Research Papers

J. Electron. Packag. 2013;136(1):011001-011001-8. doi:10.1115/1.4025863.

Intermetallic compounds (IMC) play a key role in the mechanical reliability of solder joints. The present work investigates the diffusion-induced stress developed in the Cu pad/IMC/solder sandwich structure during a solid-state isothermal aging process. An analytical model and a numerical approach are proposed to predict the stress. The model consists of a Cu6Sn5 layer sandwiched between a Cu pad and a solder layer, and it is assumed that the diffusivity of the Cu atoms is much greater than that of the Sn atoms. We use the Laplace transformation method to obtain the distribution of the Cu atoms concentration. The diffusion-induced stress is determined analytically by the volumetric strain resulted from the effect of the atomic diffusion. It is found that the Cu6Sn5 layer is subjected to compressive stress due to the Cu atoms diffusion. As the diffusion time is long enough, the diffusion-induced stress shows a linear relationship with the thickness of the Cu6Sn5 layer. A finite element approach to calculate the diffusion-induced stress is proposed, and it is compared and validated by the analytical solution. The results show that the proposed approach can give a well estimation of the diffusion-induced stress in the Cu6Sn5 layer, and is also efficient in predicting the diffusion-induced stress in the structures with more complex geometry. The distribution of the Cu atoms concentration and the diffusion-induced stress in the model with a scallop-like or flat-like Cu6Sn5/solder interface are calculated by the numerical approach. The results show that the interfacial morphology of the Cu6Sn5/solder has great influence on the evolution of the Cu atoms concentration, and the diffusion-induced stress in the Cu6Sn5 layer with the scallop edge is less than that with the flat edge.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2013;136(1):011002-011002-7. doi:10.1115/1.4025673.

In this paper we introduce a hybrid fin heat sink (HFH) proposed for the thermal control of light emitting diode (LED) lighting modules. The HFH consists of the array of hybrid fins which are hollow pin fins having internal channels and integrated with plate fins. The thermal performance of the HFH under either natural or forced convection condition is both experimentally and numerically investigated, and then its performance is compared with that of a pin fin heat sink (PFH). The observed maximum discrepancies of the numerical prediction to the measurement for the HFH are 7% and 6% for natural and forced convection conditions. The reasonable discrepancies demonstrate the tight correlation between the numerical prediction and the measurement. The thermal performance of the HFH is found to be 12–14% better than the PFH for the natural convection condition. The better performance might be explained by the enlarged external surface and the internal flow via the channel of the HF. The reference HFH is about 14% lighter than the reference PFH. The better thermal performance and the lighter weight of the HFH show the feasibility as the promising heat sink especially for the thermal control of LED street and flood lighting modules.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2013;136(1):011003-011003-7. doi:10.1115/1.4025915.

SnCuNi is one of the most common ternary intermetallic compounds formed in the Sn-based solder joint, and its formation and properties can be greatly influenced by the amount of Ni. Ni can participate in the interfacial reaction and diffuse into the intermetallic compound layer from either the solder or from the pad. In this research, comparative studies of different SnCuNi intermetallic compounds were conducted using two kinds of SnCuNi solders with organic solderability preservatives pad finish and a SnCu solder with electroless nickel/immersion gold pad finish. In the former case, Ni can only diffuse into the intermetallic compound from the solder matrix, while in the latter the Ni is only from the metallization layer on the Cu base. Scanning electron microscopy and transmission electron microscopy were employed to inspect the morphologies and interfacial microstructures of the intermetallic compounds. The thermal aging test was conducted to investigate their growth behavior under elevated temperature conditions. Mechanical strength after different aging hours was also evaluated via high speed ball pull test.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2013;136(1):011004-011004-13. doi:10.1115/1.4026051.

