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Big bottleneck to the development of the LED lighting: a technical inspection of the thermal resistance

Thermal resistance is encountered resistance, heat in the heat flow path reflects capacity of heat transfer between the medium or medium size, show that the size 1 W heat caused by the temperature rise, the unit for / W or K/W. Can be explained by an analogy, if the heat is equal to the current, the temperature difference is equal to the voltage, the thermal resistance is equal to the resistance. Usually, LED devices in the application, structure thermal resistance distribution for the chip substrate, the substrate and the LED bracket bonding layer, LED stent, LED device external heat dissipation body and free space, thermal resistance, thermal resistance channel in series. 
LED as a new energy saving lamps and lanterns of lamps and lanterns is in the process of lighting will be 30-40% of its electricity into light, all the rest into heat energy, thermal energy makes us have to focus on the existence of the thermal resistance of LED packaging device. In general, the higher the power of the LED, LED thermal effect, the more obvious, problems caused by the thermal effect is apparent, for example, chip hot red shift phenomenon; High junction temperature permanent damage to the chip; Phosphor layer of the light-emitting efficiency and accelerated aging; Color temperature drift phenomenon; The thermal stress caused by mechanical failure, etc. These are directly affect the LED luminous efficiency, wavelength, positive pressure drop, and service life. LED lamps and lanterns of heat dissipation has become a big bottleneck of the development of. 
How should evaluate LED encapsulation device heat level? 
Application, for example, 
1. Packaging devices thermal resistance test 
A: (1) test method 
Testing thermal resistance in the process of packaging products general heat path for the chips - solid crystal layer - bracket or substrate - solder paste - aided test base board - thermal connection materials. 
According to the test, the thermal resistance that can be concluded as the following graph, can be read out to test the product total thermal resistance (the entire heat dissipation path) is 7.377 K/W. The method to test the thermal resistance should be according to the structure of the test samples, determine the curve of thermal resistance layer, to obtain accurate thermal resistance of encapsulation device. This method is more suitable for SMD encapsulation device. 
(2) test method 2: 
And method of a different, need after two thermal resistance test, the method comparison of thermal resistance, can be accurate to device substrate shell, without the supplementary numerical tests substrate. 
Two tests respectively: measurement for the first time, device directly contact with the substrate, heat sink; A second measurement device and the base plate heat sink with heat conduction double-sided adhesive in the middle. Due to changes in two heat path only happens outside components encapsulation shell, so the structure function on the parting of the two measurements is the shell of the device. As shown in the figure below the curve of the change, can get an accurate thermal resistance of the device. This method is suitable for COB packaging device. 
(3) using structure function identification device structure 
Regular, chip, support or substrate, test auxiliary substrate or cold plate thermal resistance and the heat capacity of the three layers are relatively small, and the solid crystal layer and the connection material thermal resistance of thermal conductivity and heat capacity are relatively large. 
Functions such as the following structure shows that structure function on the place of the closer y represents the actual heat conduction path close to the chip structure of the active region, and the more far from where the y axis represents the heat conduction path has the structure of the source area far from. Integral structure function is the heat capacity, thermal resistance function, curve on the flat area representative components of the internal thermal resistance, heat capacity, small structure, steep area representative device internal thermal resistance is small, the structure of the heat capacity is big. Differential structure function, the peaks and troughs of the inflection point is the parting of the two kinds of structure, facilitate recognition device of the layers of internal structure. At the end of the structure function and its value tends to be a vertical asymptote, at this time on behalf of the heat conduction in the air layer, due to the volume of air infinity, so the heat capacity is infinite. From the origin to the asymptote is x value between junction area to the air environment of thermal resistance, thermal resistance is the steady state conditions. 
2. Packaging defects inside the device 

Compared the above two devices section structure, solid crystal layer shows obvious difference. The following figure, the left is normal product, the right for solid crystal layer of defective products. 


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