Cell K. Y. Wong

Investigation of moisture diffusion in electronic packages by molecular dynamics simulation

Moisture-related failure is one of the main concerns in the integrated circuit (IC) package design. To minimize such failure in multi-layered electronic assemblies and packages, it is important to develop a better understanding of the reliability at a molecular level. In this paper, molecular dynamics (MD) simulations were conducted to investigate the respective moisture diffusion into the epoxy molding compound (EMC) and at the EMC/Cu interface. Moisture diffusion coefficients into the bulk EMC material and at the EMC/Cu interface can be derived from the mean-squared displacements calculated from MD simulations. The MD results showed that the seepage along the EMC/Cu interface is more prevalent when compared to moisture diffusion into the bulk EMC and, thus, rendering it a dominant mechanism causing moisture induced interfacial delamination in plastic packages.

Study on Thermal Interface Material with Carbon Nanotubes and Carbon Black in High-Brightness LED Packaging with Flip-Chip

MWNT and carbon black were used to improve the thermal performance of TIM in high-brightness light emitter diode (HB-LED) packaging. Nitric acid treatment of CNT and optimized mixing procedure were used to help disperse the CNT and carbon black. SEM images indicated the uniform dispersion and strong bonding of fillers in the epoxy resin. 100% improvement of the thermal conductivity was achieved by the developed thermal interface material (TIM) with 2 wt% MWNT and 10 wt% carbon black. The curing temperature of 140°C and glass transition temperature of 147°C indicated it was suitable to be used in HB-LED packaging. The blue color flip chip LED packages were fabricated to evaluate the thermal performance of TIM. The output light power of the 1×1mm 2 HB-LED device with the developed TIM can achieve 62mW with the input current of 300mA.

Validation of forcefields in predicting the physical and thermophysical properties of emeraldine base polyaniline

We report a molecular modelling study to validate the forcefields [condensed-phase optimised molecular potentials for atomistic simulation studies (COMPASS) and polymer-consistent forcefield (PCFF)] in predicting the physical and thermophysical properties of polymers. This work comprises of two key steps: (1) generating and validating the molecular model in predicting the material properties of the bulk amorphous emeraldine base polyaniline and (2) modelling the glass–rubber transition of the polymer. From all the molecular dynamics simulation results, it clearly shows that the more recent COMPASS forcefield provides a higher accuracy in predicting the polymer properties than PCFF, and it enables a more accurate prediction of condensed-phase properties (density, glass transition temperature, solubility parameters, etc.) in a broad range of temperature for various applications.

Density-Functional Calculation of Methane Adsorption on Graphenes

The adsorption behaviors of methane adsorbed on different graphenes (pristine, and B-, N-, P-, and Al-doped monolayer and multilayer) are analyzed using density-functional theory. The results demonstrate that the sensing performance of graphene as a methane sensor strongly depends on the selection of dopants and the number of layers. The adsorption energy on monolayer P-doped or Al-doped graphene shows about one order of magnitude higher than that with other dopants. In addition, graphenes doped with different impurities show different responses to the charge transfer. A further analysis indicates that the multilayer structure has a positive effect on the adsorption energy on the pristine, B-doped, and N-doped graphene, while the P-doped or Al-doped graphene shows a significant decrease with the increase in the number of layers. Moreover, the multilayer structure has a minor effect on the charge transfer. Based on the combined effects on the adsorption energy and the charge transfer, Al-doped monolayer graphene is the optimal candidate for methane sensing.

A New Approach in Measuring Cu–EMC Adhesion Strength by AFM

Copper-epoxy molding compound (Cu-EMC) interface is known to be one of the weakest interfaces in an electronic package exhibiting delamination during reliability test. Thiol compound which bonds readily and forms a self-assembly monolayer (SAM) with copper is proposed to improve interfacial adhesion between copper and EMC. Conventional adhesion evaluation involves force measurement in macro-scale. However, inconclusive or even contradictive results are common in those tests because of uncontrollable surface conditions such as contamination and, in particular, roughness. To eliminate the roughness effect and reflect the true chemical bonding condition, an Si wafer was used as a substrate in the experiments. This study involves the use of an atomic force microscope (AFM) in characterizing the nanoscale adhesion force in a Cu-SAM-EMC system. Findings were used as the criteria in selecting a SAM candidate. A thiol compound having a carbonyl group is shown to be the best adhesion promoter from the measurement. The nanoscale AFM results are shown to be consistent with the result of macroscopic shear tests. It has been demonstrated, with SAM treatment on a cleaned copper surface, that the fracture force between Cu-EMC samples is improved from 119 to 195N

Molecular dynamics simulation of thermal cycling test in electronic packaging

Interfacial failure under thermal cycling conditions is one of the main concerns in package design. To minimize such failure in multi-layered electronic assemblies and packages, it is important to develop a better understanding of the reliability at a molecular level. In this paper, molecular dynamics (MD) simulations were conducted to investigate the interfacial energy of the epoxy molding compound (EMC) cuprous oxide system during the thermal cycling test. In order to investigate the effect of the cuprous oxide content in the copper substrate on the interfacial adhesion, two kinds of MD models were examined in this study. The results revealed that the cuprous oxide content in the copper substrate had a large effect on the interfacial adhesion between the EMC and copper which is consistent with the experimental observation.

