Kok Fah Choo

Enhancement of Hotspot Cooling With Diamond Heat Spreader on Cu Microchannel Heat Sink for GaN-on-Si Device

The diamond heat spreader has been directly attached between the test chip and the Cu microchannel heat sink for thermal performance enhancement of the GaN-on-Si device. In the fabricated test vehicle, the small heater is used to represent one unit of transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the performance. Two types of simulation models have been constructed in COMSOL, considering the multiphysics features and temperature-dependent material properties. The submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. The heating power, which is concentrated on eight tiny heaters of size 350 × 150 μm2, is varied from 10 to 50 W. With the diamond heat spreader attached to the liquidcooled microchannel heat sink, the maximum heater temperature can be reduced by 11.5%-22.9%, while the maximum gate temperature can be reduced by 8.9%-18.5%. Consistent results from the experimental and simulation studies have verified the enhancement of the hotspot cooling capability using directly attached diamond heat spreader.

Thermal Management of Hotspots With a Microjet-Based Hybrid Heat Sink for GaN-on-Si Devices

The direct-die-attached cooling solution with a diamond heat spreader and hybrid Si heat sink has been developed for hotspot cooling of a GaN-on-Si device. The hybrid heat sink combines the benefits of microchannel flow and microjet impingement. In the fabricated test chip, the small hotspot is used to represent one unit of a GaN transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the thermal and fluidic performances. Two types of simulation models have been constructed using the commercial Finite Element Method software COMSOL, using the multiphysics features and temperature-dependent material properties. A submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. Various heating powers 10-150 W are loaded on eight tiny hotspots of size 450 × 300 μm (heat flux on each hotspot 0.93-13.89 kW/cm2). An overall spatially averaged heat transfer coefficient of 11.53 × 104 W/m2K has been achieved in the microjet-based hybrid heat sink. Consistent results from the experimental and simulation studies have verified the high heat dissipation capability of the designed cooling solution. Several simulations have been conducted to investigate the effects of the heat sink structure and dimensions on the performances for hotspot thermal management.

Thermal management of hotspots using upstream laminar micro-jet impinging array

The problem of heat removal is likely to become more severe due to the presence of hotspots in the integrated circuit chip. The heat dissipation capability of the upstream laminar micro-jet impinging array is investigated for hotspot cooling. Micro-jet impingement array cooling is an effective method of using liquids to cool electronics where high convective heat transfer rates are required. Several simulations have been implemented on the thermal structure of 4 tiny inline-aligned hotspots to evaluate the heat dissipation capability of the laminar micro-jet impinging array. The effects of the jet diameter, jet pitch and jet-to-wall distance on the Nusselt number, heat convection coefficient, Reynolds number and thermal resistance are studied. The limit of the dissipated heat fluxes of the considered thermal structure are evaluated for the hotspots of different sizes.

Thermal characterization and modelling of a gallium arsenide power amplifier MMIC

Thermal characterization of high power microwave devices is important for determining their reliability. Exceeding the optimal temperature will have a detrimental effect on the performance and reliability of these devices. However, temperature characterization of submicron features is often challenging and numerical simulations are often used. In this paper, a detailed finite element thermal model of a power amplifier Monolithic Microwave Integrated Circuit (MMIC) was developed and analyzed to obtain the peak operating junction temperature. Although detailed models would give more accurate results, they usually require more computational effort and time. Hence, a simplified finite element thermal model was also developed and its results compared with those for the detailed model. It was found that the results from the simplified model are higher than those from the detailed model by about 3°C to 8°C at 1W/mm and 1.5W/mm respectively. The temperature distributions of actual power amplifier MMIC devices were measured using IR thermography and thermoreflectance (TR) thermography. It was found that the temperature measured using TR thermography agreed very well with the FEA results but those obtained using IR thermography did not.

GaN-on-Si hotspot thermal management using direct-die-attached microchannel heat sink

GaN-on-Si device posts new challenge to thermal management with its highly concentrated heat flux dissipation from HEMT. In order to characterize GaN-on-Si hotspot and to develop an effective cooling solution, a customized thermal test chip is built with highly doped resistors to produce tiny (150 × 350 μm2) hotspots. Direct-die-attached copper microchannel heat sink is used instead of the conventional slap on heat sink to significantly improve the heat removal rate from the device. By using Infra-red (IR) thermography, the current experimental work demonstrated the cooling capability of microchannel heat sink up to 11.9 kW/cm2 hotspot heat flux with a maximum hotspot temperature of 175 °C.

