303978-1-4799-5296-0/14/$31.00 © 2014 IEEE

PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Application of IRT NDT for Ensuring Heat Robustness of LED Modules

A. Andonova, G. Angelov, Y. Georgiev, and Tihomir Takov

Abstract – The reliable operation of LED modules is toughly dependent on structural and technology solutions as well as on numerous natural and operational phenomena that impede thermal control. In the paper, various applications of InfraRed Thermography (IRT) for ensuring thermal robustness of LED modules are examined. Experimental data from IRT measurements of LED modules at different stages of pre-service quality to in-service degradation of LED modules or structure under operating conditions are presented and analyzed.

I. INTRODUCTION

The demand for light emitting diodes (LEDs) is expected to grow by 5 percent per annum until 2016 and by 3 percent per annum afterwards until 2020 [1]. Generally, the high quality LEDs are robust devices that can operate in excess of 100 000 hours, when properly operated. However, similarly to any other device, they gradually lose their performance over time. LED modules robustness can be ensured when drive currents and operating temperatures are held within certain limits as specified by the manufacturer.

It is well known that heat is the biggest barrier at the design phase that limits the spread of higher-brightness LEDs. The reliable performance of LED modules depends on pre-service quality and in-service degradation or structure under operating conditions [2]. Therefore, the challenge for realization of high performance LED sources therefore is the use of advanced tools and methods for ensuring thermal robustness

The role of InfraRed Thermographic Non-Destructive Testing (IRT NDT) is to ensure integrity [3] and hence, reliability of LED module or LED structure. Besides, IRT NDT can also be used to monitor the in-service degradation to avoid premature failure of the LED modules/structures.

In the present paper, we focus on thermal robustness, we discuss why it is important for the LED modules and how to apply the IRT NDT methods. We define a set of rules for ensuring thermal robustness of LED modules. We briefly analyze the advantages of modern thermography and the abilities of passive and active methods as a promising approach for thermal testing of LED modules.

Three techniques to determine the thermal severity of

LED modules through thermal image analysis have been used: 1) direct interpretation by identifying the maximum temperature for each of LED modules and evaluating their condition based on the ΔT criteria; 2) histogram or histogram distance for finding the similarity between two objects; and 3) gradient analysis of the segmented region [4].

II. THERMAL ROBUSTNESS OF LEDS

Modern LED modules gained wide application to high reliability and failure-resistant systems (approx. ¼ of all applications) in sensorics, medical, military, and automotive technologies.

Typically, a LED module consists of many interconnected LED chips along with other passive components. With this respect, the LED module can be considered as a system. The thermal robustness is determined by numerous factors such as the small dimensions of LEDs, the fact that they are placed very close to each other, the numerous variants of models for achieving given brightness.

In broad sense, the robustness is defined as sustainability to “tolerable” heat behavior regardless of any contingency situations such as lack of resources, communication failure, invalid or stressing actions, etc. To ensure thermal robustness means to ensure minimum dispersion of the heat parameters of the system.

The operation of LED modules as an individual device or as a part of a system is related to a number of natural phenomena that cause problems for the heat control. The a priori uncertainty is one of the most pronounced problems. When using the robustness approach in a priori known (given) limits of uncertainty, a relation between the quality of the LED system and the degree of uncertainty in operational conditions exists.

II. IRT NDE IN THERMAL TESTING OF LED MODULES

Thermal robustness has a direct link to thermal

characterization. Today’s IRT is unique for its ability for 2D non-destructive and non-contact temperature measurement in real time. It uses various procedures for thermal control – passive and active, static and dynamic [3]. In passive IRT the tested subject is characterized with temperature field form during its operation. The active

A. Andonova, G. Angelov, Y. Georgiev and Tihomir Takov are with the Department of Microelectronics, Faculty of Electronic Engineering and technologies, Technical University of Sofia, Kliment Ohridski 8, 1797 Sofia, Bulgaria, E-mail: ava@ecad.tu-sofia.bg , angelov@ecad.tu-sofia.bg

