mian tao, ricky lee department of mechanical & aerospace engineering center for advanced...
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Mian TAO, Ricky LEEDepartment of Mechanical & Aerospace Engineering
Center for Advanced Microsystems PackagingLED-FPD Technology R&D Center at Foshan
Hong Kong University of Science and Technology
Non-uniform Junction TemperatureDistribution on LED Chip
Measurement, Characterization, Effects
12th 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 104 W/cm3
Intel I7 Xeon: 1.7 x 1.7 x 0.01 cm, 130W, 0.5 x 104 W/cm3
Importance of Thermal Management
Performance of an LED device is closely related to Tj
Instant Effects - Optical PerformanceQuantum efficiency droop and radiant output deceaseRed shifting of blue lightCorrelated color temperature (CCT) of white light from
RGB rises with Tj
Long Term Effects - Degradation and Reliability ProblemsLED chip, nucleation and growth of micro-cracks Chip bonding layerEncapsulant
[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.054506,054506-8, Mar 2005
CREE EZ900 LED Chip Specification [1]
The CCT of the white light from the combination RGB shifting with Tj increasing [2]
Instant Effects of Tj
Degradation of chip bonding layer by high Tj 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 157-161, ISSN 0022-0248,[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. 2012
Degradation in encapsulation
The excessive deformation induced by high Tj may cause delamination [3]
Long Term Effects of Tj
Lifetime and Tj
Source: Lumileds
Evolution of LED Packaging for Thermal Management
Tj Ts
Evaluation of Thermal Resistance & Network
Typical SMD LED
Tj
Tc
Tb
Ts
Rchip
Rbonding
Rleadframe
Tj
Tc
Tb
Ts
Rchip
Rbonding
Rsubstrate
Typical HP LED
L
TTkA
x
TkAq sj
q: Heat Flow Rate (W or kg·m2/s3)R, : Thermal Resistance (C/W)k: Thermal Conductivity (W/m·C)h: Film Coefficient (W/m2·C)
q
TT
kA
LR sj
sajs
assjajja R
q
TTTT
q
TTR
)( as TThAq
q
TT
hAas
1
LED Chips Are Getting Bigger & Hotter
Source: Epistar
Power (W)
Volume (cm3)
Power Density (W/cm3)
0.3
0.26 x 10-4
1.18 x 104
0.4
0.37 x 10-4
1.08 x 104
1.4
0.79 x 10-4
1.77 x 104
2.8
1.61 x 10-4
1.74 x 104
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 chip1.5 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 Tj still uniformly distributed over the chip?How to assess the non-uniform Tj, if any?
Chip Carrier : Carrying LED chip
LED Chip :
p-n Junction on topHeat is conducted downward
DEFECTS IN BONDING LAYER
LED p-n Junction
Emitting LightGenerating Heat
Heat Flow PathThe Bonding Layer
– Why Is It Important?Chip bonding layer provides primary
•Mechanical Fixture
•Thermal Path (Encapsulant is thermal insulating)
Conventional Tj Measurement Method
Electrically, considering the LED to be an diodeUtilizing temperature characteristics of a diodeMost commonly used
Thermally, considering the LED to be an object IR thermography temperature measurement
Quantitatively measuring the radiance power emittedCalculating the surface temperature from radiance power
What is the advantage?IR thermography can provide the distribution of Tj
What are the disadvantages?Exposed junctionIR 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, Vf, can be expressed as
where n is the ideality factor, I is the forward current, Tj is the absolute temperature of the p–n junctionCsat, Vo and A are the three fitting parameters
Vf is a temperature sensitive parameter Define K as
Vf under a certain forward current is linear proportion to Tj
The LED itself can be used as a sensor to monitor Tj
(2)
(1)
K-factor Calibration
NO
Place the sample in a thermostat and provide a sensing current, Isense
Set the thermostat to an intended temperature (e.
g. 30 ºC to 90 ºC)
Wait until thermal equilibrium (fluctuation of Vf is less than 0.1%)
Perform linear regression among all
these Vf - Tj data points
Tj is the same as the thermostat temperature
Record the Vf and Tj
Sufficient data points?
