Showing papers in "Journal of Electronic Packaging in 2022"

Shear and Fatigue Properties of Lead-Free Solder Joints: Modeling and Microstructure Analysis

Abstract: The reliability of SAC-based solder alloys has been extensively investigated after the prohibition of lead in the electronics industry owing to their toxicity. Low-temperature solder (LTS) alloys have recently received considerable attention because of their low cost and reduced defects in complex assemblies. The shear and fatigue properties of individual solder joints were tested using an Instron micromechanical testing system in this research. Two novel solder alloys (Sn-58Bi-0.5Sb-0.15Ni and Sn-42Bi) with low melting temperatures were examined and compared with Sn-3.5Ag and Sn-3.0Ag-0.8Cu-3.0Bi. The surface finish was electroless nickel-immersion gold (ENIG) during the test. Shear testing was conducted at three strain rates, and the shear strength of each solder alloy was measured. A constant strain rate was used for the cyclic fatigue experiments. The fatigue life of each alloy was determined for various stress amplitudes. The failure mechanism in shear and fatigue tests were characterized using scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS). The results revealed that Sn-3.0Ag-0.8Cu-3.0Bi had superior shear and fatigue properties compared to other alloys, but was more susceptible to brittle failure. The shear strain rate affected the failure modes of Sn-3.0Ag-0.8Cu-3.0Bi, Sn-58Bi-0.5Sb-0.15Ni, and Sn-42Bi; however, Sn-3.5Ag was found to be insensitive. Several failure modes were detected for Sn-3.5Ag in both shear strength and fatigue tests.

Sensitivity Coefficient-Based Inverse Heat Conduction Method for Identifying Hot Spots in Electronics Packages: A Comparison of Grid-Refinement Methods

Abstract: Estimating the distribution and magnitude of heat generation within electronics packages is pivotal for thermal packaging design and active thermal management systems. Inverse heat conduction methods can provide estimates using measured temperature profiles acquired using infrared imaging or discrete temperature sensors. However, if the heater locations are unknown, applying a fine grid of potential heater locations across the surface where heat generation is expected can result in prohibitively large computation times. In contrast, using a more computationally efficient coarse grid can reduce the accuracy of heat flux estimations. This paper evaluates two methods for reducing computation time using a sensitivity-coefficient method for solving the inverse heat conduction problem (IHCP). One strategy uses a coarse grid that is refined near the hot spots, while the other uses a fine grid but only considers heater locations near the hot spots. These methods are compared using input temperature maps acquired from a “numerical experiment,” where the outputs of a three-dimensional (3D) steady-state thermal model in FloTHERM are used for input temperatures, and temperature maps procured using infrared microscopy on a real electronics package, using sensitivity coefficients calculated with the FloTHERM model. Compared to the coarse-grid method, the fine-grid method is found to reduce computation time without significantly reducing accuracy, making it more convenient for designing and testing electronics packages. It also avoids the problem of “false hot spots” that occurs with the coarse-grid method. Overall, this approach provides a mechanism to predict hot spot locations during design and testing and a tool for active thermal management.

Thermal spreading resistance of GaN HEMTs with heat source heating studied by hybrid Monte Carlo-diffusion simulations

Abstract: Exact assessment of thermal spreading resistance is of great importance to the thermal management of electronic devices, especially when completely considering the heat conduction process from the nanoscale heat source to the macro-scopic scale heat sink. The existing simulation methods are either based on con-vectional Fourier’s law or limited to small system sizes, making it difficult to accurately and efficiently study the cross-scale heat transfer. In this paper, a hybrid phonon Monte Carlo-diffusion method that couples phonon Monte Carlo (MC) method with Fourier’s law by dividing the computational domain is adopted to an-alyze thermal spreading resistance in ballistic-diffusive regime. Compared with phonon MC simulation, the junction temperature of the hybrid method has the same precision, while the time costs could be reduced up to 2 orders of magnitude at most. Furthermore, the simulation results indicate that the heating scheme has a remarkable impact on phonon transport. The thermal resistance of the heat source (HS) scheme can be larger than that of the heat flux (HF) scheme, which is opposite from the prediction of Fourier’s law. In the HS scheme, the enhanced phonon-boundary scattering counteracts the broadening of the heat source, leading to a stronger ballistic effect as the heat source thickness decreases. The conclusion is verified by a one-dimensional thermal resistance model. This work has opened up an opportunity for the fast and extensive thermal modeling of cross-scale heat transfer in electronic devices and highlighted the influence of heating schemes.

