Zonghe Lai

Effect of Ag particle size on electrical conductivity of isotropically conductive adhesives

The present work is to introduce nanoparticles in micro-sized metal particles to study particle distribution in polymer matrix. Previous examinations of the silver-filled particles reveal that the micro-sized particle fillers appear as full density silver flakes, while nanoparticle fillers appear as highly porous agglomerates, similar to open-cell foams. Actually little work has been carried out to study the cross-sectional area of a particle-particle-contact in isotropically conductive adhesives (ICA). In this study, transmission electron microscope is chosen as a main measure to analyze the distribution of different-sized particles. The percentage of the nanoparticles varies from 20 wt% and 50 wt% to full percentage within micro-sized particles, and the total metal content in epoxy resin is 70 wt%. So the change of contact area and contact behavior with various volume ratio of nano-sized and micro-sized particles was investigated. At the same time, the electrical resistivity was measured, which is compared with the different level of the filler loading.

Overview of conductive adhesive joining technology in electronics packaging applications

This paper presents an overview of the current status of use of conductive adhesives in various electronics packaging applications. Strong emphasis is placed on recent developments in surface mount and flip-chip technology, as these methods in combination with conductive adhesives represent the latest developments in the area of electronics packaging. It is concluded that little practical use of conductive adhesives in surface mount has been found. In flip-chip applications, both isotropically and anisotropically conductive adhesives (ICAs and ACAs) have been used in real applications. Greater use is expected in the near future in this fast developing area.

1.58 mu m InGaAs quantum well laser on GaAs

We demonstrate the 1.58 mu m emission at room temperature from a metamorphic In0.6Ga0.4As quantum well laser grown on GaAs by molecular beam epitaxy. The large lattice mismatch was accommodated through growth of a linearly graded buffer layer to create a high quality virtual In0.32Ga0.68As substrate. Careful growth optimization ensured good optical and structural qualities. For a 1250x50 mu m(2) broad area laser, a minimum threshold current density of 490 A/cm(2) was achieved under pulsed operation. This result indicates that metamorphic InGaAs quantum wells can be an alternative approach for 1.55 mu m GaAs-based lasers

Experimental and theoretical characterization of electrical contact in anisotropically conductive adhesive

Electrical conduction through anisotropically conductive adhesive (ACA) is caused by deformation of metal fillers under pressure and heat. In this work, the hardness of the electrical particles under various deformation degrees was determined by nano-indentor measurements and the electrical resistance of the electrical contacts was measured under various deformation degrees. Theoretical model and simulation have been developed for the microscopic mechanism of the electrical conduction through metal fillers in the anisotropically conductive adhesive. By comparing with experimental data it is concluded that the deformation of the metal filler in our ACA is plastic even at rather low external load. Further theoretical simulation reveals two important aspects of the conductance characteristics. The conductance is improved by increasing the external load but the dependence of the conductance on the spatial position of the metal filler becomes stronger. Design and optimization of the ACA with respect to the absolute value of the electric conductance and its dependence on the spatial position of the metal filler are of essential importance for the electronics packaging application of the anisotropically conductive adhesives.

Effect of bump height on the strain variation during the thermal cycling test of ACA flip-chip joints

Flip chip joining using anisotropically conductive adhesive (ACA) has become a very attractive technique for electronics packaging. Many factors can influence the reliability of the ACA flip-chip joint. Bump height, is one of these factors. In this work, the strain development during the thermal cycling test of flip-chip joining with different bump heights was studied. The effect of bump height is significant in the interface between the bumps and the pads. Bigger volume area of high strain is found for higher bump in the interface between the bumps and the pads. Our calculations show that there is practically no effect of the bump height on the strain variation in the bumps and in the pads.

