不同芯片尺寸LED热分析

Thermal Analysis of GaN-Based Light Emitting Diodes With Different Chip SizesLianqiao Yang,Jianzheng Hu,Lan Kim,and Moo Whan Shin,Member,IEEEAbstract—In this paper,we present the thermal,electrical,and optical analyses of light emitting diode(LED)packages with dif-ferent chip sizes.The LED packages under investigation employed the same configuration of package components,except for the chip sizes.The forward current was found to increase with the chip size at the same forward voltage due to the area increase of current spreading.The luminousflux and optical power were found to increase with the chip size at the same current density.The thermal analysis was made by the transient thermal measurement and thermal simulation using thefinite volume method.It was demonstrated that the thermal resistance decreased with the chip size under the same package conditions both by simulation and experiment.The bulk thermal resistance and spreading thermal resistance were combined together to give out a quantitative inves-tigation of the partial thermal resistance variation.Moreover,the spreading thermal resistance was found to have a great effect on the total thermal resistance of LED packages.Index Terms—Chip size effect,finite volume method(FVM), light emitting diode(LED),spreading thermal resistance,tran-sient thermal measurement.I.I NTRODUCTIONS INCE theirfirst introduction in1962[1],light emitting diodes(LEDs)have found a huge application market which is increasing rapidly because of various advantages,such as being environment friendly,having long lifetime,saving en-ergy,having vivid color,etc.[2]–[4].GaN-based blue LEDs are particularly attractive devices for a variety of applications, including traffic signals,head lamps,full-color displays,mini projectors,back light units,and so on.In order to fulfill the application in general lighting,high-power LEDs keep attract-ing many researchers’interest[5]–[8].Even though great im-provements have been made on epitaxial growth and quantum efficiency,it is still difficult for a small single chip to fulfill the increasing high power requirements.To implement high-power LEDs,there are two options for the application engineer: multichip and chip size.Considering the complex electrical circuit and fabrication process,the effect of gold wires on optical efficiency,and correspondingly lower reliability,the variation of chip size and electrode shape is a simple yet good choice instead of many small chips.However,with theManuscript received December1,2007;revised March9,2008.Current version published October16,2008.This work was supported by the Korea Energy Management Corporation.L.Yang,J.Hu,and M.W.Shin are with the Department of Materials Science and Engineering,Myongji University,Yongin449-728,Korea(e-mail: mwshin@mju.ac.kr).L.Kim is with LG Display,Seoul413-811,Korea.Color versions of one or more of thefigures in this paper are available online at .Digital Object Identifier10.1109/TDMR.2008.2002357Fig.1.V–I curve of the LED packages.Insets are the micrographs ofelectrode patterns for different chip sizes.increase of chip size and input power,the thermal problemis becoming more and more important because of the nega-tive effect on the optical performance,reliability,and lifetime[8]–[10].Moreover,despite the broad application in LED in-dustry,few reports can be found about the chip size effectson the electrical,optical,and thermal performances of LEDpackages.In this paper,a series of GaN-based LED chips withthe same epilayer structures but different sizes were utilized forthe LED packages by using a surface mount device method,and then,we qualitatively and quantitatively investigated theelectrical,optical,and thermal behavior of LED packages atthe same working condition.The investigation was given out bycombining transient thermal measurement,thermal simulation,and optical and electrical measurement.II.E XPERIMENTThe GaN-based blue LED chips with the same epilayerstructures but different sizes were used for this investigation.Asshown in the inset graph of Fig.1,electrode distribution was op-timized to fulfill uniform current spreading and avoid hot spotsfor different chip sizes.The LED packages employed the sameconfiguration of package components