Nanocomposite adhesives: Mechanical behavior with nanoclay

The major objective for this research was to examine the role of epoxy-clay nanocomposites in the area of epoxy bonding to porous stone (granite) substrates. Two bisphenol A epoxy systems were selected based on the prior work that determined optimal adhesive properties from a larger set of epoxy systems to determine the role of viscosity on the intercalation and exfoliation of the clay tactiods in the epoxy resin. The systems were characterized and mechanically tested at varying levels of intercalated and exfoliated organic clay tactiods. In the first stage of the work, epoxy-clay systems were characterized by wide-angle X-ray diffraction (WAXD) to detect inter-laminar distances of clay layers and to determine if the mixing procedures had indeed dispersed and exfoliated the clay layers sufficiently. The second stage of the work involved examining mechanical properties of the epoxy-nanoclay systems. Fracture behavior was studied using granite stone substrates in notched double lap configuration. Compressing a wedge between the cover plates induced the fracture. Fracture toughness was approximated by the load at fracture. Tensile properties were measured using cast dog bone tensile samples. The better layered silicate nanocomposite performance was seen with the lower viscosity resin. The most noticeable improvements in mechanical properties for the lower viscosity resin system were found to be maximum stress, elastic modulus, and yield stress. Increased toughness and stress whitening at 1% by weight nanoclay loading revealed that the clay can act as a shear-yielding toughening agent in this epoxy system.     

High conductivity of isotropic conductive adhesives filled with silver nanowires

As an alternative to lead-bearing solder, isotropic conductive adhesives (ICA) have been utilized for many years in microelectronic packaging. In this study, silver nitrate (AgNO₃) as precursor, N-N-dimethylformamide (DMF) as solvent and reducing agent, preparing silver (Ag) nanowires in the nanoporous templates formed by the controlled hydrolysis and condensation of butyl titanate (Ti(OC₄H₉)₄). The Ag nanowires were characterized by X-ray diffraction and transmission electron microscopy. An isotropic conductive adhesive (ICA) has been developed by adding Ag nanowires as conductive filler. Bulk resistivity and shear strength of the ICA are measured and compared with those of conventional ICA filled with micrometer-sized Ag particles (about 1 μm) and nanometer-sized Ag particles (about 100 nm). It is found that the ICA filled with lower content of Ag nanowires exhibits lower bulk resistivity and higher shear strength than ICA filled with micrometer-sized Ag particles and nanometer-sized Ag particles. Possible conductive mechanisms of the ICA are discussed.     

Analysis of the nonlinear behavior of adhesives in bonded assemblies—Comparison of TAST and Arcan tests

This paper describes a study in which the shear behavior of a structural epoxy adhesive has been measured using the standard thick adherend shear test (TAST) specimen and a modified Arcan test. A numerical study of the TAST test taking into account the nonlinear behavior of the adhesive and the finite deformations of the adhesive joint, shows that there is a localization of plastic zones close to the adhesive–substrate interface near the free edge of the adhesive. Experimental tests carried out with steel and aluminum substrates and with various adhesives also show that failure initiates in this region. These edge effects in the TAST fixture can lead to an incorrect analysis of the behavior of the adhesive (for instance, underestimation of the shear stress in the joint at failure), particularly when an adhesive failure mode is dominating. The modified Arcan fixture provides a more homogeneous stress state. A similar improvement of the TAST fixture is proposed.     

国内外导电胶的发展现状

由于铅对人体及环境有害,使得铅锡焊料的使用受到了限制.目前无铅焊料和导电胶被认为是理想的铅锡焊料替代品.本文介绍国内外无铅连接材料的最新进展,重点论及作为环境友好型的无铅连接材料的导电胶及其组成、应用和最新进展.     

Novel conductive adhesives for surface mount applications

Electrically conductive adhesives (ECAs) have been explored as a tin/lead (Sn/Pb) solder alternative for attaching encapsulated surface mount components on rigid and flexible printed circuits. However, limited practical use of conductive adhesives in surface mount applications is found because of the limitations and concerns of current commercial ECAs. One critical limitation is the significant increase of joint resistance with Sn/Pb finished components under 85°C/85% relative humidity (RH) aging. Conductive adhesives with stable joint resistance are especially desirable. In this study, a novel conductive adhesive system that is based on epoxy resins has been developed. Conductive adhesives from this system show very stable joint resistance with Sn/Pb‐finished components during 85°C/85% RH aging. One ECA selected from this system has been tested here and compared with two popular commercial surface mount conductive adhesives. ECA properties studied included cure profile, glass transition temperature (T_g), bulk resistivity, moisture absorption, die shear adhesion strength, and shift of joint resistance with Sn/Pb metallization under 85°C/85% RH aging. It was found that, compared to the commercial conductive adhesives, our in‐house conductive adhesive had higher T_g, comparable bulk resistivity, lower moisture absorption, comparable adhesion strength, and most importantly, much more stable joint resistance. Therefore, this conductive adhesive system should have better performance for surface mount applications than current commercial surface mount conductive adhesives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 399–406, 1999     