Interfacial Adhesion Study for SAM Induced Covalent Bonded Copper-EMC Interface by Molecular Dynamics Simulation

Copper-epoxy molding compound (Cu-EMC) interface is known to be the weakest joint in the electronic packages, which causes delamination during a reliability test. A prime reason is the lack of adhesion between Cu and the epoxy compound. To solve the problem, a self-assembly monolayer (SAM) is introduced to improve adhesion of copper-epoxy system. Thiols/disulfides, which can effectively deposited on Cu surface, were selected as the SAM to act as surface modifier to Cu substrate. The selection of thiols/disulfides candidate with an appropriate tail group is essential for the adhesion enhancement. To promote adhesion, the SAM structures should be able to form high density covalent bonds with the EMC. This paper focuses on the use of molecular dynamics (MD) simulation to study the covalent bonds effect on the adhesion in Cu-SAM-EMC system. The data is used as a means to select SAM candidates with good interfacial adhesion strength. In this study, MD simulation models of the Cu-SAM-EMC system with covalent bonding and optimized SAM density were built to evaluate the interfacial bonding energy between the EMC and SAM coated substrate. The results show that the interfacial bonding energy changed with different SAM coatings on the Cu substrate. The modelling results were compared with the experimental button shear data. The shear test shows that when Cu substrates have been coated with SAM with amine/amide groups, adhesion increased significantly. A consistent qualitative trend is observed in the results calculated by the MD simulations. The sheared samples, which were analysed by the time-of-flight secondary ion mass spectrometer techniques, further confirmed the existence of covalent bonds at the interface. This proves that the covalent bonding at the interface is a key mechanism in enhancing the interfacial adhesion. This work illustrates that MD can help in understanding the behavior of bonding of SAM to polymer at the molecular scale. The MD can be a useful tool to select SAM structure for adhesion promotion in a Cu-SAM-EMC system.

Thiol-based self-assembly nanostructures in promoting interfacial adhesion for copper-epoxy joint

Adhesion promotion between copper-epoxy interfaces without roughening the copper substrates is critical for new generation electronic devices. This paper demonstrates a pronounced adhesion promotion of a copper-epoxy joint from 4.8 J m−2 for the untreated samples to 159 J m−2 for the interfaces prepared with a thiol-based self-assembly molecular layer (SAM). The 33-fold improvement was related to the presence of nanostructures with the SAM treatment. The adhesion enhancement is attributed to both chemical bonding between copper and epoxy and the formation of nanosized features on copper substrates. The thiol promoter enhances the interfacial adhesion with no roughening of the substrates.

Functionalization-induced changes in the structural and physical properties of amorphous polyaniline: a first-principles and molecular dynamics study

In this paper, we present a first-principles and molecular dynamics study to delineate the functionalization-induced changes in the local structure and the physical properties of amorphous polyaniline. The results of radial distribution function (RDF) demonstrate that introducing -SO3−Na+ groups at phenyl rings leads to the structural changes in both the intrachain and interchain ordering of polyaniline at shorter distances (≤5 Å). An unique RDF feature in 1.8–2.1 Å regions is usually observed in both the interchain and intrachain RDF profiles of the -SO3−Na+ substituted polymer (i.e. Na-SPANI). Comparative studies of the atom-atom pairs, bond structures, torsion angles and three-dimensional structures show that EB-PANI has much better intrachain ordering than that of Na-SPANI. In addition, investigation of the band gap, density of states (DOS), and absorption spectra indicates that the derivatization at ring do not substantially alter the inherent electronic properties but greatly change the optical properties of polyaniline. Furthermore, the computed diffusion coefficient of water in Na-SPANI is smaller than that of EB-PANI. On the other hand, the Na-SPANI shows a larger density than that of EB-PANI. The computed RDF profiles, band gaps, absorption spectra, and diffusion coefficients are in quantitative agreement with the experimental data.

Investigation of adhesion properties of Cu-EMC interface by molecular dynamic simulation

Cu-EMC (copper-epoxy molding compound) interface is known to be the weakest joint in the electronic packages, which causes delamination during reliability test. A prime reason is the lack of adhesion between Cu and epoxy compound. To solve the problem, self-assembly monolayer (SAM) is used to improve adhesion of copper-epoxy system. This paper focuses on simulation of adhesion in Cu-SAM-EMC system. In this study, molecular models of bi-material system, which consists of SAM and Cu, were built to evaluate the adhesion force of the Cu-SAM system. Interfacial energy density was derived from molecular dynamics (MD) simulation. The newly developed condensed phase optimization molecular potentials for atomistic simulation studies (COMPASS) force field enables prediction of adhesive strength of bi-material system in organic-metal oxide interface. The MD results were compared against shear load obtained from button shear tests. It has been illustrated that MD can provide a reasonable qualitatively prediction of the relative interfacial strength of the selected SAM samples. It is concluded that MD is an effective tool in screening SAM candidates.