2.2 3D ceramic substrate preparation technology process and characteristics
Many microelectronic devices (such as accelerometers, gyroscopes, deep-ultraviolet LEDs, etc.) chips are very sensitive to air, moisture, dust, etc. For example, LED chips can theoretically operate for more than 100,000 hours, but water vapor erosion can significantly shorten their lifetime (even to a few thousand hours). In order to improve the performance (especially the reliability) of these microelectronic devices, the chips must be encapsulated in a vacuum or protective gas to achieve a hermetic package (the chip is placed in a closed cavity, isolated from external oxygen, moisture, dust, etc.). Therefore, a three-dimensional substrate with a cavity (dam) structure must first be prepared to meet the requirements of packaging applications. At present, the common three-dimensional ceramic substrates are: High/Low Temperature Co-fired Ceramic Substrate (HTCC/LTCC), MultilayerSintering Ceramic Substrate (MSC), direct bonding three-dimensional ceramic substrates (MSC), and direct bonding three-dimensional ceramic substrates (MSC). MSC), Direct Adhere Ceramic Substrate (DAC), Multilayer Plated Ceramic Substrate (MPC), and Direct Molding Ceramic Substrate (DMC), etc.
High/Low Temperature Co-fired Ceramic Substrate (HTCC/LTCC): The HTCC substrate is prepared by adding ceramic powder (Al2O3 or AlN) to organic binder and mixing it well to become a paste-like ceramic paste, then scraping the ceramic paste into flakes using a squeegee, and then forming the flakes into blanks through a drying process; then drilling through-holes according to the design of the line layer, using screen printing metal paste for Then, the holes are drilled according to the design of the line layer, and the metal paste is screen-printed to lay out the lines and fill the holes, and finally the embryonic layers are stacked and sintered in a high-temperature furnace (1600°C), as shown in Figure 16. Due to the high temperature of the HTCC substrate preparation process, the choice of conductive metals was limited to metals with high melting points but low conductivity (e.g., W, Mo, and Mn), resulting in high production costs. In addition, due to the limitation of screen printing process, the HTCC substrate line accuracy is poor, and it is difficult to meet the demand of high precision packaging. However, HTCC substrates have high mechanical strength and thermal conductivity [20 W/(m-K) ~ 200 W/(m-K)] and stable physical and chemical properties, which make them suitable for high-power and high-temperature device packaging, as shown in Figure 17 (a).
In order to reduce the HTCC preparation process temperature and improve the conductivity of the line layer, LTCC substrates have been developed. Similar to the HTCC preparation process, LTCC is prepared by adding a certain amount of glass powder to the ceramic paste to reduce the sintering temperature and using metallic pastes such as Cu, Ag and Au with good conductivity, as shown in Figure 17 (b). Yuan et al. used CaO-BaO-Al2O3-B2O3-SiO2/AlN system to develop LTCC substrates with a thermal conductivity of 5.9 W/(m-K), a dielectric constant of 6.3, a dielectric loss of 4.9 × 10-3, and a bending strength of 178 MPa when the AlN component content was 40%. The bending strength of the LTCC substrate prepared using Li2O-Al2O3-SiO2/Al2O3 system was 155 MPa and the dielectric loss was 2.49 × 10-3.
Although LTCC substrates have the above-mentioned advantages, the addition of glass powder to the ceramic paste results in a low thermal conductivity of the substrate [typically 3 W/(m-K) to 7 W/(m-K)]. In addition, as with HTCC, the LTCC substrate uses screen printing technology to produce metallic lines, which may cause alignment errors due to screen spreading problems, resulting in low accuracy of metallic line layers; moreover, there are also differences in shrinkage ratios in the stacked sintering of multilayer ceramic blanks, which affects the yield rate and to some extent limits the development of LTCC substrate technology. Sim et al. improved the LTCC substrate packaging form by removing the insulating layer between the chip and the metal substrate, and the simulation and experimental results showed that the thermal resistance was reduced to 7.3 W/(m-K) to meet the demand of high-power LED packaging.
Multi-layer Sintered 3D Ceramic Substrate (MSC) Technology Process and Features
Unlike HTCC/LTCC substrates, which are prepared in a single process, Taiwan Sun Rise has prepared MSC substrates using the multiple sintering method. The process flow is shown in Figure 18, where a thick film printed ceramic substrate (TPC) is prepared first, followed by multiple screen printing of the ceramic paste on the flat TPC substrate to form a cavity structure, and then sintered at high temperature to produce the MSC substrate sample shown in Figure 19. Since the ceramic paste sintering temperature is generally around 800°C, the lower TPC substrate line layer must be able to withstand such high temperatures to prevent defects such as delamination or oxidation during the sintering process. As can be seen from the above, the TPC substrate line layer is prepared by high temperature sintering of metal paste (generally 850°C ~ 900°C), which has good high temperature resistance and is suitable for subsequent preparation of ceramic cavities by sintering. The cavity structure and the flat substrate are inorganic ceramic materials with matching coefficients of thermal expansion, so there is no delamination and warpage during the preparation process. The disadvantage is that the lower TPC substrate line layer and the upper cavity structure are screen-printed wiring, and the graphic accuracy is low; at the same time, the thickness (depth) of the prepared MSC substrate cavity is limited due to the screen-printing process. Therefore, MSC three-dimensional substrate is only suitable for small size and low precision requirements of electronic device packaging.
Direct bonding three-dimensional ceramic substrate (DAC) process flow and characteristics.
The above mentioned HTCC, LTCC and MSC substrate line layers are prepared by screen printing with low precision, which is difficult to meet the requirements of high precision and high integration packaging, so the industry proposes to prepare 3D ceramic substrates by molding cavities on high precision DPC ceramic substrates. Since the metal circuit layer of DPC substrate will be oxidized, blistered or even delaminated at high temperature (over 300°C), the preparation of 3D ceramic substrate based on DPC technology must be carried out at low temperature. Taiwan Asperity Corporation (ICP) proposes to use the gluing method to prepare 3D ceramic substrates, and the sample is shown in Figure 20. The metal ring and the DPC ceramic substrate were first processed, and then the metal ring was aligned with the DPC substrate by organic adhesive and then cured by heating, as shown in Figure 21. Because of the good fluidity of the adhesive, the gluing process is simple, low cost and easy to realize mass production, and all the preparation processes are carried out at low temperature without damaging the circuit layer of the DPC substrate. However, due to the poor heat resistance of organic adhesives, the coefficient of thermal expansion between the curing body and metal and ceramic, and non-gas-tight materials, DAC ceramic substrates are mainly used in the packaging of electronic devices with high requirements for line accuracy, but low requirements for heat resistance, gas-tightness, reliability, etc.
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