Multiscale approach optimization on surface wettability change on rough surface

Surface wettability is known that is not only governed by chemical structure but also by the surface geometrical structure. A multiscale approach on rough surface wettability study was presented in this paper. The wettability study of photo-switched trans and cis isomers of azobenzene on different substrates was first calculated by molecular dynamics calculations. Different chemical structures and configurations were input into the molecular model to get equilibrated structures. Contact angle is then estimated and input into finite element model with roughness factor included. The parameters were input into the FLUENT software to estimate the respective surface wettability for each individual trans and cis configuration on different rough surface. The simulated wettability results were found to be in good correlation with experimental measures. This multiscale approach provides an opportunity to study the combined effects of surface interaction at molecular scale, and micron scale surface roughness, on the wettability of a rough surface. It enables the prediction of contact angle of liquid media on rough surfaces which will be a powerful tool in the selection and optimization of material and substrate surface structure to control the hydrophobicity/hydrophilicity at liquid/solid interface. ©2010 IEEE

Using novel materials to enhance the efficiency of conductive polymer

Conductive polymers have a vast market in integrated circuits (IC) and microsystems packaging to enhance mechanical, thermal, electrical performance, and cost effectiveness[1]. Isotropically conductive adhesives (ICAs) have been explored for attaching encapsulated surface mount components on rigid and flexible printed circuits [2]. However, the practical use of conductive adhesives in surface mount applications is limited because of the weak electric conductivity. Jiang et al [3] used nano-sized silver particles as a candidate for conducting filers in order to reduce the sintering temperature, but the contact resistance is still high. Some groups [4, 5] studied a series of methods such as using carboxylic acid group containing chemicals as surfactants to enhance the conductivity of ICAs in a variety of conditions, but because the micron-sized silver fillers have a high sintering temperature, the enhancement in conductivity is still limited. In order to further improve the conductivity of ICAs and minimize the cost, we experimented on a series of materials for silver surface pretreatment. We noticed an about 20 times improvement in conductivity of the modified ICA than the control sample (75% silver content in all samples). The volume resistivity of the optimum formulation reached the level of 10^-6 Ω·cm. We also analyzed the adhesion strength and thermal property of the modified ICA material. The study indicated that both the electrical properties and the mechanical property were improved without negatively affecting the other physical properties, and they are both remain stable after subjecting to the 85°C and 85% relative humidity conditioning test. © 2008 IEEE

UV-activated surface modification of photo-cleavage polymer for contact printing applications

Polymer electronics is an emerging technology for the last decade. For cost-efficient mass production and for thin, flexible polymer electronic systems, large area patterning processes may be an interesting option as an economic production method and will potentially play an important role in polymer electronics manufacturing. High resolution patterning methods for defining the separation between electrodes in electronic devices are important in manufacturing. The control of surface wettability during contact printing is an interesting approach because of its wide variety of applications. Stimuli-responsive surfaces make it possible to control the wettability of the surface and have been demonstrated by various methods, including UV light-irradiation. Herein, a new strategy was demonstrated using free radical initiator to induce mold release between PDMS mold and the resins under UV irradiation. For example, by applying a thin layer of benzoyl peroxide (BPO) on PDMS surface, an increase of contact angle is achieved after UV irradiation. This method can be used as a transfer mechanism from mold to substrate. It was noticed that sufficient time of BPO deposition for the PDMS mold surface treatment is required for this strategy. Optimum concentration of BPO and suitable solvent system are concerns in the effectiveness of surface treatment. From this study, some preliminary insight in studying the controlling factors for the UV activation of free radicals on PDMS surface was shown. It can be shown that the molecular structure, polarity of materials, UV sensitivity of the free radical initiators, and solvent used, have direct effect on the efficiency of the wettability change under the UV irradiation. By knowing the controlling factors of UV assisted stimuli responses, printing can be improved and be applied in many other cases. © 2008 IEEE