This paper presents an approach for reducing detailed numerical models of electronic equipment into compact thermal-fluid models. These compact models have been created using network analogies representing mass, momentum and energy transport to reduce computational demand, preserve manufacturer intellectual property, and enable software independent exchange of information between supplier and integrator. A strategic approach is demonstrated for a steady state case from reduction to model integration within a global environment. The compact model is robust to boundary condition variation by developing a boundary condition response matrix for the network layout. A practical example of electronic equipment cooled naturally in air is presented. Solution times were reduced from ∼100 to ∼10−3 CPU hours when using the compact model. Nodal information was predicted with O(10%) accuracy compared to detailed solutions. For parametric design studies, the reduced model can provide 1800 solutions in the same time required to run a single detailed numerical simulation. The information generated by the reduction process also enhances collaborative design by providing the equipment integrator with ordered initial conditions for the equipment in the optimization of the global design. Sensitivity of the compact model to spatial variations on the boundary node faces has also been assessed. Overall, the compact modeling approach developed extends the use of compact models beyond preliminary design and into detailed phases of the product design lifecycle.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2013;136(1):011005-011005-6. doi:10.1115/1.4026164.

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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011006-011006-6. doi:10.1115/1.4026171.

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 MIL-STD-883 H 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.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011007-011007-7. doi:10.1115/1.4026352.

The peak junction temperature has a profound effect on the operational lifetime and performance of high powered microwave devices. Although numerical analysis can help to estimate the peak junction temperature, it can be computationally expensive and time consuming when investigating the effect of the device geometry and material properties on the performance of the device. On the other hand, a closed-form analytical method will allow similar studies to be done easily and quickly. Although some previous analytical solutions have been proposed, the solutions either require over-long computational times or are not so accurate. In this paper, an accurate closed-form analytical solution for the junction temperature of power amplifier field effect transistors (FETs) or monolithic microwave integrated circuits (MMICs) is presented. Its derivation is based on the Green's function integral method on a point heat source developed through the method of images. Unlike most previous works, the location of the heat dissipation region is assumed to be embedded under the gate. Since it is a closed-form solution, the junction temperature as well as the temperature distribution around the gate can be easily calculated. Consequently, the effect of various design parameters and material properties affecting the junction temperature of the device can be easily investigated. This work is also applicable to multifinger devices by employing superposition techniques and has been shown to agree well with both numerical and experimental results.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011008-011008-12. doi:10.1115/1.4026351.

This paper investigates the thermal and fluid dynamic characteristics due to multiple miniature axial fans with blade chord and span length scales less than 10 mm, impinging air onto finned surfaces. A coupled approach, utilizing both experimental and numerical techniques, has been devised to examine in detail the exit air flow interaction between cooling fans within an array. The findings demonstrate that fans positioned adjacently in an array can influence heat transfer performance both positively and negatively by up to 35% compared to an equivalent single fan—heat sink unit operating standalone. Numerical simulations have provided an insight into the flow fields generated by adjacent fans and also the air flow interaction with fixed fan motor support structures downstream. A novel experimental approach utilizing infrared thermography has been developed to locally assess the validity of the numerical models. In particular, an assessment on implementing compact lumped parameter fans and fans modeled with full geometric detail is shown for two configurations that are impinging air onto finned and flat surfaces. Overall, the study provides an insight into fan cooled heat sinks incorporating multiple miniature axial fans and general recommendations for improving current numerical modeling approaches.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011009-011009-7. doi:10.1115/1.4026414.

Thermal grease, as a thermal interface material (TIM), has been extensively applied in electronic packaging areas. Generally, thermal greases consist of highly thermally conductive solid fillers and matrix material that provides good surface wettability and compliance of the material during application. In this study, the room-temperature liquid metal (a gallium, indium and tin eutectic, also called Galinstan) was proposed as a new kind of liquid filler for making high performance TIMs with desired thermal and electrical behaviors. Through directly mixing and stirring in air, liquid metal micron-droplets were accidentally discovered capable to be homogeneously distributed and sealed in the matrix of methyl silicone oil. Along this way, four different volume ratios of the liquid metal poly (LMP) greases were fabricated. The thermal and electrical properties of the LMP greases were experimentally investigated, and the mechanisms were clarified by analyzing their surface morphologies. The experimental results indicate that the original highly electrically conductive liquid metal can be turned into a highly electrically resistive composite, by simply blending with methyl silicone oil. When the filler content comes up to 81.8 vol. %, the thermal conductivity, viscosity and volume resistivity read 5.27 W/(m · °C), 760 Pa · s and 1.07 × 107 Ω m, respectively. Furthermore, the LMP greases presented no obvious corrosion effect, compared with pure liquid metal. This study opens a new approach to flexibly modify the material behaviors of the room-temperature liquid metals. The resulted thermally conductive however highly electrically resistive LMP greases can be significant in a wide variety of electronic packaging applications.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011010-011010-7. doi:10.1115/1.4026536.

