Fatigue and failure behaviors of silver-filled electronically-conductive adhesive joints subjected to elevated humidity

Conductive adhesives are used in electronics packaging applications for hybrid, die-attach and display assemblies. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). An important issue is the cyclic fatigue behavior of conductive adhesive joints under elevated humidity environments, in which failures may occur due to cyclic mechanical and/or thermal stresses. This paper addresses the effect of elevated humidity levels on the fatigue and failure behaviors of ECAs. For this purpose, joints were prepared using stainless-steel adherend specimens and a commercial ECA, and tested under monotonic and cyclic fatigue conditions, at two humidity levels, namely 20% and 90% relative humidity at 28°C. Furthermore, joint failure mechanisms were analyzed using optical techniques, and joint conductivity measurements. Load versus number of cycles (P–N) curves were generated using these specimens at three different load ratios (R), namely 0.1, 0.5 and 0.9, at a cyclic frequency of 150 Hz. The P–N curves were parallel and the failure modes were found to be predominantly interfacial, accompanied by a significant decrease in joint conductivity.     

Fatigue and failure behavior of silver-filled electronically-conductive adhesive joints subjected to elevated temperatures

The use of adhesives to replace mechanical connectors and other joining methods has enjoyed rapid growth in recent years. There are a number of issues of concern in the design of joints bonded using electronically-conductive adhesives (ECAs). One of these is the cyclic fatigue behavior of conductive adhesive interconnects under different environmental conditions, in which fatigue failure might occur due either to mechanical or thermal stresses varying in a cyclic manner. This paper addresses the effect of elevated temperatures on the fatigue and failure behavior of ECAs. For this purpose, joints were prepared using stainless steel adherend specimens bonded with a commercial ECA, and tested using monotonic and cyclic loadings, at two elevated temperatures, namely 50^°C and 90^°C. When the temperature was increased to 90^°C, close to the glass transition temperature of the adhesive, we observed consistently parallel fatigue curves at different load ratios (R = P _min /P _max) for joints, as in the case of 50^°C test condition, along with significant reduction in fatigue lives. Joint failure mechanisms were also analyzed using optical techniques, and joint conductivity measurements.     

Electrical Properties of Pre-Alloyed Cu-P Containing Electrically Conductive Adhesive

In this study, a pre-alloyed Cu-P powder with a trace amount of P (0.002 at.%) was used as a metallic filler in a phenolic resin-based electrically conductive adhesive (ECA). The electrical property of the Cu-P-filled ECA was investigated for long-term stability and reliability by aging at high temperature exposure at 125°C and 85°C/85% RH for 1000 h, respectively. Results showed that the electrical resistivity of the Cu-P-filled ECA could be maintained consistently low after high temperature exposure at 125°C for 1000 h or aging at 85°C/85% RH for 1000 h, compared with the rapidly increased resistivity of Cu-filled ECA over time. A significantly low final resistivity at an order of magnitude of 10^?4 Ω·cm could be maintained in Cu-P-filled ECA even after aging at 85°C/85% RH for 1000 h.     

Environmental aging effects on thermal and mechanical properties of electrically conductive adhesives

This article investigates environmental aging effects on thermal and mechanical properties of three model electrically conductive adhesives (ECAs). A combination of several experimental techniques including thermogravimetric analysis (TGA), water uptake measurements, dynamic mechanical analysis (DMA), and stress-strain dogbone testing has been utilized throughout this study. Samples were aged at 85°C and 100% RH for periods of up to 50 days, and some of the samples were dried at 150°C after aging. Results obtained on aged samples with and without drying suggest that the conductive adhesives may have experienced both reversible and irreversible effects during environmental aging. Both plasticization, which is reversible, and further crosslinking and thermal degradation, which are irreversible, are indicated upon exposure of ECAs to the hot/wet environment.     

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.     

导电胶粘剂的现状与进展

导电胶作为无铅材料的一种,是连接材料Pb/Sn合金的理想替代品。文章介绍了导电胶的组成、导电机理、现状、应用以及发展前景。     

The Sintering Behavior of Electrically Conductive Adhesives Filled with Surface Modified Silver Nanowires

Electrically conductive adhesives (ECAs) filled with sintered silver nanowires were prepared and the effect of different curing conditions on the electrical property of the ECAs was discussed. Silver nanowires with a diameter of 50–60 nm and a length of 2–3 μm were successfully synthesized through a polyol process and surface functionalized with dicarboxylic acid. Morphology studies showed that surface modified silver nanowires began to sinter at 200°C and became shorter and thicker, and eventually formed large chunks at higher temperatures. The conductive adhesives filled with 75 wt% of silver flakes and nanowires (3:2 weight ratio) were cured at different temperatures using two kinds of catalysts. The volume resistivity of the conductive adhesives cured at 300°C without a catalyst reached 5.8 × 10 ^–6 Ω cm. The dramatic improvement in the conductivity of the ECA is due to the sintering of silver nanowires and the high solid content resulting from the partial evaporation of polymer components.     

