Analysis and modeling of underfill flow driven by capillary action in flip-chip packaging

Flip-chip underfilling is a technology by which silica-filled epoxy resin is used to fill the micro-cavity between a silicon chip and a substrate, by dispensing the liquid encapsulant at elevated temperatures along the periphery of one or two sides of the chip and then allowing capillary action to draw the material into the gap. Since the chip, underfill material, and substrate solidify together as one unit, thermal stresses on solder joints during the temperature cycling (which are caused by a mismatch in the coefficients of thermal expansion between the silicon chip and the organic substrate) can be redistributed and transferred away from the fragile bump zone to a more strain-tolerant region. Modeling of the flow behaviour of a fluid in the underfill process is the key to this technology. One of the most important drawbacks in the existing models is inadequate treatment of non-Newtonian fluids in the underfill process in the development of both analytical models and numerical models. Another important drawback is the neglect of the presence of solder bumps in the existing analytical models. This thesis describes a study in which a proper viscosity constitutive equation, power-law model, is employed for describing the non-Newtonian fluid behaviour in flip-chip package. Based on this constitutive equation, two analytical models with closed-form solutions for predicting the fluid filling time and fluid flow front position with respect to time were derived. One model is for a setting with two parallel plates as an approximate to flip-chip package, while the other model is for a setting with two parallel plates within which an array of solder bumps are present. Furthermore, a numerical model using a general-purpose finite element package ANSYS was developed to predict the fluid flow map in two dimensions. The superiority of these models to the existing models (primarily those developed at Cornell University in 1997) is confirmed based on the results of the experiments conducted in this study. This thesis also presents a finding of the notion of critical clearance in the design of a flip-chip package through a careful simulation study using the models developed. The flip-chip package design should make the clearance between solder bumps larger than the critical clearance

Modeling of positive-displacement dispensing process

Fluid dispensing is a method by which fluid materials are delivered to the targeted boards in a controlled manner and has been extensively applied in various packaging processes in the electronics assembly industry. In these processes, the flow rate of the fluid dispensed and/or the fluid amount transferred onto a board are two important performance indexes. Due to the involvement of the compressibility and non-Newtonian behaviour of the fluid being dispensed, modeling the fluid dispensing process has proven to be a challenging task. This thesis presents a study on the modeling of the positive displacement dispensing process, in which the linear displacement of a piston is used to dispense fluid. Also, this thesis presents an evaluation of different designs of the fluid dispensing system based on the axiomatic design principles. At first, the characterization of the flow behaviour of fluids used in the electronic packaging industry is addressed. Based on the previous experiments conducted in the author’s lab, a 3-parameter Carreau model for the fluid Hysol FP4451 is derived for use in the present study. Then, taking into account fluid compressibility and flow behaviour, a model is developed to represent the dynamics of the flow rate of the fluid dispensed. The resulting model suggests that the dynamics of the flow rate in the positive displacement dispensing process is equivalent to that of a second order system. Based on the model developed, the influences of the fluid compressibility and the process parameters such as the dispensing time and needle temperature are investigated by simulations. In the positive dispensing process, it is noticed that the fluid amount dispensed out of needle is different from the fluid amount finally transferred to the board, if the fluid amount dispensed is very small. This difference is considered one major problem affecting dispensing performance. In order to determine the fluid amount transferred to the board, a 3-step method is developed in the present study, based on existing theories of liquid bridges and Laplace’s equation. Simulations are conducted based on the developed method to study the influence of surface tension and initial fluid amount on the final fluid amount transferred onto the board. Finally, this thesis presents a new approach to evaluate and compare different designs of the fluid dispensing system, namely air-pressure, rotary-crew, and positive- displacement. In this approach, the axiomatic design principles, i.e., the Independence Axiom and the Information Axiom, are employed. This approach can be used not only to evaluate existing dispensing systems, but also to design new dispensing systems

Plastic Ball Grid Array Encapsulation Process Simulation on Rheology Effect

The integrated circuit should be encapsulated for protection from their intended environment. This paper presents the flow visualization of the plastic ball grid array (PBGA) chip encapsulation process considering of the rheology effect. In the molding process, encapsulant flow behavior is modeled by Castro-Macosko viscosity model with considering curing effect and volume of fluid technique is applied for melt front tracking. The viscosity model is written into C language and compiled using User-Defined Functions into the FLUENT analysis. Three types of Epoxy Molding Compound namely case 1, 2, and 3 were utilized for the study of fluid flow inside the mold cavity. The melt front profiles and viscosity versus shear rate for all cases are analyzed and presented. The numerical results are compared with the previous experimental results and found in good conformity. In the present study, case 1 with greater viscosity shows the higher air trap and higher pressure distributions.

