Micro-pin Fin Arrays Research Articles

Investigation on the thermal and hydrodynamic performances of a micro-pin fin array heat sink for cooling a multi-chip printed circuit board

Multi-chip printed circuit boards (PCBs) with integrated functionalities are widely used in large servers. This study investigates the thermal design of a PCB with nine different chips by using a micro-pin fin array heat sink. Four types of micro-pin fin arrays, namely, the square-pin fin array (SPFA), round-pin fin array (RPFA), truncated-pyramid-pin fin array (TPPFA), and truncated-conical-pin fin array (TCPFA), are considered, with the TPPFA providing the best thermal and hydrodynamic performances. The fabricated TPPFA heat sink was experimentally studied, achieving a maximum temperature of 63.3 °C, a temperature nonuniformity of 6.5 °C, and a pressure drop of 3.1 kPa at a total heat transfer rate of 540 W. Numerical simulations are conducted to examine the detailed temperature and flow distributions in the heat sink, exhibiting relatively uniform temperatures in the areas with micro-pin fin arrays and beneficial secondary flows in the downstream of the micro-pin fins. Further numerical simulations are performed to reveal the importance of the type of the coolant and the temperature dependence of the thermophysical properties, as well as the validity of the field synergy principle in analyzing the thermal performance of the heat sink considered in this work.

Thermal performance enhancement of a micro-jet heat sink via parametric investigated micro pin fin arrays

The thermal management issue of high electronic chips with heat flux is one of the most challenge during the development of electronic devices. In this work, the design of micro pin fin arrays (MPFA) of micro-jet heat sink was proposed to achieve effective cooling of electronic chips. The MPFA with different cross-sectional shapes were used to improve the thermal performance of the heat sink. The results show that heat sink design with MPFA exhibit excellent cooling performance while consuming approximately the same amount of power as smooth surface. In addition, the hexagonal cross-section has better thermal performance, the temperature uniformity of the heating surface can be improved by up to 70 % and the average temperature can be reduced by 7–20K. Moreover, the manufacturing cost and cooling performance per unit cost were investigated and the results show that the square cross-section is the most commercially viable. Based on this, the square cross-section was parametrically studied, including the length to height ratio and fins number. It was founded that the heat sink exhibits better thermal performance for a length-to-height ratio of 1.4 and a number of fins of 169. Finally, the ultimate heat flux of the electronic chip can be withstood was also tested, and the optimal heat sink design can handle heat flux of 400W/cm2. Therefore, the heat sink with MPFA design can significantly improve the thermal performance without consuming the pump power, which can be used to cooling the high electronic chips with high heat flux in the future.

Temperature drop for CO2 gas flowing through multilayer micro-pin fin arrays

A reasonable theoretical calculation model is established, and an appropriate correlation formula is selected to calculate the flow and heat transfer of CO2 gas flowing through micro-pin fin arrays in a micro-cooler respectively under different working conditions. The inlet pressure of CO2 gas is 1.0 MPa, 1.5 MPa, and 2.0 MPa respectively, and the inlet temperature is 12 °C and 16 °C respectively. The results show that the cold end temperature of the gas decreases with the increase of CO2 gas inlet pressure and the decrease of inlet temperature, and the lowest temperature reaches -29.93 °C. The calculated results have the same trend as the experimental data, and the calculated values are consistent with the experimentally measured values, which demonstrates the calculation model’s logic.

