A Review on Transient Thermal Management of Electronic Devices

A Review on Transient Thermal Management of Electronic Devices John Mathew Department of Mechanical Engineering, Much effort in the area of electronics thermal management has focused on developing Indian Institute of Technology Bombay, cooling solutions that cater to steady-state operation. However, electronic devices are Mumbai 400076, India increasingly being used in applications involving time-varying workloads. These include e-mail: [email protected] microprocessors (particularly those used in portable devices), power electronic devices such as insulated gate bipolar transistors (IGBTs), and high-power semiconductor laser Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 Shankar Krishnan1 diode arrays. Transient thermal management solutions become essential to ensure the performance and reliability of such devices. In this review, emerging transient thermal Department of Mechanical Engineering, management requirements are identiﬁed, and cooling solutions reported in the literature Indian Institute of Technology Bombay, for such applications are presented with a focus on time scales of thermal response. Mumbai 400076, India Transient cooling techniques employing actively controlled two-phase microchannel heat e-mail: [email protected] sinks, phase change materials (PCM), heat pipes/vapor chambers, combined PCM-heat pipes/vapor chambers, and ﬂash boiling systems are examined in detail. They are compared in terms of their thermal response times to ascertain their suitability for the thermal management of pulsed workloads associated with microprocessor chips, IGBTs, and high-power laser diode arrays. Thermal design guidelines for the selection of appropriate package level thermal resistance and capacitance combinations are also recommended. [DOI: 10.1115/1.4050002]

1 Introduction microelectronic devices seldom operate under steady-state conditions. Figures 1(a) and 1(b) demonstrates the temporal variation in As electronic devices continue to undergo intensifying minia- CPU workloads of a D2461 Siemens—Fujitsu server. The power turization [1,2], power densities persist in being on a rising trend. consumption of the 4 GHz AMD Athlon 64 dual-core processor Heat fluxes over 102 W/cm2 are currently generated by microproc- can be seen to vary continuously with time as it performs an essors [3]. Moreover, hot spots in chips involve localized heat online music search and download operation [40]. fluxes of 1 kW/cm2 or more, leading to excessive local tempera- Another development in the microelectronic industry is the tures [4,5]. Rising user demand for augmented computational per- growing shift toward portable handheld devices such as smart- formance and functionalities has been fueling the development of phones and tablets [41,42]. These devices exhibit long durations high-power microprocessors. Apart from IC devices, power semi- of low-power consumption for regular applications and short conductor devices such as insulated gate bipolar transistors durations of high-power consumption during process-intensive (IGBTs) and laser diode arrays also generate heat fluxes above applications such as video calling/recording [43–45]. 1 kW/cm2 [6–9]. Operation at excessive temperatures impairs Compared to microprocessor chips, a more pronounced tran- device performance and reliability and ultimately causes their fail- sient thermal behavior is exhibited by power semiconductor devi- ure. The different failure modes of microelectronic devices, ces. Rapid fluctuations in junction temperatures are experienced namely mechanical, electrical, and corrosion, are linked to high during high-frequency switching operations. Highly transient ther- operating temperatures [10]. Pecht and Gu discussed the various mal characteristics are also witnessed in high-power semiconduc- failure mechanisms identified in electronic products. Fatigue due tor laser diode arrays [46]. to temporal temperature oscillations leads to failure at locations Cooling solutions that work well under steady-state conditions such as the die-attach, wire bonds, solder leads, bond pads, and may not offer desired thermal characteristics when devices are vias [11]. Similarly, in the case of power electronic devices, high- operated under transient workloads. The primary focus of steady- frequency switching operation results in periodic temperature state cooling systems is to reduce the overall thermal resistance swings. The resulting thermomechanical stresses lead to the fail- (R) of the package so that maximum power can be dissipated ure of the constituent semiconductor components and thereby without exceeding the maximum temperature limit (T ) of the undermine device reliability. Transient thermal management tech- j,max device. The thermal capacitance of the package (C), though, is not niques that reduce temperature swings, for such applications considered. Such a policy of minimizing R without optimizing C involving time-varying power loads, therefore become inevitable. results in the following drawbacks: There has been considerable progress in implementing various cooling techniques to address the thermal management challenges (1) Highly efficient compact cooling devices with low R usu- associated with high heat flux devices. A summary of these tech- ally have low C [47,48]. Packages with low C exhibit sud- niques is listed in Table 1. As can be inferred, developments in den changes in temperature in response to pulsed single-phase, two-phase cooling, jet impingement, and spray cool- workloads. The consequent thermal fatigue effects can be ing have enabled the dissipation of ultrahigh heat fluxes detrimental to device reliability. [9,23,34,35,38,39]. (2) Steady-state cooling systems developed based on Tj,max is While much research work in conventional thermal manage- often over-designed as the average heat dissipation rates of ment deals with cooling under steady-state device operation, most electronic devices are usually lower during operation. (3) Any attempt to augment device performance, for a package 1Corresponding author. designed with a specific R, is constrained by power limita- Contributed by the Electronic and Photonic Packaging Division of ASME for tions governed by Tj,max. publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received September 22, 2020; final manuscript received January 22, 2021; published online The limitations of steady-state-based cooling systems under August 6, 2021. Assoc. Editor: Ercan Dede. pulsed workloads were demonstrated by Jankowski and

Journal of Electronic Packaging Copyright VC 2022 by ASME MARCH 2022, Vol. 144 / 010801-1 Table 1 List of conventionally employed thermal management techniques and their performance capabilities as reported in literature

Maximum h Cooling Maximum q00 (W/m2K)/Minimum R technique Configuration Authors Coolant (W/cm2) (K/W)

Single-phase Remote Tuckerman and Pease Water 790 __/0.09 microchannel [12] Remote Prasher et al. [13] Water 58.33 __/0.41 Remote Lee et al. [14] Water – 22,000/__ Remote Kosar and Peles [15] Water 167 55,000/__ Remote Colgan et al. [16] Water 500 200,000/__ Remote Wang et al. [6] Water 13.64 __/0.006 Ethylene glycol/water 13.64 __/0.003 Embedded Zhou et al. [9] Water 127.5 64,000/__ Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 Two-phase cooling Remote Kosar et al. [17] R-123 312 68,000/__ (forced convection in Remote Zhu et al. [18] Water 969 210,000/__ microchannels) Remote David et al. [19] Water 82 300,000/__ Remote Fazeli et al. [20] Water 380 260,000/__ Remote Kandlikar et al. [21] Water 506 193,000/__ Embedded Wang et al. [6] R134a 13.64 29,000/0.003 Embedded Green et al. [22] Air/FC-72, water 100, 1000 (hot spot) __/__ Embedded Drummond et al. [23] HFE-7100 910 28,200/ Two-phase cooling Pool boiling Li and Peterson [24] Water 350 __/__ (natural convection) Pool boiling Rahman et al. [25] Water 230 210,000/__ Pool boiling Jaikumar and Kandli- Water 394 713,000/__ kar [26] Thermosyphon Raju and Krishnan Water, water-sodium 31.3 52,000/__ [27] lauryl sulfate surfactant Jet impingement Ndao et al. [28] R134a 275 150,000/__ cooling Rau et al. [29] HFE-7100 205.8 19,000/__ Joshi and Dede [30] R-245fa 218 97,800/__ Wu et al. [31] Water 160 41,377/__ Capillary-fed boiling Palko et al. [32] Water 1280 630,000/__ Spray cooling Pautsch and Shedd FC-72 77.8 13,000/__ [33] Fabbri et al. [34] Water 93 25,000/__ Mudawar et al. [35] HFE-7100 263 __/__ Tan et al. [36] R134a 165 39,000/__ Zhou et al. [37] R410a 330 300,000/__

McCluskey [48], who examined the transient thermal response of 2 Emerging Transient Thermal Management a semiconductor power electronics package when cooled using Requirements five different configurations (cases 1–5 as illustrated in Fig. 2(a)). The overall thermal resistances (R) and thermal capacities (C)of The transient thermal issues associated with different micro- the configurations were in decreasing orders of their extents of electronic devices vary in terms of their operating conditions and cooling integration. As seen from Fig. 2(b), the maximum temper- the time scales involved. In this section, the transient thermal atures of cases 3 and 5 were, respectively, 30% and 207% higher management challenges associated with microprocessor chips, than that of case 1, while their temperature swings were higher by IGBTs, and high-power laser diode devices are examined in 106% and 401% as the period of the pulses approached their ther- detail. mal time constants (s ¼ RC). From this study, it is evident that highly efficient steady-state cooling solutions with low thermal 2.1 Microprocessors. While the ongoing trend toward dimin- resistances (R) and thermal capacities (C) are not capable of miti- ishing transistor sizes is driven by performance enhancements gating thermal transients during the pulsed load operation, espe- associated with smaller transistors, transistor downscaling is lim- cially when the frequency of the pulses is comparable to the ited by rising leakage currents, and the proportion of static power thermal time constant of the package. A similar observation was to the total power consumed is rising considerably [49]. This also made by Meysenc et al. [47], who compared the transient thermal leads to the problem of dark silicon, where the active area of the responses of an integrated microchannel liquid heat exchanger, a chip is minimized to limit the amount of heat generation. Process- liquid cold plate, a forced convection heat sink, and a natural con- ors are, therefore, thermally constrained to operate below their vection heat sink (in increasing order of thermal resistance (R) rated peak performance levels [49]. and thermal capacity (C)). The results of this study are discussed Traditional chip cooling designs are based on thermal design in detail in Sec. 2.2. It becomes pertinent to employ dynamic ther- power (TDP), which is defined for steady-state heat dissipation. mal management techniques that can mitigate thermal transients. TDP is estimated as In this review, emerging transient thermal management requirements are identified, and different transient cooling techniques Tj;max Tambient TDP ¼ (1) reported in the literature are examined. Based on their heat dissi- Rtot pation capabilities and time scales of thermal response, viable cooling solutions for electronic devices such as microprocessors, where Tj,max, and Tambient are the maximum allowable junction IGBTs, and high-power laser diode arrays are recommended. temperature and ambient temperature, respectively, while Rtot is

