Lianhua Fan

Effects of nano-sized particles on electrical and thermal conductivities of polymer composites

Polymer composite materials, for their cost-effectiveness and design flexibility, have been widely employed in electronic packaging industry. They possess unique characteristics combining the low-temperature processability of organic polymer matrix and the various functionalities endowed by the other components in the composites. Electrically conductive adhesives (ECAs) have been explored as an environment friendly interconnection technique. While they have many potential advantages for surface mount and flip chip applications, typical ECA materials suffer from several critical issues to be used as a drop-in replacement for lead-containing solders. In an attempt to understand and improve the thermomechanical properties of ECA materials, nano-sized silver particles were introduced into the conventional ECA compositions. The influence of nano particles on bulk resistivity is reported in this paper, as maintaining an acceptable conductivity is essential for high performance and environmentally benign interconnections. It was found that the bulk resistivity of ECA formulations strongly depended on the contents of silver flake and nano particles, as well as the particle morphology and surface properties. The thermal conductivity of alumina based composite samples was also affected upon the inclusion of nano alumina particles. Both the electrical and thermal conductivities of the polymer composites containing nano particles would be determined by the contacts of microsized particles and interfaces that involve nano particles along the conduction paths.

MEAM molecular dynamics study of lead free solder for electronic packaging applications

The modified embedded atom method (MEAM) was employed in conjunction with molecular dynamics (MD) simulations to investigate whether a physical mixture of nano-Sn and nano-Ag particles at a prescribed ratio would achieve the same intermixing as a nano-Sn/Ag alloy. A Sn sphere and a Ag sphere both with a diameter of 4 nm were prepared by cutting the perfect bulk lattice structure. After energy minimization and structural relaxation, the two spheres were placed next to each other with a gap of 3 A. The simulation was then performed at 500 K for 3 ns. Simulation results showed that the nano-Ag sphere still maintained its crystalline structure and no significant diffusion between Sn and Ag was observed. Further simulations were performed at 800 K, 850 K, 900 K and 1000 K, respectively, in order to obtain the activation energy of interdiffusion between An and Sn. It was then predicted that 118 ns was required for Sn and Ag to mix with each other at 500 K. The results implied that a physical mixture of nano-Sn and nano-Ag particles may satisfy the requirements of lead free solder for low temperature (~500 K) reflow applications.

Study on underfill/solder adhesion in flip-chip encapsulation

Underfill materials are employed in flip-chip assemblies to enhance solder joint reliability performance. We have studied the adhesion strength of two underfill samples with tin/lead (Sn/Pb) eutectic solder and tin/copper (Sn/Cu) lead-free solder, benchmarked with a copper surface. It was found that the adhesion of underfills and both solder materials was about 1/3 of the adhesion between underfills and copper. The effect of temperature and humidity aging as well as flux residue on adhesion strength was also investigated. A loss of adhesion was observed after the pressure cooker test, but 85/spl deg/C/85% RH aging and flux residue revealed only a slight influence on adhesion strength. Surface analysis was performed on solid surfaces including copper, Sn/Pb eutectic solder, Sn/Cu lead-free solder and cured underfills by using the three-liquid-probe three-component surface tension method with a goniometer. The surface tension of liquid underfills was measured by the pendent drop method, and their contact angles on copper, Sn/Pb eutectic solder and Sn/Cu lead-free solder were also measured with a goniometer. The thermodynamic work of adhesion for underfills with copper and solder surfaces of different conditions was then calculated following these two surface analysis approaches. It was found that the thermodynamic work of adhesion was not correlated with the lap shear strength of underfills with copper and solder materials. Thus, the wetting property of an underfill on a substrate is not the determining factor for its practical adhesion strength. Various possible techniques for improving the adhesion of underfills and solder materials were then considered, and the use of additives in underfill formulations was experimented. However, we have not observed any significant effect of adhesion strength enhancement from any of these additives. Further tests of these additives with the base underfill formulation seemed to reveal a slight possibility to enhance adhesion of underfills and solders by proper manipulation of the underfill and/or flux formulation.

Study on B-stage properties of wafer level underfills

The flip-chip on organic substrates has relied on the underfill to enhance the solder joint reliability. The invention of wafer level underfill technology has greatly improved the production efficiency of the flip-chip process. Nevertheless, because of the unique curing characteristics of the wafer level underfill, there is a need for a fundamental understanding of the underfill curing process. In this study, we have explored two underfill formulations and have investigated their curing properties and the gelation behavior. The B-stage feasibility of these two underfills was investigated based on modeling of the curing process and characterization of the material properties. It was found that the epoxy/anhydride formulation was not suitable for wafer level underfill application, due to its low degree of cure at gelation and its low Tg after the B-stage. The underfill formulation based on epoxy/anhydride system was optimized for good dicing property and curing latency in a reflow process. A successful wafer level underfill material and process were demonstrated.

Adhesion of underfill and components in flip chip encapsulation

The underfill material is a polymeric adhesive used in flip chip packaging. It encapsulates the solder joints by filling the gap between a silicon die and an organic substrate or board. Within a typical flip chip structure, there are interfaces between the various components, namely, substrate, solder mask, flux residue, underfill encapsulant and die passivation layer, etc. Maintaining a good adhesion condition, both as-made and after temperature/humidity aging, is vital for these interfaces because of the expected performance of the flip chip device, where the underfill material is employed to enhance the reliability of the flip chip interconnect. We have studied the adhesion strength between the various components for different process variables as measured with the lap shear and die shear test configurations. The effects of the assembly factors, i.e. solder mask, flux residue, underfill, and die passivation, etc., were evaluated and the adhesion strength was found to depend greatly on these factors. The die shear strength of a passivated die assembled onto an organic board coated with a solder mask was much higher after using a no-clean flux on the solder mask than for the assembly without such a no-clean flux. The influence of some accelerated aging tests on the adhesion durability was also investigated. A die passivation layer of benzocyclobutene exhibited better capability in retaining the die shear strength than a passivation layer of silicon nitride or polyimide, especially for the initial aging period. The knowledge obtained in this study should provide insights into the interfacial adhesion in the flip chip assembly structure.

