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Thermal analysis of power LED employing dual interface method and water flow as a cooling system
Thermochimica Acta 523 (2011) 237–244
Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal analysis of power LED employing dual interface method and water flow as a cooling system P. Anithambigai a,∗ , K. Dinash a , D. Mutharasu a , S. Shanmugan a , Choon Kim Lim b a b
Nano Optoelectronics Lab, School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Research and Development Laboratory, Globetronics Sdn. Bhd., 11900 Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 5 May 2011 Accepted 2 June 2011 Available online 13 June 2011 Keywords: Junction to board thermal resistance Junction to ambient thermal resistance Dual interface method Water flow system Optical performance
a b s t r a c t Thermal transient measurement based on structure function evaluation was used to measure the thermal resistance. The study signifies the importance of dual interface method in determining the exact point of separation between the board and the LED package. For a constant ambient temperature which was maintained at 28.2 ± 1.0 ◦ C at 700 mA, the junction to board thermal resistance obtained was 10.84 K/W. In addition, an experimental set up has been reported in this work having a constant water flow beneath the external heat sink. More emphasis has been given in studying the effect of change of such measurement environment on the junction to board thermal resistance. It was revealed that the junction to board thermal resistance was not affected but the total real thermal resistance from junction to ambient was reduced significantly by 55.6% upon cooling with water. A study on the effect of light output on the total thermal resistance was performed and it was revealed that the efficiency and the reliability of an LED are strongly dependent on the optical properties of the device. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Light emitting diode (LED) technology has been progressing rapidly for the past few decades. Generally, LEDs conveniently eliminate the limitations of incandescent lamps by providing more light per watt. LEDs do last longer because they do not have filaments that will burn out. Besides, LEDs are known for high efficiency, good reliability, variable colours and low power consumption [1,2]. However, despite the rapid progress of this technology, overheating of these LEDs upon higher input current has become a major drawback [3]. Thus, thermal management of these devices is becoming more and more crucial as poor thermal management would cause excessive rise in the junction temperature and subsequently would either cause a complete thermal runaway of the device or breakdown of the chip [4]. Generally in an LED, the heat generated in the p–n junction will first be conducted from the chip to the metal core printed circuit board (MCPCB) followed by an external heat sink and finally into the ambient via convection. This is assuming it is a perfect one-dimensional heat flow. However, it is impossible to assume a perfectly one-dimensional heat flow as there is heat loss through surrounding lead frame, epoxy and lens [5]. However, this loss is reported to be insignificant as the material thermal conductivity is very small [6].
∗ Corresponding author. E-mail address: [email protected] (P. Anithambigai). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.06.001
Junction to case thermal resistance (RthJC ) of an LED or any power semiconductor device is an often employed concept in thermal engineering. Since power semiconductors are prone to ever increasing power dissipation levels, achieving a low RthJC is very significant. The conventional procedure of measuring RthJC is by using the thermocouple measurement. Often this produces inaccurate results since it is difficult to place the thermocouple exactly at the required surface of case of the package. Szabo et al. presented a method of finding the junction to case thermal resistance by comparing boundary conditions of different interface layers [7]. A comparison of with and without thermal grease was reported in [7] and structure function based evaluation favoured in determining the exact point of separation between package and case. The same dual interface method was reported in 2008 using 3 different boundary conditions; liquid metal (gallium–indium–tin alloy), thermal grease and ceramic plate and thermal grease only as interface materials in each boundary condition. The importance of choosing the correct interface material with the right thermal conductivity properties and the specification of the thickness of the interface material were emphasized [8]. It was reported again in 2010 that interface materials like mylar sheet or ceramic are more suitable for smaller chip semiconductor devices [9] which means that the size of the chip is a factor for obtaining optimized RthJC with chosen interface material. It was reported by Andras et al. that if the chip is too large, which covers a large surface area of the module; it may cause a huge additional thermal resistance to the system. However, if the chip is too small, the inserted interface layer might not significantly change the structure function. In short,
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thermal resistance is defined as the ability of a component to transfer heat, and thermal management is dependent predominantly on the thermal resistance of the package. In addition to this, neglecting optical power when determining an LED’s thermal resistances yields values which are much lower than the reality. Since about 10–40% of the supplied energy leaves the system as light, the optical properties of the device are not negligible. A combined thermal and radiometric measurement set up was proposed in [10]. This method has been practiced until today [11] by many research groups subsequently. In this work, the dual interface method was employed to identify the junction to board thermal resistance. In addition, a water flow was reported as a cooling system for LEDs thermal characterization. Optical properties of the sample were studied too in order to understand the dependence of total thermal resistance with the optical output of the device. 2. Theoretical background The thermal resistance of a semiconductor device is generally defined as in Eq. (1) according to JEDEC standard 51-1 [12]: RthJX =
TJ − TX PH
(1)
where RthJX is the thermal resistance from device junction to the specific environment, TJ is the device junction temperature under steady state condition, TX is the reference temperature for a specific environment and PH is the power dissipated in the device. As it is widely known, the light output of an LED strongly depends on the operating conditions. The higher the supplied current, the more light is generated by LEDs. However, when the forward current increases or when the LEDs are driven at a constant current source, the temperature gradient increases and eventually causes a drop in the light output. The dependence of thermal resistance with optical power and temperature of an LED [13] is given in given in Eq. (2): Rth
