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.     

Stress-free temperature in a mixed-adhesive joint

Adhesive joints used in supersonic aircraft fuselage need to withstand low (?55°C), as well as high (200°C) temperatures. However, there are no adhesives suitable for the whole temperature range. A solution would be a joint with a combination of a low-temperature adhesive and a high-temperature adhesive, called a mixed-adhesive joint. In a bonded joint, the thermal stresses are generated essentially by the different thermal expansion properties of the adhesive and the adherends and, to a lesser extent, by the shrinkage of the adhesive produced by curing. The case of a mixed-adhesive joint is more complicated because there are two adhesives with different glass transition temperatures (T _g). To determine the stress-free temperature in a mixed adhesive joint, sandwich specimens of aluminium–adhesive–CFRP (carbon-fibre-reinforced plastic) were fabricated and the thermal strains were measured with strain gauges. In a mixed adhesive joint, two stress-free temperatures were found: the stress-free temperature of the high temperature adhesive, which is its cure temperature, and the stress-free temperature of the low temperature adhesive, which is its T _g.     

Effects of Temperature and Loading Rate on the Mechanical Properties of a High Temperature Epoxy Adhesive

The variation of the mechanical properties of adhesives with temperature and strain rate is one of the most important factors to consider when designing a bonded joint due to the polymeric nature of adhesives. It is well known that adhesive strength generally shows temperature dependence. Moreover, in many structural applications, the applied loads can be dynamic and the design of the joint requires the knowledge of the high loading rate mechanical behaviour of the adhesive. In this study, the combined effect of the temperature and test speed on the tensile properties of a high temperature epoxy adhesive was investigated. Tensile tests were performed at three different test speeds and various temperatures (room temperature (RT) and high temperatures (100, 125 and 150°C)). The glass transition temperature (T _g) of the epoxy adhesive investigated is approximately 155°C. The ultimate tensile stress decreased linearly with temperature (T) while increased logarithmically with the loading rate, which is in the accord with the Airing's molecular activation model.     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

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     

Aging and performance of structural film adhesives. IV. A modified epoxy with improved ambient temperature shelf life

It appears that improved latency in nitrile epoxy adhesives can be achieved by replacing the commonly used urea accelerator with one of much reduced solubility but similar chemical activity. A new, commercially available film adhesive claimed to possess a shelf life of 1 year at 22°C, excellent resistance to humidity prior to cure, and a service temperature range of ?55 to 120°C has been examined. Areas studied included characterization, cure kinetics, adhesive performance, and shelf life. Its composition was found to be a variant of the 120°C curing nitrile—epoxy systems using a urea-accelerated dicyandiamide cure. Two novel features were noted: the presence of bisphenol S and the urea 1,1'-p-phenylene-bis(3,3-dimethyl) urea. The relatively high shear strength found at elevated temperatures could be related to the adhesive's high T_g of 155°C resulting from the addition of bisphenol S and the improved room-temperature latency was due to the ultralow solubility of the urea accelerator. Good durability was obtained from aluminum joints prepared using a simple silane pretreatment. Aging experiments indicated a shelf life of at least 3 months at 23°C under both dry and wet conditions, and it is predicted that adhesives of this type of composition would be stable for at least 1 year at normal domestic refrigeration temperatures of 5–10°C. © 1993 John Wiley & Sons, Inc.     

Fatigue Crack Growth Rates in Adhesive Joints Tested at Different Frequencies

Mode I fatigue crack growth tests were conducted on joints bonded with a filled adhesive (A) at 20 Hz and 2 Hz and on joints bonded with a filled and toughened adhesive (B) at 20 Hz, 2 Hz, 0.2 Hz and 0.02 Hz. Strain energy release rate, G, and J-integral were evaluated based on elastic and elastoplastic finite element analyses (FEA) of the joints bonded with adhesive A and B, respectively. For the configurations considered, J was found to be path-independent and did not differ much from G. The fatigue crack growth rate (FCGR), da/dN, in the joints bonded with adhesive A was relatively independent of frequency while it increased with decreasing frequency at given δ for the joints bonded with adhesive B. The fatigue processes in both adhesives involved the cracking of the filler particles and subsequent linkage of the resultant microcracks. The process zone in adhesive B is larger than that in adhesive A and it increases with decreasing frequency. It is suggested that this variation in process zone size can account for the observed fatigue behaviour. The fatigue crack growth velocity, da/dt, was also calculated for the joints bonded with adhesive B and the variation of da/dt with test frequency at given δG is much smaller than the variation in da/dN, suggesting a creep effect in the fatigue crack growth.     

