Su-Tsai Lu

IMC growth reaction and its effects on solder joint thermal cycling reliability of 3D chip stacking packaging

Abstract The study aims at assessing the growth reaction of the Ni3Sn4 intermetallic compound (IMC) during bonding process and its dependences on the thermal-cycling reliability of the Cu/Ni/SnAg micro-joints of an advanced 3D chip stacking package under accelerated thermal cycling (ATC) loading. The growth reaction of the IMC during bonding process is also predicted through experiment and classical diffusion theory, and the relation between the IMC thickness and bonding process temperature and time is derived according to the predicted activation energy of the chemical reaction between Sn and Ni by experiment. Moreover, the micro-joint reliability prediction is made using finite element (FE) analysis incorporated with an empirical Coffin–Manson fatigue life prediction model and also ATC experimental test. To facilitate the FE modeling, the temperature-dependent thermoelastic properties of both single crystal and polycrystalline Ni3Sn4 IMC are characterized through molecular dynamics simulation and the Voigt–Reuss bound and Voigt–Reuss–Hill approximation. Results show that monoclinic single crystal Ni3Sn4 reveals a high elastic anisotropy or direction dependence of elasticity. The diffusion reaction of Sn and Ni exhibits that a longer bonding process time and a higher bonding temperature could not only increase the IMC thickness but also vary its surface morphology. In addition, the thermal–mechanical performance of the micro-joints is strongly affected by the geometry and material of IMC layer, where IMC with a thicker thickness, a less Young’s modulus, a smaller CTE and even a more rounded surface morphology can better the reliability.

Hygro-thermo-mechanical Behavior of Adhesive-BasedFlexible Chip-on-Flex Packaging

Although adhesive-based chip-on-flex (COF) packaging technologies have many advantageous features, such as flexibility and compatibility with standard semiconductor and microelectronics packaging processes, the low hygro-thermal resistance leads to reliability concerns. Thus, finite element (FE) modeling and experimental testing have been used to investigate the effects of temperature and humidity conditions on the hygro-thermo-mechanical behavior of a thin flexible anisotropic conductive adhesive (ACA)-based COF packaging technology. The investigation starts from process modeling of the thermo-mechanical behavior of the technology during the ACA bonding process. The validity of the process modeling is demonstrated by temperature and warpage experiments. Furthermore, three-dimensional (3-D) transient moisture diffusion FE analysis through a thermal–moisture analogy based on the “wetness” technique is performed to evaluate the moisture distribution, in which the moisture properties of the polyimide (PI) substrate are obtained through a moisture absorption experiment. Then, the effect of the moisture properties of the ACA adhesive and PI substrate on the moisture diffusion behavior is examined. Finally, following process modeling, 3-D hygro-thermo-mechanical FE analysis under a constant temperature and humidity condition is undertaken to assess the influence of hygro-thermal aging and stress relaxation of the ACA adhesive on the long-term contact performance of the interconnects.

Crystal size and direction dependence of the elastic properties of Cu3Sn through molecular dynamics simulation and nanoindentation testing

Abstract The study aims at exploring the elastic properties of orthorhombic Cu 3 Sn crystals through a proposed molecular dynamics (MD) simulation model based on the modified embedded atom method (MEAM) and nanoindentation testing. The focuses of the study are placed on their dependence on the crystal size and direction. The electronic nature of single crystal Cu 3 Sn is also examined by using first-principles calculations based on density function theory (DFT). According to continuum mechanics, the elastic stiffness coefficients of the single crystal Cu 3 Sn are derived from the calculated energy, and used in the generalized Hook’s law in compliance form to compute the associated elastic constants. The simulated elastic properties are compared with the results of the published first-principles calculations. For comparison with the present nanoindentation finding and the other published experimental data, the effective elastic properties of the polycrystalline Cu 3 Sn together with their size dependence are also derived using the Voigt–Reuss bounds and Voigt–Reuss–Hill average based on the calculated single crystal data. The simulation results show that the orthorhombic Cu 3 Sn crystals exhibit a high elastic anisotropy, which has been also confirmed by the electronic structure analysis, and also a strong size and direction dependence of elasticity. In addition, the calculated effective elastic properties of the polycrystalline Cu 3 Sn agree well with the present nanoindentation results and the published theoretical/experimental data.