Dynamics of long bubbles propagating through cylindrical micro-pin fin arrays

Dynamics of long bubbles propagating through cylindrical micro-pin fin arrays

Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

The two-phase flow boiling in microchannels is an attractive cooling strategy for high-power density electronic/optoelectronic devices, while the exist of sub-millimeter “hot spots” characterized by ultra-high heat fluxes would degrade the reliability and safety for commercial and consumer applications. In this paper, the bubble nucleation and evolution characteristics of subcooled flow boiling in high aspect ratio flat microchannels were experimentally studied with the aid of high-speed visualization and infrared temperature measurement at localized heat fluxes exceeding 1.5 kW/cm2. Dielectric liquid HFE-7100 was used at mass flow rates ranging from 11.1 to 178.2 kg/(m2·s). Results show that the explosive boiling process was accompanied by an approximately linear bubble growth rate of about 20 m/s at about the first half of growth stage, and the time duration for overall bubble growth was less than 200 μs. The bubble nucleation was suppressed dramatically on the smooth bottom surface of the channel even at a localized “hot spot” heat flux above 2.5 kW/cm2. The introduction of staggered short micro-pin fin arrays at the bottom wall significantly lowered the threshold heat flux and onset temperature of bubble nucleation and reinforced the bubble growth to larger sizes. It is attributed to the reduction in the temperature gradient of thermal boundary layer adjacent to the heated wall, and it would be further reduced by increasing the micro-pin fin denseness. Even at quite high superheats on smooth and micro-structured surfaces, the heterogeneous boiling mechanism was proved to still work.

Effects of Thin Film Heat Spreader on Hot Spots Mitigation in Heat Sinks

Abstract The effects of graphene platelets and diamond-based thin film heat spreaders on maximum temperature of integrated electronic circuits were investigated. A fully three-dimensional conjugate heat transfer analysis was performed to investigate the effects of thin film material and thickness on the temperature of a hot spot and temperature uniformity on the heated surface of the integrated circuit when subjected to forced convective cooling. Two different materials, diamond and graphene, were simulated as materials for thin films. The thin film heat spreaders were applied to the top wall of an array of micro pin fins having circular cross sections. The integrated circuit with a 4 × 3 mm footprint featured a 0.5 × 0.5 mm hot spot located on the top wall, which was also exposed to a uniform background heat flux of 500 W cm−2. A hot spot uniform heat flux of magnitude 2000 W cm−2 was centrally situated on the top surface over a small area of 0.5 × 0.5 mm. Both isotropic and anisotropic properties of the thin film heat spreaders made of graphene platelets and diamond were computationally analyzed. The conjugate heat transfer analysis also incorporated thermal contact resistance between the thin film and the silicon substrate. It was found that isotropic thin film heat spreaders significantly reduce the hot spot temperature and increase temperature uniformity, allowing for increased thermal loads. Furthermore, it was found that thickness of the thin film heat spreader does not have to be greater than a few tens of microns.

A remarkable CHF of 345W/cm2 is achieved in a wicked-microchannel using HFE-7100

Boiling heat transfer of dielectric fluids is a promising cooling technique for thermal management of microelectronic systems. However, the critical heat flux achievable is generally low, because of the poor thermophysical properties of these fluids. To address this dilemma, we propose a new cooling concept to substantially enhance global liquid supply during phase-change process using enhanced capillary-driven force. Additionally, dedicated vapor pathways are designed among a bank of micro-pillars to facilitate vapor removal. In this work, new wicks comprised of silicon micro-pinfin arrays are explored to significantly enhance the flow boiling heat transfer performance. To examine the functionalities of this wick, experiments on HFE-7100 were carried out with mass velocities varying from 247 to 3,465 kg/m2s. To explore the enhancement mechanisms and to analyze the capillary-assisted flow boiling process, visualization studies were conducted. The results indicate that sustainable evaporation induced by wick microstructures and efficient liquid supply are the enhancement mechanisms compared to parallel microchannels with solid walls. It is found that the overall heat transfer coefficient is substantially increased up to 75%. Remarkably, a high critical heat flux (CHF) of approximately 345 W/cm2 is recorded at G = 3,465 kg/m2s at coolant inlet temperature of ∼20 °C. Equally importantly, this noticeable enhancement of CHF value is associated with drastically decreased pressure drops compared to microchannels decorated with μ-pinfin fences.

