Analysis of Process Difference Between Thick Film and Thin Film of LED Heat Dissipation Substrate
LED modules are now widely used in electronic related products. With the expansion of applications and the continuous improvement of lighting systems, the demand for high power has increased rapidly since 1990, especially in the case of white light and high power type. The LED power used in the lighting system has not only 1W, 3W, 5W or even more than 10W, so the heat dissipation performance of the heat dissipation substrate has become the most important issue. The main factors affecting LED heat dissipation include the material and design of LED die, die carrier, chip package and module. The thermal energy accumulated by LED and its package materials is mostly transmitted in conduction, so LED crystal The design and material of the pellet substrate and LED chip package have become the main key.
2, the impact of the heat sink substrate on the LED module
LEDs have seen red LEDs since 1970, and have since evolved into blue and green light. The initial use is mostly on some labels, such as indications on household appliances, and since 2000, white high-power LEDs have been used. Appeared, let the use of LEDs begin to enter another stage, such as outdoor large-view version, backlight of small display, etc. (Figure 1), but with the rapid evolution of high power, it is expected that after 2010, automotive lighting, The demand for indoor and special lighting is increasing, but the requirements for heat dissipation of these high-power lighting devices are becoming more stringent. Because ceramic substrates have high heat dissipation capability and high heat resistance and air tightness, The ceramic substrate is one of the most commonly used substrate materials for high-power LEDs.
However, the most common ceramic substrates currently on the market are ceramic heat-dissipating substrates made of LTCC or thick film technology. This type of product is subject to the bottleneck of screen printing technology, which makes it impossible to match the higher-order welding. , Eutectic or Flip chip packaging, and the ceramic heat sink substrate developed by the thin film process technology provides high alignment precision products in response to the development of packaging technology.
2.1, the choice of heat sink substrate
In terms of the development of the LED die-carrying substrate, the substrate material of the conventional PCB has a highly commercial feature in terms of carrying the die, and has considerable influence in the initial stage of LED development. However, with the increase of LED power, the heat dissipation capability of the LED substrate has become one of its important material properties. For this reason, the ceramic substrate has gradually become the main heat-dissipating substrate material for high-performance LEDs (as shown in Table 1), and gradually Accepted by the market and widely used. In recent years, in addition to the material properties of the ceramic substrate itself, the requirements for the line width, wire diameter, metal surface flatness and adhesion of the metal lines on the substrate are increasing, so that the ceramics prepared by the conventional thick film process are required. The substrate is gradually used, and a thin film type ceramic heat dissipation substrate has been developed. This paper will analyze the difference between the thick film and the film process and the product characteristics of the ceramic heat dissipation substrate.
Table 1, heat dissipation coefficient of various materials
3, ceramic heat sink substrate
From traditional PCB (FR4) boards to ceramic substrates, LEDs are constantly evolving to higher power requirements. At this stage, the metal lines of ceramic substrates are mostly formed by thick film technology. However, the alignment accuracy of thick film printing makes it Can not keep up with the advancement of LED packaging technology, the main factor is the use of eutectic and flip chip packaging technology in the thermal design of higher power LED components, the introduction of these technologies can not only use high luminous efficiency LED dies It can also greatly reduce the thermal resistance value and make the joint degree more perfect, so that the power of the overall operation is relatively improved. However, the application of these two bonding methods requires the basis of accurate metal circuit design. Therefore, the thin film ceramic heat dissipation substrate with the exposure lithography as the alignment mode becomes the mainstream of the precision circuit design.
3-1, thick film printing ceramic base plate
Thick film processes mostly use screen printing to form lines and graphics. Therefore, the integrity of the line pattern and the accuracy of the line alignment tend to be significantly progressively different as the number of printing increases and the tension of the screen changes. Influencing the accuracy of the alignment on the subsequent packaging process; further, as the component size continues to shrink, the size and resolution of the screen printing also has limitations. As the size shrinks, the difference in the size of each unit in the screen printing The difference between (uniformity) and metal thickness will also become more apparent. In order to continuously reduce the size of the line and the strict requirements of precision, the production technology of the LED heat sink substrate is bound to continue to improve. Therefore, the introduction of the thin film process has become one of the improvement methods, but there are only a handful of domestic manufacturers of mature ceramic substrate thin film metallization process technology. To this end, ICP, which is a thin-film component, analyzes its process and product characteristics for its own-developed film substrates and conventional thick-film substrates (see Table 2 below).