Process Development for Printed Copper with Surface Mount Devices On Inkjet Metallization

Abstract: Printed electronics is a fastest growing and emerging technology that have shown much potential in several industries including automotive, wearables, healthcare, and aerospace. Its applications can be found not only in flexible but also in large area electronics. Inkjet technology has gained much attention due to its low-cost, low material consumption, and capability for mass manufacturing. The preferred conductive metal of choice has been mostly silver due to its excellent electrical properties and ease in sintering. However, silver comes to be expensive than its counterpart copper. Since copper is prone to oxidation, much focus has been given towards photonic sintering that involves sudden burst of pulsed light at certain energy to sinter the copper Nanoparticles. With this technique, only the printed material gets sintered in a matter of seconds without having a great impact on its substrate. With all the knowledge, there is still a large gap in the process side with copper where it is important to look how the print process affects the electrical and mechanical properties of copper. With the process developed, the resistivity of printed copper was found to be 5 times the bulk copper. In regards to adhesion to the polyimide film, mechanical shear load to failure was found to be within 15-20 gF. To demonstrate the complete process, commercial-off-the-shelf components are also mounted on the additively printed pads. Statistically, control charting technique is implemented to understand any process variation over long duration of prints.

Hybrid Monte Carlo-Diffusion Studies of Modeling Self-Heating in Ballistic-Diffusive Regime for GaN HEMTs

Abstract: Exact assessment of self-heating is of great importance to the thermal management of electronic devices, especially when completely considering the cross-scale heat conduction process. The existing simulation methods are either based on convectional Fourier's law or limited to small system sizes, making it difficult to deal with non-continuum thermal transport efficiently. In this paper, a hybrid phonon Monte Carlo-diffusion method is adopted to predict device temperature in ballistic-diffusive regime. Heat conduction around the heat generation region and boundaries are simulated by phonon Monte Carlo (MC) method, while the other domain is by Fourier's law. The temperature of the hybrid method is higher than that of Fourier's law owing to phonon ballistic transport, and the calculation efficiency of the hybrid method is remarkably improved compared with phonon MC simulation. Furthermore, the simulation results indicate that the way of modeling self-heating has a remarkable impact on phonon transport. The junction temperature of the heat source (HS) scheme can be larger than that of the heat flux (HF) scheme, which is opposite to the result under Fourier's law. In the HS scheme, the enhanced phonon-boundary scattering counteracts the broadening of the heat source, leading to a stronger ballistic effect and higher temperatures. The conclusion is verified by a one-dimensional analytical model. This work has opened up an opportunity for the fast and extensive thermal simulations of cross-scale heat transfer in electronic devices and highlighted the influence of heating schemes.

A Reduced Model Based On Proper Generalized Decomposition for the Fast Analysis of Igbt Power Modules Lifetime

Abstract: A reduced weakly-coupled thermo-mechanical model based on the Proper Generalized Decomposition method was developed for the numerical analysis of power modules. The employed model reduction method enabled to obtain, in a preliminary offline phase, the solution of the thermo-mechanical problem over a large range of design parameters, with much time saving compared to a classical (brute force) multi-resolution finite element method. In an online post-processing phase, the power module lifetime, modeled with a strain-life law, was then computed in a straightforward manner by rapidly evaluating the solution for any value of the parameters. Sensitivity analysis was conducted to select parameters values leading to acceptable module lifetimes with respect to given criteria. A robust design study was also performed to illustrate the performance of the proposed approach.

