Mian TAO, Ricky LEE Department of Mechanical & Aerospace Engineering Center for Advanced Microsystems Packaging LED-FPD Technology R&D Center at Foshan Hong Kong University of Science and Technology Non-uniform Junction Temperature Distribution on LED Chip Measurement, Characterization, Effects 12 th International Symposium on Microelectronics and Packaging KINTEX, Ilsan, Korea 15 October, 2014 ISMP 2014

Chip Heat Density Challenge Source: DARPA Typical HP LED: 0.1 x 0.1 x 0.01 cm, 1W, 1 x 10 4 W/cm 3 Intel I7 Xeon: 1.7 x 1.7 x 0.01 cm, 130W, 0.5 x 10 4 W/cm 3

Importance of Thermal Management  Performance of an LED device is closely related to T j  Instant Effects - Optical Performance  Quantum efficiency droop and radiant output decease  Red shifting of blue light  Correlated color temperature (CCT) of white light from RGB rises with T j  Long Term Effects - Degradation and Reliability Problems  LED chip, nucleation and growth of micro-cracks  Chip bonding layer  Encapsulant

[1] Specification of CREE EZ900 LED Chip [2] Chhajed, S.; Xi, Y.; Li, Y-L; Gessmann, Th; Schubert, E.F., "Influence of junction temperature on chromaticity and color-rendering properties of trichromatic white-light sources based on light-emitting diodes," Journal of Applied Physics, vol.97, no.5, pp , , Mar 2005 CREE EZ900 LED Chip Specification [1] The CCT of the white light from the combination RGB shifting with T j increasing [2] Instant Effects of T j

Degradation of chip bonding layer by high T j under thermal cycling [4] [3] Jianzheng Hu, Lianqiao Yang, Woong Joon Hwang, Moo Whan Shin, Thermal and mechanical analysis of delamination in GaN-based light-emitting diode packages, Journal of Crystal Growth, Volume 288, Issue 1, 2 February 2006, Pages , ISSN , [4] Guangchen Zhang; Shiwei Feng; Zhou Zhou; Jing Liu; Jingwan Li; Hui Zhu, "Thermal Fatigue Characteristics of Die Attach Materials for Packaged High- Brightness LEDs," Components, Packaging and Manufacturing Technology, IEEE Transactions on, vol.2, no.8, pp.1346,1350, Aug Degradation in encapsulation The excessive deformation induced by high T j may cause delamination [3] Long Term Effects of T j

Lifetime and T j Source: Lumileds

Evolution of LED Packaging for Thermal Management

TjTj TsTs Evaluation of Thermal Resistance & Network Typical SMD LED TjTj TcTc TbTb TsTs R chip R bonding R leadframe TjTj TcTc TbTb TsTs R chip R bonding R substrate Typical HP LED q: Heat Flow Rate (W or kg·m 2 /s 3 ) R,  : Thermal Resistance (C/W) k: Thermal Conductivity (W/m·C) h: Film Coefficient (W/m 2 ·C)

LED Chips Are Getting Bigger & Hotter Source: Epistar Power (W) Volume (cm 3 ) Power Density (W/cm 3 ) x x x x x x x x 10 4

Wire Bonding (electrical connection) and Encapsulation (protection) Chip Carrier (High thermal conductivity material) Formation of Chip Bonding Material (Solder/Adhesive) Chip Bonding (Pick&Place and Curing) Package of Wire-Bonded LED Light Heat The p-n junction of a LED chip 1.5 mm 10  m 60 mil blue LED chip, lateral type, sapphire substrate  Optical  Light source  25% of the total power input  Emitting upward  Thermal  Heat source  75% of the total power input  Conducted downward  Electrical  A common diode  Temperature characteristic

Chip Carrier (High thermal conductivity material) Formation of Chip Bonding Material (Solder) Package of Flip-chip LED Chip Bonding (Thermal Compression / Reflow) Encapsulation (protection)