TJ − TC = Pel − Popt
(2)
where Pel is the electrical power and Popt is the optical power.Considering optical power in the thermal resistance calculation according to Eq. (2) yields the real thermal resistance values. The sensitivity of the sample can be obtained from the voltage drop with junction temperature as given in Eq. (3), where the slope is known as the K factor [14]: K=
TJ VF
using one main heat flow path which can be considered as a direct model of thermal system. The cumulative structure functions are generated from the Cauer-network equivalent RC model. It is a function of thermal capacitance versus thermal resistance. Cumulative structure function gives information on the volume of the material inside a package in which the heat spreads. Meanwhile, the derivative of cumulative structure function forms a function which is proportional to the area of the heat spreading cone. This derivative function is called differential structure function. Both cumulative and differential structure functions enable one to determine the partial thermal resistance of the heat flow path and the die attach failures inside a package [18]. 3. Experiments A commercial 3W blue LED, originally manufactured with a 1 in2 of MCPCB per emitter was tested for its thermal characterization. The MCPCB acts as an electrical interconnect, as well as a thermal heat sink interface. However, this MCPCB can attain a very high temperature of 70 ◦ C. Therefore, the device under test (DUT) was mounted on an external heat sink which was of an aluminium block with fins. Calibration process was carried out using the conventional oil bath set up with appropriate insulation of the sample. Paraffin oil was used as the medium as it has very low vapour pressure of 0.5 kPa. Upon high temperature heating, the liquid would not evaporate. Since the vapour pressure is very small, hence the volume of the oil bath can be kept constant throughout the experiment. The calibration was carried out using a sensor current of 1 mA. Such a small current was employed in order to avoid self heating effect [19]. The ratio between the temperature and the forward voltage drop determined the temperature sensitive parameter (TSP) which is the K factor. 3.1. Dual interface method The chip size of the LED in the present study is approximately 1 mm × 1 mm. Therefore, the dual interface method is applicable. Thermal paste and mylar tape were used as the interface materials in boundary conditions, BC1 and BC2 respectively. The interface materials (TIM) were placed between the MCPCB and the external heat sink as shown in Fig. 1.
(3)
where TJ is the change in junction temperature and VF is the change in forward voltage. The interface materials are chosen based on the realization of dual interface methodology. Usually ceramic, silicon grease or mylar tape will be the predominant choices. Considering the surface area of the package and the chip is important in setting the boundary conditions. If the chip size is small in a few mm2 in dimension compared to the package, then employing dual interface method is suitable. Thus, the insertion of the interface material would not cause major changes in the structure function. However, if the chip size is large (usually for higher power LEDs) then using dual interface method might be challenging as the interface material might cause a large thermal resistance value [9]. Structure functions are defined as a graphical representation of the RC-model of thermal systems. It was previously known as Protonotarios–Wing function [15]. Since 1988, structure function based evaluation of the thermal transient response functions [16] gave ways for new avenues in the thermal transient testing of microelectronics structures [17]. Structure functions are generated
Fig. 1. Schematic diagram of an LED mounted on an MCPCB with the illustration of thermal interface material between the MCPCB and the external heat sink.
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Fig. 2. The schematic diagram of the water flow set up.
3.2. Water flow as a cooling system
4.2. Determination of RthJ-B from dual interface method
The experimental set up was designed in such a way that the water flows from one side to the other side of the heat sink. The heat sink used in the present study was an aluminium block of surface dimension of (2.5 cm × 2.5 cm) and a height of 1.5 cm. The thickness of the layer which was in contact with the MCPCB was approximately 0.1 cm and the remaining height of the heat sink was the fins which allow even faster cooling system. The package was mounted on the heat sink with thermal paste as the interface material. The external heat sink was immersed into the water as shown in Fig. 2. The temperature of the water was maintained constant at 27.8 ± 1.0 ◦ C throughout the experiment.
Fig. 4 represents the cumulative structure function of 3W blue LED driven at 700 mA. It is a complete thermal profile of the sample recorded between the point of heat generation and point of heat dissipation. Each layer inside the package is represented well in the corresponding structure function. P is the point where the heat has been generated. The steep section between S and T has a high thermal capacitance value indicating the region is a good conductor. U corresponds to the solder mask of the package. Region between U and V is a conducive path on top of a printed circuit board. This again gives a relatively high thermal capacitance. The spreading of heat is visible from V onwards. The large thermal resistance value from V and W shows the well spreading of heat upon aluminium base of the heat sink. After W is the flow of heat into the ambient. The actual point of separation between the package and the board can be determined by employing the dual interface method. The structure functions of the corresponding boundary conditions were compared in Fig. 5. As mentioned earlier, the heating
3.3. Radiometric measurements Radiometric measurements were carried in order to obtain the optical power. The sample was mounted on a peltier cell fixture and measurements were carried out in an integrating sphere. The optical measurements were done for a series of driving current at specific temperatures. Thermal properties of the sample were tested with thermal transient tester (T3ster). T3ster master software was used to capture the cooling transients. The transients were fed into structure functions. The DUT was heated up for 30 min and the cooling transients were captured for 800 s for all measurements. In order to obtain well repeatable results, the DUT was pressed against the external heat sink at constant pressure of 300 kPa. Additionally, an experimental set up has been reported to study the effect of water which flows through the fins of the external heat sink on the thermal properties of the sample. 4. Results and discussion 4.1. K-factor determination A forward voltage versus temperature plot was obtained from the calibration process. The gradient of the plot was the K factor. At a sensor current of 1 mA, the K factor obtained was 2.75 mV/◦ C as shown in Fig. 3.