Research on Reliability of Ni/Sn/Cu(Ni) Copper Pillar Bump Under Thermoelectric Loading

Abstract: With the development of packaging devices towards high performance and high density, electronic devices are subjected to thermos-electric stresses under service conditions, which has become a particularly important reliability problem in microelectronics packaging. The reliability of the chip under thermo-electric stresses is studied in this paper. Firstly, thermo-electric coupling experiments were carried out on two solder joint structures of Ni/Sn3.5Ag/Cu and Ni/Sn3.5Ag/Ni. The interface evolution of solder joints under different current densities was analyzed. The reliability of the two structures under thermo-electric stresses was compared and analyzed. After that, three-dimensional finite element analysis was employed to simulate the current density, Joule heat, and temperature distribution of the flip chip. Finally, through the combination of experiment and simulation, the distribution of Joule heat and temperature of the chip was analyzed. The results show that the Ni/Sn3.5Ag/Ni structure has better reliability than the Ni/Sn3.5Ag/Cu structure under thermal-electric coupling. In addition, when the Ni layer was used as the cathode side, and the current density was higher than 5×104A/cm2, the dissolution failure of the Ni layer occurred in two structures. Because the higher current density generated a large amount of Joule heat where the current was crowded, resulting in excessively high temperature and rapid dissolution of the Ni barrier layer.

Assessing the SAC305 Solder Joint Fatigue in BGA Assembly Using Strain-Controlled and Stress-Controlled Approaches

Abstract: One of the crucial factors in determining the reliability of an electronic device is fatigue failure of the interconnecting solder joints. In most cases, large bulk samples are used to study the fatigue characteristics of the solder materials. Real solder joints often encountered in ball grid array (BGA) components have only been considered in limited investigations. In this study, a specialized sandwich BGA test vehicle with a 3×3 solder joint was connected to the two substrates. The alloys were tested at room temperature using an Instron micromechanical tester in both the stress-controlled and strain-controlled methods. The tests were performed at a constant strain rate. Four stresses and four strain levels of the solder alloy Sn-3.0Ag-0.5Cu (SAC305) were examined using organic solderability preservative (OSP) and electroless nickel-immersion silver (ENIG) surface finishes. The work per cycle and plastic strain range were computed based on a systematic recording of the stress-strain (hysteresis) loops of each sample. A novel approach based on inelastic work is developed to calculate the fatigue life of a BGA assembled test vehicle. The results of the stress-controlled and strain-controlled tests indicated that the OSP surface finish outperformed the ENIG surface finish. Regardless of the testing process and surface finish, the Coffin-Manson and Morrow energy models were acceptable for SAC305.

A New Structure of Ceramic Substrate to Reduce the Critical Electric Field in High Voltage Power Modules

Abstract: The blocking voltage level of silicon carbide (SiC) can reach 10 to 25 kV, which will significantly increase the power density and capacity of power modules. However, high voltage can induce high electric field, increase the risk of partial discharge (PD) and threaten the insulation reliability. This paper focuses on the triple points between the metal electrode, silicone gel, and ceramic in power modules. The influencing factors of electric field at different triple points are fully analyzed. PD experiments are performed and the results show that interface between silicone gel and ceramic is a weak area of insulation. Therefore, this paper demonstrates that area of weak insulation and high electric field meet at the triple point. To solve this problem, a new structure of ceramic substrate is proposed, which isolates the interface area from high electric field. At the same time, the new structure can significantly reduce the high electric field reinforcement.

Electromigration Analysis of Solder Joints for Power Modules Using an Electrical-Thermal-Stress-Atomic Coupled Model

Abstract: Abstract The larger current densities accompanying increased output of power modules are expected to degrade solder joints by electromigration. Although previous research has experimentally studied electromigration in solder, die-attach solder joints in Si-based power modules have not been studied because the average current density of the die-attach solder is much smaller than the threshold of electromigration degradation. However, in die-attach solder, the electromigration degradation may appear where current crowding occurs. This report describes electromigration analysis of die-attach solder joints for Si-based power modules using an electrical-thermal-stress-atomic coupled model. First, we validate our numerical implementation and show that it can reproduce the distributions of vacancy concentrations and hydrostatic stress almost the same as the analytical solutions even at current densities assuming current crowding. We then simulate the die-attach solder joint with an Si-based power device and a substrate. Due to current crowding, the current density at the edge of the solder exceeds the electromigration threshold. Unlike general electromigration phenomena, the vacancy concentration increases at the center and decreases at the edges of the solder joint, regardless of whether it is on the cathode side or anode side, due to the longitudinal driving force in the solder joint generated by the current crowding. Creep strain increased remarkably at the anode edge and the cathode center. The absolute vacancy concentration clearly increased with increasing current density and size ratio. Creep strain significantly increased with increasing current density, size ratio, and temperature.

