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### Editorial

J. Electron. Packag. 2018;140(2):020201-020201-2. doi:10.1115/1.4039660.
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The Reviewer of the Year Award is given to reviewers who have made an outstanding contribution to the journal in terms of the quantity, quality, and turnaround time of reviews completed during the past 12 months. The prize includes a Wall Plaque, 50 free downloads from the ASME Digital Collection, and a one year free subscription to the journal.

Commentary by Dr. Valentin Fuster

### Guest Editorial

J. Electron. Packag. 2018;140(2):020301-020301-1. doi:10.1115/1.4039963.
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InterPACK is a premier international forum for exchange of state-of-the-art knowledge in research, development, manufacturing, and applications of micro-electronics packaging. It is the flagship conference of the ASME Electronic and Photonic Packaging Division (EPPD) founded in 1992 as an ASME–JSME joint biannual conference.

Commentary by Dr. Valentin Fuster

### SPECIAL SECTION PAPERS

J. Electron. Packag. 2018;140(2):020901-020901-7. doi:10.1115/1.4039136.

Particulate thermal interface materials (TIMs) are commonly used to transport heat from chip to heat sink. While high thermal conductance is achieved by large volume loadings of highly conducting particles in a compliant matrix, small volume loadings of stiff particles will ensure reduced thermal stresses in the brittle silicon device. Developing numerical models to estimate effective thermal and mechanical properties of TIM systems would help optimize TIM performance with respect to these conflicting requirements. Classical models, often based on single particle solutions or regular arrangement of particles, are insufficient as real-life TIM systems contain a distribution of particles at high volume fractions, where classical models are invalid. In our earlier work, a computationally efficient random network model (RNM) was developed to estimate the effective thermal conductivity of TIM systems (Kanuparthi et al., 2008, “An Efficient Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials,” IEEE Trans. Compon. Packag. Technol., 31(3), pp. 611–621; Dan et al., 2009, “An Improved Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials,” ASME Paper No. InterPACK2009-89116.) . This model is extended in this paper to estimate the effective elastic modulus of TIMs. Realistic microstructures are simulated and analyzed using the proposed method. Factors affecting the modulus (volume fraction and particle size distribution (PSD)) are also studied.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020902-020902-8. doi:10.1115/1.4039137.

In this paper, the impact of direct liquid cooling (DLC) system failure on the information technology (IT) equipment is studied experimentally. The main factors that are anticipated to affect the IT equipment response during failure are the central processing unit (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 this 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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020903-020903-9. doi:10.1115/1.4039265.

Transient liquid phase sintering (TLPS) is a novel high-temperature attach technology. It is of particular interest for application as die attach in power electronic systems because of its high-melting temperature and high thermal conductivity. TLPS joints formed from sinter pastes consist of metallic particles embedded in matrices of intermetallic compounds (IMCs). Compared to conventional solder attach, TLPS joints consist to a considerably higher percentage of brittle IMCs. This raises the concern that TLPS joints are susceptible to brittle failure. In this paper, we describe and analyze the cooling-induced formation of vertical cracks as a newly detected failure mechanism unique to TLPS joints. In a power module structure with a TLPS joint as interconnect between a power device and a direct bond copper (DBC) substrate, cracks can form between the interface of the DBC and the TLPS joint when large voids are located in the proximity of the DBC. These cracks do not appear in regions with smaller voids. A method has been developed for the three-dimensional (3D) modeling of paste-based TLPS sinter joints, which possess complex microstructures with heterogeneous distributions of metal particles and voids in IMC matrices. Thermomechanical simulations of the postsintering cooling process have been performed and the influence of microstructure on the stress-response within the joint and at the joint interfaces have been characterized for three different material systems (Cu + Cu6Sn5, Cu + Cu3Sn, Ni + Ni3Sn4). The maximum principal stress within the assembly was found to be a poor indicator for prediction of vertical crack formation. In contrast, stress levels at the interface between the TLPS joint and the power substrate metallization are good indicators for this failure mechanism. Small voids lead to higher joint maximum principal stresses, but large voids induce higher interfacial stresses, which explain why the vertical cracking failure was only observed in joints with large voids.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020904-020904-5. doi:10.1115/1.4039138.
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PARC, a Xerox Company, is developing a low-cost system of peel-and-stick wireless sensors that will enable widespread building environmental sensor deployment with the potential to deliver up to 30% energy savings. The system is embodied by a set of radio-frequency (RF) hubs that provide power to automatically located sensor nodes and relay data wirelessly to the building management system (BMS). The sensor nodes are flexible electronic labels powered by rectified RF energy transmitted by the RF hub and can contain multiple printed and conventional sensors. The system design overcomes limitations in wireless sensors related to power delivery, lifetime, and cost by eliminating batteries and photovoltaic devices. Sensor localization is performed automatically by the inclusion of a programmable multidirectional antenna array in the RF hub. Comparison of signal strengths as the RF beam is swept allows for sensor localization, reducing installation effort and enabling automatic recommissioning of sensors that have been relocated. PARC has already demonstrated wireless power and temperature data transmission up to a distance of 20 m with 71 s between measurements, using power levels well within the Federal Communications Commission regulation limits in the 902–928 MHz industrial, medical and scientific (ISM) band. The sensor's RF energy harvesting antenna achieves high performance with dimensions of 5 cm × 9.5 cm.

Topics: Sensors
Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020905-020905-12. doi:10.1115/1.4039301.

