Excellent Heat Spreading Research Articles

Thin PDMS-on-Sacrificial-PCB Devices

Poly(dimethylsiloxane) (PDMS) devices must often be so thin (e.g., <100 μm) and, therefore, so fragile that their mechanical release becomes challenging. As a simple solution, PDMS devices are frequently released by dissolving an underlying sacrificial resist. However, there may be several other issues, including temperature gradients during fabrication, electrical characterization, handling, co-integration with electronics, fabrication time, and cost. Here, we show that conventional printed circuit boards (PCBs) can be ideal sacrificial carriers, which can also perform useful functions during fabrication. The thick (e.g., 35 μm) and thermally conductive PCB copper layer behaves as an excellent heat spreading plate during temperature-sensitive process steps (e.g., PDMS curing and sintering of conductive inks) and enables pre-release electrical stimulations and characterizations of PDMS, which may help during process development. Experiments and finite element method (FEM) simulations confirm that the PCB copper layer can improve temperature uniformity. The UV-protecting film attached to the PCB can be cut to constitute a frame for easy handling. The PCB photoresist enables the straightforward release of PDMS by acetone, which is among the most environmentally friendly chemicals. Contacts can be opened by a simple bump-cut-peel strategy. As proofs of concept, we demonstrate pre-release capacitive characterization of PDMS for evaluating the ability to fabricate thin, but uniform and robust devices and post-release resistive characterization of stretchable strain sensor or interconnects encapsulated in PDMS. The proposed approach is simple, cheap, and environmentally friendly, can improve temperature uniformity throughout the pre-release process steps, enables pre-release electrical characterizations, can simplify co-integration with electronics and can be generalized to other elastomers.

Enhanced thermal performance of phase change material-integrated fin-type heat sinks for high power electronics cooling

We report the enhanced cooling performance of the phase change material (PCM)-integrated fin-type heat sink compared to conventional fin-type heat sink in high power electronics with two localized hot spots. The PCM-integrated fin-type heat sink is fabricated by embedding the phase change composite to the base plate of the heat sink. As an effort to effectively utilize thermal capacitive effects of PCM, the phase change composites with paraffin infiltrated to copper foams are deployed within circular hole arrays in the base plate, which is subsequently covered by a graphite sheet, to achieve excellent heat spreading characteristics. Considering the cooling environments of commercial high power electronics (insulated-gate bipolar transistor (IGBT)), thermal performance of the PCM-integrated and the conventional fin-type heat sinks is experimentally and numerically investigated upon the heating powers of 400∼800 W. While the PCM-integrated fin-type heat sinks have similar heat sink thermal resistance with the conventional fin-type heat sinks, the PCM-integrated fin-type heat sinks exhibit an effective time delay up to ∼27.3% of the hot-spot temperature rise until 80 ℃ of the heat sinks in reduced cooling conditions, showing the potential as an effective thermal managing platform of the PCM-integrated heat sinks in convection-limited cooling environments.

The effect of GaN epilayer thickness on the near-junction thermal resistance of GaN-on-diamond devices

Gallium nitride (GaN) heteroepitaxially integrated with diamond (GaN-on-diamond) is promising for high-power electronics due to the excellent heat spreading capability of diamond. A number of past works have examined the thermal properties of GaN-on-diamond devices, particularly the diamond thermal conductivity and the thermal boundary resistance (TBR) between the GaN and diamond, as well as the impact of these two properties on the thermal resistance of GaN-on-diamond devices. Much less investigated, however, is the effect of the thickness of the GaN epilayer on the thermal resistance of GaN-on-diamond devices. Here, we examine this effect through combining finite element simulations with calculations using a semi-classical phonon transport model. The latter considers phonon scattering on defects and interfaces and is utilized here to predict the in-plane thermal conductivity of the GaN epilayer versus layer thickness. This aims at considering the thermal spreading resistance within the GaN in a more accurate manner, which also depends on the layer thickness. Our simulation results indicate that with increasing GaN layer thickness the device thermal resistance monotonically decreases until it reaches the minimum at GaN thicknesses of ∼3.6 and ∼5.8 μm for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. A typical GaN thickness of 1 µm can provide a fairly low device thermal resistance, approximately 6 and 20% higher than the minimum thermal resistances for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. Reducing the GaN thickness below 1 µm can substantially increase the device thermal resistance, particularly when the GaN/diamond TBR is high, predominantly due to the increasing contribution of the interface. Increasing the GaN thickness above 1 µm can decrease the device thermal resistance but this change is not significant. Our findings presented here can offer a more in-depth understanding of near-junction thermal transport in GaN-on-diamond transistors.

Evaluation of Core Hopping on POWER7

Controlling and limiting the peak runtime temperature of a microprocessor has the potential to reduce chip leakage power consumption, improve chip reliability, reduce provisioned cooling, and reduce operational costs. Previously, the technique of core hopping has been proposed as one method to reduce peak runtime temperature. However, there is no study that shows core hopping is beneficial for modern many-core microprocessors found in high-end servers. This paper thoroughly examines heat spreading in the 8-core POWER7 microprocessor which provides 5 temperature sensors per core. We find that the POWER7 heatsink has excellent heat spreading capabilities which negates the benefits of core hopping. We conclude that high-performance servers with similar thermal solutions will also not see benefit from core-hopping methods.

Heat Spreader Based on Room-Temperature Liquid Metal

This study reports a high-performance heat spreader based on room-temperature liquid metal coolant. Conceptual cooling experiments show that liquid metal heat spreader owns particularly excellent heat spreading performance. In order to evaluate the driving features of liquid metal, a miniaturized electromagnetic pump with high reliability and low power consumption was fabricated and tested. Extreme experiments were performed and showed that liquid metal heat spreader could overwhelm all the latest typical advanced spreading technologies and serve as an ultimate heat spreading solution under extremely high heat flux density condition.

Degradation Analysis of InGaN Laser Diodes

The current status of InGaN-MQW-laser diodes developed at Osram OS is presented. These lasers are grown on n-conducting SiC, enabling a vertical current path, cleaved facets and excellent heat spreading. The temperature rise during cw operation is measured for different mountings. A p-side up mounted diode with thermal resistance of 18 K/W showed 143 h of cw lasing at 1 mW optical power (T = 25 °C). DC and pulsed aging shows current as main degradation reason compared to heat for InGaN-LDs. Photoluminescence spectra of the quantum wells are being compared before and after degradation caused by current.