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approach in IRT assumes utilization of additional thermal source for simulation (load) of the subject. Defects in materials detected with IRT NDE could be active, i.e. they can emit or absorb heat. Defects that before testing have uniform temperature (usually equal to the ambient temperature) are passive because they do not raise “useful” temperature gradients. For detection of such defects the entire (or part of) object should be heated or cooled. The term “stationay” (constant in time) is related to the shape of the temperature dependence versus time. The active defects in passive IRT procedures are stationary by rule. The passive defects in active procedures cause dynamic temperature fields and the testing results depend on time of testing.

In our testing experiment we have used passive IRT with active IRT (pulsed and locking).

III. EXPERIMENTAL SETUP

We have performed multiple experiments for testing

temperature fields during the development phase of LED modules in HIC AD and Octa Light companies. For part of the modules, we have done thermal simulation with FLOTHERM software packet. The problems with the impact of emissivity coefficient over IRT measurements results are solved with various approaches depending on each particular case – emissivity map, calibration with black body, and coating with film with known emissivity. The IRT system includes infrared camera ThermaCam SC 640, PC and software (ThermoVision SDK – control of the system, ThermaCam Researcher – thermogram processing in real time). The supplementary processing is carried out in Matlab and our custom developed software [4]. We have used 24° and 45° close-up lenses. The infrared camera detector is 640×420 pixels with range of 7.5÷13µm [5].

IV. EXPERIMENTAL RESULTS

A. Calibration of thermal models

The validity of the model can be estimated by comparison of measurement results obtained by the physical experiments and the simulation results. We have presented the investigation results of light emitting source realized by Luxeon REBEL White-1W type of diode, mounted over thick film dedicated structure on ceramic substrate with dimensions 16.93 × 12.70 × 0.635 mm.

Simulation data from FLOTHERM are shown in Table 1. Simulation results from IRT measurements are given for three points ((T1, T2 and T3 on the LED side and T4, T5 and T6 on the opposite side). The data could be compared for each pixel. In the same table the calculated values for the temperature of the junction at different percentage of thermal dissipation of generated power are given. In result of IRT measurement, the model is calibrated for 75% thermal power dissipation.

TABLE I THERMAL SIMULATION DATA OF TESTED LED STRUCTURE Thermal loss 65% 70% 75% 80% 85%

Tj, 0C (calculated) 53.48 53.91 54.33 54.75 55.17

top

face

T1,0C 44,1 47,7 49.3 49,3* 50,9 52,5

T20C 43,3 45,1 46,6

46,8* 48,0 49,4

T30C 42,8 44,4 45,8

45,9* 47,1 48,5

botto

m fa

ce T4

0C 46,9 48,6 50,3 50,5* 52,0 53,6

T50C 44,3 45.8 47,3

47,0* 48,7 50,2

T60C 42,9 44,4 45,7

45,5* 47,1 48,5

* measured temperature Simulation results at 75% thermal dissipation as well

as the thermogram for the measurement of the opposite side are given in Table. 1. In Table 1 are also given the data for the control points. For the measurement, the test structure is rotated at 180° counter clockwise. Without calibration, this model gives error between 2 °C to 5 °C in different points.

”Black Electrical Tape” approach is used to compensate the reflectivity of the surface on the bottom face. The tape can be seen on the thermogram in Fig.1 [6].

Fig. 1. Thermal model and thermogram of the tested LED structure.

B. Measurement of the junction temperature and reliability forecast

When thermal modeling and IRT measurement are

combined, the highest temperature of the junction for multichip LED module at the system level can be forecasted [6].

