Four challenges in thermal management in communication devices

Chip scale packages have seen a growth of over two times in the last two years and are expected to have more demands in the booming telecommunications market. This rapid growth of chip scale packages combined with green packaging solutions has made the implementation of lead free solders for chip scale package applications inevitable. But the effect of various lead free solders on the wetting behavior and their mechanical strength and the under bump metallization (UBM) integrity are also important to achieve a reliable chip scale package. This paper presents a methodology of implementation of lead free solder in the first and second level interconnects as a replacement for eutectic tin lead (SnPb) solder in chip scale packages developed for wireless applications. No clean solder paste made with alloys Sn/Ag (Supplier a), Sn/Ag/Cu (Supplier B), CASTIN (Supplier C) and SnBi (Supplier D) from various vendors have been evaluated for the implementation. The process evaluation conducted in this research study investigated the following aspects i) occurrence of voids ii) Effect of lead free solder on the UBM metallization, iii) wetting behavior of the lead (Pb) free solders, iv) thermal aging and shear strength of the, lead free solders in first and second level process and v) accelerated thermal cycle experiments on assembled lead free solder samples.

Three dimensional (3D) integration offers numerous electrical advantages like shorter interconnection distances between different dies in the stack, reduced signal delay, reduced interconnect power and design flexibilities. The main enabler of 3D integration is through-silicon-vias (TSVs) and stacking of multiple dies. Irrespective of these advantages, thermal management in 3D stacks poses significant challenges for the implementation of 3D integrated circuits. Even though extensive research work has been done in understanding the thermal management in two dimensional (2D) planar circuits for the past several decades, 3D integration offers a new set of challenges in terms of thermal management, which makes it difficult to readily apply the thermal management strategies available for 2D planar circuits. Over the past decade, some work has been done in thermal analysis and management of 3D stacks but still, knowledge is scattered and a comprehensive understanding is lacking. This research work focuses on bringing together the limited work on thermal analysis and thermal management in 3D vertically integrated circuits available in the literature. A compilation and analysis of the results from investigations on thermal management in 3D stacks is presented in this review with special emphasis on experimental studies conducted on different thermal management strategies. Furthermore, 3D integration technologies, thermal management challenges, and advanced 2D thermal management solutions are discussed.

Aircraft thermal management: Practices, technology, system architectures, future challenges, and opportunities

Despite its numerous advantages, the silicon-on-insulator (SOI) technology presents significant challenges in thermal management and exacerbates the local hotspot issue in microelectronic devices. The introduction of the buried oxide (BOX) layer hinders vertical heat dissipation, making conventional cooling solutions based on air and liquid coolers inefficient. To address this problem, we for the first time experimentally demonstrate the holey silicon-based lateral thermoelectric cooler (TEC) to provide thermal management for the SOI-based devices. By redistributing heat laterally, this lateral TEC overcomes the limitation of poor vertical heat dissipation caused by the BOX layer, meanwhile taking advantage of sustained temperature gradient from the hotspot. In our devices, vertical holey silicon fabricated by deep reactive ion etching (DRIE) is situated in the SOI device layer, acting as an efficient thermoelectric material compatible with conventional microfabrication processes. Neck size as small as 1.26 μm is created to enhance phonon boundary scattering, resulting in low in-plane thermal conductivity while retaining good electrical characteristics (i.e., electrical conductivity and Seebeck coefficient) from bulk silicon. The TEC electrodes (i.e., Peltier heater and cooler) deposited by e-beam evaporation align laterally with the holey silicon, enabling active heat removal in lateral direction. In addition to the TEC components, a coupled heating and temperature sensing system is implemented on the top surface using nickel thin-film structures with various dimensions. Two types of TEC designs are fabricated: asymmetric “S” type (3000 × 3000 μm2) for optimal temperature reduction and symmetric “R” type (2250 × 3000 μm2) for CMOS transistor-compatible design. All devices are generated in an SOI wafer with the following dimensions: device layer : oxide layer : handle layer = 25:5:500μm. Thermoelectrical measurement and the cross-checking IR thermography measurement are performed. While the majority of characterization is still working in progress, preliminary IR measurement results indicate large metal-silicon contact resistance is existed which hinders the TEC cooling. With further process control, optimal device demonstration and substantial cooling performance are expected in the future. In general, compared to the existing microscale TECs designed at a packaging level, our TEC allows for direct integration with the SOI-based architecture including power electronics, MEMS and microprocessors, which shows promise of a more compact and efficient cooling system for SOI-based devices.