Characterization and material model definition of toughened adhesives for finite element analysis

Some toughened adhesives used for structural joints are characterised by non-linear behaviour prior to failure that may significantly influence the entire joint response. The determination of appropriate and accurate material models for use in analysis and design phases covering both nonlinearities and final material rupture constitutes one of the main challenges for the utilisation of adhesives and for offering designers the same confidence level as that offered by other joining techniques.The present research proposes the utilisation of both elasto-plastic and continuum damage models as a combination that can fully reproduce the mechanical response of toughened adhesives in finite element (FE) analysis. In this context, the Drucker-Prager exponential model has demonstrated to provide accurate fits with the nonlinearities of these materials, allowing the real plastic behaviour of the adhesives to be adjusted in the computational models with a high degree of correlation. On the other hand, a continuum damage model has been proposed to simulate the final material failure process introducing a displacement-based damage parameter into the constitutive equation of the damaged material. The definition of the parameters associated with the mentioned models has been carried out through the execution of an experimental programme combining traction and torsion tests, described in the present paper as part of the study developed. The research is finally completed with an experimental and FE analysis of a specific bonded joint that allows the operation of the material model to be checked in a real application.     

Strength of epoxy-resin-based insulation systems in transverse tension and shear using two novel test pieces

A novel test specimen type has been developed to generate engineering data on bonded aluminium surfaces. The test is designed to simulate conditions in a superconducting coil, operating at 4.2 K, in a region where a glass/epoxy composite laminate is loaded primarily in through-thickness tension and in shear. We include the effects of thermal contraction from the cure temperature to the operating temperature of 4.2 K. The test specimen avoids the large stresses associated with the edges of the composite in other through-thickness tension test pieces and results calculated from the measured failure loads of specimens are the peak stresses in the centre of the specimen.     

各向异性导电胶的研究与应用现状

随着微电子工业的发展,导电胶替代传统的锡铅焊料已经成为一种发展趋势,本文介绍了各向异性导电胶(ACA)的组成和分类、3种导电机理及国内外导电胶的研究现状和发展方向,特别是在各向异性导电胶填料表面功能化处理方面的研究现状和在液晶显示屏(LCD)、发光二极管(LED)组装应用上的进展。     

Study on the properties of conductive adhesives in various curing manners for MCM applications

In this work, the properties of solvent and solvent-free electrically conductive adhesives (ECAs) in four curing manners and after solder reflows are investigated for multi-chip module applications. The curing behaviors and thermal degradation of solvent and solvent-free ECAs are also studied by differential scanning calorimeter and thermogravimetric analysis, respectively. The bulk resistivity of ECAs in four curing manners is significantly different, even if they are cured at the same temperature and time. The conductive trends of solvent and solvent-free ECAs are also different. The good and poor conductive properties of solvent ECA are obtained at curing temperature starting from room temperature and setting temperature, respectively. However, the results of solvent-free ECA are opposite. All the bulk resistivity and the coefficients of thermal expansion of solvent and solvent-free ECAs tend to decrease after solder reflows. The reasons are studied by thermomechanical analyzer, thermogravimetric analysis, topography and deformation measurement, and scanning electron microscopy.     

Electrical and mechanical properties of electrically conductive adhesives from epoxy,micro-silver flakes,and nano-hexagonal boron nitride particles after humid and thermal aging

In this study, we incorporated micro-silver flakes and nano-hexagonal boron nitride (BN) particles into a matrix resin to prepare electrically conductive adhesives (ECAs). The humid and thermal aging results under a constant relative humidity level of 85% at 85 °C revealed that the aged ECAs containing 3 wt% of nano-hexagonal BN particles had high reliability. The contact resistance was low and the shear strength high. Nano-hexagonal BN particles have a good effect on the reliability of ECAs that can be used to improve the properties of ECAs.     

Tensile strength of two-part epoxy paste adhesives: Influence of mixing technique and micro-void formation

Two-part epoxy paste adhesives are frequently used to bond metals and composite materials in many structural applications. After mixing two reactive parts (by weight or volume ratio), adhesive paste is applied to the substrate surfaces and cured at elevated temperatures. Air-entrapment during mixing and/or application process often produces micro-voids in the adhesive bondlines and influences the strength of the bonded joints. In this work, void formation was investigated using two adhesive mixing techniques: (a) dual-cartridge and static-mixer with a dispenser and (b) hand-mix. Flat adhesive sheets were cured by mixing a two-part epoxy adhesive, and bulk specimens with notches were cut using CNC-machining. Using X-ray microtomography scans, the micro-voids were detected and material porosity was evaluated. Furthermore, tensile tests were performed on the specimens and two-dimensional digital image correlation (2D DIC) was employed to analyse the surface strain concentrations near the notches. The fracture surfaces were examined using optical and scanning electron microscopy. The results indicated that mixing technique influences the formation of micro-voids and thus the tensile strength of two-part epoxy paste adhesives.     

