VIBRATION-INDUCED DROPLET ATOMIZATION HEAT TRANSFER CELL FOR HUGH- HEAT FLUX APPLICATIONS

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1 VBRATON-NDUCED DROPLET ATOMZATON HEAT TRANSFER CELL FOR HUGH- HEAT FLUX APPLCATONS S.N. Hffington, W.Z. Black & A Glzr Woodruff School of Mchanical Enginring Gorgia nstitut of Tchnology, 771 Frst Dr. Lov Bldg. Atlanta, GA 3332 Phon: (44) Fax: (44) sam.hffinaon@,m.gatch.du ABSTRACT This papr dscribs a uniqu two-phas cooling mthod that includs a closd hat transfr cll, similar to a thrmosyphon that can b usd to cool microlctronic packags. Th cooling mthod is basd upon a Vibration-nducd Droplt Atomization, or VDA, procss that can gnrat small liquid droplts insid a closd cll and propl thm onto a hatd surfac. Th VDA tchniqu involvs th violnt brak-up of a liquid film into a showr of droplts by vibrating a pizolctric actuator and acclrating th liquid film at rsonant conditions. Th droplts continually coat th surfac with a thin liquid film, which vaporats on th hatd surfac, and th vapor is condnsd on th intmal surfacs of th hat transfr cll as wll as th liquid working fluid. Th condnsd liquid is rturnd via gravity to th pizolctric actuator whr it is again atomizd. A VDA hat transfr cll 5 mm in diamtr and 2 mm thick was constructd. Tst data dscribd in this study includ th hat transfr charactristics and cooling capabilitis for a small-scal cll that is suitabl for cooling a dsktop microprocssor during th bum-in portion of th manufacturing procss. Th VDA procss producs droplts of rlativly uniform diamtr, and th droplts hav sufficint momntum to rach th rmotly locatd hatd sourc. Hat fluxs as high as 2 W/cm2 hav bn masurd whn a chilld watr hat xchangr is usd as th xtrnal hat rmoval dvic. KEY WORDS: two-phas liquid cooling, high hat flux, vibration-inducd droplt atomization, bum-in NTRODUCTON n th microlctronics industry, advancs in tchnology hav brought about an incras in transistor dnsity and fastr lctronic chips. As lctronic packags incras in spd and capability, th lvl of hat flux that must b dissipatd to maintain rasonabl chip tmpraturs has also risn. Cooling lvls ar projctd to rach th 1-15W rang according to th SA Packaging Tchnology Roadmap []. To provid rliabl cooling for this xpctd lvl, th singl lvl intgratd modul undr dvlopmnt by th Packaging Rsarch Cntr at th Gorgia nstitut of Tchnology is xpctd to dissipat btwn 2 and 1W/cm2 [2]. Two-phas hat transfr, involving th vaporation of a liquid in a hot rgion and th subsqunt condnsation of th rsulting vapor in a cold sction, can provid th larg hat fluxs ndd for microlctronic packags to oprat at accptabl tmpratur lvls. 3y changing th phas of th working liquid, a two-phas hat transfr cooling schm can transport high hat transfr rats across modratly small tmpratur diffrncs. Hat pips and thrmosyphons ar xampls of fficint hat transfr dvics that xploit th bnfits of two-phas hat transfr [3-51. mmrsion cooling involving pool boiling of a dilctric working fluid on th surfac of th packag is anothr xampl of two-phas cooling tchnology usful for microlctronic applications [6]. A cooling modul basd on th VDA principl is a nw tchnology capitalizing on th tinfits of two-phas cooling whil improving on hat pip prformanc by liminating th wicking structur and mploying a mor activ mans to transport th liquid phas back to th hat sourc [7]. Without a wick structur, th opration of a VDA cll is not limitd by th capability of th wick to continually supply th boilr sction with liquid. Furthrmor, th VDA cll has th potntial to improv upon th prformanc of a thrmosyphon, bcaus it crats a condition of thin film boiling in th vaporator sction and it can minimiz tmpratur gradints that caus potntially dangrous thrmal strsss. Th vaporation of a thin film prvnts th formation of an insulating vapor blankt that can xist in pool boiling situations. Also, th atomizd droplt momntum is sufficint to propl th liquid droplts through th vapor layr and to sprad th impinging liquid ini. a thin film on th hatd surfac. Whil th VDA cll has advantags ovr hat pips and thrmosyphons du to th production of a thin vaporating film, th VDA cll, lik a thrmosyphon, has on limitation bcaus it must b opratd in it narly horizontal orintation P. A schmatic of on typ of VDA cll is shown in Figur 1. Th cll consists of a vibrating tirivr that crats a showr of small diamtr scondary droplts by braking up a largr /2/$1. 22 EEE intr Socity Confrnc on Thrmal Phnomna

primary drop that forms on th drivr and improv th fficincy of th brakup procss. An optional orific plat can b placd ovr th drivr to rgulat th thicknss of th liquid film on th drivr.