The Effects of Pre-Bond Moisture on the Fracture Behaviour of Adhesively-Bonded Composite Joints

The results of an investigation into the effects of pre-bond moisture absorbed by fibre-composite substrates prior to bonding with various structural epoxy adhesives are presented. Substrates were bonded in the as-received condition (where substrates had been exposed to atmospheric moisture for periods of greater than three months) and were also bonded in the fully-dried condition (after drying under vacuum at 105°C for 28 days). Additionally, substrates were conditioned by water submersion for various durations prior to bonding. Double cantilever beam tests were performed on the resulting joints to determine the adhesive fracture energy, G _IC. The effect of pre-bond moisture on the glass transition temperature of the adhesive was also determined. One adhesive was shown to exhibit an extreme sensitivity to pre-bond moisture. A severe reduction in fracture energy accompanied a change in the fracture morphology and T_g. Other adhesives were shown to be relatively insensitive to the levels of pre-bond moisture introduced.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

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.     

Scanning electron fractography of lap-shear joints

Fracture surfaces of Epon 901/B-3 bonded aluminum alloy joints in the lap-shear configuration were studied using scanning electron microscopy. Major differences in the appearance of the fracture surface from those reported (8) for tensile loaded joints at 23°C are produced either by cyclic loading at 23°C or a change in test temperature to ?196°C. Fracture in tensile loaded joints at ?196°C is a brittle single step process in the opening mode in which rapid crack extension occurs throughout the joint with very little adhesive flow. Tensile fatigue fracture at 23°C is in the opening mode but crack extension is complicated by extensive adhesive flow throughout the entire joint.     

Effect of Adhesive Ductility on Cyclic Debond Mechanism in Composite-to-Composite Bonded Joints

An investigation of an adhesively bonded composite joint with a brittle adhesive was conducted to characterize both the static and fatigue debond growth mechanism under mode I and mixed mode I-II loadings. The bonded system consisted of graphite/epoxy adherends bonded with FM-400 adhesive. Two specimen types were tested: (1) a double-cantilever-beam specimen for mode I loading and (2) a cracked-lap-shear specimen for mixed mode I-II loading. In all specimens tested, failure occurred in the form of debond growth either in a cohesive or adhesive manner. The total strain-energy-release rate is not the criterion for cohesive debond growth under static and fatigue loading in the birttle adhesive as observed in previous studies with the ductile adhesives. Furthermore, the relative fatigue resistance and threshold value of cyclic debond growth in terms of its static fracture strength is higher in the brittle adhesive than its counterpart in the ductile adhesive.     

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of ?150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10^?2 and 8.33 × 10^?4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of ?150°C.     

Degradation of single lap adhesively bonded composite joints due to hot water ageing

Joints, which are the most critical part of fibre-reinforced epoxy plastic structures, can be exposed to continuous hydrothermal action. In order to estimate their long-term performance, an accelerated ageing process was performed on adhesively bonded joints of glass-fibre-reinforced epoxy plastics with [0/90/45/?45]_s fibre orientations. Changes in the static tensile properties of single lap shear samples due to hot-wet exposure were investigated for one- and two-week immersion periods and at three different water temperatures (50°C, 70°C, and 90°C). Both the ageing temperature and immersion time were found to be influential on load–displacement characteristics, maximum failure loads, and apparent failure modes of joints bonded with Loctite Hysol-9466 epoxy type adhesive. Due to the hydrothermal exposure, maximum failure loads, distance to failure values, and stiffness of joints decreased by a certain amount in proportion to the immersion time and temperature. While unaged samples and those aged at 50°C and 70°C exhibited mainly light fibre-tear (LFT) failures, the samples treated at 90°C ruptured through the material cross section in stock-break (SB) failure mode.     

Wedge test of carbon‐nanotube‐reinforced epoxy adhesive joints

Epoxy adhesives reinforced with carbon nanotubes (CNTs) were developed. The distribution of the CNTs in the epoxy matrix was observed with transmission electron microscopy. Joints were formed by unclad 2024‐T3 aluminum adherents bonded with the CNT‐filled epoxy adhesives. The durability of the joints was studied with a wedge test under water at 60°C. The addition of CNTs to the epoxy greatly improved the adhesive joint durability. The initial crack length of the joint with 1 wt % CNTs, which was obtained before the wedge specimen was put into water, was only about 7% of that with neat epoxy. After immersion of the specimens in 60°C water, the joint with neat epoxy failed after 3 h, but all of the joints adhered with different fractions of CNTs were still bound together after the experimental time of 90 h. The significant enhancement by CNTs of the adhesive joint durability was mainly attributed to the high mechanical properties of the CNTs and their ability to resist water. Nevertheless, the experimental results also reveal that the durability of the joints showed an optimum value at approximately 1 wt % CNTs, beyond which a decrease in the property was observed. In addition, the failure mechanism of the joints was also investigated in terms of interfacial failure and cohesive failure. Cohesive dominated failure was found for the joint bonded with 1 wt % CNT‐filled epoxy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Optimization study of hybrid spot-welded/bonded single-lap joints

