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IN THIS ISSUE

Editorial

J. Electron. Packag. 2010;132(3):030201-030201-1. doi:10.1115/1.3493694.
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This issue of the Journal of Electronic Packaging has six papers selected from the second international conference on thermal issues in emerging technologies theory and applications. This conference (ThETA2) was held in Cairo, Egypt, December 17–20, 2008. The objectives of the conference are to encourage dissemination of information and research in areas related to transport and emerging technologies in various domains. The main conference objective is to address the growing impact of thermal issues on many advanced and emerging technologies including Microelectronics, Nanotechnology, Smart Materials, Micro-Electro-Mechanical Systems, Biomedical Engineering, and Green Energy. The importance of transport in these areas is becoming a dominant factor in determining the performance of such technologies. The ThETA2 conference was very successful and attracted many prominent researchers from the US, Europe, and the Far East to participate and interact with researchers from the Middle East, in general, and Egypt in particular. The seven papers presented in this special issue represent some of the best ideas presented at the conference, and deemed worthy of archival publication in the JEP after the papers were reviewed again by the JEP with Professor Mohamed-Nabil Sabry serving as the special guest editor for the issue. The ThETA2 executive committee consisted of Bernard Courtois, TIMA Lab., Grenoble, France, Yogendra Joshi, Georgia Inst. Technology, USA, Waturu Nakayama, Tokyo Inst. Technology, Japan, Mohamed-Nabil Sabry, U. Française d' Egypte, Bahgat Sammakia, Binghamton U., USA, and Mamdouh Shoukri, McMaster U., Canada. The chairmen for the conference were Mohamed-Nabil Sabry and Bernard Courtois.

Commentary by Dr. Valentin Fuster

Research Papers

J. Electron. Packag. 2010;132(3):031001-031001-9. doi:10.1115/1.4002161.

Optical noninvasive temperature measurement techniques, such as interferometry, are particularly advantageous in obtaining temperature information noninvasively from enclosed low velocity flows induced by thermal sources, as commonly arise in electronic systems. The single greatest restriction in the application of interferometry as a standard measurement methodology has been the enormous cost associated with the optical equipment required. This cost is due to the quality of the optics required, which exhibits an exponential dependence on size. Digital Moiré subtraction is a technique, which removes the restriction on the use of high quality optics, thereby, enabling reasonably large fields of view. In this paper, a digital Moiré subtraction interferometer configuration is presented with a 140 mm field of view. First, the ability of the interferometer to accurately measure the free convection temperature field about an isothermal horizontal cylinder is examined through a comparison with measurements from literature using classical interferometry. The technique is then applied to the thermal interaction between 2D components representing BGAs mounted on a vertical printed circuit board (PCB). Qualitative and quantitative evaluation of the interferograms show the significant influence of in-plane PCB conductivity on the temperature field about the PCB. The spacing to length ratio above, which upstream components on a PCB experience enhanced cooling, is reduced from 4 to 3 for a PCB with a high effective in-plane conductivity $(15 W/m K)$.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031002-031002-8. doi:10.1115/1.4001853.

Thermal design requirements are mostly driven by the peak temperatures. Reducing or eliminating hot spots could alleviate the design requirement for the whole package. Combination of solid-state and liquid cooling will allow removal of both hot spots and background heating. In this paper, we analyze the performance of thin film $Bi2Te3$ microcooler and the 3D SiGe-based microrefrigerator, and optimize the maximum cooling and cooling power density in the presence of a liquid flow. Liquid flow and heat transfer coefficient will change the background temperature of the chip but they also affect the performance of the solid-state coolers used to remove hot spots. Both Peltier cooling at interfaces and Joule heating inside the device could be affected by the fluid flow. We analyze conventional Peltier coolers as well as 3D coolers. We study the impact of various parameters such as thermoelectric leg thickness, thermal interface resistances, and geometry factor on the overall system performance. We find that the cooling of a conventional Peltier cooler is significantly reduced in the presence of fluid flow. On the other hand, 3D SiGe cooler can be effective to remove high power density hot spots up to $500 W/cm2$. 3D microrefrigerators can have a significant impact if the thermoelectric figure-of-merit, $ZT$, could reach 0.5 for a material grown on silicon substrate. It is interesting to note that there is an optimum microrefrigerator active region thickness that gives the maximum localized cooling. For liquid heat transfer coefficient between 5000 and $20,000 W m−2 K−1$, the optimum is found to be between $10 μm$ and $20 μm$.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031003-031003-5. doi:10.1115/1.4001854.

