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Article
Publication date: 21 September 2010

Bo Tao, Zhouping Yin and Youlun Xiong

From the viewpoint of degree of cure, the purpose of this paper is to find how to improve the reliability of flip‐chip packaging modules based on an anisotropically…

Abstract

Purpose

From the viewpoint of degree of cure, the purpose of this paper is to find how to improve the reliability of flip‐chip packaging modules based on an anisotropically conductive adhesive film (ACF) interconnection process.

Design/methodology/approach

The work begins by revealing the correlation of adhesive strength and contact resistance of flip‐chip joint interfaces with the degree of cure of the ACF. The effect of different degrees of curing on the electrical and mechanical properties of some typical ACF‐interconnected joints is studied, and the optimum degree of cure is suggested to achieve highly reliable ACF joints, where the performance variations of the adhesion strength and contact resistance are considered simultaneously. First, the degradation data of the contact resistance of some ACF assemblies, bonded with several degrees of cure, is collected during a standard high‐hydrothermal fatigue test. The resistance distribution is verified using a two‐parameter Weibull model and the distribution parameters are estimated, respectively. After that, a reliability analysis method based on the degradation data of contact resistance is achieved, instead of the traditional failure time analysis, and the reliability index, as well as the mean‐time‐to‐degradation of the ACF joints, as a function of the degree of cure, is deduced, through which the optimum degree of cure value and recommend range are suggested.

Findings

Numerical analysis and calculations are performed based on the experiments. Results show that the optimum degree of cure to achieve highly reliable joints is 83 per cent, and the recommend range is from 82 to 85 per cent for the ACF tested (considering a 95 per cent confidence interval).

Originality/value

The paper provides important support for optimizing the curing process for various ACF‐based packaging applications, such as chip‐on‐glass packaging of liquid crystal displays and flip‐chip bonding of radio frequency identification, etc.

Details

Soldering & Surface Mount Technology, vol. 22 no. 4
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 1 March 2005

K.K. Lee, N.H. Yeung and Y.C. Chan

Anisotropic conductive adhesive film (ACF) is used for very fine pitch applications in the microelectronics industry, such as flip chip (FC) technology. During the bonding…

Abstract

Purpose

Anisotropic conductive adhesive film (ACF) is used for very fine pitch applications in the microelectronics industry, such as flip chip (FC) technology. During the bonding process, bumps on the chip and pads on the substrate are first aligned and then heat and pressure are applied so as to apply thermal energy to the ACF for curing and to cause permanent plastic deformation of the conductive particles. Consequently, a permanent electrical and mechanical contact is formed between the bumps and the pads. The purpose of this study is to investigate the effect of the size and location of any voids in the ACF during subsequent solder reflow processes necessary for SMT component attachment. The paper also investigates the use of a protective aluminium cover during such reflow cycles, which reduces the temperature inside the ACF, and therefore, the stresses inside the ACF, especially when voids exist.

Design/methodology/approach

In this study, the ACF is a temperature dependent material having various void sizes, entrapped within the ACF. A finite element method is used to analyse the stresses within a FC on flex assembly.

Findings

The results indicate that the smaller the void, the larger the stress concentration. Also, the von Mises stress was found to be larger in the upper portion of the ACF, near the chip, than in the lower portion of the ACF nearer to the flexible substrate. This implies that the four corners of a void are seen to be the most likely site for crack initiation and propagation and therefore, for failure to occur. Moreover, the temperature profile of the reflow cycle and the locations of the voids have also been shown to affect the stress level within the ACF.

Originality/value

The value of this paper is to show how the presence of voids may affect the reliability of a FC assembly.

Details

Soldering & Surface Mount Technology, vol. 17 no. 1
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 12 August 2019

Lei Guo, Lien Zhu, Lei Ma, Jian Zhang, QiuYu Meng, Zheng Jin, Meihua Liu and Kai Zhao

The purpose of this paper is to prepare a spherical modifier-modified activated carbon fiber of high specific capacitance intended for electrode materials of supercapacitor.

Abstract

Purpose

The purpose of this paper is to prepare a spherical modifier-modified activated carbon fiber of high specific capacitance intended for electrode materials of supercapacitor.