Brief Historical Perspective in Thermal Management and the Shift Toward Management at the Nanoscale

Due to the toxicity of lead (Pb), Pb-containing solder alloys are being phased out from the electronics industry. This has lead to the development and implementation of lead-free solders. Being an environmentally compatible material, the lead-free Sn-3.0Ag-0.5Cu (wt.%) solder alloy is considered to be one of the most promising alternatives to replace the traditionally used Sn-Pb solders. This alloy composition possesses, however, some weaknesses, mainly as a result of its higher melting temperature compared with the Sn-Pb solders. A possible way to decrease the melting temperature of a solder alloy is to decrease the alloy particle size down to the nanometer range. The melting temperature of Sn-3.0Ag-0.5Cu lead-free solder alloy, both as bulk and nanoparticles, was investigated. The nanoparticles were manufactured using the self-developed consumable-electrode direct current arc (CDCA) technique. The melting temperature of the nanoparticles, with an average size of 30 nm, was found to be 213.9°C, which is approximately 10°C lower than that of the bulk alloy. The developed CDCA technique is therefore a promising method to manufacture nanometer-sized solder alloy particles with lower melting temperature compared with the bulk alloy.

Expansion in technology urges for better advancement, thus resulting in miniaturization of electronic products with rising concern for the reliability of electronic packaging material. Lead-free solder, being one of the most prominent alternatives in the electronic packaging industry, is constantly exposed to harsh conditions, which are especially exacerbated with smaller solder joints and a closer pitch. Hence, with the effort of attaining a more reliable solder alloy, research has been intensively executed to overcome the hurdle of maximizing the potential of SAC solders. The scope of the review thus focuses on identifying the aptitude of bismuth-doped SAC solders by analyzing their microstructure evolution in isothermal aging while understanding their thermal and mechanical stability in different fatigue tests. In the earlier days, Bismuth was found to realize a better melting point when interacting with the tin matrix due to its unique solid solution-strengthening mechanism. Bismuth-doped solders can also induce a more robust solder joint with smaller IMC particles and a thinner interfacial layer that enables significant improvement in fatigue resistance compared to traditional SAC alloys. Therefore, the review concludes that bismuth-doped SAC solder tends to outshine the conventional alternative as well as offering immense advancement in thermal and mechanical properties, portraying them as a potential alternative for the assembly of high-reliability electronic products, especially in industries with extreme conditions such as aviation, automotive, and military.

Purpose – The aim of the present study was to gather and review all the important properties of the Sn–Ag–Cu (SAC) solder alloy. The SAC solder alloy has been proposed as the alternative solder to overcome the environmental concern of lead (Pb) solder. Many researchers have studied the SAC solder alloy and found that the properties such as melting temperature, wettability, microstructure and interfacial, together with mechanical properties, are better for the SAC solder than the tin – lead (SnPb) solders. Meanwhile, addition of various elements and nanoparticles seems to produce enhancement on the prior bulk solder alloy as well. These benefits suggest that the SAC solder alloy could be the next alternative solder for the electronic packaging industry. Although many studies have been conducted for this particular solder alloy, a compilation of all these properties regarding the SAC solder alloy is still not available for a review to say. Design/methodology/approach – Soldering is identified as the metallurgical joining method in electronic packaging industry which uses filler metal, or well known as the solder, with a melting point < 425°C (Yoon et al., 2009; Ervina and Marini, 2012). The SAC solder has been developed by many methods and even alloying it with some elements to enhance its properties (Law et al., 2006; Tsao et al., 2010; Wang et al., 2002; Gain et al., 2011). The development toward miniaturization, meanwhile, requires much smaller solder joints and fine-pitch interconnections for microelectronic packaging in electronic devices which demand better solder joint reliability of SAC solder Although many studies have been done based on the SAC solder, a review based on the important characteristics and the fundamental factor involving the SAC solder is still not sufficient. Henceforth, this paper resolves in stating all its important properties based on the SAC solder including its alloying of elements and nanoparticles addition for further understanding. Findings – Various Pb-free solders have been studied and investigated to overcome the health and environmental concern of the SnPb solder. In terms of the melting temperature, the SAC solder seems to possess a high melting temperature of 227°C than the Pb solder SnPb. Here, the melting temperature of this solder falls within the range of the average reflow temperature in the electronic packaging industry and would not really affect the process of connection. A good amendment here is, this melting temperature can actually be reduced by adding some element such as titanium and zinc. The addition of these elements tends to decrease the melting temperature of the SAC solder alloy to about 3°C. Adding nanoparticles, meanwhile, tend to increase the melting temperature slightly; nonetheless, this increment was not seemed to damage other devices due to the very slight increment and no drastic changes in the solidification temperature. Henceforth, this paper reviews all the properties of the Pb-free SAC solder system by how it is developed from overcoming environmental problem to achieving and sustaining as the viable candidate in the electronic packaging industry. The Pb-free SAC solder can be the alternative to all drawbacks that the traditional SnPb solder possesses and also an upcoming new invention for the future needs. Although many studies have been done in this particular solder, not much information is gathered in a review to give better understanding for SAC solder alloy. In that, this paper reviews and gathers the importance of this SAC solder in the electronic packaging industry and provides information for better knowledge. Originality/value – This paper resolves in stating of all its important properties based on the SAC solder including its alloying of elements and nanoparticles addition for further understanding.