010801-2 / Vol. 144, MARCH 2022 Transactions of the ASME involve user-interactive applications. For bursty workloads, implementing a steady-state thermal design with thermal throttling leads to an increase in latency and thereby reduces the quality of service (QoS). If the device is instead subjected to short-duration computational bursts surpassing the TDP, its performance can be augmented considerably without reaching Tj,max limits. QoS can accordingly be enhanced by improving user responsiveness [55]. Rotem et al. [54] demonstrated Intel’s Turbo-Boost technology in a quad-core IntelVR CoreTM 2 Duo 2860QM processor having a rated TDP of 45 W. The thermal responses of the chip under steady-state and transient (turbo) workloads were compared for a notebook device (large form factor) and a portable device

(small form factor), as shown in Figs. 3(a) and 3(b), respectively. Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 To facilitate turbo-operation, the thermal capacitance of the device (consisting of the chip, heat sink, and enclosure) was employed as a buffer. Under turbo-operation, the notebook chip offered a performance enhancement of 34% was obtained over the 20 s duration compared to steady-state operation (as shown in Fig. 3(a)). Similarly, Raghavan et al. [53] presented the concept of computational sprinting that enhances the user experience for interactive applications. Whenever a demand for increased user responsiveness arises, all processor cores are made to run simultaneously (parallel sprinting) such that high performance is delivered. To prolong the sprint duration, Raghavan et al. [53] employed PCM to limit the rise in Tj over a brief period of time during which the PCM melts. Higher computational performance can be sustained when the duration of computational sprinting is extended by employing a PCM to store thermal energy.

2.2 Insulated Gate Bipolar Transistors. Owing to their high operating voltage and current capacities as well as their high switching speeds, IGBTs are extensively used as a power con- verter, power inverter, and uninterrupted power supply (UPS) devices. They are primarily employed for traction applications and power generation, and management (solar PVs, wind energy, Fig. 1 Temporal power consumption of a 4 GHz AMD Athlon 64 power grid) [56]. dual core processor (of a D2581 Siemens—Fujitsu server) during an online music search and download operation involving The electrical and thermal characteristics of IGBTs exhibit (a) 100 user requests per minute and (b) 800 user requests per transient behavior as these devices involve high-frequency switch- minute (adapted from Ref. [40]) ing operations. Frequencies can reach up to 150 kHz under soft- switching conditions [57]. As the switching frequency increases, the switching losses and heat dissipation rates also increase the total thermal resistance between the junction and ambient. [58,59]. The cyclic variation of junction temperature with periodic TDP signifies the maximum power that can be dissipated by the on-off power conditions causes thermomechanical stresses in the chip within the limit of Tj,max. IGBT die components. Such stresses affect device reliability and For steady-state thermal designs not utilizing the thermal capac- reduce the number of cycles to failure. itance of the cooling system, device operation within the limits of Held et al. [60] described two types of failure mechanisms TDP prevents the utilization of the available thermal headroom, caused by the temperature swings during power cycling, namely which constrains device performance, whereas when the thermal lifting of bond wires and solder joint fatigue. Fast power cycling capacitance of the cooling system is used as a buffer, the device tests demonstrated that the number of cycles to failure decreases can be made to run temporarily at workloads exceeding TDP with junction temperature swings (DTj) in a logarithmic manner, before the junction temperature reaches Tj,max. One of the initial as shown in Fig. 4. Moreover, for a given DTj, the number of demonstrations of this approach was made by Cao et al. [50]. cycles to failure reduces with increasing medium junction temper- They employed liquid and solid thermal energy storage systems to ature (Tm). IGBT modules consist of direct bonded copper (DBC) facilitate the operation of portable computing devices under short substrates containing silicon chips. These chips are connected to power bursts. Such a mode of operation is currently used in Intel’s the DBC by metallic bond wires (made of aluminum). The attach- Turbo-Boost [51] and AMD’s Turbo-Core [52] technologies. This ments between the silicon chips and DCB as well as that between technology was also studied extensively by Raghavan [49], who the DCB and base plate are established by solder joints. During termed it as computational sprinting. power cycling, the varying junction temperatures cause thermo- Based on user experience, computational workloads can be mechanical stresses that arise due to the difference in thermal classified into throughput workloads and bursty workloads expansion coefficients between the silicon chips and the metallic [53–55]. While throughput workloads involve continuous compu- bond wires. Consequently, cracks are formed in the bond wires, tations for sustained user experience (examples: gaming, stream- which results in their failure. Once a bond wire lifts, the electric ing, graphics processing), bursty workloads involve intermittent current distribution to the silicon chips via the remaining bond computations (examples: browsing, interactive programs) where wires becomes nonuniform. This leads to even higher tempera- there are short periods of computational bursts followed by long tures across the chips, which exacerbate thermomechanical crack durations of idle activity. Bursty workloads are becoming increas- propagation in the remaining bond wires. Following this, failure ingly prevalent in portable device chips as such devices primarily of the solder joints commences due to thermal fatigue.

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Fig. 2 (a) Five different package conﬁgurations with increasing cooling integration from cases 1–5 (adapted from Ref. [48]) and (b) comparison of the thermal responses of cases 2 1, 3, and 5 under forced convection cooling (heff 5 10,000 W/m K) when subjected to 1 W/cm2 power pulses with duration of 1 s (adapted from Ref. [48])

Thermomechanical stresses also give rise the chip metallization. the cold plate has a thermal resistance twice as high as the micro- This phenomenon, called reconstruction, increases the metalliza- channel heat exchanger. For such power cycle frequencies, using tion resistance and leads to higher collector-emitter voltages. As a a liquid cold plate becomes more feasible in cost and result, heat dissipation rates and hence the chip temperatures manufacturability. become greater. It, therefore, becomes imperative to reduce DTj In their experimental study on the transient thermal characteris- for improving the reliability of IGBTs. tics of a liquid-cooled SiC power electronic module, Mandrusiak The swing in Tj depends on the IGBT switching frequency and et al. [62] identified that temperature swings became severe when the output frequency relative to the thermal time constant of the the frequency of the electric current excitation was comparable to IGBT module. As the output frequency reduces and becomes the thermal time constant of the module. The thermal time con- comparable to the thermal time constants, DTj becomes more sig- stant of the die (100 ms) being closest to the power supply fre- nificant [57,61]. DTj also increases with switching frequency [61]. quency (10 Hz) had the most significant influence on the Typically, IGBT modules are remotely cooled using a heat sink module’s transient thermal response. With a large thermal time mounted onto the base plate. Air-cooling, forced convection liquid constant of 10 s, the cold plate had the least influence on the tran- cooling, two-phase cooling, jet impingement, and spray cooling sient thermal sensitivity. are the conventionally employed cooling techniques [56]. Insulated gate bipolar transistors cooling systems must there- While the cooling performances of these techniques have been fore be designed such that the module thermal time constants are enhanced in terms of lowering the thermal resistance, the thermal sufficiently higher than the power cycle periods to lessen the tem- capacities of the cooling systems are often not considered for con- poral variation in Tj. As IGBT switching frequencies and output trolling the fluctuations in Tj. Meysenc et al. [47] examined the frequencies are generally in the order of tens of kHz and tens to transient response of different IGBT cooling systems, namely an hundreds of Hz, respectively, cooling solutions should be able to integrated microchannel liquid heat exchanger, a liquid cold plate, respond within a time scale range of 102 to 101 s. a forced convection heat sink, and a natural convection heat sink, under a periodic load. It was identified that the thermal time constants of the cooling systems decreased with increasing cooling 2.3 High-Power Semiconductor Laser Diodes. Semicon- performance. While the integrated microchannel heat exchanger ductor laser diodes are being used extensively for telecommunica- reduced the average Tj on account of its low thermal resistance, its tions, solid-state laser optical pumping, industrial machining, poor thermal inertia led to higher swings in Tj. As shown in image scanning, laser printing, medical surgery, and military Fig. 5, both the liquid cold plate and microchannel heat exchanger applications. With optical efficiencies generally in the range of offer relatively similar cooling performance below 10 s. However, 30-50%, laser diode bars involve high-power densities exceeding