Thermal design and management of micro-pin fin heat sinks for energy-efficient three-dimensional stacked integrated circuits

Three-dimensional stacked electronics have substantially improved the electrical performance of integrated circuits. However, given the geometrical complexity and high pressure drop they entail, thermal management difficulties and energy requirements are exacerbated owing to the inapplicability of thermal management schemes. In this study, the thermal and hydrodynamic characteristics of various micro-pin fin arrays were investigated to maximize heat dissipation while minimizing the energy consumption. Specifically, a 10 × 10 mm2 micro-pin fin array was fabricated on an eight-inch silicon wafer via microfabrication. A Pyrex cover was bonded anodically with the top side of the micro-pin fins to prevent leakage, and a titanium/gold thin film serpentine heater was used to supply uniform heat flux on the backside of the micro-pin fin array. Subsequently, the heat transfer and pressure drop in the micro-pin fin heat sinks were obtained experimentally with various micro-pin fin geometries having pin diameter Df = 38–100 µm, transverse pin spacing ST = 74–301 µm, longitudinal pin spacing SL = 74–301 µm and pin height Hf = 90–207 µm. Thereafter, the geometrical and operational effects on heat transfer and pressure drop were investigated based on a consolidated database cumulated from the literature. Altogether, 256 data points from 21 geometrical combinations were explored from existing relevant studies to obtain optimized geometric and operating conditions in the micro-pin fin arrays over a wide range of geometrical and operating conditions: Reynolds number Re = 35–491.3, heat flux q" = 0–114 W/cm2, pin diameter Df = 38–559 µm, pin spacing S = 74–800 µm, and pin height Hf = 90–845 µm. Subsequently, new empirical correlations based on the consolidated database were formulated to describe the Nusselt number and fanning friction factor in the micro-pin fin arrays. These correlations provide suitable predictions in comparison with those based on extant correlations.

An artificial neural network model for predicting frictional pressure drop in micro-pin fin heat sink

Abstract This study is part of two studies conducted for developing artificial neural-network-based tools for predicting the thermal and hydraulic performance of micro-pin fin heat sinks used for high-heat-flux electronic devices. The thermal design of high-heat-flux electronics requires a strong understanding of both pressure drop and heat transfer coefficient. Increasing the pressure drop in a cooling system significantly increases the required pumping power and decreases the system energy efficiency, in addition to considerably increasing temperature nonuniformity and causing reliability issues. Micro-pin fin heat sinks can help in the thermal management of high-heat-flux electronic systems owing to their effective heat transfer characteristics, namely, a large surface area and passage flow turbulence generation, and the requirement of lower pumping power compared with the microchannel heat sink. Studies conducted over the past decade have revealed that the thermal and hydraulic performance of micro-pin fin heat sinks are highly dependent on their geometric and operational parameters. However, a universal approach to predicting the frictional pressure drop, which influences the amount of power required, in pin fin arrays for various operating conditions and geometric shapes has not been developed so far. In this study, a trained artificial neural network (ANN) was used to develop a universal model for predicting the friction factor of micro-pin fin arrays. The friction factor correlation was predicted from 1,651 experimentally determined friction factor data points obtained from 22 studies. The relationship between a wide range of geometric and operating conditions and the hydraulic performance was investigated for accurately training the ANN. Furthermore, the universal model was analyzed by comparing values predicted by it with values obtained in other experimental studies. The model was found to show superior performance compared with other regression-method-based correlations.

Reprint of: Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

Meta-Study on Integrated Cooling of Modern Integrated Circuits using Microfluidics

The substantial increase in the transistor density of integrated circuits (ICs) in recent times has allowed considerable improvements in computing power. With increasing transistor and power density, the heat produced by modern ICs has increased significantly. This in turn has negative effects on the performance, reliability, and power consumption of the ICs. A solution to the IC’s complications caused by overheating is integrated cooling, in which cooling fluid is delivered through microchannel heat sinks on the backside of an IC. This meta-study will investigate two microfluidic cooling technologies. First, implementing varied size microfluidic channels close to the silicone substrate of the IC. Additionally, a micro-pin fin heat sink is integrated into the ICs’ fluidic microchannels. Different sized pin fins were used, to achieve a wider understanding of the application of pin fins in microfluidic cooling and compare the thermal performances of each cooling method. Integrated cooling subverts the need for suboptimal thermal interfaces and bulky heat-sinks, as well as reducing the intensity of localised hotspots commonly present in high-power electronics. Further, by locating the main heat exchange medium closer to the die of an IC, we reduce the number of thermal interfaces. This meta-study suggests that cylindrical micro-pin fin arrays with pitch longitude and latitude of 60μm and 120μm, are more thermally efficient than plain microfluidic cooling channels.

