Youmin Yu

Compact Thermal Resistor-Capacitor-Network Approach to Predicting Transient Junction Temperatures of a Power Amplifier Module

Junction temperature is an important issue for a semiconductor package, influencing the packages thermal, mechanical, and reliability performance. An accurate prediction of junction temperature provides informative guidance in design, development and operation of the package. A compact thermal resistor-capacitor (RC) network approach is presented in this paper to accurately predict transient junction temperatures. The thermal RC network in this approach is a nongrounded Foster network. This approach consists of extraction of the thermal Foster network and prediction of the transient junction temperature response to a given power input using the extracted network. The network extraction part is based on Kirchhoffs current law and Laplace transformation technique, and uses the Foster network to facilitate changes of the RC network structure. The temperature prediction part is a direct substitution-and-calculation process, and therefore is fast to carry out. Since Laplace transforms are directly or indirectly available for most power inputs, their transient temperatures may be predicted by the proposed approach. Superposition is employed in cases where the Laplace transform of a given power input is not directly found in Laplace tables, or where the junction temperature is affected by multiple heat sources. The proposed approach is demonstrated with a power amplifier (PA) module; predicted junction temperatures are accurate in both single and multiple heat source cases.

Enhancing Heat Transfer of Air-Cooled Heat Sinks Using Piezoelectrically-Driven Agitators and Synthetic Jets

Air-cooled heat sinks prevail in microelectronics cooling due to their high reliability, low cost, and simplicity. But, their heat transfer performance must be enhanced if they are to compete for high-flux applications with liquid or phase-change cooling. Piezoelectrically-driven agitators and synthetic jets have been reported as good options in enhancing heat transfer of surfaces close to them. This study proposes that agitators and synthetic jets be integrated within air-cooled heat sinks to significantly raise heat transfer performance. A proposed integrated heat sink has been investigated experimentally and with CFD simulations in a single channel heat sink geometry with an agitator and two arrays of synthetic jets. The single channel unit is a precursor to a full scale, multichannel array. The agitator and the jet arrays are separately driven by three piezoelectric stacks at their individual resonant frequencies. The experiments show that the combination of the agitator and synthetic jets raises the heat transfer coefficient of the heat sink by 80%, compared with channel flow only. The 3D computations show similar enhancement and agree well with the experiments. The numerical simulations attribute the heat transfer enhancement to the additional air movement generated by the oscillatory motion of the agitator and the pulsating flow from the synthetic jets. The component studies reveal that the heat transfer enhancement by the agitator is significant on the fin side and base surfaces and the synthetic jets are most effective on the fin tips.Copyright

Microfabrication of short pin fins on heat sink surfaces to augment heat transfer performance

Plate-fin heat sinks have been a successful technology in electronics cooling. Thermal performance of such heat sinks, however, has been driven to improve due to increasing heat generation in modern electronics devices. This paper proposes to introduce short pin fins on surfaces of plate-fin heat sinks to address such challenges. A microfabrication approach based on photolithography and electroplating technologies is devised to fabricate short copper pin fins on copper plates. The photolithography implements desired patterns of pin fins, and the electroplating enables pin fins to directly grow out of the base plate. A series of pin-fin coupons were fabricated using the devised method. A heat transfer test was designed to evaluate heat transfer augmentation by the pin fins. Fabricated coupons were tested in a rectangular channel and their thermal conductance and channel pressure drop were measured. A Design of Experiments (DoE) procedure via the Taguchi method was employed to find the influence of four factors: pin-fin height, diameter, spacing, and cross sectional shape, on the combination of thermal conductance and channel pressure drop for the coupons of different pin-fin parameters. Compared with similar plain coupons, pin-fin coupons of the best design parameters increase the thermal conductance by 78.3 % with only 7.8% increase of channel pressure drop. The devised micro-pin-fin fabrication has been proved as an effective approach to augmenting heat transfer of air-cooled plate-fin heat sinks.

