Shirong Huang

Two-dimensional hexagonal boron nitride as lateral heat spreader in electrically insulating packaging

The need for electrically insulating materials with a high in-plane thermal conductivity for lateral heat spreading applications in electronic devices has intensified studies of layered hexagonal boron nitride (h-BN) films. Due to its physicochemical properties, h-BN can be utilised in power dissipating devices such as an electrically insulating heat spreader material for laterally redistributing the heat from hotspots caused by locally excessive heat flux densities. In this study, two types of boron nitride based heat spreader test structures have been assembled and evaluated for heat dissipation. The test structures separately utilised a few-layer h-BN film with and without graphene enhancement drop coated onto the hotspot test structure. The influence of the h-BN heat spreader films on the temperature distribution across the surface of the hotspot test structure was studied at a range of heat flux densities through the hotspot. It was found that the graphene-enhanced h-BN film reduced the hotspot temperature by about 8–10°C at a 1000 W/cm2 heat flux density, a temperature decrease significantly larger than for h-BN film without graphene enhancement. Finite element simulations of the h-BN film predict that further improvements in heat spreading ability are possible if the thermal contact resistance between the film and test chip are minimised.

Characterization for graphene as heat spreader using thermal imaging method

Monolayer graphene was synthesized through thermal chemical vapor deposition (TCVD) as heat spreader in electronic packaging. Platinum (Pt) was functioned as hot spot of thermal testing chip. The hot spot temperature of thermal testing chip bonded onto a heat sink measured by an FLIR infrared thermograph was decreased by about 5 °C from 52 °C to 47 °C when driven at a heat flux about 1280W/cm2 with a graphene heat spreader attached. It is conceivable that further improvements to the cooling performance of graphene heat spreader can be made by optimizing the synthesis parameters and transfer process.

Development and characterization of graphene-enhanced thermal conductive adhesives

In this paper, a graphene enhanced thermal conductive adhesive (G-TCA) was developed for thermal management of power devices. The developed G-TCA has many advantages, including high thermal conductivity, lower density, good dispensing ability, and cost effective. To fabricate G-TCAs, few-layer graphene was utilized as fillers to improve the thermal conductivity of the TCA. The graphene nanosheets were fabricated through a high-speed shear mixing process in a mixed solvent. Compared to many reported liquid exfoliation process, the graphene fabrication process shows many advantages, such as high process efficiency, mass production, low-cost, clean and safe process. G-TCA sample with a hybrid filler ratio of 73% Ag and 3% graphene shows the highest thermal conductivity of 8 W/m K, which is almost four times higher than reference TCAs. A Joule heating setup was built to simulate G-TCAs function in a real electronic component and demonstrate the superior heat dissipation properties of the G-TCAs. Viscosities of the G-TCA samples were regulated in an acceptable range of many dispensing processes to be able to make uniform and fine patterns. Therefore, the developed G-TCA could be widely used for thermal management of power devices and electronic packaging area to decrease their working temperatures and extend the lifetime of devices.

Graphene based heat spreader for high power chip cooling using flip-chip technology

Monolayer graphene was synthesized through thermal chemical vapor deposition (TCVD) as heat spreader for chip cooling. Platinum (Pt) serpentine functioned as hot spot on the thermal testing chip. The thermal testing chip with monolayer graphene film attached was bonded using flip-chip technology. The temperature at the hot spot with a monolayer graphene film as heat spreader was decreased by about 12°C and had a more uniform temperature compared to those without graphene heat spreader when driven by a heat flux of about 640W/cm2. Further improvements to the cooling performance of graphene heat spreader could be made by optimizing the synthesis parameters and transfer process of graphene films.

Hotspot test structures for evaluating carbon nanotube microfin coolers and graphene-like heat spreaders

The design, fabrication, and use of a hotspot-producing and temperature-sensing test structure for evaluating the thermal properties of carbon nanotubes, graphene and boron nitride for cooling of electronic devices in applications like 3D integrated chip-stacks, power amplifiers and light-emitting diodes is described. The test structure is a simple meander-shaped metal resistor serving both as the hotspot and the temperature thermo-meter. By use of this test structure, the influence of emerging materials like those mentioned above on the temperature of the hotspot has been evaluated with good accuracy (±0.5°C).

