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 from lctronic componnts during thir opration is dsirabl for rliabl and optimal dsign of cooling systms and slction of th appropriat hat rmoval mthodologis, as hat dissipation from a componnt in an lctronic packag not only influncs its own prformanc but also affcts th prformanc of nighboring componnts, lading to thir failur. th hlp of which an Error Indicator is dvlopd. In ordr to validat and prov th ffctivnss of this nw mthod, xprimnts ar carrid out and xhibitd a clos match btwn th masurd and xprimntal valus. MEASUREMENT METHOD Considr a 2-nod thrmal rsistanc ntwork as shown in figur 1 blow Progrssivly, th dmand for highr powr dnsity and fficincy in lctronic products is incrasing. This lads to a gratr nd for accurat mans of masuring th dissipativ losss of powr and th knowldg of this hat dissipation from lctronic componnts is a critical input for thrmal dsignrs. Convntional tchniqus for masuring th hat dissipation includ th intgration of th product of masurd voltag and currnt and th us of insulatd calorimtry chambrs to masur th hat flow from th dvics. Calorimtric mthods hav bn usd to mak loss masurmnts of lctric machins or standalon componnts such as powr lctronic dvic, transformr, frrit and capacitor tc. A mthod proposd [1] in this articl, to masur hat dissipation of componnts on a printd circuit board (PCB) undr oprating conditions, utilizs a hat flux snsor and thmistors, and calculats hat dissipation from a thrmal standpoint. Masurmnt is pron to rror du to intrfrnc of th nighboring componnt (hat sourcs) and this is compnsatd by carrying out rror analysis, with Figur 1. 2-Nod Thrmal Rsistanc Ntwork [1] Thr ar 2 nods, A and B that ar hat sourcs. Nods A and B ar connctd to ach othr and to th ambint sink. Nod A dtails: Hat dissipation: Q 0 Tmpratur: T 0 Hat dissipation from nod A to sink: Q a0 Nod B dtails: Hat dissipation: Q 1 Hat dissipation from nod B to sink: Q a1 Also, T a rprsnts ambint tmpratur and Q ab is th hat xchang btwn nods A and B.
5 FEATURED ARTICLE For this ntwork, hat dissipation from nod A to sink is givn in quation 1. Th tmpratur diffrnc btwn tmpratur at nod A and ambint tmpratur is basically a combination of hat dissipation at nod A and som hat from nod B and is givn in quation 2 blow. Equation 3 shows that th rsistanc 1/R 1 is proportional to rsistancs only. Q a0 = Q 1 Q 0 Q 0 (1) Q 1 = 1 R 1 (T 0 ) (2) 1 = R 1 R (3) a1 Thrmal rsistanc ntwork shown in figur 2 blow dpicts diffrnt componnts on PCB. Nods labld as A, B, C, D and E ar dissipating hat whil othr nods shown do not dissipats hat. Nod A is th Dvic Undr Tst (DUT) whos hat dissipation is masurd by this mthod. Plas not that this tchniqu is ignoring all radiation hat transfr gain and loss. Th abov quation has two unknowns and two masurabl paramtrs. K is an unknown constant and is th rciprocal of thrmal rsistanc from T 0 to T a. QE is anothr unknown constant. Q is th total hat dissipation from DUT and E is th ffct of othr hat sourcs on th PCB. Q m in th quation is basically a masurd quantity. It is part of total hat dissipation Q which is drawn away through componnts cas top and not through th ntwork. This is don and masurd using thrmolctric modul, which draw diffrnt amounts of hat from th componnt cas top and monitors th chang in Q m. As Q m changs, T 0 also changs and is also monitord. Figur 3 shows th rsult of calculation basd on quations 1-3 whr QE is th prdictd hat dissipation for th DUT. Of this, Q is th actual hat dissipation and E is artificial part. Equation 4 has two unknowns K and QE. Q m can b varid using a TEC which rsults in diffrnt T 0. By taking two sts of data QE can b calculatd. Figur 3. Calculation Graph [1] Figur 2. Thrmal Rsistanc Ntwork for th Gnral Cas [1] As xplaind in th prvious sction, (T 0 ) is basically proportional to th hat dissipation at nod A and combination of hat dissipation from othr nods in th ntwork, lik from nod B, C, D, E and is givn by: Q E = Q m K (T 0 ) (4) TEST METHOD Hat flux snsor, Thrmolctric cooling modul (TEC), thrmistor ar usd in tsting th ntir PCB assmbly which is placd in a wind tunnl with high cooling air flow rat. Th arrangmnt is as shown in figur 4 whrin, hat flux snsor is placd btwn th DUT and th TEC with activ hat sink. A total of fiv thrmistors ar usd and attachd on othr sid of PCB (sid othr than whr DUT is mountd). On thrmistor is attachd xactly blow th DUT Qpdia ISSUE 95