A high time-resolution measurement of flow field in a glass plate with glass cover with two holes was done by a micro-DHPTV system. The two holes mentioned here were also inside the photo-curable resin. Particle measurement was carried out during a 2 s time period that covered the time it took for the curing of resin. This curing time was theoretically calculated from the irradiation flux of the Ultraviolet (UV) source employed in the experiment. Moreover, temperature dependence of photo-curable resin was evaluated by measuring the resin at changing temperatures. Consequently, the tracking of seeding particles could instantaneously be recorded. The 3-D displacements, obtained from the particles being tracked, took place primarily along the direction of the depth. These values were found to be in good agreement with the theoretically calculated displacement values obtained from UV irradiation flux. Moreover, the displacement during photo-curing appeared to be proportional to the increase in temperature.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031004-031004-7. doi:10.1115/1.4001856.

The nontrivial issues associated with calculating the steady state heat spreading effects generated by a heat source on top of a multilayer assembly such as a printed circuit board are discussed. It is argued that problems arise with the interpretation of heat spreading effects due to a misconception about the meaning of often-quoted flux limits and especially the physical meaning of thermal resistance. The usefulness of a number of approaches that are generally in use to analyze heat spreading effects is discussed and it is shown that the popular series-resistance approach has severe limitations. A number of test cases are covered in detail and the results testify to this assertion.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031005-031005-4. doi:10.1115/1.4001855.

This paper discusses a pulsating heat pipe (PHP) using a self-rewetting fluid. Unlike other common liquids, self-rewetting fluids have the property that the surface tension increases with temperature. The increasing surface tension at a higher temperature can cause the liquid to be drawn toward a heated surface if a dry spot appears and thus to improve boiling heat transfer. In experiments, 1-butanol and 1-pentanol were added to water at a concentration of less than 1 wt % to make self-rewetting fluid. A pulsating heat pipe made from an extruded multiport tube was partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability at different input power levels. The experiments showed that the maximum heat transport capability was enhanced by a factor of 4 when the maximum heater temperature was limited to $110°C$. Thus, the use of a self-rewetting fluid in a PHP was shown to be highly effective in improving the heat transport capability of pulsating heat pipes.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031006-031006-14. doi:10.1115/1.4001858.

Concurrency and exchanging design knowledge among thermal and IT management are required to achieve an energy efficient operational data center. In this paper, a design approach is presented to bring adaptability and concurrency for coordinated minimization of cooling and IT power consumption in data centers. The presented approach is centered on a proper orthogonal decomposition based reduced order thermal modeling approach, and power profiling of the IT equipment to identify the optimal parameters of the air cooling systems along with optimal dynamic workload distribution among the servers. The method is applied to a data center cell with different rack and server architectures. The results show that the design approach results in 12–70% saving in the total energy consumption of the data center cell for various scenarios, compared with a baseline design.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031007-031007-7. doi:10.1115/1.4002009.

Natural convection cooling provides a reliable, cost-effective, energy-efficient and noise-free method to cool electronic equipment. However, the heat transfer coefficient associated with natural convection mode is usually insufficient for electronic cooling and it requires enhancement. Chimneylike flows developed within the cabinets of electronic devices can provide better mass flow and heat transfer rates and can lead to greater cooling efficiency. Constraints in the design of natural convection cooling systems include efficiency of packing, aesthetics, and concerns of material reduction. In this paper, methods based on computational fluid dynamics are used to study the effects of parameters such as (1) vertical alignment of the slots, (2) horizontal alignment of slots, (3) area of slots, (4) differential slot opening, and (5) zonal variation in heat generation on natural convection cooling within such design constraints. Insights thus derived are found useful for designing an energy-efficient and ecofriendly cooling system for electronic devices.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031008-031008-5. doi:10.1115/1.4002010.

The effects of underfill selection on flip chip reliability were always a complex issue. Mechanical optimization of the underfill performance, achieved by the addition of appropriate fillers, is invariably a tradeoff between the adhesion and the coefficient of thermal expansion (CTE) and, thus, also between in-plane and out-of-plane stresses. Another critical concern is the degradation of the underfill in processing and/or long term exposure to operating temperatures and ambient humidity. This is strongly affected by the chemical compatibility with combinations of solder mask, chip passivation, and flux residues. The latter is believed to be responsible for our observation of interactions with the solder alloy, too. As for the effects of glass transition temperatures and CTE, we find materials that were close to optimum for eutectic SnPb to be very far from the best options for lead free joints. We report on two sets of systematic experiments. The first addressed the performance of combinations of underfills, no-clean fluxes, and solder alloys in a JEDEC level 3 moisture sensitivity test. The second one involved thermal shock testing of flip chip assemblies underfilled with one of five different materials after soldering with SnCu, SAC305, and SnPb.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031009-031009-6. doi:10.1115/1.4002012.