Heat Transfer Analysis of Supercritical Methane in Microchannels with Different Geometric Configurations On High Power Electromechanical Actuator

Abstract: The issue of regenerative cooling is one of the most important key technologies of flight vehicles, which is applied into both the engine and high-power electrical equipment. One pattern of regenerative cooling channels is the microchannel heat sinks, which are thought as a prospective means of improving heat removal capacities on electrical equipment of smaller sizes. In this paper, three numerical models with different geometric configurations, namely straight, zigzag, and sinusoid respectively, are built to probe into the thermal hydraulic performance while heat transfer mechanism of supercritical methane in microchannel heat sinks for the heat removal of high-power electromechanical actuator is also explored. In addition, some crucial influence factors on heat transfer such as inlet Reynolds number, operating pressure and heating power are investigated. The calculation results imply the positive effect of wavy configurations on heat transfer and confirm the important effect of buoyancy force of supercritical methane in channels. The heat sinks with wavy channel show obvious advantages on comprehensive thermal performance including overall thermal performance parameter ? and thermal resistance R compared with that of the straight one. The highest Nu and average heat transfer coefficient am appear in the heat sink with zigzag channels, but the pumping power of the heat sink with sinusoidal channels is lower due to the smaller flow loss.

Numerical Modeling of the Wave Soldering Process and Experimental Validation

Abstract: One of the most important procedures in the electronics industry is the assembly of electronic components onto printed circuit boards (PCBs) through the soldering process. Among the various soldering methods available, wave soldering is a very effective technique. In this process, the components are placed onto the PCB, which, subsequently, is coated with a flux and then passed across a preheat zone. In the end, the assembly is moved by the conveyor and passed over the surface of the molten solder wave in order to create a reliable connection both mechanically and electrically. Although this process has been frequently used, there are soldering defects that remain unsolved and continue to emerge, such as the missing of surface-mount components in the PCB after the soldering process. Aiming to understand if such defects are related to the force exerted by the solder wave in the PCB, a numerical and experimental study was performed in this article. For this purpose, a computational fluid dynamic model was developed by using the fluent software to describe the interaction between the solder jet and the PCB with the integrated circuits, and the multiphase method, volume of fluid, was also applied to track the solder–air interface boundary. The results obtained were numerically validated by using an experimental setup designed and built to this end. In general, the data obtained showed to be in good agreement and it was concluded that the force exerted by the solder wave is approximately 0.02 N.

Using a Multi-Inlet/Outlet Manifold to Improve Heat Transfer and Flow Distribution of a Pin Fin Heat Sink

Abstract: The increased power consumption and continued miniaturization of high-powered electronic components has presented many challenges to their thermal management. To improve the efficiency and reliability of these devices, the high amount of heat that they generate must be properly removed. In this paper, a three-dimensional numerical model has been developed and experimentally validated for several manifold heat sink designs. The goal was to enhance the heat sink's thermal performance while reducing the required pumping power by lowering the pressure drop across the heat sink. The considered designs were benchmarked to a commercially available heat sink in terms of their thermal and hydraulic performances. The proposed manifolds were designed to distribute fluid through alternating inlet and outlet branched internal channels. It was found that using the manifold design with 3 channels reduced the thermal resistance from 0.061 to 0.054 °C/W with a pressure drop reduction of 0.77 kPa from the commercial cold plate. A geometric parametric study was performed to investigate the effect of the manifold's internal channels width on the thermohydraulic performance of the proposed designs. It was found that the thermal resistance decreased as the manifold's channel width decreased, up until a certain width value, below which the thermal resistance started to increase while maintaining low pressure drop values. Where the thermal resistance significantly decreased in the 7 channels design by 16.4% and maintained a lower pressure drop value below 0.6 kpa.

Multi-physics Models of a Low-voltage Power Semiconductor System-in-package for Automotive Applications

Abstract: Power converters and semiconductor devices are spreading their application fields, due to new renewable energy and automotive frameworks. In the electrified vehicles context, the even more stringent requirements, both in terms of performances and reliability, pose new challenges in the design phase of power switches. This paper analyzes, by means of finite-element simulations, a low-voltage power semiconductor system-in-package devoted to automotive applications, which integrates a MOSFET-based half bridge and a controller. Three simulation physical domains integrated in a unique flow are considered: thermo-mechanical, electromagnetic, and thermal numerical models. The aim is to develop a new comprehensive methodology which starts with a thermo-structural simulation of the package, then computes the on-state resistance and parasitic components to assess the electrical behavior of the package. Finally, a simulation check is made to verify if the power device performances are thermally consistent with applicative conditions.