Assessing Ink Transfer Performance of Gravure-Offset Fine-Line Circuitry Printing

In this study, the printing mechanism and performance of gravure-offset fine-line circuitry printing technology are investigated in terms of key printing parameters through experimental and theoretical analyses. First, the contact angles of the ink deposited on different substrates, blankets, and gravure metal plates are experimentally determined; moreover, their temperature and solvent content dependences are analyzed. Next, the ink solvent absorption and evaporation behaviors of the blankets at different temperatures, times, and numbers of printing repetitions are characterized by conducting experiments. In addition, while printing repeatedly, the surface characteristics of the blankets, such as the contact angle, vary with the amount of absorbed ink solvent, further affecting the ink transfer performance (ratio) and printing quality. Accordingly, the surface effect of the blanket due to ink solvent absorption on the ink contact angle is analyzed. Furthermore, the amount of ink transferred from the gravure plate to the blanket in the “off process” and from the blanket to the substrate in the “set process” is evaluated by conducting a simplified plate-to-plate experiment. The influences of loading rate (printing velocity), temperature, and solvent content on the ink transfer performance are addressed. Finally, the ink transfer mechanism is theoretically analyzed for different solvent contents using Surface Evolver. The calculation results are compared with those of the experiment.

Temperature effects on ink transfer performance of gravure offset printing for fine-line circuitry

The study attempts to characterize the influences of temperature-dependent ink transfer performance of gravure offset printing for fine-line circuitry through experimental analysis. First of all, the temperature- and solvent content-dependent contact angle of ink deposited on different substrates, blankets and gravure metal plates are measured using contact angle measurement instrument. Subsequently, to examine the temperature effects on the amount of ink transfer from the gravure plate and to the blanket at off process and from the blanket to substrate at set process, an experiment is set up and carried out. At last, a theoretical prediction of the ink transfer behavior at different solvent content and temperature is also conducted using Surface Evolver. The prediction results are compared with the experimental data.

Influence of IMC surface geometry and material properties on micro-bump reliability of 3D Chip-on-Chip interconnect technology

This study aims at investigating the growth reaction of the Ni3Sn4 IMC during thermocompression bonding process, the anisotropic elastic constants of the IMC, and the effects of the material properties and surface geometry or morphology on the interconnect reliability of a three-dimensional (3D) Chip-on-Chip (CoC) interconnect technology with Cu/Ni/SnAg micro-bumps subject to accelerated thermal cycling (ATC) loading. The research starts from the investigation of the growth reaction of the Ni3Sn4 IMC during thermocompression bonding process through experiment and classical diffusion theory. The relationship between the Ni3Sn4 IMC thickness and bonding temperature/time is derived based on the predicted activation energy of the chemical reaction of the IMC layer by experiment. Next, the elastic stiffness coefficients of single crystal monoclinic Ni3Sn4 are calculated through molecular dynamics (MD) simulation using the polymer consistent force field (PCFF). The degree of anisotropy in the Ni3Sn4 crystal system is also confirmed by the electronic structure of single crystal Ni3Sn4 using first-principles calculation based on density function theory (DFT). For comparison with the published experimental data and also use in the subsequent reliability analysis, the effective elastic properties of polycrystalline Ni3Sn4 are derived using the Voigt-Reuss bound and Voigt-Reuss Hill average based on the calculated elastic stiffness coefficients. At last, 2D plane strain finite element (FE) analysis together with an empirical Coffin-Manson fatigue life prediction model are performed to predict the interconnect reliability of the 3D CoC interconnect technology. The computed results are compared with the ATC experimental data to demonstrate the effectiveness of these two FE models. The dependence of the interconnect reliability on the thickness, material properties and surface geometry or morphology of the Ni3Sn4 IMC is addressed.