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Fig. 5 Comparison of normalized peak temperatures of different cooling systems with time period of IGBT power losses (adapted from Ref. [47])

called spectral shift. Wavelengths can increase at a rate of 0.27 nm/C for an 808 nm semiconductor laser array [65]. Such wavelength shifts are particularly detrimental when semiconductor laser diode arrays are used as pump sources for solid-state laser devices. Increasing temperature can also result in spectral broadening. This happens when nonuniform temperatures across the laser diode, usually higher at the active regions in the center of the diode bar while lower at the edges, lead to lower red-shifting of the laser beams generated by the emitters located at the edges. Carter et al. [66] developed a thermal model to analyze the transient thermal response of a quasi-continuous wave GaAs laser diode. It was shown that temperature oscillations were highest at the active Fig. 3 Comparison of junction temperature responses under a regions (with responses in microseconds) of the diode emitters steady-state load and a bursty load (Intel Turbo-Boost) for an while they got dampened toward the substrate (with responses in Intel Core 2 Duo 2860QM processor used in (a) a notebook milliseconds) due to thermal inertia effects. Also, high laser diode device with large form factor (adapted from Ref. [54]) and (b)a bar temperatures generate thermal stresses that cause mechanical portable device with small form factor (adapted from Ref. [54]) expansion and variations in the refractive indices of the optical elements. These effects impair the laser beam quality [46]. Laser diode bars involve highly transient operating conditions as laser beams are usually emitted in short-duration pulses. Such quasi-continuous waves consist of high-intensity pulses with periods of 10 ls–1 ms. Consequently, temperature variations of about 1–10 C can occur in microseconds [67]. To improve the performance and reliability of laser diode arrays, thermal system designs should strive to limit temperature oscillations in the active areas. While a number of thermal management techniques such as flow boiling [64], microchannel liquid cooling [68–70], thermoelectric cooling [71,72], spray cooling [73,74], and jet impingement [75] have been demonstrated to dissipate the high heat fluxes generated by laser diode arrays, they are based on the steady-state cooling objective of minimizing thermal resistance. To address the transient thermal challenges of laser diode arrays, cooling systems should tune both the thermal resistance and thermal capacitance to reduce temperature fluctuations and provide a response in a timescale of milliseconds.

Fig. 4 Effect of junction temperature swing (DTj) and medium 3 Transient Cooling Solutions junction temperature (Tm) on the number of cycles to failure In Secs. 3.1–3.5, research developments associated with differ- (adapted from Ref. [60]) ent transient thermal management techniques are reviewed.

1 kW/cm2 [46,63,64]. Such heat losses arise due to optical conver- 3.1 Actively Controlled Two-Phase Microchannel Cooling. sion inefficiencies during the lasing process, bulk thermal resistan- Although two-phase forced convection in microchannel heat sinks ces, and contact resistances. The rise in temperature with heat has demonstrated high heat power dissipation rates with nearly generation increases the threshold current and reduces the slope uniform spatial device temperatures, they perform well mostly efficiency. Higher threshold currents, in turn, cause greater heat during a steady-state operation involving stable boiling condi- generation [46]. Increasing temperature also causes the peak tions. Operating such devices under certain heat flux and mass wavelength of the emitted laser to rise (redshift), a phenomenon flux conditions trigger boiling instabilities that result in early dry-

Journal of Electronic Packaging MARCH 2022, Vol. 144 / 010801-5 out followed by CHF. The pressure drop-mass flux curve under a 3.2 Phase Change Materials. Phase change materials fixed heat flux, two-phase operation in the region where the curve (PCMs) are traditionally used for applications requiring thermal has a negative slope results in excursive or Ledinegg instabilities. energy storage (such as for solar thermal energy storage, offset- If the pressure drop supply curve (of the pump) has a less negative ting peak cooling/heating loads for HVAC systems, and spacecraft slope than the pressure drop demand curve, the two-phase opera- thermal systems [82,83]). Based on their material composition, tion becomes unstable and can shift to a superheated condition. PCMs are generally classified as inorganic, organic, or metallic. Excursive instabilities can also lead to pressure drop oscillations. As PCMs can suppress temperature rises during power surges, These flow fluctuations arise due to compressible volumes located they have received much interest in the transient thermal manage- upstream of the microchannels [76–78]. Two-phase flow stability ment of electronic devices, in particular mobile devices. also depends on the prevalent two-phase flow regime in the microchannels. Boiling instabilities are more pronounced under a slug 3.2.1 Nonmetallic Phase Change Materials. Vesligaj and flow regime where elongated vapor slugs expand rapidly in the Amon [84] assessed the thermal response of a technical informa- microchannels and cause vapor backflow. With progression to an tion assistant (TIA) wearable computer involving a PCM-based

annular flow regime at higher heat fluxes, boiling stability thermal control unit (TCU). When subjected to a power duty Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 improves. A marked rise in instabilities is seen near CHF condi- cycle, the TIA with TCU showed reductions in the maximum sur- tions. The use of two-phase microchannel heat sinks for transient face temperature and temperature swing compared to the TIA electronics cooling is less reported in the literature. without TCU. Zhang et al. [77] demonstrated that two-phase microchannel Hodes et al. [85] investigated the use of two types of PCMs, cooling could be used for transient heat load applications when namely Thermasorb-122 and n-alkane tricosane, for the thermal flow boiling instabilities are actively controlled. Two active flow management of a mobile handset. The device operating time could instability control techniques were considered to enable stable be extended considerably, by up to five times and three times boiling operation under varying heat inputs. For the first type of using tricosane and Thermsorb-122, respectively, when compared feedback control, the inlet valve opening was adjusted, while for to the configuration without PCM. the second type, the supply pump was controlled. A considerable A computational study was undertaken by Krishnan and Gari- reduction in flow oscillations was attained upon initiating active mella [86] to evaluate the transient thermal performance of a control of either the inlet valve opening or supply pump. When PCM-based energy storage system (Eicosane) subjected to pulsed the inlet valve opening was adjusted using active feedback con- heat inputs. The transient thermal response for 12 cases involving trol, the system pressure increased by 1.01 kPa in about 1.7 s when different pulsed power cycles, container aspect ratios, and heater the heat input was raised from 1 kW to 1.5 kW. The thermal locations was simulated. It was identified that the container with capacity of the channel walls was found to have a significant aspect ratio 1 subjected to bottom heating resulted in the highest effect on the transient two-phase flow characteristics. While the thermal energy storage and the lowest maximum temperatures, frequency of pressure-drop flow oscillations was passively low- whereas the configurations with top heating offered the poorest ered with increasing wall thermal capacity, its amplitude initially thermal performance. increased and then decreased with wall thermal capacity. When To enhance the thermal performance of PCM-based transient subjected to stepwise heat inputs, minimal wall temperature and cooling, Krishnan et al. [87] developed a novel hybrid heat sink in flow oscillations were obtained at high wall thermal capacity con- which PCMs were integrated with a plate-fin heat sink. The tips of ditions. In this work, it was also shown that the working fluid the thin aluminum fins of the hybrid heat sink were immersed in a selected for two-phase microchannel cooling should have a PCM. Compared to the heat sink operated without PCM, the smaller liquid-to-vapor density ratio so that vapor bubbles depart hybrid heat sink enabled continuous heat dissipation with tempo- with smaller sizes (relative to the channel size). This can assist in ral fin temperatures maintained in a narrower temperature range reducing boiling instabilities. Despite its higher latent heat and under high and low air-cooling conditions. thermal conductivity, pure water can result in more considerable While pure PCMs can suppress device temperature rises during two-phase flow instabilities as the vapor bubble sizes are much power surges, their poor thermal conductivities lead to a nonuni- greater, whereas organic coolants such as HFE-7100 and R-134a, form melting process. This, in turn, results in uneven temperature demonstrated to have smaller liquid-to-vapor densities than water distribution. To address this drawback, some investigators studied (at a given temperature), can mitigate instabilities. the impact of integrating herringbone type graphite nanofibers Another class of working fluids viable for two-phase micro- [88] and extended surfaces [43,89] with low-conductivity PCMs channel cooling is self-rewetting alcohol/water binary mixtures for transient thermal performance. They found that the PCM heat that offer smaller bubble departure diameters and improved rewet- sinks prolonged the duration to steady-state conditions and low- ting on account of solutal and thermal Marangoni effects. Such ered the peak temperature. Kandasamy et al. [90] examined the coolants have been demonstrated to reduce boiling instabilities effectiveness of employing heat sinks embedded with PCMs to when compared to pure water. The water vapor bubbles generated cool mobile electronic devices. Jaworski [91] integrated a lauric in microchannels tend to have relatively larger bubble diameters, acid PCM with a pin fin heat spreader to promote heat penetration and they expand rapidly in the confined microchannel passages. In into the PCM. It was computationally demonstrated that the PCM- contrast, bubble sizes are smaller for alcohol/water binary mix- based pin-fin heat sinks lowered the temperature excursion rate tures and do not coalesce so readily [79,80]. compared to the non-PCM pin-fin heat sink. Active control of two-phase microchannel flow instabilities was also implemented by Bhide et al. [81], who introduced flow pulsa- 3.2.2 Metallic Phase Change Materials. In their experimental tions in the two-phase loop using a solenoid valve. When com- work, Yoo and Joshi [92] examined the use of metallic PCM- pared to flow boiling of water in a trapezoidal microchannel based finned heat sinks for the thermal management of transient without active control, pressure-drop fluctuations were seen to workloads. Aluminum plate and pin-fin heat sinks, with Inalloy reduce considerably when the pulsed flow was introduced in the 158 PCM (50 Bi/27Pb/13Sn/10Cd) embedded internally in the fin loop. Two-phase flow stability improved as the frequency of the structures, were considered. The plate-fin heat sink with PCM was flow pulsation increased. able to lower the steady-state temperature compared to its non- While two-phase microchannel cooling can be used under tran- PCM counterpart. In the case of fan integrated heat sinks, the sient heat input conditions when flow boiling instabilities are PCM-based plate and pin fin heat sinks required shorter fan oper- actively controlled, the effectiveness of this technique under more ating times compared to their non-PCM counterparts. rapidly varying workloads, for instance, during the pulsed power Shao et al. [45] examined the transient thermal response of a inputs associated with IGBTs and laser diode arrays, as well as silicon thermal test chip (TTC) with alloy PCM integrated into a during turbo-operation in microprocessors, needs to be explored. Samsung Galaxy S3 smartphone. The temperature response of the