An Active Heat Sink System With Piezoelectric Translational Agitators and Micro Pin Fin Arrays

Conventional heat sink systems with blowers or fans are approaching maximum thermal management capability due to dramatically increased heat dissipation from the chips of high power electronics. In order to increase thermal performance of air-cooled heat sink systems, more active or passive cooling components are continually being considered. One technique is to agitate of the flow in the heat sinks to replace or aid conventional blowers. In the present study, an active heat sink system that is coupled with a piezoelectric translational agitator and micro pin fin arrays on the heat sink surfaces is considered. The piezoelectric translational agitator generates high frequency and large displacement motion to a blade. It is driven by an oval loop shell that amplifies the small displacement of the piezo stack actuator to the several-millimeter range. The blade, made of carbon fiber composite, is easily extended to a multiple-blade system without adding much mass. The micro pin fin arrays were created with the LIGA photolithography technique. The cooling performance of the heat sink system was demonstrated in single-channel and multiple-channel test facilities. The singlechannel test results show that the active heat sink with the agitator operating at a frequency of 686 Hz and peak-to-peak displacement of 1.4 mm achieved a low thermal resistance of 0.053 C/W in a channel with a 7.9 m/sec flow velocity. Different configurations of the translational agitator with multiple blades were fabricated and tested in a 26-channel, full-size heat sink. Vibrational characteristics are also provided.© 2012 ASME

Comparison of Heat Transfer Enhancement by Actuated Plates in Heat-Sink Channels

This paper investigates heat transfer enhancement of an air-cooled plate-fin heat sink by introducing actively-driven agitating plates within its channels. The investigation was computationally conducted with a single actuated plate in a single channel constructed as two fin wall surfaces and one fin base surface. As air flows through the channel, the plate is vibrated transversely to agitate the channel flow and thereby enhance heat transfer. The channel flow and the actuated plate are considered to be driven by a fan and a piezoelectric stack, respectively. A Coefficient of Performance (COP), ratio of total heat dissipated from the fin channel to total electric power to drive the fan and the agitator plate, is employed to evaluate overall heat transfer enhancement. A short plate, i.e. a plate is only placed at the entrance of the channel, has been shown to possess higher COP than a longer plate, i.e. a plate that is extended to be over most of the channel. For the short plate, COP is higher when it is actuated than when it is stationary. Detailed turbulence-kinetic-energy contours indicate that the higher COPs are due to turbulence generated along the plate edges and streamwise acceleration and deceleration of the bulk channel flow; both are induced by the vibration of the plate. Within regions where the plate is present, the generated turbulence and the acceleration and deceleration augment heat transfer. For a short plate, the turbulence and unsteadiness are transported downstream of the actuated plate to increase heat transfer in that region. However, such turbulence and unsteadiness are drawn out of the channel without full benefit of agitation and heat transfer enhancement when the plate is long, as the plate’s trailing edge is already close to the channel exit. This leads to a conclusion that the short plate is a better choice for active heat transfer enhancement.Copyright

A Computational Study of Active Heat Transfer Enhancement of Air-Cooled Heat Sinks by Actuated Plates

Heat transfer performance of air-cooled heat sinks must be improved to meet thermal management requirements of microelectronic devices. The present paper addresses this need by putting actuated plates into channels of a heat sink so that heat transfer is enhanced by the agitation and unsteadiness they generate. A proof-of-concept exercise was computationally conducted in a single channel consisting of one base surface, two fin wall surfaces, and an adiabatic fourth wall, with an actuated plate within the channel. Air flows through the channel, and the actuated plate generates periodic motion in a transverse direction to the air flow and to the fin surface. Turbulence is generated along the tip of the actuated plate due to its periodical motion, resulting in substantial heat transfer enhancement in the channel. Heat transfer is enhanced by 61% by agitating operation for a representative situation. Translational operation of the plate induces 33% more heat transfer than a corresponding flapping operation. Heat transfer on the base surface increases sharply as the gap distance between it and the plate tip decreases, while heat transfer on the fin wall surface is insensitive to the tip gap. Heat transfer in the channel increases linearly with increases of amplitude or frequency. The primary operational parameter to the problem is the product of amplitude and frequency, with amplitude being slightly more influential than frequency. The analysis shows that the proposed method can be used for modern levels of chip heat flux in an air-cooled model forestalling transition to liquid or phase-change cooling.Copyright

Heat Transfer Enhancement by Synthetic Jet Arrays in Air-Cooled Heat Sinks for Use in Electronics Cooling