2 primary drop that forms on th drivr and improv th fficincy of th brakup procss. An optional orific plat can b placd ovr th drivr to rgulat th thicknss of th liquid film on th drivr. n this typ of dsign, th drivr jcts small scondary droplts through th hols in th orific plat. Th scondary droplts ar proplld toward th hatd surfac whr thy form a thin film of liquid on th hatr. Th liquid fim vaporats and fills th intrior cavity of th cll with vapor. Th xtrior surfacs of th cll ar coold via hat transfr to th ambint and th vapor condnss on ths surfacs. Th condnsd liquid is thn rturnd via gravity to th drivr whr it onc again is atomizd and th procss is rpatd. Th ntir cll can b vry small and th drivr rquirs only milliwatts of nrgy to oprat. Oprating th pizolctric drivr ovr a narrow rang of frquncis producd a rliabl spray of drops, whil at othr frquncis atomization was inhibitd crating a dangrous dry out of th hatd surfac insid th cll. Th frquncis that wr accptabl wr dtrmind by xprimntation. uizolctric drivr Pizolctric drivr Fig. 1 Schmatic of VDA hat transfr cll Svral VDA hat transfr clls that us forcd air convction as th global cooling mthod hav bn constructd and tstd [ Hat fluxs ovr 1W/cmZ ar dissipatd whil kping th hatr tmpratur blow 1 C whil using standard cooling fans. Ths VDA hat transfr clls hav usd both watr and FC-72 as th working fluid, and thy hav provn mor ffctiv than solid mtallic conductors of qual volum [ 121. This rsarch invstigats a diffrnt VDA hat transfr cll dsign that uss a circulating chilld watr loop, instad of a fin array and fan, for th global cooling mthod. This liquid coold cll is primarily dsignd for controlld cooling of microprocssor packags during th burn-in procss associatd with microlctronic packag production. THE VDA PROCESS n ordr to dsign an fficint hat transfr cll, it is important to undrstand th VDA procss. Th VDA cll includs a mtallic disc coatd with a pizolctric matrial that is nrgizd with a sinusoidal varying voltag. Th rsulting vibrations ar capabl of acclrating a singl liquid droplt or liquid film to an xtnt that th fluid has sufficint vlocity to brak-up or atomiz into numrous smallr scondary droplts. Th scondary droplts ar proplld upward and thy ar capabl of impacting a rmotly locatd hatd surfac. Th frquncy and amplitud of th voltag usd to nrgiz th pizolctric transducr ar important paramtrs that must b proprly controlld in ordr to achiv th liquid brakup phnomna that is ssntial to th VDA procss. Fig. 2 VDA procss schmatic and photograph of jctd scondary droplts from th orific plat Th schmatic and photograph in Figur 2 illustrat th VDA atomization procss usd in this rsarch invstigation. An aluminum pizolctric drivr that was 31 mm in diamtr was placd blow a brass prforatd plat. This prforatd orific plat containd hols 1.59 mm in diamtr spacd on 3.18 mm cntrs in a squar grid. Th dimnsions and spacing of th hols in th orific plat wr slctd to optimiz th production of scondary droplts from th hols in th orific plat. Th thin vrtical spacing btwn th orific plat cratd surfac tnsion forcs that wr sufficint to pump watr onto th drivr surfac from a rsrvoir surrounding th drivr. This liquid layr was acclratd by th motion of th drivr which rsultd in atomization of th liquid through th hols in th orific plat. Scondary droplts wr producd 22 ntr Socity Confrnc 49 on Thrmal Phnomna