6 During tsting, only cooling rat can b controlld, whras othr two factors ar out of masurmnt control. In ordr to compnsat th ffct of rror ovrall and achiv highly accurat masurmnt, an Error Indicator is rquird which can b usd during tsting and rval th masurmnt mthod rror. Figur 4. Tsting Apparatus [1] at th cntr and four othr thrmistors ar attachd at th mid points of th four dgs of DUT. TEC usd is modulatd to draw diffrnt amounts of hat from th DUT. Th intnt is, th hat flow should b maximum from th cntr and should sprad from cntr to th sids. To achiv this, th avrag tmpratur from th four surrounding points of DUT is lowr than or qual to that in th middl of DUT. This tsting st up and mthod is OK if th DUT is th only hat sourc on th PCB, but dos not hold tru if thr ar hat sourcs othr than DUT. In that cas, as shown in quation 4, intrfrnc from narby hat sourcs (E) coms in to pictur and should b considrd to avoid th obvious rror in hat dissipation stimation. A concpt of Isothrm Hirarchy is usd to dfin an rror indicator. Figur 5 shows a topology of hat transfr from DUT and othr n hat sourcs flowing through a hirarchy of isothrms to ambint sink. Figur 6 shows th isothrms hirarchy on PCB plan and thrmal rsistanc ntwork. Lt T b locats at th cntr of DUT. Lt T n b th 1st isothrm surrounding th DUT only, T n1 b th 2nd isothrm surrounding th DUT and first closst nighbor hat sourc, T n2 b th 3rd isothrm surrounding th DUT and th first two closst nighbor hat sourcs, so on and so forth, till T nn circls all th n nighbor hat sourcs. It is not ncssary for T ni to b highr than T ni1. Hat flow Q bi (i=1,..., n) will altr dirction to comply with th law of hat flowing from a high tmpratur nod to a lowr tmpratur nod. Figur 7 is th quivalnt ntwork for all th hat sourcs and is basically a transformation of figur 6. NEED FOR ERROR INDICATOR AND ITS DERIVATION From quation 2 and quation 4 abov, th intrfrnc trm E is givn as E = Q 1 (5) Figur 5. A Topology of Hat Flow to Ambint [1] Looking at th intrdpndncy of th paramtrs in quation 5, thr ar thr factors that affct th thortical masurmnt rror and raiss th nd to driv an Error Indicator: 1. Whn hat flow from narby sourc (Q 1 ) is smallr, th rror bcoms smallr 2. Whn th narby hat sourc is far away from DUT or th board in plan conductivity is lss, thn is largr and th rror bcoms smallr 3. Whn thr is highr cooling rat nar Q 1, is smallr and th rror bcoms smallr Figur 6. n-nod Thrmal Rsistanc Ntwork [1] Qpdia ISSUE 95
7 FEATURED ARTICLE ( R ( R ) R b a0 ) R Q = (T b )(whnq m =0) - (T n )(whnq m = 0) (9) Figur 7. Equivalnt Thrmal Rsistanc Ntwork [1] All th narby hat sourcs ar rplacd with an quivalnt hat sourc Q xt. Rarranging quation 4 and including T b, w gt quation 6 as follows, Q = Q m R 1 R b ( ) R Q xt ( ) R 1 R b ( ) R (T b ) (6) Th rror prcntag calculatd using th abov quations and applying thm to xprimntal st up, is th masurmnt mthod rror du to th narby hat sourcs intrfrnc. Comparison btwn Simulations and Masurmnt using Error Indicator Tn cass of diffrnt powr lvls and diffrnt distanc of sourcs from th DUT, varying PCB conductivitis and ambint cooling rats ar simulatd with ANSYS Icpak. Figurs 8, 9, 10, 11 show four simulatd cass rsults with diffrnt narby hat sourcs on PCB. Th rror prcntag du to nighbor hat sourcs Q xt is givn in quation 7, Error% = Q R xt R Q 1 R b ( ) Figur 8. Cas 0 No Nighbor Hat Sourc [1] R Q xt = ( R ( R ) R b a0 ) R (7) Whn TEC draws all th hat from th componnt cas top, Q - Q m is clos to zro. Thrfor, R Q xt = T n (whn Q m = Q) (8) Whn TEC draws almost non of th hat through cas top, Q is clos to (Q - Q m ). Equation 9 thrfor is: Figur 9. Cas 1 Rmot Nighbor Hat Sourcs [1] Qpdia ISSUE 95
8 Figur 13. Error % Indicator Effctivnss [1] Figur 10. Cas 2 Intrmdiat Nighbor Hat Sourcs [1] As indicatd by Equation (7), th modl rror bfor compnsation is affctd by narby hat sourcs and thir distancs to DUT, PCB conductivity, and ambint cooling rat. Th rrors bfor compnsation ar plottd in Figur 12 from cas simulation rsults. SUMMARY This nw mthod of masuring th hat flux from a componnt on PCB undr oprating condition is innovativ and practical way to stimat componnt hat loss with rasonabl rror. Comparing this mthod with traditional calorimtric mthod, it taks lss ffort to stup tsts by allowing hat sprading through PCB and modulating hat from th cas top. Figur 11. Cas 3 Immdiat Nighbor Hat Sourcs [1] REFERENCES 1. Zhongwi, Qi., A Mthod to Masur Hat Dissipation from Componnt on PCB, Gnral Elctric Tchnology Infrastructur Halthcar 2. Sajith, V., Balakrishna, C., Sobhan, P., Charactrization of Hat Dissipation from a Microprocssor Chip Using Digital Intrfromtry 3. Zhang, Y., Janhs, T., Powr Elctronics Loss Masurmnt Using Nw Hat Flux Snsor Basd on Thrmolctric Dvic With Activ Control Figur 12. Error (bfor compnsation) Snsitivity [1] Qpdia ISSUE 95