Yung-Yu Hsu

Design of nanowire anisotropic conductive film for ultra-fine pitch flip chip interconnection

As the prediction that the I/O pitch will decrease from 60 um in 2004 to 20 um beyond 2010 by ITRS roadmap, flip chip interconnection by traditional ACF containing conductive particles with micro-meter size will face more and more challenges. One of many possible solutions is using high aspect-ratio metal posts or flake instead of conductive particles for electrical interconnection between chip and substrate. But this interconnection by metal posts is less reliable compared with elastic conductive particles. Therefore we develop a new type of conductive film composed of nanowires and polymer. Unlike some other composed material by blending nanowires, tubes, powders in polymer, the arrangement of nanowires in polymer is highly ordered in X, Y, and Z direction for anisotropic conductance. In order to achieve high reliability performance of this novel package, the structure design of flip chip package constructed by nanowires/polymer conductive film was evaluated by stress simulation and related D.O.E analysis. In this research, series of finite element models were established based on the D.O.E. (design of experiment) matrix. The four factors including thickness of nanowires/polymer composed film, volume ratio of nanowires in nanowires/polymer composed film, CTE and Youngs modulus of polymer were used in this D.O.E. matrix. The full factorial DOE matrix was applied to optimize the response of peeling stress. These results indicated that volume ratio of nanowires was the major factor. The other important factor was film thickness. Besides the above stress analysis, we also demonstrated the production of nanowires/polymer composed film. Now we can obtain the silver nanowires/polyimide composed films with diameter of nanowire about 200nm and maximum film thickness up to 50 um. The X-Y insulation resistance is about 4~6 GOmega and Z-direction resistance including the trace resistance (3mm length) is less than 0.2Omega

The effect of pitch on deformation behavior and the stretching-induced failure of a polymer-encapsulated stretchable circuit

The deformation behavior and failure mechanisms of parallel-aligned, horseshoe-patterned, stretchable conductors encapsulated in a polymer substrate were investigated by numerical and experimental analyses. A design guideline for the optimal pitch between the conductors was proposed through numerical analysis, and two extreme cases—fine and coarse pitches—were investigated by in situ experimental observations. The experimental results demonstrate that the stretchable conductors enable elongation up to 123 and 135% without metal rupture for the fine and coarse pitches, respectively. The difference between these numbers is much smaller (12%) than expected from the simulations. It is found and confirmed by a modified simulation model that the reason for this is interfacial delamination, the onset of which depends on the pitch of the conductors and occurs before metal rupture of the conductors. The definition of stretchability of the electronic interconnects is discussed based on the facts that two different failure mechanisms occur: interfacial delamination and metal rupture.

Fabrication of Nanowire Anisotropic Conductive Film for Ultra-Fine Pitch Flip Chip Interconnection

As the prediction that the I/O pitch will decrease from 60 um in 2004 to 20 um beyond 2010 by ITRS roadmap, flip chip interconnection by traditional ACF containing conductive particles with micro-meter size will face more and more challenge. One of many possible solutions is using high aspect-ratio metal posts or flake instead of conductive particles for electrical interconnection between chip and substrate. But this interconnection by metal posts is less reliable compared with elastic conductive particles. Therefore we develop a new type of conductive film composite of nanowires and polymer. Unlike some other composed material by blending nanowires, tubes, powders in polymer, the arrangement of nanowires in polymer is highly ordered in X, Y, and Z direction for anisotropic conductance. In this paper, we used AAO templates to obtain silver and cobalt nanowires array by electrodeposition. And then low viscous PI were spread over and filled into the gaps of nanowires array after surface treatment. The bi-metallic Ag/Co nanowires could keep parallel during fabrication by magnetic interaction between cobalt and applied magnetic field. Now we can obtain the silver and cobalt nanwires/polyimide composite films with diameter of nanowire about 200nm and maximum film thickness up to 50 um. The X-Y insulation resistance is about 4~6 GΩ and Z-direction resistance including the trace resistance (3mm length) is less than 0.2Ω. Besides, we also demonstrated the evaluation of this nanowire composite film by stress simulation. In this study, series of finite element models were established based on D.O.E. and the response of peeling stress was in a range from approximated 10 MPa to 20 MPa when the modulus and CTE of polymer was assumed as 3 GPa and 50 ppm.

Reliability assessment of stretchable interconnects

In this paper, we comprehensively investigate the fatigue life and the failure modes of horseshoe-patterned stretchable interconnects, through both experimental and numerical analysis. The experimental results demonstrate that the fatigue life of a horseshoe-patterned stretchable interconnect embedded into a silicone matrix is able to resist up to 3000 cycles for a uniaxial elongation of 10%. By increasing the magnitude of the uniaxial elongation up to 30%, the lifetime drops rapidly to 85 cycles. A power law curve fitting relating the elongation versus the number of stretching cycles (E-N curve) is proposed based on the abovementioned experimental results. Moreover, combining the numerical modeling with the experimental results, a modified Coffin-Manson equation for fatigue life prediction is proposed for further evaluation of reliability performance. Micrographs and the correlated numerical simulations of the non-encapsulated stretchable intereconnects provide the experimental evidences and numerical explanations of the three-step failure processes.

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.

Design and analysis of a novel fine pitch and highly stretchable interconnect

Purpose – The purpose of this paper is to demonstrate electromechanical properties of a new stretchable interconnect design for “fine pitch” applications in stretchable electronics.Design/methodology/approach – A patterned metal interconnect with a zigzag shape is adhered on an elastomeric substrate. In situ home‐built electromechanical measurement is carried out by the four‐probe technique. Finite element method is used to analyze the deformation behavior of a zigzag shape interconnect under uniaxial tensile loading.Findings – The electrical resistance remains constant until metal breakdown at elongations beyond 40 percent. There is no significant local necking in either the transverse or the thickness direction at the metal breakdown area as shown by both scanning electron microscopy micrographs and resistance measurements. Micrographs and simulation results show that a debonding occurs due to the local twisting of a metal interconnect, out‐of‐plane peeling, and strain localized at the crest of a zigzag...

Design and performance of metal conductors for stretchable electronic circuits

In this paper we review the mechanical properties and reliability results of stretchable interconnections used for electronic applications. These interconnections were produced by a Moulded Interconnect Device (MID) technology in which a specially designed metal interconnection if fully embedded with an elastic material such as polyurethane or silicone. In order to get a first impression of the expected damage in the interconnections, this research employs Finite Element Modelling (FEM) to analyse the physical behaviour of stretchable interconnects under different loading conditions. Moreover, the fatigue life of a copper interconnect embedded into a silicone matrix has been evaluated using the Coffin-Manson relation and FEM.

Thermo-mechanical analysis of flexible and stretchable systems

This paper presents a summary of the modeling and technology developed for flexible and stretchable electronics. The integration of ultra thin dies at package level, with thickness in the range of 20 to 30 ¿m, into flexible and/or stretchable materials are demonstrated as well as the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. These technologies can achieve mechanically bendable and stretchable subsystems. The base substrate used for the fabrication of flexible circuits is a uniform polyimide layer, while silicones materials are preferred for the stretchable circuits. The method developed for chip embedding and interconnections is named Ultra Thin Chip Package (UTCP). Extensions of this technology can be achieved by stacking and embedding thin dies in polyimide, providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the relatively rigid polyimide by a soft and elastic silicone material. We have shown through finite element modeling and experimental validation that an appropriate thermo mechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages.

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.