Conductive adhesives for high-frequency applications

In this paper, we present results from measurement and simulation of epoxy-based anisotropically conductive adhesive joints (ACA). We studied two different types of connection: flip-chip bonded Si test chip and a transmission line gap bridged by Cu foil. Test chips were mounted on three substrates: rigid FR-4 PCB, flexible PCB and high frequency teflon-based duroid substrate. Equivalent electrical models are discussed based on physical considerations and parameters were fitted to measurement data in the HP MDS high-frequency CAD tool. A HP8510 network analyser was used to measure S-parameters on rigid FR-4 and flex boards in the 500 MHz-8 GHz frequency range for both flip-chips and bridges, and for duroid mounted flip-chips and bridges over 1-30 GHz. ACA microstructures were studied by cross-sectioning and SEM. LF results for 500 MHz-8 GHz indicated that ACA flip-chip joints and bridge joints on FR-4 or flex can be used. For HF applications (1-30 GHz), ACA flip-chip joints and bridge joints on duroid can be used. For flip-chip joints and bridge joints, ACA was as good as or better than the solder joint for FR-4 over 45 MHz-2 GHz and for duroid substrates over 1-30 GHz. The largest contribution to transmission loss is due to Si chip resistivity in the flip-chip assembly. HP momentum analysis showed that for duroid, Si-chip crosstalk also gave power losses, which is not the case for FR-4 or flex. HP momentum analysis of flip-chip on FR-4 and flex showed that FR-4 and flex substrate losses also should be considered. Different ACA particle sizes and materials made little difference to ACA joint electrical behaviour.

Process development and adhesion behavior of electroless copper on liquid crystal polymer (LCP) for electronic packaging application

Liquid crystal polymer (LCP) has potentially a very wide application as substrate material in electronic packaging applications because of its unique advantages. The work in this paper was performed to realize the metallization of LCP for the purpose of board fabrication, and to study the adhesion between deposited copper and LCP. A homogenous electroless plated copper layer on LCP with 4 to 5 /spl mu/m thickness was achieved, while it increased up to 40 /spl mu/m with the subsequent electroplating. The timescale of etching, deposit ion rate, and pH value were gradually changing during the plating process and the influences on copper layer quality were investigated. The adhesion force of the copper-LCP layer system was measured by a shear-off-method. Scanning electron microscopy (SEM) was used to check the surface morphology after etching and the interface after shearing on both the backside of the copper layer and the LCP side. The relationship between the shear-off adhesion of copper and the time of chemical etching before plating was examined, and the optimal etching time is discussed. Heat treatment after plating was used, and it was shown that this significantly improved the adhesion strength.

Microstructure investigation of Sn-0.5Cu-3.5Ag and Sn-3.5Ag-0.5Cu-0.5B lead-free solders

Identifies the intermetallics formed in Sn‐0.5Cu‐3.5Ag and Sn‐3.5Ag‐0.5Cu‐0.5B (wt%) lead‐free solders, and the influence of boron on these precipitates. SEM, TEM and SIMS were employed to reveal the difference of microstructure in both solder alloys. It was found that the intermetallics formed were Ag3Sn and Cu6Sn5. Both solders were found to have a dispersion structure of Ag3Sn particles with network‐shaped subgrains. The microstructure of Ag3Sn was also found to become finer and more uniform in Sn‐3.5Ag‐0.5Cu‐0.5B solder, due to the addition of boron, while there was little effect from the boron on the Cu6Sn5 phase.

Templated Growth of Covalently Bonded Three‐Dimensional Carbon Nanotube Networks Originated from Graphene

A template-assisted method that enables the growth of covalently bonded three-dimensional carbon nanotubes (CNTs) originating from graphene at a large scale is demonstrated. Atomic force microscopy-based mechanical tests show that the covalently bonded CNT structure can effectively distribute external loading throughout the network to improve the mechanical strength of the material.

Development of conductive adhesive joining for surface-mounting electronics manufacturing

This paper presents the results of a study of process development for conductive adhesives as solder replacement. The main objective of the work was to investigate the potentials for using conventional surface-mounting equipment for component assembly with conductive adhesives. Two processes have been studied: one which uses both anisotropically and isotropically conductive adhesives and one which uses isotropically conductive adhesives only. The results from the work show that currently available surface-mounting machinery can be used for the conductive adhesive joining process. However, further work is needed to optimize the processing conditions. Transmission electron microscopy analysis of the adhesive joints after temperature cycling and humidity testing shows that oxide layer formation on metal surfaces can be one of the mechanisms which causes decrease in the electrical performance of the joint. >