3 that wr much smallr than th diamtr of th hols in th orific plat. Th orific plat providd a robust continuous VDA procss by rgulating th mass of watr on th drivr surfac whil not allowing th drivr to bcom ovrloadd or starvd for watr. This droplt atomization procdur nsurd a continual supply of liquid droplts to th hatr surfac. VDA HEAT TRANSFER CELL Svral diffrnt VDA clls that usd a fan and fin array for transfrring hat to th nvironmnt hav bn built and tstd to valuat thir hat transfr charactristics [ n ordr to rmov th highr hat fluxs associatd with th bum-in manufacturing procss, a hat transfr cll was dsignd that rplacd th fan and fins with a chilld watr loop for th global cooling systm. Th bum-in procss provids svral luxuris that do not xist whn dsigning a cooling schm for a dsktop application. Evn though hat fluxs during th burn-in procss ar highr than thos that xist during th opration of a typical microprocssor, a cooling schm for th bum-in procss is not as rstrictd by conomic and siz constraints. Thus, a chilld watr loop maintaining a cool condnsr surfac is a viabl dsign option. -72 Fig. 3 Schmatic of a VDA hat transfr cll dsignd for th bum-in procss of microprocssor packags A schmatic of th VDA hat transfr cll using a liquid chilld loop is shown in Figur 3. Th cll has an ovrall diamtr of 5 mm and th hight is 2 mm. A coppr coil containing circulating chilld watr was locatd insid th cll. Th coppr tubing has an outsid diamtr of 3.2 mm and an innr diamtr of 2 mm. Th pizolctric drivr was locatd 6.35 mm abov th bottom of th cll laving a small backing cavity bhind th drivr. Thr stainlss stl tubs with an outr diamtr of 2 mm wr usd to connct th backsid of th drivr to th topsid. Ths tubs wr usd to qualiz th prssur on both sids of th drivr whn th cll was opratd undr vacuum conditions. Th cll consistd of two sctions connctd with an O-ring. Th bottom sction containd th drivr and coil assmbly, and th top allowd for th thrmal tst vhicl connction. Aftr th aluminum cll was machind, it was black anodizd to prvnt th aluminum surfac from corroding whr it was in contact with watr. Th cll was instrumntd with ight thrmocoupls making it possibl to masur tmpraturs within th cll vn undr vacuum conditions. A prssur transducr was also connctd to th cll to masur th intrnal gag prssur during th hat transfr xprimnts. Th cll was chargd with watr or FC-72 to provid a liquid rsrvoir in th bottom of th cll. This liquid pool also providd a uniformly thick layr of fluid ovr th: top of th drivr. Th pizolctric drivr was 31 mm in dianitr, and it was activatd by a 25 V, sinusoidal sourc. A prforatd orific plat with uniformly spacd hols was cntrally locatd ovr th drivr. Th small spacing btwn th drivr and orific plat assurd a rliabl flow of liquid to th surfac of th drivr. Th frquncy and mount of liquid insid th cll wr varid until a continuous VDA procss was obsrvd on th drivr. For th VDA cll shown in Figur 3, a strong VDA procss was xprincd whn th cll was chargd with 12 to 15 ml of liquid and it was vibratd at a frquncy btwn.5 and 1.2 khz. HEAT TRANSFER RESULTS Svral hat transfr