Friday, September 6, 2013

LED cooling substrate design and process analysis

1.       Brief introduction
LED module now is widely used in electronics-related products, with the expanding range of applications and development of lighting systems, from about 1990 high power requirements are rapid rise, especially in the form of white high power. The lighting system is now use  LED Power more than 1W, 3W, 5W even reach more than 10W. Therefore, the thermal performance of heat dissipation substrate has become the most important issues. The main factors affecting LED cooling includes LED chip, chip carrier board, chip packaging and module materials and design, the accumulated heat of the LED and packaging materials mostly dissipated by conduction. So the LED chip substrate and the LED chip package designs and materials has become the main key.

2.       Heat dissipation substrate effect to LED module
Red LED light appear from 1970,then soon evolution to the blue and green. The early mostly use on some signs, such as the directions on the appliances. To the beginning of 2000, the LED's use began to enter another phase as the emergence of white high-power LED. Look like a large-scale outdoor version, a small display backlight. But with the rapid evolution of high-power LED, estimated that from 2010, the automotive lighting, indoor lighting and special lighting demand will increasing. However, these high power lighting equipment are more stringent to the thermal performance requirements. The ceramic substrate is currently the most commonly used in high-power LED of the substrate material which is a high cooling capacity and high heat resistance, air tightness.
However, the ceramic substrates are commonly make of LTCC or thick-film technology on current market. This type of product is subject to the preparation of screen printing technology, makes its alignment accuracy can’t fit more advanced welding. While the ceramic substrates which use of thin-film technology provides a product of high alignment accuracy.

3.       Thermal substrate selection
Traditional PCB substrate material features a highly commercialized, the early development of the LED has a considerable influence. However, with improved power of LED, The LED substrate cooling capacity will become an important feature of the material, therefore, high-performance ceramic substrate has become the main cooling LED substrate material, and gradually accepted by the market. In recent years, in addition to the material properties of the ceramic substrate itself issues to be considered, the metal on the substrate Line width, diameter, metal surface roughness and adhesion requirements on the increase, making a prepared traditional ceramic thick film technology substrate gradually not enough for use, and thus developed a thin-film ceramic substrates, This article will focus on the thick and the thin film process and product characteristics to analyze the differences.

4.       Ceramic substrates
From the traditional PCB (FR4) board, and now the ceramic substrate, LED constantly to the needs of the development of higher power, This stage ceramic substrate metal lines are forming by thick film technology. However, thick film printing alignment accuracy make it impossible to keep up with advances of LED packaging technology. The main factor in the higher power LED element thermal design is the use of flip-chip and eutectic packages Technology. The introduction of these technologies can not only use high luminous efficiency of the LED chip, but also can greatly reduce the thermal resistance and allows engagement is more perfect, so that the overall operation of the power are relatively improved. However, the application of these two bonding method needs to have precise basis for the design of metal lines, position the exposure lithography method for thin-film ceramic substrates becomes the precise circuit design mainstream.

4.1       Thick film printing ceramic substrate
Thick film technology are mostly formed graphic and line using screen printing. Therefore, the integrity and accuracy on the bit lines of line graph tend to increase as the number of printing screen tension changes with significant differences in progressivity. This result will affect the subsequent packaging process on position accuracy. As device dimensions continue to shrink, screen printing graphic size and resolution has its limitations, as the reduced size, screen printing graphics presented in each unit size difference (uniformity) and the metal thickness differences will become more apparent. In order to be able to continue to shrink circuit size and accuracy, LED heat sink substrate is bound to continue to improve production technology. Thus the introduction of thin film technology has become one of the ways to improve, however, only a handful of manufacturers has a mature film metallized ceramic substrate technology in china internal.
4.2       Thin film technology applied to ceramic substrates
The introduction of thin film technology is solved circuit size reduction process. The combination of high vacuum coating technology and photolithography technique, Line pattern size can be significantly reduced, and at the same time meet the precise requirements on the bit line.
5.       Conclusion

We have two different LED cooling substrates to make a difference in the analysis process. Thin film technology ceramic substrates with high equipment and technology. Integrated materials development threshold required, such as exposure, vacuum deposition, development, evaporation, sputtering plating and electroless plating techniques. In the current market scale, film products cost relative higher, but once the market reaches a certain level, they will be reflected in the cost structure, thick film technology on price differences will significantly shorten relatively.

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