Rajesh R. Gomatam

A study on the effects of surface roughness on the strength of single lap joints

In order to gain an insight into the aspects of surface roughness, surface profile, adhesive viscosity, and surface morphology, experiments were conducted using two different steels (1018 cold-rolled and hot-rolled weld steels). Four different surface modifications were performed on these samples by sand-blasting at 552 kPa from a distance of 25.4 mm, and etching for 2 and 10 min with a chemical recipe containing chromic acid. Two different adhesives were used, namely Epon 815 and Epon 830 with viscosities of 5-7 and 170-225 Poise, respectively, to assess the effect of viscosity in bonding to adherends with different surface topographies. Contact angles were measured on all surfaces using the two adhesives. To gain an insight into the effect of surface topography, scanning electron microscopy (SEM) was utilized to observe the modified surfaces at different magnifications. Both average and maximum peak-to-valley height values were measured using a profilometer and surface profiles were also obtained for all the samples. The single lap joint strength and displacement values were measured at two different crosshead speeds of 1 and 100 mm/min, to assess the interrelationship between the failure mechanisms and joint displacements, surface topography and adhesive viscosity. The results of our work showed a general decrease in the contact angle values with average surface roughness for the liquid, Epon/DETA, thermoset/hardener combination on both cold- and hot-rolled steel specimens. When tested at a 100 mm/min loading rate, and compared with the 1 mm/min loading rate condition the cold-rolled specimens, which had smaller surface roughness averages in comparison with the hot-rolled condition, exhibited a higher reduction in failure load values. It is believed that this indicates a higher proportion of interfacial failure in cold-rolled specimens at higher loading rates.

A novel cumulative fatigue damage model for electronically-conductive adhesive joints under variable loading

This paper describes a novel cumulative fatigue damage life predictive model aimed at providing the design engineer an easy fatigue life predictive tool using experimental data. This encompasses an integrated approach of joint testing, analysis, and modeling. For this purpose, joints were prepared using stainless steel adherend specimens and a commercial silver-filled electronically conductive adhesive, and tested under monotonic and cyclic fatigue conditions, at 28°C, 20% relative humidity. Load–number of cycles (P–N) curves were generated for these specimens using load ratio R = 0.1 and R = 0.5 (R = P min/P max = σmin/σmax) at a cyclic frequency of 150 Hz. Fatigue life and failure behaviors due to cumulative fatigue damage were assessed using a novel experimental program. Experimental results revealed that most of the fatigue damage occurs during the latter part of the fatigue process and the combination of lower load ratio and higher maximum applied load has the most detrimental effect on the failure of the joint. Finally, a novel analytical model for fatigue life prediction under variable cyclic conditions was proposed using experimental data and a detailed stress analysis.

Dynamic fatigue and failure behavior of silver-filled electronically conductive adhesive joints at ambient environmental conditions

Electronically-conductive adhesives (ECAs) have been used for electronics packaging applications. Today this technology is used in electronics for laptop computers, camcorders, watch electronics, hard-drive suspensions and in various other electronic equipments. Even though ECAs have excellent potential for being efficient and less costly alternative to lead-solder interconnects, 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 the fatigue behavior due either to mechanical or thermal stresses varying in a cyclic manner. This study intends to address the fatigue and failure behavior of ECAs under ambient operating conditions. For this purpose, joints were prepared using stainless steel adherend specimens bonded with a commercial ECA, and tested using monotonic and cyclic loadings at ambient environmental conditions (28°C and 20% relative humidity). S–N curves were generated using these specimens at four different load-ratios (R), namely 0.1, 0.3, 0.5, 0.7 and 150 Hz cyclic frequency. The S–N curves were not parallel, exhibited non-linear behavior with diminishing slope at higher R values.

The effects of stress state, loading frequency and cyclic waveforms on the fatigue behavior of silver-filled electronically-conductive adhesive joints

Conductive adhesives and filled adhesive systems, in general, are used in a variety of engineering applications. There are a number of issues of concern in the design of joints bonded using electronically conductive adhesives (ECAs) and subjected to cyclic loading. These include the effects of stress state and cyclic parameters (frequency and waveform) of mechanical and/or thermal loading on the fatigue failure behavior of adhesively-bonded joints. In order to study the effects of these parameters on joint behavior, two different joint geometries were designed and tested under a spectrum of fatigue and environmental conditions. The results of our work indicated a profound influence of the stress state and cyclic waveform type on the fatigue strength of the joints. Lowering the cyclic load frequency was also found to reduce the fatigue life of the bonded joints.