Study and characetrization of plastic encapsulated packages for MEMS

Technological advancement has thrust MEMS design and fabrication into the forefront of modern technologies. It has become sufficiently self-sustained to allow mass production. The limiting factor which is stalling commercialization of MEMS is the packaging and device reliability. The challenging issues with MEMS packaging are application specific. The function of the package is to give the MEMS device mechanical support, protection from the environment, and electrical connection to other devices in the system. The current state of the art in MEMS packaging transcends the various packaging techniques available in the integrated circuit (IC) industry. At present the packaging of MEMS includes hermetic ceramic packaging and metal packaging with hermetic seals. For example the ADXL202 accelerometer from the Analog Devices. Study of the packaging methods and costs show that both of these methods of packaging are expensive and not needed for majority of MEMS applications. Due to this the cost of current MEMS packaging is relatively high, as much as 90% of the finished product. Reducing the cost is therefore of the prime concern. This Thesis explores the possibility of an inexpensive plastic package for MEMS sensors like accelerometers, optical MEMS, blood pressure sensors etc. Due to their cost effective techniques, plastic packaging already dominates the IC industry. They cost less, weigh less, and their size is small. However, porous nature of molding materials allows penetration of moisture into the package. The Thesis includes an extensive study of the plastic packaging and characterization of three different plastic package samples. Polymeric materials warp upon absorbing moisture, generating hygroscopic stresses. Hygroscopic stresses in the package add to the thermal stress due to high reflow temperature. Despite this, hygroscopic characteristics of the plastic package have been largely ignored. To facilitate understanding of the moisture absorption, an analytical model is presented in this Thesis. Also, an empirical model presents, in this Thesis, the parameters affecting moisture ingress. This information is important to determine the moisture content at a specific time, which would help in assessing reliability of the package. Moisture absorption is modeled using the single phase absorption theory, which assumes that moisture diffusion occurs freely without any bonding with the resin. This theory is based on the Fick\u27s Law of diffusion, which considers that the driving force of diffusion is the water concentration gradient. A finite difference simulation of one-dimensional moisture diffusion using the Crank-Nicolson implicit formula is presented. Moisture retention causes swelling of compounds which, in turn, leads to warpage. The warpage induces hygroscopic stresses. These stresses can further limit the performance of the MEMS sensors. This Thesis also presents a non invasive methodology to characterize a plastic package. The warpage deformations of the package are measured using Optoelectronic holography (OEH) methodology. The OEH methodology is noninvasive, remote, and provides results in full-field-of-view. Using the quantitative results of OEH measurements of deformations of a plastic package, pressure build up can be calculated and employed to assess the reliability of the package

Modeling and off-line control of fluid dispensing for electronics packaging

Fluid dispensing is a method by which fluid materials are delivered to substrates, boards or work-pieces in a controllable manner. This method has been widely used in various packaging processes in the electronics manufacturing industry. In these processes, the flow rate of fluid dispensed and the profile of fluid formed on a board are the two most important performance variables to be controlled consistently. This research presents a comprehensive study on the modeling and control of the time-pressure dispensing processes. First of all, the characterization of the rheological behaviour of fluids for electronics packaging is addressed from both time-independent and time-dependent perspectives. Under the assumption that the pressure in the dispensing syringe has reached a steady-state status, a model representative of the steady-state flow rate of fluid dispensed is developed. To represent the profile of fluid formed on a board, the spreading of fluid on a board is addressed and a solution to this problem is established. To consider the influence of time-dependent fluid behaviour in fluid dispensing, a method of applying model updating technique is developed in this study. Based on this method, an off-line control of the dispensing process is developed to improve the consistency in the flow rate of fluid dispensed, which is broken by the time-dependent fluid behaviour. Taking into account air compressibility and the fluid inertia, a model is developed to represent the dynamics of the flow rate of fluid dispensed, which shows that the dynamics is sensitive to the air volume in the syringe. Based on the model, the inconsistency in the fluid amount dispensed due to the variation of the air volume in the syringe over a dispensing process is investigated, and an off-line control is developed to alleviate the amount inconsistency. Experiments on a typical commercial dispensing system are designed and carried out to verify the effectiveness of the models and the off-line control developed in this study. It is shown that the model results have an excellent agreement with the experimental results. Also, with the introduction of the off-line control, the consistency in both the flow rate and the amount of fluid dispensed can be significantly improved