This paper intends to address an important gap between reliability standards and the physics of how components respond to real use conditions using a knowledge-based qualification (KBQ) process. Bridging the gap is essential to developing test methods that better reflect field performance. With the growth in importance of automotive market and the wide usage of electronics in this market, vibration-induced failures was chosen for this study. MIL-STD-810G and ISTA4AB are couple of industry standards that address the risk of shipping finished goods to a customer. For automotive electronic products that are exposed to vibration conditions all through their life, USCAR-2 and GMW3172 are more relevant. Even though the usage models and transportation duration for shipping fully packaged systems is different from automotive electronics, the source of energy (road conditions), driving the risks, are similar. The industry standards-based damage models appear to be generic, covering a wide variety of products and failure modes. Whereas, the KBQ framework, used in this paper, maps use conditions to accelerated test requirements for only two failure modes: solder joint fatigue and socket contact fretting. The mechanisms were chosen to be distinct with different damage metric and drivers. The process is intended to explain how industry standards reflect field risks for two of the risks relevant for automotive electronics.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020906-020906-6. doi:10.1115/1.4040088.

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 μm and pore diameter of 5 μm. 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/cm2 (at 16.2 °C superheat) or about three times higher than that of smooth Si surface (80 W/cm2 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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):020907-020907-12. doi:10.1115/1.4039974.
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With recent advancements in additive manufacturing (AM) technology, it is possible to deposit copper conductive paths and insulation layers of an electric machine in a selective controlled manner. AM of copper enables higher fill factors that improves the internal thermal conduction in the stator core of the electric machine (induction motor), which will enhance its efficiency and power density. This will reduce the motor size and weight and make it more suitable for aerospace and electric vehicle applications, while reducing/eliminating the rare-earth dependency. The objective of this paper is to present the challenges associated with AM of copper coils having 1 × 1 mm cross section and complex features that are used in producing ultra-high efficiency induction motor for traction applications. The paper also proposes different approaches that were used by the authors in attempts to overcome those challenges. The results of the developed technologies illustrate the important of copper powder treatment to help in flowing the powder easier during deposition. In addition, the treated powder has higher resistance to surface oxidation, which led to a high reduction in porosity formation and improved the quality of the copper deposits. The laser powder direct energy deposition (LPDED) process modeling approach helps in optimizing the powder deposition path, the laser power, and feed rate that allow the production of porosity free thin wall and thin floor components. The laser powder bed fusion (LPBF) models identify the optimum process parameters that are used to produce test specimens with >90% density and minimum porosity.

Commentary by Dr. Valentin Fuster

### Research Papers

J. Electron. Packag. 2018;140(2):021001-021001-12. doi:10.1115/1.4039475.

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 D = 46.5 μm; spacing, S ∼ 100 μm; and height, H ∼ 110 μm. Deionized single-phase water with mass flow rates of $m˙$ = 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/cm2 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 (∼Re1.04) and Fanning friction factor (∼Re−0.52) 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 μm and fin spacing S = 50–500 μm, respectively, with the Reynolds number, Re < 300.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):021002-021002-11. doi:10.1115/1.4039706.

The present paper proposes a proof of concept of a completely passive thermosyphon for cooling of power electronics. This thermosyphon is composed of an evaporator to cool down a four-heater pseudo-transistor module and a natural air-cooled condenser to reject the heat into the environment. R1234ze, R1234yf, and R134a are used as the working fluids with charges of 524, 517, and 566 g, respectively, for the low charge tests, and 720, 695, and 715 g for the high charge tests. It has been demonstrated that the refrigerant R1234ze with a low charge is not a good solution for the cooling system proposed here since low evaporator performance and fluid instability have been detected at moderate heat fluxes. In fact, R1234ze needed a larger charge of refrigerant to be safely used, reaching a transistor temperature of $53 °C$ at a heat load of $65 W$. R1234yf and R134a, on the other hand, showed good results for both the low and the high charge cases. The maximum temperatures measured, respectively, were $52 °C$ and $48 °C$ at $65 W$ for the low charge case and $55 °C$ and $47 °C$ at $62 W$ for the high charge case. The corresponding values of overall thermal resistances of the thermosyphon for the working fluids R1234yf and R134a at the maximum heat load are very similar, being in the range of $0.44−0.46 K/W$.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2018;140(2):021003-021003-18. doi:10.1115/1.4039749.

This study examined the cause of nonwetted regions of the gold (Au) finish on iron-nickel (Fe–Ni) alloy lids that seal ceramic packages using the 80Au-20Sn solder (wt %, abbreviated Au–Sn) and their impact on the final lid-to-ceramic frame solder joint. The Auger electron spectroscopy (AES) surface and depth profile techniques identified surface and through-thickness contaminants in the Au metallization layer. In one case, the AES analysis identified background levels of carbon (C) contamination on the surface; however, the depth profile detected Fe and Ni contaminants that originated from the plating process. The Fe and Ni could impede the completion of wetting and spreading to the edge of the Au metallization. The Au layer of lids not exposed to a Au–Sn solder reflow step had significant surface and through-thickness C contamination. Inorganic contaminants were absent. Subsequent simulated reflow processes removed the C contamination from the Au layer without driving Ni diffusion from the underlying solderable layer. An Au metallization having negligible C contamination developed elevated C levels after exposure to a simulated reflow process due to C contamination diffusing into it from the underlying Ni layer. However, the second reflow step removed that contamination from the Au layer, thereby allowing the metallization to support the formation of lid-to-ceramic frame Au–Sn joints without risk to their mechanical strength or hermeticity.

Commentary by Dr. Valentin Fuster