Accurate thermal characterization of a GaN PA MMIC using Thermoreflectance thermography

The reliability of power microwave devices is often determined by its peak operating junction temperature. In this paper, an accurate thermal characterization of a Gallium Nitride power amplifier Monolithic Microwave Integrated Circuit device using Thermoreflectance thermography is presented. The measured gate temperatures using Thermoreflectance thermography are compared with measured temperatures using Infrared thermography and calculated temperatures from finite element analysis.

Development of package level hybrid silicon heat sink for hotspots cooling

In this paper, the fabrication of package level silicon microchannel heat sink for hotspot thermal management is presented. These include the design, micro fabrication process and chip level integration of a hybrid silicon heat sink, which integrates jet impingement, microchannel cooling technologies. The fabrication of hybrid heat sink is proposed by bonding two Si chips which patterned with nozzle and microchannel structures separately. The nozzle array is fabricated using though silicon vias (TSV) process. This nozzle plate is used to generate jet impingement effect into the microchannel heat sink. On the other hand, the microchannel heat sink consists of micro fins and channels which are fabricated using deep reactive ion etch (DRIE) process. The micro fins increase the area for convective heat transfer while the micro channels serve as the liquid conduit to carry the intense heat away from the heat source. Two silicon chips are bonded using thermal compression bonding (TCB) process. For the packaging, the integration of thermal chip and diamond heat spreader onto silicon heat sink is performed using gold-tin eutectic bonding through TCB process. In this paper, the major fabrication steps and critical process parameters will be discussed in details along with the hydraulic test and thermal analysis.

Submerged liquid jet impingement cooling

In this paper, submerged liquid jet array impingement is proposed as a solution for the thermal management of the high performance electronics. Experiments were done using a submerged jet array of 285 nozzles of 0.5 mm diameter spaced 5.5 mm apart, machined on a 5 mm thick nozzle plate, with an impingement height of 3 mm. The temperature difference between the average heat source temperature and the inlet temperature decreases with the flow rate and increases with the heating power. The experimental Nusselt numbers agree well with the reported data in previous studies. Numerical studies were also conducted on a selected 16-nozzle module using the renormalization group (RNG) k-ε turbulence model. The temperature and velocity fields were obtained, and the average heat transfer coefficients from the numerical studies show good agreement with the experimental results.

Large area spray cooling by inclined nozzles for electronic board

To cool a 1 kW 6U electronic card, an innovative inclined spray chamber with multiple nozzles has been developed and investigated with a closed-loop refrigeration system. A large heated surface (12.3 × 15.5 cm2) was sprayed by four gas-assisted nozzles with an inclined angle of 39° relative to the normal direction. A reasonable spray coverage area can be obtained by the inclined spray chamber while enabling a relatively lower spray chamber height than that required by a normal spray chamber. R134a was implemented as the working fluid in this study. The mass flow rate, pressure drop across the nozzles, and the spray chamber pressure were varied experimentally, and the results suggest that the increases of the mass flow rate, the pressure drop across the nozzles, and the spray chamber pressure can improve the thermal performance of the inclined spray. The average heated surface temperature can be maintained within 20.0°C, and the maximum heat transfer coefficient of 4742.2 W/m2·K can be achieved at a suitable working condition.

Measurement of MMIC gate temperature using infrared and Thermoreflectance thermography

Thermal characterization of high power microwave devices is important for determining their reliability. Exceeding the optimal temperature will have a detrimental effect on the performance and reliability of these devices. In this paper, the temperature a power amplifier (PA) Monolithic Microwave Integrated Circuit (MMIC) was measured using the traditional Infrared (IR) thermography technique and an emerging technique called Thermoreflectance (TR) thermography. The measured results were compared to those calculated using finite element analysis (FEA). It was found that temperatures measured using TR thermography agreed very well with FEA results, whereas temperatures measured using IR thermography did not. This could be attributed to the presence of reflective and low emissivity surfaces on the PA MMIC and the inadequate spatial resolution of the IR camera.