Metal-coated Polymer Particles Research Articles

Contact area measurement of micron-sized metal-coated polymer particles under compression

The contact problem of a sphere under compression by a rigid plate is pertinent to understanding electrical contact formation in anisotropic conductive adhesive (ACA) applications. However, no work has experimentally verified existing contact models for the metal-coated sphere-plate contact problem. Herein, compression tests on metal-coated polymer spheres were performed by flat punch nanoindentation with in-situ scanning electron microscopy (SEM) monitoring, and the resultant contact area was measured using SEM images. The obtained results were compared to Tatara and Abbott-Firestone models, along with finite element analysis (FEA). The measured contact area falls between Tatara and Abbott–Firestone models, indicating a mixture of elastic and plastic deformation. The contact area as a function of force is dominated by the polymer core and independent of the metal coating for the thin metal coatings tested. The work provides guidelines for bonding parameters for ACA assembly.

Resistance Analysis of Spherical Metal Thin Films Combining Van Der Pauw and Electromechanical Nanoindentation Methods

Although micron-sized metal-coated polymer particles are an important conductive filler material in anisotropic conductive adhesives, the resistance of the particles in an adhesive is not well understood. In this study, a van der Pauw method for spherical thin films is developed and applied to determine the resistivity of 30 μm silver-coated poly(methyl methacrylate) (PMMA) particles. The resistivity is used to interpret resistance contributions in single particle electromechanical nanoindentation measurements, which simulate the compression particles undergo in application. The resistivity was found to be coating thickness dependent for thin films in the range 60–270 nm. Estimation of the resistance of the metal shell using the measured resistivity did not account for the total resistance measured in electromechanical nanoindentation. We therefore deduce a significant contribution of contact resistance at the interfaces of the particle. The contact resistance is both coating thickness and particle deformation dependent.

Electromechanical characterization of individual micron-sized metal coated polymer particles

Micron-sized polymer particles with nanoscale metal coatings are essential in conductive adhesives for electronics assembly. The particles function in a compressed state in the adhesives. The link between mechanical properties and electrical conductivity is thus of the utmost importance in the formation of good electrical contact. A custom flat punch set-up based on nanoindentation has been developed to simultaneously deform and electrically probe individual particles. The set-up has a sufficiently low internal resistance to allow the measurement of sub-Ohm contact resistances. Additionally, the set-up can capture mechanical failure of the particles. Combining this data yields a fundamental understanding of contact behavior. We demonstrate that this method can clearly distinguish between particles of different sizes, with different thicknesses of metal coating, and different metallization schemes. The technique provides good repeatability and physical insight into the behavior of these particles that can guide adhesive design and the optimization of bonding processes.

Deformation and fracture of nano-sized metal-coated polymer particles: A molecular dynamics study

The mechanical behavior of Ni-coated polyethylene (PE) nanoparticles subjected to compression loading is systemically investigated by classical molecular dynamics simulation. Results show that the Ni coatings on PE nanoparticles lead to a densification of particle surface and remarkably enhance the compression strength. The particle size-effect and the coating thickness effect on the compression responses in terms of compressive strength and particle burst are observed. Burst of Ni-coated PE nanoparticles initiates at the high stress-concentrated grain-boundaries zone of polycrystalline Ni-shell, and propagates along the compression direction to the flattened contact surface, resulting in release of core PE molecules.

Size dependent efficiency of photophoretic swimmers.

We investigate experimentally the efficiency of self-propelled photophoretic swimmers based on metal-coated polymer particles of different sizes. The metal hemisphere absorbs the incident laser power and converts its energy into heat, which dissipates into the environment. A phoretic surface flow arises from the temperature gradient along the particle surface and drives the particle parallel to its symmetry axis. Scaling the particle size from micro to nanometers, the efficiency of converting optical power into motion is expected to rise with the reciprocal size for ideal swimmers. However, due to the finite size of the metal cap, the efficiency of a real swimmer reveals a maximum depending sensitively on the details of the metal cap shape. We compare the experimental results to numerical simulations.