Joining of components with structural adhesives is currently one of the most widespread techniques for advanced structures (e.g., aerospace or aeronautical). Adhesive bonding does not involve drilling operations and it distributes the load over a larger area than mechanical joints. However, peak stresses tend to develop near the overlap edges because of differential straining of the adherends and load asymmetry. As a result, premature failures can be expected, especially for brittle adhesives. Moreover, bonded joints are very sensitive to the surface treatment of the material, service temperature, humidity and ageing. To surpass these limitations, the combination of adhesive bonding with spot-welding is a choice to be considered, adding a few advantages like superior static strength and stiffness, higher peeling and fatigue strength and easier fabrication, as fixtures during the adhesive curing are not needed. The experimental and numerical study presented here evaluates hybrid spot-welded/bonded single-lap joints in comparison with the purely spot-welded and bonded equivalents. A parametric study on the overlap length (L_O) allowed achieving different strength advantages, up to 58% compared to spot-welded joints and 24% over bonded joints. The Finite Element Method (FEM) and Cohesive Zone Models (CZM) for damage growth were also tested in Abaqus^® to evaluate this technique for strength prediction, showing accurate estimations for all kinds of joints.     

Cleavage and Static Toughness

The cleavage of adhesive joints allows the experimental study of the process of fracture in the low speed range. The value of the fracture energy deduced from the fracture length is the static toughness of the adhesive. This value, which determines the endurance limit of the joint, is much larger than can be explained by the current theories. It depends on the surface treatment of the substrate and results from the damage of the adhesive bonds. To take into account these results, the equation describing the fracture of adhesive joints as it was proposed by A. N. Gent and J. Schultz has to be extended. When that is done, it applies to viscoelastic adhesives, whether pressure sensitive or hot melts, and probably also to cross-linked adhesives.

If G is the fracture energy of the joint, the equation G = G ₀ + α K ₂ ·v^a accounts for most experimental results and even for the fatigue of adhesive joints.     

Analysis of adhesively-bonded T-joints by experimentation and cohesive zone models

Within the scope of adhesively-bonded joints, one of the joint types having industrial application is the T-joint, for example, in marine applications (joining of panels to the hull and connecting the glass-fibre composite hull with anti-flood panels) and aeronautical applications (wing panels, fuselage sections). This work aims to experimentally and numerically study, by cohesive zone models (CZM), the behaviour of T-joints under peel loads. The experimentally evaluated adhesives are the Araldite^® AV138 (high ultimate strength but brittle) and Araldite^® 2015 (less stress to failure but ductile and more flexible). The joint strength is evaluated with different L-shaped adherends’ thickness (t_P2). With the numerical analysis, the stress distributions, damage evolution and strength are studied. Additionally, a purely numerical study compared joints with or without adhesive filling at the curvature of the L-shaped adherends, and an extremely ductile adhesive (Sikaforce^® 7752) was additionally evaluated. The experimental tests validated the numerical results and showed that CZM is an accurate technique for the study of T-joints. It was also shown that the geometry of the L-parts, the presence of filler adhesive and the type of adhesive have a direct influence on the joint strength. In fact, in this particular joint configuration, the ductile but with lower ultimate strength adhesive Sikaforce^® 7752 clearly outperforms the two adhesives with higher mechanical properties but less ductility.     

Steel-Epoxy Composite Joints Bonded with Nano-TiO2 Reinforced Structural Acrylic Adhesive

This article reports a study on the effect of TiO₂ nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nano-TiO₂ in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with adding the filler content up to 3 wt.%, after which it decreased with adding more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. Differential scanning calorimeter analysis revealed that glass transition temperature (T_g) values of the adhesives rose with increasing the nano-filler content. The equilibrium water contact angle decreased for adhesives containing nanoparticles. Scanning electron microscope micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.     

Aging and performance of structural film adhesives. I. A comparison of two high-temperature curing,epoxy-based systems

The room temperature aging of two epoxy adhesives, both of which are cured at 177°C and contain the moisture sensitive resin triglycidyl (4-aminophenol), has been examined. It has been found that hydrolysis of this resin is the major cause of reduction in epoxide content during aging. This in turn is largely responsible for the deterioration in the performance, especially at high temperatures, of bonded joints made with aged adhesive. The advantages of using high purity resins in adhesive formulations have been demonstrated.