Jet Dispensing of Liquid Metal as a Thermal Interface Material

Gallium-based liquid metals (LMs) are materials that possess some unique properties. Just like any other metals, they have high thermal conductivity and low interfacial resistance, but they are in a liquid phase at room temperature. In contrast to mercury alloys, gallium alloys are non-toxic. They don’t evaporate, and they can’t be inhaled. The viscosity of those alloys is very similar to water, but they are six times as dense as water. All those properties make gallium-based liquid metals very good candidates for thermal interface material (TIM) in electronics applications. On the other hand, the reaction and incompatibility of those alloys with some metals is one of the challenges for this type of TIM. The other challenge is that those materials are not just thermally, but also electrically conductive, and that is not a desirable property for TIM. A suitable barrier that will prevent any leakage of LMs and the best way to apply the appropriate volume of LM in high-volume production (HVP) would be one of the most important things for any application. A key challenge is applying the liquid metal consistently through a traditional dispensing method due to its property and behavior which involves high surface tension. Through advanced dispensing techniques like jetting technology, liquid metal can be applied reliably on a flat, uneven surface or in arrays of minuscule confined spaces or cavities. This paper highlights the dispense quality, weight repeatability from one substrate to another, and valve hardware stability. This paper also addresses the challenges faced during dispensing of liquid metal in high volume manufacturing, and how to achieve desired bondline thickness with jet dispensing for higher throughput and process reliability.

Gallium alloys have some unique physical properties and attributes. Gallium alloys melt near or below room temperature, making them liquid metals. These liquid metals are non-toxic and have a very low vapor pressure. This liquid metal “softness” opens up an entire arena of applications to interface with human skin. In addition, when exposed to air the liquid gallium alloy forms an oxide skin, enabling printing and writing with this liquid metal. A review of these and other properties and potential applications of liquid metal gallium alloys will be the focus of this paper. Gallium-based liquid metals are not new to the world of thermal interface materials (TIMs). However, their purpose has primarily served overclocking gamers. As thermal demands continue to grow for high-performance computing chips, traditional polymer-based TIMs like greases and phase change materials are reaching their limits for use. Their relatively low thermal conductivity means they are not able to dissipate enough heat for these emerging applications. This has generated a renewed interest in gallium-based liquid metals. The application of gallium based liquid metals TIMs will be discussed in the appendix.

Thermal interface materials (TIMs), as typical thermal functional materials, are highly required to possess both high thermal conductivity and low Young's modulus. However, the naturally synchronized change in the thermal and mechanical properties seriously hinders the development of high-performance TIMs. To tackle such a dilemma, a strategy of codoping solid fillers and liquid metal fillers into polymer substrates is proposed in this study. This strategy includes a large amount of liquid metals that play the role of thermal paths and a small amount of uniformly dispersed solid fillers that further enhance heat conduction. Through the synergistic effect of the liquid metal and solid fillers, the thermal conductivity can be improved, and Young's modulus can be kept small simultaneously. A typical TIM with a volume of 55% gallium-based liquid metal and 15% copper particles as fillers has a thermal conductivity of 3.94 W/(m·K) and a Young's modulus of 699 kPa, which had the maximum thermomechanical performance coefficient compared with liquid metal TIMs and solid filler-doped TIMs. In addition, the thermal conductivity of the solid-liquid metal codoped TIM increased sharply with an increase of liquid metal content, and Young's modulus increased rapidly with an increase of the volume ratio of copper and polymer. The high-low-temperature cycling test and large-size light-emitting diode (LED) application demonstrated that this TIM had stable physical performance. The synergistic effect of the solid fillers and liquid metal fillers provides a broad space to solve the classic tradeoff issue of the mechanical and thermal properties of composites.