The preparation and performance of high-temperature adhesives for graphite bonding

Two kinds of high-temperature adhesives (HTAs) were prepared. One was composed of phenol-formaldehyde (PF) resin and boron carbide (PF+B₄C), the other was composed of PF resin, B₄C and fumed silica (PF+B₄C+SiO₂). Graphite materials were bonded by the above adhesives and heat-treated at temperatures ranging from 200 to 1500 °C. The joining strength was tested at room temperature. The results show that the graphite joints exhibit satisfactory bonding strength and that ceramics fillers show a marked property modification effect. The strength of graphite joints bonded by PF+B₄C and PF+B₄C+SiO₂ adhesive and treated at 1500 °C are 9.3 and 17.1 MPa, respectively. The property modification mechanism of ceramics fillers is also discussed in this paper. A strong chemical bonding force is introduced at the bonding interface and the volume shrinkage is restrained, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

Electrical characterization of individual metal‐coated polymer spheres used in isotropic conductive adhesives

Isotropic conductive adhesives (ICAs) filled with metal‐coated polymer spheres (MPS) have been proposed to improve the mechanical reliability compared to conventional ICAs filled with silver flakes. The electrical properties of MPS play an important role in the electrical performance of macroscopic MPS‐based ICAs. This article deals with the electrical characterization of individual MPS using a nanoindentation‐based flat punch method, in which the resistance and the deformation of single MPS were monitored simultaneously. Four groups of silver‐coated polymer spheres (AgPS) with identical polymer cores but different silver coating thicknesses were tested. The resistance of AgPS decreases gradually with increasing deformation degree of particles, and increases when the deformation of particles is reduced. In addition, the resistance of individual AgPS is dependent on the physical properties of the silver coating, such as thickness, uniformity, and porosity. The thicker the silver coating is, the lower and more stable the resistance of AgPS is. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43764.     

Experimental study of the influence of adhesive reinforcement in lap joints for composite structures subjected to mechanical loads

The paper deals with experimental investigations on reinforcing the adhesive in single lap joints subjected to mechanical loads such as tensile, bending, impact and fatigue. The adhesive used for bonding was an epoxy reinforced with unidirectional and chopped glass fibres as well as micro-glass powder. The adherends were glass reinforced composite laminates. The bonding surfaces were prepared before joining. In the case of unidirectional fibres in the adhesive region, the fibre orientations considered were 0°, 45° and 90°. The volume fraction of fibres in the adhesive layer in all the cases was 30%. The volume fractions of micro-glass powder were 20%, 30% and 40%. The tensile, bending, impact and fatigue tests on the prepared specimens were conducted according to ASTM standards. The results show that except the 90° unidirectional orientation, reinforcing the adhesive with glass fibres or powder increases the joint strength. The use of volume fraction of 30% of micro-glass powder gave the best performance in the above loading conditions. The fatigue life increased by 125%, the ultimate joint strength in tension increased by 72%, the bending ultimate joint strength increased by 112% and the impact joint strength increased by 63%. The microstructure of the debonded area was examined and three modes of failure could be observed namely cohesive failure, light fibre-tear failure and thin layer cohesive failure.     

Silver coating of spindle- and filament-type magnetic particles for conductive adhesive applications

Spindle-type hematite and magnetite particles and nickel filaments have been coated uniformly with silver by using chemical reduction. The effects of the reaction time and the concentration and nature of the reducing agent on the particle coatings were investigated at constant temperature. The reaction time had a pronounced effect on the nature of the coated surface layer. Coated and core particles were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDAX). The electrical properties of the particles were also characterized. The conductivity of nickel filaments which can be oriented under a magnetic field for anisotropic electric conduction increased from 302 to 877 S/cm at 3.5 kPa pressure when coated with silver.     