Development of environmentally friendly nonanhydride no-flow underfills

Most no-flow underfill materials are based on epoxy/anhydride chemistry. Due to the sensitizing nature, the use of anhydride is limited and there is a need for a no-flow underfill using nonanhydride curing system. This paper presents the development of novel no-flow underfill materials-based on epoxy/phenolic resin system. Epoxy and phenolic resins of different structures are evaluated in terms of their curing behavior, thermo-mechanical properties, viscosity, adhesion toward passivation, moisture absorption and the reliability in flip-chip underfill package. The influence of chemical structure and the crosslinking density of the resin on the material properties is investigated. The assembly with nonanhydride underfill shows high reliability from the thermal shock test. Solder wetting test has confirmed the sufficient fluxing capability of phenolic resins. Results show that epoxy/phenolic system has great potential for an environmentally friendly and highly reliable no-flow underfill.

Effect of interface on thermal conductivity of polymer composite

The thermal conductivity of alumina particle filled polystyrene was characterized at different temperatures. A silane coupling agent, /spl gamma/-glycidoxypropyltrimethoxy silane (GPS), was applied to functionalize the surface of alumina filler, in an attempt to investigate the effect of polymer/filler interface on composite thermal conductivity. Thermal conductivity measurements at different temperatures showed that the addition of 10 wt% alumina filler can enhance the thermal conductivity by about 23%, and the functionalization of alumina particle using silane coupling agent can increase the thermal conductivity by another 6.5%. Both 85/spl deg/C/85%RH temperature/humidity aging and -50/spl deg/C /spl sim/ +80/spl deg/C thermal shock testing exerted almost negligible effect on the composite conductivity, indicating the polymer/filler interface was stable and no interface debonding occurred for the specific alumina polymer composite system studied.

Lotus Effect coating and its application for microelectromechanical systems stiction prevention

The stiction problem is one of the major factors that limit the widespread use and reliability of microelectromechanical systems (MEMS). The fundamental mechanism to prevent stiction is either increasing the surface roughness, or coating MEMS surfaces with hydrophobic materials. By nature, the Lotus Effect coating is a good combination of rough surface and hydrophobic materials. In this work, the feasibility of using Lotus Effect coatings to prevent MEMS stiction was proposed and investigated. Lotus Effect surfaces were either prepared by creating nanoscale rough structures on a hydrophobic surface with plasma techniques, or by coating functional self-assembled monolayers (SAMs) on nanoporous silica substrates. The effects of surface chemistry and structure on the superhydrophobicity of Lotus Effect materials were studied. The relationship between surface structure, surface wetting, and surface stiction properties of a Lotus Effect surface was also investigated.

Use of dispersant in high K polymer-ceramic nano-composite to improve manufacturability and performance of integral capacitors

Integral or embedded capacitor technology could increase packaging density, improve electrical performance and reduce assembly cost compared with traditional discrete capacitor technology. Developing a successful dielectric material that satisfies electrical, reliability and processing requirements is one of the major challenges for incorporating capacitors into the large-area substrates. Polymer-ceramic nano-composites have been of great interest as the high dielectric constant (K) material because they combine the processability of polymers with the desired electrical properties of ceramics. Nevertheless, there are some technical barriers for the polymer-ceramic composites to be used in the organic substrates. Most significantly, for a very high dielectric constant of about 150 as reported so far by our group, a necessary rather high ceramic filler loading (85% by volume) gave problems in well dispersion of the ceramic fillers within the organic matrix, and there was almost no adhesion towards other layers in the printed circuit board structure. In order to develop polymer-ceramic nano-composites with a dielectric constant as high as possible together with compatibility toward manufacturing process of organic printed circuit boards, we have introduced dispersants into the formulations. Comprehensive formulation techniques have resulted in a much higher dielectric constant (e.g., typically over 65 at a ceramic loading of 40% by volume) as well as excellent adhesion performance.

Fundamental understanding of conductivity establishment for electrically conductive adhesives

Use of Electrically conductive adhesives has been considered an environment friendly interconnection technique, one of the two potential alternatives. Fundamental understanding of conductive adhesives has lagged far behind their commercial utilization. For isotropically conductive adhesives (ICAs), the conductive particles in the adhesives are responsible for the electrical interconnection, while the polymer matrix mainly provides the mechanical interconnection. We have found from our previous studies that there is obvious relationship between the polymerization shrinkage of epoxy curing and the electrical conductivity finally established with the material. In this paper, several epoxy resin based curing systems were used as the matrices for the ICAs. They exhibited different curing peak temperatures, which would enable us to investigate the effects of curing process upon the resultant bulk resistivity of the ICAs. To eliminate the complex effect of lubricants in silver flakes, a spherical silver powder was used as the conductive filler. The experimental results indicated a strong correlation between bulk resistivity and curing temperature or curing kinetics, which could be explained by a proposed mechanism on the establishment of electrical conductivity of ICAs during the curing process.