xprimnts wr conductd with two diffrnt hat sourcs. Both hatrs wr thrmal i.st vhicls providd by th ntl Corporation. Th smallr hatr had a di ara of 1.18 cm2 whil th di of th largr hatr had a surfac ara of 4.72 cm'. Th procdur for complting a hat transfr tst was th sam for both hatrs. Aftr th hatr had bn attachd to th top sction of th cll with a silicon salant, th cll was chargd with ml of working fluid. Th capillary pumping action providd by th location of th orific plat abov th pizolctric drivr allowd for atomization at a wid rang of' working liquid volums. f watr was th working fluid, th cll was thn vacuatd until th prssur in th cll rachd approximatly 2.5 kpa. Th prssur insid th cll was initially on atmosphrr if FC-72 was th working fluid. Aftr charging th cll, th chilld watr supply was activatd and allowd to rach :stady-stat. Th pizolctric drivr was first nrgizd to form droplts followd by activation of th hlatr. Powr was supplid to th di surfac that was uniformly hatd by mans of a dirct currnt powr supply. Tmpratur snsing diods mbddd in th silicon di wr usd to masur and rcord th tmpratur of th di as th powr lvl to th hatr was vand. n th first xprimnt, th flow rat of th c'hilld watr through th coppr coil was cliangd whil maintaining th tmpratur of th liquid at a constant 2 C. Figur 4 shows th ffct of changing th watr flow rat from 1.5 to 25 ml/sc on th di tmpratur ;and hat transfr rat from th di. Th working fluid insid th VDA hat transfr cll for both of ths tsts was watr. For both flow rat conditions th prssur insid th cll varid from 2.8 to and th tmpratur insid th cll incrasd from 22 to 41 C. Th rsults indicat idntical di prformanc whn th di tmpratur is lss than about 5 C, rgardlss of th cooling watr flow rat. Howvr, as th di tmpratur incrass, it is abl to dissipat a gratr amount of hat at th largr watr flow rats. Whn th di tmpratur is 1 C, it is abl to dissipat 17% mor hat at th highr watr flow rat ntr Socity Confrnc on Thrmal Phnomna

4 14, loo] s a 2-. a Q=1.5 mlls Q=25 nus O E Hat Transfr [wl Fig. 4 Effct of chilld watr flow rat for liquid coold VDA hat transfr cll Th ffct of changing th circulating bath tmpratur in th coppr coil on th di tmpratur and hat transfr rat is shown in Figur 5. Thr diffrnt bath tmpraturs (2, 4 or 6 C) ar plottd, and th flow rat for all thr tsts was hld constant at 25 mysc. As th tmpratur of th cooling watr incrasd, th prssur insid th cll incrasd and th hat dissipatd dcrasd for a givn di tmpratur. At a bath tmpratur of 2"C, th prssur incrasd from 2.8 to 6.9 Wa and th tmpratur incras from 22 to 41 C. Th prssur incrasd from 9. to 18.7 kpa and th tmpratur incras from 47 to 6 C whn th bath tmpratur was 4 C. Whn th bath tmpratur was 6"C, th prssur incrasd from 26.2 to 36.5 kpa and th tmpratur incras from 67 to 74 C. Sinc th hat transfr goal during th bum-in procss was OOW at a di tmpratur of lss than OO"C, ths rsults indicat that th VDA hat transfr cll is capabl of mting th thrmal rquirmnts of th bum-in procss. Th cll is capabl of dissipating 15OW whil kping th di at 1 C for a consrvativ chilld watr flow rat of 25mL/sc and a bath tmpratur of 2 C A A 5 A Tc=G(PC Hat Transfr DN] Fig. 5 Effct of cooling watr tmpratur for liquid coold VDA hat transfr cll