010801-6 / Vol. 144, MARCH 2022 Transactions of the ASME TTC-based smartphone was compared with the original smart- Nevertheless, the larger weights of metallic PCMs compared to phone under single and periodic short-duration power bursts. The nonmetallic PCMs put them at a disadvantage despite their supe- ability of the TTC-based smartphone to lower the peak tempera- rior thermal performance. Boteler et al. [96] addressed this trade- ture and temperature swing by 16 C and 21.3 C, respectively, off by developing a module in which a high thermal conductivity demonstrated the effectiveness of using a metallic alloy PCM for metallic PCM, Gallium, is combined with a lightweight organic such power cycles. PCM, PT-29, such that their time scales of thermal response are The effectiveness of using low melting point gallium PCM for matched. In such a hybrid module, the heat dissipated by the chip managing thermal shocks of the order of 100 W/cm2 was explored is initially removed rapidly by adjacent Ga (higher k) as it under- by Yang et al. [93]. For a fixed fin height and fin width fraction, goes a phase change (primary melting) following which the unab- an increase in the fin number resulted in a greater extent of phase sorbed heat is transferred to Pt-29 (low k) that undergo a slower change and temperature uniformity in the PCM. phase change process (secondary melting). Among the hybrid Cu/ From the above studies, it is seen that organic and inorganic Ga/Pt-29 configurations, case 11 (Ga/Pt-29) offered lower temper- PCMs involve slower thermal responses that lead to longer dura- atures than pure Cu (for about 89 s) while it weighed 50% and

tions for heating/cooling during melting/resolidification. As will 25% lesser than pure Cu and Ga, respectively. Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 be shown later in Sec. 4.1, the organic PCMs reviewed in this section are found to have response times >10 s/ C under heat fluxes 2 3.3 Heat Pipes/Vapor Chambers. Heat pipes/vapor cham- in the range of 0.1–10 W/cm . This is due to their lower thermal bers are passive thermal transport devices that effectively transfer conductivities and nonuniform melting characteristics. The supe- heat over short distances from a heat source to a heat sink using rior thermophysical characteristics of metallic PCMs make them phase change heat transfer. As heat transfer in heat pipes/vapor suitable candidates for transient thermal management applica- chambers occurs via phase change, nearly isothermal conditions tions. This is discussed in detail by Shamberger and Bruno [94]. are maintained across these devices [97,98]. Heat pipes/vapor Figure 6 illustrates the higher phase change heat transfer rates into chambers are used for thermal transport applications in spacecraft, metallic PCMs compared to nonmetallic PCMs (salt hydrates and waste heat recovery, HVAC, solar thermal energy storage, and paraffin). The figure of merit (FOMq), as defined by Shamberger electronics cooling [99]. [95] in a previous study, was found to be primarily dependent on More lately, heat pipes/vapor chambers are increasingly being the thermal conductivity (kl) and volumetricpffiffiffiffiffiffiffiffi latent heat of fusion used to cool portable electronic devices such as laptops, tablets, (Lv) of the PCM such that FOMq klLv. Accordingly, the signif- and smartphones [100]. This is being made possible due to the icantly higher FOMq associated with metallic PCMs signifies development of high-performance ultrathin heat pipe devices inte- quicker heat removal from the heat source compared to nonmetal- grated into such small form-factor devices. Ultrathin flat heat lic PCMs. As will be discussed in Sec. 4.1, some of the metallic pipes and loop heat pipes with thicknesses in the ranges of PCMs reviewed in this section are found to offer response time- 0.35–0.6 mm and 0.6 mm–1.2 mm, respectively, were demon- 2 s < 0.1 s/ C even at high heat fluxes (10–100 W/cm ). This dem- strated for practical applications [100–102]. As portable devices onstrates their potential to serve as compact thermal buffer involve user-interactive applications that lead to intermittent short devices that can mitigate short-duration power bursts passively. computational bursts followed by long idle times, transient ther- Another important aspect of PCM-based transient thermal man- mal analyses of heat pipes/vapor chambers under varying work- agement systems is the melting temperature associated with the loads become important. PCM. Considering the case of maintaining safe device operating temperatures of about 80 C, it is desirable to have PCMs with 3.3.1 Fixed Conductance Heat Pipes/Vapor Chambers. melting temperatures in the range of 60–70 C. Metallic PCMs, Patankar et al. [44] studied the transient thermal performance of such as those studied by Shao et al. [45] and Yoo and Joshi [92], vapor chambers compared to metal spreaders in terms of the gov- appear to be suitable for transient electronics cooling applications erning mechanisms involved. A performance metric was defined as they involve melting temperatures of 58 C and 70 C, to ascertain the transient thermal performance of the vapor cham- respectively. ber under different vapor core thicknesses and heating power inputs. From this analysis, it was found that vapor chambers with vapor core thicknesses in the range of 50–100 lm outperformed the copper spreader for time scales greater than 5 s, while those with vapor core thicknesses in the range of 200–300 lm offered better thermal performance when operated at time scales either below 2–4 s or above 30–50 s. In their subsequent work, Patankar et al. [103] developed guidelines on the design of vapor chambers for transient operation, particularly the thicknesses of the vapor core, wall, and wick, as well as the choice of working fluid. Vapor chambers used for transient operation should possess high thermal capacity and high in-plane vapor core thermal conductivity. While thicker vapor cores were found to perform better under steady-state conditions, an optimum vapor core thickness was found to exist for transient operating conditions. The numerical study also determined working fluid choice that resulted in a superior vapor chamber thermal performance under transient conditions. A high vapor core thickness (small wall thickness and minimum wick thickness based on capillary limit) and vapor phase figure of merit were earlier shown to improve the vapor chamber steady-state performance due to the Fig. 6 Heat fluxes into different types of PCMs (metallic and greater in-plane conductance of the vapor core. However, both nonmetallic) as a function of time under a wall temperature high in-plane vapor core conductance and total vapor chamber increment of 10 C. Time commences from the inception of melting. Steady-state heat transfer rates associated with spray thermal capacity were required to enhance transient thermal per- cooling (Sp), Boiling (B), forced convection (FC), and free formance for transient conditions. This included the volumetric convection (free), for different working fluids (air, oil, or water), thermal capacity of the working fluid’s liquid phase in addition to are shown for comparison. (Reproduced with permission from the vapor core thickness and vapor phase figure of merit. The Ref. [94]. Copyright 2020 by Elsevier). vapor chamber’s transient thermal performance with different