Defects in Chip Bonding Layer - I  Chip was bonded to leadframe by adhesive (silver- filled epoxy)  Insufficient dosage of adhesive  Chip was bonded to a silicon carrier by soldering  Void inside the bonding layer  Flip-chip LED was bonded to a silver plated board by Au- Sn eutectic bonding  Void inside the bonding area

Defects in Chip Bonding Layer - II Defects in bonding layer,  Is T j still uniformly distributed over the chip?  How to assess the non-uniform T j, if any? Chip Carrier : Carrying LED chip LED Chip :  p-n Junction on top  Heat is conducted downward DEFECTS IN BONDING LAYER LED p-n Junction  Emitting Light  Generating Heat Heat Flow PathThe Bonding Layer – Why Is It Important? Chip bonding layer provides primary Mechanical Fixture Thermal Path (Encapsulant is thermal insulating)

Conventional T j Measurement Method Electrically, considering the LED to be an diode  Utilizing temperature characteristics of a diode  Most commonly used Thermally, considering the LED to be an object  IR thermography temperature measurement  Quantitatively measuring the radiance power emitted  Calculating the surface temperature from radiance power  What is the advantage?  IR thermography can provide the distribution of T j  What are the disadvantages?  Exposed junction  IR emissivity influences the temperature measurement

Modeling of I-V Characteristics of LED  Based on Shockley diode model and ignoring the voltage consumed on the serial resistance, the LED forward voltage, V f, can be expressed as where n is the ideality factor, I is the forward current, T j is the absolute temperature of the p–n junction C sat, Vo and A are the three fitting parameters  V f is a temperature sensitive parameter  Define K as  V f under a certain forward current is linear proportion to T j The LED itself can be used as a sensor to monitor T j (2) (1)

K-factor Calibration NO Place the sample in a thermostat and provide a sensing current, I sense Set the thermostat to an intended temperature (e. g. 30 ºC to 90 ºC) Wait until thermal equilibrium (fluctuation of V f is less than 0.1%) Perform linear regression among all these V f - T j data points T j is the same as the thermostat temperature Record the V f and T j Sufficient data points? YES Thermostat providing a controllable temperature  The I sense typically is 1mA,  I sense is so small that cannot raise the T j  The K factor is -1.6 mV/K  The resulted relationship between T j change and V f change is If the K factor of a sample under a certain current is calibrated,  T j can be obtained from  V f

T j Measurement by V f Method  Measure the V f change in cooling  T j change can be known from the K factor  In a cooling process, the sample will finally be cooled to T amb Measurement Procedures : 1.Drive the sample under an operating current, I drive, and the T j would be raised up 2.After thermal equilibrium, I drive is switched to I sense and the corresponding V f is recorded as the V sense,1 3.After current switching, The temperature of the junction begins to descend 4.Wait till the junction is cooled down to T amb and the corresponding V f is recorded as the V sense,2 5.The T j can be calculated Current Level Time Measure V sense,2 I drive =350 mA I sense =1 mA cooling Junction Temperature Time Measure V sense,2 T J,drive ≈ 80 ℃ cooling TT Measure V sense,1 T amb

Sample Description  Two types of LED chips with the p-n junction on top side  Two types of chip carrier  Adhesive chip bonding  Surface mounted on MCPCB  No encapsulant K1 Emitter leadframe 5050 leadframe B LED Chip Vertical – SiC A LED Chip Lateral – Sapphire

IR Thermography Setup FLIR-E63 IR Camera for Thermography Thermostat providing a known temperature The sample glued on the thermostat by thermal grease The V f method was implemented by the T3Ster System

 The sample was placed in the thermostat  The temperature of the thermostat was set to be 70 ºC  Thermal equilibrium was achieved  The IR thermography images were captured  IR Emissivity, , is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature.  A true black body should have an ε = 1  Any real object should have ε < 1 IR Thermography with one emissivity True IR Thermography Emissivity substantially influences the temperature measurement results Consideration of Emissivity