High current density (HCD) operation in proton exchange membrane fuel cells (PEMFCs) presents significant water and thermal management challenges, mainly due to excessive production and uneven distribution of water and heat. Constructing an in-plane temperature difference (TD) in the cathode to match the water–gas state variations is a promising approach. This study innovatively reconstructs the cathode liquid cooling channels of a hydrogen-air PEMFC to create an in-plane TD, experimentally investigating various designs across different inlet relative humidities (RH), with deeper mechanism analysis, including electrochemical impedance spectroscopy (EIS) and local current density distribution. Results show that a positive temperature difference (PTD) enhances HCD performance, improving voltage and current uniformity by 10% and 2% at 2100 mA/cm2, while the enhancement weakens with increasing inlet humidity. Reverse temperature differences (RTDs) persistently have an adverse effect, which worsens with higher difference amplitude and RH. Mechanism exploration for TDs reveals that PTD improves membrane hydration upstream and reduces flooding downstream under low humidity conditions while tending to cause flooding upstream under high humidity conditions. RTD causes drying upstream and exacerbates downstream flooding, which is more pronounced at higher humidity. This research provides novel insights into optimizing thermal management at HCDs in low-humidity environments like automotive applications and high-humidity environments.

Heat exchangers are essential components in a wide range of industrial applications, enabling effective thermal energy transfer between fluids. This article aims to clarify the fundamental principles of heat exchangers, emphasizing their design, operation, and diverse applications across various sectors. By simplifying complex concepts such as the effectiveness-NTU method, logarithmic mean temperature difference (LMTD), and the classifications of heat exchangers, this piece provides a thorough understanding tailored for engineering students and professionals. We estimate the comprehension of a student reading this article to be around 70-80%. The discussion is further enriched with practical examples and real-world case studies that illustrate the role of heat exchangers in industries including chemical processing, power generation, and HVAC systems. These examples serve to connect theoretical principles with practical applications, thereby enhancing comprehension among readers. The article also highlights the significance of heat exchangers in fostering energy efficiency and sustainability, addressing contemporary challenges in thermal management. This resource serves both as a foundational guide for students and a reference for educators and industry professionals, aiming to deepen the understanding of heat exchangers. It aspires to equip future engineers with the skills necessary to innovate and optimize thermal systems, ultimately contributing to advancements in engineering practices.

The paper introduces a microreactor with high thermal insulation properties, which has been developed for integration with standard planar-type solid oxide fuel cells (SOFCs) in portable power generation applications. While planar SOFCs offer high efficiency and energy density, their use has been largely limited to stationary applications due to challenges in thermal management and slow start-up times. Our microreactor overcomes these barriers by providing an effective thermal insulation system, allowing SOFCs to operate efficiently in a compact, portable format. We designed a cantilevered structure using yttria-stabilized zirconia (YSZ) to minimize thermal conduction and combined it with a multilayer insulation (MLI) system to suppress thermal radiation loss. This flexible cantilevered structure prevents cracking under thermal stress and maintains high temperatures up to 700 °C, ensuring reliable operation. Additionally, the MLI system features an inherent safety mechanism: when the insulation structure is damaged by a drill, the loss of thermal insulation causes a rapid temperature drop, bringing the system below the hydrogen explosion threshold temperature within 5 minutes, thus preventing potential hazards. Our prototype successfully demonstrated handheld power generation using a button-type metal-supported SOFC, achieving a rapid start-up time of just 5 minutes and driving a motor. This breakthrough offers a new platform for miniaturized SOFC technology, bridging the gap between stationary and portable energy solutions.