Reliability prediction in electronic packages using molecular simulation

Reliability of electronic packages is a great concern to packaging design engineers. During its design life, packages experience a wide range of temperature variations. The mismatch in coefficient of thermal expansion between the different layers in the packages can generate high interfacial stresses due to these thermal loading. If these stresses exceed the limiting value, delamination will occur. The present study is focused on the reliability of the epoxy molding compound (EMC) and cuprous oxide coated copper substrate. In order to verify whether the interfacial adhesion is dominant by the content of cuprous oxide on the cooper substrate, two models were built to simulate the thermal cycling test with a constant cuprous oxide and a changing content of cuprous oxide on the copper substrate. The thermal cycling test was conducted with a given thermal profile. The adhesion strength between EMC and cuprous oxide coated copper substrate at different thermal cycles was evaluated using the button shear test (BST). A simple molecular model of a bi-material system, which consists of EMC and cuprous oxide coated copper substrate, was built to evaluate the interfacial energy of the Cu-EMC system. In order to dramatically reduce the computational time, the system was modeled with a limited number of atoms. A preset strain was applied to the model representing a forcing step as the EMC material was pulled away from the copper substrate. Equilibration was conducted to relax the whole system before the next strain step proceeded. The procedure was repeated using different strains. The interfacial energy at different thermal cycles was evaluated. The variation of the interfacial energy indicated the change in the adhesion strength between EMC and cuprous oxide coated copper during the thermal cycling test. The simulation results revealed that the cuprous oxide content in the copper substrate had a large effect on the adhesion between EMC and copper, which is consistent with the experimental observation. © 2005 IEEE

A new method to predict delamination in electronic packages

Interfacial delamination, due to the presence of dissimilar material systems, is one of the primary concerns in electronic package design. The mismatch in coefficient of thermal expansion between the different layers in the packages can generate high interfacial stresses due to thermal loading during fabrication and assembly. The present study is focused on the delamination at the Epoxy Molding Compound (EMC)/copper interface. Different EMC materials molded on copper leadframe were tested with different shear height. The stresses at the interface were evaluated using data from the button shear test (BST). Conventional failure criteria are not able to explain the stress results observed from the button shear test data. In this study, a multi-scale model was built to determine the interfacial energy between EMC and copper substrate. The interfacial material properties were evaluated from the interaction energy between EMC and Cu substrate. The interaction of EMC and Cu can be measured using the atomic force microscope (AFM). The force-distance curve obtained directly from AFM measurement is used to determine the interfacial material properties. The properties were input to the multi-scale model. Experimental force from the BST was applied to the model. The interfacial tensile stress and shear stress were evaluated and were used to calculate the interfacial energy. An energy-based failure criterion for delamination was set up. In order to benchmark the delamination failure criterion, two electronic packages, SOT #1 and SOT #2 were studied to investigate delamination in the soldering reflow process. Based on the proposed method, the predicted results were found to be consistent with those from C-SAM measurement. © 2005 IEEE

Molecular design of reliable epoxy-copper interface using molecular dynamic simulation

Despite the fact that epoxy has continuously used as encapsulant in electronic packaging, its joint with copper-based substrate is prone to delaminate during reliability test. A prime reason is the lack of adhesion between Cu and epoxy compound. To solve the problem, self-assembly molecular structure (SAM) is adopted to improve adhesion of epoxy-copper system. In seeing that hydrophobic behaviour of the SAM structure may hinder moisture diffusion along the interface, further work in terms of the molecular structure of the SAM candidates is conducted in this study. This work aims at investigating the moisture effect on the interfacial adhesion with different types of SAM modified interfaces through molecular dynamic (MD) simulations. This study uses MD model as a tool to 1) predict the interfacial adhesion of the SAM modified interface in moisture sensitive condition; 2) investigate the moisture diffusion behavior of the modified interfaces under moisture sensitive conditions. The results demonstrate a reasonable qualitative co-relation between the MD prediction and the TDCB tested data. Nevertheless, without the experimental adsorption data for the SAM material, the moisture diffusion coefficient obtained from MD study cannot explain the adhesion degradation after aging. ©2010 IEEE