Design/methodology/approach

In this study, phenolic-based microspheres are taken as modifiers to prepare PAN-based fiber composites by electrospinning, pre-oxidation and carbonization. Pearl-chain structures appear in RFC/ACF composites, and pure polyacrylonitrile fibers show a dense network. The shape and cross-linking degree are large. After the addition of the phenolic-based microspheres, the composite material exhibits a layered pearlite chain structure with a large porosity, and the RFC/ACF composite material is derived because of the existence of a large number of bead chain structures in the composite material. The density increases, the volume declines and the mass after being assembled into a supercapacitor as a positive electrode material decreases. The specific surface area of RFC/ACF composites is increased as compared to pure fibers. The increase in specific surface area could facilitate the diffusion of electrolyte ions in the material. Owing to the large number of bead chains, plenty of pore channels are provided for the diffusion of electrolyte ions, which is conducive to enhancing the electrochemical performance of the composite and improving the RFC/ACF composite and the specific capacitance of the material. The methods of electrochemical testing on symmetric supercapacitors (as positive electrodes) are three-electrode cyclic voltammetry, alternating current impedance and cycle stability.

Findings

The specific capacitance value of the composite material was found to be 389.2 F/g, and the specific capacitance of the electrode operating at a higher current density of 20 mA/cm2 was 11.87 F/g (the amount of the microsphere modifier added was 0.3 g). Using this material as a positive electrode to assemble into asymmetrical supercapacitor, after 2,000 cycles, the specific capacitance retention rate was 87.46 per cent, indicating excellent cycle stability performance. This result can be attributed to the fact that the modifier embedded in the fiber changes the porosity between the fibers, while improving the utilization of the carbon fibers and making it easier for electrolyte ions to enter the interior of the composites, thereby increasing the capacitance of the composites.

Originality/value

The modified PAN-based activated carbon fibers in the study had high specific surface area and significantly high specific capacitance, which makes it applicable as an efficient and environment-friendly absorbent, as well as an advanced electrode material for supercapacitor.

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Article
Publication date: 1 August 2002

C.M. Lawrence Wu and M.L. Chau

This paper presents a reliability assessment of adhesive joints using chip‐on‐glass (COG) technology which was conducted by testing samples at various aging temperatures…

Abstract

This paper presents a reliability assessment of adhesive joints using chip‐on‐glass (COG) technology which was conducted by testing samples at various aging temperatures and at high humidity.The range of aging temperatures took into account the glass transition temperature (Tg) of the adhesive films. The effects of high temperature and high humidity on the bond strength of flip‐chip‐on‐glass joints were evaluated by shear testing as well as by microstructural examination.It was found that aging generally caused a decrease in shear strength while the aging temperature was below the glass transition temperature of ACF. When the aging temperature was slightly above the Tg of the ACF, a significant decrease in shear strength was observed. Moreover, results from scanning electronic microscopy revealed the presence of some voids near the component bumps, resulting in high stresses at the high aging temperature. DSC results showed that the ACF was not fully cured, allowing moisture absorption more seriously than a fully cured ACF, leading to joint degradation.

Details

Soldering & Surface Mount Technology, vol. 14 no. 2
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 7 September 2015

Lilan Gao, Hong Gao and Xu Chen

This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the…

Abstract

Purpose

This review paper aims to provide a better understanding of formulation and processing of anisotropic conductive adhesive film (ACF) material and to summarize the significant research and development work for the mechanical properties of ACF material and joints, which helps to the development and application of ACF joints with better reliability in microelectronic packaging systems.

Design/methodology/approach

The ACF material was cured at high temperature of 190°C, and the cured ACF was tested by conducting the tensile experiments with uniaxial and cyclic loads. The ACF joint was obtained with process of pre-bonding and final bonding. The impact tests and shear tests of ACF joints were completed with different aging conditions such as high temperature, thermal cycling and hygrothermal aging.

Findings

The cured ACF exhibited unique time-, temperature- and loading rate-dependent behaviors and a strong memory of loading history. Prior stress cycling with higher mean stress or stress amplitude restrained the ratcheting strain in subsequent cycling with lower mean stress or stress amplitude. The impact strength and adhesive strength of ACF joints increased with increase of bonding temperature, but they decreased with increase of environment temperature. The adhesive strength and life of ACF joints decreased with hygrothermal aging, whereas increased firstly and then decreased with thermal cycling.

Originality/value

This study is to review the recent investigations on the mechanical properties of ACF material and joints in microelectronic packaging applications.