YES
Thermostat providing a controllable temperature
The Isense typically is 1mA,
Isense is so small that cannot raise the Tj
The K factor is -1.6 mV/K
The resulted relationship between Tj change and Vf change is
If the K factor of a sample under a certain current is calibrated, Tj can be obtained
from Vf
Tj Measurement by Vf Method Measure the Vf change in cooling Tj change can be known from the K
factor In a cooling process, the sample will
finally be cooled to Tamb
Measurement Procedures : 1. Drive the sample under an operating
current, Idrive, and the Tj would be raised up
2. After thermal equilibrium, Idrive is switched to Isense and the corresponding Vf is recorded as the Vsense,1
3. After current switching, The temperature of the junction begins to descend
4. Wait till the junction is cooled down to Tamb and the corresponding Vf is recorded as the Vsense,2
5. The Tj can be calculated
Cu
rren
t L
evel
Time
MeasureVsense,2Idrive=350 mA
Isense=1 mA
cooling
Jun
ctio
n
Tem
per
atu
re
Time
Measure Vsense,2
TJ,drive ≈ 80 ℃
cooling T
Measure Vsense,1
MeasureVsense,1
Tamb
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 L
ED
Ch
ip
Ver
tica
l – S
iC
A L
ED
Ch
ip
Lat
eral
– S
app
hir
e
IR Thermography Setup
FLIR-E63 IR Camera for Thermography
Thermostat providing a known temperature
The sample glued on the thermostat by thermal grease
The Vf 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
40
50
60
70
80
90
IR Thermography with one emissivity
40
50
60
70
80
90
True IR Thermography
Emissivity substantially influences the temperature measurement results
Consideration of Emissivity
IR Thermography Calibration
Calibration Process
Example :
Tobj = 40/50/60/70/80/90 °C
The Timg are shown below
30 40 50 60 70 80 90 100
Wire bonding (Gold)
Trace (Gold)
Junction (GaN)
Only the chip area was focused
Calibration Procedures :1. Place the sample on the thermostat2. Set the thermostat to a desired temperature3. Wait until thermal equilibrium. This temperature is denoted as Tobj
4. Capture an IR image and obtain the image temperature, Timg of every pixel5. Repeat step 2 ~ 4
40
50
60
70
80
90
Calibration Results A program was developed to perform the calibration Pixel by pixel calibration For illustration, the pixel at the center Tobj = 40/50/60/70/80/90 °C Timg = 39.9/41.3/53.5/60.6/67.5/74.7 °C (set = 1)
The fitting goodness is excellent
4 4
Timg4 = aTobj
4 + b
Full Image Calibration
0
0.5
1 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
30 40 50 60 70 80 90 100
30 40 50 60 70 80 90 100
Before calibration
After calibration
Calibrated Image of an Operating LED
Operating Condition : Thermostat - 40 °C; Idrive = 350/700/1000 mA
Bef
ore
Cal
ibra
tion
Aft
er C
alib
rati
on
The calibration can effectively eliminate the influences from the emissivity
Validation of Tj Measurement Methods
Tj measured by Vf methodMost widely used and trustedThe Tj given by the Vf method is considered to be correct
Considering the IR methodAccuracy is uncertainTemperature on the surfaceThe average temperature on the surface is used for
comparison
Define the relative error as :Tj, relative error = (Tj, IR - Tj, Vf ) / Tj, Vf
Validate the IR method by the relative error
Tj Measurement Results Comparison
Test Vehicle A LED 45mil 5050Driving Current mA 350 700 1000
Thermostat Temperature
ºC
40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0
Tj (Vf method) ºC
50.3 60.6 71.0 81.2 66.7 77.4 88.1 99.3 85.0 96.6 108.0 120.1
Tj,avg (IR method) ºC
50.1 59.3 69.5 79.7 65.3 74.7 85.4 96.2 81.7 92.0 103.3 114.3
ErrorºC
-0.2 -1.3 -1.5 -1.5 -1.4 -2.7 -2.7 -3.1 -3.3 -4.6 -4.8 -5.7
Relative Error % -0.4% -2.1% -2.1% -1.8% -2.1% -3.5% -3.1% -3.1% -3.9% -4.7% -4.4% -4.8%Test Vehicle A LED 45mil K1 Emitter
Driving Current mA 350 700 1000Thermostat
TemperatureºC 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0
Tj (Vf method) ºC 45.8 55.9 66.0 76.4 55.6 66.0 76.3 86.8 66.4 77.0 88.2 98.9 Tj,avg (IR method) ºC 45.6 54.8 64.3 76.3 57.8 66.1 75.5 85.2 66.2 74.4 83.9 93.6
Error ºC -0.2 -1.0 -1.7 -0.2 2.2 0.1 -0.8 -1.6 -0.2 -2.7 -4.4 -5.4 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 Idrive Use different thermostat temperatures to imitate different Tamb
Tj Measurement Results Comparison