Temperature Uniformity Enhancement and Flow Characteristics of Embedded Gradient Distribution Micro Pin Fin Arrays Using Dielectric Coolant for Direct Intra-Chip Cooling

Targeting at reducing temperature gradient in the direct intra-chip cooling, embedded gradient distribution micro pin fin arrays are fabricated. The flow characteristics and temperature uniformity performance are experimentally investigated using dielectric coolant (HFE7100). It is found the dependence of friction factor on Re is weak after reaching a critical Re (293) with a correlation of f~Re−0.14. Obvious flow fluctuations appear with steady stagnant symmetric vortices and unsymmetrical vortices by the experimentally validated numerical results, attributing to the abrupt rise of pressure drop. Moreover, the present gradient micro-pin fin arrays show great strengths in improving thermal performance and lowering temperature gradient. The thermal resistance and maximum temperature difference reduce by 34.6% and 8 K at the Qv of 100 ml/min and qw of 40 W/cm2 compared to the uniform micro pin fin arrays for a same pumping power. Notably, the local temperature uniformity is also improved by gradient distribution micro pin fin arrays with absolute temperature deviations less than 2 K from x=1 mm to x=10 mm. The local Nu based on experimental data show a rapid increase (29%) in the second junction zone where unsteady wakes and flow acceleration occur, bringing considerable enhancement in mixing and local heat transfer.

Experimental and numerical study on convective boiling in a staggered array of micro pin-fin microgap

The flow patterns, heat transfer and pressure drop of convective boiling of dielectric fluid, FC-72, in a micro-gap are investigated experimentally. The surface of the microgap is enhanced with a staggered array of micro pin-fins. The enhanced surface of the microgap with the size of 10 × 10 mm2 is subject to an electrical heat source. The micro-pin-fins are etched as cubic columns of 100 μm size and arranged in a staggered arrangement with 400 μm pitch in both transverse and longitudinal directions. The inside of the micro pin-fins is nucleated with a cavity of cylindrical shape with diameter of 60 μm and an opening with a size of either 15 μm or 45 μm width. The opening of the cavities of the micro-pin-fins is aligned toward the down-stream. For the case of single-phase flow, a numerical analysis is performed, and the pressure drop and velocity fields are investigated in the micro-gap. The experiments were performed for various mass fluxes ranging from 94 to 275 kg/m2s and heat flux ranging from 0 to 10 W/cm2, and at two saturation temperatures of 35 and 50 °C. For the case of single-phase heat transfer, the experimental results are compared with the pin-fins having 45 μm cavity opening and found that the effect of cavity opening is significant when the mass flux is high. The surface-superheat at the onset of boiling is reduced by reducing the cavity opening-width from 45 to 15 μm. The microgap having nucleation cavity with cavity opening-width of 15 μm results in a 3.44 °C smaller surface-superheat than that of 45 μm opening-width. The convective heat transfer coefficient increases with the increase of mass flux, regardless of the flow type. Moreover, the variation of the pin-fin opening-widths does not influence the convection heat transfer rate. For single-phase flow, the heat flux shows a negligible effect on pressure drop, and the pressure drop increases with increasing mass flux. For two-phase flow, the pressure drop increases drastically with increasing heat flux. A smaller cavity opening-width results in a smaller pressure drop when the mass flux is high. The flow observation shows two distinct flow patterns in the microgap at the beginning of the bubble nucleation.

