1. On the Standardization of Thermal Characterization of LEDs András Poppe Mentor Graphics MicReD Division, Budapest, Hungary and Budapest University of Technology & Economics, Department of Electron Devices, Hungary Clemens Lasance Philips Research, Eindhoven, The Netherlands MicReD Division

2. 2 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Why to deal with thermal issues? ► Reliability is connected to thermal issues  life time (failure mechanisms are thermally assisted)  mechanical stress ► Optical properties strongly depend on temperature  spectra  emitted flux / efficiency / efficacy Courtesy of J. Schanda

3. MicReD Division 3 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Main drive for thermal characterization ► Fair comparison with competitors' data ► In an ideal world: provide customer support from a manufacturer point of view from a customer point of view Are temperatures within specs? Starting point: T j = R th j-ref  P + T ref ► T ref from (often unspecified) measurements (own responsibility) ► P from estimated power dissipation (own responsibility) ► R thj-ref from Component Data Sheets (other’s responsibility) If T j calculated > T specification  Redesign! ► In case of lighting: specified lumens at operating temperature

4. MicReD Division 4 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 The main reason for standardization Exactly because of the many uncertainties in the simple equation, everything can be quoted by vendors. Standardization = define strict rules. It is a consequence of living in a world that is based on money, not on honesty. Differences and similarities: LED vs IC ► Similarities process of how to establish thermal standards ► Differences  The good news: LEDs are thermally much less complex  The bad news: T j plays a major role not only in thermal, but also in lighting design  we need multi-domain models

5. MicReD Division 5 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 A few words about thermal resistance ► Rearrange the equation: R th j-ref = (T j – T ref )/P ► Original definition in the JEDEC JESD51-1 document ► Accurate; the questions are:  what is the dissipated power?Issue for LEDs…  what is the T X reference temperatureUse cold-plate!

6. MicReD Division 6 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 A few words about thermal resistance ► Alternate formulation: instead of spatial difference, let us build a temporal difference: T J1 = R th J-X  P H1 + T X (1a) T J2 = R th J-X  P H2 + T X (1b) If t 1 =0 and t 2 =   R th J-X =  T J /  P H T J2 – T J1 = R th J-X  (P H2 ­ – P H1 ) (1b-1a) Let T J1 =T J (t 1 ) and T J2 =T J (t 2 ): R th J-X =[T J (t 2 ) – T J (t 1 ) ] / (P H2 ­ – P H1 ) R th J-X =  T J (t) /  P H If P H2 = 0, then T J2 = T X – see (1b)

7. MicReD Division 7 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Extend the R th concept to transient R th J-X =  T J (t) /  P H ► t 1 = 0; t 2 = changes from 0+ to  ► Keep reference temperature constant (though, falls out) ► Record  T J (t) very densely ► With known  P H dissipation, series of R th -s corresponding to each t instance is obtained: This is preferred for LEDs (e.g. compliance to CIE 127-2007) R thJX (t) =  T J (t) /  P H is called Z th curve  heating curve  cooling curve

8. MicReD Division 8 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 How to standardize R th j-ref ? ► Questions one may ask:  How is T j defined? Can it be measured in practice? IR / electrical test method If not, how to derive it from another measurement?  What is T ref ? Can it be unambiguously defined and easily measured in practice? cold-plate setups  How is P defined? Corrected for non-thermal contributions? P opt as per CIE-127:2007  What is the physical meaning of R th ?  Is R th really temperature-independent?  What is the variance in the published data per manufacturer?

9. MicReD Division 9 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 "Lumen maintenance" / life time ~5 years ago By now Vendors usually mean degradation of Φ V /Φ V0 but not stated explicitly ► Nowadays life-time of LEDs is defined according to the degradation of their light output in terms of Φ V total luminous flux ("lumens")

10. MicReD Division 10 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 "Lumen maintenance" / life time ► Nowadays life-time of LEDs is defined according to the degradation of their light output in terms of Φ V total luminous flux ("lumens")

11. MicReD Division 11 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Life time ► L70: degradation of the relative light output by 30%  It is already considered as a failure ► B50: when 50% of the population already fails ► Life-time metrics Lxx and Bxx are often used (to be standardized – by whom?)