Journal of Electronic Packaging MARCH 2022, Vol. 144 / 010801-7 vapor core thicknesses was compared between two working fluids, range by altering the thermal conductance in response to changes namely water, and methanol. Compared to water, methanol in heating power input and ambient conditions. Their use for the resulted in a smaller peak to mean temperature at the optimum precision temperature control of photonics systems was demon- vapor core thickness on account of the larger thermal capacities strated by Cleary et al. [108]. As presented in Fig. 7(a), the VCHP and the higher vapor phase figure of merit. is similar to a conventional constant conductance heat pipe except The transient response of a flat copper heat pipe subjected to that it contains a noncondensable gas reservoir after the condenser heating power inputs exceeding the capillary limit was studied by section. Based on the vapor saturation pressure inside the vapor Baraya et al. [104]. The temperature response of the 150 mm core, the noncondensable gas expands into or withdraws from the (L) 9 mm (W) 0.62 mm (H) heat pipe was then examined condenser section, thereby blocking or exposing the condenser when a heating power of 10 W, higher than the capillary limit (of area, respectively. By changing the condenser area available for 5.1 W), was supplied for 10 s from an initial steady-state heating heat transfer, the thermal conductance of the heat pipe is altered. power of 3 W. Sharp increases in the evaporator and condenser For the passive operation of the VCHP only configuration, evapo- temperatures were seen when the heating power was stepped up. rator temperatures were reasonably controlled for varying heating

An inflection point and plateau were observed in the transient power inputs at a fixed ambient temperature of 65 C. When the Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 temperature profiles of the evaporator and condenser, respec- VCHP was operated in active mode (involving a heated reservoir), tively, 5.25 s after the 10 W pulse was supplied. These points were the evaporator temperature remained near 70 C for much of the identified to indicate the occurrence of dry-out in the heat pipe. ambient temperature range. When the heating power input was reduced to the initial steady- On similar lines, Liu et al. [109] developed a silicon vapor state value of 3 W, the evaporator temperature was 12 C higher chamber whose thermal resistance could be tuned passively to than the temperature before the supply of the 10 W input. This facilitate its use as a thermal switch for applications involving temperature hysteresis was an indication that dry-out had taken nonuniform and transient heat loads. The device, as presented in place. For the pulsed heat input, which is below the time to dry- Fig. 7(b), is similar to a conventional vapor chamber with the out duration, no inflection point was seen in the evaporator tem- addition of noncondensable gas (NCG) in the vapor core. As the perature profile. There was no temperature hysteresis after the NCG pressure was increased, the device thermal resistance heating power was lowered to the initial steady-state level. This became higher. This behavior became more pronounced with demonstrated that the heat pipe could be operated above the capil- decreasing heat input. At higher heat input, the NCG thermal lary limit for short durations without reaching dry-out. resistance weakened as the vapor mass fraction gradient became To address the transient thermal management challenges asso- greater. For the NCG pressure of 12 kPa, the device thermal resist- ciated with starter-alternators used in automobiles, Harmand et al. ance was reduced by four times as the heat input was lowered [105] proposed flat heat pipes. In their numerical study, the tran- from its minimum to maximum level. Such a vapor chamber could sient thermal performance of the flat heat pipe device was com- thereby be used as a protective device against surges in heating pared with a solid copper plate and hollow copper plate. Under a power. The device was also able to clamp the temperature differ- periodic power supply, the flat plate heat pipe offered a lower ence between the evaporator and condenser such that it remained peak temperature and evaporator temperature rise compared to the unchanged with heat input. hollow copper plate, and smaller evaporator-condenser tempera- In order to control the amount of heat transfer and its direction, ture differences compared to the solid copper plate. Moreover, Zhou et al. [110] employed a vapor chamber device as a thermal being lighter, the flat heat pipe was identified to be advantageous diode and a thermal switch. A 2 cm (L) 2 cm (W) 6.3 cm (H) for transient electronics cooling applications. copper vapor chamber with a super-hydrophilic evaporator and In subsequent work, the transient thermal response of vapor super-hydrophobic condenser was used for this purpose. The chambers involving nanofluid-based working fluids was studied by working fluid used was water. When the heat was supplied to the Hassan and Harmand [106]. They observed computationally that super-hydrophilic section, phase change occurred, and the con- the Cu-based nanofluid offered the lowest peak temperatures com- denser liquid droplets returned by bouncing off the superhydro- pared to the CuO- and Al2O3-based nanofluids. The temperature phobic condenser surface. However, when the heat was supplied responses of the nanofluid-based vapor chambers were compared to the superhydrophobic surface, the condensed liquid could not under different volumetric fractions, nanoparticle diameters, wick return from the super hydrophilic surface. As a result, thermal porosities, and thickness, with a copper block. Lower peak tempera- resistance was higher as the heat was transported through the tures were obtained using the nanofluids. This observation became vapor core without sustained phase change. In effect, the vapor pronounced with decreasing Cu nanoparticle diameter and increas- chamber functioned as a thermal diode. The vapor chamber was ing volumetric fraction. Compared to the vapor chamber with pure also configured to function as a thermal switch by means of water, the use of Cu nanofluids was seen to enhance evaporation adjusting the saturation temperature to the desired level. When the and condensation mass flow rates. These mass flow rates were evaporator temperature reached the set saturation temperature, found to increase with smaller nanoparticle diameters, wick thick- phase change occurred, and the vapor chamber was in an “on” ness and porosity. The improved evaporation rates with the nano- state. Conversely, when the evaporator temperature was lower fluids were attributed to their higher thermal conductivities. A than the set saturation temperature, the vapor chamber was in an larger liquid velocity in the wick was obtained with reducing Cu “off” state. The desired vapor saturation temperature was attained nanoparticle diameter, wick thickness, and porosity. The liquid by controlling the amount of residual noncondensable gas in the velocities for the nanofluid with the smallest Cu nanoparticles vapor core and the amount of charged working fluid. (10 nm diameter) were greater than those for pure water. A thermal switch-based active thermal management system for The startup and shut-down thermal characteristics of a flat copper power electronic devices was developed by Yang et al. [111]. The heat pipe were studied by Wang and Vafai [107] through experiments thermal switch, involving a 1 mm x 5 mm x 30 mm channel con- and numerical analysis. Higher convective heat transfer coefficients at taining a liquid metal droplet (Galinstan), was integrated with sin- the condenser outer surface led to a reduction in the maximum wall gle and dual GaN devices attached to a PCB. Heat was dissipated temperatures and the duration to reach a steady-state. While the maxi- from the system in two paths, namely via the front end of the GaN mum temperature rise increased with an increase in heat flux or a devices subjected to forced convection liquid cooling and via the decrease in the convective heat transfer coefficient, heat flux had a rear end of the PCB cooled by natural convection. By adjusting more pronounced effect on the temperature rise. the position of the liquid metal droplet, the system could operate in thermally “on” or “off” states. When the droplet made contact 3.3.2 Variable Conductance Heat Pipes/Vapor Chambers and with metal conductors connected to the single GaN device, the Thermal Switches. Variable conductance heat pipes (VCHPs) thermal switch was “on” and about 70% or more of the heat input offer the capability of maintaining device temperatures within a was dissipated through the frond-end. Conversely, when the

010801-8 / Vol. 144, MARCH 2022 Transactions of the ASME Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021

Fig. 7 Schematic images of (a) a variable conductance heat pipe (adapted from Ref. [108]) and (b) a variable conductance vapor chamber (adapted from Ref. [109])

droplet was made to break contact with the metal conductors, the The combination of a flat heat pipe and PCM was explored by switch was “off” and heat dissipation via the front end was Weng et al. [113] and Zhuang et al. [114]. Under different peri- reduced. For the dual GaN device system, the thermal switch was odic power inputs, it was seen that the composite heat pipe was able to achieve uniform junction temperatures for both GaN devi- able to lower the maximum to minimum temperature compared to ces by adjusting the droplet’s position relative to them. With a the conventional heat pipe (without PCM). A similar study based switching speed of 1 Hz, the thermal switch demonstrated the on copper heat pipe combined with a PCM was assessed by Behi potential to offer quick thermal responses under transient loads. et al. [115] through experimental and computational studies. Yang et al. [116] developed a novel cooling device that com- 3.4 Combined Phase Change Material–Heat Pipe/Vapor bined a low melting point metal PCM (LMPM), E-BiInSn, with Chamber. Researchers have considered complementing the high high thermal conductivity flat heat pipes (FHP) containing finned thermal energy storage capacity of PCMs with the high thermal plates at the condenser sections. Under periodic heating power conductivity of a heat pipe/vapor chamber by combining the two inputs (1000 W for 10 min followed by 15 min of no power), tem- to form composite heat pipes. While most configurations reported peratures for the LMPM—FHP device (with the air-cooling radia- in the literature involve PCM integrated with the heat pipe exter- tor) were maintained within 50 C–85 C as opposed to nally, a few involve PCM embedded within the vapor chamber. temperature swings of 20–160 C associated with the non-PCM Both configurations are examined in Secs. 3.4.1 and 3.4.2. FHP device. In their experimental work, Robak et al. [117] employed heat 3.4.1 Combined Phase Change Material—Heat Pipe/Vapor pipes to enhance the melting and solidification rates of a paraffin Chamber Configurations Involving External Phase Change Mate- PCM (n-octadecane). Compared to the PCM-only module, the rial Integration. Li et al. [112] studied the prospect of combining heat pipe- and fin-assisted PCM modules improved charging and a liquid metal PCM (LMPCM), gallium with a flat heat pipe discharging thermal performance. (FHP) to improve the heat storage effectiveness of the PCM. It was demonstrated that the LMPCM-FHP modules offered smaller 3.4.2 Combined Phase Change Material—Heat Pipe/Vapor temperature changes with time as well as the lowest peak temper- Chamber Configurations Involving Internal Phase Change Mate- atures at steady-state. High heat spreading facilitated by the FHP rial Integration. While PCMs embedded with metallic fins or car- enabled heat to diffuse more effectively into the entire PCM. bon foams offer an enhanced thermal conductivity, the total mass Under normal and heavy usage operations, the FHP-LMPCM con- of the PCM-based heat sink increases as the metallic fins or car- figuration was able to maintain stable temperatures in the ranges bon foams account for a considerable part of the heat sink mass. of 30–59 C and 22.5–31 C, respectively. To improve the phase change performance of PCMs without