except for the chip sizes.Moreover,the details of the LED package are shown in the insetgraph of Fig.2.The fabricated LED packages were evaluatedby electrical,optical,and thermal measurement.The V–I curveand optical performance were measured with a Keithley Model238Source-Measure Unit and integrating sphere,respectively.Moreover,the thermal resistance was measured in a transientthermal tester(T3Ster,MicRed,Ltd).The measurement of the 1530-4388/$25.00©2008IEEEFig.2.Luminousflux and optical power of LED packages with current density of350mA/mm2and ambient temperature of25◦C.Inset graph is the schematic structure of LED package.thermal resistance is based on the RC structure function theory. The K factor,the temperature calibration parameter,was mea-sured in the temperature range of20◦C to80◦C with a1-mA sensor current.Transient measurement was started to record the cooling curve after driving the samples for10min at the ambient temperature of25◦C to reach a thermal stabilization. Details of the transient thermal measurement and evaluation can be found elsewhere[11].The driving current density of 350mA/mm2was used for the package with different chip sizes.Moreover,the extracted input powers from the experi-ment were0.204,0.297,0.427,0.627,and1.401W for the LED packages with chip side lengths of15,20,24,28,and40mil, respectively.III.A NALYTICAL AND N UMERICALS IMULATIONS FOR LED SThermal modeling of the LED packages was done in FLOTHERM.FLOTHERM includes a computationalfluid dynamics solver,which carries out a full3-D solution of the Navier–Stokes equations for mass,momentum,and energy conservation using thefinite volume method[5].The same dimensions as the real packages were used in the modeling.The modeled LED package has a dimension of8×8×2mm3and is composed of GaN-based blue chip,die attach,lead frame, mold,heat slug and reflector,and epoxy lens.Detailed structure of epilayers in the chip was simplified,and the effect of gold wire was not considered in modeling.Thermal conductivities used for the chip,die attach,mold,heat slug and reflector cup,lead frame,and epoxy lens are56,7.5,0.23,401,201, and0.17W/m·◦C,respectively.Thermal conductivities are assumed to be independent of temperature due to relatively small temperature change during the whole experiment.As a boundary condition during thermal simulation,the heat transfer coefficient is important to ensure an accurate result[12].In our simulation,the heat transfer coefficient was calculated in real time by FLOTHERM itself.The bottom of the heat slug was fixed at25◦C,and the initial ambient temperature was set at 25◦C.The heating power extracted from the experiment was utilized for the simulationprocess.Fig.3.Cooling curves of the LED packages at the same current density.Inset graph is forward voltage versus temperature plot for the calculation of the K factors in LED packages.IV.R ESULTS AND D ISCUSSIONSThe V–I characteristic and optical performance of LEDs with different chip sizes are shown in Figs.1and2,respectively. The forward current was found to increase with the chip size at the same forward voltage.As the chip size increased,the current spreading was effectively enlarged and thus reduced the series resistance.This can be evidenced by the chip size and the optimized electrode distribution as shown in the inset graph of Fig.1.The luminousflux increased from1403to6140mlm, and the optical power increased from35to216mW when the chip side length increased from15to40mil at the same current density of350mA/mm2.It can be seen that increasing the chip size is an effective method to obtain high brightness and high optical power with the optimized design of the electrode even if the higher junction temperature is expected for the bigger chip size package.The K factors were calculated based on(1),and the mea-surement results are shown in the inset graph of Fig.3.It can be seen that all the packaged LED samples exhibited good linear relationships between the voltages and temperatures.Therefore, the following measurement results of thermal resistance are accurate and reliable.The obtained K factors are1.482,1.235, 1.337,1.182,and1.621mV/◦C for LED packages with chip side lengths of15,20,24,28,and40,respectively andK=ΔD VF/ΔD TJ(1) whereΔD VF is the