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 in recent years for interconnection applications. Even though ECAs have excellent potential for being efficient and less costly alternative to solder joining in electronic components, they still suffer from a number of problems relative to durability and design to meet specific needs. These include issues with the formulations of the conductive adhesive and its interactions with the substrate surface. In order to study these problems, we prepared two different adherends varying in surface characteristics and bonded them with three different conductive adhesive formulations with varying particle loadings, shapes and sizes of conductive nickel fillers. Joints were also prepared with two different adhesive thicknesses, and the effects were discussed in Part I of this paper. This part discusses the effects of filler surface properties, volume fraction and bonding under different levels of pressure to gain insight into the influence of these parameters on the joint strength, deformation and joint conductivity. Our results showed that chemical etching of particles, which changes the shape of the particles, has a profound effect on the joint strength, deformation and electrical conduction behavior. We note that prolonged period of chemical etching of spherical Ni-110 particles with hydrochloric acid causes irregularities in the shape of the nickel particles, and an increase of 60% in the joint resistance. Higher bonding pressure generally resulted in lower failure load and ultimate displacement values, indicating the possibility of squeeze flow for the neat resin under high pressure. Incorporation of larger particles in the adhesive formulation yielded a more efficient resin squeeze mechanism.

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.

Effects of various adherend surface treatments on fatigue behavior of joints bonded with a silver-filled electronically conductive adhesive

Conductive adhesives have been used in a variety of electronic packaging applications. This paper presents an investigation into the effects of various adherend surface treatments on the fatigue and failure behaviors of adhesively-bonded joints. For this purpose, single-lap joints were fabricated using specimens with adherend surfaces modified employing various chemical and mechanical modification techniques, and tested under a spectrum of fatigue and environmental conditions. The results of our work indicate a profound influence of the adherend surface on both the fatigue behavior and also the moisture ingress mechanism into the joint. Finally, experiments were conducted to assess the effect of adherend surface condition on the moisture ingress mechanism.

A comprehensive fatigue life predictive model for electronically conductive adhesive joints under constant-cycle loading

This paper describes a novel fatigue life prediction methodology aimed at providing the design engineer an easy fatigue life predictive tool using experimental data for thermo-mechanical load cyclic fatigue under constant maximum load (P max) and load ratio (R = P min/P max = σmin/σmax). This encompasses an integrated approach to joint testing, analysis and modeling. Utilizing the proposed methodologies, we aim to predict the changes in fatigue life of the adhesive, based on the whole spectrum of test variables including temperature, humidity and load ratio. For this purpose, joints were prepared using stainless steel adherend specimens and a commercial silver-filled electronically conductive adhesive, and tested under monotonic and cyclic fatigue conditions, at 28°C, 20% relative humidity, 50°C, 90°C and elevated humidity levels. Load–number of cycles (P–N) curves were generated using two specimen geometries at two different load ratios (R), at a cyclic frequency of 150 Hz. Using the experimental data, a life predictive methodology was developed and validated. Furthermore, the usefulness of the above-mentioned fatigue life predictive capability was extended to varying stress states.

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part I: Verification of the method for flat surfaces

A mathematical procedure is developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F (x, y, z) = 0. In this paper, the flat interface problem, i.e. y = 0 surface is considered for verification of the method by comparison with the FEA method. A comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.

Behavior of electronically conductive filled adhesive joints under cyclic loading. I. Experimental approach

Electronically conductive adhesives have received a great deal of attention in recent years for interconnection applications. Although they have great potential for being a more efficient and less costly alternative to solder joining in electronic components, there are still a number of problems in the areas of durability and design to meet specific needs. Unfortunately, the usefulness of this technique has been limited due to lack of understanding of environmental effects such as exposure to high moisture and/or temperature during mechanical fatigue loading, as faced in the service environment. Furthermore, the environmental effects mentioned may, themselves be acting in cyclic form. The objective of this paper is to add to the fundamental understanding of fatigue degradation in these joints, and to identify the dominant fatigue mechanisms for different service environment regimes, including cyclic mechanical loading under elevated temperature, and humidity. The scope of this study involves in-depth analysis and assessment of fatigue mechanisms and fatigue modes for a wide variety of parameters, i.e. humidity, temperature, pre-conditioning, stress ratio, and frequency. Failure surfaces are examined to identify degradation mechanisms in the adhesive interlayer by optical and SEM techniques. Measurements and observations are related to damage processes, failure modes, and the results are assessed with respect to the relevance of existing failure theories and criteria. It is expected that much improved fatigue life, fail-safe capability, and reduced manufacturing costs will be realized for electronically conductive adhesives.