Journal of Electronic Packaging MARCH 2022, Vol. 144 / 010801-9 heat storage device, depending on the operating conditions. Com- pared to a PCM heat sink embedded with aluminum ﬁns, the PCM-vapor chamber conﬁguration weighed 52.9% lower for the same quantity of PCM. Similarly, Lee et al. [119] embedded PureTemp25VR PCM into an aluminum vapor chamber, in ten aluminum drawers stacked horizontally with a spacing of 0.32 cm between them. As shown in Fig. 8(b), the space between the drawers served as vapor con- duits. Screen mesh that lined the vapor chamber and drawer walls facilitated the wicking of condensed liquid to the evaporator section. The device enabled operation under heat spreading or heat storage modes. A novel integrated vapor chamber—thermal energy storage

device (VCTES) was developed by Kota [120] for the thermal Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 management of pulsed heat loads. The VCTES was purposed to serve as the heat sink of a spray cooling module. When the electronic device generated power pulses, vapor generated during two-phase cooling in the spray cooling module was directed to the VCTES, where it condensed upon transferring its latent heat of vaporization to a PCM in the condenser section. The heat stored by the PCM was then dissipated to the ambient during the time between the power pulses. While the liquid–vapor phase change process in the spray cooling module facilitated rapid heat transfer from the heat source, the solid–liquid phase change process during vapor condensation enabled slower heat dissipation to the ambient over the long idle state period. Experiments were performed on a VCTES involving four vertically oriented TES columns that were partially within the vapor chamber and partially extended into the ambient. A graphite foam (PocofoamVR ) embedded with parafﬁn wax (PolywaxVR ) constituted each TES column. The temperature response of the system was examined under heat ﬂuxes of 40 W/ cm2 generated in 16 s and 32 s pulses. Compared to a VCTES with no PCM, the VCTES with PCM was able to lower vapor temperature rise.

3.5 Flash Boiling. Engerer [67] demonstrated that temperature overshoots were best suppressed during rapid cooling with high initial rates. Compared to conventional cooling techniques such as pool boiling, single-phase and two-phase microchannel cooling, jet impingement, and spray cooling, flash boiling was identified to be a promising candidate that can offer rapid cooling rates in 100 ms–10 s. Flash boiling occurs when a fluid is suddenly depressurized below its saturation pressure. During this process, the fluid gets superheated and enters a metastable state. Once nucleation is initi- ated (either by heterogeneous or homogeneous nucleation), the liquid begins to undergo rapid phase-change [121–126]. The phenomenon of flash boiling has been studied for understanding boiling liquid exploding vapor expansion (BLEVE) accidents associated with pressurized vessels that rupture [123,124], for improving fuel atomization to enhance combustion efficiencies in Fig. 8 Schematic images of combined PCM-vapor chamber engines [127], and more recently, for rapid cooling of electronic configurations involving (a) micro-encapsulated PCMs embed- devices involving power pulsations [46,128–130]. ded in vapor core of the vapor chamber (adapted from Ref. [118]) and (b) PCM contained in aluminum drawers provided Engerer et al. [129] demonstrated the potential of using this within the vapor core of the vapor chamber. Operation of the phenomenon for the transient cooling of electronic devices involv- device under heat spreading and heat storage modes is shown ing high power pulsed loads. In their experimental work, it was (adapted from Ref. [119]). shown that the transient flash cooling process takes place rapidly within a time span of 1 s. The cooling device was able to maintain temperatures within 6 5 C, even at a high heat flux of 104 W/ increasing the total mass of the PCM-based heat sink, Yun et al. cm2. In earlier work, Engerer et al. [128] studied flash boiling of [118] integrated a paraffin PCM within a vapor chamber (as methanol in chambers with and without porous foam structures. depicted in Fig. 8(a)). Microencapsulated PCM beads, laid inside These high thermal conductivity structures were provided to the vapor chamber, assisted in storing thermal energy and wick increase the convective heat transfer area and bubble nucleation the condensed liquid to the evaporator section. At temperatures sites. Two types of porous foams were considered—a graphitic exceeding the melting point of the PCM, heat supplied to the foam and a carbon-boron-nitrogen foam (CBN). A maximum tem- vapor chamber was stored in the PCM. When the temperature perature drop of 21 C was attained in 65 ms using the CBN foam. dropped below its melting point, the PCM would release its stored Flash boiling was also demonstrated by Shah et al. [130]tobea heat energy to the vapor chamber working fluid. The PCM vapor promising cooling solution for addressing the high power den- chamber was able to serve both as a heat spreader as well as a sities associated with silicon interconnect fabric (Si-IF) based

010801-10 / Vol. 144, MARCH 2022 Transactions of the ASME devices. In their experimental study, flash cooling of a Si-IF- with fast response. The majority of the organic PCMs and com- based device involving a single copper terminal block was exam- bined PCM (organic/metallic)—heat pipes/vapor chambers ined using methanol as the working fluid. The transient thermal exhibit slower thermal response times (>10 s/C) with lower heat response of the copper tube and silicon wafer were examined dissipation rates in the range of 0.1–75 W/cm2. Table 3 classifies under four flash cooling pulse timings. As the flash cooling pulses the different transient cooling techniques into groups based on were applied more frequently, the average silicon wafer tempera- their thermal response time and maximum heat flux ranges. ture as well as the temperature variations of the silicon wafer and copper tube became lower. In the flash boiling studies reported previously, methanol was 4.2 Guidelines on the Appropriate Selection of Package used as the working fluid because of its large latent heat of vapori- Thermal Resistance and Thermal Capacitance. It follows from zation and boiling point close to standard temperature and pres- the Sec. 4.1 that the thermal response time of a transient cooling sure conditions [67]. package is governed by the thermal resistance (R) and thermal capacitance (C) of the system. In particular, the thermal time constant of the system (s ¼ RC) relative to the time period of the 4 Evaluation of Transient Thermal Management power pulsations, dictates the amplitude and time period of the Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 Techniques junction temperature under a pulsed load. The goal of a transient thermal management package should be to reduce junction tem- In this section, the different transient thermal management tech- perature swings as well as maintain the temperature amplitude niques examined in the Sec. 3 are compared in terms of their time within maximum limits. This objective is of paramount impor- scales of thermal response. This is followed by a recommendation tance from the standpoint of thermal reliability. As discussed in of the package thermal resistance (R) and capacitance (C) suitable Sec. 2.2, the system must be designed to have a thermal time con- for cooling transiently operated microprocessor chips, IGBTs, and stant (s) that is sufficiently higher than the time period of the high-power laser diode arrays. power pulsations. Referring to the illustration in Fig. 10, transient thermal man- 4.1 Analysis of Thermal Response Times. The suitability of agement systems should aim at selecting appropriate R and C val- a given transient thermal management technique for a given appli- ues such that (1) the junction temperature swing DTj is lowered, cation should be determined by how quickly or slowly it can dissi- and (2) the total time period of thermal response (tth,r) is matched pate a given pulsed load. For high-frequency power pulsations, with the time period of the pulsed load (tpulse). Here, tpulse is the the cooling package should be able to quickly remove the heat time interval between two successive heat pulse inputs as they from the heat source during the heat loading interval, store it, and increase from low to high levels. In the case of applications thereafter release the stored thermal energy to the ambient during involving short-duration pulsed loads, slow thermal responses can the heat unloading interval, such that the system returns to its lead to incomplete discharging of the stored thermal energy dur- original low-temperature state in time to receive the subsequent ing the cooling cycle (heater “off”). This will lead to an accumula- heat pulse. If a slow thermal response cooling system is used for tion of thermal energy due to the inability of the system to such an application, insufficient time available to discharge the completely dissipate the absorbed heat pulse before the arrival of stored heat to the ambient will result in a net accumulation of ther- the next pulse. To demonstrate this, a simple one-dimensional mal energy within the system. Nevertheless, packages with slower thermal model is used to examine the thermal response of three thermal response times would be suitable for applications involv- types of packages, case 1 involving low C (2 J/K) and high R ing moderate or low-frequency power pulsations. Table 2 com- (15 K/W), case 2 involving high C (50 J/K) and low R (10.1 K/W), pares the heat flux, temperature increment and decrement ranges, and case 3 involving low C (2 J/K) and low R (10.1 K/W). As and the corresponding time intervals for different transient ther- depicted in Fig. 11(a), the model consists of a 100 mm 100 m mal management approaches under thermal charging (heating package with a 10 mm 10 mm heat source at the bottom. The power “on”) and discharging (heating power “off”). Thermal top surface is subjected to natural convection ambient conditions 2 response time, sr, defined as the duration per unit change in tem- (h ¼ 10 W/m K, T1 ¼ 35 C). This corresponds to an ambient perature (tDT peak/DTpeak), is thereafter estimated to signify the thermal resistance of 1/(hA) ¼ 10 K/W. It should be noted that the nature of the thermal response that the cooling system exhibits total package thermal resistance R includes the ambient natural when subjected to heat loading/unloading. Here, tDT peak pertains convection thermal resistance. A pulsed load, Qin (t), involving a to the transient temperature response of the reported thermal man- heat input of 200 W for 200 ms followed by 0 W for 800 ms, is agement system. It is the duration over which the peak junction applied at the heat source. The junction temperature, Tj,i, is ini- temperature increases from minimum to maximum when the heat tially at ambient temperature, T1, at time t ¼ 0 s. An equivalent R- input is turned “on.” Conversely, it is the duration over which the C thermal circuit of the model is shown in Fig. 11(a). By using a peak junction temperature drops from maximum to minimum lumped capacitance analysis, the transient junction temperature Tj when the heat input is turned “off.” The time durations ton and toff can be determined using provided in Table 2 are the durations over which the heating power is “on”/“off,” respectively. DT is the maximum-to- Tj;i T1 t peak T ¼ Q Q ðÞe RC R þ T (2) minimum temperature difference during the heating power “on” j in in R 1 and “off” phases. The thermal response times of various transient thermal man- Figure 11(b) shows the applied transient heat pulse, while agement techniques under thermal charging and discharging are Fig. 11(c) presents the corresponding temperature responses for presented in Figs. 9(a) and 9(b), respectively, with respect to the the three Cases. In case 1, the high R and low C result in a rapid base heat flux. It can be seen that heat pipes/vapor chambers, temperature excursion (large DTj) during the heating cycle (heater metallic and hybrid PCMs, flash boiling, and actively controlled “on”) and in an incomplete dissipation of the stored thermal two-phase microchannel cooling offer faster responses to heat energy during the cooling cycle (heater “off”). As a result of the input. Metallic PCMs and vapor chambers show response times thermal energy accumulated in each heating cycle, the peak junc- within 0.1 s/C while flash boiling, actively controlled two-phase tion temperature can be seen to rise with each subsequent cycle microchannel cooling, and hybrid PCMs demonstrate response whereas for case 2 involving low R and high C, the temperature times between 0.1 and 2.5 s/C. From the standpoint of actual rise during each heating cycle is suppressed. Moreover, there is a applications, such as microprocessors, IGBTs, and laser diode lesser accumulation of thermal energy during the cooling cycle as 2 arrays, which involve high heat fluxes >100 W/cm , metallic the stored heat is dissipated more readily. Case 3, which involves PCM and flash boiling appear to be promising cooling solutions a low R and low C, exhibits a similar thermal response as case 1.