Test Vehicle B LED 36mil 5050Driving Current mA 350 500 700
Thermostat Temperature
ºC 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0
Tj (Vf method) ºC 52.7 62.7 73.0 83.1 59.8 69.9 80.1 90.4 70.7 80.8 90.9 101.5 Tj,avg (IR method) ºC 53.0 62.5 72.4 82.2 59.9 69.5 79.5 89.2 70.8 79.8 89.5 99.5
Error ºC 0.3 -0.2 -0.6 -0.8 0.0 -0.4 -0.7 -1.2 0.1 -1.0 -1.4 -2.0 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 Vehicle B LED 36mil K1 EmitterDriving Current mA 350 500 700
Thermostat Temperature
ºC 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0 40.0 50.0 60.0 70.0
Tj (Vf method) ºC 48.4 58.5 68.7 79.3 52.8 62.8 73.8 84.7 60.2 70.8 81.4 92.6 Tj,avg (IR method) ºC 48.6 58.3 68.4 78.2 53.9 63.2 73.2 83.9 60.6 70.6 80.9 91.1
Error ºC 0.2 -0.1 -0.3 0.3 1.1 0.4 -0.7 -0.8 0.4 -0.1 -0.5 -1.5 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 Idrive Use different thermostat temperatures to imitate different Tamb
Discussion on Measurement Results
The IR method for Tj measurement is validated
Two main factors affecting the errorJunction temperatureChip type – Sapphire/SiC
Higher Tj results in larger error IR method tends to underestimate the Tj
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 Tj,max
Sample Description
Sample-1 Sample-2
Sample-5Sample-4
Sample-3
Sample-64Corner
Sheared and flipped over
Sample-1 1000mA
Sample-4 350mA
Sample-4 700mA
Sample-4 1000mA
Sample-1 350mA
Sample-1 700mA
Sample-7 350mA
Sample-7 700mA
Sample-7 1000mA
In every sample with insufficient adhesive, the maximum temperature exists at the corner of the chip (denoted by a red circle)
Non-uniform Tj phenomena were
observed
IR Thermography Inspection - I
Similar phenomenon was observed in the Sample 4Corner as well
4Corner 700mA4Corner 350mA 4Corner 1000mA
Sample under Test 4Corner
IR Thermography Inspection - II
Summary of Tj Measurement Results
The factors influencing the temperature
Tj,max Overall thermal resistanceDefects in the bonding layer
Tj,center or Tj,corner
Overall thermal resistance Introduce the temperature difference
of these two feature temperatures
Tj = Tj,max – Tj,center
or Tj = Tj,max – Tj,corner
Corner or Center Temperature
Summary of Temperature Differences
Tj changes with the area of the defects
Tj can be used to assess the non-uniformity of Tj
The Tj 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 Tj
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
AlNi
Sn
AlNi
AlNi
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 chipSapphire substrate
Silicon Wafer Similar to HASL
Without Solder Paste
Surface Finishing : Gold
IR Thermography Inspection
Testing Condition : Idrive = 700/800/900/1000/1100/1200 mA
Tamb = 30 °C
30 40 50 60 70 80 90 100 110
Before Calibration
After Calibration
Results of IR method
Tj Correlation between IR and Vf Method
Different Isense results in different Tj
Not observed in normal samples Caused by the non-uniformity of Tj
Chose this case for further investigation
Lower Sensing Current (Isense = 0.2 mA) Results in Higher Tj
Higher Sensing Current (Isense = 0.7 mA) Results in Lower Tj
Dual Isense Method for Detecting the Non-uniformity of Tj
Tj Measurement with Different Isense
Idrive = 1100 mA
Tmax = 85.8 ºC
Tcorner = 61.1 ºC
Tavg = 71.0 ºC
Tmax
Tavg
Dual Isense Method for Different Samples
Sample 1 Sample 2 Sample 3 Test Condition : Idrive = 1100 mA
Tamb = 30 °C
Unit - ºC No. 1 No. 2 No. 3
IR Method
Tmax 92.6 94.8 88.7
Tcorner 73.1 82.3 79.1
Tj, IR 19.5 12.5 9.6
Vf Method
Tj (0.2 mA) 88.7 89.9 82.2
Tj (0.7 mA) 85.3 87.5 80.6
Tj,Vf 3.4 2.4 1.6
Tj, IR – Non-uniformity
Tj,Vf – Dual Isense Method
Mechanism of Multiple Isense Method
Contacting Insulated
4 LEDs are in parallel
One contacting thermostat The other were insulated Thermostat raise the temperature
from 0 °C to 90 °C Isource = 40 mA, Ihot = 13 mA
Isource = 40 A, Ihot = 19 A
Non-uniform Isense Distribution Induced by Non-uniform Tj
Concluding Remarks
IR thermography for Tj distribution measurement was implemented with calibration.
Tj measurement using forward voltage was performed to validate the calibrated IR thermography method.
The non-uniform Tj 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 Tj characterization is under development.
Campus of HKUST