change of forward voltage andΔD TJ is the junction temperature change of the LED.Fig.3shows the cooling curves of the LED package,and Fig.4shows the comparison of the simulated and measured thermal resistance.The inset graph of Fig.4shows the simu-lated temperature distribution when chip side length is28mil. From the simulation in FLOTHERM,the net heatflow in each direction for different components was calculated,and it was found that more than98%of the heat was dissipated through the heat slug to ambient for all the cases;therefore,a1-DY ANG et al.:THERMAL ANALYSIS OF GaN-BASED LIGHT EMITTING DIODES WITH DIFFERENT CHIP SIZES573Fig.4.Simulated and measured thermal resistance.Inset graph is the simu-lated temperature distribution when chip side length is 28mil.assumption can be regarded to be accurate enough for the thermal analysis of the LED package.The calculated thermal resistance from simulation was obtained byR th =ΔT/P =(T J −T A )/P(2)where R th ,ΔT ,T J ,T A ,and P are thermal resistance,junction temperature rise,junction temperature,ambient temperature,and input power,respectively.From Fig.3,it can be seen that the stabilized junction temperature rises are 13.9◦C,14.6◦C,17.8◦C,18.5◦C,and 32.7◦C for the chip side lengths of 15,20,24,28,and 40mil,respectively.As mentioned previously,the input powers are 0.204,0.297,0.427,0.627,and 1.401W,correspondingly.It can be clearly seen that the junction temperature rise increases much more slowly than the input power.The lower increase rate of junction temperature is due to the decrease of thermal resistance.The measured thermal resistances are 67.6◦C /W,48.7◦C /W,42.4◦C /W,29.5◦C /W,and 23.2◦C /W for an LED with chip size side lengths of 15,20,24,28,and 40mil,respectively as shown in Fig.4.Moreover,the calculated thermal resistances from the simulation are 65.2◦C /W,44.7◦C /W,36.6◦C /W,22.3◦C /W,and 15.3◦C /W,correspondingly.From both the experiment and simulation results,it was demonstrated that the thermal resistance decreased with the chip size under the same package conditions.It was also found that the variation of chip size has great effect on the total thermal resistance of the LED package,and the details will be discussed thoroughly in the following discussions.Fig.5shows the differential structure functions of LED packages.The differential structure function is defined as the derivative of the cumulative thermal capacitance (C Σ)with respect to the cumulative thermal resistance (R Σ)and is ex-pressed in (3).Equation (3)can be further transformed to (4)by definitionK (R Σ)=dC Σ/dR Σ(3)K (R Σ)=cAdx/(dx/λA )=cλA2(4)Fig.5.Differential structure functions of LED package with different chip sizes at the same currentdensity.parison of R th _cd :Calculated bulk thermal resistance,calcu-lated bulk and spreading thermal resistance,and experiment results.where c is the volumetric heat capacitance,λis the thermal conductivity,and A is the cross-sectional area of the heat flow.Therefore,thermal resistance change caused by c ,λ,or A can be observed in the plot of differential structure function.By using the differential structure functions,the thermal re-sistance change in different components of the LED package can be recognized.Moreover,the partial thermal resistances from chip to die attach (R th _cd )decreased from 39.1◦C /W to 23.2◦C /W,17.4◦C /W,9.5◦C /W,and 4.9◦C /W when chip side lengths increased from 15to 20,24,28,and 40mil,as shown in Fig.5.As stated earlier,the LEDs employed the same packaging components except for the chip size.There-fore,the change of R th _cd plays a major role in the total thermal resistance variation as expected.Two reasons are believed to be responsible for the decrease of R th _cd .One is the bulk thermal resistance,and the other one is the spreading thermal resistance.The bulk thermal resistance can be calculated with (5)for different packaging componentsR thb =t/(λ·A )(5)where R thb is the bulk thermal resistance,t is the thickness,λis the thermal conductivity,and A is the thickness.However,574IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY ,VOL.8,NO.3,SEPTEMBER 2008there is a largish deviation in numerical value between the bulk thermal resistance and experiment results as shown in Fig.6.The contribution