Zhi Yang

With smaller and denser transistors, the physical flow of electrons may inhibit the performance of the device over time by forming voids and cracks at interconnects due to electromigration (EM). Circuit designs that fail to meet EM specifications may lead to catastrophic failures and SI/PI performance degradation. One way of mitigating EM is to use multiple vias between layers of copper traces to reduce the current crowding effect. However, the quantities of vias may affect the current density… Show more

With smaller and denser transistors, the physical flow of electrons may inhibit the performance of the device over time by forming voids and cracks at interconnects due to electromigration (EM). Circuit designs that fail to meet EM specifications may lead to catastrophic failures and SI/PI performance degradation. One way of mitigating EM is to use multiple vias between layers of copper traces to reduce the current crowding effect. However, the quantities of vias may affect the current density and current redistribution inside critical joints. Current studies mainly focus on predicting the EM time-to-failure (TTF) based on the empirical Black’s equation. However, this method may not give enough insights about void formation and crack propagation, and reflects the current redistribution that could impact the TTF. In this study, we compared the EM lifetime of ball grid array (BGA) test vehicles with different structural designs and developed a methodology to consider the diffusion of atoms in solder joints based on Multiphysics field migration to study the current redistribution influence of vias. Moreover, crack propagation was also simulated to understand the failure mechanism. BGA traces without vias and with 8 vias are stressed under 5A, 7A, and 9A at 150 degrees C to compare the EM performance. Moreover, each test structure is manufactured with two different surface finishes: A and B. Based on the experimental results, finite element analysis (FEA) simulations based on Atom Flux Divergence (AFD) were performed to compare with the experiment results. It was found that the current crowding effect could be significantly reduced by 8 vias Show less

In this article, we design, demonstrate, and characterize a 3-D printed package-level polymer jet impingement cooling solution on a 23×23 mm 2 thermal test chip. The experimental hardware results for a nozzle pitch of 2 mm show that, with 1-kW power dissipation, at a coolant (deionized (DI) water) flow rate of 3 liters per minute (LPM), the measured average chip temperature increase is ~65 °C with a cooler pressure drop of 0.15 bar between the inlet and outlet connections. It is also shown that… Show more

In this article, we design, demonstrate, and characterize a 3-D printed package-level polymer jet impingement cooling solution on a 23×23 mm 2 thermal test chip. The experimental hardware results for a nozzle pitch of 2 mm show that, with 1-kW power dissipation, at a coolant (deionized (DI) water) flow rate of 3 liters per minute (LPM), the measured average chip temperature increase is ~65 °C with a cooler pressure drop of 0.15 bar between the inlet and outlet connections. It is also shown that bare die cooling without lid [and thermal interface material (TIM)] shows better cooling performance than the lidded package. Second, an advanced 3-D printed manifold with an additional flow redistribution structure is demonstrated. The experimental results show that the improved design achieves a better chip temperature uniformity compared to the reference design, showing a reduction of the chip temperature gradient with a factor of 4 and 2.3 for a flow rate of 0.5 and 3 LPM, respectively, while no significant impact on the cooler pressure drop was measured. The numerical modeling studies predict an additional 15.4% thermal performance improvement, by reducing the nozzle pitch from 2 to 1 mm, for a flow rate of 3 LPM. Show less

Laminates or sequential build-up (SBU) laminate comprised of dielectric materials, metal traces, and metal vias not only serve as the mechanical support for the silicon integrated circuits (ICs), but also electrically connect ICs to ball grid array (BGA); by way of an embedded power delivery structure. Electrical current required to power the ICs is carried through spatially distributed metal traces and vias. The non-uniformity in the power distribution may induce hotspots due to parasitic… Show more

