Hsien-Chie Cheng

On the Thermal–Mechanical Behaviors of a Novel Nanowire-Based Anisotropic Conductive Film Technology

Extensive understanding and management of the thermal-mechanical characteristics of novel packaging designs during the bonding process are indispensable to the realization of the technologies. Thus, this paper attempts to explore the bonding process-induced thermal-mechanical behaviors of an advanced flip chip (FC) electronic packaging. FC packaging employs a novel anisotropic conductive film, which is a thin composite film composed of polymer matrix and thousands of millions of highly oriented, 1-D silver (Ag) nanowires on the scale of 200 nanometers in diameter. For carrying out the process simulation, a process-dependent finite element (FE) simulation methodology that integrates both thermal and nonlinear contact FE analyses and a special meshing scheme is applied. The material properties of the nanoscale Ag wires are first explored using molecular dynamics (MD) simulations. By the characterized material properties of the Ag nanowires, the effective material properties of the composite film are derived through two theoretical approaches: 1) the rule-of-mixture (ROM) technique and 2) the proposed FE method-based approach. The predicted results by these two approaches are extensively compared with each other to examine the feasibility of using the widely used ROM technique for such cases. In addition, the validity of the proposed process-dependent FE simulation methodology is also confirmed through three experiments: 1) micro-thermocouple measurement of temperature; 2) Twyman-Green Moire interferometry measurement of out-of-plane deformations; and 3) portable engineering Moire interferometry measurement of in-plane deformations. Throughout the investigation, the effectiveness of the novel interconnect technology is demonstrated. Good agreement with the experiments is also obtained. It is found that the technology may ensure good electrical performance and structural integrity, not only at room temperature but even at elevated temperature, based on its substantial contact stresses but minor peeling stresses on the bonding line, together with a moderate, process-induced warpage on the substrate.

Modified Nosé-Hoover thermostat for solid state for constant temperature molecular dynamics simulation

Nose-Hoover (NH) thermostat methods incorporated with molecular dynamics (MD) simulation have been widely used to simulate the instantaneous system temperature and feedback energy in a canonical ensemble. The method simply relates the kinetic energy to the system temperature via the particles momenta based on the ideal gas law. However, when used in a tightly bound system such as solids, the method may suffer from deriving a lower system temperature and potentially inducing early breaking of atomic bonds at relatively high temperature due to the neglect of the effect of the potential energy of atoms based on solid state physics. In this paper, a modified NH thermostat method is proposed for solid system. The method takes into account the contribution of phonons by virtue of the vibrational energy of lattice and the zero-point energy, derived based on the Debye theory. Proof of the equivalence of the method and the canonical ensemble is first made. The modified NH thermostat is tested on different gold nanocrystals to characterize their melting point and constant volume specific heat, and also their size and temperature dependence. Results show that the modified NH method can give much more comparable results to both the literature experimental and theoretical data than the standard NH. Most importantly, the present model is the only one, among the six thermostat algorithms under comparison, that can accurately reproduce the experimental data and also the T^3-law at temperature below the Debye temperature, where the specific heat of a solid at constant volume is proportional to the cube of temperature.

Low-Temperature Thermal Conductivity of Short Single-Walled Carbon Nanotubes Using a Modified Nosé-Hoover Thermostat

The study aims at predicting the low-temperature thermal conductivity of short single-walled carbon nanotubes (SWCNTs) through a modified Nosé-Hoover (NH) thermostat method incorporated with nonequilibrium molecular dynamics (NEMD) simulation. The thermostat method accounts for the phonon effects by virtue of the lattice vibrational and zero-point energy to accurately capture the quantum effects at temperatures below Debye temperature. Using this method, the dependences of the tube length, diameter, chirality, and temperature on thermal conductivity are examined, and the size-dependent phonon transport phenomena in the SWCNTs are also studied. The effectiveness of the proposed model is demonstrated through comparison with the results of the standard NH- and “massive” NH chain (MNHC)-based NEMD models with or without quantum corrections as well as the literature experimental and theoretical data. The results showed that the proposed model is the only one among those five that allows better agreement with the published experimental and theoretical predictions as well as the T λ-dependence (λu2009=u20092–3) of thermal conductivity at low temperature due to the dominated phonon boundary scattering. Even with quantum corrections, by which the solution can be substantially improved, the standard NH- and MNHC-based models remain unable to follow the experimental data at temperatures below 300 K. Due to ballistic–diffusive phonon transport, the low-temperature thermal conductivity of SWCNTs with lengths greater than 5 nm shows strong dependence on tubes length, diameter, and chirality.