For th prvious two figurs, th VDA spray impactd dirctly on th coppr intgratd hat spradr (HS) that covrd th di. Th rsults in Figur 6 indicat th ffct of rmoving th HS and spraying dirctly on th di surfac. For th tst rsults shown in Figur 6, th watr in th chilld loop circulatd at 25 ml/sc and was maintaind at 2"C, and th small thrmal tst vhicl was usd. Th prssur insid th cll incrasd from 2.8 to 6.9 Wa, and th intrnal tmpratur also incrasd from 22 to 41 C. Th HS surfac ara for th smallr di was 9.61 cm', and diffrnc in ara btwn th bar di and th HS was narly 8.5 cm'. Th bar di tmpratur incrass with incrasing hat powr until a critical hat transfr rat is rachd at approximatly 5W. Byond this lvl, larg incrass in di tmpratur ar xprincd for slight incrass in powr. Th critical hat transfr rat in which th HS was in plac could not b dtrmind with th smallr di bcaus th maximum oprating tmpratur of 12 C was xcdd bfor th critical hat transfr rat was rachd. Ths rsults indicatd that th addd ara associatd with th HS plays a largr rol than th rduction in thrmal contact rsistanc that can b achivd by rmoving th HS. 14 E 121 E Bar di Hat Transfr DN] Fig. 6 Effct of intgratd hat spradr for liquid coold VDA hat transfr cll Som tsts wr also conductd with th largr thrmal tst vhicl, and ths rsults ar prsntd in Figur 7. Ths tsts wr compltd with th HS rmovd, th chilld watr was maintaind at a flow rat of 25mL/sc and a tmpratur of 2 C. Watr and FC-72 wr usd as th working fluids and th initial prssur in cll was 4.1 kpa for watr, and kpa for FC-72. Whn watr was usd, th prssur in th cll incrasd from 4.1 to 11.7 kpa, and for FC-72 th prssur in th cll incrasd from 1.5 to 12.5 Wa. Th data plottd in Figur 7 obviously shows that watr has suprior hat transfr proprtis, and it is a bttr choic for a working fluid in a VDA hat transfr cll. Th critical hat transfr rat whn FC-72 was usd as a working fluid was 6W whn th di rachd 1 C. Th critical hat transfr rat could not b rachd for th cas whn watr was usd as a working fluid bfor th di rachd its uppr tmpratur limit of 12 C. With watr as th working fluid, ovr 3W was dissipatd whil kping th di at a tmpratur of 118 C. Standard uncrtainty analysis dtrmind th hat transfr masurmnt accurat to within 4.5% and th di tmpratur was within *"C of th valu rportd. 22 ntr Socity Confrnc 41 1 on Thrmal Phnomna

5 14 a *Oj - loo O Hat Transfr w] Fig. 7 Effct of working fluid for liquid coold VDA hat transfr cll CONCLUSONS A nw hat transfr cll basd on a droplt atomization mchanism and suitabl for cooling small microlctronic packags during th burn-in manufacturing procss is proposd. A vibrating drivr provids continual supply of small diamtr droplts that ar proplld toward th hatd surfac whr thy form a thin liquid film. Th liquid vaporats on th hatr and th vapor condnss on th scondary droplts and on th cool innr surfacs insid th cll. Th condnsd liquid rturns to th drivr whr it is ratomizd. nitial hat transfr rsults utilizing watr as a working fluid and a simpl cll dsign hav shown that a cll basd on th VDA procss can provid a hat transfr rat ovr 3 W. Rducing th flow rat insid th chilld loop insid th cll slightly rducd th hat flux at a givn hatr di tmpratur. n addition, for a givn hat transfr