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Review Article

J. Electron. Packag. 2016;138(3):030801-030801-13. doi:10.1115/1.4033375.
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Most solders used in electronic systems have low-melting temperature and hence experience significant amount of creep deformation throughout their life-cycle because typical operational and test conditions represent high homologous temperature. Phenomenological and mechanistic models used in the literature for predicting creep response of both bulk and grain scale specimens are reviewed in this paper. The phenomenological models reviewed in this paper are based on purely empirical observations of the creep deformation behavior or derived from qualitative interpretation of the underlying microscale mechanisms. These models have some intrinsic disadvantages since they do not have explicit mechanistic dependence on microstructural features. Therefore, the constitutive relations derived using the above models are difficult to extrapolate beyond the test conditions. This paper also reviews how some of the above limitations can be mitigated by using mechanistic or microstructurally motivated models. Mechanistic models are capable of estimating the material creep response based on the detailed physics of the underlying mechanisms and microstructure. The microstructure and constitutive response of the most popular family of lead-free solders, namely, SnAgCu (SAC) solders, are significantly different from those of previously used eutectic Sn37Pb solder. The creep deformation in Sn37Pb solder occurs primarily through diffusion-assisted grain-boundary sliding. In SAC solder joints, dislocation-based creep deformation mechanisms such as glide, climb, detachment, and cross-slip appear to be the dominant mechanisms in coarse-grained joints. Mechanistic creep models are therefore based on the deformation mechanisms listed above.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):030802-030802-23. doi:10.1115/1.4034037.
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Recent advances in flip chip technology such as wafer bumping, package substrate, flip chip assembly, and underfill will be presented in this study. Emphasis is placed on the latest developments of these areas in the past few years. Their future trends will also be recommended.

Commentary by Dr. Valentin Fuster

Research Papers

J. Electron. Packag. 2016;138(3):031001-031001-8. doi:10.1115/1.4033376.

It is important to control the skin temperatures of smartphones, tablets, and wearable electronics. The comfort levels of the skin temperatures are determined by surveys with subjective evaluations on a limited number of configurations and/or materials. This study is the first attempt to develop a characterization tool that is objective, repeatable, and predictable. The first step is to apply a gel finger that replaces the human finger to measure temperatures after gel finger's contact with the skin, i.e., the case of a mobile system. The second step is to establish a model calibrated by the experimental results. The calibrated model can be used to simulate different effects on the temperatures at the interface between the gel finger and the skin. The temperatures of the polycarbonate skin “felt” by the gel finger are always lower than those of the aluminum skin. The difference could reach 2–6 °C depending on heat spreading inside the system and heat sources in the finger. The difference can be reduced from 6 to 3 °C by using a novel casing with thin film metal on polymer. The transient periods are approximately 100–200 s with shorter transient periods for the aluminum skin.

Topics: Temperature , Aluminum , Skin
Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031002-031002-11. doi:10.1115/1.4033464.

Generally, porous jump (PJ) model is used for rapid air flow simulations (without resolving the tile pore structure) through perforated floor tiles in data centers. The PJ model only specifies a step pressure loss across the tile surface, without any influence on the flow field. However, in reality, the downstream flow field is affected because of the momentum rise of air due to acceleration through the pores, and interaction of jets emerging from the pores. The momentum rise could be captured by either directly resolving the tile pore structure (geometrical resolution (GR) model) or simulated by specifying a momentum source above the tile surface (modified body force (MBF) model). Note that specification of momentum source obviates the need of resolving the tile pore geometry and, hence, requires considerably low computational effort. In previous investigations, the momentum source was imposed in a region above the tile surface whose width and length were same as the tile dimensions with a preselected height. This model showed improved prediction with the experimental data, as well as with the model resolving the tile pore geometry. In the present investigation, we present an analysis for obtaining the momentum source region dimensions and other associated input variables so that the MBF model can be applied for general cases. The results from this MBF model were compared with the GR model and good agreement was obtained.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031003-031003-7. doi:10.1115/1.4033415.