Thermo-Mechanical Topology Optimization of 3D Heat Guiding Structures for Electronics Packaging

Abstract: Heterogeneous and complex electronic packages may require unique thermo-mechanical structures to provide optimal heat guiding. In particular, when a heat source and a heat sink are not aligned, conventional thermal management methods providing uniform heat dissipation may not be appropriate. Here we present a topology optimization method to find thermally conductive and mechanically stable structures for optimal heat guiding under various heat source-sink arrangements. To exploit the capabilities, we consider complex heat guiding scenarios and 3D serpentine structures to carry the heat with corner angles ranging from 30° to 90°. While the thermal objective function is defined to minimize the temperature gradient, the mechanical objective function is defined to maximize the stiffness with a volume constraint. Our simulations show that the optimized structures can have a thermal resistance of less than 32% and stiffness greater than 43% compared to reference structures with no topology optimization at an identical volume fraction. The significant difference in thermal resistance is attributed to a thermally dead volume near the sharp corners. As a proof-of-concept experiment, we have created 3D heat guiding structures using a selective laser melting technique and characterized their thermal properties using an infrared thermography technique. The experiment shows the thermal resistance of the thermally optimized structure is 29% less than that of the reference structure. These results present unique capabilities of topology optimization and 3D manufacturing in enabling optimal heat guiding for heterogeneous systems and advancing the state-of-the-art in electronics packaging.

Stochastic Finite Element Thermal Analysis of a Ball Grid Array Package

Abstract: This paper presents a straight-forward finite element approach for the quantification of electronic package thermal performance under uncertainty. The method makes use of high accuracy sensitivity calculations and a gradient-based minimization method. The approach was applied to the thermal analysis of a ball grid array (BGA) package under uncertainty to illustrate its capabilities. The effect of uncertainty in the heat source, heat transfer coefficient, ambient temperature, and thermal conductivities of the component materials on the probability of exceeding a specified average junction temperature at the die-heat-spreader interface was studied. In addition, the performance and accuracy of two different methods for computing the required sensitivities were compared. Results showed that the average junction temperature probability was more sensitive to some system parameters over others, providing crucial information for selecting the manufacturing tolerance of BGA package components. For parameters identified as especially sensitive, selecting components with tighter tolerances will reduce uncertainty and increase the overall reliability. And for less sensitive parameters, selecting larger tolerance could help reduce manufacturing costs.

Evolution of Anand Parameters for Thermally Aged SAC Leadfree Alloys at Low Operating Temperature

Abstract: Abstract In extreme environmental applications, such as aerospace and automotive, electronics may endure high or low operating temperatures during service, handling, and storage. An electronic assembly may experience strain rates of 1–100 per sec of strain and ambient temperatures of –65 to +200 °C. Electronic assembly's temperature depends mainly on location, energy dissipation, and thermal architecture. Electronic assemblies in automotive applications may be located underhood, on engine, on-transmission, and or in wheel well. Study of property evolution of solders used for interconnection is important for assurance of reliability. Degradation in material properties for lead-free solder alloys can be caused by change in microstructure due to variation in temperatures. There is need for data on the effect of operating period and operating conditions on the material properties. Addition of dopants in Sn-Ag-Cu (SAC) alloys has been shown to improve mechanical properties and minimize deterioration due to aging at lower strain rates. SAC-Q is formulated with Sn-Ag-Cu with addition of Bi (SAC+Bi). It has been observed that adding Bismuth (Bi) to SAC alloy can play an important role to make the solder alloy resistant to aging-induced degradations. In author's prior research the evolution of Anand parameters and materials properties for SAC solder (SAC105, SAC305, and SAC-Q) at high temperatures and high strain rates has been studied. However, data on thermally aged SAC solder alloys at high strain rate levels at low operating temperatures are not available in published literature. In this paper, materials characterization of thermally aged SAC (SAC105 and SAC-Q) solder at low operating temperatures (−65 °C to 0 °C) and at high strain rates (10–75 per sec) has been studied. Stress–strain curves have been measured at low operating temperatures using impact hammer-based tensile tests with cooling chamber. The fabricated SAC lead-free solder specimen was isothermally aged up to 6 months at 50 °C before testing. Anand viscoplastic model has been used to compute nine Anand parameters to describe the material constitutive behavior. Anand Model parameters evolution due to thermal aging has been studied for SAC solders. The computed nine Anand parameters from experimental data then were used to simulate the tensile test to predict the stress–strain curve and compared to experimental stress–strain curves to verify the accuracy of the model. A good correlation was found between experimental data and Anand-predicted data.