Assessing polymeric nanocomposites and advanced cooling techniques for thermal management of next-generation power electronics

The field of power electronics devices has seen two significant trends in recent years: rapid miniaturization of devices and the replacement of silicon-based devices with wide bandgap semiconductor materials-based devices (Silicon Carbide (SiC), Gallium Nitride (GaN)). The end result of these advancements are devices that need advanced cooling technologies to dissipate ultrahigh high and concentrated heat loads. Multiple advanced thermal management solutions such as liquid cooling, jet, and spray impingement have been proposed as potential solutions. The present dissertation quantifies the benefits of key advanced cooling techniques for thermal management of power electronics packages. An analytical modeling framework based on a thermal resistance circuit has been utilized to estimate the maximum heat flux that can be dissipated from a power electronics package, and the junction temperatures at varying levels of power dissipation. Analysis was conducted for heat sinks made of copper (k=400 W/mK) and a polymer (k=20 W/mK). The developed modeling framework takes into account heat spreading in both lateral directions while capturing the influence of material properties on the spreading angle. The model can, therefore, be considered to capture 3D effects as well. Additionally, 3D Finite Element Analysis (FEA) simulations have been carried out to compare with the findings of the analytical model. This dissertation also studies the influence of polymeric encapsulants of varying thermal conductivities on the resulting temperature distributions in the package via steady 2D coupled electro-thermal simulations. Overall, the methodology and results presented in this dissertation provide insights for selecting optimal combinations of thermal management technologies and advanced polymeric materials, based on the heat dissipation requirements of power electronics packages.Mechanical Engineerin

CFD Simulation Of Underfill Encapsulation Process In Flip Chip Packaging With Various Dispensing Methods

The major trend in electronic industry is to make the products smarter, lighter, functional and highly compact, at the same time cheaper. This trend has necessitated stringent packaging requirements and the flip-chip technology has emerged as a promising option to tackle this issue. However, a serious issue in flip-chip packaging is the difference in the coefficient of thermal expansion between the silicon chip and the organic substrate, which generates thermo-mechanical stresses and causes fatigue in solder joints. This problem is effectively solved by the underfill process in which the space between the silicon die and the PCB is filled with the underfill encapsulant that redistributes the induced stresses thereby enhancing the solder joints reliability

Modeling of epoxy dispensing process using a hybrid fuzzy regression approach

In the semiconductor manufacturing industry, epoxy dispensing is a popular process commonly used in die bonding as well as in microchip encapsulation for electronic packaging. Modeling the epoxy dispensing process is important because it enables us to understand the process behavior, as well as determine the optimum operating conditions of the process for a high yield, low cost, and robust operation. Previous studies of epoxy dispensing have mainly focused on the development of analytical models. However, an analytical model for epoxy dispensing is difficult to develop because of its complex behavior and high degree of uncertainty associated with the process in a real-world environment. Previous studies of modeling the epoxy dispensing process have not addressed the development of explicit models involving high-order and interaction terms, as well as fuzziness between process parameters. In this paper, a hybrid fuzzy regression (HFR) method integrating fuzzy regression with genetic programming is proposed to make up the deficiency. Two process models are generated for the two quality characteristics of the process, encapsulation weight and encapsulation thickness based on the HFR, respectively. Validation tests are performed. The performance of the models developed based on the HFR outperforms the performance of those based on statistical regression and fuzzy regression

An experimental assessment of computational fluid dynamics predictive accuracy for electronic component operational temperature