Metals have long been used as thermal interface materials (TIMs). Due to their high reliability and thermal conductivity, metal TIMs are excellent solutions for heat dissipation in electronic systems, especially for more challenging applications. Thermal conductivity and interfacial resistance are the most important properties of TIM. One of the biggest obstacles for using metal TIMs is interfacial resistance. Most metal TIMs are quite stiff and require compressive force to maintain necessary contact with active components to lower the interfacial resistance. With devices becoming smaller, consuming more power, and producing more heat, finding the right TIM becomes a highly critical step in any electronic systems application. Recently, liquid metal TIMs have gained popularity, especially for thermal management of high-performance computing semiconductor applications such as in central processing units (CPUs), graphics processing units (GPUs), and multi-chip modules (MCMs). Due to their fluid nature, liquid metal TIMs do not need to be compressed to maintain even contact, and they can accommodate imperfections in the neighboring components. The newest metal TIMs are made of liquid metal paste (LMP). These gallium-based, high viscosity materials maintain all the good properties of liquid metals but also offer some improved mechanical properties. A key challenge lies in applying LMPs consistently through various dispensing techniques which could be traditional, like time-pressure versus advanced jetting technology. Both dispensing techniques will be compared based on dispensing quality, weight repeatability on substrates, and valve-hardware stability. This paper addresses the challenges faced during LMP dispensing in high-volume manufacturing, and how to maintain constant volume over the product with good dispense quality leading to higher throughput and process reliability.

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Effective heat conduction has become the most urgent technological challenge in the electronics industry. Currently, alumina (Al2O3) is widely used as a thermal filler in thermal interface materials (TIMs). However, Al2O3-based TIMs generally exhibit relatively low thermal conductivity and high interfacial resistance, which can be attributed to both the low intrinsic thermal conductivity of Al2O3 and the significant Kapitza resistance between filler particles. To address this issue, this study introduces fine-sized aluminum nitride (AlN) fillers into spherical Al2O3 to enhance the thermal conduction network. Additionally, liquid metal can form a "liquid bridge" between filler particles and optimize the interfacial contact, effectively reducing the interfacial thermal resistance between the filler particles. The resulting TIM with a filler loading of 88.3 vol % exhibits an isotropic thermal conductivity of 14.06 W m-1 K-1 and thermal resistance as low as 0.24 K cm2 W-1 under a packaging pressure of 40 psi. Moreover, the composite retains excellent electrical insulation with a volume resistivity of 2.11 × 1013 Ω cm. Due to the wetting effect of the LM, the resulting composite demonstrates excellent plasticity properties, making it well-suited for heat transfer applications in complex or irregular packaging structures. The performance test shows that this TIM outperforms current advanced commercial counterparts, highlighting its significant potential for applications in irregular shape thermal management systems.

In recent years, considerable attention has been paid to gallium-based liquid metal alloys in their use as thermal interface materials for GPU, APU, and HPC in PC and super computers due to its considerably high thermal conductivity and low contact thermal resistance. Gallium-based Liquid metal paste containing low content of metal particles (LMP-MPs) to enhance thermal performance and controllable BLT of the prepared liquid metal composites for thermal interface materials (TIMs) prepared by in-situ introducing gallium oxide into the liquid metal alloys will be reported in our recent research. In this paper, we will report effect of the composition, particle size/types, and bond line thickness on thermal properties of the LMP-MPs. Thermal properties of the LMP-MPs were measured at 50o C using a thermal tester, TIMA5, based on ASTM-D5470 test methodology. It was found in the present study that the composition and types of metal particles, and BLT have significant influence on thermal conductivity, and the conductivity values increase with an increase of the content of metal particles and BLT. However, thermal resistance varies with the change of composition and BLT of LMP-MPs. Scanning electron microscope (SEM) was used to characterize microstructure of LMP-MPs with different metal particles and sizes. Micrographic morphology of the LMPs-MPs shows continuous frame structure observed with SEM, which could keep liquid metals from spreading out and stabilize the phase. We eventually found the LMP-MPs have the better wetting and adhesion properties on bare Cu, glass and bare Si surface than liquid metal alloys that endows good printability to the LMP-MPs. Continuous monitoring of thermal resistance of LMP-MPs at 45o C and 89o C using a house-made thermal tester shows stable thermal resistance after a few months, which indicates LMP-MPs have excellent thermal reliability in ambient atmosphere.