2D heat dissipation materials for microelectronics cooling applications

The need for faster and smaller, as well as more reliable and efficient consumer electronic products has resulted in microelectronic components that produce progressively more heat. The resultant reliability issues from the increased heat flux are serious and hinder technological development. One solution for microelectronics cooling applications is 2D materials applied as heat spreaders and these include monolayer graphene, graphene based films, and monolayer hexagonal boron nitride and BN based films. In addition, thermal performances of the graphene heat spreader were also studied under different packaging structures, including wire bonding, cooling fins and flip chips. Finally, 2D hexagonal Boron nitride (h-BN) heat spreaders, fabricated by different methods, were characterized by different thermal characterization methods, such as resistance temperature detector (RTD) and Infrared (IR) methods. In conclusion, these new novel 2D materials developed show great potential for microelectronics cooling applications.

Preventing aging of electrically conductive adhesives on metal substrate using graphene based barrier

The Electronic Conductive Adhesive (ECA) is a promising material as a substitute of traditional tin-lead solder, with many advantages outperforming tin-lead containing solder such as environmentally friendly, requiring much lower processing temperature and a much finer pitch. However, one critical problem related to ECA application is that the contact resistance increases significantly during an aging test, particularly at 85°C/85% relative humidity (RH) when ECA is bonded onto non-noble metal surfaces due to galvanic corrosion. Recently, it has been reported that graphene has an interesting and wonderful property, impermeability; it can prohibit most molecules from going through due to the fact that graphene is one of the most impermeable barrier materials, and it has become a potential candidate for anti-corrosion applications. Some papers studying the application of graphene for anti-corrosion of steel and other metals have already been published. In this paper, graphene film [12] barriers were introduced between the ECA and metal pad to alleviate the galvanic corrosion problem, which can lead to the deterioration of contact resistance, then the aging test @85°C/85% was performed for 500 hours. It was found that the contact resistance increased quickly during the first 200 hours of aging test for samples without graphene film barriers, while the contact resistance for samples with graphene barrier remained stable. Samples with graphene film barriers showed a smaller shift of the contact resistance than those without graphene barrier. The results indicated that the graphene film barrier can be used to improve ECAs reliability, especially @85°C/85% conditions.

Enhanced Cold Wall CVD Reactor Growth of Horizontally Aligned Single-walled Carbon Nanotubes

HASynthesis of horizontally-aligned single-walled carbon nanotubes (HA-SWCNTs) by chemical vapor deposition (CVD) directly on quartz seems very promising for the fabrication of future nanoelectronic devices. In comparison to hot-wall CVD, synthesis of HA-SWCNTs in a cold-wall CVD chamber not only means shorter heating, cooling and growth periods, but also prevents contamination of the chamber. However, since most synthesis of HA-SWCNTs is performed in hot-wall reactors, adapting this well-established process to a cold-wall chamber becomes extremely crucial. Here, in order to transfer the CVD growth technology from a hot-wall to a cold-wall chamber, a systematic investigation has been conducted to determine the influence of process parameters on the HA-SWCNT’s growth. For two reasons, the cold-wall CVD chamber was upgraded with a top heater to complement the bottom substrate heater; the first reason to maintain a more uniform temperature profile during HA-SWCNTs growth, and the second reason to preheat the precursor gas flow before projecting it onto the catalyst. Our results show that the addition of a top heater had a significant effect on the synthesis. Characterization of the CNTs shows that the average density of HA-SWCNTs is around 1 - 2 tubes/μm with high growth quality as shown by Raman analysis.

Infrared emissivity measurement for vertically aligned multiwall carbon nanotubes (CNTs) based heat spreader applied in high power electronics packaging

Vertically-aligned multiwall carbon nanotubes were deposited on silicon substrate by low pressure chemical vapor deposition (LPCVD), which can be utilized as heat spreaders in high power electronic packaging due to their remarkable thermal conductivity. The infrared emissivity of the vertically aligned multiwall carbon nanotubes was then characterized based on the FLIR SC600 infrared imaging system. The average infrared emissivity of the multiwall carbon nanotubes sample was about 0.92, which agrees well with experimental results reported before. Scanning electron microscopy (SEM) images of the multiwall carbon nanotubes were further analyzed to explain its high emissivity, and the reason can be attributed to the homogeneous sparseness and aligned structure of the vertically aligned multiwall carbon nanotubes.

Reliability of graphene-based films used for high power electronics packaging

Graphene-base film was fabricated with chemical conversion process, including graphene oxide (GO) prepared by Hummers method, graphene oxide reduced with L-ascorbic acid (LAA), graphene based film deposited by vacuum filtration process. Meanwhile, the functionalization of the graphene-based film was performed to decrease the thermal interface resistance between the graphene-based film and substrate. Characterization data showed that the graphene-based film possessed high reliability after 500 hours under 85°C aging test. In summary, the graphene-based film can be a promising solution in thermal management of high power electronics.