Details

Soldering & Surface Mount Technology, vol. 27 no. 4
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 1 June 2005

M.J. Rizvi, Y.C. Chan, C. Bailey, H. Lu and A. Sharif

Anisotropic conductive film (ACF) is now an attractive technology for direct mounting of chips onto the substrate as an alternative to lead‐free solders. However, despite…

Abstract

Purpose

Anisotropic conductive film (ACF) is now an attractive technology for direct mounting of chips onto the substrate as an alternative to lead‐free solders. However, despite its various advantages over other technologies, it also has many unresolved reliability issues. For instance, the performance of ACF assembly in high temperature applications is questionable. The purpose of this paper is to study the effect of bonding temperatures on the curing of ACFs, and their mechanical and electrical performance after high temperature ageing.

Design/methodology/approach

In the work presented in this paper, the curing degree of an ACF at different bonding temperatures was measured using a differential scanning calorimeter. The adhesion strength and the contact resistance of ACF bonded chip‐on‐flex assembly were measured before and after thermal ageing and the results were correlated with the curing degree of ACF. The ACF was an epoxy‐based adhesive in which Au‐Ni coated polymer particles were randomly dispersed.

Findings

The results showed that higher bonding temperatures had resulted in better ACF curing and stronger adhesion. After ageing, the adhesion strength increased for the samples bonded at lower temperatures and decreased for the samples bonded at higher temperatures. ACF assemblies with higher degrees of curing showed smaller increases in contact resistance after ageing. Conduction gaps at the bump‐particle and/or particle‐pad interfaces were found with the help of scanning electron microscopy and are thought to be the root cause of the increase in contact resistance.

Originality/value

The present study focuses on the effect of bonding temperatures on the curing of ACFs, and their adhesion strength and electrical performances after high temperature ageing. The results of this study may help the development of ACFs with higher heat resistance, so that ACFs can be considered as an alternative to lead‐free solders.

Details

Soldering & Surface Mount Technology, vol. 17 no. 2
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 7 September 2015

Hong Gao, Jianhua Ma, Lilan Gao, Dunji Yu and Jinsheng Sun

The purpose of this paper is to determine: how the thermal cycling aging affects the ratcheting behavior of anisotropic conductive adhesive film (ACF); how the loading…

Abstract

Purpose

The purpose of this paper is to determine: how the thermal cycling aging affects the ratcheting behavior of anisotropic conductive adhesive film (ACF); how the loading conditions and loading history affect the ratcheting strain and strain rate of ACF with different thermal cycling aging histories.

Design/methodology/approach

The ACF of CP6920F was cured at 190°C in an electro-thermal vacuum drying apparatus for 30 s. The cured specimens were put into the thermal cycling chamber (−40-150°C) for aging to 25, 50, 100, 200 and 500 cycles. A series of uniaxial ratcheting tests of aged ACF after different thermal cycles was carried out under stress control at 80°C.

Findings

The ACF subjected to larger number of thermal aging cycles exhibits less ratcheting strain under the same loading conditions. The ACF with the same thermal cycling aging history shows more ratcheting strain and a higher ratcheting strain rate when loaded under a larger mean stress or stress amplitude or a lower loading rate. The ratcheting behavior of aged ACF is found to be more sensitive to the lower loading rate. The higher mean stress (or stress amplitude) enhances the deformation resistance and consequently restrains the ratcheting strain of subsequent cycling with a lower mean stress (or stress amplitude). The prior lower loading rate accelerates the plastic deformation more significantly than the higher one.

Originality/value

The influencing trends of thermal cycling aging, loading condition and loading history on ratcheting behavior of ACF are obtained, which is important for the design and safety assessment of ACF joints.

Details

Soldering & Surface Mount Technology, vol. 27 no. 4
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 1 January 2006

G. Dou, Y.C. Chan, J.E. Morris and D.C. Whalley

The resistance, capacitance and inductance of anisotropic conductive film (ACF) connections determine their high frequency electrical characteristics. The presence of…

Abstract

Purpose

The resistance, capacitance and inductance of anisotropic conductive film (ACF) connections determine their high frequency electrical characteristics. The presence of capacitance and inductance in the ACF joint contributes to time delays and cross‐talk noise as well as simultaneous switching noise within the circuit. The purpose of this paper is to establish an experimental method for estimating the capacitance and inductance of a typical ACF connection. This can help to provide a more detailed understanding of the high frequency performance of ACF assemblies.