of spreading thermal resistance was believed to be responsible for the sizable deviation.When the heat flows across an interface between two components with different cross-sectional areas,constriction/spreading thermal resistance should be considered.The term constriction is used to describe the case where heat flows into a narrower region,and spreading is used to describe the opposite case.It has been known that the thermal resistance considering constriction/spreading effects can be calculated in (6)with the isothermal boundary condition at the bottom surface of the LED package [13]R c =1λπ·a · h n a n +w n ·h n ·(1−a n )/(w n +a n )+(w n −h n )·a n(6)where R c is the thermal resistance considering the constriction/spreading effect,λis the thermal conductivity of the bigger part,a n is the dimensionless contact area ratio a/b (a is the equivalent radius of the source part;b is the equivalent radius of the dissipation part),w n is the normalized plate thickness t/b ,and h n is defined as w n when w n is smaller than 1and equal to 1when w n is bigger than 1.For the LED packages under investigation,the spreading effect from chip to die attach should be considered.Moreover,for the specific cases we investigated,a is the equivalent chip radius (l 2/π;here,l is defined as the chip side length),λis the thermal conductivity of die attach-ment (7.5W/m ·◦C),b is the die attachment radius (2.5mm),t is the thickness of die attachment (0.2mm),w n equals 0.08(t/b ),and h n equals 0.08because w n is smaller than 1.The comparison of the three cases R th _cd ,only considering bulk thermal resistance (R thb _chip +R thb _dieattach ),together with spreading thermal resistance (R thb _chip +R c _dieattach ),and the measured results,is shown in Fig.6.From the comparison,we can see that the thermal resistance considering the spreading effects is much closer to the measured results than the gener-ally accepted,which only considers bulk thermal resistance.The spreading thermal resistance was found to have a great effect both on the partial and total thermal resistances,and should be taken into consideration during the thermal design of LED packages.However,there is still unneglectable numer-ical deviation between experiment and calculation even if the spreading effect were taken into consideration.The fix angle assumption of heat spreading induced calculation error [13],nonideal interface [5],and heat dissipation in other dimensions,and the temperature dependence of thermal conductivity for LED packaging materials [14]were believed to be responsible for the deviation in numerical value.V .C ONCLUSIONIn this paper,we investigated the electrical,optical,and thermal performance of LED packages with different chip sizes at the same working conditions.The forward current was found to increase with the chip size at the same forward voltage.Moreover,the optical flux and power increased with the chipsize at the same current density.The thermal resistance wasfound to decrease with the chip size both by experiment and simulation.The bulk and spreading thermal resistance were combined together to quantitatively analyze the partial thermal resistance variation,and the spreading thermal resistance was found to have great effect on the total thermal resistance.R EFERENCES[1]N.Holonyak,Jr.and S.F.Bevacqua,“Coherent (visible)light emissionfrom Ga (As 1−x P x )junctions,”Appl.Phys.Lett.,vol.1,no.4,pp.82–83,Dec.1962.[2]C.P.Kuo,R.M.Fletcher,T. 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J.Park,“Temperature measurement of visible light-emittingdiodes using nematic liquid crystal thermography with laser illumination,”IEEE Photon.Technol.Lett.,vol.16,no.7,pp.1706–1708,Jul.2004.[9]J.Hu,L.Yang,and M.W.Shin,“Electrical,optical and thermal degrada-tion of high power GaN/InGaN light-emitting diodes,”J.Phys.D,Appl.Phys.,vol.41,no.3,p.035107,Feb.2008.[10]N.Narendran and Y .Gu,“Life of LED-based white light sources,”J.Display Technol.,vol.1,no.1,pp.167–171,Sep.2005.[11]G.Farkas,Q.van V oorst Vader,A.Poppe,and G.Bognár,“Thermalinvestigation of high power optical devices by transient testing,”IEEE pon.Packag.Technol.,vol.28,no.1,pp.45–50,Mar.2005.[12]T.H.Lee,L.Kim,W.J.Hwang,C.C.Lee,and M.W.Shin,“Thermalanalysis of GaN-based LEDs using the finite element method and unit temperature profile approach,”Phys.Stat.Sol.