ENVIRONMENTAL AGING EFFECTS ON THERMAL AND MECHANICAL PROPERTIES OF ELECTRICALLY CONDUCTIVE ADHESIVES

This article investigates environmental aging effects on thermal and mechanical properties of three model electrically conductive adhesives (ECAs). A combination of several experimental techniques including thermogravimetric analysis (TGA), water uptake measurements, dynamic mechanical analysis (DMA), and stress-strain dogbone testing has been utilized throughout this study. Samples were aged at 85°C and 100% RH for periods of up to 50 days, and some of the samples were dried at 150°C after aging. Results obtained on aged samples with and without drying suggest that the conductive adhesives may have experienced both reversible and irreversible effects during environmental aging. Both plasticization, which is reversible, and further crosslinking and thermal degradation, which are irreversible, are indicated upon exposure of ECAs to the hot/wet environment.     

Improvement of adhesion and electrical properties of reworkable thermoplastic conductive adhesives

The adhesion and electrical properties of a PAE-2-based isotropically conductive adhesive (ICA) material were investigated. PAE-2 (thermoplastic polymer)-based ICAs, as well as a commercial epoxy-based ICA, showed dramatic reduction in adhesion to a NiAu surface after the 85°C/85% RH aging test, while the adhesion to a SnPb surface did not change much. To improve the adhesion strength of PAE-2 to a Ni:Au surface after the humidity aging test, coupling agents and epoxy blends were used. The formulation with a higher amount of epoxy showed a higher adhesion strength. The combined use of epoxy blending and coupling agents was effective in increasing the adhesion strength of PAE-2 before aging and also in maintaining adhesion at a 50% level after humidity aging. However, its adhesion strength to NiAu was reduced to less than 50% after 85°C/85% RH aging. PAE-2-based ICAs showed excellent reworkability.     

The interrelationships between electronically conductive adhesive formulations,substrate and filler surface properties,and joint performance. Part I: the effects of adhesive thickness

Electronically conductive adhesives (ECAs) have received a great deal of attention for interconnection applications in recent years. Even though ECAs have excellent potential for being efficient and less costly alternative to solder joining in electronic components, they still possess a number of problems with respect to durability and design to meet specific needs. One of the issues that requires understanding is regarding the optimum adhesive thickness (AT) to be used. This study addresses this issue in relation to the formulations of the conductive adhesives and their interactions with adherend surfaces. For this purpose, two different adherends varying in surface characteristics were utilized along with three different conductive adhesive formulations with varying particle loadings, and shapes and sizes of conductive nickel fillers. Joints were also prepared with two different AT values, to gain insight into the influence of AT on the joint strength, deformation and joint conductivity. As the AT was increased, only a small reduction in failure load and ultimate displacement values were observed with unetched adherends. With etched adherends, however, a small increase in joint stretchability was evident with higher adhesive thickness tested at a lower crosshead speed. When the AT was increased, we also noted a corresponding increase in the initial joint resistance.     

微电子互连用导电胶研究进展

导电胶具备无铅、分辨率高、柔韧性好、加工工艺简单、低温操作等优点而使其成为替代传统Sn/Pb钎料的理想的微电子互连材料。本文对微电子互连用导电胶的组成、分类、机理和研究现状等进行了综述。为进一步研究开发新型性/价比高的微电子互连用导电胶提供思路。     

High performance electrically conductive adhesives from functional epoxy,micron silver flakes,micron silver spheres and acidified single wall carbon nanotube for electronic package

To develop high performance electrically conductive adhesives (ECAs), bi-modal ECAs were prepared by a matrix resin, micron silver flakes and micron silver spheres, and tri-modal ECAs were prepared by a matrix resin, micron silver flakes, micron silver spheres and acidified single wall carbon nanotube (ASWCNT). With the increase of nano silver spheres, the bulk resistivity of bi-modal ECAs decreased firstly and then increased while tri-modal ECAs' bulk resistivity firstly increased and then decreased with the increase of ASWCNT due to different electrically conductive channels were formed in them. After aged for 500 h under humid and thermal cycle of constant humidity level of 85% relative humidity at 85 °C, the contact resistance shift of bi-modal ECAs was more than 20% and that of tri-modal ECAs was less than 15% showing tri-modal ECAs had lower and more stable contact resistance. The humid and thermal surroundings had bad effect on the mechanical properties of bi- and tri-modal ECAs, after aged for 500 h, they both were reduced about 50–65%. And a bi-modal ECAs and a tri-modal ECAs were optimized and investigated in detail which can be used in electronic packaging.     

Processing and shape effects on silver paste electrically conductive adhesives (ECAs)

Various amounts of silver flakes and dendrites were used as conductive fillers in an electrically conductive adhesive (ECA) resin with DPM, BCA and xylene as diluent to help uniform distribution of filler particles in the matrix. Due to the fact that the higher the temperature, the higher the shrinkage rate of the polymer resin and, consequently, the larger the connecting area in-between fillers, a better curing condition for processing silver filled ECA was found to be a relatively higher curing temperature. The mechanism of conductivity achievement in conductive adhesives was analyzed by comparing processing conditions, resistivity and microstructures. In addition, the influence of adding nano-sized silver particles on the resistivity of the conductive adhesives was also investigated and the addition of nano-sized silver particles resulted in a lower percolation threshold for ECAs.     