A synthetic jet is an intermittent jet which issues through an orifice from a closed cavity over half of an oscillation cycle. Over the other half, the flow is drawn back through the same orifice into the cavity as a sink flow. The flow is driven by an oscillating diaphragm, which is one wall of the cavity. Synthetic jets are widely used for heat transfer enhancement since they are effective in disturbing and thinning thermal boundary layers on surfaces being cooled. They do so by creating an intermittently-impinging flow and by carrying to the hot surface turbulence generated by breakdown of the shear layer at the jet edge. The present study documents experimentally and computationally heat transfer performance of an array of synthetic jets used in a heat sink designed for cooling of electronics. This heat sink is comprised of a series of longitudinal fins which constitute walls of parallel channels. In the present design, the synthetic jet flow impinges on the tips of the fins. In the experiment, one channel of a 20-channel heat sink is tested. A second flow, perpendicular to the jet flow, passes through the channel, drawn by a vacuum system. Surface- and time-averaged heat transfer coefficients for the channel are measured, first with just the channel flow active then with the synthetic jets added. The purpose is to assess heat transfer enhancement realized by the synthetic jets. The multiple synthetic jets are driven by a single diaphragm which, in turn, is activated by a piezoelectrically-driven mechanism. The operating frequency of the jets is 1250 Hz with a cycle-maximum jet velocity of 50 m/s, as measured with a miniature hot-film anemometer probe. In the computational portion of the present paper, diaphragm movement is driven by a piston, simulating the experimental conditions. The flow is computed with a dynamic mesh using the commercial software package ANSYS FLUENT. Computed heat transfer coefficients show a good match with experimental values giving a maximum difference of less than 10%. The effects of amplitude and frequency of the diaphragm motion are documented. Changes in heat transfer due to interactions between the synthetic jet flow and the channel flow are documented in cases of differing channel flow velocities as well as differing jet operating conditions. Heat transfer enhancement obtained by activating the synthetic jets can be as large as 300% when the channel flow is of a low velocity compared to the synthetic jet peak velocity (as low as 4 m/s in the present study).Copyright

Impacts of solder voids on PQFN packages' thermal and mechanical performances

Power Quad Flat No-lead (PQFN) packages are extensively used in automotive applications. Features such as solder die attach material, thick copper lead frame, exposed heat sink and heavy gauge aluminum wire, are adopted to ensure good thermal management and reliability performance.

The Effects of Agitator Blade Geometry and Configuration for Augmenting Heat Transfer by Agitation in Channel Flows

Translationally oscillating blades, called agitators, can be used to thoroughly mix the flow inside heat exchanger channels such as those in an electronics module heat sink. Generally, throughflow is provided with an induction fan. Agitation is implemented inside the channel by using either multiple agitator blades, agitator blades with notched edges, full-length long-blade agitators or short-blade agitators. The power needed to drive the agitator blades is dependent on the agitation velocity, geometry and mass. The performance features of a 50mm long agitator blade operating at an oscillation frequency of 500Hz, a 15mm short agitator blade operating at a frequency of 1000Hz, and two blades of length 15mm operating at a frequency of 500 Hz have been compared. Also, runs with other geometric changes, like those with added notches at the tip of the agitator, are made to explore their benefits. The intent is that the notches generate additional vorticity at the channel inlet, which is convected downstream enhancing heat transfer as it passes. Thus, this study numerically finds directions toward optimal agitator configurations and geometries that would give heat transfer augmentation without excessive power input. It was found that a multiple agitator blade configuration containing two short blade agitators operating at frequency 500Hz gives the best performance in terms of heat transfer augmentation when power consumption is considered. Heat flux plots on the channel wall and turbulence kinetic energy plots within the channel have been used to explain the mechanisms of heat transfer augmentation for the various cases.Copyright

Development of Synthetic Jet Arrays for Heat Transfer Enhancement in Air-Cooled Heat Sinks for Electronics Cooling

Synthetic jet arrays driven by a piston-diaphragm structure with a translational motion were fabricated. A piezo-bow actuator generating large translational displacements at a high working frequency was used to drive the jets. Vibration analysis with a laser vibrometer shows the peak-to-peak displacement of the piston inside the jet cavity of about 0.5 mm at the second resonant vibrational frequency of 1,240 Hz. In this driving condition, the peak velocity of a 20-orifice jet array reaches 45 m/s for each orifice with a total power consumption of 1.6 W. Heat transfer performance of the jet array was tested on a 100-mm-long single channel of a 26-channel heat sink. The synthetic jet flow impinges on the tips of the fins. A cross flow through the channel enters from the two ends of the channel, and exits from the middle. Results show that the activation of jets generates a unit-average heat transfer enhancement of 9.3% when operating with a channel flow velocity of 14.7 m/s, and 23.1% when operating with a channel flow velocity of 8 m/s. The effects of various choices for orifice configuration and different dimensionless distances from the fin tips, z/d, on jet performance were evaluated. By decreasing the length of the fin channel from 100 mm to 89 mm and reducing the orifice number of the jet array from 20 to 18, jet peak velocities of about 54 m/s can be obtained with the same power consumption, and a heat transfer enhancement of 31.0% from the jets can be achieved on the 89-mm-long heat sink channel with a flow velocity of 8 m/s.Copyright