|High Power LED cooling has always been a problem plaguing LED technical staff, in Figure 1 for the current use of high power LED package structure, LED chip will be packaged in a thermal substrate, and the call letters of gold, plastic, this LED package structure With lightweight, high thermal conductivity and simple circuit, etc., can be used in outdoor and indoor lighting. The choice of substrate, the alumina (Al2O3) and silicon (Si) are currently available in the application materials, including alumina substrate by an insulator, heat conduction must be designed by copper plating thickness of 75um; The silicon is an excellent conductor of heat, but poor insulation, the surface must be insulated.
Figure 1, LED package structure
Alumina substrates and silicon-based panels used in the present have switched high power LED package, the LED luminous efficiency to be improved, the use of LED lighting is still hot on the important issues that must be addressed, so LED thermal function must be carefully analyzed. Function must be used to analyze the thermal conductivity meter to measure resistance, the following analysis will segment LED package structure, including the chip layer, the bonding layer and the substrate layer, to analyze each thermal resistance, analysis tools, is now recognized worldwide as the most accurate The T3Ster equipment.
In this paper a simple analytical alumina substrates and silicon-based LED board is encapsulated in thermal performance (T3Ster instrument measured), in which proper nouns are defined as follows:
Rth: thermal resistance, the unit is (oC / W), the formula for the T / KA;
T: thermal conductivity of the substrate thickness (um);
K: thermal conductivity of the substrate thermal conductivity (W / mC);
A: Thermal Area (mm x mm).
Figure 2, E Company alumina substrate thermal resistance of LED package
Alumina used in the LED package is mainly because of high insulation and aluminum oxide materials can be made small in size and the components, however, alumina substrates used in electronic components because of low thermal conductivity of alumina (about 20K / W) , resulting in high thermal resistance. Figure 2 T3Ster resistance test instrument company alumina substrate package E LED (LED area: 1 x 1 mm; LED emitter: 3.15x3.5 mm)) results in 25 oC ambient temperature test, the encapsulation layer thermal resistance as follows:
1, Chip: 2 oC / W
2, Bonding layer: 3 oC / W
3, alumina substrate: 20 oC / W (high thermal resistance, substrate production of poor)
When a small electrical current 175mA into the 1 x 1 mm ² of LED chips, alumina substrate, the temperature rise due to thermal resistance is 10.5oC (= 20x175mAx3.0V), heat conduction the LED chip is not easy; when 350mA power flow into the 1 x 1 mm ² of LED chips, alumina substrate, the temperature rise due to thermal resistance is 23oC (= 20 x 350mAx3.3 V), alumina substrate at this time will not be able to heat the LED chips, LED chips will have a lot of bad light; and When a large electrical current into the 500 mA in 1 x 1 mm ² of LED chip, the alumina substrate temperature rise due to thermal resistance is about 36 oC (20 x (500 Ax3.6 V), alumina substrate at this time will not the thermal conductivity of the LED chips, LED chips will be rapid light decay. Therefore, LED chip package If you choose to alumina substrates, because of its high thermal resistance, package components are only suitable for use in low power (~ 175 mA, about 0.5W).
Figure 3, R company alumina substrate thermal resistance of LED package
Figure 3 T3Ster alumina substrate resistance test instrument package LED (LED area: 1 x 1 mm; LED emitter: 3.15x3.5 mm)) results in 25 oC ambient temperature test, the package thermal resistance layer as follows:
1, Chip: 2 oC / W
2, Bonding layer: 1.5 oC / W
3, alumina substrate: 4.7 oC / W (alumina substrate thickness: 400um, copper thickness 75um)
When a small electrical current into the next 175mA, the substrate temperature rise due to thermal resistance 4.7 oC; when the current to 350mA, the substrate temperature is 6.9 oC, when the flow of 500mA Power Pops, the substrate temperature of about 8.1oC, and when 700mA flow into the next great power, substrate temperature rise due to thermal resistance is about 11oC.
When the alumina substrate temperature greater than 8 o C, alumina substrate at this time would be difficult to heat the LED chips, LED chips will be rapid light decay. LED chip is encapsulated in alumina substrate, the package components are only suitable for the beginning of the use of power (~ 350mA, about 1watt).
Figure 4, VisEra Si substrate thermal resistance of LED package
Application of silicon-substrate for the LED package shown in Figure 4, in the small size of 3.37 x3.37 mxm2 area, with rapid thermal performance, since the use of aluminum oxide can significantly solve the resulting problem of the high thermal resistance. Thermal resistance of precision measurement equipment (T3Ster) measured the thermal resistance, ambient temperature 25 oC in the test, the package thermal resistance layer as follows:
1, Chip: 1 oC / W
2, Bonding layer: 1.5 o C / W
3, Si substrate: 2.5 o C / W
When the high current (700 mA) leads to the 1 x 1 mm ² of LED chip, silicon plate temperature rise due to thermal resistance 6. 3 oC (= 2.5 x 700mA x 3.6 V), Si substrate will quickly heat the LED chips, LED chips only have a small amount of bad light. Excellent thermal conductivity of silicon by the thermal conductivity of the LED chips, LED chips promise will only be a small light decline. Therefore, LED chip package on a silicon plate used for high power (~ 700 mA, about 3 W). Because the Si substrate is encapsulated in silicon production and LED panels, high technical difficulty, only a few companies have. LED integrated package of silicon substrate material as a thermal resistance lower than the alumina substrate materials used in high-power, the silicon plate is a good choice.
The overall thermal resistance of LED package T3Ster experimental results shown below:
LED Si Package: VisEra: 5 o C / W
LED aluminum, E Company: 25 o C / W
LED aluminum packaging, R Company: 8.2 o C / W