Microfluidic Cooling of a 14-nm 2.5-D FPGA With 3-D Printed Manifolds for High-Density Computing: Design Considerations, Fabrication, and Electrical Characterization

The 2.5-D integration is becoming a common method of tightly integrating heterogeneous dice with dense interconnects for efficient, high-bandwidth inter-die communication. While this tight integration improves performance, it also increases the challenge of heat extraction by increasing aggregate package powers and introducing thermal crosstalk between the adjacent dice. In this article, a microfluidic heat sink is used to cool a 2.5-D Stratix 10 GX field-programmable gate array (FPGA) consisting of an FPGA die surrounded by four transceiver dice. The heat sink utilizes a heterogeneous micropin-fin array with micropin-fin densities that are tailored to the local heat fluxes of the underlying dice. Enabled through a 3-D printed enclosure for fluid delivery, the assembled heat sink has a total height of 6.5 mm, including the tubes used for fluid delivery. The heat sink is tested in an open-loop system with deionized water as a coolant, and thermal performance is compared against a high-end air-cooled heat sink. Improvements in die temperatures, computational density, and thermal coupling between the dice are observed. The effect of the FPGA power on the surrounding transceiver die temperatures was reduced by a factor of ${10}\times$ to over ${100}\times$ when compared with the air-cooled heat sink.

Single-phase thermal and hydraulic performance of embedded silicon micro-pin fin heat sinks using R245fa

Aggressive thermal management strategies such as liquid cooling have become essential for high-performance three-dimensional (3D) integrated circuit (IC) chips. Micro-pin fin arrays integrated between stacks can provide superior thermal performance with relatively less pumping power compared to microchannel cooling. In this work, we experimentally studied the single-phase heat transfer and pressure drop characteristics of micro-pin fin arrays. Three different samples consisting of 31–131 rows of cylindrical micro-pin fins with pin diameters Dh = 45–100 μm, center-to-center pin spacings S = 74–298 μm, and pin height Hf ∼ 200 μm were tested. Dielectric fluid R245fa was used as the working fluid with mass flow rates ṁ = 14.7–181.6 g/min and corresponding Reynolds numbers Re = 35–481.3. The heat fluxes ranged from 2.5 W/cm2 to 48.7 W/cm2, and the inlet fluid temperature was maintained at ambient temperature in the range of 22.2–25.3 °C. The local heater temperature distributions, average heat transfer characteristics, and pressure drops for various geometries of the embedded microfluid pin–fin arrays were determined. The experimentally determined heat transfer coefficient varied with both the mass flow rate and pin spacing with an averaged heat transfer coefficient of up to 18.2 kW/(m2·K). Full-scale conjugate simulations with a turbulence model were conducted using ANSYS Fluent to validate the experimental results for the three cases. A comparison with the numerical model showed mean absolute errors of 9.1% for the heat transfer and 14.3% for the pressure drop.

Performance analysis of an array of square micro-fins

Array of micro pin fin heat sinks shows higher thermal efficiency in high heat flux and critical devices such as aerospace, microelectronic etc. In this work, an array of micro square pin fins has been designed and for this copper is selected as workpiece material. Moreover, a numerical investigation of thermal performances and mechanical behavior of the designed fins has been carried out. The effects of Reynolds numbers on velocity profile and heat transfer performance have also been studied. Additionally, the equivalent (von-Mises) stress along with the structural and total deformations have been observed at the desired pressure applied on the surface of the pin fins. Numerical simulations with similar parametric conditions have also been conducted on a plate having the same dimensions without fins, and a comparison has been made with a plate having micro-fins. It has been observed that the array of square micro pin fins gives a better thermal performance than that of without fins.

Integrated Circuit Cooling Using Heterogeneous Micropin-Fin Arrays for Nonuniform Power Maps

As microelectronic system density continues to increase, cooling with conventional technologies continues to become more challenging and is often a limiter of performance and efficiency. The challenge arises due to both large heat fluxes generated across entire chips and packages, and localized hotspots with even higher heat flux. In this paper, nonuniform micropin-fin heat sinks are investigated for the cooling of integrated circuits with nonuniform power maps. Four heterogeneous micropin-fin samples were fabricated and tested in single-phase experiments with deionized water to investigate the effectiveness of local micropin-fin clustering for the cooling of hotspots. Cylindrical and hydrofoil micropin-fins were tested, as well as two types of heterogeneous arrays: those with pin-fins clustered directly over the hotspot and those with the high density cluster spanning the entire width of the channel to prevent flow bypass around the cluster. Samples were tested with a uniform nominal heat flux of 250 W/cm2 as well as a hotspot heat flux of 500 W/cm2. Local micropin-fin clustering was found to be an effective method of reducing local thermal resistance with a modest pressure drop penalty.