Laminates or sequential build-up (SBU) laminate comprised of dielectric materials, metal traces, and metal vias not only serve as the mechanical support for the silicon integrated circuits (ICs), but also electrically connect ICs to ball grid array (BGA); by way of an embedded power delivery structure. Electrical current required to power the ICs is carried through spatially distributed metal traces and vias. The non-uniformity in the power distribution may induce hotspots due to parasitic Ohmic heating; especially in regions with high current density. Electrical packaging engineers need effective tools to identify, quantify, and mitigate hot spots in the laminate. Thermoelectrical multiphysical simulation provides a robust platform integrating the electrical, and thermal analyses for the study of joule heating in a complex design. Conventional simulations simplify detailed laminate wiring layout as a single planar with effective orthogonal material properties. Such simplification provides a solution to the inherent simulation challenges encountered with a complex design (i.e. tiny characteristic lengths, high aspect ratios, excessive computational time and resources). However, simplification comes with a price. Information required to optimize the detail trace and via wiring physical design is unavailable in a solution incorporating an effective laminate; laminate joule heating as well as non-uniform trace wiring are left out. The laminate temperature profile is averaged based on effective material properties. Without accurate joule heating evaluation, the hotspots cannot be identified or quantified. Overheating inside the laminate compromises signal speed and integrity, raises reliability concerns, and may even trigger catastrophic damage of dielectric material breakdown.

This work introduces an iterative approach integrating the detailed laminate electrical computer aided design (ECAD) and package design to simulate the joule heating with minimum simplification. Show less

Phase change materials (PCMs) possessing ideal properties, such as superior mass specific heat of fusion, low cost, light weight, excellent thermal stability as well as isothermal phase change behavior, have drawn considerable attention for thermal management systems. Currently, the low thermal conductivity of PCMs (usually less than 1 W mK−1) greatly limits their heat dissipation performance in thermal management applications. Hexagonal boron nitride (h-BN) is a two-dimensional material known… Show more

Phase change materials (PCMs) possessing ideal properties, such as superior mass specific heat of fusion, low cost, light weight, excellent thermal stability as well as isothermal phase change behavior, have drawn considerable attention for thermal management systems. Currently, the low thermal conductivity of PCMs (usually less than 1 W mK−1) greatly limits their heat dissipation performance in thermal management applications. Hexagonal boron nitride (h-BN) is a two-dimensional material known for its excellent thermally conductive and electrically insulating properties, which make it a promising candidate to be used in electronic systems for thermal management. In this work, a composite, consisting of h-BN nanosheets (BNNSs) and commercialized paraffin wax was developed, which inherits high thermally conductive and electrically insulating properties from BNNSs and substantial heat of fusion from paraffin wax. With the help of BNNSs, the thermal conductivity of wax–BNNS composites reaches 3.47 W mK−1, which exhibits a 12-time enhancement compared to that of pristine wax (0.29 W mK−1). Moreover, an 11.3–13.3 MV m−1 breakdown voltage of wax–BNNS composites was achieved, which shows further improved electrical insulating properties. Simultaneously enhanced thermally conductive and electrically insulating properties of wax–BNNS composites demonstrate their promising application for thermal management in electronic systems. Show less

Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectronic, telecommunications, and military devices. However, studies of broadband transparent conductors and their controlled modulation are scarce. In this study, we report that reversible transmittance modulation has been achieved with sandwiched nanocarbon thin films (containing carbon nanotubes (CNTs) and reduced graphene oxide (rGO)) via electrochemical… Show more

Optical transparent and electrical conducting materials with broadband transmission are important for many applications in optoelectronic, telecommunications, and military devices. However, studies of broadband transparent conductors and their controlled modulation are scarce. In this study, we report that reversible transmittance modulation has been achieved with sandwiched nanocarbon thin films (containing carbon nanotubes (CNTs) and reduced graphene oxide (rGO)) via electrochemical alkali-ion intercalation/deintercalation. The transmittance modulation covers a broad range from the visible (450 nm) to the infrared (5 μm), which can be achieved only by rGO rather than pristine graphene films. The large broadband transmittance modulation is understood with DFT calculations, which suggest a decrease in interband transitions in the visible range as well as a reduced reflection in the IR range upon intercalation. We find that a larger interlayer distance in few-layer rGO results in a significant increase in transparency in the infrared region of the spectrum, in agreement with experimental results. Furthermore, a reduced plasma frequency in rGO compared to few-layer graphene is also important to understand the experimental results for broadband transparency in rGO. The broadband transmittance modulation of the CNT/rGO/CNT systems can potentially lead to electrochromic and thermal camouflage applications. Show less