Showing papers in "Journal of Electronic Packaging in 2018"

Low Temperature Cu–Cu Bonding Technology in Three-Dimensional Integration: An Extensive Review

Abstract: Arguably, the integrated circuit (IC) industry has received robust scientific and technological attention due to the ultra-small and extremely fast transistors since past four decades that consents to Moore’s law. The introduction of new interconnect materials as well as innovative architectures has aided for large-scale miniaturization of devices, but their contributions were limited. Thus, the focus has shifted toward the development of new integration approaches that reduce the interconnect delays which has been achieved successfully by three-dimensional integrated circuit (3D IC). At this juncture, semiconductor industries utilize Cu–Cu bonding as a key technique for 3D IC integration. This review paper focuses on the key role of low temperature Cu–Cu bonding, renaissance of the low temperature bonding, and current research trends to achieve low temperature Cu–Cu bonding for 3D IC and heterogeneous integration applications. [DOI: 10.1115/1.4038392]

Thermal Metamaterials for Heat Flow Control in Electronics

Abstract: Rapid advancement of modern electronics has pushed the limits of traditional thermal management techniques. Novel approaches to the manipulation of the flow of heat in electronic systems have potential to open new design spaces. Here, the field of thermal metamaterials as it applies to electronics is briefly reviewed. Recent research and development of thermal metamaterial systems with anisotropic thermal conductivity for the manipulation of heat flow in ultra-thin composites is explained. An explanation of fundamental experimental studies on heat flow control using standard printed circuit board (PCB) technology follows. From this, basic building blocks for heat flux cloaking, focusing, and reversal are reviewed, and their extension to a variety of electronics applications is emphasized. While device temperature control, thermal energy harvesting, and electrothermal circuit design are the primary focus, some discussion on the extension of thermal guiding (TG) structures to device-scale applications is provided. In total, a holistic view is offered of the myriad of possible applications of thermal metamaterials to heat flow control in future electronics. [DOI: 10.1115/1.4039020]

Technological Advances to Maximize Solar Collector Energy Output: A Review

Abstract: Since it is highly correlated with quality of life, the demand for energy continues to increase as the global population grows and modernizes. Although there has been significant impetus to move away from reliance on fossil fuels for decades (e.g., localized pollution and climate change), solar energy has only recently taken on a non-negligible role in the global production of energy. The photovoltaics (PV) industry has many of the same electronics packaging challenges as the semiconductor industry, because in both cases, high temperatures lead to lowering of the system performance. Also, there are several technologies, which can harvest solar energy solely as heat. Advances in these technologies (e.g., solar selective coatings, design optimizations, and improvement in materials) have also kept the solar thermal market growing in recent years (albeit not nearly as rapidly as PV). This paper presents a review on how heat is managed in solar thermal and PV systems, with a focus on the recent developments for technologies, which can harvest heat to meet global energy demands. It also briefs about possible ways to resolve the challenges or difficulties existing in solar collectors like solar selectivity, thermal stability, etc. As a key enabling technology for reducing radiation heat losses in these devices, the focus of this paper is to discuss the ongoing advances in solar selective coatings and working fluids, which could potentially be used in tandem to filter out or recover the heat that is wasted from PVs. Among the reviewed solar selective coatings, recent advances in selective coating categories like dielectric-metal-dielectric (DMD), multilayered, and cermet-based coatings are considered. In addition, the effects of characteristic changes in glazing, absorber geometry, and solar tracking systems on the performance of solar collectors are also reviewed. A discussion of how these fundamental technological advances could be incorporated with PVs is included as well.

