Thermal Resistance Measurements and Thermal Transient Analysis of Power Chip Slug-Up and Slug-Down Mounted on HDI Substrate

Transcription

1 Intl Journal of Microcircuit and Electronic Packaging Thermal Reitance Meaurement and Thermal Tranient Analyi of Power Chip Slug-Up and Slug-Down Mounted on HDI Subtrate Claudio Sartori Magneti Marelli - Electronic Sytem Diviion Viale Carlo Emanuele II, Venaria Italy Phone:: Fax: claudioartori@venariamarelliit Abtract Thermal reitance meaurement on power chip lug-up and lug-down configuration are performed in i work Two different package et up are analyzed to reduce e contact ermal reitance effect Cooling tranient behavior i analyzed and an analytical expreion wi two time contant i hown to be uitable to decribe e meaured tranient wi good approximation Thermal reitance and heat capacitance are evaluated from e equivalent electrical circuit Contraint to ue lug-up configuration intead of lug-down are etablihed in i paper Key word: Thermal Reitance, Heat Capacitance, Tranient Time Contant, Slug-down, and Slug-up Introduction configuration lug-up wi no ermal via, ince e portion of ubtrate under e component become a ueful area for oer device On e oer hand, heat diipation for lug-up could be le effective an lug-down and conequently condition of applicability of lug-up mut be etablihed The comparion between ee two different configuration i made by meauring e ermal reitance of chip and analyzing e ermal tranient during cooling of e device In i way, it would be poible to identify which configuration can be ued and which are e contraint The aim of i work i to compare ermal behavior of a power chip () device in two configuration available: lugup and lug-down In lug-down configuration, heat i diipated wi a great efficiency, ince metallic lug i in contact wi ermal via High Denity Interconnection (HDI) ubtrate give e opportunity to Printed Circuit Board (PCB) deigner to increae e number of device (or equivalently e number of Input/Output connection) at could be aembled According to i point of view, a lug-down device reduce e ueful area on e ubtrate, ince e portion of PCB under e component cannot be ued to mount oer device for e preence of ermal via For i reaon, it could be intereting to analyze e Thermal Reitance Meaurement In Figure, e cheme of apparatu ued for ermal reitance meaurement i reported All e device under invetigation have a ermal enitive integrated diode whoe forward voltage i linearly related to e temperature of e device Alo, a reitance i preent in e device in order to increae e chip temperature by Joule effect when voltage i applied By mean of Gd voltage generator and by chooing e appropriate value of reitance R, a contant current of 00µA i The International Journal of Microcircuit and Electronic Packaging, Volume 4, Number, Firt Quarter, 00 (ISSN ) 84 International Microelectronic And Packaging Society

2 Thermal Reitance Meaurement and Thermal Tranient Analyi of Power Chip Slug-Up and Slug-Down Mounted on HDI Subtrate upplied to e integrated diode of e component C under meaurement Thi current i meaured by Ad ampere-meter In ee condition, e temperature increae!t (computed repect to environment temperature) i related to e decreae of forward voltage drop!v ( meaured by Vd voltmeter) of diode by e relation:!t!v/c where c05 mv/ C Aluminium Cae Slug-down Air HDI ubtrate Gp Ap S Vp C R Vd Ad Gd Thermal via Figure Slug-down configuration Thin Thermal Pad Figure Scheme of apparatu ued to meaurer Aluminium Cae Slug-up HDI Subtrate Cloing e witch S, e integrated reitance in C begin to heat e device uing power upplied by DC voltage generator Gp Thi power i eaily computed by e product between e current meaured by Ap ampere-meter and e voltage meaured by Vp voltmeter Thi power diipated by Joule effect caue an increae in e temperature of C When ermal equilibrium i reached, e witch S could be opened and e tranient i recorded by a computer The ermal reitance value R i given by e following equation, Figure 3 Slug-up configuration 4 Meaurement Reult and Dicuion Air V R () cp The power P i evaluated in e ame time when e witch S i opened and!v i e difference between e reading of Vd when S i opened and e voltage drop when e diode i at ambient temperature 3 Package