3-2, the film process is applied to the ceramic substrate
The introduction of thin film technology can solve the process bottleneck of the above-mentioned line size reduction. Combined with high vacuum coating technology and yellow light lithography technology, the circuit pattern size can be greatly reduced, and the precise line alignment requirements can be met at the same time. The low difference (high uniformity) is the result that is not easily achieved by traditional screen printing. Under the requirement of high thermal conductivity, the film processing technology of ICP has overcome the bottleneck of the precision of the thick film process at present. Figure (2) is a simple flow chart of the film process, in the blank ceramics. After pre-treatment on the substrate (alumina/aluminum nitride), seeding is applied, photoresist coating, exposure and development, thickening of the desired lines (electroplating/electroless plating), and finally passing The film and etching steps shape the line, and the products prepared by the process have higher line precision and better flatness of the metal plating surface. Figure (3) shows the optical microscopy image of the metal line of the ç‘·Spo film substrate product and the traditional thick film product. It can be clearly seen that the thick film printing circuit has obvious pits on the surface and the flatness of the lines is not good. In contrast, the metal lines prepared by the thin film process are not only clear in color but straight lines.
The difference in geometric accuracy between the above thick/film metal lines, coupled with the thick film line, is caused by the progressive tolerance of the array pattern due to the screen problem of the screen, making the thick film printing product easier on subsequent wafer placement. Causes the offset of the part or the edge finding abnormality. In other words, the alignment of the thick film printing products and the accuracy of the wiring are not precise enough, which limits the process margin of the chip packaging process. However, thin film process products can greatly improve their phenomena.
However, from the perspective of product cost structure, the process equipment (yellow lithography) of the film product shown in Table 2 is higher in cost than the thick film product in the production environment (dust-free or clean room), but the metal circuit of the film process is more. Thick copper materials, compared to thick film printing thick silver, the material cost is relatively low, therefore, it can be expected that when the metallization of the ceramic substrate by the thin film process, the economic scale, The cost will gradually approach the thick film product.
Figure 2, film process
Figure 3 Difference between thick film and film line
3-2-1, alumina ceramic substrate
The above part is for the different parts of the process, and the other related to the heat dissipation is the substrate material. The material used for the LED heat dissipation substrate is mainly ceramic, and the alumina ceramic substrate should be easy to obtain. The lower cost material is the main material currently used on components. However, thick film technology or thin film technology is used to prepare metal lines on alumina ceramic substrates, and the metal line and the substrate have no significant adhesion or characteristics. The difference, and the two main processes show the main difference is that the film process can provide smaller line sizes and higher graphics accuracy that cannot be achieved by thick film technology under the requirement of line size reduction.
3-2-2, aluminum nitride ceramic substrate
On the premise of higher power LED applications, aluminum nitride (170-230W/mK) with high thermal conductivity will be the preferred material for heat-dissipating substrates, but thick-film printed metal layers (such as high-temperature silver glue) need to go through High temperature (higher than 800oC) sintering process, which is performed in an atmospheric environment, which may cause an oxide layer between the metal line and the aluminum nitride substrate, thereby affecting the adhesion between the line and the substrate; however, the film process is 300. Prepared under the conditions of Â°C and below, no doubt about the formation of oxides and poor adhesion, but also the advantages of line size and high precision. The thin film process creates more application space for high-power aluminum nitride ceramic LED heat sink substrates.
Above, we have made the difference analysis of the LED heat-dissipating substrate on two different processes. The film-prepared ceramic heat-dissipating substrate has high equipment and technology, and needs to integrate material development thresholds such as exposure, vacuum deposition, development, and evaporation. (Evaporation), sputtering (Sputtering) plating and electroless plating technology, the current market scale, the relative cost of film products is relatively high, but once the market size reaches a certain level, it will certainly be reflected in the cost structure, relative The difference between the price and the thick film process will be greatly shortened.
Under the development trend of high-performance, high-quality products and high-production LED ceramic substrates, the selection of high-heat-dissipation and high-precision thin film process ceramic substrates will become a trend to overcome the current thick film process. The bottleneck that the product cannot break through. Therefore, the expected thin film ceramic substrate will gradually be applied to high-power LEDs, and with the rapid development of high-power LEDs, the economic scale, regardless of high-power LED die, thin film ceramic heat sink substrate, packaging process cost, etc. Both will be greatly reduced, which will further accelerate the quantification of high-power LED products.
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