LED semiconductor lighting chips work without ultraviolet or infrared light, so their light doesn't take away heat, so the temperature keeps going up.
In order to reduce the working temperature of LED, to extend the life of LED lamp, the thermal energy generated when LED is luminous must be exported in time.
Every link of the LED from chip to radiator must be fully considered for heat dissipation.
Improper design of any link can cause serious heat dissipation problems.
Influence of temperature on LED lamp
The LED's light decay indicates its life span, and as it is used, the brightness gets darker and darker until it finally goes out.
It is usually defined as a time of 30% decay for its lifetime.
The relationship between LED temperature and life is shown in figure 1. From the figure, we can see that the life of LED lamp is shortened with the increase of working temperature.
1 influence of temperature on LED lamp
FIG. 2 shows the relationship between the junction temperature and luminescence amount. If the luminescence rate is 100% when the junction temperature is 25 degrees, the luminescence rate drops to 95% when the junction temperature rises to 50 degrees.
100 degrees is now down to 80%;
At 150 degrees it's only 68%.
The relationship between junction temperature and luminescence amount
LED temperature cause analysis
Leds generate heat because only about 20-30% of the electricity they add is converted to light energy, while much of it is converted to heat.
The LED junction temperature is caused by two factors.
(1) the recombination rate of carriers in the PN region is not 100%, that is, when electrons and holes are combined, not all of them produce photons. The product of leakage current and voltage is the heat energy generated in this part.
But the internal photon efficiency is now close to 90%, so this part of the thermal energy is not the main factor behind the LED junction temperature.
(2) the main factor leading to the LED junction temperature is that the internal compound generated photons cannot all be emitted to the outside of the chip to transform the heat energy. At present, the external quantum efficiency is only about 30%, most of which is converted into heat.
LED heat dissipation can be achieved in the following ways :(1) heat dissipation from air;
(2) thermal energy is directly exported from the circuit board;
(3) the heat energy is exported via the gold wire;
(4) if the process is eutectic and Flip chip, the heat energy will be exported via the through-hole to the circuit board of the system.
Among the above methods of heat dissipation, the selection of heat dissipation substrate material and the packaging method of LED grains play an important role in the management of LED heat dissipation.
Ceramic heat dissipation substrate
LED heat dissipation base plate has metal and ceramic base plate.
At present, LED products generally use metal substrate, because the metal substrate material is mainly aluminum or copper, low cost and relatively mature technology.
However, ceramic substrate has better thermal conductivity and heat dissipation performance than metal substrate, which is the most suitable solution for high power LED heat dissipation at present.
The requirement of illumination for heat dissipation and stability is much higher than that of electronic products such as TV and computer. Even if the cost ratio of ceramic base plate is higher than that of metal base plate, international manufacturers including CREE, osram, philips and Japan asification also use ceramic base plate as LED grain heat dissipation material.
At present, the high-power LED radiator base plate commonly used in the market is shown in FIG.3. Its structure is generally aluminum base plate: its lowest layer is aluminum metal layer with a thickness of about 1.3mm.
The aluminum layer is about 0.1mm thick polymer insulation layer;
The top layer is welding circuit and copper circuit.
Due to the extremely low thermal conductivity of the insulation layer, even if the thermal conductivity of aluminum is relatively high, the insulation layer will become the heat dissipation bottleneck of the structure's base plate, affecting the heat dissipation effect of the entire base plate.
Secondly, the insulation layer is unable to withstand high-temperature welding, which limits the optimization of packaging structure, affects the implementation of packaging technology, and is not conducive to LED cooling.
3 ceramic heat dissipation substrate
Ceramic substrate refers to the special process board that directly bonds copper foil to ALN or AL2O3 ceramic substrate surface (single or double) at high temperature.
The ultra-thin composite substrate made by this process has excellent electrical insulation performance, high thermal conductivity, high adhesion strength and excellent soft soldering performance, and it can be etched with a variety of graphics, with very large current-carrying capacity.
At present, ceramic substrate has become the foundation of interconnection technology and high-power electronic circuit structure, gradually replacing aluminum substrate.
Table 1 is a comparison table between ceramic heat dissipation base plate and metal heat dissipation. Due to the new heat conducting materials and new ceramic heat dissipation base plate with internal structure, various defects of aluminum metal base plate are eliminated, thus greatly improving the overall heat dissipation effect of the base plate.
4. Comparison table of heat dissipation between ceramic base plate and metal
There are three kinds of ceramic substrates: low-temperature co-firing multilayer ceramics, thick film ceramic substrates and thin film ceramic substrates. In traditional high-power LED components, thick film or low-temperature co-firing ceramic substrates are mainly used as grain heat dissipation substrates to combine the gold wire and LED grain with the ceramic substrates.
Thick film ceramic substrate is produced by screen printing technology. The material is first printed on the substrate by using a spatula, then sintered, dried and laser. At present, the main manufacturer of thick film ceramic substrate is dahao, Kowloon and other companies.
Generally speaking, the line produced by screen printing is more likely to be inaccurate and rough due to problems such as screen printing.
In addition, the shrinkage ratio of multilayer ceramics is affected by the laminated sintering process, which limits the resolution of the process.
Low temperature co-firing multilayer ceramic technology to ceramic as substrate material, should be about 30% ~ 50% of the first glass materials and inorganic alumina powder and mixing organic adhesives become mud slurry, and then put the sheet with scraper scraping paste, through the dry process thin will flake pulp formation of embryo, and then according to the requirements of each layer design guide hole, as the layers of signal transmission, LTCC screen printing technology, the internal wiring is used in embryo birth do fill in the hole and printed circuit, respectively, both inside and outside the electrode can be formed by silver, copper, gold and other metals, finally will do laminated layers,
Placed in the middle of 850 ~ 900 ℃ sintering furnace sintering molding.
In recent years, the thin film ceramic substrate has been developed as a heat dissipation substrate for LED grains due to the network problems in the thick film making process and shrinkage ratio problems after multilayer lamination and sintering.
The thin film thermal substrate is made by electrochemical deposition, spattering, and photolithography.
With the advent of ceramic substrate products, the thermal application industry has opened a new chapter.
Due to the excellent heat dissipation characteristics of ceramic substrate, with the improvement of production technology and equipment, the cost performance of products has been greatly improved, and the application field of LED industry has been expanded.
The success of the development of ceramic substrates will serve as outdoor lighting and indoor lighting products, making the future development of LED industry more extensive.