Showing papers in "Journal of Electronic Packaging in 2014"

Review and Projections of Integrated Cooling Systems for Three-Dimensional Integrated Circuits

Abstract: In an effort to increase processor speeds, 3D IC architecture is being aggressively pursued by researchers and chip manufacturers. This architecture allows extremely high level of integration with enhanced electrical performance and expanded functionality, and facilitates realization of VLSI and ULSI technologies. However, utilizing the third dimension to provide additional device layers poses thermal challenges due to the increased heat dissipation and complex electrical interconnects among different layers. The conflicting needs of the cooling system requiring larger flow passage dimensions to limit the pressure drop, and the IC architecture necessitating short interconnect distances to reduce signal latency warrant paradigm shifts in both of their design approach. Additional considerations include the effects due to temperature nonuniformity, localized hot spots, complex fluidic connections, and mechanical design. This paper reviews the advances in 3D IC cooling in the last decade and provides a vision for codesigning 3D IC architecture and integrated cooling systems. For heat fluxes of 50‐100W/cm 2 on each side of a chip in a 3D IC package, the current single-phase cooling technology is projected to provide adequate cooling, albeit with high pressure drops. For future applications with coolant surface heat fluxes from 100 to 500W/cm 2 , significant changes need to be made in both electrical and cooling technologies through a new level of codesign. Effectively mitigating the high temperatures surrounding local hot spots remains a challenging issue. The codesign approach with circuit, software and thermal designers working together is seen as essential. The through silicon vias (TSVs) in the current designs place a stringent limit on the channel height in the cooling layer. It is projected that integration of wireless network on chip architecture could alleviate these height restrictions since the data bandwidth is independent of the communication lengths. Microchannels that are 200lm or larger in depth are expected to allow dissipation of large heat fluxes with significantly lower pressure drops. [DOI: 10.1115/1.4027175]

Three-Dimensional and 2.5 Dimensional Interconnection Technology: State of the Art

Abstract: Three-dimensional (3D) packaging with through-silicon-vias (TSVs) is an emerging technology featuring smaller package size, higher interconnection density, and better performance; 2.5D packaging using silicon interposers with TSVs is an incremental step toward 3D packaging. Formation of TSVs and interconnection between chips and/or wafers are two key enabling technologies for 3D and 2.5D packaging, and different interconnection methods in chip-to-chip, chip-to-wafer, and wafer-to-wafer schemes have been developed. This article reviews state-of-the-art interconnection technologies reported in recent technical papers. Issues such as bump formation, assembly/bonding process, as well as underfill dispensing in each interconnection type are discussed. [DOI: 10.1115/1.4026615]

Variable Fin Density Flow Channels for Effective Cooling and Mitigation of Temperature Nonuniformity in Three-Dimensional Integrated Circuits

Abstract: The surface temperature of integrated circuit (IC) chips cooled with a single-phase liquid flow increases along the flow direction following the increase in the liquid temperature. Increasing the heat transfer coefficient along the flow direction is an effective way to enhance the cooling performance while mitigating the temperature nonuniformity and high pressure drop concerns. This investigation evaluates numerically the cooling performance of different flow channel designs suitable in 3D IC applications with channel heights restricted to 100 lm. Internal configurations featuring offset strip fins with variable fin density and variable spacing ribs were studied in an effort to minimize the temperature nonuniformity while maintaining a relatively low pressure drop. The performance of 13 different designs for the variable-fin-density configuration and three different rib configurations have been evaluated and compared with two baseline cases, corresponding to a smooth flow channel and a flow channel with continuous fins. All of the analyzed internal configurations are contained within a flow channel of 100 lm height and 910 lm width. A coolant chip formed by nine flow channels for the dissipation of 200 W of a 3D IC with a surface area of 1 cm is the base for this investigation. The best performing configuration resulted in a temperature variation of less than 30 K with a pressure drop of 34 kPa as compared to a temperature variation of 38 K and a pressure drop of 144 kPa with continuous fins and 51 K and 21 kPa for a smooth flow channel. [DOI: 10.1115/1.4027091]

Intermittent Failures in Hardware and Software

Abstract: Intermittent failures and no fault found (NFF) phenomena are a concern in electronic systems because of their unpredictable nature and irregular occurrence. They can impose significant costs for companies, damage the reputation of a company, or be catastrophic in systems such as nuclear plants or avionics. Intermittent failures in systems can be attributed to hardware failures or software failures. In order to diagnose and mitigate the intermittent failures in systems, the nature and the root cause of these failures have to be understood. In this paper we have reviewed the current literature concerning intermittent failures and have a comprehensive study on how these failures happen, how to detect them and how to mitigate them.

