Kyung-Woon Jang

High reliable non-conductive adhesives for flip chip CSP applications

Chip Scale Packages (CSP) have been adapted for mobile phones, DVC, PC cards, PDAs and various other applications due to the robustness, cost effectiveness, and high reliability of CSP packages. For first level interconnection in CSP, wire bonding with encapsulation, Au stud bump by conductive adhesive, and solder bump interconnection with underfill process are mostly used. Non-conductive adhesives (NCA), widely used in display packaging and fine pitch flip chip packaging technology, are recommended as one of the most suitable interconnection materials for flip chip CSPs. NCA interconnection in flip chip assembly have many advantages such as easier processing, good electrical performance, lower cost, and low temperature processing. In this paper, we have developed film type NCA materials for flip chip assembly on organic substrates such as FR-4 printed circuit boards (PCBs) or BT resin. NCAs are generally a mixture of epoxy polymer resin without any fillers, and have high CTE values like conventional underfill materials used to enhance thermal cycling reliability of solder flip chip assembly on FR-4 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 modified NCA interconnection in flip chip CSP showed highly reliable interconnection when exposed to various environmental tests.

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.

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.

Epoxy/BaTiO/sub 3/ composite films and pastes for high dielectric constant and low-tolerance embedded capacitors fabrication in organic substrates

Epoxy/BaTiO/sub 3/ composite embedded capacitor films (ECFs) were newly designed for high dielectric constant and low-tolerance (less than /spl plusmn/5%) embedded capacitor fabrication for organic substrates. In terms of material formulation, ECFs are composed of a specially formulated epoxy resin and latent curing agent, and in terms of a coating process, a comma roll coating method is used for uniform film thickness in large area. The dielectric constant of ECF in high frequency range (0.5/spl sim/3 GHz) is measured using the cavity resonance method. In order to estimate dielectric constant, the reflection coefficient is measured with a network analyzer. The dielectric constant is calculated by observing the frequencies of the resonant cavity modes. Calculated dielectric constants in this frequency range are about 3/4 of the dielectric constants at 1 MHz. This difference is due to the decrease of the dielectric constant of the epoxy matrix. The dielectric relaxation of barium titanate (BaTiO/sub 3/: BT) powder is not observed within measured frequency. An alternative material for embedded capacitor fabrication is epoxy/BaTiO/sub 3/ composite embedded capacitor paste (ECP). It uses similar materials formulation like ECF and a screen printing method for film coating. The screen printing method has the advantage of forming a capacitor partially in the desired part. However, the screen printing makes surface irregularities during mask peel-off. Surface flatness is significantly improved by adding some additives and by applying pressure during curing. As a result, a dielectric layer with improved thickness uniformity is successfully demonstrated. Using epoxy/BaTiO/sub 3/ composite ECP, a dielectric constant of 63 and specific capacitance of 5.1 nF/cm/sup 2/ were achieved.

Effects of Anisotropic Conductive Film Viscosity on ACF Fillet Formation and Chip-On-Board Packages

In this paper, the effects of anisotropic conductive film (ACF) viscosity on ACF fillet formation and, ultimately, on the pressure cooker test (PCT) reliability of ACF flip chip assemblies were investigated. The ACF viscosity was controlled by varying the molecular weight of the epoxy materials. It was found that the ACF viscosity increased as the increase of molecular weight of the epoxy materials. However, there was little variation of the thermomechanical properties among the evaluated ACFs with different viscosites. Also, the results showed that the ACFs have no differences in moisture absorption rate, die adhesion strength, and degree-of-cure. In scanning electron microscopy images, the lower ACF viscosity resulted in the smoother ACF fillet shape and the higher fillet height. From the results of PCT, the ACF flip chip assembly with the smoother fillet shape showed better reliability in terms of contact resistance changes. After 130 h of PCT, the flip chip assembly with lower ACF viscosity also showed a lesser degree of delamination at the ACF/chip interface.

A Study of Hygrothermal Behavior of ACF Flip Chip Packages With Moiré Interferometry

A primary factor of anisotropic conductive film (ACF) package failure is delamination between the chip and the adhesive at the edge of the chip. This delamination is mainly affected by the thermal shear strain at the edge of the chip. This shear strain was measured on various electronic ACF package specimens by micro-Moire interferometry with a phase shifting method. In order to find the effect of moisture, the reliability performance of an adhesive flip-chip in the moisture environment was investigated. The failure modes were found to be interfacial delamination and bump/pad opening which may eventually lead to total loss of electrical contact. Different geometric size specimens in terms of interconnections were discussed in the context of the significance of mismatch in coefficient of moisture expansion (CME) between the adhesive and other components in the package, which induces hygroscopic swelling stress. The effect of moisture diffusion in the package and the CME mismatch were also evaluated by using the Moire interferometry. From Moire measurement results, we could also obtain the stress intensity factor K. Through an analysis of deformations induced by thermal and moisture environments, a damage model for an adhesive flip-chip package is proposed.