(B),vol.241,no.12,pp.2681–2684,Oct.2004.[13]F.N.Masana,“A closed form solution of junction to substrate thermal re-sistance in semiconductor chips,”IEEE pon.,Packag.,Manuf.Technol.A ,vol.19,no.4,pp.539–545,Dec.1996.[14]L.Yang,J.Hu,and M.W.Shin,“Investigation of thermal measure-ment variables in high power GaN-based LEDs,”Solid State Phenom.,vol.124–126,pp.483–486,2007.Lianqiao Yang received the B.S.degree from Wuhan University,Wuhan,China,in 2004and the M.S.degree from Myongji University,Yongin,Korea,in 2006.She is currently working toward the Ph.D.degree in the Department of Materials Science and Engineering,Myongji University.Her research interest is the thermal analysis of high-power LED packages and systems.Ms.Yang is a recipient of an Excellent For-eign Students Scholarship from the Korea Research Foundation.Y ANG et al.:THERMAL ANALYSIS OF GaN-BASED LIGHT EMITTING DIODES WITH DIFFERENT CHIP SIZES575Jianzheng Hu was born in Yicheng,China,in 1976.He received the B.S.and M.S.degrees from Wuhan University,Wuhan,China,in 2001and 2004,re-spectively.He is currently working toward the Ph.D.degree in the Department of Materials Science and Engineering,Myongji University,Yongin,Korea.His research interests are thermal and mechanical design for optoelectronic packages and reliability test and analysis of high-power LEDs.Mr.Hu is a recipient of an Excellent For-eign Students Scholarship from the Korea ResearchFoundation.Lan Kim received the B.S.degree in computer engi-neering from Dalian Nationalities University,Dalian,China,in 2002.She received the M.S.and Ph.D.degrees in material science and engineering from Myongji University,Yongin,Korea,in 2004and 2008,respectively.She is currently with LG Display,Seoul,Korea.Her research interest is the thermal analysis of high-power LED packages andsystems.Moo Whan Shin (M’07)was born in Seoul,Korea,in 1960.He received the B.S.degree in metallurgy from Yonsei University,Seoul,in 1986and the M.M.S.E.and Ph.D.degrees in materials science and engineering from the North Carolina State Univer-sity,Raleigh,NC,in 1988and 1991,respectively.While in graduate school,he was a Research As-sistant and was with the Microelectronics Center of North Carolina,Research Triangle Park,NC,by the financial support from the BOC Group,Inc.From 1991to 1994,he was with the Department of Electri-cal and Computer Engineering,North Carolina State University as a Research Associate and contributed to the development of wide band-gap semiconductor devices such as SiC MESFET,SiC JFET,and Diamond IGFET for high-power,high-frequency,and high-temperature applications.From 1994to 1995,he was the Senior Engineer (Engineer III)of the Electrical Engineering and Applied Physics Department,Case Western Reserve University,Cleveland,OH,where he was engaged in research work in the field of high-power and high-frequency GaN and SiC devices.Since 1995,he has been with the Department of Materials Science and Engineering,Myongji University,Yongin,Korea,where he is currently a Professor.He is currently a Director of the Creation Research Institute,Myongji University.He has published more than 180journal and conference papers related to semiconductor devices for high-temperature and high-power applications.He is the holder of 12patents on thermal design of high-power LEDs and their applications.He has been a technical advisor to Seoul Semiconductor Company.His primary current research interests are the thermal packaging of high-power GaN-based LEDs.Dr.Shin has served as a Chair of the International Seminar on Light Emitting Diodes for the years 2005and 2006.He has also been a Chair of the Advisory Committee for the LEDEXPO,Korea,since 2005.Dr.Shin is a lifetime member of the Institute of Microelectronics and Packaging Society-Korea and the editorial committee of the Korean Institute of Metals and Materials.He was a steering committee member of several conferences including the International Symposium on Blue Lasers and Light Emitting Diodes and the International Conference on Properties and Applications of Dielectric Materials.He served as a committee member of the National Science and Technology Development,Korea,in 1999.。