Thermal Analysis and Experimental Validation of Laminar Heat Transfer and Pressure Drop in Serpentine Channel Heat Sinks for Electronic Cooling

Abstract: In this paper, a thermal resistance network analytical model is proposed to investigate the thermal resistance and pressure drop in serpentine channel heat sinks with 180 deg bends. The total thermal resistance is obtained using a thermal resistance network model based on the equivalent thermal circuit method. Pressure drop is derived considering straight channel and bend loss because the bends interrupt the hydrodynamic boundary periodically. Considering the effects of laminar flow development and redevelopment, the bend loss coefficient is obtained as a function of the Reynolds number, aspect ratios, widths of fins, and turn clearances, through a three-regime correlation. The model is then experimentally validated by measuring the temperature and pressure characteristics of heat sinks with different Reynolds numbers and different geometric parameters. Finally, the temperature-rise and pressure distribution of the thermal fluid with Reynolds numbers of 500, 1000, and 1500 are examined utilizing this model. [DOI: 10.1115/1.4027508]

A Note on the Normalized Approach to Simulating Moisture Diffusion in a Multimaterial System Under Transient Thermal Conditions Using ansys 14 and 14.5

Abstract: Moisture can have significant effects on the performance and reliability of electronic components. Accurately simulating moisture diffusion is important for designers and manufacturers to obtain a realistic reliability evaluation. Beginning with version 14, ANSYS is capable of simulating diffusion and related behaviors, such as hygroscopic swelling, with newly developed elements. However, a normalized approach is still required to deal with the discontinuity of concentrations at the material boundaries, and normalization of the moisture concentration in transient thermal conditions is tricky. Case studies have shown that normalizing the moisture concentration with respect to a timeor temperature-dependent material property will lead to erroneous results. This paper readdresses the issues of performing diffusion simulations under transient thermal conditions and more general anisothermal conditions (temporally and spatially), and suggests an easy-to-use approach to cope with the limitations of the current version for users in the electronic packaging industry. [DOI: 10.1115/1.4026661]

Constructal Theory Based Geometric Optimization of Wavy Channels in the Low Reynolds Number Regime

Abstract: To obtain better fluid mixing and higher heat transfer in the low Reynolds number regime, various wavy fins are employed in heat sinks (heat exchangers) for electronic cooling applications. However, it was reported in previous works that in the low Reynolds number regime there are no remarkable differences in the thermal performance of a straight-plate and a wavy-wall channel. In this study, the constructal theory is applied to optimize the geometry of wavy-wall channels of an electronic heat sink, where the objective is to minimize the global thermal resistance. The domain has three degrees of freedom: The interplate-spacing (S), the wavelength ratio (lambda(1)/lambda(2)), and the amplitude ratio (a(1)/a(2)). The two times minimized global thermal resistance indicates that the thermal-hydraulic performance of the wavy channels is unaffected by the amplitude ratio, while the wavelength ratio and interplate separation have strong impacts on the overall performance. In addition, the thermal performances at four Reynolds numbers are evaluated, and it is found that the constructal-wavy channels can exhibit much better thermal performance in the low Reynolds number regime. (Less)

Advances in the Fabrication Processes and Applications of Wafer Level Packaging

Abstract: Fast development of wafer level packaging (WLP) in recent years is mainly owing to the advances in integrated circuit fabrication process and the market demands for devices with high electrical performance, small form factor, low cost etc. This paper reviews the advances of WLP technology in recent years. An overall introduction to WLP is presented in the first part. The fabrication processes of WLP and redistribution technology are introduced in the second part. Reliability problems of WLPs, such as the strength of solder joints and reliability problems concerning fan-out WLPs are introduced in the third part. Typical applications of WLP technologies are discussed in the last part, which include the application of fan-out WLP, 3D packaging integrating with WLP technologies and its application in microelectromechanical systems (MEMS).