Flexible thermoplastic conductive adhesive with high reliability

Isotropically conductive adhesives (ICAs) have always been a hot spot in producing UHF RFID tag antennas with low cost for mass-production and low conductor loss.[1] As an interesting motif of flexible electronic component, they can also be integrated into sensors or as biocompatible electronic components in tissue engineering applications.[2] Considering the cost-effectiveness and the requirements for making those devices which can remain high conductivity under external stress forces, polyurethane (PU) is an interesting motif for making the dispersant material, as it can provide maximized elongation at break, moisture resistance, and excellent impact resistance and reliability. They are also safe to human health, which have a blooming market in tissue regeneration materials.[3, 4] Severe deformation of the matrix can result in the rearrangement of the conductive silver fillers, which can result in the variation of the electrical resistivity of the elastic ICA. Based on our prior research work on the ultralow silver filling rate of ICAs, we evaluated the electrical performance of the elastic ICA materials and their mechanical properties.[5] From the experimental result, we observed that the ICAs with this elastic polymer dispersant showed stable electrical resistivity after aging for 168 hours and even at low filler content level (e.g. 30%). Read range of the RFID tags showed that these ICAs are feasible for printed antennas. © 2009 IEEE

Delamination control in electronic packaging using the energy method

Interfacial delamination, due to the presence of dissimilar material systems, is one of the primary concerns in electronic package design. The mismatch in the coefficient of thermal expansion between different layers in packages can generate high interfacial stresses when subjected to thermal loading during fabrication and assembly. Many of the failure criteria were developed to solve the problems with a precrack. However, in real electronic packages, the size and location of the cracks or/and delamination cannot be predicted. It is not easy to use the traditional fracture criteria to deal with complicated 3-D delamination problems. The delamination of copper leadframe/Epoxy Molding Compound (EMC) was selected in the study. The allowable stresses of the interface were evaluated by the Button Shear Test (BST). In this paper, the critical load acting on the upper part of the button shear sample was measured at a certain shear height and a finite element model was used to evaluate the interfacial stresses for different material system. An energy-based method is proposed by deriving the energy to initiate each of the tensile and shear modes of failure across the interfaces of the button shear test samples for the chosen interfacial material system. In order to benchmark the delamination failure criterion, two electronic packages, SOT #1 and SOT #2 were studied to investigate delamination in the soldering reflow process. Four kinds of interfaces in two packages were investigated under solder reflow respectively. The button shear test was done to obtain the critical force for each material system, and the allowable strain energy density U_C of each material system was evaluated. By comparing the calculated interfacial energy density to U_C of each material system, delamination at each interface can then be predicted. The predicted results were consistent with those from C-SAM

Thermal cycling simulation in electronic packages using molecular dynamic method

Reliability under thermal cycle conditions is one of the main concerns in electronic packaging design. The cause of this concern arises from the mismatch in coefficient of thermal expansion between the different layers in a package leading to high interfacial stresses induced by thermal loading during fabrication and assembly. If these stresses exceed the limiting value, delamination will occur. The present study is focused on the reliability of the interfacial failure between conductive polymer (silver epoxy) and gold coated on sample die. The thermal cycling test was conduced with a given thermal profile. The adhesion strength between epoxy and gold coated die subjected to at different thermal cycles was evaluated using the button sheer test (EST). A simple molecular model of a bi-material system, which consists of epoxy and gold, vas built to evaluate the interfacial energy of the gold-epoxy system. In order to dramatically reduce the computational time, the system was modeled with a limited number of atoms. A preset strain was applied to the model representing a forcing step as the epoxy material was pulled away from the gold substrate. Equilibration was conducted to relax the whole system before the next strain step proceeded. The procedure was repeated using different strains. The interfacial energy induced in the interface under different thermal cycles was evaluated. The trend of the interfacial energy indicates the change of the adhesion strength between epoxy and gold during the thermal cycling test. The simulation results can be benchmarked by the results of BST subjected to the same number of thermal cycles. The simulation result demonstrated that the basic molecular model could be used to predict reliability in the package design. © 2005 IEEE