Broad societal needs have focused attention on technologies that can effectively dissipate huge amount of heat from high power density electronic devices. Liquid metal cooling, which has been proposed in recent years, is fast emerging as a novel and promising solution to meet the requirements of high heat flux optoelectronic devices. In this paper, a design and implementation of a practical liquid metal cooling device for heat dissipation of high performance CPUs was demonstrated. GaInSn alloy with the melting point around $10°C$ was adopted as the coolant and a tower structure was implemented so that the lowest coolant amount was used. In order to better understand the design procedure and cooling capability, several crucial design principles and related fundamental theories were demonstrated and discussed. In the experimental study, two typical prototypes have been fabricated to evaluate the cooling performance of this liquid metal cooling device. The compared results with typical water cooling and commercially available heat pipes show that the present device could achieve excellent cooling capability. The thermal resistance could be as low as $0.13°C/W$, which is competitive with most of the latest advanced CPU cooling devices in the market. Although the cost (about 70 dollars) is still relatively high, it could be significantly reduced to less than 30 dollars with the optimization of flow channel. Considering its advantages of low thermal resistance, capability to cope with extremely high heat flux, stability, durability, and energy saving characteristic when compared with heat pipe and water cooling, this liquid metal cooling device is quite practical for future application.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031010-031010-4. doi:10.1115/1.4002298.

The thermal and optical characteristics of phosphor converted white light-emitting diodes (LEDs) with different phosphor concentrations ranging from $4 wt %$ to $13 wt %$ are investigated. The light output of LEDs with higher phosphor concentration is found to have larger degradation in constant current compared with pulse current than that with lower phosphor concentration. In addition, the junction temperatures of phosphor converted white LEDs raise with increasing phosphor concentration, so that the decreased phosphor conversion efficiency is observed both in pulse and constant current modes. The physical mechanisms for these observations are discussed. This study elucidates the phosphor dependent optical and thermal behavior of phosphor converted white LEDs.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031011-031011-5. doi:10.1115/1.4002299.

As a solid electroluminescent source, white light emitting diode (LED) has entered a practical stage and become an alternative to replace incandescent and fluorescent light sources. However, due to the increasing integration and miniaturization of LED chips, heat flux inside the chip is also increasing, which puts the packaging into the position to meet higher requirements of heat dissipation. In this study, a new interconnection material—nanosilver paste is used for the LED chip packaging to pursue a better optical performance, since high thermal conductivity of this material can help improve the efficiency of heat dissipation for the LED chip. The bonding ability of this new die-attach material is evaluated by their bonding strength. Moreover, high-power LED modules connected with nanosilver paste, $Sn3Ag0.5Cu$ solder, and silver epoxy are aged under hygrothermal aging and temperature cycling tests. The performances of these LED modules are tested at different aging time. The results show that LED modules sintered with nanosilver paste have the best performance and stability.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2010;132(3):031012-031012-6. doi:10.1115/1.4002451.

This paper presents a simulation method to evaluate the thermal fatigue life of a power module. A coupled electrical-thermal analysis was performed to obtain the nonuniform temperature distribution of electric current. Then, a thermomechanical analysis was carried out based on the temperature distribution from the electrical-thermal analysis. Since crack propagation can change the route of heat transfer, a crack path simulation technique was used to investigate the fracture behavior of the power module. The crack initiates in the solder joint below the Al bonding wire of the insulated gate bipolar transistor module and propagates by increasing the diameter. The effect of the bonding type on power cycling fatigue life is also discussed. The fracture process was found to depend on the type of bonding. Lead frame bonding was found to be more effective than wire bonding.

Commentary by Dr. Valentin Fuster

Silver flip-chip joints between silicon (Si) chips and copper (Cu) substrates were fabricated using a solid-state bonding process without any solder and without flux. The bonding process was performed at $250°C$, compatible with typical reflow temperature for lead-free solders. During the bonding process, there was no molten phase involved. The Ag joints fabricated consisted of only pure Ag without any intermetallic compound (IMC). Thus, reliability issues associated with IMCs and IMC growth do not exist anymore. Silver has the highest electrical conductivity and highest thermal conductivity among all metals. It is also quite ductile and able to deform to release stresses caused by thermal expansion mismatch. Flip-chip joints of high aspect ratio can be accomplished because the joints stay in a solid state during the bonding process. It looks like that silver is the ultimate joining material for flip-chip as well as through-Si-via interconnect technologies. In this study, the solid-state bonding process was first developed using a pure Ag foil to bond a Si chip to a Cu substrate in one step. The bonding strength on two interfaces, Si/Ag and Ag/Cu, passes the MIL-STD-883G Method 2019.7. To demonstrate Ag flip-chip interconnects, Si chips were electroplated with Ag bumps, followed by the solid-state bonding process on Cu substrates. The flip-chip bumps are well bonded to the Cu substrate. It would take some time for this new technology to be probably accepted and utilized in production. On the other hand, the preliminary results in this study show that Ag flip-chip joints can indeed be fabricated at $250°C$.