A series of experiments was conducted to investigate participant thermal responses to different surface temperatures, from 34 to 44 °C, for a simulated tablet computer in different ambient temperatures (13 °C, 23 °C, and 33 °C). Two subjective measures, thermal sensations and thermal comfort, were reported by the participants. Within the same ambient temperature, participants' thermal sensation and discomfort scores were positively correlated with the increase of surface temperature (higher surface temperatures gave warmer sensations). Thermal comfort also decreases with the increase of surface temperature in the tested range. In addition, ambient temperature moderated the effect of surface temperature on participants' thermal sensation scores. The higher surface temperature of 44 °C was rated warmer at 33 °C than 13 °C, but lower surface temperatures (34–38 °C) were rated less warm at 33 °C than 13 °C. On the other hand, all the surface temperatures were perceived less uncomfortable in an environment at 13 °C environment than at 33 °C. The findings can be used to set limits for future tablet computer heat dissipation designs to improve user's thermal experiences.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031004-031004-13. doi:10.1115/1.4033487.

Multi-microchannel evaporators with flow boiling, used for cooling high heat flux devices, usually experience transient heat loads in practical applications. These transient processes may cause failure of devices due to a thermal excursion or poor local cooling or dryout. However, experimental studies on such transient thermal behavior of multi-microchannel evaporators during flow boiling are few. Thus, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of flow conditions. The effects on the base temperature behavior of the test section, heat flux magnitude, mass flux, inlet subcooling, outlet saturation temperature, and fluid were investigated. The transient base temperature response, monitored by an infrared (IR) camera, was recorded simultaneously with the flow regime acquired by a high-speed video camera. For cold startups, it was found that reducing the inlet orifice width, heat flux magnitude, inlet subcooling, and outlet saturation temperature but increasing the mass flux decreased the maximum base temperature. Meanwhile, the time required to initiate boiling increased with the inlet orifice width, mass flux, inlet subcooling, and outlet saturation temperature but decreased with the heat flux magnitude. For periodic variations in heat flux, the resulting base temperature was found to oscillate and then damp out along the flow direction. Furthermore, the effects of mass flux and heat flux pulsation period were insignificant.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031005-031005-13. doi:10.1115/1.4033558.

The performances of various transverse-flow double-layer microchannel heat sink configurations were evaluated compared to those of parallel-flow heat sink configurations via conjugate heat transfer analysis. For the analysis, three-dimensional Navier–Stokes and energy equations for steady incompressible laminar flow were solved using a finite-volume solver. Water with temperature-dependent thermophysical properties was used as a coolant. The thermal resistances were evaluated for various flow configurations of both cross-channel and parallel-channel designs with identical geometric parameters and total flow rate. Changes in the microchannel flow direction lead to remarkable changes in thermal resistance and temperature uniformity. A transverse-flow configuration exhibited the best overall performance among the tested flow configurations in terms of the thermal resistance, temperature uniformity, and pressure drop.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031006-031006-4. doi:10.1115/1.4033923.

We examined the effect of the design parameters of a through-silicon via (TSV) on the thermomechanical stress distribution at the bottom of the TSV using finite element analysis. Static analyses were carried out at 350 °C to simulate the maximum thermomechanical stress during postplating annealing. The thermomechanical stress is concentrated in the lower region of a TSV, and the maximum stress in silicon occurs at the bottom of the TSV. The TSV diameter and dielectric liner thickness were two important determinants of the maximum stress in the silicon. The maximum stress decreased with decreasing TSV diameter, whereas the effect of aspect ratio was negligible. A thick dielectric liner is advantageous for lowering the maximum stress in silicon. The minimum dielectric thickness resulting in a maximum stress less than the yield stress of silicon was 520, 230, and 110 nm for via diameters of 20, 10, and 5 μm, respectively. The maximum stress also decreased with the thickness of the copper overburden.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031007-031007-5. doi:10.1115/1.4034062.

