Stanley Y.Y. Leung

Performance of Printed Polymer-Based RFID Antenna on Curvilinear Surface

The performance of flexible printed RFID tags affixed onto cylindrical containers is dependent on the inductive behavior of the bent antenna on the tag. Conductive polymeric coil antennas were screen printed onto flexible substrates, and the coil resistances, the inductances, and the S-parameters of the antenna coils were measured and analyzed. The RFID dies were mounted onto the antenna coils and the read ranges were characterized as a function of curvature. The results show that the coil inductance decreased slowly with increasing curvature, and the maximum read range of the tags was markedly reduced with the curvature. The decrease in the coil inductance and the maximum read range were hypothesized to vary with the projected bent coil area instead of the geometric coil area. Experimental results confirmed that the maximum read range of an RFID tag affixed on a curvilinear surface can be predicted by the classical inductive coupling model with the bent projected coil area. On the basis of the experimental and analytical results, a reading reliability factor of two is proposed as a design parameter for flexible RFID tags.

Geometric and Compaction Dependence of Printed Polymer-Based RFID Tag Antenna Performance

Radio frequency identification (RFIDs) tag with printed antennas are lower in costs, but have lower performance than those with metal antennas. Printed antennas can replace metal ones if the performance is increased without raising cost. The performance of printed antennas can be increased if the series resistance in the antennas is lowered. The resistance is dependent on the line thickness and the resistivity of the conductive ink. Printed antennas with different line thicknesses were fabricated to investigate the effect of compaction and thickness on the resistance. The resistance of the printed antenna coils was decreased by more than 40% after compaction, while the inductance and the parasitic capacitance were unchanged. RFIDs with compacted printed antennas were found to have significantly increased the read range. RFIDs with thick printed antennas were fabricated and tested. These RFIDs were shown to have read ranges comparable to the RFIDs with copper wire antennas. Moreover, a geometry-independent plateau for the read range was found. The presence of a plateau is valuable for thick-line printed antenna design since the plateau will enable the usage of lower precision high volume printing techniques to lower tag fabrication cost.

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.

Influence of chemistry and applied stress on reliability of polymer and substrate interfaces

Epoxy-based underfills in flip-chip assembly have been widely employed to enhance electronic package reliability. Addition of coupling agent in the underfill encapsulant can increase the adhesive bonding by introducing chemical bonding across the interface. The stability of this interfacial bonding is depended on the active chemicals and residual stress from curing and thermal mismatch present at the interface. The effects of chemicals and stresses have been independently observed to accelerate debonding. A model of the combined influence of stress and chemistry on the debonding rate has been proposed, but data on the combined influence of chemical and stress are not available. In this study, the stress-assisted interfacial debonding of epoxy adhesives is quantified. Underfill adhesives with silane coupling agent, titanate coupling agent, and zirconate coupling agent were characterized. Basic material properties including the curing behavior, coefficient of thermal expansion, glass transition temperature, elastic modulus and moisture absorption profile were measured by differential scanning calorimetry, thermal mechanical analysis, 3-point bending test and dynamic mechanical analysis. Debonding rates of adhesives under varied applied stress conditions were characterized using tapered double cantilever beam specimens. The implications of the data and the kinetic parameters on material choices are discussed with respect to electronic packaging reliability.

The role of water in delamination in electronic packages: degradation of interfacial adhesion

Polymeric electronic packages subjected to standard Joint Electron Device Engineering Council (JEDEC) reliability testing are known to exhibit weakening and failures at the polymeric adhesive interfaces. Coupling agents are typically used as additives in epoxy-based materials to improve package reliability. Coupling agent chemistry and environment conditions, including pH, temperature and applied stress, are known factors that affect the rate of adhesion degradation and jeopardize the long-term reliability of the package. In this study, the subcritical interfacial debonding process is described. The debonding rates of polymers with silane, titanate and zirconate coupling agents were characterized at different temperatures by shear fracture tests and tapered double cantilever beam tests under mechanical loading and simultaneous exposure to controlled acidic environments. An analytical procedure was developed to delineate the material parameters governing adhesion degradation. Elevated temperature and acidity were shown to have a strong effect on package reliability, but mechanical loading was found to have a minimal effect on the rate of adhesion degradation. The effects of the JEDEC testing conditions on interfacial bond degradation are discussed using the chemical kinetic model.

Establishment of the coarse grained parameters for epoxy-copper interfacial separation

Atomistic coarse grained parameters were calculated from a non-equilibrium molecular dynamics simulation of the separation of an epoxy-copper interface. The methodology to determine the interaction energy and the equilibrium distance between the interfacial materials at a minimum energy is established. The traction-displacement relations of the separation under the influence of time taken for atomic interaction, displacement step, and molecular size have been studied. The study illustrates that the control of the time step in the molecular dynamics models is important to ensure a proper separation simulation. The result shows close matching with the thermodynamics work of adhesion. An analytical scheme to determine the coarse grained parameters from the relations is discussed. The proposed methodology contributes to the interpretation of interfacial adhesion beyond the continuum framework.