Fig. 3. K factor calibration using oil bath set up.
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Fig. 4. Cumulative structure function of Luxeon star 3W blue LED driven at 700 mA.
Fig. 5. Dual interface method of Luxeon star 3W blue LED measured with thermal paste and mylar tape: (a) cooling curve and (b) cumulative structure function.
process was carried out for 30 min and the corresponding cooling transients were captured for 800 s. The measurement was carried out in a still air chamber at ambient temperature of 28.2 ± 1.0 ◦ C.
The transients of both boundary conditions run together until approximately 1.62 s as the main trajectory of the heat flow goes through the same structure as shown in Fig. 5(a). As observed in Fig. 5(b), both the functions start to diverge at around 10.29 K/W,
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Fig. 6. Graph of junction to ambient thermal resistance with increasing driving current.
then run almost closely parallel before they diverge distinctively. The starting point of the divergence is a significant value. It is very close to the thermal resistance value where the heat fills up the minimum volume of the case. The heat dissipates to the ambient at a total junction to ambient thermal resistance of 25.65 K/W which corresponds to the total volume of the heat spreader of the package. Comparing both boundary conditions, a slight deviation at 10.84 K/W is observed in the graph which reveals an inflexion point at that region. This is the point where the heat enters the interface material. This point is assumed to be the exact point of RthJ-B where the heat spreading is more significant at the board of the package [7–9]. The close parallel running of the structure functions was caused by the difference in the area of the heat-spreading cone due to the material property of the interface materials. Point X denotes the point where the heat reaches the heat sink for BC1. In BC2 , heat reaches the heat sink only at point Y. The difference in thermal resistance that had to be overcome by the heat to reach the heat sink has been labelled clearly in Fig. 5(b). It can be observed that the heat was able to reach the external heat sink with less thermal resistance in BC1 compared to in BC2 which passes through mylar sheet. A lower thermal resistance has been achieved in BC1 due to the material property of the thermal paste. Thermal
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paste used in BC1 was a grease-like silicone material, heavily filled with heat conductive metal oxides. It is a good conductor which has a better heat spreading effect compared to the mylar tape which is an insulator. This variation can also be explained in terms of the thermal conductivity of the interface material. Thermal conductivity of thermal paste and mylar tape was >1.22 W/(m K) [20] and 0.16 W/mK [21] respectively. It is a fact that thermal resistance is inversely proportional to thermal conductivity. Therefore, for higher thermal conductivity of thermal paste, the RthJ-A achieved was lower in BC1 compared to BC2 . Besides, the variation might be due to contact thermal resistance phenomenon. The surface of the external heat sink cannot be assumed as perfectly smoothed. Meanwhile, the thermal paste was in semi liquid phase. Thus, when spread onto the surface of the external heat sink, the semi liquid thermal paste fills up almost all the micro gaps between the surface of the heat sink and the MCPCB. This does not occur in mylar interface. Since, mylar tape is a solid interface material, it would not be able to fill up the micro gaps leaving behind a less contact with the heat sink surface [22]. Less contact between heat sink and MCPCB results in more thermal resistance for mylar tape compared to thermal paste. Fig. 6 shows the graphical representation of RthJ-A with increasing driving current for both boundary conditions. The RthJ-A was found to decrease with increasing driving current in both boundary conditions. The decrease in RthJ-A with current is due the relationship between driving current and temperature rise. Higher driving current, supplies more power to the device. Higher power generates more heat in the junction. Since heat flow is always through a temperature gradient, the dissipation is faster for higher temperature rise. Thus, in order to reach thermal equilibrium even faster upon higher input power, the thermal resistance reduces as the driving current increases. However, the variation in the numerical values of the RthJ-A is not very significant when compared between thermal paste and mylar tape conditions. This was maybe due to inhomogeneous spreading of the thermal paste onto the surface of the external heat sink. In short, from dual interface method, the RthJ-B was obtained to be 10.84 K/W at 700 mA. 4.3. Studying the effect of water flow on the total thermal resistance Fig. 7 shows the differential structure function of the sample under still air environment and water flow condition in the two cir-
Fig. 7. Differential structure function of Luxeon Star 3W blue LED with and without water flow as a cooling system.
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Fig. 8. Schematic diagram of a diode.