Decription Every package meaured i placed over a temperature controlled urface et to 5 C In Table, reult of ermal reitance meaurement of package #, # and #3 are reported Table Thermal reitance meaurement reult Chip and package R ( C/W) Vb (V) Va (V) Power (W) T ( C) Slug-down A - package # Slug-down B - package # Slug-up A - package # Slug-up B - package # Slug-down A - package # Slug-down B - package # Slug-up A - package # Slug-up B - package # Slug-down A - package # Slug-down B - package # Slug-up A - package # Slug-up B - package # The package configuration for lug-down, and lug-up, i reported in Figure, and 3, repectively The metallic cae i made by aluminum The heat conduction i poible in downward direction only for lug-down and upward direction only for lug-up; on e oer ide, e ermal convection in air in i In e ird and four column of Table, forward drop voltage at e beginning (Vb) of tranient, and at ambient tempera- work will be neglected repect to conduction A tated in e introduction, heat diipation for e lug-up ture (Va), repectively, are reported Then, in e lat two column, e power diipated and e whole temperature rie of configuration could be le effective an lug-down ince, a hown in Figure 3, in ermal pad mut be ued between e device are reported In every package, lug-up A alway ha a metallic lug and aluminum package to reduce contact ermal lower R reitance On e contrary, in lug-down configuration, e value an lug-up B and lug-down A alway ha a greater R metallic lug i in contact wi HDI ermal via at pread value an lug-down B Thee difference can be undertood examining e reciprocal poition of device in e ermal power to aluminum cae PCB For example, lug-up A i cloet to a crew (at fixe e For every package teted, ere are two lug-up and two lugdown and in e following ey will be identified by letter A and PCB to e aluminum cae) an lug-up B Therefore, it i poible lug-up A experience a bigger preure toward cae an B The International Journal of Microcircuit and Electronic Packaging, Volume 4, Number, Firt Quarter, 00 (ISSN ) International Microelectronic And Packaging Society 85

3 Intl Journal of Microcircuit and Electronic Packaging lug-up B, and e heat tranmiion could be better for e former an for e latter Thee meaurement how how i critical e aembly of device repect to ermal behavior: a light bending of PCB can caue a bad contact between power chip and metallic cae deteriorating e heat tranfer efficiency Moreover, from ee data, lug-up A ha R value le an lug-down A (package # and #): but i reult eem to be trongly affected by contact ermal reitance In order to reduce ee unwanted effect in a imple way, a new configuration i conidered: e ame ickne of ermal pad have been added to e package bo for lug-down, and lug-up, a hown in Figure 4, and 5, repectively The R value meaured in i modified et up for package # are reported in Table R value for lug-down component are reduced by one half and about one ird for lug-up Now, e ermal reitance of lug-down i about one half of lug-up one The improvement in heat diipation i due to e following two factor: a) adding ermal pad, heat ha two way to flow away upward and downward, b) e planarity of PCB i improved u ha a better contact wi aluminium cae, conequently, contact reitance are reduced Anyway, e difference between lugup and lug-down configuration now i evident: lug-down ha a better ermal exchange wi aluminum cae an lug-up: ermal via, preent in e lug-down configuration only, play a very important role and ey are fundamental to obtain low R value Thee reult how at good value of R can be obtained wi lug-up too, but contact reitance mut be reduced in a very accurate way Therefore, for i particular package deign, if great ermal power diipation i needed, lug-down configuration i preferred Indeed from e above reult, it eem to be very difficult to obtain a ermal reitance value le an C/W for lug-up configuration Added ermal pad Table Thermal reitance value meaured in modified configuration Chip and package R ( C/W) Vb (V) Va (V) Power (W) T ( C) Slug-down A package # Slug-down B package # Slug-up A package # Slug-up B package # Tranient Analyi Thermal tranient of device under teting can be analyzed in order to evaluate time contant t of ermal ytem In i way e heat