Adhesion and delamination have been pervasive problems hampering the performance and reliability of micro-and nano-electronic devices. In order to understand, predict, and ultimately prevent interface failure in electronic devices, development of accurate, robust, and efficient delamination testing and prediction methods is crucial. Adhesion is essentially a multi-scale phenomenon: at the smallest scale possible, it is defined by the thermodynamic work of adhesion. At larger scales, additional dissipative mechanisms may be active which results in enhanced adhesion at the macroscopic scale and are the main cause for the mode angle dependency of the interface toughness. Undoubtedly, the macroscopic adhesion properties are a complex function of all dissipation mechanisms across the scales. Thorough understanding of the significance of each of these dissipative mechanisms is of utmost importance in order to establish physically correct, unambiguous, values of the adhesion properties, which can only be achieved by proper multi-scale techniques. The topic “Advances in Delamination Modeling” has been split into two separate chapters: this chapter discusses the atomistic aspects of delamination, while the preceding chapter deals with the atomistic aspects of interface separation. The chapter starts with a concise overview of molecular simulation strategies. Next, examples are provided which represent actual materials being developed for electronic packaging: (1) the prediction of thermomechanical properties of an epoxy molding compound (EMC) and the adhesion properties of an EMC/copper interface by means of MD and CG MD approaches; (2) the modeling of wetting, adhesion, and reliability cycling of die attach and via fills; (3) model scaling to discrete element modeling (DEM) for understanding underfill flow; (4) CG modeling of an epoxy molding compound which relates to the first example; (5) molecular modeling of silicate layers used in planarization and encapsulant layers for flat panel displays; (6) mesoscale modeling of diffusion of organic bases which is of concern to photoresist poisoning; and (7) the prediction of thermomechanical properties of a low-k dielectric material, SiOC:H.

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Purpose
- The purpose of this paper is to discuss the possibility of using soldering process for the bonding of chip-on-flexible (COF) light-emitting diode (LED) packages to heat sinks. The common bonding materials are thermal conductive adhesives. For thermal performance and reliability concerns, Tin-Bismuth (SnBi) lead-free solder paste was used for the connection of the COF packages and the Cu heat sinks by a soldering process in this study. Meanwhile, the geometrical effect of the SnBi solder layer on the thermal performance was also investigated.
Design/methodology/approach
- The effects of the bonding materials and the area of the solder layers on the thermal performance of the LED modules were investigated by finite element simulation and experimental tests.
Findings
– The SnBi soldered modules show much lower thermal resistance at the bonding layers than the adhesive-bonded LED module. Vertical heat transfer from the LED chips to the heat sinks is the primary heat dissipation mode for the SnBi soldered modules. Thus, the LED module with local solder layer shows similar LED thermal performance with the full-area soldered module. Meanwhile, the local soldering process decreases the possibility to form randomly distributed defects such as the large area voids and residue flux in the solder layers.
Research limitations/implications
– The research is still in progress. Further studies mainly focus on the reliability of the samples with different bonding materials.
Practical implications
– COF package is a new structure for LED packages. This study provides a comparison between SnBi solder and adhesive material on the thermal performance of the LED. Meanwhile, the authors optimized the geometrical design for the solder layer. The study provides a feasible bonding process for COF packages onto heat sinks.
Originality/value
– This study provides a soldering process for the COF LED packages. The thermal performance of the LED light source was improved significantly by the new process.@en

Light Emitting Diode (LED) luminaires and lamps are energy-saving and environmental friendly alternatives to traditional lighting products. However, current luminous flux depreciation test at luminaire and lamp level requires a minimum of 6000 h testing, which is even longer than the product development cycle time. This paper develops an accelerated test method for luminous flux depreciation to reduce the test time within 2000 h at an elevated temperature. The method is based on lumen maintenance boundary curve, obtained from a collection of LED source lumen depreciation data, known as LM-80 data. The exponential decay model and Arrhenius acceleration relationship are used to determine the new threshold of lumen maintenance and acceleration factor. The proposed method has been verified by a number of simulation studies and experimental data for a wide range of LED luminaire and lamp types from both internal and external experiments. The qualification results obtained by the accelerated test method agree well with traditional 6000 h tests.@en

Lumen depreciation is one of the major failure modes in light-emitting diode (LED) systems. It originated from the degradation of the different components within the package, being the LED device or chip, the driver, and the optical materials (including phosphorous layer). This chapter describes the state of the art of the degradation mechanism for these components and how they contribute to the lumen depreciation of the LED package as a whole.@en

We proposed a molecular modeling methodology to study the protonic acid doping of emeraldine base polyaniline which can be used in gas detection. The commerical forcefield COMPASS was used for the polymer and protonic acid molecules. The molecular model, which is capable of representing the polyaniline doping with the aqueous hydrochloric acid, was built by Monte Carlo method. The initial entire systems were equilibrated using a new protocol, which aims at creating a final structure with realistic density and low-potential energy characteristics. The doping process is modeled by a combined molecular mechanics and molecular dynamics simulation technique. The radial distribution functions of doped emeraldine salt and the relationships including pKa/pH and doping percentage/pH, were computed and compared with the experimental data. This method contributes a novel molecular modeling approach to select and evaluate the conducting polymers in chemical sensor applications.@en

Gas sensors fabricated with nanowires as the detecting elements are powerful due to their many improved characteristics such as high surface-to-volume ratios, ultrasensitivity, higher selectivity, low power consumption, and fast response. This paper gives an overview on the recent process of the development of nanotechnology and nanowire-based gas sensors. The two basic approaches, top-down and bottom-up, for synthesizing nanowires are compared. The conduction mechanisms, sensing performances, configurations, and sensing principles of different nanowire gas sensors and arrays are summarized and discussed. Meanwhile, an emerging nanowires fabrication method and a self-powered nanowire pH sensor are highlighted. The scientific and technological challenges in the field are discussed at the end of the review.@en