Advanced Natural Convection Cooling Designs for Light-Emitting Diode Bulb Systems

Abstract: The movement to light-emitting diode (LED) lighting systems worldwide is accelerating quickly as energy savings and reduction in hazardous materials increase in importance. Government regulations and rapidly lowering prices help to further this trend. Today's strong drive is to replace light bulbs of common outputs (60 W, 75 W, and 100 W) without resorting to compact fluorescent (CFL) bulbs containing mercury while maintaining the standard industry bulb size and shape referred to as A19. For many bulb designs, this A19 size and shape restriction forces a small heat sink which is barely capable of dissipating heat for 60 W equivalent LED bulbs with natural convection for today's LED efficacies. 75 W and 100 W equivalent bulbs require larger sizes, some method of forced cooling, or some unusual liquid cooling system; generally none of these approaches are desirable for light bulbs from a consumer point of view. Thus, there is interest in developing natural convection cooled A19 light bulb designs for LEDs that cool far more effectively than today's current designs. Current A19 size heat sink designs typically have thermal resistances of 5–7 °C/W. This paper presents designs utilizing the effects of chimney cooling, well developed for other fields that reduce heat sink resistances by significant amounts while meeting all other requirements for bulb system design. Numerical studies and test data show performance of 3–4 °C/W for various orientations including methods for keeping the chimney partially active in horizontal orientations. Significant parameters are also studied with effects upon performance. The simulations are in good agreement with the experimental data. Such chimney-based designs are shown to enable 75 W and 100 W equivalent LED light bulb designs critical for faster penetration of LED systems into general lighting applications.

Modeling of Two-Phase Evaporative Heat Transfer in Three-Dimensional Multicavity High Performance Microprocessor Chip Stacks

Abstract: Three-dimensional (3D) stacking of integrated-circuit (IC) dies increases system density and package functionality by vertically integrating two or more dies with area-array through-silicon-vias (TSVs). This reduces the length of global interconnects and the signal delay time and allows improvements in energy efficiency. However, the accumulation of heat fluxes and thermal interface resistances is a major limitation of vertically integrated packages. Scalable cooling solutions, such as two-phase interlayer cooling, will be required to extend 3D stacks beyond the most modest numbers of dies. This paper introduces a realistic 3D chip stack along with a simulation method for the heat spreading and flow distribution among the channels of the evaporators. The model includes the significant sensitivity of each channel's friction factor to vapor quality, and hence mass flow to heat flux, which characterizes parallel two-phase flows. Simulation cases explore various placements of hot spots within the stack and effects which are unique to two-phase interlayer cooling. The results show that the effect of hot spots on individual dies can be mitigated by strong interlayer heat conduction if the relative position of the hot spots is selected carefully to result in a heat load and flow which are well balanced laterally.

The Strength of High-Temperature Ag–In Joints Produced Between Copper by Fluxless Low-Temperature Processes

Abstract: Two copper (Cu) substrates were bonded using silver (Ag) and indium (In) and annealed at 200-250 °C to convert the joints into the solid solution (Ag) for enhanced strength and ductility. Cu-Cu pair was chosen so that the samples break in the joint during shear test. The upper Cu was electroplated with 15 μm Ag. The lower Cu was plated with 15 μm Ag, followed by In and 0.1 μm Ag to inhibit indium oxidation. Two designs were implemented, using 8 μm and 5 μm In, respectively. The Cu substrates were bonded at 180 °C in 100 mTorr vacuum without flux. Afterwards, samples were annealed at 200 °C for 1000 h (first design) and at 250 °C for 350 h (second design), respectively. Scanning electron microscope with energy dispersive X-ray analysis (SEM and EDX) results indicate that the joint of the first design is an alloy of mostly (Ag) with micron-size Ag2In and (ζ) regions, and that of second design has converted to a single (Ag) phase. Shear test results show that the samples are very strong. The breaking forces far exceed requirements in MILSTD- 883H standards. Fracture incurs inside the joint and is a mix of brittle and ductile modes or only ductile mode. The joint solidus temperatures are 600 °C and 900 °C for the first and second designs, respectively. Copyright © 2014 by ASME.