cled regions. The measurement was carried out for driving current of 500, 600, 700, 800, 900 and 1000 mA. For each driving current, the same methodology of heating and cooling process was carried out. From Fig. 7, it was found that the transients of still air conditions take a wide range of thermal resistance. However, for water flow condition, it was found that there is a significant reduction in the total thermal resistance. The RthJ-A in water flow condition has been decreased more than 30% in average for each driving current compared to still air condition. Correspondingly, there is a variation in the shifting pattern of the structure functions of both measurement environments. As the driving current increases, the transients shift towards left for still air environment and towards right for water flow condition. In conjunction with this, a schematic diagram of a diode is shown in Fig. 8 which can be modeled as resistor and a diode in series. Generally, for the still air environment, when current passes through the diode, the power dissipated through the resistor is proportional to its value (P = I2 R). Resistance on the other hand is proportional to the temperature. This causes resistance to increase, then the power dissipated is increased again causing the temperature to rise. This process keeps occurring until heat generated by the system is equivalent to the heat lost from the system. Furthermore, the higher the supplied current, the more light is generated by LEDs. When the forward current increases or when the LEDs are driven at a constant current source, the temperature gradient increases and eventually causes a drop in the light output. Fig. 9 shows a graph of emitted optical power (Popt ) with junction temperature for each forward current employed in this study. From Fig. 9(a), it was observed that the optical performance reduces with increasing junction temperature for the series of increasing forward current. On the other hand, the dependence of Popt on the forward current was studied. Based on Fig. 9(b) the Popt at specific junction temperature at the 30th minute of the heating time was plotted against forward current. It was observed that the Popt consumption increases when the operating current is more. The dependence of power and temperature of an LED is given in given in Eq. (2) as discussed above. The Popt has to be subtracted from the electrical power (Pel ) in order to obtain the heating power (Pheat ) which has been fed into the package and eventually obtain the real thermal resistance values. The dependence of Pheat as well as the total thermal resistance with forward current has been presented in Fig. 9(c). It was observed that the Pheat increases with forward current. Increasing Pheat reduces the total thermal resistance as it is inversely proportional to the supplied power. This explains the left shift of the transients in Fig. 7 in terms of the optical performance of the sample. In the case where the system has been cooled by a water flow system, the whole system is in a cooler state compared to that in still air. This enables the current to pass through the p–n junction
Fig. 9. Optical characteristics of the sample: (a) dependence of optical power on junction temperature, (b) dependence of optical power on forward current and (c) dependence of heat power and total thermal resistance on the forward current.
with less heating at the junction. Thus, a lower temperature would be generated hence less electrical resistance in the p–n junction. A difference in temperature rise between both the cases for the DUT driven at 700 mA is shown in Fig. 10. Less electrical resistance causes the system to consume less power and therefore the thermal resistance increases accordingly. This explains the shift in thermal resistance in the opposite direction of driving current for both still air and water flow conditions. Based on Fig. 9(c) the thermal resistance in the system was affected by the light output as it was observed that the total thermal resistance decreases with increasing junction temperature. Besides, the optical efficiency of the device under test was studied. Fig. 11 shows the dependence of wall plug efficiency (WPE) of the LED with junction temperature driven at 700 mA. It was observed that the efficiency of the sample degrades with increasing temperature. This shows that the light output plays a major role in determining the efficiency and the reliability of an LED.
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Fig. 10. The temperature rise difference between still air and water flow conditions for the DUT driven at 700 mA.
Fig. 11. Dependence of wall plug efficiency (WPE) of the LED with junction temperature driven at 700 mA.
RthJ-A . This prevents the junction from being overheated. Comparing the junction to board thermal resistances, the change in the numerical values was not significant compared to the junction to ambient thermal resistances. This proves that a change in measurement condition does not affect the junction to board thermal resistance but the total thermal resistance. This shows the significance of studying the junction to board thermal resistance of a semiconductor device as this parameter is the key to study the reliability of the device to transfer heat efficiently in various environments. It was reported in [24] that for early LED packages which were introduced in 1960s have a very high thermal resistance of about 250 K/W. However, packages using heat slugs made of aluminium or copper that transfer heat from the chip directly into the printed circuit board (PCB), which in turn spreads heat, has a thermal resistance of 6–12 K/W. This signifies that the obtained experimental value of 10.84 K/W in still air condition is valid which falls within the range of the theoretical value of the junction to PCB thermal resistance. 5. Conclusion
In short, the total thermal resistance for water flow increases with current, whereas it decreases in still air condition. However, the overall reduction in total thermal resistance for water flow condition is significant. At a specific driving current of 700 mA, the total thermal resistance for water flow has been reduced by 55.6% compared to still air condition. The observed thermal resistance values and the calculated real thermal resistance values by considering the optical output were tabulated in Table 1. The observed results indicate that the water flow plays a major role in reducing the thermal resistance between the region of MCPCB and into the ambient. This has been reported by Kim et al. where a heat pipe was used in analogous to the water flow concept [23]. Water that flows in one direction through the fins of the external heat sink favours in driving the heat away from the package. Faster heat dissipation results in lower total thermal resistance,
Table 1 Thermal resistances of 3W blue LED with and without water flow for a specific driving current of 700 mA. Measurement condition
Still air Water flow
Thermal resistance, RthJ-X (K/W) Junction to case RthJ-B
Junction to ambient RthJ-A
Measured
Real
Measured
Real
10.84 10.45
13.97 10.01
25.65 11.45
26.78 11.89
Thermal transient measurements were carried out for commercial 3W blue LED. Thermal paste and mylar tape were used as the two different interface materials. It was observed that dual interface method is a fast and reproducible way to measure the junction to board thermal resistance of semiconductor packages. The material properties of the interface materials diverges the trajectory of the transients and hence creates avenues for determination of RthJ-B . Having the RthJ-B been obtained, a water flow condition was studied to investigate the effect of change of measurement environment on the RthJ-B. It was revealed that the RthJ-B was not affected much by the water flow. However, the RthJ-A was reduced significantly by the flow of water beneath the heat sink. Radiometric measurements proved that the efficiency of an LED strongly depends on the optical performance and thus optical output of an LED has to be considered in obtaining the real thermal resistance values. Hence, it was demonstrated that applying water flow method effectively decreases the total real thermal resistance of an LED, and was proved to be a good solution for controlling junction temperature of high power LED systems. Acknowledgements The authors would like to acknowledge Globetronics Sdn. Bhd. and QAV technologies Sdn. Bhd. for their technical support.