The experiments are carried out with LED module consisting of 18 LEDs (6 pcs. of LXHL-BD03, LXHL-BM01 and LXHL-BR02 types). Such stable temperature profile with two selected lines in the thermogram on the package-radiator made of coated aluminum profile (Aluminum 6060 per EN573-3) is shown in Fig. 2. The average temperature of the package in thermal equilibrium is 73.2 °C (at ambient temperature of 25 °C without enforced convection). The temperature difference between measured and calculated temperature varies in the range of 2÷3 °С due to deviations in the data for the material properties of the module. Due to the steady thermal transfer

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between the junction and a selected measurement point Тmeas from the package surface (the cross point of the two lines at 73 °C is selected), there is constant difference of 18.4 °C and we can write down:

4,92734,18const =+=+= measj TT 0С (1) For the real simulation, the junction temperature

should be between 89.5÷95.5 °С.

Fig. 2. Thermal profile of the radiator in equilibrium mode

C. Diagnostics of the thermal behavior of operating LED modules.

Passive IRT can be used for auditing LEDs in real

operation conditions. In Figure 3a-3g we have shown thermograms and

thermal histograms of LED lamp diagnostics at the beginning of its operation (1e and 1f) after 8760 hours of operation – (1c and 1d), and after 17520 hours of operation – (1a and 1b). It can be seen that a correlation exist between temperature and lumen depreciation. This graph in Figure 3g illustrates the evolution of degradation processes and gives a good perspective on the quality evaluation and the robustness evaluation of LED modules both during the reliability tests and during the actual operation.

а) б)

c) d)

e) f)

g) Fig. 3. Thermograms and thermal histograms of LED lamp in different working times of its life.

IRT NDT is used to evaluate the LED modules after HTOL test. Fig. 4 shows thermogram and 3D images for the temperature distribution of two LED modules before and after test. The emerging degradation processes for one of the both modules can be seen.

Fig. 4. This is an example of the figure caption, which must come after the figure.

Under normal operating conditions LED modules can demonstrate certain self-heating with a temperature increase (with respect to ambient temperature) ranging between 20 °C and 80 °C. The current and temperature levels reached by LEDs during the time of ageing can strongly influence the degradation kinetics of LEDs. For this reason it is discussed an accurate definition of the

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operating conditions and a careful optimization of the heat dissipation process are the mandatory steps for achieving long LED lifetime by using the IRT NDT approach for hot spots evaluation. Fig. 5a-b depicts thermograms of poor LED module design and the same module after thermal design optimization. Non-uniform temperature field (caused by mounting defects) can be detected by IRT monitoring (Fig. 5c).

a) b) c) Fig. 5. This is an example of the figure caption which must come after the figure. D. Failure analysis

Active IRT can be used for failure diagnostics and analysis during the development and maintenance activities [7].

The LED, as a part of a system, can be exposed to new various environmental conditions compared to its original qualification. Due to the proximity of the different components and materials in such a system, those environmental conditions mainly originate from the system itself and on the opposite, may affect the function and reliability of the LED. Especially at higher temperatures, aggressive substances from different materials can evaporate. In Figure 4, it is shown the effect of corrosion which leads to resistance deterioration and increasing the temperature of the marked LED chip.

The effect of corrosion, which leads to resistance deterioration and increasing the temperature of the marked LED chip, is shown in Figure 6а. In Fig. 6b it is shown a thermogram where the abrupt drop of reliability of the module (under 1000 hours) is due to big non-uniformity of TIM thickness upto 40 °C between LED chips in the module.

а) b) Fig. 6 Thermogram of module with corrosion (а) and of module with non-uniformly applied TIM (b).

III. CONCLUSION

The present-day IRT NDE is a powerful and effective tool for ensuring thermal robustness of LED modules. Remote inspection and monitoring of heat distribution during the operation of LED modules gives important correlations for their thermal characterisation. It is important to perform thermal diagnosis of the LED modules for degradation evaluation and failure analysis.

ACKNOWLEDGEMENT

This research was funded by a Grant No. ДФНИ-

I01/9-3 by the National Ministry of Science and Education of Bulgaria – Scientific Research Fund.

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