Ultrasonic Bonding of Anisotropic Conductive Films Containing Ultrafine Solder Balls for High-Power and High-Reliability Flex-On-Board Assembly

New solder anisotropic conductive films (ACFs) consist of a thermosetting polymer resin and fine solder balls instead of the conventional metal particles or metal-coated polymer particles. These solder balls have lower melting temperature than conventional metal conductive particles, which enable them to be melted during ultrasonic (US) bonding, and form intermetallic alloy joints with metal pads of flex on board. In this paper, excellent solder ACF joints are demonstrated using a US-bonding method for high-power and high-reliability flex-on-board (FOB) assemblies. Ultrasonically bonded solder ACF joints were characterized in terms of their electrical properties and reliability. Solder alloy bonding was achieved using two kinds of solder balls, i.e., Sn-58Bi and SAC305 (96.5Sn-3.0Ag-0.5Cu). At the same time, the acrylic ACF resin was completely cured after 5 s of US bonding. Solder alloy ACF joints show about 20% decrease in daisy-chain electrical resistance and 100% increase in the current-carrying capability compared with conventional physical-contact-based Ni ACF joints. Solder alloy ACF joints also show significantly improved reliability with stable electrical resistances up to 24 h in an unbiased autoclave test, whereas conventional Ni ACF joints show severe electrical open failures within 4 h.

Loading rate effects on the fracture of Ni/Au nano-coated acrylic particles

Mechanical failure of monodisperse Ni/Au coated acrylic particles has been investigated by individual compression tests using nanoindentation-based technique equipped with a flat diamond punch. We have found that both fracture property and morphology of particles depend on the compression loading rate. The breaking strain of the metal coating decreases with increasing loading rate, while the breaking stress increases. Two obvious fracture patterns with cracking in meridian or latitude direction are identified according to the loading rate, and attributed respectively to tensionor bendingdominated deformation of the coating. The findings reported here give a significant guiding to the manufacture design of metal coated polymer particles for Anisotropic Conductive Adhesive (ACA) packaging.

Metal Coated Polymer Particles for Electronic Packaging

Anisotropic conductive adhesives (ACA) are widely used as interconnect materials in the manufacturing of LCD screens. To be integrated in a broader range of interconnect applications several technical and economic issues still need to be addressed. One significant challenge is to encapsulate polymer particles with well controlled modal diameter and size distribution into continuous, compact, and uniform conductive metallic shells. This presentation describes a method to deposit layers of nickel with different thickness (30 – 120 nm) onto monosized polymer particles ranging in size from 1 to 10 micrometers. The novelty of the approach consists in a pre-treatment of the epoxy functionalized polymer particles with linear polymeric amines. We show that by increasing amine chain length the distribution of the palladium catalyst, and consequently the adhesion and distribution of nickel metal deposited, can be improved dramatically. The effect of several plating parameters is discussed and illustrated.

Fracture of micrometre-sized Ni/Au coated polymer particles

Deformation and fracture of individual micrometre-sized Ni/Au coated polymer particles have been studied by a nanoindentation-based flat punch method. A wide range of test conditions has been applied to deform the coated and the uncoated particles. The compression induced cracking of the Ni/Au coating and delamination between the metal coating and the polymer core have been investigated and provide insight into the effect of nanoscale metal coating on the deformation capacity and fracture process of the particles. A three-stage deformation and fracture behaviour of the particles have been identified. The results are essential for the design of metal coated polymer particles for industrial applications.

A method for determining elastic properties of micron-sized polymer particles by using flat punch test

Micron-sized metal coated polymer particles are used in developing anisotropic conductive adhesives (ACA). The mechanical properties of polymer particles are of crucial importance for the reliability and design of ACA. In this paper we present a method to determine the mechanical properties of polymer sphere particles under large deformation by using a flat punch test-soft elastic sphere against a rigid flat. Finite element analyses have been carried out to study large deformation contacts. It has been shown that the classical Hertz solution works only for very small sphere deformation (less than 1% compression strain) and Tatara’s solution works well for sphere compression up to 20%, beyond that large strain effect should be considered. For compression less than 20%, both the compression stress–strain curves and lateral expansion strain–compression strain curves can be normalized by a function of the Poisson ratio. Based on the finite element results, explicit equations describing these relations are presented for determining the Young’s modulus and Poisson ratio of polymer particles at large deformation. The proposed method has been applied to analyze the experimental results of typical polymer particles used for conductive adhesives from a specially built capacitance-based flat punch test.