Ever-rising Integrated Circuit (IC) power dissipation, combined with reducing product development cycles times, have placed increasing reliance on the use of Computational Fluid Dynamics (CFD) software for the thermal analysis of electronic equipment. In this study, predictive accuracy is assessed for board-mounted electronic component heat transfer using both a CFD code dedicated to the thermal analysis of electronics, Flotherm, and a general-purpose CFD code, Fluent. Using Flotherm, turbulent flow modelling approaches typically employed for the analysis of electronics cooling, namely algebraic mixing length and two-equation high-Reynolds number k-e models, are assessed. As shown, such models are not specific for the analysis of forced airflows over populated electronic boards, which are typically classified as low-Reynolds number flows. The potential for improved predictive accuracy is evaluated using candidate turbulent flow models more suited to such flows, namely a one-equation SpalartAllmaras model, two-layer zonal model and two equation SST k-co model, all implemented in Fluent. Numerical predictions are compared with experimental benchmark data for a range of componentboard topologies generating different airflow phenomena and varying degrees of component thermal interaction. Test case complexity is incremented in controlled steps, from single board-mounted components in free convection, to forced air-cooled, multi-component board configurations. Apart from the prediction of component operational temperature, the application of CFD analysis to the design of electronic component reliability screens and convective solder reflow temperature profiles is also investigated. Benchmark criteria are based on component junction temperature and component-board surface temperature profiles, measured using thermal test chips and infrared thermography respectively. This data is supplemented by experimental visualisations of the forced airflows over the boards, which are used to help assess predictive accuracy. Component numerical modelling is based on nominal package dimensions and material thermal properties. To eliminate potential numerical modelling uncertainties, both the test component geometry and structural integrity are assessed using destructive and non-destructive testing. While detailed component modelling provides the à priori junction temperature predictions, the capability of compact thermal models to predict multi-mode component heat transfer is also assessed. In free convection, component junction temperature predictions for an in-line array of fifteen boardmounted components are within ±5°C or 7% of measurement. Predictive accuracy decays up to ±20°C or 35% in forced airflows using the k-e flow model. Furthermore, neither the laminar or k-e turbulent flow model accurately resolve the complete flow fields over the boards, suggesting the need for a turbulence model capable of modelling transition. Using a k-co model, significant improvements in junction temperature prediction accuracy are obtained, which are associated with improved prediction of both board leading edge heat transfer and component thermal interaction. Whereas with the k-e flow model, prediction accuracy would only be sufficient for the early to intermediate phase of a thermal design process, the use of the k-co model would enable parametric analysis of product thermal performance to be undertaken with greater confidence. Such models would also permit the generation of more accurate temperature boundary conditions for use in Physics-of-Failure (PoF) based component reliability prediction methods. The case is therefore made for vendors of CFD codes dedicated to the thermal analysis of electronics to consider the adoption of eddy viscosity turbulence models more suited to the analysis of component heat transfer. While this study ultimately highlights that electronic component operational temperature needs to be experimentally measured to quality product thermal performance and reliability, the use of such flow models would help reduce the current dependency on experimental prototyping. This would not only enhance the potential of CFD as a design tool, but also its capability to provide detailed insight into complex multi-mode heat transfer, that would otherwise be difficult to characterise experimentally

Solar Module Packaging

While global demand for photovoltaic (PV) modules has increased approximately 45 percent per year over the past decade, PV modules must be durable and inexpensive to compete with traditional energy resources. Often overlooked as a means to improve solar technology, polymer packaging is not only the key to protecting fragile solar cells from environmental factors, but is also the critical path for increasing the power performance of a PV module Solar Module Packaging: Polymeric Requirements and Selection explores current and future opportunities in PV polymeric packaging, emphasizing how it can simultaneously reduce cost, increase weatherability, and improve a PV module’s power. The book offers an insider’s perspective on the manufacturing processes and needs of the solar industry and reveals opportunities for future material development and processing. A broad survey of the polymeric packaging of solar cells, the text covers various classifications of polymers, their material properties, and optimal processing conditions. Taking a practical approach to material selection, it emphasizes industrial requirements for material development, such as cost reduction, increased material durability, improved module performance, and ease of processing. Addressing cost and profitability, the author examines the economics behind polymeric packaging and how it influences the selection process used by solar companies. Suitable for nonspecialists in polymer science, the book provides a basic understanding of polymeric concepts, fundamental properties, and processing techniques commonly used in solar module packaging. It presents guidelines for using polymers in commercial PV modules as well as the tests required to establish confidence in the selection process