Enhanced interface heat transfer based on gallium-based liquid metal infiltrated into vertically aligned copper nanowire arrays

Highly enhanced thermoelectric energy harvesting from a high-temperature heat source by boosting thermal interface conduction

Minimizing interface thermal resistance via laser surface micropatterning for enhancing wetting of gallium-based liquid metal with copper

Metal thermal interface materials (TIMs) have been used in the electronics industry for over 20 years. The thermal performance of any TIMs is defined by their thermal conductivity, bondline thickness, and interfacial resistance. Generally, metal TIMs have very high thermal conductivity and they can be applied in a thin layer. Most of the metals are hard and stiff, making interfacial resistance the biggest obstacle for metal TIMs to overcome. Over the years, many metal TIMs have been developed and used in applications to improve the performance of the components. Recently, liquid metal TIMs are getting more attention, especially in gaming and high-performance applications. They are excellent in terms of accommodating imperfections of the materials they are connecting, but their physical properties can make application risky. To overcome this challenge, a new metal TIMs material - liquid metal paste (LMP) TIM – has been developed. This paper will examine this new material’s thermal conductivity, viscosity, jetting, and dispensing performance compared with other existing metal TIMs, such as liquid metal. The application in mass manufacturing will be studied, with a solution to prevent leakage and provide higher thermal cycling performance (-40°C / +125°C) for LMP.

With the increased level of integration and miniaturization of modern electronics, high-power density electronics require efficient heat dissipation per unit area. To improve the heat dissipation capability of high-power electronic systems, advanced thermal interface materials (TIMs) with high thermal conductivity and low interfacial thermal resistance are urgently needed in the structural design of advanced electronics. Metal-, carbon- and polymer-based TIMs can reach high thermal conductivity and are promising for heat dissipation in high-power electronics. This review article introduces the heat dissipation models, classification, performances and fabrication methods of advanced TIMs, and provides a summary of the recent research status and developing trends of micro- and nanoscale TIMs used for heat dissipation in high-power electronics.

Liquid metal incorporated graphene oxide films with enhanced through-plane thermal conductivity and flame resistance

Gallium-based liquid metal alloy incorporating oxide-free copper nanoparticle clusters for high-performance thermal interface materials

Advancements within the semiconductor industry have led to an increase in microprocessor performance through continued reduction in the length scale of individual transistors. These improvements create challenging thermal management issues that are driven by a sharp increase in power densities. Current thermal interface materials (TIMs) must have extremely low interfacial contact resistance, while being highly stretchable to accommodate large deformation in semiconductor packages due to mismatch of the coefficient of thermal expansion. Several innovations in material architectures from vertically aligned carbon nanotubes, and graphite composites to solder based solid metal die attach and liquid metal TIMs have been previously proposed. However low thermal resistance and high mechanical reliability properties are still mutually exclusive in current TIMs. To address the growing thermal challenges of semiconductor industry, we introduce a novel material architecture of Liquid Metal Embedded Elastomers (LMEEs) where we embed micro-droplets of gallium-based alloys (Tm < 16°C) inside a silicone elastomer. Liquid metal droplets do not impact the overall mechanical properties of the base polymer, resulting in high stretchability, low elastic modulus, high adhesion, and high thermal conductivity (creating an extremely low thermal resistance at the interface). Furthermore, Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA) results show that LMEEs maintain their mechanical properties down to temperatures of less than -55°C, which is essential for reliability performances. In this presentation, I will address LMEEs material architecture and properties that shows high promises for effective thermal managements as a thermal interface material (TIM) in high performance semiconductor packaging.