Due to the high-end integration, the new generation of advanced functional electronics will inevitably face challenges in thermal management. Metasurface, as a new unnatural periodic structure, can not only realize highly-effective electromagnetic wave (EM) manipulation owing to its extraordinary performance but also function in various complex environments by introducing advanced functional materials. In this work, we have proposed a polarization-multiplexed flexible metasurface absorber for microwave-infrared compatible stealth in high temperature conditions. To verify the characteristics of the proposed metasurface, the high-temperature polarization-multiplexed flexible absorber has been fabricated and measured, which is stable at temperatures up to 200 °C. For microwaves, the proposed metasurface is able to achieve individual perfect absorption at different frequencies for both x- and y-polarized incidence. For infrared, the introduction of thermal insulation materials, aerogel felt, and the patterned structure of graphene assembly film (GAF) with a large surface area filling ratio contribute to reducing the infrared emissivity to 0.40 in the range of 4-22 µm. This work proposes a polarization-multiplexed flexible absorber with excellent thermal stability and durable high-temperature stealth performance, showing great potential for thermal management applications in integrated electronic components operating under high-temperature environments.

Electronic packaging for automotive applications are at particular risk of thermomechanical failure due to the naturally harsh conditions it is exposed to. With the rise of electric and hybrid electric vehicles (EVs and HEVs), combined with a desire to miniaturize, the challenge of removing enough heat from electronic devices in automotive vehicles is evolving. This paper closely examines the new challenges in thermal management in various driving environments and aims to classify each existing cooling method in terms of performance. Particular focus is placed upon emerging solutions regarded to hold great potential, such as phase-change materials (PCMs). PCMs have been regarded for some time as a means of transferring heat quickly away from the region with the electronic components and are widely regarded as a possible means of carrying out cooling in large scale from small areas, because of their high latent heat of fusion, high specific heat, temperature stability, and small volume change during phase change, etc. They have already been utilized as a method of passive cooling in electronics in various ways, but their adoption in automotive power electronics, such as in traction inverters, has yet to be fulfilled. A brief discussion is made on some of the potential areas of application and challenges relating to more widespread adoption of PCMs, with reference to a case study using computational model of a commercially available power module used in automotive applications.

The rapid advancement of CPUs has introduced significant challenges in thermal management as higher computational power generates greater heat fluxes. Liquid cooling emerges as a superior alternative to traditional air cooling for dissipating heat in high-performance systems. This study employs Computational Fluid Dynamics (CFD) and conjugate heat transfer simulations to optimize the design of a dual-inlet heat sink with flat jets for a CPU rated with a thermal design power (TDP) of 100 W. The dual-inlet configuration enhances mixing, increasing heat transfer compared to single-inlet designs. An optimal jet-to-plenum width ratio of 0.2 and 0° inlet inclination were identified, achieving a thermal resistance as low as 0.07 cm2°C/W at a flow rate 1.11×10−4 m3/s. These findings offer actionable insights to advance thermal management for highperformance CPUs.

The continuous increase of the system power and the shrinking size of portables present increasing challenges in thermal management at component and system level. Among the various cooling techniques, heat pipe technology is emerging as a cost-effective thermal design solution due to its excellent heat transport efficiency and capability. In this paper, we start with the basics of heat pipes and review their performance, reliability and cost characteristics. The use of heat pipes in portables Is illustrated in the cooling of notebooks and subnotebooks using the recently introduced 75 MHz Pentium processor in Tape Carrier Package (TCP) with a design specification of approximately 6.5 W (depending on the system configuration) with Active Thermal Feedback (ATF). Tests were conducted using test boards and TCP thermal test dies in one reference notebook and sub-notebook. After the thermal characterization of TCP in the reference notebook and sub-notebook, three configurations using heat pipes were studied. In all three cases, results showed that the use of heat pipes enables the system to handle the 6.5 W of CPU power with 4 W of board power. Although the study is conducted in notebooks and subnotebooks, the analyses of heat pipes and their usage are also applicable to other portables.