Design/methodology/approach

Experiments on the transient response of an ACF joint were performed using a digital oscilloscope capable of achieving the required ns resolution. An equivalent circuit model is proposed in order to quantify the capacitance (C) and inductance (L) of a typical ACF connection and this model is fitted to the experimental data. The full model consisted of two resistors, an inductor, and a capacitor.

Findings

The capacitance and inductance of a typical ACF connection were estimated from the measured transient response using Kirchhoff's voltage law. The method for estimation of R, L, and C from the transient response is discussed, as are the RLC effects on the high frequency electrical characteristics of the ACF connection.

Research limitations/implications

There was decay time deviation between the calculation and the experiment. It may have resulted from the skin effect in the high frequency response and the adhesive surrounding joint as well. The main reason may be the capacitance zctric lost. Further research work will be done to determine more accurately the dielectric losses in anisotropic conductive adhesive (ACA) joint.

Originality/value

This paper presents a new method to characterise the high frequency properties of ACA interconnections and will be of use to engineers evaluating the performance of ACF materials in high frequency applications.

Details

Soldering & Surface Mount Technology, vol. 18 no. 1
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 1 September 2005

L. Ali, Y.C. Chan and M.O. Alam

Anisotropic conductive film (ACF) offers miniaturization of package size, reduction in interconnection distance and high performance, cost‐competitive packaging and…

Abstract

Purpose

Anisotropic conductive film (ACF) offers miniaturization of package size, reduction in interconnection distance and high performance, cost‐competitive packaging and improved environmental impact. However, a major limitation for ACF is the instability caused by thermal warpage. The purpose of this paper is to study the effects of thermal warpage on contact resistance in real time i.e. make online measurements of contact resistance fluctuations while the assembly undergoes thermal shock.

Design/methodology/approach

The ACF assemblies are subjected to thermal cycling with different temperature profiles that have peak temperatures either below or above the glass transition temperature (Tg) of the ACF. The flex substrate used was made of polyimide film, with Au/Ni/Cu electrodes and a daisy‐chained circuit matched to the die bump pattern. The ACF used was based on epoxy resin in which nickel and gold‐coated polymer particles are dispersed. A comparative study was carried out on the results obtained.

Findings

The results showed that the glass transition temperature (Tg) of the ACF material plays an important role in the high temperature contact resistance. Above Tg, the ACF matrix becomes less viscous, which reduces its adhesive strength and allows the bumps on the chip to slide away from the pads on the substrate. Even though a flex substrate was used in this study, the sliding effect is severe at the corner bumps of the chip, where cumulative forces are generated due to the thermal expansion mismatch. For every thermal cycling profile, there is an incubation period encountered from this work that would have a significant impact in the application of ACF. After the incubation period the contact resistance increased rapidly and the assemblies were therefore no longer reliable.

Originality/value

The work in this paper focuses on contact resistance changes during thermal shock. The paper discusses the reliability issue of ACF during thermal warpage, which is useful to industries using ACF for flip‐chip assemblies.

Details

Soldering & Surface Mount Technology, vol. 17 no. 3
Type: Research Article
ISSN: 0954-0911

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Article
Publication date: 1 December 1999

Zhaowei Zhong

Reports the research and development results on flip chip on FR‐4 and ceramics, using anisotropic conductive film (ACF), anisotropic conductive paste (ACP), or eutectic…

Abstract

Reports the research and development results on flip chip on FR‐4 and ceramics, using anisotropic conductive film (ACF), anisotropic conductive paste (ACP), or eutectic solder with underfill. Several types of ACF and ACP with different types of conductive particles and adhesives were investigated. Simple but high yield procedures for reworking flip chip on board using ACP and ACF were developed. Processes for flip chip on FR‐4 and ceramic boards using eutectic solder bumps with underfill were also evaluated. The flip chips were assembled on test vehicles for temperature cycling and high‐temperature high‐humidity tests. The reliability performance of the three processes (gold bumps with ACF, gold bumps with ACP, and eutectic solder bumps with underfill) is compared.

Details

Microelectronics International, vol. 16 no. 3
Type: Research Article
ISSN: 1356-5362

Keywords

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