In order to fulfill various growing needs of application fields, the development of low-cost directly printable radio-frequency identification (RFID) tag is essential for item level tracking. Currently, there lacks an easily available way to directly write out functional consumer electronicslike typewriting on paper by an office printer. Here, we show a desktop printing of RFID tag inductors on flexible substrates via developing liquid metal ink and related working mechanisms. The directly printing inductor on various flexible substrates with extremely low cost and rapid speed was designed based on the sympathetic oscillations of multiple LC (inductor–capacitor) circuits. In order to better meet the demands of the distinct resonant circuits, a series of conceptual experiments for investigating the relationship between the character of the inductor and its parameters—shape, number of coils, line width, spacing, etc.,—have been designed. The parameters are all working upon the performance of the printed inductors by liquid metal ink printer, and the relationship laws are consistent with those of the conventional inductors. The coils number as the biggest effect factor has a linear relationship with the inductance of the spiral-type inductors. An inductor with excellent properties can be well chosen by adjusting its parameters according to various applications. The present work demonstrated the way for a low cost and easy going method in directly printing RFID tag inductors on flexible substrates.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2016;138(3):031008-031008-10. doi:10.1115/1.4034101.

Solid liquid phase-change materials (PCMs) present a promising approach for reducing data center cooling costs. We review prior research in this area. A shell-and-tube PCM thermal energy storage (TES) unit is then analyzed numerically and experimentally. The tube bank is filled with commercial paraffin RUBITHERM RT 28 HC PCM, which melts as the heat transfer fluid (HTF) flows across the tubes. A fully implicit one-dimensional control volume formulation that utilizes the enthalpy method for phase change has been developed to determine the transient temperature distributions in both the PCM and the tubes themselves. The energy gained by a column of tubes is used to determine the exit bulk HTF temperature from that column, ultimately leading to an exit HTF temperature from the TES unit. This paper presents a comparison of the numerical and experimental results for the transient temperature profiles of the PCM-filled tubes and HTF.

Commentary by Dr. Valentin Fuster

Errata

Reviewer's Recognition

J. Electron. Packag. 2016;138(3):038001-038001-1. doi:10.1115/1.4033688.

List of JEP ReviewersAlekhya AddagatlaDavid HuttKaustubh NagarkarDereje AgonaferToru IkedaLuu T. NguyenS. Ravi AnnapragadaToshitaka IshizakiMark NorthMehmet ArikNokibul IslamTuba OkutucuCharles ArvinSrinivasan KandadaiGaozhu PengMehdi Asheghi*Satish G. KandlikarEric PerfectoMunshi BasitKailash C. KarkiMark D. PoliksIbrahim BekarRobert KayXian QinBiddut BhattacharjeeH. Ezzat KhalifaEmil RahimSiddharth BhopteKyoung Joon KimRishi RajNate BlattauJoo KimSven C. RzepkaHuseyin BostanciYeong Kook KimSandip K. Saha*Cyril ButtayYeonsung KimYu-Lin ShenBing-Yang CaoSvenja KnappeSaurabh ShrivastavaNienhua ChaoMing KongAntariksh SinghSuman ChakrabortyAli KosarAshish SinhaJames ChenErkan KoseTuhin SinhaShea Chen*Desiderio KovarC. B. SobhanTz-Cheng ChiuJan KrajniakGiulia SpinatoSoonwan ChungPradeep LallYong SunPeter DebockMichael LarsonXiao-Ming TanDavid Van DillenFuliang LePradit TerdtoonRainer DudekHyoungsoon Lee*Naveenan ThiagarajanKhosrow EbrahimiY. C. Lee*Chris J. TuckVincent FioriXiaobo Li*Kumar UpadhyayulaTimothy S. FischerLi-Anne LiewJames VanGilderGeorge FlowersDapeng LiuChung-Ting WangChong Leong GanJeffery LoXiao-Dong Wang*Jivtesh GargXiaobing LuoZheyao WangVadim GektinJohn MaddoxGongnan XieJoshua GessNeha MaluJun XuOmidreza GhaffarriMike MannoHaoyue YangMohammad HamashaToni Matilla*Chao YuanTom HanftMichael MayerTieJun ZhangEnisa HarrisRajat MittalDongliang ZhaoYurong HeMasataka MochizukiJiantao ZhengQuanwen HouMohammad MotalabYi HuangKaushik Mysore**Multiple reviews.

Commentary by Dr. Valentin Fuster

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