Wafer level packages (WLPs) using anisotropic conductive adhesives (ACAs) solution for flip-chip interconnections

In this study, wafer level packages (WLPs) using anisotropic conductive adhesives (ACAs) solution for flip-chip interconnections have been newly developed and the effects of process parameters on the wafer level package performance were investigated. At first, the effect of coating process parameters such as blade gap and temperature were investigated for uniform thickness coating without voids and bubbles on a Au bumped wafer. After solvent drying and the subsequent singulation of a B-stage ACA solution coated wafer, singulated chips were flip-chip assembled on organic substrates using a thermo-compression bonding method. The reliabilities of flip chip assemblies using WLP were evaluated in terms of high temperature/humidity, and pressure cooker test and compared with those of conventional anisotropic conductive film (ACF) package. In high temperature/humidity reliability test, there was no difference of flip chip reliabilities between two types of flip chip assemblies. Furthermore, in pressure cooker test (PCT), WLP using ACAs solution showed better reliability than conventional ACF package.

Epoxy/BaTiO3 (SrTiO3) Composite Films and Pastes For High Dielectric Constant and Low Tolerance Embedded Capacitors in Organic Substrates

Epoxy/BaTiO3 composite embedded capacitor films (ECFs) were newly designed for high dielectric constant and low tolerance (less than plusmn5%) embedded capacitor fabrication for organic substrates. In terms of material formulation, ECFs are composed of specially formulated epoxy resin and latent curing agent, and in terms of coating process, a comma roll coating method is used for uniform film thickness in large area. Dielectric properties of BaTiO3 (BT) & SrTiO3 (ST) composite ECF is measured MIM capacitors with oxygen plasma etched ECFs on 10times10 cm PCBs. For 1times1 mm capacitors with 12 um thickness and 50 vol.%BTO ECFs, dielectric constant of 39, dielectric loss of 0.017, capacitance density of 2.4 nF/cm2, and less than 10% tolerance were obtained. Dielectric constant of BTOECF is bigger than that of STECF, and it is due to difference of permittivity of BT and SrTiO3 particles. Dielectric constant of BT & ST ECF in high frequency range (0.5~10 GHz) is measured using a cavity resonance method. In order to estimate dielectric constants at high frequency, the reflection coefficient is measured with a network analyzer. Dielectric constant is calculated by observing the frequencies of the resonant cavity modes. Although there was no dielectric relaxation observed at pure epoxy, for BT ECF, there is the dielectric relaxation at 5~9GHz. It is mainly due to changing of polarization mode of BT powder itself. In contrast, there is no relaxation for ST ECF up to 10 GHz. Alternative material for embedded capacitor fabrication is epoxy/BT composite embedded capacitor pastes (ECPs). It uses similar materials formulation like ECFs and a screen printing method for film coating. The screen printing method has the advantage of forming capacitor partially in desired area. But the screen printing makes surface irregularity during mask peel-off. Surface flatness is significantly improved by adding some additives and by applying pressure during curing. As a result, dielectric layer with improved thickness uniformity is successfully demonstrated. For 4times4 mm capacitors with 11 um thickness and 50 vol.% BT ECFs, dielectric constant of 39, dielectric loss of 0.023, capacitance density of2.6nF/cm2, and 22% tolerance were obtained

High- reliability nonconductive adhesives for flip chip interconnections

Non-conductive adhesives (NCA) are one of the interconnection materials widely used in display packaging and flip chip packaging technology, especially for fine-pitch interconnection. NCA interconnection in flip chip assembly has many advantages such as easier processing, good electrical performance, lower cost, and low temperature processing. We have developed new NCA materials, film type, for flip chip assembly on organic substrate such as FR-4 printed circuit boards (PCBs). NCAs are generally mixture of epoxy polymer resin without any fillers. As a result, NCAs have higher CTE values than conventional underfill materials used to enhance thermal cycling reliability of solder flip chip assembly on FR-4 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 lowered by filling of nonconductive fillers of diameter below 1 gm. The modified NCA in flip chip interconnection between gold stud bumps of a chip and metal pads of PCB substrates show highly reliable interconnection when exposed to 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 hours) and high temperature storage test (125/spl deg/C, dry condition).

Wafer-Level Packages Using Anisotropic Conductive Adhesives (ACAs) Solution for Flip-Chip Interconnections

In this paper, wafer-level packages (WLPs) using an anisotropic conductive adhesives (ACAs) solution have been newly developed for flip-chip interconnections. WLPs using ACAs (ACA-WLPs) reduce processing steps compared to WLPs using anisotropic conductive films (ACFs), because ACA solution is directly coated on a wafer without an ACF formation process on the releasing film and an ACF lamination process on the wafer. The effects of ACA coating process parameters, such as blade gap and temperature, were first investigated for a uniform thickness coating without voids on an Au-bumped wafer. After solvent drying and subsequent singulation of a B-stage ACA-coated wafer, a singulated chip was flip-chip assembled on an organic substrate using a thermo-compression bonding method. The reliabilities of flip-chip assemblies using ACA-WLPs were evaluated in terms of a high temperature/humidity test, thermal cycling test, and pressure cooker test (PCT) and compared with corresponding results of conventional ACF flip-chip assemblies. In the high temperature/humidity reliability test and thermal cycling test, there was no difference in the flip-chip reliabilities between the two types of flip-chip assemblies. Furthermore, the flip-chip assemblies using ACA-WLPs showed better PCT reliability than the conventional ACF flip-chip assemblies.