Process-dependent Thermal-mechanical Analysis and Design of a Novel Nanowire-based Anisotropic Conductive Film Assembly

The process-induced thermal-mechanical behaviors of the np ACF typed FC technology during bonding process and temperature variation are investigated. For achieving the goal, a process-dependent simulation methodology is introduced, which incorporates a transient thermal and static thermal-mechanical finite element (FE) modeling and a death-birth meshing scheme. Prior to the modeling, the mechanical properties of the npACF film are first characterized through both the rule-of-mixture technique and the proposed FE-based scheme. Moreover, the validity of the proposed transient thermal modeling is also verified by way of a micro thermocouple technique for temperature measurement. Finally, parametric FE study is performed to assess the dependence of the thermal-mechanical behaviors of the substrate and the contact stress at the I/O interconnects and the peeling stress at the nonconductive paste (NCP) on a number of geometry and material design parameters

Mechanical analysis of Ultra-Thin-Chip-on-Flex (UTCOF) with anisotropic conductive adhesive (ACA) joints

The work attempts to investigate the thermal-mechanical behaviors of an advanced flexible electronics technology during the bonding process and under four-point bending (FPB) and static bending (SB). The flexible technology implements an epoxy-based anisotropic conductive film (ACF) to form fine-pitch and reliable interconnects of Integrated circuits (IC) bumps on flex substrates. The paper starts from exploring its process-induced thermal-mechanical behaviors through process modeling using a proposed process-dependent simulation methodology. The validity of the process-modeling is confirmed through various experiments. Subsequently, the contact behaviors of the ACF joints in the flexible technology under FPB and SB are also characterized using FE modeling and a four-point probe method. The simulated contact stress is correlated with the measured contact electrical resistance.

Optimum design of contact springs used in registered jack connectors

Two basis designs of contact springs used in RJ-45 connectors with different layouts and beam thickness were sequentially examined and optimized. The first design failed to attain a feasible solution mainly due to insufficient beam thickness. The second one is simpler in layout and in pin geometry but with thicker pins. Optimum design for Type A and Type B pins based on the second was achieved efficiently using the RSM with the power-transformed response data of computed plastic strain and contact force. Both optimized models of Type A and Type B pins enjoy a reduced strain level and an elevated contact force level above a specified value of 12.5 gf.

Design optimization and analysis of a novel nanocomposite-film typed flip chip technology

This paper aims at developing an effective scheme for design optimization of a novel nanocomposite-typed flip chip (FC) technology, constructed by integrating an Ag-nanowire/polymer nanocomposite film together with a nonconductive paste (NCP) technology. The objective of the optimization problem is to achieve the optimal process-induced thermal-mechanical behaviors of the novel FC technology during the NCP bonding process through the selection of material properties, process parameters and geometry data. The process-induced thermal-mechanical behaviors are evaluated using a process-dependent simulation methodology that integrates both transient thermal and nonlinear contact FE analyses and a “death-birth” meshing scheme. The validity of the process-dependent FE simulation methodology is also confirmed through experiment. To demonstrate the effectiveness of the present design optimization approach, several design problems associated with the FC technology are performed.

On the molecular dynamics analysis of defect effect on mechanical properties and fracture behaviors of carbon nanotubes

Summary Due to the limitation of fabrication technologies nowadays, initial defects in carbon nanotubes (CNTs) are inevitably perceived particularly during the manufacturing process or chemical treatment. The investigation of the effects of initial defects existing in CNTs on their mechanical properties and fracture behaviors becomes essential for their potentiality in engineering applications. In this study, the defect effects, including number in percentage, type, and location, are explored using the molecular dynamics (MD) simulation with TersoffBrenner potential. Results show that the mechanical properties, such as the elastic modulus, failure strength and strain, are strongly affected by the defects. Moreover, the distributionand the location of defects are also the factors to influence the mechanical properties of CNTs significantly. For example, with the same amount of defects, the elastic modulus and the failure strength both vary notably due to different defect locations of defects. Not only their static/dynamic behaviors and material properties but also their fracture evolutions are discussed in this work. It turns out that the defected CNTs behave a brittle fracture characteristic. As a single bond is ruptured, the fracture will continue to propagate until all bonds around the circumference have failed. Additionally, based on the atomic-level stress distribution, it is also observed that the crack propagates along the maximum atomic-level stress region. As similar to the mechanical properties, the crack propagation path also depends on the location of initial defects considered. In summary, according to the above results achieved, it can be reasonably explained that the large variation of mechanical properties often found in CNTs as reported in literature may result from the initial defects existing in CNTs.

Theoretical and experimental characterization of process-induced thermal-mechanical behaviors of an ultra-thin chip-on-film assembly

In this study, an advanced flexible electronics technology, termed as the ultra-thin chip-on-film (UTCOF) technology, is introduced. The technology implements an epoxy-based anisotropic conductive film (ACF), a composite materials composed of an adhesive polymer resin and conductive particles of metal-coated polymer particles, to form fine-pitch and reliable interconnects of IC bumps on flex substrates. Basically, yield and reliability that determine the feasibility of any proposed novel technology are two of the most critical and essential issues. Prior to attempting to better the reliability of the technology, it is necessary and essential to well comprehend its thermal-mechanical behaviors. Hence, the main objective of the study is to investigate the process-induced thermal- mechanical behaviors of the UTCOF technology during the ACF bonding process. Furthermore, to undertake the thermal-mechanical modeling, a process-dependent simulation methodology that integrates both thermal and thermal-mechanical finite element (FE) analyses together with a death-birth meshing scheme is proposed. The validity of the modeled results is verified through a micro thermocouple experiment, a warpage measurement using a Micro Figure Measurement Instrument and laser scanner.