High-power ultra-bright LED printed circuit board2020-06-02
High-power ultra-bright LED printed circuit board
Recently, combined with the latest achievements in the field of LED lighting, interest in creating light sources based on powerful ultra-bright LEDs has increased dramatically. The luminous efficiency of semiconductor LED has reached 100lm/W. This ultra-high-brightness LED replaces the traditional incandescent lamp and is used in almost all areas of lighting technology: street lamps, automotive lighting, emergency lighting, advertising signs, LED panels, indicators, traffic lanes, traffic lights, etc. These LEDs have become an integral part of decorative lighting and dynamic systems due to their monochrome and switching speed. They can also be used in places where substantial energy savings are required,
Compared with traditional light sources, the main advantages of powerful ultra-bright LEDs are:
Long service life (tens of times or even hundreds of times longer than traditional incandescent bulbs), so the possibility of replacing burning bulbs is greatly reduced, which is especially important for hard-to-reach places;
Efficiency and high efficiency: under similar brightness, the energy consumption of LED lamps is 3-5 times less than halogen lamps or incandescent lamps;
Easy to install and compact: surface mount installation and small size LEDs allow developers and designers to choose interesting solutions to create lighting systems;
A wide range of choices for white, color and RGB LED shadows: allows you to use dynamic changes in color and brightness to realize interesting ideas for lighting or decorative lighting developers;
The low heat radiation of LED lamps allows you to install them in places where incandescent lamps cannot be used due to high heat.
Unlike traditional incandescent lamps, LEDs do not radiate heat to the surrounding space, but conduct heat in the direction of the transition from pn to the heat sink in the LED housing (usually the output of the LED or a special metal plate). Therefore, the heat removal process is more complicated and specific. The heat dissipation path is composed of many thermal resistors: “pn junction-heat sink of the housing”, “heat sink of the housing-a printed circuit board”, “a printed circuit board-heat sink”, “heat sink-environment”. As a result, the use of high-power LEDs is associated with a high probability of an excessive increase in transition temperature, which directly affects the life, reliability, and light characteristics of the LED.
Research data shows that about 65-85% of the electrical energy is converted into heat when the LED is working. However, depending on the thermal conditions recommended by the LED manufacturer, the life of the LED can be as long as 10 years. However, if you violate the thermal specifications (usually operating at a transition temperature exceeding 120 … 125°C), the life of the LED may be shortened by 10 times! And in the case of serious failure to meet the recommended thermal conditions, for example, when the LED of the emitter type is turned on for more than 5-7 seconds without a heat sink, the LED will fail even during the first turn-on process. In addition, the increase in the transition temperature causes the brightness of the glow to decrease and the operating wavelength to shift. Moreover, the polymer used to make the LED housing cannot be heated above a certain limit because the LED components (contacts, frame, crystal, lens material) have different linear expansion coefficients and may break the contact connection. Therefore, it is very important to calculate the thermal state correctly and dissipate the heat generated by the LED as much as possible.
If the power LED used is less than 0.5 watts, then conventional LEDs that assemble PCB-level FR-4 printed circuit boards are acceptable. However, when dissipating higher power, it may be necessary to use a special printed circuit board with a metal base and a dielectric layer, the thermal conductivity of which is improved, and the thermal resistance is 0.45 to 1.5 K/W.
The main manufacturers of high-power LEDs, such as Cree, Osram, Nichia, Luxeon, Seoul Semiconductor, Edison Optoelectronics, etc., have long been simplified and enabled for the expansion of LED applications, in the form of LED modules, or In the printed cluster circuit board and x with metal base (international classification IMPCB-insulated metal printed circuit board, or AL PCB-aluminum printed circuit board s). LED clusters are off-the-shelf connector plates of various shapes (star, circular, linear, rectangular, hexagonal) and one or more LED bases, lens (collimator) can be installed, simple power supply Connection and easy installation. By soldering to the contact pads of the circuit board s mounted on the circuit board e connector, a cluster connection for powering the LED driver can be achieved. Such clusters sometimes contain both limiting resistors and the power driver itself, so they can be directly connected to low-voltage power supplies.