Comparison of different flex materials in high density flip chip on flex applications

This paper presents the results from the evaluation of different types of flexible substrates for high-density flip chip application. In this work four different flexible substrates were used. The flex substrates were Espanex, Upilex and epoxy glass with 80 μm pitch and Upilex with 54 μm pitch. Two different test IC’s were used for both pitches. In test IC1 (80 μm pitch) and IC3 (54 μm pitch) the bumps were in one row and test IC2 (80 μm pitch) and IC4 (54 μm pitch) in two rows. The total amount of contacts in test IC1 was 190, in test IC2 173, in test IC3 293 and in test IC4 270. The anisotropically conductive adhesive that was used in the tests was epoxy based thermosetting adhesive film with conductive particles. The conductive particles in the adhesives were isolated soft metal-coated polymer particles. The contact resistance was measured using Kelvin four-point method and the continuity and series resistance using daisy chain structure. The reliability of the flip chip interconnections was tested in temperature cycling test and environmental test. Cross section samples were made to analyse the possible reason for failures. The results presented in this paper are from FLEXIL development project that is part of European Union IST research program.

Process Modeling and Thermal/Mechanical Behavior of ACA/ACF Type Flip-Chip Packages

Development of flip-chip-on-glass (FCOG) assembly technology using anisotropic conductive adhesive/film (ACA/ACF) is currently underway to achieve fine pitch interconnections between driver IC and flat panel display. Conductive adhesives are characterized by fine-pitch capability and more environment compatibility. Anisotropic conductive adhesive/film (ACA/ACF) is composed of an adhesive resin and conductive particles, such as metallic or metal-coated polymer particles. In contrast to a solder type flip chip interconnection, the electric current passing through conductive particles becomes the dominant conduction paths. The interconnection between the particles and the conductive surfaces is constructed by the elastic/plastic deformation of conductive particles with contact pressure, which is maintained by tensile stress in the adhesive. Although loss of electric contact can occur when the adhesive expands or swells in the Z- axis direction, delamination and cracking can occur in the adhesive layer while the tensile stress is excessive. In addition to performing processing simulations as well as reliability modeling, this research investigates the contact force that is developed and relaxed within the interconnection during the process sequence by using nonlinear finite element simulations. Environmental effects, such as high temperature and thermal loading, are also discussed. Moreover, a parametric study is performed for process design. To improve performance and reliability, variables such as ACF materials, proper processing conditions are discussed as well. Furthermore, this study presents a novel method called equivalent spring method, capable of significantly reducing the analysis CPU time and make process modeling and contact analysis of the 3D ACA/ACF process possible.

EFFECTS OF GAP HEIGHT ON CONDUCTION WITHIN ANISOTROPIC CONDUCTING ADHESIVE ASSEMBLIES

This paper explores the effect that compression of the conducting particles in Anisotropic Conductive Adhesive (ACA) assemblies has on the resistance of the joints. Particles within the same assembly were compressed by differing amounts by the action of two rigid, planar surfaces which form a wedge with the particles trapped between them. Using this method, it is found that optimum conduction is obtained when metal-coated polymer particles are compressed to a height between 80% and 100% of their original diameters (see Fig. 1 for definitions). Results on flip-chip assemblies with a pitch of 220 μm are presented. They are in agreement with the wedge experiment findings in that larger particles are shown to be able to better accommodate variations in the gap heights between chip and substrate. The wedge experiment has also been used to validate fluid flow based models of particle movement during assembly and statistical models of particle distribution after assembly is complete. The experiments also indicated the presence of thin insulating films between particles which offer an additional safety margin when considering shorting between neighbouring electrodes by clusters of particles.