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Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal analysis of power LED employing dual interface method and water flow as a cooling system P. Anithambigai a,∗ , K. Dinash a , D. Mutharasu a , S. Shanmugan a , Choon Kim Lim b a b
Nano Optoelectronics Lab, School of Physics, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia Research and Development Laboratory, Globetronics Sdn. Bhd., 11900 Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 29 December 2010 Received in revised form 5 May 2011 Accepted 2 June 2011 Available online 13 June 2011 Keywords: Junction to board thermal resistance Junction to ambient thermal resistance Dual interface method Water flow system Optical performance
a b s t r a c t Thermal transient measurement based on structure function evaluation was used to measure the thermal resistance. The study signifies the importance of dual interface method in determining the exact point of separation between the board and the LED package. For a constant ambient temperature which was maintained at 28.2 ± 1.0 ◦ C at 700 mA, the junction to board thermal resistance obtained was 10.84 K/W. In addition, an experimental set up has been reported in this work having a constant water flow beneath the external heat sink. More emphasis has been given in studying the effect of change of such measurement environment on the junction to board thermal resistance. It was revealed that the junction to board thermal resistance was not affected but the total real thermal resistance from junction to ambient was reduced significantly by 55.6% upon cooling with water. A study on the effect of light output on the total thermal resistance was performed and it was revealed that the efficiency and the reliability of an LED are strongly dependent on the optical properties of the device. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Light emitting diode (LED) technology has been progressing rapidly for the past few decades. Generally, LEDs conveniently eliminate the limitations of incandescent lamps by providing more light per watt. LEDs do last longer because they do not have filaments that will burn out. Besides, LEDs are known for high efficiency, good reliability, variable colours and low power consumption [1,2]. However, despite the rapid progress of this technology, overheating of these LEDs upon higher input current has become a major drawback [3]. Thus, thermal management of these devices is becoming more and more crucial as poor thermal management would cause excessive rise in the junction temperature and subsequently would either cause a complete thermal runaway of the device or breakdown of the chip [4]. Generally in an LED, the heat generated in the p–n junction will first be conducted from the chip to the metal core printed circuit board (MCPCB) followed by an external heat sink and finally into the ambient via convection. This is assuming it is a perfect one-dimensional heat flow. However, it is impossible to assume a perfectly one-dimensional heat flow as there is heat loss through surrounding lead frame, epoxy and lens [5]. However, this loss is reported to be insignificant as the material thermal conductivity is very small [6].
∗ Corresponding author. E-mail address: [email protected] (P. Anithambigai). 0040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2011.06.001
Junction to case thermal resistance (RthJC ) of an LED or any power semiconductor device is an often employed concept in thermal engineering. Since power semiconductors are prone to ever increasing power dissipation levels, achieving a low RthJC is very significant. The conventional procedure of measuring RthJC is by using the thermocouple measurement. Often this produces inaccurate results since it is difficult to place the thermocouple exactly at the required surface of case of the package. Szabo et al. presented a method of finding the junction to case thermal resistance by comparing boundary conditions of different interface layers [7]. A comparison of with and without thermal grease was reported in [7] and structure function based evaluation favoured in determining the exact point of separation between package and case. The same dual interface method was reported in 2008 using 3 different boundary conditions; liquid metal (gallium–indium–tin alloy), thermal grease and ceramic plate and thermal grease only as interface materials in each boundary condition. The importance of choosing the correct interface material with the right thermal conductivity properties and the specification of the thickness of the interface material were emphasized [8]. It was reported again in 2010 that interface materials like mylar sheet or ceramic are more suitable for smaller chip semiconductor devices [9] which means that the size of the chip is a factor for obtaining optimized RthJC with chosen interface material. It was reported by Andras et al. that if the chip is too large, which covers a large surface area of the module; it may cause a huge additional thermal resistance to the system. However, if the chip is too small, the inserted interface layer might not significantly change the structure function. In short,
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thermal resistance is defined as the ability of a component to transfer heat, and thermal management is dependent predominantly on the thermal resistance of the package. In addition to this, neglecting optical power when determining an LED’s thermal resistances yields values which are much lower than the reality. Since about 10–40% of the supplied energy leaves the system as light, the optical properties of the device are not negligible. A combined thermal and radiometric measurement set up was proposed in [10]. This method has been practiced until today [11] by many research groups subsequently. In this work, the dual interface method was employed to identify the junction to board thermal resistance. In addition, a water flow was reported as a cooling system for LEDs thermal characterization. Optical properties of the sample were studied too in order to understand the dependence of total thermal resistance with the optical output of the device. 2. Theoretical background The thermal resistance of a semiconductor device is generally defined as in Eq. (1) according to JEDEC standard 51-1 [12]: RthJX =
TJ − TX PH
(1)
where RthJX is the thermal resistance from device junction to the specific environment, TJ is the device junction temperature under steady state condition, TX is the reference temperature for a specific environment and PH is the power dissipated in the device. As it is widely known, the light output of an LED strongly depends on the operating conditions. The higher the supplied current, the more light is generated by LEDs. However, when the forward current increases or when the LEDs are driven at a constant current source, the temperature gradient increases and eventually causes a drop in the light output. The dependence of thermal resistance with optical power and temperature of an LED [13] is given in given in Eq. (2): Rth