Development of Polyaniline/Epoxy Composite as a Prospective Solder Replacement Material

In this paper we present a novel application of a conducting polymer, polyaniline, as a conductive filler for the development of isotropically conductive adhesives. We have developed isotropically conductive adhesives using protonic acid-doped polyaniline as the conducting filler in an anhydride-cured epoxy system. Fundamental material characterization like DSC, TGA and SEM of the samples was conducted to study their properties. Conductivity of these materials was measured by the four probe method while impact properties were studied by lap shear and drop tests. Samples were aged at 85°C/～100% RH for more than 500 h and the effects of aging were studied. Conductivity value of 10^?3 S/cm was obtained at 25% PANI filler concentration. These results demonstrate the potential of such systems to function as isotropically conductive adhesives.     

Self‐healing Ag/epoxy electrically conductive adhesive using encapsulated epoxy‐amine healing chemistry

In this study, a dual‐microcapsule epoxy‐amine self‐healing concept is used for electrically conductive adhesives (ECAs). It provides the ECA samples with the ability to recover mechanical and electrical properties automatically. Epoxy and amine microcapsules were prepared and incorporated into silver/epoxy ECAs. The healing efficiency and bulk resistivity of the undamaged, damaged, and healed specimens were measured, respectively. The optimal loading of the epoxy and amine microcapsules is 6 wt % (weight ratio 1.05), and the bulk resistivity of the healed specimens is 3.4 × 10^?3 Ω cm. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41483.     

Preparation of high peel strength and high anti-aging epoxy adhesive that used for bonding aluminum alloy without surface treatment

In the case of bonding of aluminum alloy, surface pretreatment have been widely adopted for adherends so as to achieve superior adhesive performance. However, the strict surface treatment of the aluminum alloy cannot be implemented without special equipment and the mechanical properties, corrosion resistance and aging resistance of the common adhesives cannot meet the demand without surface treatment. Here, acrylic oligomer modified by carboxyl terminated organosilicon and nano alumina were used to modify an epoxy formulation based on a classical DGEBA monomer to produce a high peel strength epoxy adhesive that can be used without surface pretreatment. The peel strength and the shear strength of the adhesive could reach 7.18?N/cm and 18.75?MPa, respectively, and could be well maintained under ?70?°C and 100?°C. The novel adhesive also has good heat aging resistance, water resistance and artificial seawater resistance. SEM and XPS were used to investigate mechanism of aging resistance of modified adhesives without surface treatment.     

MXene fillers and silver flakes filled epoxy resin for new hybrid conductive adhesives

The addition of V₂CT_x two-dimensional materials as auxiliary fillers in conductive adhesives can increase the contact area between conductive particles inside the matrix effectively reducing the resistivity of epoxy resin conductive adhesives. The V₂CT_x/Ag/rGO/MWCNTs fillers inside the epoxy resin will connect more Ag-clad Cu particles to form a conductive pathway, but its excessive content will be aggregated inside and thus increase the resistivity of the conductive adhesive. The volume resistivity of ECAs increases from 4.4 × 10⁻⁶ Ω m to 1.15 × 10⁻⁵ Ω m when the V₂CT_x/Ag/rGO/MWCNTs content of 0.1% increases to 0.34%. The Ag-clad Cu particles are interconnected inside the epoxy resin to form an electron transfer network. Inside the epoxy resin substrate Ag-clad Cu particles and V₂CT_x/Ag/rGO/MWCNTs interconnects to form a larger conductive network, so that the conductive adhesive shows good conductive properties.     

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.     

微电子封装用导电胶的研究进展

随着现代科学技术的高速发展,电子产品向小型化、便携化和集成化方向发展。导电胶作为一种"无铅、绿色和环境友好"的新型材料取代传统的Pb/Sn材料,已成为电子工业组装材料的主流。简述了微电子封装用导电胶的组成和分类、研究进展和可靠性评估,提出了导电胶在微电子封装中的技术问题,并对其发展趋势和发展前景作了展望。     

Aging and performance of structural film adhesives. III. Effect of humidity on a modern aerospace adhesive

The aging of an uncured, DGEBA-based commercial film adhesive exposed to high humidity at 40°C was monitored by high performance liquid chromatography (HPLC), thermal analysis, solubility, flow, and mechanical testing of joints. DGEBA and brominated DGEBA resins, and representative mixes of these were hydrolyzed in vitro. It is suggested that the major reaction at 40°C is polymerization, which is accelerated by moisture and leads to a crosslinked structure different to that resulting from normal cure. The poorer adhesive performance after aging is mainly the result of reduced resin flow during joint formation, although there is some contribution from the hydrolysis of epoxide groups which leads to lower crosslink density in the cured adhesive. The softening point of the uncured material and T_g of the cured material are useful indicators of the quality of this adhesive which, compared to some earlier formulations, has improved resistance to these conditions.