High lumen LED street lights require new cooling solutions other than fans

Limited by technology, the current conversion efficiency of LED is still about 20%, which means that up to 80% of the electrical energy will be converted into heat energy. Therefore, heat dissipation is always an important challenge facing LED lighting. In particular, the heat energy of LED light is concentrated in a small area. Therefore, in street lamp applications with large power input, the temperature of the LED interface is quite high. In addition, the street lamps work continuously for a long time, and the heat energy generated is even more considerable. Therefore, if the heat dissipation module cannot effectively dissipate heat, the LED street lamps will experience serious light decay, resulting in a significant shortening of the service life. With the promotion of the "Ten Cities and Ten Thousand Lamps" semiconductor lighting application project policy in mainland China in recent years, the number of LED street lamps installed has increased rapidly, and the problem of LED street lamp light decay has become more prominent.

The phenomenon of LED light decay is closely related to the heat dissipation performance. In order to effectively achieve the purpose of heat dissipation, relevant industry players have sought solutions through various channels. From the perspective of overall heat dissipation design, the heat dissipation substrate material and the LED chip packaging method are extremely critical. First of all, the operation mode of the LED heat dissipation substrate is to use the thermal conductivity of the heat dissipation substrate material itself to conduct the heat source from the LED. From the perspective of LED heat dissipation, the LED heat dissipation substrate can be subdivided into two categories: LED chip substrate and system circuit board. These two types of heat dissipation substrates carry LED chips and LED chips respectively. Among them, the LED chip substrate is mainly used as a medium to conduct the heat energy of the LED chip to the system circuit board; the system circuit board is responsible for conducting the heat energy to the heat dissipation fins, the shell or the material in the atmosphere.

Carefully select heat dissipation substrate materials and thermal conductive adhesives

Ai Sibo Electronics Engineer You Huiru pointed out that in terms of system circuit board heat dissipation, the system circuit boards of early LED products were mostly PCB-based. However, with the increasing demand for high-power LEDs such as LED street light applications, the heat dissipation capacity of PCB materials is insufficient to cope with it. Therefore, the industry has developed a high thermal conductivity aluminum substrate (MCPCB), which mainly uses the better heat dissipation characteristics of metal materials to achieve the purpose of heat dissipation of high-power products.

In addition to paying attention to the material of the heat dissipation substrate, the thermal conductive adhesive used for bonding the aluminum substrate must also be carefully selected. Dr. Ye Shengwei, deputy general manager of the R&D department of Guanpin Chemical, pointed out that some manufacturers think that the thicker the thermal conductive adhesive film or thermal conductive pad, the better, but ignore that the thicker the thermal resistance, the greater. Furthermore, the thermal conductive adhesive film or soft thermal conductive pad cannot be truly tightly fitted with the substrate, and the many pores existing on the bonding surface are another form of thermal resistance. Combining these factors, the efficiency of heat dissipation and heat conduction will be reduced, and the right thermal conductive adhesive is needed to reduce thermal resistance.

He pointed out that the soft ceramic thermal conductive adhesive used in screen printing is a soft semi-liquid. When the screen printer scraper is applied to the aluminum substrate, the thermal conductive particles will penetrate into the pores on the surface of the substrate and fill them up, thus forming a completely flat and pore-free plane. When it is bonded to the LED chip carrier, it will be completely tight, thus reducing thermal resistance, and the heat can be quickly conducted away. The ambient temperature of the LED chip will be reduced, and the light decay of the LED street lamp will naturally be delayed. He also emphasized that the soft ceramic thermal conductive adhesive used in screen printing is ductile. When it is baked at high temperature or reflowed, it will change with the thermal expansion and contraction of the aluminum substrate. The stress is very small, so it will not cause the aluminum substrate to bend or even explode.

However, You Huiru pointed out that although the system circuit board can effectively dissipate the heat generated by the LED chip to the atmosphere, the heat generated by the LED crystal must first be effectively conducted from the crystal to the system circuit board. Otherwise, as the LED power increases, the overall LED heat dissipation bottleneck will appear in the LED crystal heat dissipation substrate. Currently, the LED crystal substrate on the market is mainly based on ceramic substrates, which can be roughly divided into thick film ceramic substrates, low temperature co-fired multilayer ceramics, and thin film ceramic substrates according to different circuit preparation methods. Among them, thin film ceramic substrates are relatively new technology, which can effectively meet the wiring accuracy and sintering shrinkage ratio required by the flip chip packaging method.