TJ − TC = Pel − Popt
(2)
where Pel is the electrical power and Popt is the optical power.Considering optical power in the thermal resistance calculation according to Eq. (2) yields the real thermal resistance values. The sensitivity of the sample can be obtained from the voltage drop with junction temperature as given in Eq. (3), where the slope is known as the K factor [14]: K=
TJ VF
using one main heat flow path which can be considered as a direct model of thermal system. The cumulative structure functions are generated from the Cauer-network equivalent RC model. It is a function of thermal capacitance versus thermal resistance. Cumulative structure function gives information on the volume of the material inside a package in which the heat spreads. Meanwhile, the derivative of cumulative structure function forms a function which is proportional to the area of the heat spreading cone. This derivative function is called differential structure function. Both cumulative and differential structure functions enable one to determine the partial thermal resistance of the heat flow path and the die attach failures inside a package [18]. 3. Experiments A commercial 3W blue LED, originally manufactured with a 1 in2 of MCPCB per emitter was tested for its thermal characterization. The MCPCB acts as an electrical interconnect, as well as a thermal heat sink interface. However, this MCPCB can attain a very high temperature of 70 ◦ C. Therefore, the device under test (DUT) was mounted on an external heat sink which was of an aluminium block with fins. Calibration process was carried out using the conventional oil bath set up with appropriate insulation of the sample. Paraffin oil was used as the medium as it has very low vapour pressure of 0.5 kPa. Upon high temperature heating, the liquid would not evaporate. Since the vapour pressure is very small, hence the volume of the oil bath can be kept constant throughout the experiment. The calibration was carried out using a sensor current of 1 mA. Such a small current was employed in order to avoid self heating effect [19]. The ratio between the temperature and the forward voltage drop determined the temperature sensitive parameter (TSP) which is the K factor. 3.1. Dual interface method The chip size of the LED in the present study is approximately 1 mm × 1 mm. Therefore, the dual interface method is applicable. Thermal paste and mylar tape were used as the interface materials in boundary conditions, BC1 and BC2 respectively. The interface materials (TIM) were placed between the MCPCB and the external heat sink as shown in Fig. 1.
(3)
where TJ is the change in junction temperature and VF is the change in forward voltage. The interface materials are chosen based on the realization of dual interface methodology. Usually ceramic, silicon grease or mylar tape will be the predominant choices. Considering the surface area of the package and the chip is important in setting the boundary conditions. If the chip size is small in a few mm2 in dimension compared to the package, then employing dual interface method is suitable. Thus, the insertion of the interface material would not cause major changes in the structure function. However, if the chip size is large (usually for higher power LEDs) then using dual interface method might be challenging as the interface material might cause a large thermal resistance value [9]. Structure functions are defined as a graphical representation of the RC-model of thermal systems. It was previously known as Protonotarios–Wing function [15]. Since 1988, structure function based evaluation of the thermal transient response functions [16] gave ways for new avenues in the thermal transient testing of microelectronics structures [17]. Structure functions are generated
Fig. 1. Schematic diagram of an LED mounted on an MCPCB with the illustration of thermal interface material between the MCPCB and the external heat sink.
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Fig. 2. The schematic diagram of the water flow set up.
3.2. Water flow as a cooling system
4.2. Determination of RthJ-B from dual interface method
The experimental set up was designed in such a way that the water flows from one side to the other side of the heat sink. The heat sink used in the present study was an aluminium block of surface dimension of (2.5 cm × 2.5 cm) and a height of 1.5 cm. The thickness of the layer which was in contact with the MCPCB was approximately 0.1 cm and the remaining height of the heat sink was the fins which allow even faster cooling system. The package was mounted on the heat sink with thermal paste as the interface material. The external heat sink was immersed into the water as shown in Fig. 2. The temperature of the water was maintained constant at 27.8 ± 1.0 ◦ C throughout the experiment.
Fig. 4 represents the cumulative structure function of 3W blue LED driven at 700 mA. It is a complete thermal profile of the sample recorded between the point of heat generation and point of heat dissipation. Each layer inside the package is represented well in the corresponding structure function. P is the point where the heat has been generated. The steep section between S and T has a high thermal capacitance value indicating the region is a good conductor. U corresponds to the solder mask of the package. Region between U and V is a conducive path on top of a printed circuit board. This again gives a relatively high thermal capacitance. The spreading of heat is visible from V onwards. The large thermal resistance value from V and W shows the well spreading of heat upon aluminium base of the heat sink. After W is the flow of heat into the ambient. The actual point of separation between the package and the board can be determined by employing the dual interface method. The structure functions of the corresponding boundary conditions were compared in Fig. 5. As mentioned earlier, the heating
3.3. Radiometric measurements Radiometric measurements were carried in order to obtain the optical power. The sample was mounted on a peltier cell fixture and measurements were carried out in an integrating sphere. The optical measurements were done for a series of driving current at specific temperatures. Thermal properties of the sample were tested with thermal transient tester (T3ster). T3ster master software was used to capture the cooling transients. The transients were fed into structure functions. The DUT was heated up for 30 min and the cooling transients were captured for 800 s for all measurements. In order to obtain well repeatable results, the DUT was pressed against the external heat sink at constant pressure of 300 kPa. Additionally, an experimental set up has been reported to study the effect of water which flows through the fins of the external heat sink on the thermal properties of the sample. 4. Results and discussion 4.1. K-factor determination A forward voltage versus temperature plot was obtained from the calibration process. The gradient of the plot was the K factor. At a sensor current of 1 mA, the K factor obtained was 2.75 mV/◦ C as shown in Fig. 3.