12. MicReD Division 12 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Some degradation mechanisms ► If LEDs are driven within their specified ratings (safe operating area), failures should not happen (in principle) ► However, some users wish to know what happens in case of uprating ► Possible degradation and failure modes in this case:  Yellowing of phosphor containing encapsulations  Lens degradation  Delamination of adhesive layers  Solder joint failures What temperature really counts?

13. MicReD Division 13 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Some questions / issues ► Do quoted data reflect real-life conditions?  E.g. do quoted temperature values correspond to values that happen during operation?  Is junction temperature really the junction temperature?  If reference point temperature is quoted, does it allow fair comparison? ► Are data measured really in a correct way? ► Are the I F and T ?? reference values comparable at different vendors? ► There is no idiot-proof way of translating temperature related data to e.g. "useful lumens" under all operating conditions ► Thermal data are often provided by means of series thermal resistance models which may fail in some cases

14. MicReD Division 14 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Some questions / issues ► Usually luminous flux data are provided, which can not be used for thermal design purposes. Total radiometric flux (emitted optical power) would also be required ► Efficacy data (also called luminous efficiency or "lumen per watts") are provided, but efficiency data (emitted optical power per supplied electrical power) are not published  Term efficiency is used ambiguously

15. MicReD Division 15 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Example ► Let us assume two WPE-s (wall plug efficiency: P opt /P el )   T = 50 o C, P el = 10W  "R thel " =  T / P el = 50/10 = 5 K/W WPE = 25%  R threal =  T / (P el – P opt ) =  T / [P el  (1-WPE)] = = 50/(10  0.75) = 6.67 K/W WPE = 50%  R threal =  T / (P el – P opt ) =  T / [P el  (1-WPE)] = = 50/(10  0.5) = 10 K/W ► By neglecting P opt vendors report much nicer data than reality

16. MicReD Division 16 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 How to consider light output? P opt (T,I F ) WPE(T,I F )  V (T,I F ) steady- state electrical powering for photometric/radiometric measurements in thermal steady-state JEDEC JSD51-1 static test method compliant thermal measurement DUT LED on cold-plate detector CIE 127-2007 compliant photometric & radiometric measurement switching-off record thermal data, calculate T j and R th-real ANY THERMAL TESTER WHICH MEETS CERTAIN REQUIREMENTS ANY PHOTOMETRIC TEST SETUP WITH TEMP.CONTROL

17. MicReD Division 17 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 WPE – is not a single number ► Depends on I F and T 2008 ~2004 Set of such plots must be published for typical operating conditions. Neither is efficacy a single number; people tend to forget this.

18. MicReD Division 18 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Series thermal resistance models?! ► Usual suggestion of LED vendors on their data sheets  ► In some cases does not work for LED packages where the heat-flow path splits into multiple branches: Multiple heat-flow paths / need to be modeled, based on simulation? Assumes a single heat-flow path!

19. MicReD Division 19 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Series thermal resistance models?! ► Flotherm simulations (conduction mode) ► Element values of the series resistance model show dependence on environmental conditions ► Nonsense in some cases The underlying assumption of the single path must be checked

20. MicReD Division 20 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Multiple heat-sources ► What would be the right R th ? ► In general, any # of LEDs (LED chips) may be present in a package or in a level1 assembly Force Sense

21. MicReD Division 21 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Problem of multiple LED chips ► Individual R th vs 'ensemble' R th ? ► No way to measure R th-ji -s unless LED array is designed for thermal testability Force Sense R th-ij R th-ensamble

22. MicReD Division 22 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 If we have access to individual LEDs: ► If each LED can be measured individually: thermal resistance or impedance matrices (see e.g. T. Treurniet or M.W. Shin – SEMITHERM'06) ► The JEDEC JC15.1 committee goes this way for IC-s