capacitance can be evaluated too In Figure 6, tranient behavior of diode voltage drop (V f ) after opening witch S relative to lug-down A of package # i reported Vf(V) Meaured tranient Theoric tranient Time () Figure 6 Meaured and eoretical emal tranient In Figure 6, eoric tranient given by e following Equation () i alo reported a follow, Aluminium Cae Slug-down Thermal via Figure 4 Slug-down modified configuration Aluminium Cae Slug-up Figure 5 Slug-up modified configuration HDI Subtrate Thin Thermal Pad HDI Subtrate Added ermal pad t V f + τ ( Va Vb ) exp Vb The unknown parameter in Equation () i e time contant " at can be evaluated from e meaured value of V f by e following expreion, τ ln[ ( V f t V ) /( V b a V )] The median value (mot frequent) of ditribution of " value given by Equation (3) i choen for Equation (), at i " 5 It i clear at one time contant only cannot decribe in an accurate way e ermal tranient From Figure 6 eoretical tranient i very inaccurate epecially at e beginning of tranient, on e oer hand Equation () i very accurate for aymptotic behavior: o e computed time contant i e aymptotic " b () (3) The International Journal of Microcircuit and Electronic Packaging, Volume 4, Number, Firt Quarter, 00 (ISSN ) 86 International Microelectronic And Packaging Society

4 Thermal Reitance Meaurement and Thermal Tranient Analyi of Power Chip Slug-Up and Slug-Down Mounted on HDI Subtrate To improve e matching between meaured and eoretical tranient, a two-time contant expreion mut be ued a follow, W Junction (heat generator) t t V f ( Va Vb ) A + A exp exp + Vb τ τ (4) where " and " are e two time contant, A and A two contant whoe um i equal unity Togeer wi aymptotic time contant ( " ), e minimum value obtained from Equation (3) i choen for e econd time contant: " 85 Then, wi e choice A 085 and A 05, eoretical and meaured tranient are in good agreement a hown in Figure 7 Contant A and A are evaluated minimizing e um of quare of difference between meaured and eoretical value R C C R Aluminum Aluminium cae (environment reervoir) Figure 8 Equivalent electrical cheme of ermal ytem Table 3 Theoretical tranient paramenter evaluated for configuration wi and wiout pad Vf (V) Meaured tranient Parameter Wiout Pad Wi Pad τ 96 5 τ 7 4 A A Theoric tranient Time () Figure 7 Theoretical tranient evaluated wi Equation (4) compared to meaured tranient According to Equation (4), e ermal behavior of e package can be correctly decribed by a model wi two time contant: e greatet one repreent e aymptotic behavior, e mallet one repreent e beginning of tranient The equivalent electrical circuit of ermal flow can be ummarized a follow (Figure 8): e heat ource of e ytem i e junction in e ilicon, en ermal power goe rough e package and e metal lug (reitance and capacitance R and C ), after at ere are HDI ubtrate and ermal via for e lugdown configuration or ermal pad for lug-up configuration (reitance and capacitance R and C ) Finally, heat reache e aluminum cae at, wi good approximation, remain at ambient temperature during e whole tranient (environment reervoir) The aymptotic time contant (" ) can be related to e HDI ubtrate and ermal via for lug-down configuration and ermal pad for lug-up configuration; e mallet time contant (" ) can oerwie be related to e chip package togeer wi metallic lug When ermal pad were added, e ermal reitance wa trongly reduced In Table 3, e computed value of #", ", A and A wi and wiout pad are compared for lug-down A of package # Bo " and " are trongly reduced by e introduction of ermal pad: i mean at bo ermal reitance of chip package (R ) and ubtrate (R ) are reduced; moreover, alo, capacitance value C and C are affected by e added ermal pad To evaluate R, C, R and C, e tranient of electrical circuit of Figure 8 mut be olved The analytical expreion of e tranient can be evaluated uing tandard Laplace tranform technique and auming a tep function for input power W at goe to zero when witch S i opened a follow, V R + R R + R V0 exp α 0 + α exp( αt) + α ( α α ) R κ R κ (5) V where V 0 cw (R + R ) i e voltage drop due to e erie of two reitance R and R at diipate e power W Thi drop i equal to V a - V b, moreover, one would have R R + R Thu, V 0 /c repreent whole temperature drop of e tranient The abolute value of exponential coefficient $ and $ repreent e invere of time contant Their explicit form are of e form, 4 α + κ κ κκ 4 α κ κ κκ The International Journal of Microcircuit and Electronic Packaging, Volume 4, Number, Firt Quarter, 00 (ISSN ) International Microelectronic And Packaging Society 87 / where k R C, k R C, /k /k +/k +/R C / (6)