IR Thermography Calibration

Calibration Process Example :  T obj = 40/50/60/70/80/90 °C  The T img are shown below Wire bonding (Gold) Trace (Gold) Junction (GaN)  Only the chip area was focused  Calibration Procedures : 1.Place the sample on the thermostat 2.Set the thermostat to a desired temperature 3.Wait until thermal equilibrium. This temperature is denoted as T obj 4.Capture an IR image and obtain the image temperature, T img of every pixel 5.Repeat step 2 ~ 4

Calibration Results  A program was developed to perform the calibration  Pixel by pixel calibration  For illustration, the pixel at the center  T obj = 40/50/60/70/80/90 °C  T img = 39.9/41.3/53.5/60.6/67.5/74.7 °C (  set = 1) The fitting goodness is excellent 4 4 T img 4 = aT obj 4 + b

Full Image Calibration  The Root Mean Square Error (RMSE) of every pixels were calculated during the linear fitting  The RMSE image is shown on the left  The RMSE of most area is less than 0.5 °C  Before calibration  After calibration

Calibrated Image of an Operating LED  Operating Condition : Thermostat - 40 °C; I drive = 350/700/1000 mA Before Calibration After Calibration The calibration can effectively eliminate the influences from the emissivity

Validation of T j Measurement Methods T j measured by V f method  Most widely used and trusted  The T j given by the V f method is considered to be correct Considering the IR method  Accuracy is uncertain  Temperature on the surface  The average temperature on the surface is used for comparison Define the relative error as :  T j, relative error = (T j, IR - T j, Vf ) / T j, Vf Validate the IR method by the relative error

T j Measurement Results Comparison Test VehicleA LED 45mil 5050 Driving CurrentmA Thermostat Temperature ºC T j (V f method) ºC T j,avg (IR method) ºC Error ºC Relative Error%-0.4%-2.1% -1.8%-2.1%-3.5%-3.1% -3.9%-4.7%-4.4%-4.8% Test VehicleA LED 45mil K1 Emitter Driving CurrentmA Thermostat Temperature ºC T j (V f method)ºC T j,avg (IR method)ºC ErrorºC Relative Error%-0.4%-1.8%-2.6%-0.2%3.9%0.2%-1.0%-1.9%-0.3%-3.5%-4.9%-5.4% Test Condition :  Three different I drive  Use different thermostat temperatures to imitate different T amb

T j Measurement Results Comparison Test VehicleB LED 36mil 5050 Driving CurrentmA Thermostat Temperature ºC T j (V f method)ºC T j,avg (IR method)ºC ErrorºC Relative Error%0.7%-0.3%-0.8%-1.0%0.1%-0.6%-0.8%-1.3%0.1%-1.2%-1.5%-2.0% Test VehicleB LED 36mil K1 Emitter Driving CurrentmA Thermostat Temperature ºC T j (V f method)ºC T j,avg (IR method)ºC ErrorºC Relative Error%0.5%-0.2%-0.5%0.4%2.1%0.6%-0.9%-1.0%0.7%-0.2%-0.6%-1.6% Test Condition :  Three different I drive  Use different thermostat temperatures to imitate different T amb

Discussion on Measurement Results The IR method for T j measurement is validated  Two main factors affecting the error  Junction temperature  Chip type – Sapphire/SiC  Higher T j results in larger error  IR method tends to underestimate the T j  For the chosen LED chip, the relative error is smaller than 5%

 Sample preparation was the same as the samples in previous section  By means of controlling the adhesive dispensing, artificial defects were created  The residue adhesive material shows the area and boundary of bonding layer  The red dot denotes the location of T j,max Sample Description Sample-1Sample-2 Sample-5 Sample-4 Sample-3 Sample-6 4Corner Sheared and flipped over