国外微电子封装用导电胶力学性能研究进展

首先概述了导电胶较之于传统的Sn-Pb焊的优点、应用前景和存在的不足。然后系统地介绍了近来国外关于导电胶力学性能研究的方法和取得的成果。     

Influence of radiant exposure on degree of conversion,water sorption and solubility of self-etch adhesives

The purpose of this study was to investigate the influence of the radiant exposure on the degree of C=C conversion (DC), water sorption (WS), and solubility (S) of the Clearfil SE Bond (CSE) and Filtek LS (LS) self-etch adhesive systems. The primer of the LS (LSP), and bond agents of the CSE (CSEB) and LS (LSB) were tested. Specimens were light-cured using a light-emitting diode (LED) at different radiant exposures (6.1, 12.2, 13.9, and 27.8 J/cm²). DC (n=10) was measured using Fourier-transform infrared spectroscopy (FT-IR). WS and S were determined according to ISO 4049. Data were subjected to two-way ANOVA and Tukey's test at pre-set alpha of 0.05. The highest DC was exhibited by LSP, followed by CSEB and LSB, all with statistical difference (p<0.001). The DC was increased with higher radiant exposure and extended light-curing time (p<0.001). LSB and CSEB showed the lower WS followed by LSP, all with statistical difference (p<0.001). CSEB and LSB presented no significance difference on the S values (p>0.05) and were lower than LSP (p≤0.05). The WS and S were not influenced by the different radiant exposures evaluated (p=0.9548 and p>0.05, respectively). The monomer conversion is related to improvement on the mechanical properties of resinous material, but these properties also depend on the polymer network structure formed.     

Influence of a wollastonite microfiller and a halloysite nanofiller on properties of thermally curable pressure-sensitive structural adhesives

A thermally curable pressure-sensitive structural tape (SAT-0) was prepared using an epoxy acrylate copolymer and an epoxy resin and modified with a wollastonite microfiller (SAT-W) and a halloysite nanofiller (SAT-H). Influence of the minerals on self-adhesive features and curing behavior of the tapes as well as on shear strength of aluminum/SAT/aluminum joints has been investigated. Thermally uncured SATs containing the fillers exhibited higher T_g (+5 °C for SAT-H), adhesion (+64% for SAT-H) and tack (+10% for SAT-W) while their cohesion was lower (−22% for SAT-W and −86% for SAT-H) in relation to SAT-0. Moreover, filler addition reduced the shear strength of the Al/SAT/Al overlap joints (−6% for Al/SAT-H/Al and −22% for Al/SAT-W/Al), however, the microfiller-based systems exhibited better crack and fatigue resistance, and higher shear strength after ageing tests than SAT-0 and SAT-H (+100% after thermal ageing, +85% after exposure in a climatic chamber, and +27% after immersion in a fuel).     

Effect of Nanoclay on the Mechanical,Thermal, and Water Absorption Properties of an UP-toughened Epoxy Network

Unsaturated polyester (UP)-toughened epoxy nanocomposites were prepared, and their effective mechanical and thermal properties were studied. Two types of organo-modified montmorillonite (OMMT) clays were used to prepare the nanocomposites. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis showed the formation of exfoliated silicate layers in the UP-toughened epoxy matrix. Mechanical tests revealed that nanocomposites (containing 1 wt% OMMT clay) showed an increase in tensile strength to 13.8%, flexural strength to 10%, and impact strength to 4% compared with an UP-toughened epoxy blend. The effect of different heating rates on the curing behavior of UP-toughened epoxy nanocomposites was investigated using non-isothermal differential scanning calorimetry. The data were interpreted using the Kissinger and Flynn–Wall–Ozawa models to find the curing reaction parameter. The water uptake behavior for nanocomposites increased with the addition of OMMTs. Scanning electron microscopy micrographs indicated morphological changes in the impact fractured samples of UP-toughened epoxy nanocomposites.     

Comparison between the ENF and 4ENF fracture characterization tests to evaluate GIIC of bonded aluminium joints

ABSTRACT

Adhesively bonded joints have been increasingly used in structural applications over mechanical joints. Cohesive Zone Modelling (CZM) is the most widespread technique to predict the strength of these joints, and it uses the tensile fracture toughness (G_IC) and the shear fracture toughness (G_IIC). Different fracture characterization methods are available for shear loadings, among which the End-Notched Flexure (ENF) is undoubtedly the most popular. The 4-Point End-Notched Flexure (4ENF) is also available. This work consists of a detailed comparison between the ENF and 4ENF tests for the experimental estimation of G_IIC of bonded aluminium joints. Three adhesives were used: a strong and brittle (Araldite® AV138), a less strong but with intermediate ductility (Araldite® 2015) and a highly ductile (SikaForce®7752). Different data reduction methods were tested, and the comparison included the load-displacement (P-δ) curves, resistance curves (R-curves) and measured G_IIC. It was found that the ENF test presents a simpler setup and has a higher availability of reliable data reduction methods, one of these not requiring measuring the crack length (a) during its growth. For the 4ENF test, only one test method proved to be accurate, and the test geometry revealed to be highly affected by friction effects.