23. MicReD Division 23 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Short pulse measurements ► During in-line testing photometric/colorimetric properties are measured with a short pulse  T j = T ref = constant is assumed  In 10 ms significant junction temperature change may take place During 10 ms T j changes almost by 5 o C 1e-61e-40.01110010000 0 5 10 15 20 25 Time [s] Normalized temperature rise [°C] T3Ster Master: Zth dimco_kisf2 - Ch. 0 Question is if this causes big problems or not… A multi-domain LED model could correlate this with long term data

24. MicReD Division 24 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 PRPR P loss P D -P opt P heat = I F, T ref Φ V (I F, T j ), T j Create a 'standardised' LED model? In research phase at Budapest University of Technology Input: I F, T ref output: Φ V  dream of SSL designers Lot of work and round-robin tests would be needed

25. MicReD Division 25 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 How about existing standards? ► Is JEDEC JESD51-1 "good" for LEDs?  yes, but the "power dissipated in the device" has to be carefully specified reference temperature needs to be well established and kept constant  Possible new measurement guidelines: measure P opt according to CIE 127-2007 measure on a cold-plate, T ref = T cold-plate calculate junction temperature as follows: T J = T ref + R th  P H ► Is CIE 127-2007 "good" enough?  yes, thermal aspects of total flux measurement are precisely specified, but scattered around in the document Arrangement b of Figure 9 allows TEC-based control of LED package temperature (cold-plate), attach DUT LED to such a cold-plate  Possible measurement guidelines to standardize: 4 wire connection to DUT LED to comply with JEDEC JESD51-1 requirements

26. MicReD Division 26 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Standardization activities: JEDEC ► JEDEC JC15.1 committee: LED standardization task group  Initiated by Philips, OSRAM and Mentor Graphics MicReD  White paper published, next versions of the white paper: Eurosime'09 Electronics Cooling Magazine  Panel discussion at the 14 th THERMINIC Workshop in Rome, Italy Philips Lighting, OSRAM OS, Lumileds, MicReD, KETI  Next meeting: 20 March 2009 in San Jose, companies interested in LED business invited

27. MicReD Division 27 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Standardization activities: CIE ► Two new TC-s established, dealing with thermal aspects  TC-63: Optical Measurement of High-Power LEDs To develop a CIE recommendation on methods for the operation of high-power LEDs in DC and in pulse mode, at specified junction temperatures, for optical measurements.  TC-64: High Speed Testing Methods for LEDs To prepare a technical report on high speed testing methods for electrical, thermal and optical quantities during the production of LEDs and the conversion of the values to DC operational conditions including the related time dependent functions. ► Light and Lighting Conference with Special Emphasis on LEDs and Solid State Lighting, 27-29 May Budapest, Hungary  TC meetings

28. MicReD Division 28 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Some conclusions ► Data sheets do not provide sufficient information  e.g. for thermal design, efficiency data are also required, efficacy is of no help ► Agreement on measuring 'real' Rth of single chip LEDs is required  one should consider the real heating power not juts the supplied electrical power – would allow real thermal design  this would also provide a fair basis of comparison for LED vendors ► Agreement on the way of obtaining LEDs' junction temperature is needed  reported metrics (e.g. luminous flux) possibly should refer to this ► Standard test environments are also part of a like-with-like comparison  we believe measuring on cold-plates yields meaningful and repeatable results

29. MicReD Division 29 A. Poppe and C. Lasance: On the Standardization of Thermal Characterization of LEDs 18/03/2009 Some conclusions ► For multi-chip LED systems Rth/Zth 'matrices' can be introduced (slef/transfer impedances) as this is becoming accepted for conventional silicon devices (ICs) too  This assumes access to the individual LED chips (which is assured in certain cases like RGB modules)  Design for thermal testability of LED lighting solutions could support this  If LED chips are not accessible individually, a commonly accepted concept of 'ensemble' thermal resistance would be needed ► Easy-to-handle yet technically sound, commonly accepted multi-domain LED models would be greatly appreciated by lighting designers  Such models could partly replace conventional data sheets: vendors provide model parameters instead of data sheet charts and values