This type of cluster is used for the installation of non-standard luminaires, indoor and landscape lighting, and the creation of dynamic lighting shows-that is, you can arrange these size and shape LED clusters as elements of the LED matrix without increasing costs And place. But if the standard form is difficult or impossible to do-unsatisfactory size, LED or layout, then the spacing between the designs is customized yvaetsya, your personalized design printed circuit board trumpet-high-tech and adaptive lighting The best solution for the system.
If necessary, the cluster based on a printed circuit board with a metal base and the LEDs mounted thereon can be installed on an additional heat sink-the last link of the heat dissipation channel. The choice of the heat sink can be an aluminum heat sink or a metal shell of the product, depending on which part of the total budget of the thermal resistance “transition-environment” was “wasted” at the previous link of the thermal path.
For LEDs with a power consumption of 1 W (assuming that natural convection is not complicated at 25°C), a board must be used and the area of the aluminum base is not less than 6.5 cm 2. If the LED under the area board size corresponds roughly to a given value, when the cluster device does not require an additional heat sink, the LED is at rated operation. When working in a sealed enclosure, as well as in an environment with a higher ambient temperature, no additional cooling or additional heat sinks, or increased area board trumpets are required.
In order to provide LED chips at the recommended operating temperature, the lower substrate trumpet is set in a cluster corresponding to the applicable type through a special plastic mixture of multiple heat sinks-conductive paste or because they are called when filled with thermal grease The bumps provide good thermal contact and maximum thermal conductivity of the system.
When using other heat sinks, for example, the power supply current of the cluster with Cree XR-E LED can be increased to 1 A, the current of the XP-E LED cluster can be increased to 700 mA, XP-C, XR-C The current of the LED cluster is increased to 500 mA, which will increase the luminous intensity by about 70%.
Let’s get entangled in the printed circuit board E for high-power LED clusters-printed circuit boards and metal bases. For example, in this type of board, a metal plate is used as the base, on which one or more conductive layers of copper foil Bond with glass fiber impregnated with resin (prepreg). This card trumpet is used in products in the entire area where there is a high local or distributed heat load. The simplest type of printed circuit board is small, so the most cost-effective LEDs for effective surface mounting are printed single-layer circuit boards and with aluminum bases. In essence, the circuit board a is a single-layer printed circuit board glued to an aluminum board. The generated heat easily passes through the insulator, and then quickly dissipates through the aluminum firmly attached heat sink formed as part of the printed circuit board s.
Circuit boards of ultra-high-brightness LEDs are usually covered with black or white solder resist layers, which further increase light absorption or light reflection, respectively, which beneficially affects the temperature conditions and the design of the lamp.
Briefly consider the materials used in the circuit board and x metal base:
Copper foil-copper foil with a thickness of 35-350 microns, is the standard for the production of printed circuit boards.
Dielectric-prepreg-glass fiber impregnated with epoxy resin with a thickness of 50-150 microns. As a prepreg, it can be used as a conventional epoxy glass fiber FR-4 and a special thermally conductive composition (T-preg) with optimal thermal and electrical insulation properties. It is a special chemical-resistant structure with a high thermal conductivity of 75-200μm and is made of a special dielectric-polymer and special ceramic mixture. The polymer is selected according to the dielectric properties of the polymer, and the ceramic filler is designed to increase thermal conductivity, so the material has excellent dielectric properties and very low thermal resistance. For example, the material of this layer Bond Ply Thermal Clad IMS (Bergquist) has a volume resistivity of at least 1014 ohm•cm. When the thickness of the dielectric layer is 75 μm, the allowable operating voltage between the layers is 5.5 kV AC or higher, and the thermal conductivity is not less than 1.3 W/mK, which is sufficient for most applications.