Journal of Electronic Packaging MARCH 2022, Vol. 144 / 010801-11 1811 / 010801-12 Table 2 Comparison of transient thermal management techniques in terms of their thermal response times during heating power “on” and heating power “off” phases

Heater “on” Heater “off” Downloaded fromhttp://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdfbygueston27September2021

Thermal conductivity o.14 AC 2022 MARCH 144, Vol. PCM— PCM Tm enhancement ton Q q" DTpeak tDT peak toff Q q" DTpeak tDT peak organic type (C) elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Vesligaj and Organic — — 900.00 10.00 0.11 8.17 894.71 1800.00 0.00 0.00 8.33 1779.58 Amon [84] Hodes et al. Tricosane 48 — sp 4.00 0.25 58.75 4278.00 sp 0.00 0.00 45.40 75.50 [85] Krishnan and Eicosane 36 — 10.00 150.00 16.67 6.91 14.93 300.00 30.00 3.33 6.08 296.53 Garimella [86] PCM— Thermal organic with conductivity 00 00 internal fins/ PCM Tm enhancement Q q DTpeak tDT peak toff Q q DTpeak tDT peak 2 2 nanofibers type ( C) elements ton (s) (W) (W/cm ) ( C) (s) (s) (W) (W/cm ) ( C) (s)

Kandasamy Paraffin wax 53-57 Plate fin heat sp 6.00 0.62 53.16 1695.84 — — — — — et al. [90] sinks Fok et al. [43] n-Eicosane 36 Plate fin heat 1800.00 5.00 0.08 1.91 1786.92 600.00 0.00 0.00 0.36 586.43 sinks Fleischer et al. Paraffin 54 Embedded sp 750.00 7.27 7.40 871.82 — — — — — [88] graphite nanofibers Baby and n-Eicosane 36.5 Plate fin 9600.00 10.00 0.20 49.93 9610.70 sp 0.00 0.00 40.82 5970.87 Balaji [89] matrix heat sinks Jaworski [91] Lauric acid 41.5 Pipe fin heat — 37.50 1.25 48.49 298.26 — — — — — sinks Thermal conductivity 00 00 PCM— PCM Tm enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak metallic type (C) elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Shao et al. Roto136F 58 — 0.60 11.00 11.00 36.24 0.60 sp 0.01 0.01 34.34 5.98 [45] alloy Thermal rnatoso h ASME the of Transactions PCM— conductivity 00 00 metallic with PCM Tm enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak internal fins type (C) elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Yoo and Joshi 50Bi/27Pb/ 70 Plate fin and 60.00 35.00 0.97 3.16 57.59 6.00 0.00 0.00 1.67 12.68 [92] 13Sn/10Cd pin fin heat sinks Yang et al. Gallium 29.78 Plate fin, 1.00 10000.00 100.00 20.85 1.01 sp 0.00 0.00 16.16 1.14 [93] crossed fin and pin fin heat sinks ora fEetoi Packaging Electronic of Journal

Thermal PCM—hybrid: conductivity Downloaded fromhttp://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdfbygueston27September2021 00 00 metallic þ PCM Tm enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak nonmetallic type (C) elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Boteler [96] Gallium þ PT- 29.8/29 — sp 100.00 100.00 131.39 300.00 — — — — — 29 Thermal conductivity 00 00 Heat pipes/vapor Working enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak chambers fluid elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Patankar et al. Water — sp 4.00 4.00 27.85 150.29 — — — — — [44] Baraya et al. [104] Water — 14.00 7.00 4.55 33.64 15.25 Sp 2.00 1.30 35.23 55.84 Harmand et al. Water — 2.00 70.00 66.67 74.18 2.01 3.00 10.00 9.52 32.95 0.92 [105] Hassan and Har- Cu, CuO, and Nanoﬂuids 0.80 400.00 47.56 34.76 0.79 1.20 0.00 0.00 31.13 1.22 mand [106] Al2O3 nanoﬂuids Wang and Vafai Water — 141.53 99.35 1.40 14.32 141.53 117.76 0.00 0.00 14.25 117.76 [107] Combined organic PCM- Thermal heat pipes/ PCM conductivity 00 vapor type/working Tm enhancement ton Q q DTpeak tDT peak toff Q q" DTpeak tDT peak chambers fluid (C) elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Weng et al. Tricosane/ 42–48 — 300.00 20.00 6.12 13.88 299.20 300.00 10.00 3.06 13.06 323.27 [113] water Zhuang et al. RT55/water 55 — 600.00 20.00 3.31 11.62 574.53 120.00 0.00 0.00 9.79 138.60 [114] Behi et al. RT42/water 42 — sp 70.00 6.19 27.03 2972.32 — — — — — [115]

AC 02 o.144 Vol. 2022, MARCH Yun et al. Parafﬁn 55 — 2130.60 10.25 2.01 3.81 244.36 5.00 0.00 0.00 34.52 5.05 [118] (micro-encapsulated)/water Lee et al. PureTemp 25/ 25 — 13500.00 15.00 0.09 38.25 12977.40 sp 0.00 0.00 24.84 5000.00 [119] (heat acetone storage mode 1) Lee et al. PureTemp 25/ 25 — 4500.00 15.00 0.09 1.24 4500.00 [119] (heat acetone exchanger 2) Kota [120] Parafﬁn wax/ 113 PCM embed- 16.00 160.00 40.00 1.21 14.78 141.10 44.00 11.00 1.38 141.10

010801-13 / water ded in graphite foam 1811 / 010801-14

Combined Downloaded fromhttp://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdfbygueston27September2021 metallic Thermal PCM-heat PCM conductivity 00 00 pipes/vapor type/working Tm enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak 2 2 o.14 AC 2022 MARCH 144, Vol. chambers fluid ( C) elements (s) (W) (W/cm ) ( C) (s) (s) (W) (W/cm ) ( C) (s)