Four-Wire Bridge Measurements of Silicon van der Pauw Stress Sensors

Abstract: Under the proper orientations and excitations, the transverse output of rotationally symmetric four-contact van der Pauw (VDP) stress sensors depends upon only the in-plane shear stress or the difference of the in-plane normal stresses on (100) silicon. In bridgemode, each sensor requires only one four-wire measurement and produces an output voltage with a sensitivity that is 3.16 times that of the equivalent resistor rosettes or bridges, just as in the normal VDP sensor mode that requires two separate measurements. Both numerical and experimental results are presented to validate the conjectured behavior of the sensor. Similar results apply to sensors on (111) silicon. The output voltage results provide a simple mathematical expression for the offset voltage in Hall effect devices or the response of pseudo Hall-effect sensors. Bridge operation facilitates use of the VDP structure in embedded stress sensors in integrated circuits. [DOI: 10.1115/1.4028333]

Adhesion and Puncture Strength of Polyurethane Coating Used to Mitigate Tin Whisker Growth

Abstract: Reliability of conformal coatings used to mitigate tin whisker growth depends on their ability to contain tin whiskers. Two key material properties required to assess the reliability of a polyurethane coating are documented experimentally: adhesion strength and puncture strength. A modified blister test using a predefined blister area is employed to assess the adhesion strength and a puncture test is employed to evaluate the puncture strength of the coating. After measuring the properties at time zero, the coatings are subjected to accelerated testing conditions (high temperature/humidity storage and temperature cycling) and the degradations of the coating properties are documented. [DOI: 10.1115/1.4026922]

An Experimental Study on Heat Transfer Enhancement of Non-Newtonian Fluid in a Rectangular Channel With Dimples/Protrusions

Abstract: Dimple and protrusion play important roles in the heat transfer enhancement and flow characteristic in cooling channels, which widely employed within electronic cooling systems Non-Newtonian fluid has significant differences with Newtonian fluid, such as water, in fluid characteristic In this study, an experiment on the viscosity of three different kinds of non-Newtonian fluids, ie, xanthan gum solution, Carbopol 934 solution, polyacrylamide solution, was first accomplished to acquire the viscosity with different mass fractions Then, experimental measurements on heat transfer and friction characteristics of non-Newtonian fluid in a rectangular channel with dimples and protrusions were conducted The overall Nusselt numbers (Nu) and Fanning friction factors at different dimple/protrusion structures were obtained with various inlet flow rates and mass fractions The results show that only xanthan gum solution has the significant shear thinning effect within the concentration range of this study, and the dimples/protrusions both have great effect on the heat transfer enhancement in the rectangular channel, and that the heat transfer of the case with the protrusions and crossing arrangement can be further enhanced with the higher Nu when compared to the case with the dimples and aligned arrangement Moreover, an increase in Nu with the higher non-Newtonian fluid mass fraction is observed [DOI: 101115/14025713]

Heterogeneous Void Nucleation Study in Flip Chip Assembly Process Using No-Flow Underfill

Abstract: No-flow underfill process has exhibited a narrow feasible process window due to electrical assembly yield loss or underfill voiding. In general, the assembly yield can be improved using reflow process designed at high temperature, while the high temperature condition potentially causes serious underfill voiding. Typically, the underfill voiding can result in critical defects, such as solders fatigue cracking or solders bridge, causing early failures in thermal reliability. Therefore, this study reviews a classical bubble nucleation theory to model voids nucleation during reflow process. The established model designed a reflow process possibly preventing underfill voiding. The reflow process was validated using systematic experiments designed on the theoretical study with a commercial high I/O counts (5000>), fine-pitch (<150 μm) flip chip. The theoretical model exhibits good agreement with experimental results. Thus, this paper presents systematic studies through the use of structured experimentation designed to achieve a high, stable yield, and void-free assembly process on the classical bubble nucleation theory.

A New Analytical Approach for Dynamic Modeling of Passive Multicomponent Cooling Systems

Abstract: A new one-dimensional thermal network modeling approach is proposed that can accurately predict transient/dynamic temperature distribution of passive cooling systems. The present model has applications in variety of electronic, power electronic, photonics, and telecom systems, especially where the system load fluctuates over time. The main components of a cooling system including: heat spreaders, heat pipes, and heat sinks as well as thermal boundary conditions such as natural convection and radiation heat transfer are analyzed, analytically modeled and presented in the form of resistance and capacitance (RC) network blocks. The present model is capable of predicting the transient/dynamic (and steady state) thermal behavior of cooling system with significantly less cost of modeling compared to conventional numerical simulations. Furthermore, the present method takes into account system “thermal inertia” and is capable of capturing thermal lags in various components. The model is presented in two forms: zero-dimensional and onedimensional which are different in terms of complicacy. A custom-designed test-bed is also built and a comprehensive experimental study is conducted to validate the proposed model. The experimental results show great agreement, less than 4.5% relative difference in comparison with the modeling results. [DOI: 10.1115/1.4027509]