-indentation experiment of a conductive particle, the elastic recovery of a single conductive particle decreased as the applied load and the deformation of the conductive particle increased. The evaluated conductive particles which had the same polymer core showed the similar load-deformation behaviors regardless of the existence of Ni/Au-projections. The contact resistances of ACF joints using each conductive particle as a function of bonding pressure were measured, and the results showed that the contact resistances of ACF joints with the metal-projection-type conductive particles were lower and more stable than those with the conventional conductive particles. Especially at lower bonding pressure, ACF joints with the metal-projection-type conductive particles were much more stable and had lower contact resistances than the conventional ACF joints. According to the results of thermal cycling (T/C) reliability test, the metal-projection-type conductive particles also enhanced T/C reliability of ACF joints when compared with the conventional conductive particles.

The effects of the degree of cure of anisotropic conductive films (ACFs) on the contraction stress build-up of ACFs and ACF joints stability for chip-on-flex (COF) applications

In this paper, the effects of the degree of cure of an anisotropic conductive film (ACF) on the material properties and the contraction stress build-up of the ACF and ACF joints stability were investigated. The degrees of cure of the ACF as a function of bonding times were quantitatively obtained by a dynamic DSC study and an attenuated total reflectance/Fourier transform infrared (ATR/FT-IR) analysis. According to the results, the thickness expansion rate of the ACF as a function of temperature decreased and the storage modulus increased as the degree of cure increased. In addition, the contraction stress of partially cured ACF with the degree of cure below 40% was much smaller than that of fully cured ACF. The ACF contact resistances decreased and the ACF peel adhesion strengths increased as the degree of cure of the ACF increased. In particular, poor electrical contact was observed when the degree of cure of ACFs was below 40 %. The ultimate tensile strengths (UTSs) of the ACF increased as the degree of cure increased, and they were closely related to the ACF peel adhesion strengths. Furthermore, the ACF joints with the degree of cure below 40% were more unstable than those with the degree of cure over 90% during 85 °C and 85% relative humidity test (85 °C/85% RH).

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.

Wafer Level Packages (WLPs) using Pre-Applied Anisotropic Conductive Films (ACFs)

Wafer level package (WLP) is 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 reports on new WLP design, material and process have been accomplished according to performance and reliability requirement of the devices to be packaged. For, anisotropic conductive films (ACFs) flip chip assembly has been widely used for flat panel display (FPD) and general semiconductor packaging applications because of lower cost, higher performance and environmentally green packaging process. However, there has been no previous attempt on the wafer level flip chip assembly using ACFs. In this study, wafer level flip chip packages using pre-applied ACFs (denoted as ACF-WLPs) were investigated. After ACF pre-lamination on an electroplated Au bumped wafer, and subsequent singulation, and singulated chips were flip-chip assembled on an organic substrate using a thermo-compression bonding method. Au plated bumps were well assembled on Ni/Au pads of organic substrates. The electrical, mechanical properties and the reliabilities of ACF-WLPs were evaluated and compared with conventional ACF flip chip assemblies using thermo-compression method. Contact resistance measurement was performed after under 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 m per a bump, strong bump adhesion, and similar reliability behaviors compared with conventional ACF flip chip joints using a thermo-compression bonding. As a summary, new ACF-WLPs were successfully demonstrated for flip chip assembly, and ACF-WLPs can be widely used for many flip chip assembly appliations such as COB (chip-on-board), COF (chip-on-flex) and COG (chip-on-glass).

Enhancement of electrical stability of anisotropic conductive film (ACF) interconnections with viscosity-controlled and high T-g ACFs in fine-pitch chip-on-glass applications

This paper describes how anisotropic conductive film (ACF) properties including viscosity affect the electrical stability of ACF interconnections for fine pitch chip-on-glass (COG) applications. In this study, new ACFs for COG applications were designed by combining a high viscosity ACF layer and a low viscosity NCF layer to prevent the electrical shortage between bumps. As expected, the viscosity-controlled ACF showed better electrical insulation stability than a conventional ACF in fine pitch COG assemblies. According to the results of thermo-mechanical analysis (TMA) and dynamic-mechanical analysis (DMA), the viscosity-controlled ACF showed the improved thermo-mechanical properties such as lower coefficient of thermal expansion (CTE), higher storage modulus (E 0 ) at higher temperature region, and higher glass transition temperature (Tg) than the conventional ACF. Furthermore, hot air reliability test and pressure cooker test (PCT) results showed that the viscosity-controlled ACF with higher Tg had better hot air test and PCT reliabilities than the conventional ACF.

Wafer Level ACA Packages and their Applications to Advanced Electronic Packaging

Recently, wafer level package (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 single chip packages. Many developments on new WLP design, materials and processes have been accomplished according to the electrical, mechanical performance and reliability requirement of the devices to be packaged. For lower cost, higher performance and environmentally green packaging process, anisotropic conductive adhesives(ACAs) including films (ACFs) and pastes (ACPs) assembly has been widely used such as ultra-fine pitch flat panel display (FPD) and general semiconductor packaging applications. However, there has been no previous attempt using ACAs on WLP. In this study, wafer level packages using pre-applied ACAs on an entire wafer for flip-chip assembly on organic substrates have been investigated, and the effect of process parameters, such as ACAs lamination/coating, wafer dicing, and ACAs flip chip assembly, on the ACA wafer level package performance were investigated. After ACA lamination or coating on Au stud bumped 6 inch wafers, and subsequent singulation by diamond sawing. And then singulated chips were flip-chip assembled on an organic substrate using a thermo-compression bonding method. ACA joints between Au stud bumps and substrate metal pads using WLACA method showed stable bump contact resistance of 8~9m¿ per a bump which is the same as the typical thermocompression-bonded ACA joint. Reliability of the ACA-WLP joints are the same as that of thermocompression-bonded ACA joint.