Synergistic Toughening of Epoxy-Copper Interface Using a Thiol-Based Coupling Layer

A novel concept of tuning the fracture properties of the interface through the treatment process of the coupling layer according to the cohesive critical strain energy release rate of the epoxy is proposed for optimizing the joint strength between epoxy and copper substrate. In most coupling agent application recipes, the treatment condition design has omitted the influence of the fracture properties of the corresponding adhesive. Conceivably, excessive strengthening of the adhesive–substrate interface may not lead to optimal interfacial strength. Synergistic toughening of the interface takes place when there is simultaneous interfacial debonding and failure of adhesive under a comparable critical stress state. Under critical applied load, energy is concurrently dissipated through the fracture of the interface, the fracture in the adhesive, and possible non-reversible failure processes such as shear yielding or micro-cracking of the adhesive. These combined energy dissipation processes result in extensive energy absorption around the crack tip. The adhesive joint, therefore, becomes more crack resistant. In this study, the interfacial adhesion promotion concept with synergistic toughening was demonstrated using three different epoxy systems bonded to copper substrates modified by a thiol-based coupling layer. The coupling layer was formed by treating the copper substrate with a thiol-based coupling agent. Critical strain energy release rate of the treated tapered double cantilever beam samples in different treatment conditions was measured for each of the epoxy systems. From the failure path analysis, mixed interfacial and cohesive failure was observed. This observation indicated that extensive energy dissipation occurs around the crack tip that results in synergistic toughening of the interface. This work shows the significance of matching the fracture property of the coupling layer with the adhesive. Up to 2.3 times improvement in the critical strain energy release rate was achieved with optimized thiol treatment compared to non-optimized treatment.

Experimental investigation of time dependent degradation of coupling agent bonded interfaces

Moisture induced delamination between plastic encapsulant and substrate may lead to damage of interconnect and corrosion of metallic components. Coupling agents are widely used as additives in epoxy based encapsulants to improve the adhesive strength by introducing chemical bonding across the interface. The stability of the interfacial bonds is affected by the presence of active chemicals and stresses. Coupling agent chemistry, pH, temperature, and stresses have been observed to affect the rate of adhesion strength degradation and jeopardize the long-term reliability of packages. In this study, the subcritical interfacial debonding process under controlled chemical environment and stresses were quantified. Debonding rate of underfill adhesives with silane coupling agent, titanate coupling agent, and zirconate coupling agent were characterized by the shear fracture tests and the tapered double cantilever beams test under varied temperatures, pHs, and loadings. An analytical procedure was developed to delineate the kinetic parameters of gradual debonding. The implications of the results on subcritical failures of interfacial bonds were discussed with respect to material selection criteria and electronic package reliability.

Printed polymer based RFID antenna on curvilinear surface

Rigid-flex RFID tags with the antenna and interconnections printed on low cost flexible substrate is a low cost route for RFID production. When these printed tags are affixed onto cylindrical containers, the RF performance is affected by the bending. The effects of bending on RFID performance are investigated in this study. Conductive polymeric coil antennas with different line thicknesses and designs were screen printed onto flexible substrates for use with 13.56 MHz RFID tags. The antenna coils were mounted on curved surfaces. S-parameters of the antenna coils were measured, and coil resistances and the inductances were delineated. RFID dies were mounted onto the antenna coils and the read ranges were characterized as a function of surface curvature. While the coil inductance decreased slowly with increasing curvature, the maximum read range of the tags was dramatically reduced. On the basis of the results, design guidelines are developed for printed RFID tags and implementation issues for printed antenna coils on curvilinear surfaces are discussed.

Quantitative analysis of resistance variations in as-deposited nickel-phosphorus (NiP) embedded resistors

Embedded resistors can be fabricated by plating resistive Nickel-Phosphorus (NIP) alloy in a patterned geometry to form resistors on circuit boards. As-deposited NickelPhosphorus (NIP) alloy resistors typically have poor resistance tolerances and require tuning by costly laser trimming to meet specifications. Cost of plated resistors can be reduced if tolerances can be reduced to meet specified tolerance without laser trimming. Resistance tolerance of the resistor is determined by the plated geometry and sheet resistivities of the plated materials. In this study, a group of parameters affecting patterning accuracies, plating and sheet resistivities were vaned to evaluate their effects on the resistance tolerances of electroless-plated NiP resistors. The effect of substrate on sheet resistivity variations was also characterized. Design guidelines lo obtain NiP resistors with low tolerance without laser trimming are discussed.