Fig. 3. K factor calibration using oil bath set up.
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Fig. 4. Cumulative structure function of Luxeon star 3W blue LED driven at 700 mA.
Fig. 5. Dual interface method of Luxeon star 3W blue LED measured with thermal paste and mylar tape: (a) cooling curve and (b) cumulative structure function.
process was carried out for 30 min and the corresponding cooling transients were captured for 800 s. The measurement was carried out in a still air chamber at ambient temperature of 28.2 ± 1.0 ◦ C.
The transients of both boundary conditions run together until approximately 1.62 s as the main trajectory of the heat flow goes through the same structure as shown in Fig. 5(a). As observed in Fig. 5(b), both the functions start to diverge at around 10.29 K/W,
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Fig. 6. Graph of junction to ambient thermal resistance with increasing driving current.
then run almost closely parallel before they diverge distinctively. The starting point of the divergence is a significant value. It is very close to the thermal resistance value where the heat fills up the minimum volume of the case. The heat dissipates to the ambient at a total junction to ambient thermal resistance of 25.65 K/W which corresponds to the total volume of the heat spreader of the package. Comparing both boundary conditions, a slight deviation at 10.84 K/W is observed in the graph which reveals an inflexion point at that region. This is the point where the heat enters the interface material. This point is assumed to be the exact point of RthJ-B where the heat spreading is more significant at the board of the package [7–9]. The close parallel running of the structure functions was caused by the difference in the area of the heat-spreading cone due to the material property of the interface materials. Point X denotes the point where the heat reaches the heat sink for BC1. In BC2 , heat reaches the heat sink only at point Y. The difference in thermal resistance that had to be overcome by the heat to reach the heat sink has been labelled clearly in Fig. 5(b). It can be observed that the heat was able to reach the external heat sink with less thermal resistance in BC1 compared to in BC2 which passes through mylar sheet. A lower thermal resistance has been achieved in BC1 due to the material property of the thermal paste. Thermal
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paste used in BC1 was a grease-like silicone material, heavily filled with heat conductive metal oxides. It is a good conductor which has a better heat spreading effect compared to the mylar tape which is an insulator. This variation can also be explained in terms of the thermal conductivity of the interface material. Thermal conductivity of thermal paste and mylar tape was >1.22 W/(m K) [20] and 0.16 W/mK [21] respectively. It is a fact that thermal resistance is inversely proportional to thermal conductivity. Therefore, for higher thermal conductivity of thermal paste, the RthJ-A achieved was lower in BC1 compared to BC2 . Besides, the variation might be due to contact thermal resistance phenomenon. The surface of the external heat sink cannot be assumed as perfectly smoothed. Meanwhile, the thermal paste was in semi liquid phase. Thus, when spread onto the surface of the external heat sink, the semi liquid thermal paste fills up almost all the micro gaps between the surface of the heat sink and the MCPCB. This does not occur in mylar interface. Since, mylar tape is a solid interface material, it would not be able to fill up the micro gaps leaving behind a less contact with the heat sink surface [22]. Less contact between heat sink and MCPCB results in more thermal resistance for mylar tape compared to thermal paste. Fig. 6 shows the graphical representation of RthJ-A with increasing driving current for both boundary conditions. The RthJ-A was found to decrease with increasing driving current in both boundary conditions. The decrease in RthJ-A with current is due the relationship between driving current and temperature rise. Higher driving current, supplies more power to the device. Higher power generates more heat in the junction. Since heat flow is always through a temperature gradient, the dissipation is faster for higher temperature rise. Thus, in order to reach thermal equilibrium even faster upon higher input power, the thermal resistance reduces as the driving current increases. However, the variation in the numerical values of the RthJ-A is not very significant when compared between thermal paste and mylar tape conditions. This was maybe due to inhomogeneous spreading of the thermal paste onto the surface of the external heat sink. In short, from dual interface method, the RthJ-B was obtained to be 10.84 K/W at 700 mA. 4.3. Studying the effect of water flow on the total thermal resistance Fig. 7 shows the differential structure function of the sample under still air environment and water flow condition in the two cir-
Fig. 7. Differential structure function of Luxeon Star 3W blue LED with and without water flow as a cooling system.
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Fig. 8. Schematic diagram of a diode.