A circuit board with a metal base dielectric layer (copper core) and x are connected to a lower metal (aluminum or copper) core. The lower metal core serves as a heat sink for the entire printed circuit board s and plays from the upper layer to the lower layer (metal base) Thermal conductivity.
The main comparative characteristics of the dielectric are given in the table.
In the design of plates with metal bases, the coefficient of thermal expansion (CTE) of the substrate material plays an important role. The use of materials with large KTP at high temperatures can cause internal mechanical stress in the structure. Therefore, for high temperature applications where this parameter is critical, materials with low KTP low carbon steel substrates (thicknesses 1 and 2.3 mm) are used.
Although copper has the best thermal conductivity, aluminum is still the most commonly used material for plates with metal substrates because aluminum is cheap and importantly lightweight.
Thermal conductivity of waste aluminum substrate:
Aluminum 1100 (an analog of HELL)-222 W/mK
Aluminum 5052 (Analog AMg2.5) -138 W/mK
Aluminum 6061 (similar to AD33) -167 W/mK
Today, several large manufacturers of thermally and electrically insulating materials produce basic materials for the manufacture of printed circuit boards with metal substrates:
Bergquist (United States)
Laird (Thermagon) (United States)
The materials provided with different characteristics can meet the most demanding tastes of electronic device developers and technicians, and are expected to directly obtain economic benefits in the production stage and subsequent operations of products. These materials themselves meet the requirements of commercial and military standards and can be used in almost any field: from household appliances to military equipment.
For example, a widely used (due to the high price/mass ratio in the production of LED clusters) thermal conductive polymer material based on aluminum and ceramic-T111 (Toke) has the following characteristics:
The thickness of the aluminum base-1.5 mm
The thickness of the copper foil is 35 microns
Dielectric thermal conductivity -2.2 W/mK
Dielectric thermal resistance -0.7°C/W
Thermal conductivity of aluminum substrate (5052-Analog AMg2.5) -138 W/mK
Breakdown voltage-3 KV
Glass transition temperature (Tg) -130
Volume resistance -108MΩ×cm
Surface resistance -106MΩ
Maximum working voltage (CTI) -600V
Most processes for manufacturing printed circuit boards with metal substrates, such as etching, applying protective masks, applying protective metal coatings (HASL), marking, etc., are similar to the process of manufacturing traditional circuit boards from FR-4 , Only different in machining and contour.
Printed circuit board S is not limited to the use of high-power LEDs on metal substrates, and can also be used in any product, where heat is important and size. The use of such boards greatly simplifies the design of radio electronics, especially at high power, because heat dissipation is no longer strictly dependent on the relative position of the components and the surrounding free space board: heat is dissipated through the substrate. No additional radiators are needed-radiators, tires, etc. As a result, the degree of integration of components on the electronic board increases, and its size decreases.
Printed circuit board S has many advantages compared to traditional boards with metal alkali and E:
No need to use other radiators and special thermal paste to dissipate heat.
Reduce/eliminate the need for forced air cooling fans.
Increase the mechanical rigidity of the product.
They increase the integration of high-power equipment components that operate at high current and high voltage at high operating temperatures.
Reduce the thermal stress of all components, thereby increasing the life of components and product durability.
The cooling performance of such a plate makes it easier to organize heat removal, which beneficially affects the cost of the product.
Due to any configuration of the circuit board, they can greatly save equipment space.
These boards have excellent electromagnetic compatibility and shielding performance.
The use of such boards can improve the reliability of the equipment (mean time to failure).
The possibilities of combining on a single printed circuit board include: multiple LEDs, components mounted using standard automatic soldering techniques, and low heat-all of these combinations make it possible to create compact, high-performance light sources.
The use of cost-effective lighting systems is one of the most important areas for introducing energy-saving technologies in industry and public utilities. Today, among the lighting energy-saving technologies, the most advanced is the creation of powerful semiconductor LEDs and lighting systems based on them. According to experts, as semiconductors replace electronic lamps in their time, the introduction of new LED lighting technology will gradually almost completely replace traditional luminaires around the world. It seems that the waiting time is not long.