Accelerated Vibration Reliability Testing of Electronic Assemblies using Sine Dwell with Resonance Tracking

Abstract: In this work, a sinusoidal vibration test method with resonance tracking is employed for reliability testing of circuit assemblies. The system continuously monitors for changes in the resonant frequency of the circuit board and adjusts the excitation frequency to match the resonant frequency. The test setup includes an electrodynamic shaker with a real-time vibration control, resistance monitoring for identifying electrical failures of interconnects, and vibration logging for monitoring changes in the dynamic response of the assembly over time. Reliability tests were performed using the resonance tracking sinusoidal test method for assemblies, each consisting of a centrally mounted ball grid array (BGA) device assembled with 63Sn37Pb and SAC105 solder alloys. These tests show that the resonance tracking method gives more consistent failure times. Failure analysis for the tested devices shows the primary failure mode is “input” trace crack first, followed by fatigue through the solder for complete failure. A finite element (FE) model, correlated with experimental modal analysis, is shown to accurately estimate the circuit board deflection estimated from the harmonic vibration data. This provides a means of estimating the stresses in the electronic interconnections while accounting for the variability between test parts. These fine-tuned vibration measurement techniques and related FE models provide the building blocks for high cycle solder fatigue plots (i.e., S–N curves).

Micro Copper Pillar Interconnection Using Thermosonic Flip Chip Bonding

Abstract: The incorporation of a micro copper pillar is considered as the major interconnection method in three-dimensional (3D) integrated circuit (IC) intergradation under high-density I/O conditions. To achieve low-temperature bonding, this study investigated the thermosonic flip chip bonding of a copper pillar with a tin cap. The effect of bonding force on bonding strength was studied, and an average bonding strength 2500 g (approximately 84.8 MPa) was obtained in 2 s, at an optimized bonding force of 0.11 N per 40 μm pillar bump, and substrate temperature of 200 °C. Additionally, the effect of the bonding force on bonding interface microstructure and intermetallic compounds (IMCs) was also investigated. Tin whiskers were also observed at the bonding interface at low bonding forces.

Experimental investigation of embedded micro-pin-fins for single-phase heat transfer and pressure drop

Abstract: Three-dimensional (3D) stacked integrated circuit (IC) chips offer significant performance improvement, but offer important challenges for thermal management including, for the case of microfluidic cooling, constraints on channel dimensions, and pressure drop. Here, we investigate heat transfer and pressure drop characteristics of a microfluidic cooling device with staggered pin-fin array arrangement with dimensions as follows: diameter D1⁄4 46.5 lm; spacing, S 100 lm; and height, H 110 lm. Deionized singlephase water with mass flow rates of _ m1⁄4 15.1–64.1 g/min was used as the working fluid, corresponding to values of Re (based on pin fin diameter) from 23 to 135, where heat fluxes up to 141 W/cm are removed. The measurements yield local Nusselt numbers that vary little along the heated channel length and values for both the Nu and the friction factor do not agree well with most data for pin fin geometries in the literature. Two new correlations for the average Nusselt number ( Re) and Fanning friction factor ( Re ) are proposed that capture the heat transfer and pressure drop behavior for the geometric and operating conditions tested in this study with mean absolute error (MAE) of 4.9% and 1.7%, respectively. The work shows that a more comprehensive investigation is required on thermofluidic characterization of pin fin arrays with channel heights Hf< 150 lm and fin spacing S1⁄4 50–500 lm, respectively, with the Reynolds number, Re< 300. [DOI: 10.1115/1.4039475]

Enhanced Heat Transfer Using Microporous Copper Inverse Opals

Abstract: Enhanced boiling is one of the popular cooling schemes in thermal management due to its superior heat transfer characteristics. This study demonstrates the ability of copper inverse opal (CIO) porous structures to enhance pool boiling performance using a thin CIO film with a thickness of 10 lm and pore diameter of 5 lm. The microfabricated CIO film increases microscale surface roughness that in turn leads to more active nucleation sites thus improved boiling performance parameters such as heat transfer coefficient (HTC) and critical heat flux (CHF) compared to those of smooth Si surfaces. The experimental results for CIO film show a maximum CHF of 225 W/cm (at 16.2 C superheat) or about three times higher than that of smooth Si surface (80 W/cm at 21.6 C superheat). Optical images showing bubble formation on the microporous copper surface are captured to provide detailed information of bubble departure diameter and frequency. [DOI: 10.1115/1.4040088]

Reliability Analysis of Lead-Free Solders in Electronic Packaging Using a Novel Surrogate Model and Kriging Concept