Sample mA Sample-4 350mA Sample-4 700mA Sample mA Sample-1 350mA Sample-1 700mA Sample-7 350mA Sample-7 700mA Sample mA  In every sample with insufficient adhesive, the maximum temperature exists at the corner of the chip (denoted by a red circle) Non-uniform T j phenomena were observed IR Thermography Inspection - I

 Similar phenomenon was observed in the Sample 4Corner as well 4Corner 700mA4Corner 350mA4Corner 1000mA Sample under Test 4Corner IR Thermography Inspection - II

Summary of T j Measurement Results  The factors influencing the temperature T j,max  Overall thermal resistance  Defects in the bonding layer T j,center or T j,corner  Overall thermal resistance  Introduce the temperature difference of these two feature temperatures  T j = T j,max – T j,center or  T j = T j,max – T j,corner Corner or Center Temperature

Summary of Temperature Differences   T j changes with the area of the defects  T j can be used to assess the non-uniformity of T j  The  T j of Sample 4Corner is greater than Sample-5 (45 mil chip) even though the defect area of these two samples are close Void inside the bonding layer may intensify the non-uniformity of T j Sample-4Corner Sample-4

Chip Carrier for Soldering Chip Bonding  Focus on the void inside the bonding layer  The adhesive bonding area can be controlled  Soldering chip bonding was introduced  Silicon chip carrier fabrication  Silicon wafer  Al layer Deposition – 0.5  m  Electroless Ni Plating – 2~3  m  Electro Pure Sn Plating – 50~80  m  Different bonding pattern (size), the red area was Sn plated Silicon Wafer Al Ni Sn Al Ni Al Ni LED Chip Mark Artificial void was build in the bonding layer

Samples Description 1. A soldering compatible LED 3. LED Chip + Carrier 2. Bonded to the chip carrier by soldering 4. Glue on ceramic substrate

Bonding Interface Cross-section Inspection Sn, 0.08 mm Cu, 0.02 mm Solder well wetted on the chip Sapphire substrate Silicon Wafer Similar to HASL Without Solder Paste Surface Finishing : Gold

IR Thermography Inspection Testing Condition :  I drive = 700/800/900/1000/1100/1200 mA  T amb = 30 °C Before Calibration After Calibration

Results of IR method T j Correlation between IR and V f Method  Different I sense results in different T j  Not observed in normal samples  Caused by the non-uniformity of T j Chose this case for further investigation

 Lower Sensing Current (I sense = 0.2 mA) Results in Higher T j  Higher Sensing Current (I sense = 0.7 mA) Results in Lower T j Dual I sense Method for Detecting the Non-uniformity of T j T j Measurement with Different I sense  I drive = 1100 mA  T max = 85.8 ºC  T corner = 61.1 ºC  T avg = 71.0 ºC T max T avg

Dual I sense Method for Different Samples Sample 1Sample 2Sample 3 Test Condition :  I drive = 1100 mA  T amb = 30 °C Unit - ºCNo. 1No. 2No. 3 IR Method T max T corner  T j, IR V f Method T j (0.2 mA) T j (0.7 mA)  T j,Vf  T j, IR – Non-uniformity  T j,Vf – Dual I sense Method

Mechanism of Multiple I sense Method ContactingInsulated  4 LEDs are in parallel  One contacting thermostat  The other were insulated  Thermostat raise the temperature from 0 °C to 90 °C I source = 40 mA, I hot = 13 mA I source = 40  A, I hot = 19  A Non-uniform I sense Distribution Induced by Non-uniform T j

Concluding Remarks  IR thermography for T j distribution measurement was implemented with calibration.  T j measurement using forward voltage was performed to validate the calibrated IR thermography method.  The non-uniform T j distribution was proved by the artificial defect built in the bonding layer with the calibrated IR thermography method.  A modified electrical method with multiple sensing currents for non-uniform T j characterization is under development.

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