Experimental and Numerical Analysis of Data Center Pressure and Flow Fields Induced by Backward and Forward Crah Technology

Abstract: An increasingly common power saving practice in data center thermal management is to swap out air cooling unit blower fans with electronically commutated plug fans, the side effects of which are not fully understood. Therefore, it has become necessary to develop an overall understanding of backward curved blowers and compare the resulting flow, pressure, and temperature fields with forward curved ones in which the induced fields are characterized, compared and visualized in a reference data center. In this study, experimental and numerical characterization of backward curved blowers is introduced. Then, a physics-based Computational Fluid Dynamics model is built using the 6SigmaRoomTM tool to predict/simulate the measured fields. The parametric and sensitivity of the baseline modeling are investigated and considered. Different operating conditions are applied at the room level for experimental characterization, comparison, and illustration of the interaction between different CRAH technologies. The measured data is plotted and compared with the CFD model assessment to visualize the fields of interest. The results show that the fields are highly dependent on CRAH technology. The tile to CRAH airflow ratios for the flow constraints of scenarios 1, 2, 3, and 4 are 85.5%, 83.9%, 61%, and 59%, respectively. The corresponding leakage ratios are 14.5%, 16%, 38.9%, and 41%, respectively. Furthermore, the validated CFD model was used to investigate and compare the airflow pattern and plenum pressure distribution. Lastly, it is notable that a potential side effect of backward curved technology is the creation of airflow dead zone.

Hfe7500 Coolant Dielectric Strength Augmentation Under Convective Conditions

Abstract: Power densification and rising module heat losses cannot be managed by traditional "external-to-case" cooling solutions. This is especially pronounced in high voltage systems, where intervening layers of insulating material between the power devices and cooling solution need to be sufficiently thick to provide adequate voltage isolation. As operating voltages increase, the required thicknesses for these insulating layers become so large that they limit the ability to extract the heat. A direct cooling approach that addresses voltage separation issues represents a unique opportunity to deliver coolant to the hottest regions, while opening up the opportunity for increased scaling of power electronics modules. However technical concerns about long-term performance of coolants and their voltage isolation characteristics coupled with integration challenges impede adoption. Here, the reliability and performance of a dielectric fluid of the hydrofluoroether type, HFE7500, are examined to advance the feasibility of a direct cooling approach for improved thermal management of high-voltage, high-power module. The breakdown voltage of the dielectric fluid is characterized through relevant temperatures, flow rates, and electric fields with the ultimate goal of developing design rules for direct integrated cooling schemes.

High Temperature Degradation Modes Observed in Gallium Nitride-Based Hall-Effect Sensors

Abstract: Abstract Magnetic field sensors based on the Hall-effect have a variety of applications such as current sensing in power electronics and position and velocity sensing in vehicles. Additionally, they have benefits such as easy integration into circuits, low manufacturing cost, and linearity over a wide range of magnetic fields. However, in order to use these devices in an industrial or automotive setting, the effect of high temperatures on the reliability of the Hall-effect sensors needs to be evaluated. This study focuses on the effect of high temperature on the electrical and material properties of novel gallium nitride (GaN)-based Hall-effect sensors and the impacts on the reliability of these devices. Changes in device properties such as resistance and electrical response, as well as on the metallic contacts, are examined, using two sets of devices made with different substrates and contact metals. A probe station is used to characterize electrical responses, while an X-ray photo-electron spectrometer (XPS) and energy-dispersive X-ray (EDX) are used to characterize material interactions. The findings include saturation curves, the presence of gallium on the contacts of the octagonal device, and the activation energy of reaction responsible for resistance increase for the octagonal AlGaN/GaN devices. Additionally, the Greek cross AlGaN/GaN Hall sensors showed excellent thermal stability.