Visualization of Two-Phase Flow in Serpentine Heat Exchanger Passages With Microscale Pin Fins

Microchannel heat exchangers offer the potential for high heat transfer coefficients; however, implementation challenges must be addressed to realize this potential. Maldistribution of phases among the microchannels and the changing phase velocities associated with phase change present design challenges. Flow maldistribution and oscillatory instabilities can affect transfer rates and pressure drops. In condensers, evaporators, absorbers, and desorbers, changing phase velocities can change prevailing flow regimes from favorable to unfavorable. Geometries with serpentine passages containing pin fins can be configured to maintain favorable flow regimes throughout the component for phase-change heat and mass transfer applications. Due to the possibility of continuous redistribution of the flow across the pin fins along the flow direction, maldistribution can also be reduced. These features enable high heat transfer coefficients, thereby achieving considerable compactness. The characteristics of two-phase flow through a serpentine passage with micro-pin fin arrays with diameter 350 μm and height 406 μm are investigated. An air–water mixture is used to represent two-phase flow through the serpentine test section, and flow features are investigated using high-speed photography. Improved flow distribution is observed in the serpentine geometry. Distinct flow regimes, different from those observed in microchannels, are also established. Void fraction and interfacial area along the length of the serpentine passages are compared with the corresponding values for microchannels. A model developed for the two-phase frictional pressure drops across this serpentine micro-pin fin geometry predicts experimental values with a mean absolute error (MAE) of 7.16%.

Pressure drop and heat transfer characteristics of pin fin enhanced microgaps in single phase microfluidic cooling

Microfluidic cooling with microgaps with surface area enhancements such as pin fins can potentially achieve superior thermal performance in cooling high power microsystems. In this work, the single phase thermal/fluid characteristics of microgaps of H/W around 0.02, incorporating staggered pin fin arrays are studied. The effects of the pin fin dimensions including diameter, transversal and longitudinal spacing, and height are investigated by experimentally validated model over a range of Reynolds number (Re) 22–357. Micropin fin arrays investigated have pin diameter of 100μm, pitch/diameter ratios of 1.5–2.25, and height/diameter ratios of 1.5–2.25. Correlations of friction factor (f) and Colburn j factor for these dense arrays of micro pins have been developed. The performance of circular and square micro pin fin arrays were compared, showing that the circular array has better thermal performance under the same pumping power.

Fluid flow and heat transfer performance in a micro-reactor with non-uniform micro-pin-fin arrays for hydrogen production at low Reynolds number

To improve the flow and heat transfer performance of micro-reactors with micro-pin-fin arrays (MPFAR), a micro-reactor with non-uniform micro-pin-fin arrays (NMPFAR) was proposed. To investigate its flow and heat transfer characteristic at low Reynolds number, the Navier–Stokes and energy equations in the NMPFAR were established. Simulated results indicated that the NMPFAR has the better heat transfer performance than micro-reactors with staggered and aligned micro-pin-fin arrays. Also, the heat transfer performance of three different NMPFARs with two, four and eight kinds of pin diameter was investigated by numerical simulation. Results showed the Nusselt number is larger when the change of pin diameter in NMPFAR is less, which means the NMPFAR with two kinds of pin diameter has the better heat transfer performance. Meanwhile, the experiments were carried out for validating the simulation model, and the experimental results agreed well with the simulated ones. The research result shows that the NMPFAR does have the merit for enhancing flow and heat transfer performance of the micro-reactor with micro-pin-fin arrays.