cled regions. The measurement was carried out for driving current of 500, 600, 700, 800, 900 and 1000 mA. For each driving current, the same methodology of heating and cooling process was carried out. From Fig. 7, it was found that the transients of still air conditions take a wide range of thermal resistance. However, for water flow condition, it was found that there is a significant reduction in the total thermal resistance. The RthJ-A in water flow condition has been decreased more than 30% in average for each driving current compared to still air condition. Correspondingly, there is a variation in the shifting pattern of the structure functions of both measurement environments. As the driving current increases, the transients shift towards left for still air environment and towards right for water flow condition. In conjunction with this, a schematic diagram of a diode is shown in Fig. 8 which can be modeled as resistor and a diode in series. Generally, for the still air environment, when current passes through the diode, the power dissipated through the resistor is proportional to its value (P = I2 R). Resistance on the other hand is proportional to the temperature. This causes resistance to increase, then the power dissipated is increased again causing the temperature to rise. This process keeps occurring until heat generated by the system is equivalent to the heat lost from the system. Furthermore, the higher the supplied current, the more light is generated by LEDs. When the forward current increases or when the LEDs are driven at a constant current source, the temperature gradient increases and eventually causes a drop in the light output. Fig. 9 shows a graph of emitted optical power (Popt ) with junction temperature for each forward current employed in this study. From Fig. 9(a), it was observed that the optical performance reduces with increasing junction temperature for the series of increasing forward current. On the other hand, the dependence of Popt on the forward current was studied. Based on Fig. 9(b) the Popt at specific junction temperature at the 30th minute of the heating time was plotted against forward current. It was observed that the Popt consumption increases when the operating current is more. The dependence of power and temperature of an LED is given in given in Eq. (2) as discussed above. The Popt has to be subtracted from the electrical power (Pel ) in order to obtain the heating power (Pheat ) which has been fed into the package and eventually obtain the real thermal resistance values. The dependence of Pheat as well as the total thermal resistance with forward current has been presented in Fig. 9(c). It was observed that the Pheat increases with forward current. Increasing Pheat reduces the total thermal resistance as it is inversely proportional to the supplied power. This explains the left shift of the transients in Fig. 7 in terms of the optical performance of the sample. In the case where the system has been cooled by a water flow system, the whole system is in a cooler state compared to that in still air. This enables the current to pass through the p–n junction
Fig. 9. Optical characteristics of the sample: (a) dependence of optical power on junction temperature, (b) dependence of optical power on forward current and (c) dependence of heat power and total thermal resistance on the forward current.
with less heating at the junction. Thus, a lower temperature would be generated hence less electrical resistance in the p–n junction. A difference in temperature rise between both the cases for the DUT driven at 700 mA is shown in Fig. 10. Less electrical resistance causes the system to consume less power and therefore the thermal resistance increases accordingly. This explains the shift in thermal resistance in the opposite direction of driving current for both still air and water flow conditions. Based on Fig. 9(c) the thermal resistance in the system was affected by the light output as it was observed that the total thermal resistance decreases with increasing junction temperature. Besides, the optical efficiency of the device under test was studied. Fig. 11 shows the dependence of wall plug efficiency (WPE) of the LED with junction temperature driven at 700 mA. It was observed that the efficiency of the sample degrades with increasing temperature. This shows that the light output plays a major role in determining the efficiency and the reliability of an LED.
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Fig. 10. The temperature rise difference between still air and water flow conditions for the DUT driven at 700 mA.
Fig. 11. Dependence of wall plug efficiency (WPE) of the LED with junction temperature driven at 700 mA.
RthJ-A . This prevents the junction from being overheated. Comparing the junction to board thermal resistances, the change in the numerical values was not significant compared to the junction to ambient thermal resistances. This proves that a change in measurement condition does not affect the junction to board thermal resistance but the total thermal resistance. This shows the significance of studying the junction to board thermal resistance of a semiconductor device as this parameter is the key to study the reliability of the device to transfer heat efficiently in various environments. It was reported in [24] that for early LED packages which were introduced in 1960s have a very high thermal resistance of about 250 K/W. However, packages using heat slugs made of aluminium or copper that transfer heat from the chip directly into the printed circuit board (PCB), which in turn spreads heat, has a thermal resistance of 6–12 K/W. This signifies that the obtained experimental value of 10.84 K/W in still air condition is valid which falls within the range of the theoretical value of the junction to PCB thermal resistance. 5. Conclusion
In short, the total thermal resistance for water flow increases with current, whereas it decreases in still air condition. However, the overall reduction in total thermal resistance for water flow condition is significant. At a specific driving current of 700 mA, the total thermal resistance for water flow has been reduced by 55.6% compared to still air condition. The observed thermal resistance values and the calculated real thermal resistance values by considering the optical output were tabulated in Table 1. The observed results indicate that the water flow plays a major role in reducing the thermal resistance between the region of MCPCB and into the ambient. This has been reported by Kim et al. where a heat pipe was used in analogous to the water flow concept [23]. Water that flows in one direction through the fins of the external heat sink favours in driving the heat away from the package. Faster heat dissipation results in lower total thermal resistance,
Table 1 Thermal resistances of 3W blue LED with and without water flow for a specific driving current of 700 mA. Measurement condition
Still air Water flow
Thermal resistance, RthJ-X (K/W) Junction to case RthJ-B
Junction to ambient RthJ-A
Measured
Real
Measured
Real
10.84 10.45
13.97 10.01
25.65 11.45
26.78 11.89
Thermal transient measurements were carried out for commercial 3W blue LED. Thermal paste and mylar tape were used as the two different interface materials. It was observed that dual interface method is a fast and reproducible way to measure the junction to board thermal resistance of semiconductor packages. The material properties of the interface materials diverges the trajectory of the transients and hence creates avenues for determination of RthJ-B . Having the RthJ-B been obtained, a water flow condition was studied to investigate the effect of change of measurement environment on the RthJ-B. It was revealed that the RthJ-B was not affected much by the water flow. However, the RthJ-A was reduced significantly by the flow of water beneath the heat sink. Radiometric measurements proved that the efficiency of an LED strongly depends on the optical performance and thus optical output of an LED has to be considered in obtaining the real thermal resistance values. Hence, it was demonstrated that applying water flow method effectively decreases the total real thermal resistance of an LED, and was proved to be a good solution for controlling junction temperature of high power LED systems. Acknowledgements The authors would like to acknowledge Globetronics Sdn. Bhd. and QAV technologies Sdn. Bhd. for their technical support.
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