5 , ceramic substrate characteristics
5.1 Thermal conductivity
Thermal conductivity represents the ability of the substrate material itself to directly conduct heat, the higher the value represents the better its ability to dissipate heat. In the field of LED thermal substrate is the main role is how to effectively conduct heat from the LED chip to the system heat dissipation, in order to reduce the temperature of the LED chip, increase luminous efficiency and extend the life of the LED, therefore, the thermal conductivity of the thermal substrate has become the industry in the choice of thermal substrate, one of the important evaluation items. Review Table 1, the comparison of the four ceramic thermal substrates can be clearly seen, although the thermal conductivity of Al2O3 material is about 20 ~ 24 between, LTCC to reduce its sintering temperature and add 30% ~ 50% of the glass material, so that its thermal conductivity down to about 2 ~ 3W/mK; and HTCC because of the general co-firing temperature is slightly lower than the sintering temperature of pure Al2O3 substrate, so that its lower thermal conductivity due to the material The lower density of HTCC makes the thermal conductivity lower than that of Al2O3 substrates at about 16~17W/mK. Generally speaking, LTCC and HTCC are not as effective as DBC and DPC thermal substrates.
5.2 Operating environment temperature
The operating environment temperature refers to the highest process temperature used in the production process, and in terms of a production process, the higher the temperature used, the higher the relative manufacturing cost and the yield is not easy to control. The process temperature of LTCC/DBC is between 850~1000℃. In addition, HTCC and LTCC must be stacked after the process and then sintered, making each layer will have the problem of shrinkage ratio, in order to solve this problem related to the industry is also trying to find a solution. On the other hand, DBC has very strict requirements for process temperature accuracy, which must be in the temperature range of 1065~1085℃ to make the copper layer melt into eutectic melt and closely bond with the ceramic substrate, if the temperature of the production process is not stable enough, it will definitely cause the phenomenon of low yield. The process temperature and margin considerations, DPC process temperature of only about 250 ~ 350 ℃ to complete the production of heat dissipation substrate, completely avoid the high temperature for the material caused by the destruction or size variation phenomenon, but also exclude the problem of high manufacturing costs.
5.3 Process capability
The process capability is mainly to indicate what kind of technology is used to complete the metal circuit of various heat sink substrates, because the method of circuit manufacturing/forming directly affects the characteristics of circuit accuracy, surface roughness, alignment accuracy, etc. Therefore, under the demand of high power and small size fine circuit, the process resolution becomes one of the important items that must be considered. HTCC are using thick film printing technology to complete the line production, the thick film printing itself that is limited by the stencil tension, in general, the line surface is more rough, and easy to cause the alignment is not accurate and progressive tolerance is too large phenomenon. In addition, the multi-layer ceramic lamination sintering process, there is the problem of shrinkage ratio to be considered, which makes its process resolution is more limited. Although DBC uses micro-shadowing process to prepare metal lines, the lower limit of metal copper thickness is about 150~300um due to the limitation of its process capability, which makes the upper limit of its metal line resolution only between 150~300um (with depth to width ratio of 1:1 as the standard). In contrast, DPC is made by thin-film process, using vacuum coating, yellow micro-shadowing process to produce lines, so that the lines on the substrate can be more accurate, high surface flatness, and then use electroplating / electrochemical deposition to increase the thickness of the line, DPC metal line thickness can be designed according to the actual needs of the product (metal thickness and line resolution). Generally speaking, the resolution of DPC metal line is about 10~50um under the principle of metal line depth to width ratio of 1:1. Therefore, DPC eliminates the sintering shrinkage ratio of LTCC/HTCC and the problem of stencil tension of thick film process.
5.4. Application of Ceramic Heat Sink Substrate
Ceramic thermal substrates can vary in appearance depending on the demand and application. On the other hand, various ceramic substrates can also be made according to the different manufacturing methods of the products, to make basic distinctions.LTCC thermal substrates in the application of LED products, mostly large size high power and small size low power products, basically the appearance of most of the concave cup shape, and according to the user's needs can be made with wire frame & no wire frame two kinds of thermal substrates, concave cup shape is mainly for the packaging process using The concave cup shape is mainly designed for the packaging process using the simpler dispensing method, and using the edge of the concave cup as the path of light reflection, but the LTCC itself is limited by the process factors, making it difficult to prepare the product into a small size, in addition, the use of thick film production line, making the line accuracy is not enough to meet the high power small size LED products. And LTCC process and appearance similar to the HTCC, in the LED heat sink substrate this piece, has not been commonly used, mainly because HTCC using 1300 ~ 1600 ℃ high-temperature drying hardening, so that the increase in production costs, the relative HTCC substrate costs are also high, so the LED industry towards low-cost tendency, facing a more demanding test of HTCC.
On the other hand, DBC and DPC are not only different from LTCC/HTCC in appearance, but also different from the way LED products are packaged, DBC/DPC are flat heat sink substrates, and flat heat sink substrates can be prepared according to custom metal circuit processing, and then cut into small size products according to customer needs, supplemented by eutectic/complex crystal process, combined with a very sophisticated fluorescent powder coating Combined with the very sophisticated fluorescent powder coating technology and advanced encapsulation technology, casting film molding can significantly improve the luminous efficiency of LEDs. However, DBC products are limited by the process capability, so that the upper limit of line resolution is only 150 ~ 300um, if you want to make special fine line products, you must use the grinding process to reduce the thickness of the copper layer, but it causes the surface flatness is not easy to control and increase the additional cost and other issues, making DBC products are not easy to eutectic / complex crystal process high line accuracy and high flatness requirements of the application. DPC uses a thin film micro-imaging process to prepare the metal circuit processing, which has the characteristics of high line accuracy and high surface flatness, and is very suitable for the compound/eutectic bonding process, which can significantly reduce the wire cross-sectional area of LED products and thus improve the efficiency of heat dissipation.
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