Abstract: A novel reliability evaluation procedure of lead-free solders used in electronic packaging (EP) subjected to thermomechanical loading is proposed. A solder ball is represented by finite elements (FEs). Major sources of nonlinearities are incorporated as realistically as practicable. Uncertainties in all design variables are quantified using available information. The thermomechanical loading is represented by five design parameters and uncertainties associated with them are incorporated. Since the performance or limit state function (LSF) of such complicated problem is implicit in nature, it is approximately generated explicitly in the failure region with the help of a completely improved response surface method (RSM)-based approach and the universal Kriging method (KM). The response surface (RS) is generated by conducting as few deterministic nonlinear finite element analyses as possible by integrating several advanced factorial mathematical concepts producing compounding beneficial effect. The accuracy, efficiency, and application potential of the procedure are established with the help of Monte Carlo simulation (MCS) and the results from laboratory investigation reported in the literature. The study conclusively verified the proposed method. Similar studies can be conducted to fill the knowledge gap for cases where the available analytical and experimental studies are limited or extend the information to cases where reliability information is unavailable. The study showcased how reliability information can be extracted with the help of multiple deterministic analyses. The authors believe that they proposed an alternative to the classical MCS technique.

Regional Stiffness Reduction Using Lamina Emergent Torsional Joints for Flexible Printed Circuit Board Design

Abstract: Flexible printed circuit boards (PCBs) make it possible for engineers to design devices that use space efficiently and can undergo changes in shape and configuration. However, they also suffer from tradeoffs due to nonideal material properties. Here, a method is presented that allows engineers to introduce regions of flexibility in otherwise rigid PCB substrates. This method employs geometric features to reduce local stiffness in the PCB, rather than reducing the global stiffness by material selection. Analytical and finite element models are presented to calculate the maximum stresses caused by deflection. An example device is produced and tested to verify the models.

Hybrid Nanocomposite Thermal Interface Materials: The Thermal Conductivity and the Packing Density

Abstract: We have investigated a novel hybrid nanocomposite thermal interface material (TIM) that consists of silver nanoparticles (AgNPs), silver nanoflakes (AgNFs), and copper microparticles (CuMPs). Continuous metallic network form while AgNPs and AgNFs fuse to join bigger CuMPs upon hot compression, resulting in superior thermal and mechanical performances. The assembly temperature is as low as 125 °C due to the size effect of silver nanoparticulates. The thermal conductivity, k, of the hybrid nanocomposite TIMs is found to be in the range of 15–140 W/mK, exceeding best-performing commercial thermal greases, while comparable to high-end solder TIMs. The dependence of k on the solid packing density and the volume fraction of voids is discussed through comparing to model predictions.

Failure Analysis of Direct Liquid Cooling System in Data Centers

Abstract: In this paper, the impact of direct liquid cooling (DLC) system failure on the IT equipment is studied experimentally. The main factors that are anticipated to affect the IT equipment response during failure are the CPU utilization, coolant set point temperature (SPT) and the server type. These factors are varied experimentally and the IT equipment response is studied in terms of chip temperature and power, CPU utilization and total server power. It was found that failure of the cooling system is hazardous and can lead to data center shutdown in less than a minute. Additionally, the CPU frequency throttling mechanism was found to be vital to understand the change in chip temperature, power, and utilization. Other mechanisms associated with high temperatures were also observed such as the leakage power and the fans speed change. Finally, possible remedies are proposed to reduce the probability and the consequences of the cooling system failure.

A Wire-Bondless Packaging Platform for Silicon Carbide Power Semiconductor Devices

Abstract: As silicon carbide (SiC) power semiconductor devices continue to mature for market adoption, innovative power electronics packaging designs and materials are needed. Wire-bonding loop is one of the limiting factors in traditional module packaging methods. Wire-bondless packaging methods have been demonstrated with low losses and to allow integration of gate drive circuit. In this paper, a wire-bondless packaging platform, referred to as power overlay kiloWatt (POL-kW), for SiC devices is presented. The packaging platform is intended for motor drives and power conversion in automotive, aerospace, and renewable power applications. POL-kW module's electrical and thermal performances are first summarized from previous experimental evaluations and numerical simulations. Although some of the evaluations were made using Si and Si–SiC hybrid modules, the results are applicable to SiC modules. Compared with aluminum wire-bonds, the utilization of polyimide-based Cu via interconnections resulted in much reduced parasitic inductance, contributing to significantly lower switching loss and less voltage overshoot. The POL-kW module with integrated heat sinks showed low thermal resistance, which was further reduced by double-sided cooling. Recent reliability results are presented, including high-temperature storage, temperature cycling, and power cycling.