Optimization of a hybrid double-side jet impingement cooling system for high-power light emitting diodes (LEDs) was performed using a hybrid multi-objective evolutionary approach and three-dimensional numerical analysis for steady incompressible laminar flow and conjugate heat transfer using Navier–Stokes equations. For optimization, two design variables, i.e., ratios of the diameter of jet holes and the distance from the exit of upper impinging hole to chips to thickness of substrate were chosen out of the various geometric parameters affecting the performance of the cooling system. To evaluate cooling performance and pressure loss of the system, two objective functions, viz., the ratio of the maximum temperature to average temperature on the chips and pressure coefficient, were selected. Surrogate modeling of the objective functions was performed using response surface approximation. The Pareto-optimal solutions were obtained using a multi-objective evolutionary algorithm, and performances of three representative Pareto-optimal designs were discussed compared to a reference design. In the optimal designs, higher level of uniform cooling was generally achieved with higher pressure coefficient. The Pareto-sensitivity analysis between the objective function and design variable was also performed.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011011-011011-10. doi:10.1115/1.4026540.

Cold aisle containment is used in air cooled data centers to minimize direct mixing between cold and hot air. Here, we present room level air flow field investigation for open, partially and fully contained cold aisles. Our previous investigation for rack level modeling has shown that consideration of momentum rise above the tile surface, due to acceleration of air through the pores, significantly improves the predictive capability as compared to the generally used porous jump model. The porous jump model only specifies a step pressure loss at the tile surface without any influence on the flow field. The momentum rise could be included by either directly resolving the tile's pore structure or by artificially specifying a momentum source above the tile surface. In the present work, a modified body force model is used to artificially specify the momentum rise above the tile surface. The modified body force model was validated against the experimental data as well as with the model resolving the tile pore geometry at the rack level and then implemented at the room level. With the modified body force model, much higher hot air entrainment and higher server inlet temperatures were predicted as compared to the porous jump model. Even when the rack air flow requirement is matched with the tile air flow supply, considerable hot air recirculation is predicted. With partial containment, where only a curtain at the top of the cold aisle is deployed and side doors are opened, improved cold air delivery is suggested.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011012-011012-5. doi:10.1115/1.4026616.

To promote heat dissipation in power electronics, we investigated the thermal conduction performance of Sn-Bi solder paste between two Cu plates. We measured the thermal resistance of Sn-Bi solder paste used as thermal interface material (TIM) by laser flash technique, and a thermal resistance less than 5 mm2 K/W was achieved for the Sn-Bi TIM. The Sn-Bi solder also showed a good reliability in terms of thermal resistance after thermal cycling, indicating that it can be a promising candidate for the TIM used for power electronics applications. In addition, we estimated the contact thermal resistance at the interface between the Sn-Bi solder and the Cu plate with the assistance of scanning acoustic microscopy. The experimental data showed that Sn-Bi solder paste could be a promising adhesive material used to attach power modules especially with a large size on the heat sink.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011013-011013-6. doi:10.1115/1.4026626.

Hygroscopic and thermal expansion behavior of advanced polymers is investigated when subjected to combined high temperature and moisture conditions. An enhanced experimental–numerical hybrid procedure is proposed to overcome the limitations of the existing methods when used at temperatures above the water boiling temperature. The proposed procedure is implemented to measure the hygrothermal strains of three epoxy molding compounds and a no-filler underfill over a wide range of temperatures including temperatures beyond the water boiling temperature. The effects of moisture content on the glass transition temperature (Tg) and coefficient of thermal expansion (CTE) are evaluated from the measurement data. A formulation to predict the Tg change as a function of moisture content is also presented.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2014;136(1):011014-011014-5. doi:10.1115/1.4026639.

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.

Commentary by Dr. Valentin Fuster

Technology Review

J. Electron. Packag. 2014;136(1):014001-014001-9. doi:10.1115/1.4026615.

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.

Commentary by Dr. Valentin Fuster

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