5 Intl Journal of Microcircuit and Electronic Packaging To find relation among e computed parameter R, ", ", A, A and R, C, R, C, one mut compare expreion (4) and (5) Immediately, one would have $ -/", $ -/" and R + R α α Rκ + α R + R A α α Rκ + α A (7) Solving e ytem of Equation (6) and (7), one would find, κ A κ ; κ ( α α) α ( α + α ) ( α α ) 4/ ; R κ R ( α α ) ( α + α ) 4( α + α / κ ) Finally, one would have: R R - R, C k /R, C k /R In Table 4, computed value of ermal reitance and capacitance for lug-down A and lug-up B of package # wi and wiout added ermal pad are reported Table 4 Computed value of ermal reitance and capacitance Slug-down A wiout pad Slug-down A wi pad Slug-up B wiout pad Slug-up B wi pad R ( C/W) R ( C/W) R ( C/W) C (J/ C) C (J/ C) Thermal pad introduced in lug-down configuration have trongly reduced ermal reitance R relative to chip package and metallic lug, becaue now ere i anoer pa for diipating heat: lug-down can pread ermal power in upward direction too Thermal reitance R ha alo been reduced ince ermal pad have increaed contact area wi metallic cae and reduced contact reitance: in i configuration, togeer wi e ubtrate urface wi ermal via ere i alo ermal pad urface in contact wi aluminium cae Thermal capacitance related to chip package C i practically unchanged wi added ermal pad; on e oer hand, C heat capacitance ha become twice e original value, ince added ermal pad have a greater heat capacitance an air Alo, R and R are reduced by introducing ermal pad in lug-up configuration, but e reduction i le evident an in lug-down configuration, ince in e ubtrate under e device, ere are no ermal via For lug-up configuration, e preferred pa of heat diipation remain upward direction, alough downward heat diipation i clearly increaed The ame conideration developed for heat capacitance value in lug-down configuration can be applied to lug-up: C value i twice e original one, ince air i ubtituted by ermal pad 6 Concluion In e package deign examined in i work, lug-down device ha a lower ermal reitance value an lug-up The difference between ee value can vary very much, depending on accuracy of aembly of chip: contact ermal reitance play a very important role in ermal performance of e ytem Anyway, when good aembly condition are reached, ermal reitance of lug-up i about twice e value for lug-down Thermal via preent in lug-down configuration are fundamental to reach very low R value, erefore, for i kind of package, lugdown i preferred to lug-up when a ermal reitance value not exceeding C/W i needed For application where a greater R value an i i allowed, lug-up can be ued It mut be remarked at i reult i valid for i package deign only: for example, e ubject of a future work could be a configuration where a heat ink i oldered directly on lug-up On e oer hand, e meodology followed in i paper i uitable to analyze different kind of package Thermal tranient analyi how at behavior of e package can be decribed wi good approximation wi two time contant only: e greatet one decribing e aymptotic behavior, e mallet one characterizing e very beginning of tranient When time contant are known, heat capacitance can be evaluated and ueful information can be gained to improve e ermal behavior of e ytem Acknowledgment The auor would like to ank G Scarano and G Fabbri for ueful technical uggetion in performing ermal reitance meaurement; R Brignolo, E Autelli, M Bezza and D Campinoti for reviewing and encouraging him to prepare i paper Reference D J Dean, Thermal Deign of Electronic Circuit Board and Package, Electrochemical Publication, Ayr (Scotland), Chapter 7, pg 94, 985 Mohan, Undeland, Robbin Power Electronic, John Wiley, Second Edition, New York, New York, Chapter 9, pp , 995 The International Journal of Microcircuit and Electronic Packaging, Volume 4, Number, Firt Quarter, 00 (ISSN ) 88 International Microelectronic And Packaging Society