Interfacial Compounds Characteristic and Its Reliability Effects on SAC305 Microjoints in Flip Chip Assemblies

Abstract: This study mainly focuses on site effects of the Ni pad interface on intermetallic compounds (IMCs) characteristic during assembly reflowing, and attempts to provide a reasonable explanation for this particular finding. Besides, the changes of the resulting IMCs characteristic are characterized during thermal shock (TS) cycling, and their potential influences on thermal–mechanical reliability of microjoints are evaluated experimentally and numerically. The results show that the site on the Ni pad interface of silicon chip has great influence on interfacial reaction products, i.e., interfacial IMCs. After bumps soldering, a great amount of larger diamond-shaped (Cu, Ni)6Sn5 compounds were densely packed at the edge region, while some smaller ones were only scattered at the center region. Moreover, substantial particle-shaped (Ni, Cu)3Sn4 compounds as well as some rod-shaped ones emerged at the spaces between the (Cu, Ni)6Sn5 compounds of the center region. More importantly, such site effects were remained in the microjoints during TS cycling, which induced the formation of larger protruding (Cu, Ni)6Sn5 compounds. Finite element (FE) simulation results showed that the stress was mainly concentrated at the top of the protruding (Cu, Ni)6Sn5 compounds, which can be a critical reason to cause the crack occurrence. Furthermore, the underlying mechanism of the interfacial IMCs characteristic induced by the site effects was attempted to propose during bumps soldering.

Reactive Joining of Thermally and Mechanically Sensitive Materials

Abstract: Reactive joining, i.e., utilization of an exothermal reaction to locally generate the heat required for soldering or brazing, represents an emerging technology for flexible and benign joining of heat-sensitive materials, e.g., for microelectromechanical systems (MEMS) applications. However, for successful reactive joining, precise control of heat production and heat distribution is mandatory in order to avoid damaging of the components during the process. For the exemplary case of borosilicate glass, the reactive joining process for a both thermally and mechanically sensitive material is developed. Employing various nondestructive and destructive testing methods, typical problems which can occur upon reactive joining are identified, e.g., exposure of the joining zone to excessive temperatures, experience of thermal shock by the substrate due to sudden temperature increase, and generation of residual stresses in substrate and soldering zone. Utilizing the results of nondestructive and destructive testing, procedures for successful reactive joining of borosilicate glass, silicon and aluminum oxide are provided.

Correlation of Warpage Distribution with the Material Property Scattering for Warpage Range Prediction of PBGA Components

Abstract: In recent years, due to the increased size of ball grid array (BGA) devices, the assembly of BGAs on printed circuit boards through surface mount technology has encountered unprecedented challenges from thermal warpage. The excessive warpage of BGAs in the reflow process may cause manufacture problems and even the risk of failure. Thus, it is essential to acquire warpage values and corresponding distribution ranges of BGAs before the surface mount technology process. In order to avoid assembly failure, theoretically, it is necessary to guarantee that all BGA devices meet the acceptance requirement of relevant standards. Generally, a large number of samples should be measured to obtain a relatively reliable warpage data distribution in the reflow temperature range, which makes this test quite costly and extremely time consuming. This study proposes another method to estimate the BGA warpage value and its possible corresponding range from the material property point of view. Because the mechanism of BGA warpage is related to the coefficient of thermal expansion (CTE) mismatch between the different materials, the warpage data scattering can be correlated with the scattering of material properties through finite element method (FEM) analysis. With a known mean value and range of material properties, the warpage value and corresponding distribution range can be solved. A sensitivity study is also presented in this paper. The accuracy of the proposed method is evaluated and the corresponding warpage data fluctuation range is estimated. From the comparison of the simulation and experiment results, determining the material properties could lead to a reasonable prediction of warpage in both the qualitative and quantitative sense. The proposed methodology for BGA warpage estimation can be used for academic research and industrial applications.