Li et al. [112] Gallium/water 29.77 — 600.00 7.25 7.25 30.70 598.61 600.00 0.00 0.00 26.28 587.39 Yang et al. E-BiInSn/ 60.2 Plate ﬁn heat 600.00 1000.00 10.00 34.14 594.07 900.00 0.00 0.00 33.51 889.74 [116] water sink Thermal conductivity 00 00 Flash Working enhancement ton Q q DTpeak tDT peak toff Q q DTpeak tDT peak boiling fluid elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Engerer et al. Methanol Graphite foam 5.06 126.00 104.00 29.10 4.93 — — — — — [129] Shah et al. Methanol — sp 18.25 28.52 10.60 20.20 — — — — — [130] Actively controlled Thermal con- two-phase ductivity 00 00 microchannel Working enhancement ton Q q DTpeak tDTpeak toff Q q DTpeak tDT peak cooling fluid elements (s) (W) (W/cm2) (C) (s) (s) (W) (W/cm2) (C) (s)

Zhang et al. R-134a — 5.00 1894.43 11.84 29.07 4.83 5.00 915.30 5.72 27.86 4.95 [77] rnatoso h ASME the of Transactions Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021

Fig. 9 Comparison of the thermal response times of different transient thermal management techniques with respect to base heat ﬂux under (a) heating power “on” and (b) heating power “off” phases. Refer to Table 2 for references indicated.

The peak junction temperature increases rapidly under a pulsed Ts,max within the level of human comfort (45 C) is more chal- power input on account of its small thermal capacity. lenging than the regulation of Tj,max in these applications. As such For applications in which only the junction temperature is lim- devices are increasingly being operated in transient mode on ited, the package should be designed with a low thermal resistance account of the heavy usage of responsive-based applications, a (R) and high thermal capacitance (C). This will facilitate the higher C will be able to lower the rise in Tj over the duration of a reduction of the maximum temperature amplitude as well as sup- power burst. Moreover, a higher R will limit heat dissipation to press temperature increment rates. Such a thermal design will suit the ambient such that Ts does not exceed Ts,max. To achieve a high applications such as data server microprocessors, IGBTs, and system level R, it should be noted that the thermal resistance high-power laser diode arrays. However, for hand-held computing between the cooling element (which temporarily stores the ther- devices such as smartphones and tablets, both junction and skin mal energy) and device skin needs to be made high to reduce heat temperatures, Tj and Ts, need to be limited. In fact, maintaining dissipation to the ambient. The thermal resistance between the

Journal of Electronic Packaging MARCH 2022, Vol. 144 / 010801-15 Table 3 Classiﬁcation of transient thermal management techniques based on thermal response time, sr, and maximum heat ﬂux dissipated, q00

sr (s/ C) q00 (W/cm2) <0.1 0.1–10 >10

0.1–10 — Group 3: organic Group 6: organic PCM, organic PCM PCM, organic PCM with internal fins/ with internal fins/ nanofibers nanofibers, combined PCM—heat pipes/ vapor chambers 10–100 Group 1: heat pipes/ Group 4: actively con- Group 6 vapor chambers, trolled two-phase metallic PCMs microchannel cooling, Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021 heat pipes/vapor chambers, flash boiling >100 Group 2: metallic Group 5: flash boil- — PCMs ing, hybrid PCMs

cooling element and skin can be increased by the use of air gaps Thermal design guidelines on appropriate R and C combina- or low-k insulation layers (such as aerogel) [131,132], whereas the tions for different applications are depicted in Fig. 12. Thermal local thermal resistance between the heat source and cooling ele- systems for microprocessor-based portable devices should be ment should be minimized so that the heat pulse can be rapidly designed with sufficiently high R and high C values (indicated by transferred to the cooling element where it is stored temporarily I). In the case of IGBTs, high-power semiconductor laser diode before being dissipated to the ambient via the skin. arrays, and microprocessors for data centers, thermal system A desirable solution is to have a thermal system that enables designs should employ low R and high C (as indicated by II) such tuning of its R and C based on the given pulsed load condition. that the thermal time constant, s ¼ RC, is higher than the period of The use of variable conductance heat pipes/vapor chambers the pulsed load. It should be noted that this is applicable for cool- [108,109], whose R and C can be varied by adjusting the pressure ing solutions in which the working fluid/cooling medium remains of the noncondensable gas, is an attractive approach for transient within the control volume of interest. This may not be applicable heat load applications. Similarly, the use of vapor chambers with for cooling systems such as liquid metal thermal switches in internal thermal energy storage appears to be promising on which the working fluid/cooling medium moves in and out of the account of their ability to operate in heat spreading and heat stor- control volume. For low-frequency power loads that are more or age modes [119]. less steady-state in nature, thermal systems should have low R and C (indicated by III). Applications requiring thermal insulation can use systems with high R and low C (indicated by IV).

5 Conclusions Traditional thermal management of microelectronic devices based on steady-state heat transfer often results in over-designed cooling systems that are sized based on peak loads. They can also result in undesirably large temperature swings when employed under pulsed heat load conditions. Large temperature variations with time cause thermo-mechanical fatigue effects that jeopardize device reliability. In this review, the need for a paradigm shift toward the development of cooling solutions based on transient thermal characteristics is outlined initially. Thereafter, the thermal management challenges and requirements associated with three types of microelectronic devices, namely microprocessors, IGBTs, and high-power semiconductor laser diode arrays, are identified in detail. The transient thermal management capabilities and developments associated with actively controlled two-phase microchannel cooling, PCMs, heat pipes/vapor chambers, combined PCM—heat pipe/vapor chamber configurations, and flash boiling are then assessed extensively. The effectiveness of these techniques for microprocessor chips, IGBTs, and high-power laser diodes is evaluated based on their thermal response times and the maximum heat flux dissipated. Metallic/hybrid PCMs and flash boiling are found to be suitable cooling solutions for applications involving short- Fig. 10 Schematic plots of a pulsed heat load (q00(t) with cycle duration high-power pulses. An important aspect of transient time period of tpulse) and ideal junction temperature response thermal management is the appropriate selection of the overall (Tj (t) with total thermal response time of tth,r)) of a transient thermal resistance (R) and capacitance (C) of the cooling pack- cooling package age. The thermal time constant must be designed to be larger

010801-16 / Vol. 144, MARCH 2022 Transactions of the ASME Downloaded from http://asmedigitalcollection.asme.org/electronicpackaging/article-pdf/144/1/010801/6737531/ep_144_01_010801.pdf by guest on 27 September 2021

Fig. 12 Thermal design guidelines on the selection of system level thermal resistance (R) and capacitance (C)

Funding Data Institute Postdoctoral Fellowship IIT Bombay (Funder ID: 10.13039/501100005808).

Nomenclature C ¼ thermal capacitance (J/K) CHF ¼ critical heat flux CPU ¼ central processing Unit h ¼ heat transfer coefficient (W/m2 K) IGBT ¼ insulated gate bipolar transistor k ¼ thermal conductivity (W/mK) L ¼ latent heat of fusion (J/Kg) PCM ¼ phase change material Q ¼ heating power (W) q00 ¼ heat flux (W/cm2) R ¼ thermal resistance (K/W) sp ¼ single pulse t ¼ time (s) T ¼ temperature (C) TDP ¼ thermal design power

Greek Symbols q ¼ density (kg/m3) s ¼ thermal time constant (s) sr ¼ thermal response time (s/ C) Fig. 11 (a) One-dimensional thermal model of transient cooling package used for analyzing thermal response under three Subscripts cases involving different thermal R and C values. Equivalent thermal R–C network shown on right. (b) Proﬁle of applied heat- 1 / amb ¼ ambient 00 2, ing power pulse with time (Qin,peak 5 200 W/qpeak 5 200 W/cm i ¼ initial with “on” period of 0.2 s and “off” period of 0.8 s). (c) Junction in ¼ input temperature response (Tj) for three types of packages, namely j ¼ junction case 1 (high R,lowC: R 5 15 K/W, C 5 2 J/K), case 2 (low R, high m ¼ melting C: R 5 10.1 K/W, C 5 50 J/K), and case 3 (low R,lowC: R 5 10.1 max ¼ maximum K/W, C 5 2 J/K). out ¼ output s ¼ skin r ¼ response th ¼ thermal than the time period of the power cycles in order to reduce temperature variations. Cooling systems offering tunable R and C show much promise for future transient thermal management Superscript applications as they can dynamically adapt to time-varying “ ¼ per unit area workloads. References Acknowledgment [1] Wong, P. H.-S., Akarvardar, K., Antoniadis, D., Bokor, J., Hu, C., King-Liu, T.-J., Mitra, S., Plummer, J. D., and Salahuddin, S., 2020, “A Density Metric The Institute Postdoctoral Fellowship funding provided by IIT for Semiconductor Technology [Point of View],” Proceedings of the IEEE, Bombay is gratefully acknowledged by the authors. IEEE, 108(4), pp. 478–482.

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