rat, th di tmpratur was incrasd as th tmpratur of th watr in th cooling loop was incrasd. Rmoval of th intgratd hat spradr placd on top of th di dos not incras th hat transfr prformanc bcaus th bnfit of th additional ara of th hat spradr outwighs th drawback of th addd thrmal contact rsistanc btwn th di and hat spradr. Comparativ tsts using watr at rducd prssurs and FC-72 at atmosphric prssur show th clar advantags of utilizing watr as a working fluid insid th VDA hat transfr cll. Ths prliminary tsts hav indicatd that a VDA-basd hat transfr cll has a potntial to cool small-scal lctronic dvics during th bum-in procss, and VDA clls can provid cooling capabilitis that ar within th rang rquird for most modm microprocssor packag dsigns. [2] Gorgia nstitut of Tchnology, 1995, Packaging Rsarch Cntr, Third Annual Rviw, Atlanta Gorgia, pp [3] Ptrson, G. P., 1994, An ntroduction to Hat Pips, John Wily & Sons, nc.,, Nw York, pp [4] Kiwra, E. W., and P. C. Waynr, A Small Scal Thrmosyphon for mmrsion Cooling of a Disc Hat Sourc, Hat Transfr in Elctronic Equipmnt, ASME Symposium HTD-Vol. 57, Bar-Cohn, A. (Ed.), Amrican Socity of Mchanical Enginrs, Nw York, pp [5] Mudawar,., T. A. ncropra, and F. P. ncropra, 1987, Microlctronic Cooling by Fluorocarbon Liquid Films, Proc. nt. Symposium of Cooling Tchnolagy for Elctronic Equipmnt. [6] Arik, M. and A. Bar-Cohn, 1998, mmrsion Cooling of High Hat Flux Microlctronics with Dilctrid Aquids, 4 nt. Symposium and Exhibition on Advancd Packaging- Matrials-Procsss-Proprtis,.4tlanta, pp [7] Smith, M. K., A. Jams, B. Vukasinovic, ant1 A. Glzr, 1998, Vibration-nducd Dropltt Atomization, submittd for publication. [8] Dnny, D. L., W. Z. Black, J. G. Hartly, and A. Glzr, 1996, High Hat Flux Boiling with Monodisprs Watr Droplts, Procss, Enhancd, ;and Multiphas Hat Transfr, R. M. Manglik and A. D. Kraus, ds., Bgl Hous, nc., Nw York, pp [9] Hflington, S.N., W.Z. Ellack, and A. Glzr, 2, Vibration-nducd Droplt Cooling of Microlctronic Componnts, THERM 2 Procdings, Vol. 2, Las Vgas, NV, pp [lo] Hffigton, S.N., W.Z. 13lack, and A. Glzr, 21, Vibration-nducd Droplt Atomization Hat Transfr Cll for Microlctronic Thrmal Managmnt, 13 Europan Microlctronics and Packaging Confrnc & Exhibition, Strasbourg, Franc. [ll] Hflington, S.N., W.Z. 13lack, and A. Glzr, 21, Vibration-nducd Droplt Atomization Hat Transfr Cll for Cooling of Microlctronic Componnts, Th Pacific RidASME ntrnational Elctronic Packaging Tchnical Confrnc & Exhibition, Kauai, Hawaii. [12] Hfington, S.N., W.Z. Black, and A. Glzr, 22, Vibration-nducd Droplt Atomization Hat Transfr Cll for High-Hat Flux Dissipation., Thrmal Challugs in Nxt Gnration Elctronic Systms (THERMES-22), Santa F, NM. ACKNOWLEDGEMENTS This work was supportd by DARPA through BAA REFERENCES [] SA Smiconductor ndustry Association, 1994, Th National Tchnology Roadmap for Smiconductor: Tchnology nds , Smiconductor ndustry Association, Washington DC ntr Soclty Confrnc on Thrmal Phnomnsa

MEASURING HEAT FLUX FROM A COMPONENT ON A PCB

MEASURING HEAT FLUX FROM A COMPONENT ON A PCB INTRODUCTION Elctronic circuit boards consist of componnts which gnrats substantial amounts of hat during thir opration. A clar knowldg of th lvl of hat dissipation